**Abbreviations**

BA—N6-benzyladenine; 2,4-D—2,4-dichlorophenoxyacetic acid; GA3—gibberellic acid; IAA—Indole-3-acetic acid; IBA—indole-3-butyric acid; MS—Murashige and Skoog (1962) medium; NAA—α-Naphthaleneacetic acid; PCs—phenolic compunds; TPC—total phenolic content; TFC—total flavonoid content; FRP—ferric reducing power.


*4.3. Validation of Reconstruction*

To assess the quality of the extrapolation resulting from our data fusion method, we validate it by analyzing 5 different regions (as shown in Figure 4); each region has at least an area of 8 × 8 degrees (200 × 200 pixels). For each of these areas a mask representing a cloud structure (of the typical size and shape found in chlorophyll images) is defined (the clouds are generated using a real MODIS SST 9-km daily image of 1 January 2006, and are kept fixed in time). Those masks have a surface of about 30% of the region on which they will be applied. To assess the quality of the fusion algorithm, we proceed in three steps. In the first step, points lying in the masked area of a daily Chl-*a* map are removed (notice that there will be additional missing points in the Chl-*a* map because of the gaps in the original remote sensing product; for instance, in the EP area (Figure 4) the average percentage of missing points is 85%). In the second step, the fusion algorithm is applied to the masked Chl-*a* map using the corresponding daily SST map as a template, extrapolating to the missing values. Finally, the extrapolated values are compared to the available original ones on the masked area for each image during the entire year 2006 (365 daily images). We require a minimum of 5% of original Chl-*a* values existed inside the masked area to perform the cross validation. This strategy allows comparing the original Chl-*a* and the retrieved one at the available masked points, and therefore the extrapolation ability of the fusion method.

The performance of the algorithm is studied in different Chl-*a* regimes: oligotrophic regions (Central Atlantic, CA and Pacific in front of California, CL) and eutrophic regions in higher latitudes (North Atlantic, NA), coastal upwelling areas (Benguela upwelling, BG) and the Equator (Equatorial Pacific, EP). NA region is defined by [19.61◦W–9.19◦W; 51.65◦N–57.91◦N] and 22% of the area is masked, EP region is defined by [142.54◦W–130.04◦W; 5.02◦S–0.40◦N] (29% of the area is masked), CL region is defined by [125.87◦W–117.53◦W; 22.48◦N–30.82◦N] (31% of the area is masked), BG region is defined by [9.56◦E–17.90◦E; 27.53◦S–19.19◦S] (35% of the area is masked) and CA is defined by [40.44◦W–30.03◦W; 22.48◦N–30.82◦N] with a 34% of the area being masked.

Examples of one daily image validation are shown in Figure 4 for each region. A quantitative measure of the quality of the reconstruction of the chlorophyll is given in terms of four parameters: the root mean square (rms), bias (mean) and standard deviation (std) of the reconstructed error and the correlation coefficient (r) between the masked and reconstructed values. In the case of the central

Atlantic and California regions, the concentration of chlorophyll is smaller, followed by the values found in the North Atlantic, the Equatorial Pacific and the Benguela upwelling where highest values of Chl-*a* concentration are found. Correlations coefficients between L4 and original Chl-*a* decrease for the regions where lower pigment concentrations are found. Better statistics are associated to areas of high primary production.

The validation for the entire year 2006 is summarized in Table 1; the mean seasonal values for each one of the statistical parameters and clouds are included. Our data fusion method succeeds in filling gaps with mean annual correlation coefficients ranging from 0.58 to 0.81 for the studied period and artificial clouds. Oligotrophic regions (North Atlantic and California) have mean regression coefficients which are significantly lower than the mean correlation coefficients values for the equatorial Pacific, the North Atlantic and the Benguela upwelling regions respectively. The smaller performance in oligotrophic regions is probably due to the small horizontal gradients of Chl-*a* there together with a larger signal-to-noise ratio. However, the absolute error of the reconstruction is always moderate. A slightly mean positive bias is systematically found, meaning that the L4 estimates are smaller than the original Chl-*a* values, probably due to the smoothing generated by the weighting function used in the fusion algorithm [16].

The seasonal segregation of validation results highlights a worse performance during the January-February-March period, with mean correlation coefficients ranging from 0.15 to 0.47 (Central Atlantic and Benguela upwelling, respectively). During the rest of seasons, the validation results greatly improve with correlation coefficients ranging from 0.67 to 0.94 in spring (AMJ), from 0.77 to 0.94 in summer (JAS) and from 0.74 to 0.94 in fall season (OND). In the winter season, the strengthening of winds and deepening of the mixed layer in mid and high latitudes create a large supply of nutrients from the deep ocean to the surface, which will be available for phytoplankton to proliferate in the following seasons. In equatorial and coastal upwelling regions (Equatorial Pacific and Benguela, respectively), the upwelling intensity also varies seasonally depending on wind strength and direction and the vertical structure of the water column. Under these circumstances, nutrient availability, vertical mixing and the depth of the mixed layer play crucial roles in the distribution of Chl-*a*, which could not be only explained by horizontal advection and the distribution of surface temperature.

