Since the cells containing TNW gave high conversion efficiencies, we examined the effects of content of TNW in the electrode composed of TNW and P-25 upon the conversion efficiency with variation in TNW from 0 % to 100 %. Content of TNW was defined as percentage of titanium atoms of TNW in the total titanium atoms included in the titania electrode. Table 3 shows performance of DSSCs with various TNW content, i.e., *J*sc, *V*oc, *FF* and *η*, together

PCE of the cells including TNW are higher than those cells without TNW, indicating that TNW is useful to attain high efficiency except 100% TNW case. When the film thickness of 100% TNW cells increased larger than 5 m, peel off of the films with cracks was observed by SEM images as shown in Fig. 15, resulting that less than 6% of PCE were observed as shown in Table 3 and Fig. 14. Thus, mixing of TNW with P-25 nanoparticles is important to

Since the amount of adsorbed dyes is another important factor to affect PCE, the amounts of adsorbed dyes for the cells with various TNW contents are shown against film thickness in Fig. 16. The amount of adsorbed dye in the cells containing TNW from 0 % to 50 % locates in the same straight line regardless of the difference in TNW contents, except 100 % TNW

Table 3. Performance of dye sensitized solar cells with various TNW content.

**3.4 Effects of content of TNW on the properties of dye-sensitized solar cells**

with film thickness. Effect of TNW content on PCE is shown in Fig. 14.

for recombination reactions.

make robust films.

which shows higher adsorbed amounts.

For the impedance concerning with platinum electrode, equation (6) was assumed:

$$Z\_{Pt} = \frac{1}{1 + i\alpha r\_{pt}C\_{pt}}\tag{6}$$

where, *r*pt and *C*pt represent the resistance at the Pt surface and the capacitance at the Pt surface, respectively. For the impedance concerning with tri-iodide diffusion, finite Warburg impedance equation, i.e., equation (7) was assumed:

$$Z\_N = R\_{13-} \frac{1}{\sqrt{\frac{i\alpha}{\left(D\_{13-} \;/\,\delta^2\right)}}} \tanh\sqrt{\frac{i\alpha}{\left(D\_{13-} \;/\,\delta^2\right)}}\tag{7}$$

The calculated solid curves in Fig. 13 agree quite well with the plotted experimental data. The characteristics shown in Table 2 are following three points which show strong tendency for the highly efficient cells. 1) The resistance for the electron transport from the titania electrode to the conducting glass electrode *R*w becomes smaller with increasing conversion efficiency. 2) The ratio of the resistance for the recombination reactions against the resistance for the transport rate to the conducting glass electrode (*R*k/*R*w) becomes large, and the rate constant of recombination reactions *k*eff becomes smaller with increasing conversion efficiency. 3) The values of Con, which represents constant inversely proportional to the

Where, Con=*k*B*T*/*qAn* [cms-1], where *k*B [JK-1] represents Boltzmann constant, *T* [K] is absolute temperature, *q* [C] is elementary charge, *A* [cm2] is area of the cell and *n* [cm-3] is electron density. Con: constant inversely proportional to the electron density, *D*eff [cm2s-1]: diffusion coefficient of electron, *L* [cm]: film thickness of TiO2 electrode, *k*eff [s-1]: reaction rat constant of recombination reactions, *R*I3- []: diffusion resistance of I3- , *D*I3- [cm2s-1]: diffusion coefficient of I3- , *δ* [cm]: thickness of the electrolyte phase, *R*Pt []: resistance of Pt electrode, *C*Pt [F]: capacity of Pt electrode, *R*sub []: resistance of substrate, *R*w[]: resistance for electron transport in the TiO2 electrode, *R*k []: resistance for recombination reaction.

Table 2. Determined parameters concerning with electron transport by impedance spectroscopy for three kinds of cells

1

where, *r*pt and *C*pt represent the resistance at the Pt surface and the capacitance at the Pt surface, respectively. For the impedance concerning with tri-iodide diffusion, finite Warburg

*i rC*

3 2

P25 only P25+PEG TNW+(P25+PEG)

*i D*

2

The calculated solid curves in Fig. 13 agree quite well with the plotted experimental data. The characteristics shown in Table 2 are following three points which show strong tendency for the highly efficient cells. 1) The resistance for the electron transport from the titania electrode to the conducting glass electrode *R*w becomes smaller with increasing conversion efficiency. 2) The ratio of the resistance for the recombination reactions against the resistance for the transport rate to the conducting glass electrode (*R*k/*R*w) becomes large, and the rate constant of recombination reactions *k*eff becomes smaller with increasing conversion efficiency. 3) The values of Con, which represents constant inversely proportional to the

Con= 2.65 0.28 0.163

= 0.0006 0.00014 0.00008

= 0.0042 0.0025 0.002

= 23 13.8 10

I3-= 5.1 7 7.5

I3-= 0.000015 0.000003 0.000005

= 0.005 0.005 0.005

Pt= 1.5 4.7 3.65

Pt= 0.00005 0.00007 0.00005

sub= 7.9 10 6.7

w= 14.8 5 4.08

k= 27.4 8.11 8.15

= 2.42×1017 2.29×1018 3.94×1018

Where, Con=*k*B*T*/*qAn* [cms-1], where *k*B [JK-1] represents Boltzmann constant, *T* [K] is absolute temperature, *q* [C] is elementary charge, *A* [cm2] is area of the cell and *n* [cm-3] is electron density. Con: constant inversely proportional to the electron density, *D*eff [cm2s-1]: diffusion coefficient of electron, *L* [cm]: film thickness of TiO2 electrode, *k*eff [s-1]: reaction rat constant of recombination

the electrolyte phase, *R*Pt []: resistance of Pt electrode, *C*Pt [F]: capacity of Pt electrode, *R*sub []: resistance of substrate, *R*w[]: resistance for electron transport in the TiO2 electrode,

Table 2. Determined parameters concerning with electron transport by impedance

w= 1.48 1.62 2

, *D*I3- [cm2s-1]: diffusion coefficient of I3-

, *δ* [cm]: thickness of

1 tanh

3

*I*

/

(7)

*pt pt*

(6)

For the impedance concerning with platinum electrode, equation (6) was assumed:

1 *Pt*

3

*I*

*D*

/

*i Z R*

*Z*

impedance equation, i.e., equation (7) was assumed:

Deff

L

keff

R

D

R

C

R

Rk/R

R

R

n

reactions, *R*I3- []: diffusion resistance of I3-

*R*k []: resistance for recombination reaction.

spectroscopy for three kinds of cells

*N I*

Table 3. Performance of dye sensitized solar cells with various TNW content.

electron density, becomes smaller with increasing conversion efficiency, i.e., Con value increases in the order of P-25 only > (P-25+PEG) > TNW+(P-25+PEG). Therefore the electron density *n* increases with increasing conversion efficiency. So, the characteristics of highly efficient cells are high electron density, small resistance for the electron transport to the conducting glass electrode, and large ratio of the resistances *R*k/*R*w with small rate constant for recombination reactions.

## **3.4 Effects of content of TNW on the properties of dye-sensitized solar cells**

Since the cells containing TNW gave high conversion efficiencies, we examined the effects of content of TNW in the electrode composed of TNW and P-25 upon the conversion efficiency with variation in TNW from 0 % to 100 %. Content of TNW was defined as percentage of titanium atoms of TNW in the total titanium atoms included in the titania electrode. Table 3 shows performance of DSSCs with various TNW content, i.e., *J*sc, *V*oc, *FF* and *η*, together with film thickness. Effect of TNW content on PCE is shown in Fig. 14.

PCE of the cells including TNW are higher than those cells without TNW, indicating that TNW is useful to attain high efficiency except 100% TNW case. When the film thickness of 100% TNW cells increased larger than 5 m, peel off of the films with cracks was observed by SEM images as shown in Fig. 15, resulting that less than 6% of PCE were observed as shown in Table 3 and Fig. 14. Thus, mixing of TNW with P-25 nanoparticles is important to make robust films.

Since the amount of adsorbed dyes is another important factor to affect PCE, the amounts of adsorbed dyes for the cells with various TNW contents are shown against film thickness in Fig. 16. The amount of adsorbed dye in the cells containing TNW from 0 % to 50 % locates in the same straight line regardless of the difference in TNW contents, except 100 % TNW which shows higher adsorbed amounts.

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

δ

nanoparticles.

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 111

TNW [%] 0 0 055 5 10 10 10 10 Con= 0.309 0.397 0.4 0.316 0.257 0.33 0.22 0.224 0.277 0.23 *D* eff = 0.000038 0.000093 0.00024 0.0000747 0.000095 0.00025 0.00009 0.000085 0.00022 0.00015 *L* = 0.001 0.00264 0.0035 0.00191 0.00191 0.0032 0.0018 0.0018 0.002 0.002 *k* eff = 13.8 7.67 13.8 7.67 7.67 13.8 7.67 7.67 16.3 13.8 *R* I3- = 7.7 7 7 8 7.6 6.5 9.1 8.1 5.1 6.4 *D* I3- = 0.00001 0.000004 0.00000295 0.000007 0.0000068 0.00000328 0.0000027 0.0000057 0.0000045 0.0000045

 = 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 *R* pt = 3.5 6.7 4.8 15 5.8 3.5 4.25 3.4 7 4.6 *C* pt = 0.00006 0.000046 0.000055 0.0000313 0.0000675 0.00005 0.00005 0.00006 0.00004 0.000065 *R* sub = 7.96 8.9 9.8 12 11.79 10.4 8.69 8.92 8.5 9.8 *R* k/*R* w= 2.75 1.74 1.42 2.67 3.4 1.77 3.62 3.42 3.37 2.72 *R* w= 8.13 11.3 5.83 8.08 5.17 4.22 4.4 4.74 2.52 3.07 *R* k= 22.4 19.6 8.28 21.6 17.5 7.47 15.9 16.2 8.5 8.33 *n* = 2.17×1018 1.69×1018 1.67×1018 2.11×1018 2.6×1018 2.03×1018 3.04×1018 2.99×1018 2.42×1018 2.91×1018 TNW [%] 28 28 50 50 50 50 100 100 100 Con= 0.165 0.225 0.206 0.217 0.251 0.15 0.161 0.154 0.185 *D* eff = 0.0001 0.00012 0.00014 0.00018 0.00015 0.000104 0.000072 0.000023 0.00015 *L* = 0.002 0.002 0.00139 0.00139 0.0022 0.0022 0.000925 0.000505 0.0009 *k* eff = 10 12.5 18.65 19.9 7.67 5 8 10.32 18 *R* I3- = 5.6 5.8 6.9 6.9 9.5 11.5 14.5 13.4 11 *D* I3- = 0.000007 0.0000055 0.000005 0.0000053 0.000003 0.0000035 0.000011 0.0000096 3.52E-06 *δ* = 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 *R* pt = 2 2.8 3 9.85 5 3.88 4 5.8 5.7 *C* pt = 0.0001 0.00006 0.000038 0.00006 0.0000577 0.000045 0.000052 0.00005 0.0000433 *R* sub = 5.6 5.8 9.12 10.1 8.21 8.07 8 10.7 11 *R* k/*R* w= 2.5 2.4 3.89 4.68 4.04 4.3 10.5 8.74 10.3 *R* w= 3.3 3.75 2.04 1.68 3.68 3.17 2.07 3.38 1.11 *R* k= 8.25 9 7.94 7.84 14.9 13.6 21.8 29.5 11.4 *n* = 3.89×1018 2.85×1018 3.25×1018 3.08×1018 2.67×1018 4.46×1018 4.16×1018 4.35×1018 3.62×1018 Table 4. Parameters determined by EIS analysis for the cells with various TNW contents.

This higher adsorption of 100 % TNW is attributed to the smaller diameter of TNW of 3-7 nm, which is much smaller than the diameter of P-25 of 23 nm. The specific surface area of P-25 and the mixture of 28 % TNW with P-25 after calcinations at 773 K for 30 min. were 45 m2/g and 48 m2/g, respectively. These values of specific surface area are much smaller than that of 100 % TNW which is 78 m2/g after calcinations. This difference in specific surface area between 28 % TNW with P-25 and pure 100 % TNW corresponds well to the difference in adsorbed dye amount between from 0 % to 50 % TNW with P-25 and pure 100 % TNW. These findings suggest some interesting structural change in the surface of the mixture of TNW and P-25. However, the reason why the cells containing different TNW content from 0% to 50 % locates in the same straight line in Fig. 16 is not well understood at present.

Resistance for electron transport from titania electrode to the transparent conducting glass electrode *R*w are plotted against TNW content in Fig. 17a. *R*w values decrease steeply up to 10 % and become gradual decrease after 20 % of TNW content. This decrease indicates clearly that electron transport in the titania electrode is improved by mixing TNW with P-25

The ratios of *R*k representing the resistance for the recombination reactions between electrons in the titania electrode and I3- in the electrolyte to *R*w are plotted against TNW

Fig. 14. Effect of content of TNW on power conversion efficiency

Fig. 15. Top views of 100% TNW films. Left: 5 μm, right: 11μm thickness.

Fig. 16. Relationship between film thickness and the amount of dye.

**0% 5% 10% 28% 50% 100%**

0 % 5 % 10% 28% 50% 100%

**[m]**

**0 10 20 30 40**

**0 10 20 30 40**

**Filn thoickness**

F ilm thickness [μm」

**[m]**

**Film thickness**

Fig. 14. Effect of content of TNW on power conversion efficiency

Fig. 15. Top views of 100% TNW films. Left: 5 μm, right: 11μm thickness.

**0.E+00**

Fig. 16.  Parameters determined by EIS analysis for the cells with various TNW contents.

This higher adsorption of 100 % TNW is attributed to the smaller diameter of TNW of 3-7 nm, which is much smaller than the diameter of P-25 of 23 nm. The specific surface area of P-25 and the mixture of 28 % TNW with P-25 after calcinations at 773 K for 30 min. were 45 m2/g and 48 m2/g, respectively. These values of specific surface area are much smaller than that of 100 % TNW which is 78 m2/g after calcinations. This difference in specific surface area between 28 % TNW with P-25 and pure 100 % TNW corresponds well to the difference in adsorbed dye amount between from 0 % to 50 % TNW with P-25 and pure 100 % TNW. These findings suggest some interesting structural change in the surface of the mixture of TNW and P-25. However, the reason why the cells containing different TNW content from 0% to 50 % locates in the same straight line in Fig. 16 is not well understood at present.

Resistance for electron transport from titania electrode to the transparent conducting glass electrode *R*w are plotted against TNW content in Fig. 17a. *R*w values decrease steeply up to 10 % and become gradual decrease after 20 % of TNW content. This decrease indicates clearly that electron transport in the titania electrode is improved by mixing TNW with P-25 nanoparticles.

The ratios of *R*k representing the resistance for the recombination reactions between electrons in the titania electrode and I3- in the electrolyte to *R*w are plotted against TNW

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

thiocyanate decreased redox potential of I-/I3-

efficiency was obtained.

2008).

**Current density [mA/cm**

**-3]**

(Fig. 19)

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 113

efficiency with complex titania electrode made of titania nanowires and P-25. Recently, we attained the same conversion efficiency 9.33 % using different electrolyte, i.e., 0.6M 1-butyl 3-methyl imidazolium iodide, 0.1M guanidium thiocyanate, 0.05M I2, 0.5M tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85:15) for a complex titania electrode made of titania nanowires, titania nanoparticles (3-5 nm in diameter) and P-25.

**0 0.2 0.4 0.6**

Fig. 19. I-V curve obtained for a cell with a complex electrode composed of network structure of single-crystal-like titania nanowires, titania nanoparticles and P-25

In our previous paper (Adachi et al. 2004), we used an electrolyte composed of 0.1 M of LiI, 0.6 M of1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M of I2, 1 M of 4-tert-butylpyridine in methoxyacetonitrile and got 9.33 % conversion efficiency with short circuit current density *J*sc=19.2 mA/cm2, open circuit voltage *V*oc=0.72 V and fill factor 0.675. In the recent results, *V*oc value 0.8 V is larger than that of previous one 0.72 V, because guanidium

short circuit current density *J*sc=16.8 mA/cm2 than that of our previous one, and the same

Highly crystallized titania nanorods (TNR) have been synthesized by hydrothermal process using blockcopolymer (F127) and surfactant cetyltrimethylammonium bromide (CTAB) as a mixed template (Jiu et al. 2006). TNR with 100-300 nm in length and 20-30 nm in diameter was obtained. A high-resolution TEM (HRTEM) image of single TNR shows that titanium atoms align perfectly in titania anatase crystalline structure with no lattice defect, and the surface of TNR is facetted with the TiO2 anatase {101} faces (Yoshida et al. 2008). The fringes are {101} planes of anatase TiO2 with a lattice spacing of about 0.351 nm, which agrees with the value recorded in JCPDS card. The highly crystallized titania nanorods prepared successfully were used to fabricate a titania electrode of DSSCs. The complex electrodes were made by the repetitive coating-calcining process: 3 layers of titania nanoparticles (3-5 nm in diameter) were first coated on FTO conducting glass, followed by 8 layers of mixed gel composed of titania nanorods and titania nanoparticles. A high light-to-electricity conversion efficiency of 8.93 % was achieved (Yoshida et al.

*J*sc=16.8 mA/cm2 *V*oc=-0.800 V FF=0.694 efficiency=9.33%

**-Voltage[V]**

**0.8**

in the electrolyte. Unfortunately, we got lower

content in Fig. 17b. The ratio of *R*k/ *R*w increases with increase in TNW content. This shows that TNW restrains the recombination reactions between electrons and I3 - and contributes to collect electrons properly to the transparent conducting glass electrode. The findings shown in Fig. 17 a, b bring the high electron density in the titania electrode as shown in Fig. 17c.

Fig. 17. a) Relationship between *R*w and TNW content, b) relationship between *R*k/*R*w and TNW cointent and c) relationship between electron density and TNW content.

Thus, the conclusion deduced from the experiments of three kinds of cells, i.e., small resistance for the electron transport to the conducting glass electrode, large value of resistance ratio *R*k/*R*w, and high electron density in the titania electrode as the characteristics of highly efficient cells, was confirmed again by the experiments of variation in TNW content.

#### **3.5 Some examples of our highly crystallized one-dimensional TiO2 nanoscale materials for fabricating highly efficient dye-sensitized solar cells**

We succeeded in the preparation of titania nanorods (TNR) (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains (see Fig. 18 ), which have been newly synthesized. We applied these materials for DSSCs.

Fig. 18. TEM image of titania nanochains.

We present highly crystallized one-dimensional titania nanoscale materials are effective to attain high light-to-electricity conversion yield. As shown in our previous paper (Adachi et al. 2004), network structure of single crystal-like titania nanowires can be synthesized successfully by the oriented attachment mechanism. We attained 9.33 % conversion

content in Fig. 17b. The ratio of *R*k/ *R*w increases with increase in TNW content. This shows

collect electrons properly to the transparent conducting glass electrode. The findings shown in Fig. 17 a, b bring the high electron density in the titania electrode as shown in Fig. 17c.

> **0 20 40 60 80 100 TNW content [%]**

Fig. 17. a) Relationship between *R*w and TNW content, b) relationship between *R*k/*R*w and

Thus, the conclusion deduced from the experiments of three kinds of cells, i.e., small resistance for the electron transport to the conducting glass electrode, large value of resistance ratio *R*k/*R*w, and high electron density in the titania electrode as the characteristics of highly efficient cells, was confirmed again by the experiments of variation

We succeeded in the preparation of titania nanorods (TNR) (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains (see Fig. 18 ), which have been newly synthesized. We applied these materials for DSSCs.

We present highly crystallized one-dimensional titania nanoscale materials are effective to attain high light-to-electricity conversion yield. As shown in our previous paper (Adachi et al. 2004), network structure of single crystal-like titania nanowires can be synthesized successfully by the oriented attachment mechanism. We attained 9.33 % conversion

and contributes to

**0 20 40 60 80 100 TNW content [%]**

**0.E+00 1.E+18 2.E+18 3.E+18 4.E+18 5.E+18**

*n* **[cm-3]**

that TNW restrains the recombination reactions between electrons and I3-

a b c

TNW cointent and c) relationship between electron density and TNW content.

**3.5 Some examples of our highly crystallized one-dimensional TiO2 nanoscale**

**materials for fabricating highly efficient dye-sensitized solar cells**

R k/R w **[-]**

in TNW content.

*R***w [**Ω**]**

> **0 20 40 60 80 100 TNW content [%]**

Fig. 18. TEM image of titania nanochains.

efficiency with complex titania electrode made of titania nanowires and P-25. Recently, we attained the same conversion efficiency 9.33 % using different electrolyte, i.e., 0.6M 1-butyl 3-methyl imidazolium iodide, 0.1M guanidium thiocyanate, 0.05M I2, 0.5M tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85:15) for a complex titania electrode made of titania nanowires, titania nanoparticles (3-5 nm in diameter) and P-25. (Fig. 19)

Fig. 19. I-V curve obtained for a cell with a complex electrode composed of network structure of single-crystal-like titania nanowires, titania nanoparticles and P-25

In our previous paper (Adachi et al. 2004), we used an electrolyte composed of 0.1 M of LiI, 0.6 M of1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M of I2, 1 M of 4-tert-butylpyridine in methoxyacetonitrile and got 9.33 % conversion efficiency with short circuit current density *J*sc=19.2 mA/cm2, open circuit voltage *V*oc=0.72 V and fill factor 0.675. In the recent results, *V*oc value 0.8 V is larger than that of previous one 0.72 V, because guanidium thiocyanate decreased redox potential of I-/I3 in the electrolyte. Unfortunately, we got lower short circuit current density *J*sc=16.8 mA/cm2 than that of our previous one, and the same efficiency was obtained.

Highly crystallized titania nanorods (TNR) have been synthesized by hydrothermal process using blockcopolymer (F127) and surfactant cetyltrimethylammonium bromide (CTAB) as a mixed template (Jiu et al. 2006). TNR with 100-300 nm in length and 20-30 nm in diameter was obtained. A high-resolution TEM (HRTEM) image of single TNR shows that titanium atoms align perfectly in titania anatase crystalline structure with no lattice defect, and the surface of TNR is facetted with the TiO2 anatase {101} faces (Yoshida et al. 2008). The fringes are {101} planes of anatase TiO2 with a lattice spacing of about 0.351 nm, which agrees with the value recorded in JCPDS card. The highly crystallized titania nanorods prepared successfully were used to fabricate a titania electrode of DSSCs. The complex electrodes were made by the repetitive coating-calcining process: 3 layers of titania nanoparticles (3-5 nm in diameter) were first coated on FTO conducting glass, followed by 8 layers of mixed gel composed of titania nanorods and titania nanoparticles. A high light-to-electricity conversion efficiency of 8.93 % was achieved (Yoshida et al. 2008).

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

Mechanism. *J. Am. Chem. Soc.*, 126, pp. 14943-14949

Spectroscopy: J. Phys. Chem. B 110, pp. 13872-13880

ZrO2 catalysts. *Catalysis Today*. 45, pp. 179-183.

ZrO2. *Catalysis Today*. 114, pp. 40-47.

*Langmuir*. 20, pp. 11223-11233.

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*Science*, 288, 494-497.

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*Chemistry*, 198, pp. 52-59

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cells. *ACS NANO*, 3, pp. 3103-3109

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Adachi, M. Murata, Y. Takao, J. Jiu, J. Sakamoto, M. & Wang, F. (2004). Highly Efficient Dye-

Adachi, M. Sakamoto, M. Jiu, J. Ogata, Y. Isoda, S. (2006). Determination of Parameters of

Aneggi, E.; de Leitenburg, C. Dolcetti, G & Trovarelli, A. (2006) Promotional effect of rare

Bekyarova, E.; Fornasiero, P.; Kaspar, J. & Graziani, M. (1998) CO oxidation on Pd/CeO2–

Bumajdad, A.; Zaki, M. I.; Eastoe, J. & Pasupulety, L. (2004) Microemulsion-Based Synthesis

Chen, C-Y., Wang, M., Li, J-Y., Pootrakulchote, N., Alibabael, L., Ngoc-le, C-H., Decoppetr,

Chen, D., Huang, F. Cheng, Y-B. & Caruso, R. A. (2009) Mesoporous anatase TiO2 beads

Colvin, V. L., Schlamp, M. C., & Alivisatos, A. P. (1994) Light-emitting diodes made from

De Faria, L. A. & Trasatti, S. (1994) The Point of Zero Charge of CeO2. J. Colloid Interface

Fuhrer, M. S., Nygard, J., Shih, L., Forero, M., Yoon, Y. -G., Mazzoni, M. S. C., Choi, H. J.,

Grinis, L., Dor, S., Ofir, A., & Zaban, A. (2008) Electrophoretic deposition and compression

Hamann, T. W., Farha, O. K. & Hupp, J. T. (2008) Atomic layer deposition of TiO2 on aerogel

Han, W-Q.; Wu, L. & Zhu, Y. (2005) Formation and Oxidation State of CeO2-x Nanotubes. *J.*

performance dye-sensitized solar cells. *Adv. Mater.*, 21, pp. 2206-2210 Chiang, Y. M.; Lavik, E. B., Kosacki, I.; Tuller, H. L. & Ying, J. Y. (1996) Defect and transport properties of nanocrystalline CeO2−x. *Appl. Phys. Lett.* 69, pp. 185-187. Colodrero, S., Mihi, A., Häggman, L. Ocana, M. Boschloo, G. Hagfeldt, A., & Miguez, H.

efficiency of dye-sensitized solar cells. *Adv. Mater*., 21, pp. 764-770

Sensitized Solar Cells with Titania Thin Film Electrode Composed of Network Structure of Single-Crystal-Like TiO2 Nanowires Made by "Oriented Attachment"

Electron Transport in Dye-Sensitized Solar Cells Using Electrochemical Impedance

earths and transition metals in the combustion of diesel soot over CeO2 and CeO2–

of CeO2 Powders with High Surface Area and High-Temperature Stabilities.

J-D., Tsai, J-H., Grätzel, C., Wu, C-G., Zakeeruddin, M., & Grätzel, M. (2009) Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar

with high surface areas and controllable pore sizes: A superior candidate for high-

(2009) Porous one-dimensional photonic crystals improve the power-conversion

cadmium selenide nanocrystals and a semiconducting polymer Nature, 370, pp.

Ihm, J., Louie, S. G., Zettl, A., McEuen, P. L. (2000) Crossed nanotube junctions

of titania nanoparticle films for dye-sensitized solar cells. *J. Photochem. Photobio. A:*

templates: New photoanodes for dye-sensitized solar cells. *J. Phys. Chem. C,* 112, pp.

Fig. 20. I-V curve obtained for the cell composed of one-dimensional chains of titania nanoparticles mixed with fine titania nanoparticles (3 - 5 nm in diameter).