The power density spectra (PDS) is computed as in [23] inside the boxes shown in in Figure 5 (top) for each daily file and then the median for the entire period 2006 is calculated. The spatial spectral analysis of MODIS Chl-*a*, MODIS L3 SST and Level 4 Chl-*a* products under the SPURS and STP regions (red and green boxes in Figure 5 (top)) shows that the actual spatial resolution of MODIS Chl-*a* and MODIS L3 SST is about 10 km and, and 15 km for the L4 Chl-*a* product (Figure 5 (bottom)). The intermittent character of MODIS Chl-*a* and the lower resolution of L4 Chl-*a* explain the vertical shift between their PDS.

**Table 1.** Mean seasonal results of daily reconstruction validation under artificial clouds for year 2006.

**Figure 4.** Artificial clouds generated for validation purposes in the Central Atlantic (CA), Benguela upwelling (BG), California Pacific (CL), Equatorial Pacific (EP) and North Atlantic (NA) (**top left panel**). Validation of reconstruction under artificial clouds for MODIS Chl-*a* concentration. For CA region day 95, BG region day 159, CL day 90, EP day 193, and NA day 193 of year 2006.

**Figure 5.** (**top**) Areas for power density spectra (PDS) computation: Red box represent SPURS region (22◦–28◦N and 28◦–60◦W) and green box represent STP region (27◦–33◦S and 106◦–170◦W). (**bottom**) Median of PDS for the year 2006 of MODIS L3 SST, MODIS Chl-*a*, and Level 4 Chl-*a* for the SPURS region (**left**) and the STP region (**right**).


*3.6. Limitations*

There are several limitations that may be considered given our conclusions:

Firstly, the described interactions of DMSO in the concentrations used may have interfered with the neuroprotective effects of both tested substances. However, this may only account for the highest concentration of 2.0 Vol.% of DMSO and not for the lower concentrations, not showing significant effects on the outcome in control as well as OGD. Secondly, we did not use a specific imaging technique to depict calcium concentrations. By applying well-established concentrations of nimodipine and amiloride, we are confident that an effective channel blockade was achieved, preventing calcium influx via these ion channels. Thirdly, we planned smaller groups for vehicle controls and control groups from the beginning to minimise the number of required animals. Thus, some control groups have limited sample size compared to treatment groups, and further power would be eligible regarding our observation of a possible neuroprotective effect of DMSO at a concentration of 1.0 Vol.%. Lastly, we did not investigate the effect of nimodipine in 2.0 Vol.% DMSO. Hence, it is not possible to draw specific conclusions from our observation of enhanced damage of combined treatment with nimodipine and amiloride at 2.0 Vol.% DMSO.

#### **4. Materials and Methods**

#### *4.1. Mediums*

Preparation medium (Gey's balanced salt solution (Sigma-Aldrich, Munich, Germany), 5 mg/mL D-(+)Glucose (Roth, Karlsruhe, Germany)) [63] was used for initial slice manufacturing. The growth medium used for slice culturing consisted of 50% Eagle minimal essential medium with Earle's salts (Sigma-Aldrich), 25% Hank's balanced salt solution (Sigma-Aldrich), 25% heat inactive horse serum (Sigma-Aldrich) with additional 5 mg/mL D-(+)Glucose (Roth, Karlsruhe, Germany), 1 Vol.% antibiotic/antimycotic solution [penicillin G GIBCO™, 10,000 units/mL, streptomycin sulphate 10 mg/mL, amphotericin B 25 μg/mL] (Thermo Fisher Scientific, Waltham, MA, USA), 5 μL/mL medium L-Glutamine solution (Sigma-Aldrich) and 10 μL/mL HEPES buffer solution (Sigma-Aldrich) [63]. For experiments, the experimental medium (75% Eagle minimal essential medium with Earle's salts (Sigma-Aldrich), 25% Hank's balanced salt solution (Sigma-Aldrich) with additional, 5 mg/mL D-(+)Glucose (Roth, Karlsruhe, Germany), 1 Vol.% antibiotic/antimycotic solution [penicillin G GIBCO™, 10,000 units/mL, streptomycin sulphate 10 mg/mL, amphotericin B 25 μg/mL] (Thermo Fisher Scientific, Waltham, MA, USA), 5 μL/mL L-Glutamine solution (Sigma-Aldrich) and 10 μL/mL HEPES buffer solution (Sigma-Aldrich)) or OGD medium (75% Eagle minimal essential medium with Earle's salts (Sigma-Aldrich), 25% Hank's balanced salt solution (Sigma-Aldrich) with additional 1 Vol.% antibiotic/antimycotic solution [penicillin G GIBCO™, 10,000 units/mL, streptomycin sulphate 10 mg/mL, amphotericin B 25 μg/mL] (Thermo Fisher Scientific, Waltham, MA, USA), 5 μL/mL L-Glutamine solution (Sigma-Aldrich) and 10 μL/mL HEPES buffer solution (Sigma-Aldrich)) were used, respectively. In essence, OGD medium is simply experimental medium without D-(+)Glucose.

#### *4.2. Slice Preparation and Cultivation*

The experiments in this article were strictly conducted according to institutional and governmental guidelines (TierSchG) with institutional permission by the animal protection representative of the Institute of Animal Research at the RWTH Aachen University Hospital and the local institutional committee (LANUV North Rhine-Westphalia, TV-11141A4). After decapitation of 4–7-day-old mice pups (C57BL/6N from Charles Rivers Laboratories, Sulzfeld, Germany and from Janvier Labs, La Rochelle, France, *n* = 138), their brains were extracted and instantaneously immersed into ice-cold preparation medium. The hippocampus slices were prepared using an already established method [58,63]. In brief, the brains were sagittally divided in half, and the frontal pole, as well as the cerebellum, was resected. Using a McIlwain Tissue Chopper (The Mickle Laboratory Engineering Co. ltd. [Now: Cavey Laboratory Engineering Co. ltd], Gomshall, UK), the brains were sliced into 400 μM thick slices from which the hippocampus was then carefully dissected. The hippocampus slices were then transferred onto MilliCell tissue culture inserts (MilliCell-CM, Millipore Corporation, Billerica, MA, USA) placed in 1 mL growth medium. Slices were cultivated at 37 ◦C and 5% CO2 for 14 days with the growth medium exchanged one day after the preparation and every following third day. On average, 9.1 slices per pup were prepared. Slices of one animal were allocated in two wells, which were then randomly allocated to the experimental groups to avoid allocation of slices of the same animal to only one experimental group. Overview in Figure 6a.

**Figure 6.** Experimental design. (**a**) Slice preparation, culturing for 14 days and baseline propidium iodide (PI) imaging. (**b**) OGD experiments with overview of experimental groups. (**c**) Incubation for 72 h after OGD, PI imaging for cell death assessment at 72 h, analysis of experimental groups. Created with BioRender.com.


**2. Tourism competitiveness: measurement indicators**

Since the last decade of the twentieth century, an increasing number of researchers have focused on knowing the causes that make a geographical enclave become a preferred tourist destination above others, and the reasons for their greater or lesser tourist attraction potential. In the past decade of the present century, the proposals have been focused on producing models that measure, by means of a specific empirical methodology, the competitiveness of the tourism destinations [1–4].

In agreement with [5], there are three main reasons why the mentioned scientific interest has been inspired: firstly, the increasing importance that the tourism sector has in the regional, national and global economies; secondly, the increasing competition between tourism destinations, although they have hardly changed over the years [13,14]; and finally, the benefits of tourism for the economies seem to be clear in the short term, but their effects in the long term do not appear to be as great, or at least there is insufficient empirical evidence that supports it [15–17].

The first section of this chapter reviews the conceptual models of tourism competitiveness. This is followed with an examination of the empirical studies developed to date, highlighting the main existing indicators on tourism competitiveness that have come to be used at the international level, putting emphasis on the applied methodology.


**Appendix A**

**Table A1.** Fish that the A'tolan Amis spear frequently.

**Table A1.** *Cont.*

**No. Photo Amis Name (Meaning)** 9 Kahungto'ay (pipe fish) 10 Lilateng (vegetable) 11 Mitilidan niliwa/salisin ira (chief of fish, shaman) 12 Lihok 13 Matopa'ay a Lihok (stupid Liohk) 14 Fafoy (wild pig)

**Table A2.** Traditional and contemporary coastal names in the A'tolan marine territory (from north to south).

#### **References**

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).


**3.2.2 Gene identification in crop species**

The sequencing and assembly of large and complex crop genomes remains a valuable goal, but at the moment, a significant amount of knowledge can be gained from low coverage shotgun sequencing of these genomes. In this contest, the second generation technologies of sequencing are particularly suitable to know genes and gene promoters in crop plants that are homologous to related species. Therefore, designing polymerase chain reaction (PCR) primers to the read pairs enables the amplification and sequencing of the gene and corresponding genomic region in the target species. This approach to gene discovery offers the potential to identify genes, gene promoters and polymorphisms in a wide range of agronomically important crop species (Bracci *et al.*, 2011).

Microarray represents functional genomic approaches that have revolutionized global gene expression profiling. In fact they allow studying the entire gene complement of the genome in a single experiment (Duggan *et al.*, 1999; Li *et al.*, 2005). At the moment, cDNA and oligonucleotide microarrays have been widely used in plants, such as *Arabidopsis,* rice, maize, strawberry, petunia, ice plants and lima bean, to study and compare global gene expression levels in specific organs and/or tissues under controlled physiological conditions.

In olive, the genomics information present on the international database NCBI, concerning the identification and characterization of functional genes are prevalently based on EST identification and they are predominantly related to pollen allergens and characteristics of olive fruit.

*Olea europaea* trees are widely distributed throughout the Mediterranean basin and therefore their pollen is one of the most prevalent causes of respiratory allergy such as allergic rhinitis and allergic asthma in the Mediterranean region and some other countries between late April and early June (Kalyoncu *et al.*, 1995). Olive pollen is also responible of allergic inflammation of the upper and/or lower airways that may persist after the pollination season is over (Quiralte *et al.*, 2005).