We have newly synthesized titania nanochains as shown in Fig. 18. Highly crystallized titania nanoparticles with diameter of around 10 nm combine with each other and make chains. The obtained white solid product was mixed with spherical titania nanoparticles (3-5 nm in diameter) synthesized using F127 reported in our previous paper (Jiu et al. 2004, Jiu et al. 2007) to fabricate titania film electrodes. The I-V curve of the cell is shown in Fig. 20. The obtained light-to-electricity conversion yield of the cell was 9.2%.

All three kinds of one-dimensional titania nanoscale materials mentioned above show high light-to-electricity conversion yield around 9%, suggesting strongly that highly crystallized one-dimensional titania materials are essentially important for attainment of high efficient dye-sensitized solar cells.

#### **3.6 Conclusions of 3rd section**

## **4. References**

114 Smart Nanoparticles Technology

*J*sc **= 19.8 mA/cm2**

*V*oc **= 0.8 V** *FF* **= 0.704**

= 9.19 %

Fig. 20. I-V curve obtained for the cell composed of one-dimensional chains of titania

η

nanoparticles mixed with fine titania nanoparticles (3 - 5 nm in diameter).

The obtained light-to-electricity conversion yield of the cell was 9.2%.

i.e., very small *R*w is indispensable for the highly efficient cells.

is large. 3) Electron density *n* in the titania electrode is high.

nanochains, show high power conversion efficiency about 9 %.

**0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7**

**-Voltage [V]**

We have newly synthesized titania nanochains as shown in Fig. 18. Highly crystallized titania nanoparticles with diameter of around 10 nm combine with each other and make chains. The obtained white solid product was mixed with spherical titania nanoparticles (3-5 nm in diameter) synthesized using F127 reported in our previous paper (Jiu et al. 2004, Jiu et al. 2007) to fabricate titania film electrodes. The I-V curve of the cell is shown in Fig. 20.

All three kinds of one-dimensional titania nanoscale materials mentioned above show high light-to-electricity conversion yield around 9%, suggesting strongly that highly crystallized one-dimensional titania materials are essentially important for attainment of high efficient

1. Many researchers familiar with EIS measurement know that highly efficient dyesensitized solar cells show small total resistance of the cell, i.e., small Nyquist spectrum. They also know that largest arc of Nyquist plot represents the resistance for recombination reactions *R*k. This apparent conflict is solved clearly by theoretical consideration through recognition that the large value of the ratio *R*k/*R*w is essentially important for the highly efficient cells, and the absolute value of *R*k is not important,

2. The experimental results of I-V and EIS measurements of the three kinds of cells made of P-25 only, P-25+PEG, and TNW+P-25+PEG and also cells made of various content of TNW with P-25+PEG clearly showed the following three points as characteristics of highly crystallized 1-dimensional titania nanoscale material TNW. 1) Resistance of electron transport in the titania electrode *R*w is small. 2) The ratio of resistance *R*k/*R*w

3. All cells composed of three kinds of highly crystallized 1-dimensional titania nanoscale materials, i. e., network structure of titania nanowires, titania nanorods, and titania

**0**

**5**

**10**

**Current density [mA/cm2]**

dye-sensitized solar cells.

**3.6 Conclusions of 3rd section**

**15**

**20**

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

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**6**

 *Australia*

**Experimental and Theoretical Study of**

, Yusuf Valentino Kaneti and Aibing Yu

**Low-Dimensional Iron Oxide Nanostructures**

*School of Materials Science and Engineering, University of New South Wales, Sydney,*

Iron oxide has many phases, including 16 pure phases (e.g., FeO, Fe3O4), 5 polymorphs of FeOOH (e.g., -FeOOH, -FeOOH) and 4 kinds of Fe2O3 (e.g., -Fe2O3, -Fe2O3). Because of their unique properties (optical, electronic, magnetic), they have found many applications in the areas of catalysts, magnetic recording, sorbents, pigments, flocculants, coatings, gas sensors, lubrications, and biomedical applications (e.g., magnetic resonance imaging, drug

Many efforts have been made in the synthesis (co-precipitation, hydrothermal, microemulsion, and sol-gel method), structural characterization, and functional exploration, as well as fundamental understandings of iron oxide nanostructures. Despite some success, several challenges still exist regarding the synthesis, strcture, properties, and fundamental understanding of the iron oxides. the grand challenge is how to efficiently synthesize iron oxides with controlled morphology, size and functionality, and how to fundamentally understand the formation and growth mechanisms, structure, and interaction forces. Therefore, the development of simple but effective experimental and theoretical strategies to

To fundamentally understand the nanoscale system, theoretical methods should exist. Computational modeling is one of the most important enabling techniques in nanotechnology and material research. It can increase the pace of discovery across the entire scientific scope, and reduce the cost in the development and commercialization of technologies and materials. Various computational approaches have been developed and used to predict the materials properties (e.g., electronic, magnetic, optical) at different length and time scales. For example, at an atomic scale, density functional theory (DFT) is widely used for binding energy calculation, while at a microscopic scale, molecular dynamics (MD)

This Chapter will give a brief overview of the experimental and theoretical methods conducted on iron oxide nanostructures, particularly for low-dimensional iron oxide nanoparticles. This includes: (i) several representative methods for iron oxides nanomaterials in Sections 2 and 3; (ii) surface modified iron oxide nanostructures by

Jeffrey Yue, Xuchuan Jiang

**1. Introduction**

delivery and therapy).

Corresponding Author

overcome the challenges is still imperative.

are able to provide insights into atomic/molecular systems.

nanotube and nanowire arrays for oxidative photoelectrochemistry. *J. Phys. Chem. C*, 113, pp. 6327-6359

## **Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures**

Jeffrey Yue, Xuchuan Jiang , Yusuf Valentino Kaneti and Aibing Yu *School of Materials Science and Engineering, University of New South Wales, Sydney, Australia*

## **1. Introduction**

118 Smart Nanoparticles Technology

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and surface reactions of pyridine on pure and doped ceria catalysts as studied by

TiO2, ZrO2, and CeO2: IR Spectroscopic Assessment of Impacts of the Surface

Micro/Nanocomposite Structure and Its Application for Water Treatment and CO

Iron oxide has many phases, including 16 pure phases (e.g., FeO, Fe3O4), 5 polymorphs of FeOOH (e.g., -FeOOH, -FeOOH) and 4 kinds of Fe2O3 (e.g., -Fe2O3, -Fe2O3). Because of their unique properties (optical, electronic, magnetic), they have found many applications in the areas of catalysts, magnetic recording, sorbents, pigments, flocculants, coatings, gas sensors, lubrications, and biomedical applications (e.g., magnetic resonance imaging, drug delivery and therapy).

Many efforts have been made in the synthesis (co-precipitation, hydrothermal, microemulsion, and sol-gel method), structural characterization, and functional exploration, as well as fundamental understandings of iron oxide nanostructures. Despite some success, several challenges still exist regarding the synthesis, strcture, properties, and fundamental understanding of the iron oxides. the grand challenge is how to efficiently synthesize iron oxides with controlled morphology, size and functionality, and how to fundamentally understand the formation and growth mechanisms, structure, and interaction forces. Therefore, the development of simple but effective experimental and theoretical strategies to overcome the challenges is still imperative.

To fundamentally understand the nanoscale system, theoretical methods should exist. Computational modeling is one of the most important enabling techniques in nanotechnology and material research. It can increase the pace of discovery across the entire scientific scope, and reduce the cost in the development and commercialization of technologies and materials. Various computational approaches have been developed and used to predict the materials properties (e.g., electronic, magnetic, optical) at different length and time scales. For example, at an atomic scale, density functional theory (DFT) is widely used for binding energy calculation, while at a microscopic scale, molecular dynamics (MD) are able to provide insights into atomic/molecular systems.

This Chapter will give a brief overview of the experimental and theoretical methods conducted on iron oxide nanostructures, particularly for low-dimensional iron oxide nanoparticles. This includes: (i) several representative methods for iron oxides nanomaterials in Sections 2 and 3; (ii) surface modified iron oxide nanostructures by

Corresponding Author

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 121

**Crystallographic structural features References**

Cornell and Schwertmann,

Cudennec and Lecerf, 2005

Cornell and Schuwertmann,

Cornell and Schuwertmann,

Cornell and Schuwertmann,

Mohapatra and Anand,2010

Cornell and Schuwertmann,

Weckler and Lutz, 1998

Cornell and Schuwertmann,

Mohapatra and Anand,2010

1991;

1991

1991;

1991; Garcia et al.,

2009

1991

1991

3D-structure built up with FeO3(OH) octahedra spreading along the (010) direction, with each octahedron linked to eight neighbouring octahedral by four edges and three vertices. Oxygen atoms are in tetrahedral surroundings,

either OFe3H or OFe3H (bond).

Double chains of edge linked Fe(O, OH) octahedral that share corners to form a framework containing large tunnels with square cross sections.

Arrays of close cubic -packed anions (O2-/OH-) stacked along the [150] direction with Fe3+ ions occupying the

Disordered hexagonally close-packed

Stacking of sheets of octahedrally (sixfold) coordinated Fe3+ ions. Between two close-packed layers of oxygen ions. Each oxygen ion is bonded to

Each cell of maghemite contains 32 O2 ions, 21(1/3) Fe3+ ions and 2 (1/3) O vacancies. Eight cations occupy tetrahedral sites and the remaining cations are distributed over the octahedral sites. The vacancies are confined to octahedral sites.

Inverse spinel structure with a facecentered cubic cell based on 32 O2 ions, regularly close-cubic packed along [111], with Fe2+ ions and half of the Fe3+ occupying the octahedral sites

and the other half of Fe3+ ions, occupying the tetrahedral sites.

array of anions with Fe3+ ions distributed over half the octahedral

sites in an orderly manner.

only two Fe ions.

octahedral interstices.

**Iron oxides Crystallographic system**

> Orthorhombica = 0.9956 nm, b = 0.30215 nm and c = 0.4608 nm

Monoclinic a = 1.056 nm, b = 0.3031 nm and c = 1.0483 nm

Orthorhombic a = 0.3071 nm, b = 1.2520 nm and c = 0.3873 nm

Hexagonal a = 0.293 nm and c = 0.456 nm

Hexagonal a= 0.5035 nm and c = 1.375 nm

Cubic

Cubic

a = 0.8396 nm

Table 1. Complicated phases and polymorphs of iron oxides in nature

a= 0.83474 nm

Goethite (-FeOOH)

Akaganéite (β-FeOOH)

Lepidocrocite (γ-FeOOH)

Feroxyhyte (δ'-FeOOH)

Hematite (α-Fe2O3)

Maghemite (γ-Fe2O3)

Magnetite (Fe3O4)

surfactants, polymers, silica or metals in Section 4; (iii) functional properties of such nanomaterials in gas sensing, catalysis, and biotechnology in Section 5. Moreover, in Section 6, the discussion will be extended to the theoretical modeling and simulation methods that can predict the formation and performance of nanomaterials, such as MD and DFT methods.

## **2. Iron oxide materials**

Iron is the fourth most abundant element in the Earth's crust, and iron oxides are commonly found in nature and have become the most plentiful transition metal oxides (Morrissey and Guerinot, 2009; Ilani et al., 1999). The complicated phases and features of iron oxides have been listed in **Table 1**.

Some crystalline phases of iron oxides are not very stable and can convert into others. Much work has been conducted to convert akaganéite to hematite and/or magnetite phases to pursuit good performance in catalysis and gas sensing applications. Magnetite nanorods can be produced by the conversion of iron oxyhydroxides into a thermally stable structure of hematite by heating above 400 ºC in air, or magnetite in a mixture of H2 and Ar gas (Bomati-Miguel et al., 2008). Recently, our group has simplified the phase conversion procedures among the iron oxides. The iron oxyhydroxides can directly convert into magnetite by using hydrazine as a reducing agent, and the morphology was maintained. Using this method, magnetite nanorods could be directly synthesized from akaganéite rather than using hematite as an intermediate (Yue et al., 2010).

By using hydrazine, iron(III) ions can be reduced to iron(II). The change in coordination number to the iron atom will therefore transfer from FeOH to FeO following dehydration. The structure change caused by loss of H2O will create pores or holes within the nanorod framework. Continuous reaction with hydrazine can form larger defects in the 1-D nanostructure, leading to the collapse of the framework. At the same time, the FeO6 units will reconstruct into other crystals, and the broken fractions could fuse with neighboring particles to form larger ones. However, this does not happen to hematite because of its thermally stable structure under the considered conditions. The nature of such a conversion needs further investigation. Nevertheless, the proposed approach could be used for a controlled conversion of akaganéite to magnetite nanostructures (**Fig. 1**) without hightemperature treatment. These porous magnetic structures would find more applications in electronic and magnetic areas (Yue et al., 2011).