Table 1. The olive pollen allergens (from Villalba *et al.*, 2007)

to survey the genome of Miscanthus (Swaminathan *et al.*, 2009), while Sanger, Illumina Solexa and Roche 454 sequencing are being used to characterize the genome of banana

The sequencing and assembly of large and complex crop genomes remains a valuable goal, but at the moment, a significant amount of knowledge can be gained from low coverage shotgun sequencing of these genomes. In this contest, the second generation technologies of sequencing are particularly suitable to know genes and gene promoters in crop plants that are homologous to related species. Therefore, designing polymerase chain reaction (PCR) primers to the read pairs enables the amplification and sequencing of the gene and corresponding genomic region in the target species. This approach to gene discovery offers the potential to identify genes, gene promoters and polymorphisms in a wide range of

Microarray represents functional genomic approaches that have revolutionized global gene expression profiling. In fact they allow studying the entire gene complement of the genome in a single experiment (Duggan *et al.*, 1999; Li *et al.*, 2005). At the moment, cDNA and oligonucleotide microarrays have been widely used in plants, such as *Arabidopsis,* rice, maize, strawberry, petunia, ice plants and lima bean, to study and compare global gene expression levels in specific organs and/or tissues under controlled physiological

In olive, the genomics information present on the international database NCBI, concerning the identification and characterization of functional genes are prevalently based on EST identification and they are predominantly related to pollen allergens and characteristics of

*Olea europaea* trees are widely distributed throughout the Mediterranean basin and therefore their pollen is one of the most prevalent causes of respiratory allergy such as allergic rhinitis and allergic asthma in the Mediterranean region and some other countries between late April and early June (Kalyoncu *et al.*, 1995). Olive pollen is also responible of allergic inflammation of the upper and/or lower airways that may persist after the pollination

> Ole e 1 ~ 19 Unknown Ole e 2 ~ 15 Profiling Ole e 3 ~ 9 Polcalcin Ole e 4 ~ 32 Unknown

Ole e 6 ~ 6 Unknown Ole e 7 ~ 10 Lipid transfer protein Ole e 8 ~ 19 Ca++ binging protein Ole e 9 ~ 46 1,3 β glucanase Ole e 10 ~ 10 Carbohydrate binding protein

Ole e 5 ~ 16 Cu/Zn superoxide dismutase

**Allergenic proteins name Molecular mass (kDalton) Family**

Table 1. The olive pollen allergens (from Villalba *et al.*, 2007)

(Hribova *et al.*, 2009).

conditions.

olive fruit.

season is over (Quiralte *et al.*, 2005).

**3.2.2 Gene identification in crop species**

agronomically important crop species (Bracci *et al.*, 2011).

At the moment 10 olive pollen allergens have been purified and characterized from *Olea europaea* pollen extract (Table 1). Several of these allergenic proteins, eg, Ole e 6, fail to show any homology to known protein sequences and, therefore, the biochemical function of these gene products remains unknown. Many other allergens belong to well-known families of proteins, such as profilin (Ole e 2), superoxide dismutase (Ole e 5), calciumbinding proteins (Ole e 3 and Ole e 8), lipid transfer proteins (Ole e 7) and 1,3-β-glucanases (Ole e 9) (Villalba *et al.*, 2007).

Photo 10. Cross sections of mesocarp olive fruit at level of insect injury (*Bactrocera oleae*, right: sections stained with safranin O/ azur II; left: localization of *OeCHLP* transcripts by *in situ* hybridization with dig-labelled *OeCHLP* antisense probe).

The biochemical composition of olive fruit is variable because it depends on olive variety, soil, climate, and cultivation. The virgin olive oil is overwhelmingly composed of triglycerides (>98%), along with traces of other compounds. The dominant triglyceride fatty acid species are the oleic acids (57-78%) such as palmitic, stearic, linoleic and linolenic acids (Caravita *et al.,* 2007). The other minor constituents such as alcohols, polyphenols, chlorophyll, carotenoids, sterols, tocopherols and flavonoids, contribute to the olive's organoleptic qualities, taste, flavour, and nutritional value (Perri *et al.,* 2002; Servili *et al.*, 2004). These constituents may also serve to distinguish olive oils originating from different regions. Olive oil, especially extra-virgin oil also contains small amounts of hydroxytyrosol, secoiridoids, lignans (Bianco *et al.,* 1999, 2001; De Nino *et al.,* 2005) and other compounds thought to possess anticancer properties (*i.e.*, squalene and terpenoids) (Fabiani *et al.,* 2002; Owen *et al.*, 2004). In spite of its economical importance and metabolic peculiarities, very few data are available on gene sequences controlling the main metabolic pathways. Particular attention has been paid to the genes encoding the key enzymes involved in fatty acid biosynthesis, fatty acid modification, triacylglycerol synthesis, and fat storage (Hatzopoulos *et al.,* 2002; De la Rosa *et al.,* 2003; Banilas *et al.,* 2005).

In recent years, much attention has turned to the olive fruit. In this contest, the parallel sequencing of different fruit cDNA collections has provided large scale information about the structure and putative function of gene transcripts accumulated during fruit development (Alagna *et al.*, 2009).

A nuclear gene, named *OeCHLP* (*Olea europaea* GERANYLGERANYL REDUCTASE was isolated and characterizated by Bruno *et al.,* (2009). This gene encodes a chloroplastic enzyme involved in the formation of phytolic side chain of tocopherols chlorophyll, and plastoquinones. In olive fruits *OeCHLP* gene expression was enhanced in dark fruit very likely in relation to the increase in mature fruits of the level of total tocopherols suggesting a role in the synthesis of the antioxidant. It is noteworthy that the variations in gene transcript levels that occurred during the ripening of olive fruits depend on the genotype analyzed (Muzzalupo *et al.,* 2011). In this contest, in olive fruits tocopherols confer not only nutritional value (Valk and Hornstra, 2000), but also contribute to product stability and post harvesting shelf life (Goffman and Bohme, 2001) by protecting storage oil from oxidative damage (Sattler *et al.,* 2004). *OeCHLP* was also detected in fruits attacked by *Bactrocera oleae*  pathogen as well as in fruits wounded by needle suggesting a role in protection mechanisms related to cell damage and oxidative burst induced by pathogen (photo 8 and 10) (Ebel, 1998; Klessig *et al.,* 2000; Bruno *et al.*, 2009).


**2. Physiology and biochemistry of strawberry during ripening**

Fruit ripening involves dramatic changes in the color, texture, flavor, and aroma of fleshy fruits. Both the palatability and nutritional quality of fruit are highly dependent on its consumption at an optimum stage of ripeness. However, ripe fleshy fruits are also perishable commodities, and this presents problems for fruit production, harvesting, storage, and marketing.

The general ripening programmes (**Figure 1**) displayed by strawberry typically include: (i) modification of color through the alteration of chlorophyll, carotenoid, and/or anthocyanin accumulation; (ii) modification of texture via alteration of cell turgor and cell wall structure; (iii) accumulation and modification of acids, sugars and volatiles that affect nutritional quality, aroma and flavor; and (iv) increased susceptibility to pathogens and herbivores [2]. These changes in flavor, color, aroma and texture make fruit ripening a complex process, which must be very tightly regulated. Fruit species are categorized as either climacteric or nonclimacteric, based on physiological differences in their ripening patterns [3].

Strawberry is a nonclimacteric fruit and fast growing, with a short postharvest life. During development, receptacle growth was due to a combination of cell division and cell expansion until seventh day after petal fall, and thereafter, only

#### **Figure 1.**

*Stages of fruit development [1], from left to right color development from tip toward receptacle (anthocyanin development).*

**25**

**Figure 2.**

*Fruit Physiology and Postharvest Management of Strawberry*

cell enlargement occurs [4]. Accumulation of sugars, water and synthesis of cell walls were observed until 21–28 days after petal fall [5]. In strawberry and other fruits like grapes, growth may continue after initiation of ripening process [6]. Fruit enlargement continues until it reaches 25 per cent red or more, when chlorophylls

Auxin, the first plant hormone identified, may act as an inhibitor of ripening in some nonclimacteric fruits [7, 8]. In strawberry, it appears that auxin from the externally located achenes (seeds) inhibits the ripening of the fleshy receptacle [9]. Fruit development continues till the auxin level falls below the critical level in the receptacle and achenes, thus permitting ripening [10]. Therefore, removing the achenes promotes ripening, while treating strawberries with synthetic auxins delays

During ripening, the primary cell wall of fleshy fruits shows structural and compositional change [11], which leads to loss of firmness and facilitates the attack of pathogens, enhancing postharvest decay and reducing the quality of fresh fruit. During ripening, water soluble polyuronides increases (**Figure 2**), whereas, there will be decrease in insoluble, covalently bound pectins. Concurrently, depolymerization of pectins occurs, which has been linked with the action of a number of hydrolases, mostly polygalacturonases (PG) [14]. Xyloglucan, the main component of hemicelluloses in dicotyledons, is also considered to play a vital role in cell wall structure, since it forms cross-linkages among the cellulose polymers. Xyloglucan endotransglycosidases, endoglucanases, and expansins contribute to the depoly-

Three major components of fruit organoleptic quality are flavor, sweetness, and acidity. Fruit with intense flavor also have high titratable acidity and high soluble solids [15]. Fruit soluble solids, sugars, titratable acidity, and organic acids at

*Changes in polyuronides during fruit ripening [13], changes in ethanol insoluble powder, and total polyuronides during strawberry development (o) mg ethanol insoluble powder per gram fruit fresh weight (●) mg powder*

have been completely degraded and anthocyanins begin to build up.

*DOI: http://dx.doi.org/10.5772/intechopen.84205*

**2.1 Role of hormones in fruit ripening**

**2.2 Softening of fruit during ripening**

merization of xyloglucans at the time of ripening [14].

*per individual fruit (*ە *(mg total polyuronide per individual fruit.*

ripening [11, 12].

#### *Fruit Physiology and Postharvest Management of Strawberry DOI: http://dx.doi.org/10.5772/intechopen.84205*

cell enlargement occurs [4]. Accumulation of sugars, water and synthesis of cell walls were observed until 21–28 days after petal fall [5]. In strawberry and other fruits like grapes, growth may continue after initiation of ripening process [6]. Fruit enlargement continues until it reaches 25 per cent red or more, when chlorophylls have been completely degraded and anthocyanins begin to build up.