## **3. Synthesis methods**

A variety of methods have been reported to synthesize iron oxide nanoparticles, including solid-state, liquid-phase, and gas-phase syntheses, as listed in **Table 2**. Among the synthesis approaches, liquid-phase synthesis is the most popular. The iron salts are highly soluble in water and different additives can be used in conjunction to modify the structure of the nanoparticles. Moreover, the liquid-phase synthesis is convenient for understanding ageing, recrystallization, and evolution into other shapes and sizes. It is also available for controlling experimental conditions in liquid (e.g., concentration, salt precursor, pH, temperature, surface modifiers). A few representative synthesis methods are briefly introduced in this Section, such as co-precipitation, hydrothermal and microemulsion.

surfactants, polymers, silica or metals in Section 4; (iii) functional properties of such nanomaterials in gas sensing, catalysis, and biotechnology in Section 5. Moreover, in Section 6, the discussion will be extended to the theoretical modeling and simulation methods that can predict the formation and performance of nanomaterials, such as MD

Iron is the fourth most abundant element in the Earth's crust, and iron oxides are commonly found in nature and have become the most plentiful transition metal oxides (Morrissey and Guerinot, 2009; Ilani et al., 1999). The complicated phases and features of iron oxides have

Some crystalline phases of iron oxides are not very stable and can convert into others. Much work has been conducted to convert akaganéite to hematite and/or magnetite phases to pursuit good performance in catalysis and gas sensing applications. Magnetite nanorods can be produced by the conversion of iron oxyhydroxides into a thermally stable structure of hematite by heating above 400 ºC in air, or magnetite in a mixture of H2 and Ar gas (Bomati-Miguel et al., 2008). Recently, our group has simplified the phase conversion procedures among the iron oxides. The iron oxyhydroxides can directly convert into magnetite by using hydrazine as a reducing agent, and the morphology was maintained. Using this method, magnetite nanorods could be directly synthesized from akaganéite rather than using

By using hydrazine, iron(III) ions can be reduced to iron(II). The change in coordination number to the iron atom will therefore transfer from FeOH to FeO following dehydration. The structure change caused by loss of H2O will create pores or holes within the nanorod framework. Continuous reaction with hydrazine can form larger defects in the 1-D nanostructure, leading to the collapse of the framework. At the same time, the FeO6 units will reconstruct into other crystals, and the broken fractions could fuse with neighboring particles to form larger ones. However, this does not happen to hematite because of its thermally stable structure under the considered conditions. The nature of such a conversion needs further investigation. Nevertheless, the proposed approach could be used for a controlled conversion of akaganéite to magnetite nanostructures (**Fig. 1**) without hightemperature treatment. These porous magnetic structures would find more applications in

A variety of methods have been reported to synthesize iron oxide nanoparticles, including solid-state, liquid-phase, and gas-phase syntheses, as listed in **Table 2**. Among the synthesis approaches, liquid-phase synthesis is the most popular. The iron salts are highly soluble in water and different additives can be used in conjunction to modify the structure of the nanoparticles. Moreover, the liquid-phase synthesis is convenient for understanding ageing, recrystallization, and evolution into other shapes and sizes. It is also available for controlling experimental conditions in liquid (e.g., concentration, salt precursor, pH, temperature, surface modifiers). A few representative synthesis methods are briefly introduced in this

and DFT methods.

**2. Iron oxide materials**

been listed in **Table 1**.

hematite as an intermediate (Yue et al., 2010).

electronic and magnetic areas (Yue et al., 2011).

Section, such as co-precipitation, hydrothermal and microemulsion.

**3. Synthesis methods**

Table 1. Complicated phases and polymorphs of iron oxides in nature

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 123

**Particle shape/size**

Spheres D= 2.9-3.6 nm

Nanospheres (d = 30 - 80 nm)

Nanorods (l: 400-600 nm, w =20-30 nm) Nanodiscs (d = 50 nm, thickness = 6.5

nm)

Nanospheres (d = 16 nm)

Nanoparticles (24-52 nm)

Nanoparticles (3-5 nm)

Nanorods 10-80 nm in diameter

(d= 3-20 nm)

Hollow nanospheres, (d =300 nm)

Nanowires (30 nm 1-5

Nanobelts (100 nm 7 µm)

µm)

**Features References**

Lu et al. (2007)

et al. (2007)

Li et al. (2006); Yue et al. (2010, 2011); Jiang et al. (2009)

Dong and Zhu

Vidal-Vidal et al. (2006)

Vijayakumar et al. (2000, 2001)

Zhang et al (2007); Pascal et al (1999)

González-Carreño et al. (1993)

Morber et al. (2006)

Liu

Sun et al. (2004)

(2004)

Mechanical energy to smash.

Low

Hightemperature decomposition of iron organic precursors.

Dissolve, condensation, and calcinations of alkoxides.

Reaction in two immiscible phases (water and oil).

Ultrasound to promote chemical reaction.

Electrons act as reactant with no pollution.

Spraying, aerosol evaporation, condensation, drying, and thermolysis.

Heating of a gaseous mixture of iron precursor.

Ageing of ferric and ferrous salts in a basic medium.

temperature, reaction, commonly conducted in autoclaves, and high efficiency.

**Common products**

δ-Fe2O3, Fe3O4

Fe2O3, Fe3O4

δ-Fe2O3, Fe3O4 (-FeOOH β-FeOOH

α-Fe2O3, δ-Fe2O3, Fe3O4

Fe2O3, Fe3O4

Fe2O3, Fe3O4

Fe3O4

Fe3O4

α-Fe2O3, δ-Fe2O3, ε-Fe2O3 Fe3O4

Electrochemical -Fe2O3 Nanoparticles

Table 2. Several typical synthesis methods for iron oxides

Co-precipitation α-Fe2O3, δ-

**Synthesis media**

Liquid state

**Synthesis methods**

milling

Hydrothermal α-Fe2O3,

decomposition

Sol-gel α-Fe2O3, δ-

Microemulsion α-Fe2O3, δ-

Sonochemical Fe2O3,

Gas state Spray pyrolysis δ-Fe2O3,

Laser pyrolysis or deposition

Solid state Mechanical

Thermal

Fig. 1. TEM images showing the conversion of nanorods: (A) β-FeOOH nanorods, (B) β-FeOOH nanorods calcined at 300 ºC, (C) β-FeOOH nanorods reduced with N2H4 at 80 ºC; and (D-F) the corresponding HRTEM images with labeled lattice spacing and crystal planes. Reprinted with permission from (Yue et al., 2011).

## **3.1 Co-precipitation**

One simple and efficient way is to use co-precipitation technique in solution. By this approach, iron(II) and/or iron(III) salts are first dissolved in aqueous solution, and then one alkaline media (e.g., NaOH, Na2CO3) solution is added to form precipitate. The prepared particles can be tuned to be uniform in size, shape as well as pure in its composition. Various crystalline phases of iron oxides can be produced using this method, which is controlled by experimental parameters such as types of iron salts (e.g., chloride, sulphate and nitrate), alkaline media, concentration, temperature, and pH (Iida et al., 2007).

Moreover, the phase of iron oxide(s) formed through the co-precipitate approach is often reported as goethite or hematite if iron(III) salt is used. However, the initially precipitated material is usually found as ferrihydrite, which is a thermodynamically unstable phase. The precipitate can further convert into other phases (e.g., hematite, magnetite) depending on the pH, ionic medium, and temperature. For example, Varada et al. (2002) prepared monodispersed acicular goethite particles by precipitating Fe(III) using sodium carbonate. If sodium hydroxide was used, the axial ratio of particles will increase from 60 to 230 nm. It was proposed that different bases have different ability to maintain the solution at a constant pH, where other pH levels would produce polydispersed and hematite particles. The mechanism of the growth of spherical hematite nanoparticles has been explored by Liu et al.(2007). The variation in the final pH of the solution plays a key role in the formation of hematite at different sizes. They found that the particles with diameter of 60-80 nm were obtained at pH 7, while reduced to 30-40 nm in diameter at pH 9.

Fig. 1. TEM images showing the conversion of nanorods: (A) β-FeOOH nanorods, (B) β-FeOOH nanorods calcined at 300 ºC, (C) β-FeOOH nanorods reduced with N2H4 at 80 ºC; and (D-F) the corresponding HRTEM images with labeled lattice spacing and

One simple and efficient way is to use co-precipitation technique in solution. By this approach, iron(II) and/or iron(III) salts are first dissolved in aqueous solution, and then one alkaline media (e.g., NaOH, Na2CO3) solution is added to form precipitate. The prepared particles can be tuned to be uniform in size, shape as well as pure in its composition. Various crystalline phases of iron oxides can be produced using this method, which is controlled by experimental parameters such as types of iron salts (e.g., chloride, sulphate and nitrate), alkaline media, concentration, temperature, and pH (Iida

Moreover, the phase of iron oxide(s) formed through the co-precipitate approach is often reported as goethite or hematite if iron(III) salt is used. However, the initially precipitated material is usually found as ferrihydrite, which is a thermodynamically unstable phase. The precipitate can further convert into other phases (e.g., hematite, magnetite) depending on the pH, ionic medium, and temperature. For example, Varada et al. (2002) prepared monodispersed acicular goethite particles by precipitating Fe(III) using sodium carbonate. If sodium hydroxide was used, the axial ratio of particles will increase from 60 to 230 nm. It was proposed that different bases have different ability to maintain the solution at a constant pH, where other pH levels would produce polydispersed and hematite particles. The mechanism of the growth of spherical hematite nanoparticles has been explored by Liu et al.(2007). The variation in the final pH of the solution plays a key role in the formation of hematite at different sizes. They found that the particles with diameter of 60-80 nm were obtained at pH 7, while reduced to 30-40 nm in diameter at

crystal planes. Reprinted with permission from (Yue et al., 2011).

**3.1 Co-precipitation**

et al., 2007).

pH 9.

Table 2. Several typical synthesis methods for iron oxides

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 125

Microemulsion method (surfactant-stabilized water/oil (W/O) microemulsion) has been widely used to prepare shape- and size-controlled iron oxide nanoparticles. Generally, a microemulsion is transparent, isotropic and thermodynamically stable dispersion of two immiscible phases (e.g., water and oil). When a surfactant is present in W/O system, the surfactant molecules may form a monolayer at the interface of oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the oil phase and the hydrophilic head groups in the aqueous phase (Wu et al., 2008). In a binary system such as water/surfactant or oil/surfactant, a variety of self-assembled structures can be formed, ranging from spherical and cylindrical micelles to lamellar phases or bi-continuous microemulsions depending on the molar ratio of water, oil and surfactant(s). This will be

For example, magnetite nanoparticles ~4 nm in diameter have been prepared by the controlled hydrolysis of ammonium hydroxide with FeCl2 and FeCl3 aqueous solution within the reverse micelles nanocavities generated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and heptane as a continuous oil phase (López-Quintela and Rivas, 1993). Lee and co-workers (2005) have successfully synthesized uniform and highly crystalline magnetite nanoparticles in microemulsion nanoreactors. The particle size of the prepared magnetite nanoparticles could be adjusted from 2-10 nm by varying the relative concentrations of iron salt, surfactant, and solvent. Li et al (2009) demonstrated the effect of volumetric ratios of aqueous FeCl3 solution to 1,2-propanediamine on the formation of magnetic particles, as shown in **Fig. 3**. Chin and Yaacob (2007) reported the synthesis of

Fig. 3. SEM images of the products obtained at different volume ratios of aqueous FeCl3 solution to 1,2 propanediamine: (a) without 1,2 propanediamine, (b) 3:1, (c) 1:1, (d) 1:2,

(e) 1:4, and (f) 1:5. Reprinted with permission from (Li et al., 2009).

useful for the generation of nanoparticles with different shapes and sizes.

**3.3 Microemulsion**

## **3.2 Hydrothermal and thermal decomposition methods**

Hydrothermal technique is defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under a high pressure and a temperature (6-10 atm, 100- 200 C). A hydrothermal reaction requires the iron(III) salt (e.g., iron chloride, nitrate, or sulphate), which can be dissolved in solution followed by reaction with water. This is different from the thermal decomposition reaction that generally takes place for those iron organic precursors (Fe(CO)5, Fe(acac)3, and Fe(cup)3)in an organic solvent at high temperatures (Hyeon et al., 2001; Li et al., 2004; Rockenberger et al., 1999). Both hydrothermal and thermal decomposition methods are commonly used for the synthesis of iron oxide nanoparticles.

The hydrothermal method is often performed in an autoclave, where the reaction system can exceed the boiling point of liquid(s) at normal atmospheric pressure (Jia et al., 2005). The temperature can alter the system in such a way that disrupts the thermodynamics of a material, which is governed by enthalpy (ΔH) and entropy (ΔS), and hence Gibbs free energy (ΔG). The essential role of a fluid under high temperatures is that it changes the vapor pressure of the fluid. This is also beneficial for diverse choices of solvents (polar and non-polar). The morphology and crystalline phase of iron oxides produced through this approach can vary by simply tuning reaction temperature, concentration, and additive(s) (Almeida et al., 2009; Jiang et al., 2010).

The synthesis of iron oxide nanoparticles via a hydrothermal approach can be conducted with or without the use of surfactant(s). Hematite nanoparticles have been prepared by Sahu et al*.* (1997) under conditions of pH (3-10) and 180 °C in autoclaves. In this study, the average particle size of hematite nanoparticles was found to decrease with an increase of pH. In our recent work (Jiang et al*.* 2010), we reported a facile hydrothermal route for the synthesis of monodispersed hematite nanodiscs with diameters of ~ 50 nm and thickness of ~6.5 nm in the absence of any surfactants in water at around 90 C (**Fig. 2**). The nanodiscs exhibited interesting paramagnetic property at a low temperature (20 K), but ferromagnetic at room temperature (~300 K). In addition, the hematite nanodiscs also showed lowtemperature catalytic activity in CO oxidation to CO2.

Fig. 2. A) TEM image of α-Fe2O3 nanodiscs with overlapping as pointed by arrows; B) HRTEM image showing the lattice fringe of {110} plates with spacing between two adjacent planes of 0.411 nm. Reprinted with permission from (Jiang et al. 2010).