#### **2.1 Role of hormones in fruit ripening**

*Strawberry - Pre- and Post-Harvest Management Techniques for Higher Fruit Quality*

embedded, which are popularly called seeds.

in postharvest management of strawberry.

production, harvesting, storage, and marketing.

and require cool dry storage.

fruit of strawberry is "accessory fruit" and is not a true berry. The flesh consists of the greatly enlarged flower receptacle in which many true fruits, or achenes, are

Strawberries are commercially cultivated both for immediate consumption and for processing as frozen, canned, or preserved berries or as juice. Due to the perishable nature of the berries and the improbability of mechanical picking, the fruit is generally grown near centers of consumption or processing and where sufficient labor is available. The berries are handpicked directly into small baskets and crated for marketing or put into trays for processing. Early crops can be produced under controlled conditions (glass or plastic covering). Strawberries are very perishable

To innovate a new technology for extending storage life of strawberry fruits in the postharvest area, it is necessary to understand the physiology and biochemistry of fruits. This chapter reviews fruit physiology, recent trends and future prospects

Fruit ripening involves dramatic changes in the color, texture, flavor, and aroma

The general ripening programmes (**Figure 1**) displayed by strawberry typically include: (i) modification of color through the alteration of chlorophyll, carotenoid, and/or anthocyanin accumulation; (ii) modification of texture via alteration of cell turgor and cell wall structure; (iii) accumulation and modification of acids, sugars and volatiles that affect nutritional quality, aroma and flavor; and (iv) increased susceptibility to pathogens and herbivores [2]. These changes in flavor, color, aroma and texture make fruit ripening a complex process, which must be very tightly regulated. Fruit species are categorized as either climacteric or nonclimacteric,

Strawberry is a nonclimacteric fruit and fast growing, with a short postharvest

*Stages of fruit development [1], from left to right color development from tip toward receptacle (anthocyanin*

life. During development, receptacle growth was due to a combination of cell division and cell expansion until seventh day after petal fall, and thereafter, only

of fleshy fruits. Both the palatability and nutritional quality of fruit are highly dependent on its consumption at an optimum stage of ripeness. However, ripe fleshy fruits are also perishable commodities, and this presents problems for fruit

**2. Physiology and biochemistry of strawberry during ripening**

based on physiological differences in their ripening patterns [3].

**24**

**Figure 1.**

*development).*

Auxin, the first plant hormone identified, may act as an inhibitor of ripening in some nonclimacteric fruits [7, 8]. In strawberry, it appears that auxin from the externally located achenes (seeds) inhibits the ripening of the fleshy receptacle [9]. Fruit development continues till the auxin level falls below the critical level in the receptacle and achenes, thus permitting ripening [10]. Therefore, removing the achenes promotes ripening, while treating strawberries with synthetic auxins delays ripening [11, 12].

#### **2.2 Softening of fruit during ripening**

During ripening, the primary cell wall of fleshy fruits shows structural and compositional change [11], which leads to loss of firmness and facilitates the attack of pathogens, enhancing postharvest decay and reducing the quality of fresh fruit. During ripening, water soluble polyuronides increases (**Figure 2**), whereas, there will be decrease in insoluble, covalently bound pectins. Concurrently, depolymerization of pectins occurs, which has been linked with the action of a number of hydrolases, mostly polygalacturonases (PG) [14]. Xyloglucan, the main component of hemicelluloses in dicotyledons, is also considered to play a vital role in cell wall structure, since it forms cross-linkages among the cellulose polymers. Xyloglucan endotransglycosidases, endoglucanases, and expansins contribute to the depolymerization of xyloglucans at the time of ripening [14].

Three major components of fruit organoleptic quality are flavor, sweetness, and acidity. Fruit with intense flavor also have high titratable acidity and high soluble solids [15]. Fruit soluble solids, sugars, titratable acidity, and organic acids at

#### **Figure 2.**

*Changes in polyuronides during fruit ripening [13], changes in ethanol insoluble powder, and total polyuronides during strawberry development (o) mg ethanol insoluble powder per gram fruit fresh weight (●) mg powder per individual fruit (*ە *(mg total polyuronide per individual fruit.*

maturity are quantitatively inherited [16, 17]. Numerous biochemical changes are observed during strawberry development and especially during fruit ripening [18]. The major soluble constituents of maturing and ripe strawberries are soluble sugars and organic acids [6, 19].

#### **2.3 Sugars**

The major soluble sugars in strawberries are glucose [1.4–3.1% fresh weight (FW)], fructose (1.7–3.5% FW), and sucrose (0.2–2.5% FW) [6]. Glucose and fructose concentrations increase continuously during fruit development, while sucrose accumulates mostly during maturation [19].

#### **2.4 Organic acids**

The major organic acid is citrate, and its concentration ranges from 4 to 12 mg·g−1 FW. This acid contributes greatly to fruit titratable acidity, which declines gradually during fruit development. The sugar/organic acid ratio is a major parameter of strawberry taste [6].

### **2.5 Amino acids**

Of the other soluble constituents of strawberries, amino acids may also directly affect fruit taste, as was shown by the sensory evaluation of another fleshy fruit, peach [*Prunus persica* (L.) Batsch] [20]. Moreover, some amino acids are flavor precursors [20]. The major amino acids in strawberries are asparagine, glutamine, and alanine [21]. Anthocyanins (0.5–1.5 mg/g fruit weight) are a major component of the fruit, while ascorbic acid (0.3–1.2 mg/g fruit weight) makes an important contribution to the fruit nutritional value. Among the insoluble constituents, starch is present in young fruit and disappears before ripening [5].

#### **3. Harvesting**

Harvesting is generally practiced after 3–4 months from planting. Strawberries are the sweetest when they are fully ripened on the plant. It is better to leave them on the plant for a day or two till they turn red. To ensure ripeness, taste test can be made. During harvesting berries, care should be taken as ripe ones bruise very easily. For harvesting, snap the stem just above the berry to remove them from the plant. Store harvested berries out of direct sunlight in some cool place, such as a refrigerator immediately after picking to increase the storage time. Strawberries can be consumed fresh or preserved by freezing or dehydrating and canning.

#### **3.1 Robotic harvesting**

The pericarp of a strawberry is so soft that workers must harvest the fruits carefully to avoid damage. The fruits are harvested early in the morning, before the temperature of the fruits rises and they become soft; workers need to select mature red fruits from among the many fruits that have set. These factors result in long working hours during the harvest period. Mechanical harvesting trials have been conducted on the assumption of once-over picking, but utilization of this strategy is not yet widespread [13]. The commonly used selective harvesting method requires high-tech and sophisticated robot technology. In short, it is essential to design an

**27**

precooling at 4°C and storage at 4°C, respectively.

requirements.

**Figure 3.**

**4. Precooling**

*Fruit Physiology and Postharvest Management of Strawberry*

smart robot with human-like perceptive capabilities; for example, the machine would need to analyze fruit position, assess maturity level and pick the fruit without injuring the pericarp. Basic studies on robotic harvesting were initiated with orchard fruits [22]; since then, such studies have been ongoing in a number of countries [23]. This skill has then been used for vegetable fruits. Tillett [24] reviewed various robot prototypes and clarified the importance of the manipulator design and its application to practical use. Several studies have applied robotic technology to fields in greenhouses for instance, cucumber harvesting [25], strawberry harvesting (**Figure 3**) [1] tomato harvesting [26], aubergine harvesting [27] and de-leafing [28]. However, the performance and cost have not met commercial

*Robotic harvesting in strawberry [1]: (a) high tech sophisticated harvester from a distance and (b) enlarged*

Rapid removal of field heat from freshly harvested commodities is called precooling. It slows down ripening, respiration, senescence, decay, and water loss, thus helping for quality maintenance and prolonging shelf life [29]. Rapid precooling is most essential for produce such as strawberry which has a high rate of metabolism. The process of removal of field heat can be achieved by different methods that includes room cooling (RC), forced-air cooling (FA), hydrocooling (HC) contact icing, and vacuum cooling, each differing in efficiency of heat removal. Strawberries are typically cooled used forced air cooling. Delay in cooling of harvested strawberries results in reduction of number of marketable berries due to increased water loss, softening, and losses of sugars and vitamin C [30]. Thus it is usually recommended that strawberries should be cooled to temperatures near 0°C as soon as possible (within 1 h) after harvest to limit deterioration and decay [31, 32]. However, for commercial strawberry operations, this idea is rarely achieved due to factors such as the volume of strawberries handled, cooling and handling equipment availability, and capability, economics, energy, and market conditions. Hydro-cooling is a more rapid precooling method, but strawberries are not hydro-cooled commercially, due to decay problems by the water left on the berries after Hydro-cooling [29, 32]. Park et al. [33] proved that effectiveness for keeping the freshness of strawberries was best achieved by

*DOI: http://dx.doi.org/10.5772/intechopen.84205*

*view with different parts of robotic harvester.*

*Fruit Physiology and Postharvest Management of Strawberry DOI: http://dx.doi.org/10.5772/intechopen.84205*

**Figure 3.**

*Strawberry - Pre- and Post-Harvest Management Techniques for Higher Fruit Quality*

and organic acids [6, 19].

**2.3 Sugars**

**2.4 Organic acids**

**2.5 Amino acids**

**3. Harvesting**

dehydrating and canning.

**3.1 Robotic harvesting**

eter of strawberry taste [6].

12 mg·g−1

maturity are quantitatively inherited [16, 17]. Numerous biochemical changes are observed during strawberry development and especially during fruit ripening [18]. The major soluble constituents of maturing and ripe strawberries are soluble sugars

The major soluble sugars in strawberries are glucose [1.4–3.1% fresh weight (FW)], fructose (1.7–3.5% FW), and sucrose (0.2–2.5% FW) [6]. Glucose and fructose concentrations increase continuously during fruit development, while

The major organic acid is citrate, and its concentration ranges from 4 to

gradually during fruit development. The sugar/organic acid ratio is a major param-

Of the other soluble constituents of strawberries, amino acids may also directly affect fruit taste, as was shown by the sensory evaluation of another fleshy fruit, peach [*Prunus persica* (L.) Batsch] [20]. Moreover, some amino acids are flavor precursors [20]. The major amino acids in strawberries are asparagine, glutamine, and alanine [21]. Anthocyanins (0.5–1.5 mg/g fruit weight) are a major component of the fruit, while ascorbic acid (0.3–1.2 mg/g fruit weight) makes an important contribution to the fruit nutritional value. Among the insoluble constituents, starch

FW. This acid contributes greatly to fruit titratable acidity, which declines

sucrose accumulates mostly during maturation [19].

is present in young fruit and disappears before ripening [5].

Harvesting is generally practiced after 3–4 months from planting.