## **3.3 Microemulsion**

124 Smart Nanoparticles Technology

Hydrothermal technique is defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under a high pressure and a temperature (6-10 atm, 100- 200 C). A hydrothermal reaction requires the iron(III) salt (e.g., iron chloride, nitrate, or sulphate), which can be dissolved in solution followed by reaction with water. This is different from the thermal decomposition reaction that generally takes place for those iron organic precursors (Fe(CO)5, Fe(acac)3, and Fe(cup)3)in an organic solvent at high temperatures (Hyeon et al., 2001; Li et al., 2004; Rockenberger et al., 1999). Both hydrothermal and thermal decomposition methods are commonly used for the synthesis of

The hydrothermal method is often performed in an autoclave, where the reaction system can exceed the boiling point of liquid(s) at normal atmospheric pressure (Jia et al., 2005). The temperature can alter the system in such a way that disrupts the thermodynamics of a material, which is governed by enthalpy (ΔH) and entropy (ΔS), and hence Gibbs free energy (ΔG). The essential role of a fluid under high temperatures is that it changes the vapor pressure of the fluid. This is also beneficial for diverse choices of solvents (polar and non-polar). The morphology and crystalline phase of iron oxides produced through this approach can vary by simply tuning reaction temperature, concentration, and additive(s)

The synthesis of iron oxide nanoparticles via a hydrothermal approach can be conducted with or without the use of surfactant(s). Hematite nanoparticles have been prepared by Sahu et al*.* (1997) under conditions of pH (3-10) and 180 °C in autoclaves. In this study, the average particle size of hematite nanoparticles was found to decrease with an increase of pH. In our recent work (Jiang et al*.* 2010), we reported a facile hydrothermal route for the synthesis of monodispersed hematite nanodiscs with diameters of ~ 50 nm and thickness of ~6.5 nm in the absence of any surfactants in water at around 90 C (**Fig. 2**). The nanodiscs exhibited interesting paramagnetic property at a low temperature (20 K), but ferromagnetic at room temperature (~300 K). In addition, the hematite nanodiscs also showed low-

Fig. 2. A) TEM image of α-Fe2O3 nanodiscs with overlapping as pointed by arrows; B) HRTEM image showing the lattice fringe of {110} plates with spacing between two adjacent planes of 0.411 nm. Reprinted with permission from (Jiang et al. 2010).

**3.2 Hydrothermal and thermal decomposition methods**

iron oxide nanoparticles.

(Almeida et al., 2009; Jiang et al., 2010).

temperature catalytic activity in CO oxidation to CO2.

Microemulsion method (surfactant-stabilized water/oil (W/O) microemulsion) has been widely used to prepare shape- and size-controlled iron oxide nanoparticles. Generally, a microemulsion is transparent, isotropic and thermodynamically stable dispersion of two immiscible phases (e.g., water and oil). When a surfactant is present in W/O system, the surfactant molecules may form a monolayer at the interface of oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the oil phase and the hydrophilic head groups in the aqueous phase (Wu et al., 2008). In a binary system such as water/surfactant or oil/surfactant, a variety of self-assembled structures can be formed, ranging from spherical and cylindrical micelles to lamellar phases or bi-continuous microemulsions depending on the molar ratio of water, oil and surfactant(s). This will be useful for the generation of nanoparticles with different shapes and sizes.

For example, magnetite nanoparticles ~4 nm in diameter have been prepared by the controlled hydrolysis of ammonium hydroxide with FeCl2 and FeCl3 aqueous solution within the reverse micelles nanocavities generated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and heptane as a continuous oil phase (López-Quintela and Rivas, 1993). Lee and co-workers (2005) have successfully synthesized uniform and highly crystalline magnetite nanoparticles in microemulsion nanoreactors. The particle size of the prepared magnetite nanoparticles could be adjusted from 2-10 nm by varying the relative concentrations of iron salt, surfactant, and solvent. Li et al (2009) demonstrated the effect of volumetric ratios of aqueous FeCl3 solution to 1,2-propanediamine on the formation of magnetic particles, as shown in **Fig. 3**. Chin and Yaacob (2007) reported the synthesis of

Fig. 3. SEM images of the products obtained at different volume ratios of aqueous FeCl3 solution to 1,2 propanediamine: (a) without 1,2 propanediamine, (b) 3:1, (c) 1:1, (d) 1:2, (e) 1:4, and (f) 1:5. Reprinted with permission from (Li et al., 2009).

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 127

2008). It has shown that through careful choice of the passivating and activating polymers and/or reaction conditions, polymer-stabilized iron oxide nanoparticles with tailored and

The iron oxide particles by ionic properties can be modified with functional polymer groups with COOH, NH2 (Chibowski et al., 2009; Kandori et al., 2005; Li et al., 2004). The polymer coated particles can be synthesized by the *ex situ* method, i.e. dispersion of the nanoparticles in a polymeric solution, or *in situ* method, i.e. monomer polymerization in the

Polymeric coating materials can be classified into two main classes: natural (e.g., dextran, starch, gelatin, chitosan) and synthetic (e.g., polyethylene glycol, PEG; polymethylmethacrylate, PMMA; polyacrylic acid, PAA). However, the saturation magnetization value of iron oxide nanoparticles will decrease after polymer-fictionalization. Dextran is often utilized as a coating polymer because of its stability and biocompatibility (Laurent et al. 2008). Molday and Mackenzie (Molday and Mackenzie, 1982) have reported the formation of Fe3O4 in the presence of dextran with molecular weight (MW) of 40,000. In the synthesis of dextran-coated ultra-small superparamagnetic iron oxides (USPIO), the reduction of the terminal glucose of dextran was found to be significant for controlling particle size, stability, and magnetic properties. For low molecular weight dextrans (MW,

Polyvinyl alcohol (PVA) is a hydrophilic and biocompatible polymer that can be used for particle surface modification to prevent particle agglomeration (Laurent et al. 2008). Lee et al. (1996) have modified the surface of magnetite nanoparticles with PVA by precipitation of iron salts at a high pH (13.8) to form stable magnetite colloidal dispersions, and particle size is around 4 nm. The investigators noted that the crystallinity of the magnetite nanoparticles decreased with PVA concentration increasing, although morphology and particle sizes remained. When PVA is introduced, it reacts with the surface through hydrogen bonding between polar functional groups of the polymer and hydroxylated and/or protonated surface of the iron oxide. In addition to the polymer-surface interactions, PVA is known for its hydrogen bonding interaction, resulting in hydrogel structure embedding the nanoparticles. When the PVA concentration is over the critical saturation value,

Polymerized tetraethoxysilane (TEOS) network is often used as a surface coating material for iron oxide nanoparticles as this coating can prevent aggregation in solution, improve the chemical stability, and provide better protection against toxicity (Laurent et al. 2008). Additionally, polymerized silica-coated iron oxide nanoparticles exhibited good biocompatibility and solubility in water. Silica coating can stabilize the magnetite nanoparticles in two different ways: one is by shielding the magnetic dipole interaction with the silica shell, and another one is by enhancing the coulomb repulsion of the magnetic nanoparticles. Such a silica coating increases the size of the particles and decreases the

presence of the synthesized nanoparticles (Mammeri et al., 2005; Guo et al., 2007).

<10,000), it is difficult to obtain nanoparticles with a small size of <20 nm.

agglomeration may occur for PVA-coated particles via bridging interactions.

desired properties can be synthesized.

**4.3 Polymerized amorphous silica**

saturation magnetization value.

magnetic iron oxide nanoparticles with an average particle size of <10 nm by mixing two microemulsion systems, one containing Fe2+ ions and the other containing OH– ions. The study reveals that the nanoparticles prepared by the microemulsion technique were smaller in size and higher in saturation magnetization than those nanoparticles prepared by Massart's procedure (Massart et al., 1981).

Despite some success, this microemulsion approach has some drawbacks, such as the difficulty in scale-up production, the adverse effect of residual surfactants on the properties of the nanoparticles, and the aggregation of the produced nanoparticles. Repeated wash processes and further stabilization treatment are usually required for such a reaction approach (Wu et al., 2008).

## **4. Surface modifications**

The surface modifications of nanoparticles have attracted much more attention, which can improve the surface-related properties like hydrophobic or hydrophilic. This can be achieved by using surfactants, polymers, and inorganic materials (silica).

## **4.1 Surfactants**

Surface modification with surfactant(s) is widely used for altering surface properties such as hydrophobic or hydrophilic. The use of surfactant molecules, such as oleic acid, oleylamine, or thiols (Wang et al., 2005), can easily functionalize iron oxide nanoparticles to be hydrophobic surfaces. These molecules can covalently bond to the iron atoms or clusters against particle degradation (Soler et al., 2007).

Many researches focus on the synthesis of water-soluble iron oxide nanoparticles with biocompatibility and biodegradability for biological applications. For example, one is to directly introduce the biocompatible organic molecules, e.g., amino acid (Sousa et al., 2001), vitamin (Mornet et al., 2004), and citric acid (Morais et al., 2003). Despite some advantages, the instability of small organic molecules in alkaline or acidic environment may result in agglomeration of the functionalized iron oxide nanoparticles.

Another alternative technique is to transform the oil-soluble type into water-soluble one via a ligand exchange reaction (Chen et al., 2008). The ligand exchange involves the addition of an excess of ligand(s) to nanoparticle suspension, which has stronger interaction with the nanoparticles than the original ones. Sun et al. (2003) converted the synthesized hydrophobic maghemite nanoparticles into hydrophilic ones by mixing with bipolar surfactants such as tetramethylammonium 11-aminoudecanoate. Lattuada and Hatton (2006) reported that the oleic groups initially present on the surface of magnetite nanoparticles were replaced by various capping agents containing reactive hydroxyl moieties. They also tuned the particle size in the range of 6-11 nm by varying the heating rate.

## **4.2 Polymers**

Polymer-functionalized iron oxide nanoparticles have gained much more attention due to the benefits offered by polymeric coating, which may increase repulsive forces to balance the magnetic and van der Waals attractive forces acting on the nanoparticles (Wu et al.

magnetic iron oxide nanoparticles with an average particle size of <10 nm by mixing two microemulsion systems, one containing Fe2+ ions and the other containing OH– ions. The study reveals that the nanoparticles prepared by the microemulsion technique were smaller in size and higher in saturation magnetization than those nanoparticles prepared by

Despite some success, this microemulsion approach has some drawbacks, such as the difficulty in scale-up production, the adverse effect of residual surfactants on the properties of the nanoparticles, and the aggregation of the produced nanoparticles. Repeated wash processes and further stabilization treatment are usually required for such a reaction

The surface modifications of nanoparticles have attracted much more attention, which can improve the surface-related properties like hydrophobic or hydrophilic. This can be

Surface modification with surfactant(s) is widely used for altering surface properties such as hydrophobic or hydrophilic. The use of surfactant molecules, such as oleic acid, oleylamine, or thiols (Wang et al., 2005), can easily functionalize iron oxide nanoparticles to be hydrophobic surfaces. These molecules can covalently bond to the iron atoms or clusters

Many researches focus on the synthesis of water-soluble iron oxide nanoparticles with biocompatibility and biodegradability for biological applications. For example, one is to directly introduce the biocompatible organic molecules, e.g., amino acid (Sousa et al., 2001), vitamin (Mornet et al., 2004), and citric acid (Morais et al., 2003). Despite some advantages, the instability of small organic molecules in alkaline or acidic environment may result in

Another alternative technique is to transform the oil-soluble type into water-soluble one via a ligand exchange reaction (Chen et al., 2008). The ligand exchange involves the addition of an excess of ligand(s) to nanoparticle suspension, which has stronger interaction with the nanoparticles than the original ones. Sun et al. (2003) converted the synthesized hydrophobic maghemite nanoparticles into hydrophilic ones by mixing with bipolar surfactants such as tetramethylammonium 11-aminoudecanoate. Lattuada and Hatton (2006) reported that the oleic groups initially present on the surface of magnetite nanoparticles were replaced by various capping agents containing reactive hydroxyl moieties. They also tuned the particle size in the range of 6-11 nm by varying the heating

Polymer-functionalized iron oxide nanoparticles have gained much more attention due to the benefits offered by polymeric coating, which may increase repulsive forces to balance the magnetic and van der Waals attractive forces acting on the nanoparticles (Wu et al.

achieved by using surfactants, polymers, and inorganic materials (silica).

Massart's procedure (Massart et al., 1981).

against particle degradation (Soler et al., 2007).

agglomeration of the functionalized iron oxide nanoparticles.

approach (Wu et al., 2008).

**4. Surface modifications**

**4.1 Surfactants**

rate.

**4.2 Polymers**

2008). It has shown that through careful choice of the passivating and activating polymers and/or reaction conditions, polymer-stabilized iron oxide nanoparticles with tailored and desired properties can be synthesized.

The iron oxide particles by ionic properties can be modified with functional polymer groups with COOH, NH2 (Chibowski et al., 2009; Kandori et al., 2005; Li et al., 2004). The polymer coated particles can be synthesized by the *ex situ* method, i.e. dispersion of the nanoparticles in a polymeric solution, or *in situ* method, i.e. monomer polymerization in the presence of the synthesized nanoparticles (Mammeri et al., 2005; Guo et al., 2007).

Polymeric coating materials can be classified into two main classes: natural (e.g., dextran, starch, gelatin, chitosan) and synthetic (e.g., polyethylene glycol, PEG; polymethylmethacrylate, PMMA; polyacrylic acid, PAA). However, the saturation magnetization value of iron oxide nanoparticles will decrease after polymer-fictionalization.

Dextran is often utilized as a coating polymer because of its stability and biocompatibility (Laurent et al. 2008). Molday and Mackenzie (Molday and Mackenzie, 1982) have reported the formation of Fe3O4 in the presence of dextran with molecular weight (MW) of 40,000. In the synthesis of dextran-coated ultra-small superparamagnetic iron oxides (USPIO), the reduction of the terminal glucose of dextran was found to be significant for controlling particle size, stability, and magnetic properties. For low molecular weight dextrans (MW, <10,000), it is difficult to obtain nanoparticles with a small size of <20 nm.

Polyvinyl alcohol (PVA) is a hydrophilic and biocompatible polymer that can be used for particle surface modification to prevent particle agglomeration (Laurent et al. 2008). Lee et al. (1996) have modified the surface of magnetite nanoparticles with PVA by precipitation of iron salts at a high pH (13.8) to form stable magnetite colloidal dispersions, and particle size is around 4 nm. The investigators noted that the crystallinity of the magnetite nanoparticles decreased with PVA concentration increasing, although morphology and particle sizes remained. When PVA is introduced, it reacts with the surface through hydrogen bonding between polar functional groups of the polymer and hydroxylated and/or protonated surface of the iron oxide. In addition to the polymer-surface interactions, PVA is known for its hydrogen bonding interaction, resulting in hydrogel structure embedding the nanoparticles. When the PVA concentration is over the critical saturation value, agglomeration may occur for PVA-coated particles via bridging interactions.

## **4.3 Polymerized amorphous silica**

Polymerized tetraethoxysilane (TEOS) network is often used as a surface coating material for iron oxide nanoparticles as this coating can prevent aggregation in solution, improve the chemical stability, and provide better protection against toxicity (Laurent et al. 2008). Additionally, polymerized silica-coated iron oxide nanoparticles exhibited good biocompatibility and solubility in water. Silica coating can stabilize the magnetite nanoparticles in two different ways: one is by shielding the magnetic dipole interaction with the silica shell, and another one is by enhancing the coulomb repulsion of the magnetic nanoparticles. Such a silica coating increases the size of the particles and decreases the saturation magnetization value.

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 129

Fig. 4. (A) Schematics of the hematite-gold core-shell nanorice particles. SEM (left) and TEM (right) of (B) hematite core, (C) seed particles, (D) nanorice with thin shells, and (E) nanorice

Carbon has been widely studied since its poly-morphologies as active carbon, graphite, graphene, carbon nanotubes, and fullerene bucky ball structures. They have exhibited extraordinary tensile strengths and electrical conductivity due to their covalent *sp*2 hybridized network structure. The combination of semi-conductive iron oxides and carbon may therefore enhance the electrical properties of the nanocomposite material. The method to coat carbon on the surface of iron oxide is often performed by the decomposition of a carbon source (i.e., hydrocarbons, polymer or glucose) at high temperatures under oxygen-

Carbon coated iron oxide particles have attracted much more attention. Zhang et al. (2008) demonstrated that carbon coated magnetite nanorods can be synthesized through a series of procedures. In this process, hematite nanorods were firstly synthesized by a hydrothermal method as previously mentioned. Secondly, glucose was coated onto the hematite nanorods by pyrolysis under hydrothermal conditions. Finally, the product was heated at 600 ºC under N2 to carbonize glucose and reduce hematite into magnetite simultaneously. Boguslavsky et al.(2008) reported a similar procedure, in which polydivinylbenzene (PDVB) was used as the carbon source. The PDVB coating was formed by emulsion polymerization of DVB in the presence of γ-Fe2O3, followed by annealing of the powder in a quartz tube at 1050 ºC under flowing Ar gas for 2 hours. The decomposition of the polymer in this case reduced γ-Fe2O3 to metallic Fe, which finally forms carbon coated iron (Fe/C) nanoparticles.

free environments (Tristão et al., 2010; Tristão et al., 2009; Zhang et al., 2008, 2010).

with thick shells. Reprinted with permission from (Wang et al., 2006).

**4.5 Carbon**

A commonly used method to coat iron oxide nanoparticles with silica is the well-known Stőber method, in which silica is formed *in situ* via hydrolysis and condensation of a sol-gel precursor such as TEOS. For example, Im et al*.*(2005) have reported the synthesis of silica colloids loaded with superparamagnetic iron oxide nanoparticles, which revealed that the final size of silica colloids depended upon the concentration of iron oxide nanoparticles because the size of silica was closely related to the number of seeds (emulsion drops). The lower concentration the iron oxide nanoparticles in alcohol, the larger size the obtained colloids.

Another one is aerosol-pyrolysis method, in which silica-coated magnetic nanoparticles were prepared by pyrolysis of a mixed precursor of silicon alkoxides and metal compound in a flame environment (Deng et al. 2005). Tartaj *et al.*(2001) synthesized silica-coated γ-Fe2O3 hollow spheres with size of 150 ± 100 nm by aerosol pyrolysis of methanol solution containing iron ammonium citrate and silicon ethoxide.

## **4.4 Metals**

Noble metals (e.g., Au, Ag, Pt, and Pd), possessing unique electronic and catalytic properties, can be utilized to improve the physicochemical properties of magnetic nanoparticles and applications in biomedicine. The coating of iron oxide nanoparticles with noble metals can be helpful to improve stability from aggregation, however, decrease the saturation magnetization value in some cases (Wu et al. 2008).

Several procedures have been employed to synthesize such core-shell nanostructures. For example, Mikhaylova *et al.* (2004) have prepared gold-coated superparamagnetic iron oxide nanoparticles (SPION) using a reverse micelle method. In their study, the reverse micelles were formed from surfactant, cetyltrimethylammonium bromide (CTAB), octane (the oil phase), butanol (the co-surfactant), and an aqueous mixture of FeCl3, FeCl2 and HAuCl4 solutions. They found that the Au-coated SPION retained the superparamagnetic properties for a longer period than those of starch-coated and multi-arm polyethylene glycol (MPEG) coated ones. Wang et al*.*(2005) obtained gold coated iron oxide nanoparticles, in which the pre-synthesized Fe3O4 nanoparticles were used as seeds during the reduction of gold precursor, Au(OOCCH3)3. The average size of Fe3O4 nanoparticles increases from 5.2 ± 0.5 nm to 6.7 ± 0.7 nm after coating with gold (**Fig. 4**). Fe3O4/Au and Fe3O4/Au/Ag core/shell nanoparticles with tuneable plasmonic and magnetic properties have been developed by controlling the coating thickness and materials (Xu et al. 2007).

A facile and one-pot synthesis approach has been developed by Zhang et al*.* (Zhang et al., 2010) for generating metal (Au, Pt, Ag and Au-Pt)/Fe2O3 nanocomposites assisted by lysine. Lysine, containing functional groups -NH2 and –COOH, acts as both a linking molecule to the Fe2O3 matrix and a capping agent to stabilize the noble metal nanoparticles for a good dispersion. Jiang et al. (Jiang and Yu, 2009) have demonstrated a facile synthetic method for the preparation of Pd/α-Fe2O3 nanocomposites by adding citric acid into a mixture of iron oxide nanoparticles and palladium precursor, Pd(CH3CN)2Cl2) under a reflux heating at 90°C for 2 hours. The synthesized Pd/α-Fe2O3 nanocomposites inherited the rod-like morphology of the α-Fe2O3 nanoparticles and they exhibited superior catalytic activity in CO oxidation compared with pure α-Fe2O3 nanoparticles. UV-vis measurement of the nanocomposites revealed the presence of two plasma bands centered at around 383 and 552 nm, which can be assigned to the synergistic effect of both Pd and α-Fe2O3 nanoparticles.

Fig. 4. (A) Schematics of the hematite-gold core-shell nanorice particles. SEM (left) and TEM (right) of (B) hematite core, (C) seed particles, (D) nanorice with thin shells, and (E) nanorice with thick shells. Reprinted with permission from (Wang et al., 2006).

## **4.5 Carbon**

128 Smart Nanoparticles Technology

A commonly used method to coat iron oxide nanoparticles with silica is the well-known Stőber method, in which silica is formed *in situ* via hydrolysis and condensation of a sol-gel precursor such as TEOS. For example, Im et al*.*(2005) have reported the synthesis of silica colloids loaded with superparamagnetic iron oxide nanoparticles, which revealed that the final size of silica colloids depended upon the concentration of iron oxide nanoparticles because the size of silica was closely related to the number of seeds (emulsion drops). The lower concentration the iron oxide nanoparticles in alcohol, the larger size the obtained colloids.

Another one is aerosol-pyrolysis method, in which silica-coated magnetic nanoparticles were prepared by pyrolysis of a mixed precursor of silicon alkoxides and metal compound in a flame environment (Deng et al. 2005). Tartaj *et al.*(2001) synthesized silica-coated γ-Fe2O3 hollow spheres with size of 150 ± 100 nm by aerosol pyrolysis of methanol solution

Noble metals (e.g., Au, Ag, Pt, and Pd), possessing unique electronic and catalytic properties, can be utilized to improve the physicochemical properties of magnetic nanoparticles and applications in biomedicine. The coating of iron oxide nanoparticles with noble metals can be helpful to improve stability from aggregation, however, decrease the

Several procedures have been employed to synthesize such core-shell nanostructures. For example, Mikhaylova *et al.* (2004) have prepared gold-coated superparamagnetic iron oxide nanoparticles (SPION) using a reverse micelle method. In their study, the reverse micelles were formed from surfactant, cetyltrimethylammonium bromide (CTAB), octane (the oil phase), butanol (the co-surfactant), and an aqueous mixture of FeCl3, FeCl2 and HAuCl4 solutions. They found that the Au-coated SPION retained the superparamagnetic properties for a longer period than those of starch-coated and multi-arm polyethylene glycol (MPEG) coated ones. Wang et al*.*(2005) obtained gold coated iron oxide nanoparticles, in which the pre-synthesized Fe3O4 nanoparticles were used as seeds during the reduction of gold precursor, Au(OOCCH3)3. The average size of Fe3O4 nanoparticles increases from 5.2 ± 0.5 nm to 6.7 ± 0.7 nm after coating with gold (**Fig. 4**). Fe3O4/Au and Fe3O4/Au/Ag core/shell nanoparticles with tuneable plasmonic and magnetic properties have been developed by

A facile and one-pot synthesis approach has been developed by Zhang et al*.* (Zhang et al., 2010) for generating metal (Au, Pt, Ag and Au-Pt)/Fe2O3 nanocomposites assisted by lysine. Lysine, containing functional groups -NH2 and –COOH, acts as both a linking molecule to the Fe2O3 matrix and a capping agent to stabilize the noble metal nanoparticles for a good dispersion. Jiang et al. (Jiang and Yu, 2009) have demonstrated a facile synthetic method for the preparation of Pd/α-Fe2O3 nanocomposites by adding citric acid into a mixture of iron oxide nanoparticles and palladium precursor, Pd(CH3CN)2Cl2) under a reflux heating at 90°C for 2 hours. The synthesized Pd/α-Fe2O3 nanocomposites inherited the rod-like morphology of the α-Fe2O3 nanoparticles and they exhibited superior catalytic activity in CO oxidation compared with pure α-Fe2O3 nanoparticles. UV-vis measurement of the nanocomposites revealed the presence of two plasma bands centered at around 383 and 552 nm, which can be assigned to the synergistic effect of both Pd and α-Fe2O3 nanoparticles.

containing iron ammonium citrate and silicon ethoxide.

saturation magnetization value in some cases (Wu et al. 2008).

controlling the coating thickness and materials (Xu et al. 2007).

**4.4 Metals**

Carbon has been widely studied since its poly-morphologies as active carbon, graphite, graphene, carbon nanotubes, and fullerene bucky ball structures. They have exhibited extraordinary tensile strengths and electrical conductivity due to their covalent *sp*2 hybridized network structure. The combination of semi-conductive iron oxides and carbon may therefore enhance the electrical properties of the nanocomposite material. The method to coat carbon on the surface of iron oxide is often performed by the decomposition of a carbon source (i.e., hydrocarbons, polymer or glucose) at high temperatures under oxygenfree environments (Tristão et al., 2010; Tristão et al., 2009; Zhang et al., 2008, 2010).

Carbon coated iron oxide particles have attracted much more attention. Zhang et al. (2008) demonstrated that carbon coated magnetite nanorods can be synthesized through a series of procedures. In this process, hematite nanorods were firstly synthesized by a hydrothermal method as previously mentioned. Secondly, glucose was coated onto the hematite nanorods by pyrolysis under hydrothermal conditions. Finally, the product was heated at 600 ºC under N2 to carbonize glucose and reduce hematite into magnetite simultaneously. Boguslavsky et al.(2008) reported a similar procedure, in which polydivinylbenzene (PDVB) was used as the carbon source. The PDVB coating was formed by emulsion polymerization of DVB in the presence of γ-Fe2O3, followed by annealing of the powder in a quartz tube at 1050 ºC under flowing Ar gas for 2 hours. The decomposition of the polymer in this case reduced γ-Fe2O3 to metallic Fe, which finally forms carbon coated iron (Fe/C) nanoparticles.

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 131

resonance spectroscopy (SERS) for sensing devices (Zhai et al., 2009). This effect is also of importance for bimetallic core/shell nanoparticles. As the ratio of gold to iron oxide increases, the gold character increases and the iron oxide becomes buried beneath and suppresses the dielectric effect. The increasing thickness of the shell structure will therefore

cause blue-shifting in the surface plasmon resonance (Lyon et al. 2004).

Fig. 5. Diagram of different spin arrangements in magnetic nanoparticles:

spins. Reprinted with permission from (Lu et al., 2007).

**5.2 Biomedical applications**

a) Ferromagnetism (FM), b) Antiferromagnetism (AFM), D = diameter, Dc = critical diameter, c) a combination of two different ferromagnetic phases in permanent magnets, which are materials with high remanence magnetization (Mr) and high coercively (Hc), d) Superparamagnetism (SPM), e) the interaction at the interface between a ferromagnet and an antiferromagnet producing an exchange bias effect, and f) pure anti-ferromagnetic nanoparticles with superparamagnetic relaxation arising from uncompensated surface

Many investigations have been reported the application of nanoparticles for biomedicine, such as magnetic nanoparticles for improving the quality of magnetic resonance imaging (MRI), hyperthermic treatment for malignant cells, site-specific drug delivery, cell labeling, and manipulating cell membranes (Babič et al. 2008; Catherine and Adam, 2003). These

Iron oxide nanocomposites or particle coated with biocompatible polymer(s) have shown some advantages, e.g., reducing aggregation, maintain magnetic stability, slowdown degrading process under physiological conditions, and lower toxicity (Mahmoudi et al. 2009). So far, they have shown promise for monitoring living cells by both MR and

magnetic particles can also be used for diagnosis, imaging, and drug delivery.

fluorescence imaging, as well as for drug delivery (Liong et al., 2008).

In addition, Wang et al*.* (2006) have reported the synthesis of Fe3O4/C nanocomposites by heating the aqueous solution of glucose and oleic acid-stabilized Fe3O4 nanoparticles at 170 °C for 3 hours. The results revealed that without prior surface hydrophobic modification, the magnetite nanoparticles could not be encapsulated by the carbon nanospheres, but instead only bare carbon nanospheres with the size of ~200 nm and Fe3O4 nanoparticles were obtained. The variation of glucose concentration (0.3-0.6 M) and the reaction temperature (160-180 °C) were found to have no significant effect on the morphology of the product, however, both reaction time and the amount of oleic acid-stabilized Fe3O4 nanoparticles showed significant effects. The increase in the concentration of oleic-acid stabilized Fe3O4 nanoparticles from 2.5 to 6 g/L was found to generate a product that has more embedded Fe3O4 nanoparticles increasing from 41 to 63%).

Although carbon-coated iron oxide nanoparticles may offer some advantages, such particles are often obtained as agglomerated clusters due to the lack of effective synthetic control, and lack of proper understanding on the formation mechanism. The synthesis of dispersible carbon-coated nanoparticles in isolated forms still remains a challenge in this field.

Moreover, the surface modification of iron oxide allows the attachment of biomolecules such as proteins and drugs (Mohapatra et al. 2007; Sun et al. 2007). The design of the surface modifications may be determined by factors such as ion energy and ion flux of depositing species, interface volume, crystalline size, coating thickness, surface and interfacial energy (Kim et al. 2003; Pinho et al. 2010).

## **5. Functionalities of iron oxide nanostructures**

## **5.1 Magnetic property**

The magnetic property has been extensively studied since it was discovered and explained through electronic structures of atoms. The magnetic dipole moments generated by the spin and orbital angular momenta of electrons in the Fe atom may vary between each phase of the iron oxide material. In general, magnetic behavior of a material depends on the electron spin vector or the total magnetic dipole moment. One important aspect in iron oxide nanoparticles is the unique form of magnetism called superparamagnetism. At temperature of above the blocking temperature, the magnetization behavior is identical to that of atomic paramagnets. This phenomenon will occur if particles reach below a certain size (10-20 nm), when the particle consists of a single magnetic domain, even though the material is ferro- or ferri-magnetic in bulk form (Ye et al., 2007), as shown in **Fig. 5**. Particles with this type of magnetism show high field irreversibility, high saturation field, extra anisotropy contributions, and shifted loops (Pedro et al., 2003).

For noble gold and silver nanoparticles with unique surface plasmon resonance (SPR) properties, they are often used to modify the iron oxide surfaces for generating coupled or multiple functionalities. At the nanoscale, the metallic electron cloud oscillates on the particle surface and absorbs electromagnetic radiation at a particular energy. The surface geometry of the iron oxide particles such as spheres, cubes, triangles, or rods, can therefore influence the absorption of radiation from the ultra-violet up to the near infrared spectrum (350-1200 nm). Other factors that affect the absorption are the solvent and surface functionalization. They are important contributors that can tune the exact frequency and intensity of the plasmon resonance band, which attracts them to the surface enhanced resonance spectroscopy (SERS) for sensing devices (Zhai et al., 2009). This effect is also of importance for bimetallic core/shell nanoparticles. As the ratio of gold to iron oxide increases, the gold character increases and the iron oxide becomes buried beneath and suppresses the dielectric effect. The increasing thickness of the shell structure will therefore cause blue-shifting in the surface plasmon resonance (Lyon et al. 2004).

Fig. 5. Diagram of different spin arrangements in magnetic nanoparticles: a) Ferromagnetism (FM), b) Antiferromagnetism (AFM), D = diameter, Dc = critical diameter, c) a combination of two different ferromagnetic phases in permanent magnets, which are materials with high remanence magnetization (Mr) and high coercively (Hc), d) Superparamagnetism (SPM), e) the interaction at the interface between a ferromagnet and an antiferromagnet producing an exchange bias effect, and f) pure anti-ferromagnetic nanoparticles with superparamagnetic relaxation arising from uncompensated surface spins. Reprinted with permission from (Lu et al., 2007).

## **5.2 Biomedical applications**

130 Smart Nanoparticles Technology

In addition, Wang et al*.* (2006) have reported the synthesis of Fe3O4/C nanocomposites by heating the aqueous solution of glucose and oleic acid-stabilized Fe3O4 nanoparticles at 170 °C for 3 hours. The results revealed that without prior surface hydrophobic modification, the magnetite nanoparticles could not be encapsulated by the carbon nanospheres, but instead only bare carbon nanospheres with the size of ~200 nm and Fe3O4 nanoparticles were obtained. The variation of glucose concentration (0.3-0.6 M) and the reaction temperature (160-180 °C) were found to have no significant effect on the morphology of the product, however, both reaction time and the amount of oleic acid-stabilized Fe3O4 nanoparticles showed significant effects. The increase in the concentration of oleic-acid stabilized Fe3O4 nanoparticles from 2.5 to 6 g/L was found to generate a product that has

Although carbon-coated iron oxide nanoparticles may offer some advantages, such particles are often obtained as agglomerated clusters due to the lack of effective synthetic control, and lack of proper understanding on the formation mechanism. The synthesis of dispersible

Moreover, the surface modification of iron oxide allows the attachment of biomolecules such as proteins and drugs (Mohapatra et al. 2007; Sun et al. 2007). The design of the surface modifications may be determined by factors such as ion energy and ion flux of depositing species, interface volume, crystalline size, coating thickness, surface and interfacial energy

The magnetic property has been extensively studied since it was discovered and explained through electronic structures of atoms. The magnetic dipole moments generated by the spin and orbital angular momenta of electrons in the Fe atom may vary between each phase of the iron oxide material. In general, magnetic behavior of a material depends on the electron spin vector or the total magnetic dipole moment. One important aspect in iron oxide nanoparticles is the unique form of magnetism called superparamagnetism. At temperature of above the blocking temperature, the magnetization behavior is identical to that of atomic paramagnets. This phenomenon will occur if particles reach below a certain size (10-20 nm), when the particle consists of a single magnetic domain, even though the material is ferro- or ferri-magnetic in bulk form (Ye et al., 2007), as shown in **Fig. 5**. Particles with this type of magnetism show high field irreversibility, high saturation field, extra anisotropy

For noble gold and silver nanoparticles with unique surface plasmon resonance (SPR) properties, they are often used to modify the iron oxide surfaces for generating coupled or multiple functionalities. At the nanoscale, the metallic electron cloud oscillates on the particle surface and absorbs electromagnetic radiation at a particular energy. The surface geometry of the iron oxide particles such as spheres, cubes, triangles, or rods, can therefore influence the absorption of radiation from the ultra-violet up to the near infrared spectrum (350-1200 nm). Other factors that affect the absorption are the solvent and surface functionalization. They are important contributors that can tune the exact frequency and intensity of the plasmon resonance band, which attracts them to the surface enhanced

carbon-coated nanoparticles in isolated forms still remains a challenge in this field.

more embedded Fe3O4 nanoparticles increasing from 41 to 63%).

**5. Functionalities of iron oxide nanostructures**

contributions, and shifted loops (Pedro et al., 2003).

(Kim et al. 2003; Pinho et al. 2010).

**5.1 Magnetic property**

Many investigations have been reported the application of nanoparticles for biomedicine, such as magnetic nanoparticles for improving the quality of magnetic resonance imaging (MRI), hyperthermic treatment for malignant cells, site-specific drug delivery, cell labeling, and manipulating cell membranes (Babič et al. 2008; Catherine and Adam, 2003). These magnetic particles can also be used for diagnosis, imaging, and drug delivery.

Iron oxide nanocomposites or particle coated with biocompatible polymer(s) have shown some advantages, e.g., reducing aggregation, maintain magnetic stability, slowdown degrading process under physiological conditions, and lower toxicity (Mahmoudi et al. 2009). So far, they have shown promise for monitoring living cells by both MR and fluorescence imaging, as well as for drug delivery (Liong et al., 2008).

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 133

The sensitivity of iron oxide-based nanosensors can be improved by various doping schemes as well as by changing the sensing material structure. For example, the thin film type sensors tend to exhibit higher sensitivity than bulk material sensor(s) (Mohapatra and Anand, 2010). Tao and co-workers (1999) have studied the sensing characteristics of Y2O3 doped γ-Fe2O3 towards hydrocarbon gases, H2 and CO and found that the addition of Y2O3 to γ-Fe2O3 resulted in a little difference in the sensitivity and selectivity compared with those made of pure γ-Fe2O3. Neri et al*.*(2002) have assessed the gas-sensing properties of Zndoped Fe2O3 thin films prepared by liquid phase deposition method. They observed that the addition of metal Zn can increase the sensitivity of the Fe2O3 thin film to NO2 below 250 °C.

A catalyst can attract atoms and/or molecules, and then change the surface conductivity and other properties. Different from sensing material, the catalyst often converts itself into a different species through a chemical reaction. The iron oxides (hematite and magnetite) have been applied in industry to produce chemicals with high efficiencies, such as ammonia (Haber process) and hydrocarbons (Fischer-Tropsch process) (Teja and Koh, 2009). It is expected that the nanoparticles with high surface areas can perform much better to enhance the chemical reaction rates than that of bulk states. For hematite, its thermal-dynamically

stable structure allows it for high temperature oxidation catalysis (Sivula et al., 2010).

The catalysis effect can also be enhanced by coupling metal nanoparticles on the surface (Jiang and Yu 2009; Zhong et al., 2007). Jiang et al (2009) have reported the synthesis of Pd/α-Fe2O3 nanocomposites at ambient conditions, which displayed superior lowtemperature catalytic activity toward CO oxidation to the pure α-Fe2O3 nanoparticles. It was proposed that the enhanced catalytic activity was due to the reaction between oxygen adsorbed on the reduced sites of the support (Fe2+) and CO adsorbed on Pd at the metal-

By using gold deposited iron oxide materials as a catalyst material, the oxidation and hydrogenation reaction of many organic compounds can be performed at much lower temperatures (Kung et al., 2007; Herzing et al., 2008; Lenz et al., 2009; Scirè et al., 2008). For example, Al-Sayari and co-workers (2007) have shown the dependence of the catalytic performance of Au/Fe2O3 catalyst that the non-calcined Au/Fe2O3 catalyst exhibited a high activity when pH≥ 5, whereas the activity of calcined Au/Fe2O3 catalyst was not influenced by the preparation conditions. Furthermore, the authors also noted that the catalytic activity of Fe2O3 toward CO oxidation was considerably lower than that of the Au/Fe2O3 catalyst. Maghemite and magnetite/carbon composites have been found to be good catalysts for reducing the concentration of undesirable nitrogen in acrylonitrile-butadiene-styrene (ABS) degradation oil (Brebu et al., 2001), whereas hematite can be used as a photocatalyst for the degradation of chlorophenol and azo dyes (Bandara et al., 2007), as well as a support material for gold in catalysts for the oxidation of carbon monoxide (CO) at low temperatures

The challenge of catalysis research being the reaction mechanism for these systems are still yet to be confirmed or explained, especially for the metal oxide/gold systems (Astruc et al., 2005). The reaction can be compared from titanium oxide/gold. The rutile phase of titania provides a support for gold, in which CO will convert mostly along the perimeter between the titania and

**5.4 Catalyst**

oxide interface, as shown in **Fig. 6**.

(Zhong et al., 2007).

As mentioned previously, iron oxide nanoparticles exhibit paramagnetic or superparamagnetic properties in a limited size range. Particles larger than 50 nm show superparamagnetic iron oxides (SPIO), whereas particles smaller than 50 nm show ultrasmall superparamagnetic property (USP). The smaller ones have the ability to enhance signal detection and increase resolution in the MRI (Foy et al., 2010; Tong et al., 2010). Therefore, the SPIO particles can be used for imaging tumors in the liver and spleen, while superparamagnetic particles for contrast agents for lymphography and angiography. However, the superparamagnetic particles do not retain their magnetism when the external magnetic field is removed, while other magnetic materials will become magnetized and aggregate.

In addition, the problem using magnetite or maghemite nanoparticles in clinic is often limited by the biocompatibility and toxicity of these particles (Martin et al., 2008; Pisanic Ii et al., 2007). This happens from the body's defense system, the reticulo-endothelial system (RES), trying to remove these particles from the bloodstream as they pass through the liver, spleen and lymph nodes. The rapid removal of the iron oxide nanoparticles reduces their life-time. This is why it is necessary to produce nanocomposites with special surface modifications. The surface modification of the particles allows the water-insoluble drugs to be loaded and stored for a long time (Liong et al., 2008; Son et al., 2005). Despite some progress, the challenges in using surface modified magnetic iron oxide nanoparticles still exist. More work needs to be performed in the future.

## **5.3 Gas sensing**

Gas detection with high sensitivity and selectivity is essential for controlling industrial, waste, and vehicle emissions, household activity and environmental monitoring. In the past decades, many sensor devices have been developed for various gases such as CO, CO2, O2, O3, H2, NH3 and SO2, as well as various organic vapors e.g., benzene, methanol, ethanol, amines and isopropanol (Jimenez-Cadena et al. 2007). Although semiconducting oxides have been quite useful as gas sensors, the operation at high temperatures often limits their functionality and applications. This has prompted the exploration of new materials that may offer higher sensing and selective capabilities than traditional ones.

Nanostructured metal oxides are one of the most commonly used materials for gas sensing because of the semiconductors make them possible for the electrical conductivity change when the surrounding atmosphere changes. Additionally, nanosized metal oxides exhibit high ratios of surface to volume, which favors the adsorption of gases on the particle surface, and hence increases the sensitivity in detection.

Iron oxide nanoparticles have shown good sensing capabilities toward hydrocarbon gases, CO and alcohols (Jimenez-Cadena et al., 2007; Han et al., 1996, 1999, and 2001). The studies by Zhang et al*.* (1996) and Tao et al*.* (1999) showed that γ-Fe2O3 nanosensors exhibited good sensitivity and selectivity to a range of hydrocarbon gases such as LPG, petrol and C2H2 at 380 °C, but poor sensitivity to H2 and CO. However, Nakatani and Matsuoka (1983) together with Lee and Choi (1990) reported that the γ-Fe2O3-based sensors exhibited good sensitivity to H2. This suggests that the gas-sensing characteristics of a nanosensor are related to its preparation process.

The sensitivity of iron oxide-based nanosensors can be improved by various doping schemes as well as by changing the sensing material structure. For example, the thin film type sensors tend to exhibit higher sensitivity than bulk material sensor(s) (Mohapatra and Anand, 2010). Tao and co-workers (1999) have studied the sensing characteristics of Y2O3 doped γ-Fe2O3 towards hydrocarbon gases, H2 and CO and found that the addition of Y2O3 to γ-Fe2O3 resulted in a little difference in the sensitivity and selectivity compared with those made of pure γ-Fe2O3. Neri et al*.*(2002) have assessed the gas-sensing properties of Zndoped Fe2O3 thin films prepared by liquid phase deposition method. They observed that the addition of metal Zn can increase the sensitivity of the Fe2O3 thin film to NO2 below 250 °C.

## **5.4 Catalyst**

132 Smart Nanoparticles Technology

As mentioned previously, iron oxide nanoparticles exhibit paramagnetic or superparamagnetic properties in a limited size range. Particles larger than 50 nm show superparamagnetic iron oxides (SPIO), whereas particles smaller than 50 nm show ultrasmall superparamagnetic property (USP). The smaller ones have the ability to enhance signal detection and increase resolution in the MRI (Foy et al., 2010; Tong et al., 2010). Therefore, the SPIO particles can be used for imaging tumors in the liver and spleen, while superparamagnetic particles for contrast agents for lymphography and angiography. However, the superparamagnetic particles do not retain their magnetism when the external magnetic field is removed, while other magnetic materials will become magnetized and

In addition, the problem using magnetite or maghemite nanoparticles in clinic is often limited by the biocompatibility and toxicity of these particles (Martin et al., 2008; Pisanic Ii et al., 2007). This happens from the body's defense system, the reticulo-endothelial system (RES), trying to remove these particles from the bloodstream as they pass through the liver, spleen and lymph nodes. The rapid removal of the iron oxide nanoparticles reduces their life-time. This is why it is necessary to produce nanocomposites with special surface modifications. The surface modification of the particles allows the water-insoluble drugs to be loaded and stored for a long time (Liong et al., 2008; Son et al., 2005). Despite some progress, the challenges in using surface modified magnetic iron oxide nanoparticles still

Gas detection with high sensitivity and selectivity is essential for controlling industrial, waste, and vehicle emissions, household activity and environmental monitoring. In the past decades, many sensor devices have been developed for various gases such as CO, CO2, O2, O3, H2, NH3 and SO2, as well as various organic vapors e.g., benzene, methanol, ethanol, amines and isopropanol (Jimenez-Cadena et al. 2007). Although semiconducting oxides have been quite useful as gas sensors, the operation at high temperatures often limits their functionality and applications. This has prompted the exploration of new materials that may

Nanostructured metal oxides are one of the most commonly used materials for gas sensing because of the semiconductors make them possible for the electrical conductivity change when the surrounding atmosphere changes. Additionally, nanosized metal oxides exhibit high ratios of surface to volume, which favors the adsorption of gases on the particle

Iron oxide nanoparticles have shown good sensing capabilities toward hydrocarbon gases, CO and alcohols (Jimenez-Cadena et al., 2007; Han et al., 1996, 1999, and 2001). The studies by Zhang et al*.* (1996) and Tao et al*.* (1999) showed that γ-Fe2O3 nanosensors exhibited good sensitivity and selectivity to a range of hydrocarbon gases such as LPG, petrol and C2H2 at 380 °C, but poor sensitivity to H2 and CO. However, Nakatani and Matsuoka (1983) together with Lee and Choi (1990) reported that the γ-Fe2O3-based sensors exhibited good sensitivity to H2. This suggests that the gas-sensing characteristics of a nanosensor are related to its

exist. More work needs to be performed in the future.

offer higher sensing and selective capabilities than traditional ones.

surface, and hence increases the sensitivity in detection.

aggregate.

**5.3 Gas sensing**

preparation process.

A catalyst can attract atoms and/or molecules, and then change the surface conductivity and other properties. Different from sensing material, the catalyst often converts itself into a different species through a chemical reaction. The iron oxides (hematite and magnetite) have been applied in industry to produce chemicals with high efficiencies, such as ammonia (Haber process) and hydrocarbons (Fischer-Tropsch process) (Teja and Koh, 2009). It is expected that the nanoparticles with high surface areas can perform much better to enhance the chemical reaction rates than that of bulk states. For hematite, its thermal-dynamically stable structure allows it for high temperature oxidation catalysis (Sivula et al., 2010).

The catalysis effect can also be enhanced by coupling metal nanoparticles on the surface (Jiang and Yu 2009; Zhong et al., 2007). Jiang et al (2009) have reported the synthesis of Pd/α-Fe2O3 nanocomposites at ambient conditions, which displayed superior lowtemperature catalytic activity toward CO oxidation to the pure α-Fe2O3 nanoparticles. It was proposed that the enhanced catalytic activity was due to the reaction between oxygen adsorbed on the reduced sites of the support (Fe2+) and CO adsorbed on Pd at the metaloxide interface, as shown in **Fig. 6**.

By using gold deposited iron oxide materials as a catalyst material, the oxidation and hydrogenation reaction of many organic compounds can be performed at much lower temperatures (Kung et al., 2007; Herzing et al., 2008; Lenz et al., 2009; Scirè et al., 2008). For example, Al-Sayari and co-workers (2007) have shown the dependence of the catalytic performance of Au/Fe2O3 catalyst that the non-calcined Au/Fe2O3 catalyst exhibited a high activity when pH≥ 5, whereas the activity of calcined Au/Fe2O3 catalyst was not influenced by the preparation conditions. Furthermore, the authors also noted that the catalytic activity of Fe2O3 toward CO oxidation was considerably lower than that of the Au/Fe2O3 catalyst.

Maghemite and magnetite/carbon composites have been found to be good catalysts for reducing the concentration of undesirable nitrogen in acrylonitrile-butadiene-styrene (ABS) degradation oil (Brebu et al., 2001), whereas hematite can be used as a photocatalyst for the degradation of chlorophenol and azo dyes (Bandara et al., 2007), as well as a support material for gold in catalysts for the oxidation of carbon monoxide (CO) at low temperatures (Zhong et al., 2007).

The challenge of catalysis research being the reaction mechanism for these systems are still yet to be confirmed or explained, especially for the metal oxide/gold systems (Astruc et al., 2005). The reaction can be compared from titanium oxide/gold. The rutile phase of titania provides a support for gold, in which CO will convert mostly along the perimeter between the titania and

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 135

and forces. Thus, the macroscopic properties (e.g., pressure, energy, heat capacities) can be

In our recent work, the MD method was used to explain the interactions between various goethite surfaces and surfactants of the nanorods. The simulation results of the side wall (*xy*0) surfaces with six different surfactants have been reported (Yue et al. 2010, 2011). The positively charged surfactants, CTAB (**Fig. 7**) and tetraethylammonium chloride (TEAC), were found to interact greatly with the side wall (*xy*0) of the nanorod, while the polymeric polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) and anionic surfactants (AOT) and Sodium Dodecyl Sulfate (SDS) were not suitable because of the low interaction energies among the surfaces. This is caused by the differences in the active sites on different surfaces (Kim et al. 2007). The ratios of iron and oxygen can vary greatly for different surfaces, in which the packing and exposure of atoms along a particular crystal plane will therefore determine the strength of adsorbed surface molecules. The simulation could provide quantitative information toward the interaction between surfactants and goethite surface(s),

Through a similar MD simulation, the adsorption of minerals has been explored. Kerisit et al. (2006) simulated the interactions for electrolyte solutions to determine the surface properties of monovalent ions, such as NaCl, CsCl, and CsF on the (100) goethite surface. The calculations showed a structured interfacial region is in the first 15 Å on the surface. The structure of the mineral surface will also affect the arrangement and orientation of the water molecules, and hence the diffusive properties and distribution of the ionic species. In comparison, the adsorption of sodium ions is stronger than cesium ions because the former

Fig. 7. MD simulation of CTAB molecular adsorption on the goethite crystal (010) surface at different time: (A) 0 ps, (B) 10 ps, (C) 20 ps, and (D) 50 ps. Reprinted with permission from

Similarly, MD simulation was also employed to explain the growth mechanisms of akaganéite nanorods (Yue et al., 2011), as shown in **Fig. 8**, in which the atomic concentration profiles of various anions on different crystalline surfaces were compared. With the assistance of experimental techniques such as transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD), the role of chloride ions in the lattice

and hence understand the particle formation and growth mechanisms.

can occupy an interstitial site of mineral(s) due to smaller size.

(Yue et al., 2010).

derived by means of statistical mechanics.

gold (Haruta, 2002). In other studies, it was proposed that the nature of the support material has much greater influence on the reactive properties of the deposited nanoparticle, because the active and selective sites are formed by negative gold particles (Milone et al., 2007).

Fig. 6. HRTEM image of Pd particles binding on the surface of iron oxide, in which the lattice distance is ~0.385 nm, corresponding to Pd{1 0 0} planes. Catalytic activity of Pd/α-Fe2O3 nanocomposites showing the dependence of CO oxidation (A); (B) comparison of the catalytic activity of nanoparticles with and without doped palladium. Reprinted with permission from (Jiang and Yu, 2009).

## **6. Theoretical simulations**

Beyond physical phenomena, theoretical methods have been developed and widely used to understand electronic, structure and forces of nanostructures (Cohen et al., 2008; Freund and Pacchioni, 2008; Hafner et al., 2006; Carter, 2008). Specifically, molecular dynamics (MD) method can be used for calculating interaction energies between surface modifiers and the modified matters, density functional theory (DFT) for binding energies, and Monte Carlo (MC) method for equilibrium properties (e.g., free energy, phase equilibrium) of particles. These methods have allowed researchers to understand and explain the growth mechanisms, structure, and functionalities of nanostructures (Hafner et al., 2006).

## **6.1 Molecular dynamics**

MD simulation has been widely used for the study of the molecular behaviours in liquids and solids, examining material properties, and designing new materials, particularly for nanoparticles and nanocomposites. The MD method allows one to predict the time evolution of a system of interacting particles (atoms or molecules) and estimate relevant physicochemical properties. Specifically, it can calculate and simulate the interaction energies among atoms/molecules, which can help understand atomic positions, velocities,

gold (Haruta, 2002). In other studies, it was proposed that the nature of the support material has much greater influence on the reactive properties of the deposited nanoparticle, because

the active and selective sites are formed by negative gold particles (Milone et al., 2007).

Fig. 6. HRTEM image of Pd particles binding on the surface of iron oxide, in which the lattice distance is ~0.385 nm, corresponding to Pd{1 0 0} planes. Catalytic activity of Pd/α-Fe2O3 nanocomposites showing the dependence of CO oxidation (A);

Reprinted with permission from (Jiang and Yu, 2009).

**6. Theoretical simulations**

**6.1 Molecular dynamics**

(B) comparison of the catalytic activity of nanoparticles with and without doped palladium.

Beyond physical phenomena, theoretical methods have been developed and widely used to understand electronic, structure and forces of nanostructures (Cohen et al., 2008; Freund and Pacchioni, 2008; Hafner et al., 2006; Carter, 2008). Specifically, molecular dynamics (MD) method can be used for calculating interaction energies between surface modifiers and the modified matters, density functional theory (DFT) for binding energies, and Monte Carlo (MC) method for equilibrium properties (e.g., free energy, phase equilibrium) of particles. These methods have allowed researchers to understand and explain the growth

MD simulation has been widely used for the study of the molecular behaviours in liquids and solids, examining material properties, and designing new materials, particularly for nanoparticles and nanocomposites. The MD method allows one to predict the time evolution of a system of interacting particles (atoms or molecules) and estimate relevant physicochemical properties. Specifically, it can calculate and simulate the interaction energies among atoms/molecules, which can help understand atomic positions, velocities,

mechanisms, structure, and functionalities of nanostructures (Hafner et al., 2006).

and forces. Thus, the macroscopic properties (e.g., pressure, energy, heat capacities) can be derived by means of statistical mechanics.