Strawberries are the sweetest when they are fully ripened on the plant. It is better to leave them on the plant for a day or two till they turn red. To ensure ripeness, taste test can be made. During harvesting berries, care should be taken as ripe ones bruise very easily. For harvesting, snap the stem just above the berry to remove them from the plant. Store harvested berries out of direct sunlight in some cool place, such as a refrigerator immediately after picking to increase the storage time. Strawberries can be consumed fresh or preserved by freezing or

The pericarp of a strawberry is so soft that workers must harvest the fruits carefully to avoid damage. The fruits are harvested early in the morning, before the temperature of the fruits rises and they become soft; workers need to select mature red fruits from among the many fruits that have set. These factors result in long working hours during the harvest period. Mechanical harvesting trials have been conducted on the assumption of once-over picking, but utilization of this strategy is not yet widespread [13]. The commonly used selective harvesting method requires high-tech and sophisticated robot technology. In short, it is essential to design an

**26**

*Robotic harvesting in strawberry [1]: (a) high tech sophisticated harvester from a distance and (b) enlarged view with different parts of robotic harvester.*

smart robot with human-like perceptive capabilities; for example, the machine would need to analyze fruit position, assess maturity level and pick the fruit without injuring the pericarp. Basic studies on robotic harvesting were initiated with orchard fruits [22]; since then, such studies have been ongoing in a number of countries [23]. This skill has then been used for vegetable fruits. Tillett [24] reviewed various robot prototypes and clarified the importance of the manipulator design and its application to practical use. Several studies have applied robotic technology to fields in greenhouses for instance, cucumber harvesting [25], strawberry harvesting (**Figure 3**) [1] tomato harvesting [26], aubergine harvesting [27] and de-leafing [28]. However, the performance and cost have not met commercial requirements.


**Author details**

Joy-anne N. Oliver\*, Olanrewaju Akande and Melanie Ecker Biomedical Engineering Department, Materials Science and Engineering, University of North Texas, Denton, TX, USA

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Incorporation of Novel Elements in Bioactive Glass Compositions to Enhance Implant Performance DOI: http://dx.doi.org/10.5772/intechopen.99430*


**ZnO Nanowires for Dye Sensitized Solar Cells**

Simas Rackauskas, Nadia Barbero,

Claudia Barolo and Guido Viscardi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67616

#### **Abstract**

[39] Rouxinol FP, Gelamo R V., Amici RG, Vaz AR, Moshkalev SA. Low contact resistivity and strain in suspended multilayer graphene. Applied Physics Letters. 2010;**97**:253104-1-

[40] Li M, Anderson W, Chokshi N, DeLeon RL, Tompa G. Laser annealing of laser assisted molecular beam deposited ZnO thin films with application to metal‐semiconductor‐ metal photodetectors. Journal of Applied Physics. 2006;**100**:053106‐1‐053106‐4. DOI:

253104‐4. DOI: 10.1063/1.3528354

10.1063/1.2344811

58 Nanowires - New Insights

This chapter provides a broad review of the latest research activities focused on the syn‐ thesis and application of ZnO nanowires (NWs) for dye‐sensitized solar cells (DSCs) and composed of three main sections. The first section briefly introduces DSC‐working prin‐ ciples and ZnO NW application advantages and stability issues. The next section reviews ZnO NW synthesis methods, demonstrating approaches for controlled synthesis of dif‐ ferent ZnO NW morphology and discussing how this effects the overall efficiency of the DSC. In the last section, the methods for ZnO NW interface modification with various materials are discussed, which include ZnO core‐shell structures with semiconductive or protective layers, ZnO NW hybrid structures with other materials, such as nanoparticles, quantum dots and carbon nanomaterials and their benefit for charge and light transport in DSCs. The review is concluded with some perspectives and outlook on the future developments in the ZnO nanowire application for DSCs.

**Keywords:** Nanowires, ZnO nanowire, Synthesis, dye‐sensitized solar cell, DSC, Photovoltaics, surface modification, hybrid materials

## **1. Introduction**

World energy demand is rapidly increasing; therefore, a clean and reliable energy source is fundamental to global economy. The environmental impact of the fossil fuel use urges the need for the search of alternative energy resources. Many different renewable energy technol‐ ogies were developed recently aiming for efficient solutions of clean energy supply; however, a price competitiveness is a biggest drawback. Solar energy is considered as a key solution for environmental challenge, because of its carbon neutral nature and high abundance. The con‐ version of solar energy to electricity is fulfilled by solar cells based on the photovoltaic effect.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Dye‐sensitized solar cells (DSCs) have gained widespread attention because of the ease of fab‐ rication, low production costs and tuneable optical properties, such as colour and transpar‐ ency [1]. The most attractive properties of DSCs are their low cost and simple manufacturing processes together with their advantageous attributes such as lightweight, low toxicity and good performance in diverse light conditions [2].

ZnO attracted attention as an alternative photoelectrode material for DSCs due to multiple advantages, that is, excellent optoelectronic properties, low cost, easy synthesis, non‐toxicity and others. Moreover, ZnO has the diversity of one‐dimensional (1D) structures, which sug‐ gest attractive approach for photoelectrode scaffolding.

ZnO nanowire (NW) application as DSC photoelectrode shows multiple advantages, such as higher electron mobility and increased optical way because of refraction; moreover, addi‐ tional functionalities such as flexible DSCs with the help of ZnO NW photoelectrode can also be obtained. The efficiency of ZnO nanowire DSCs is rapidly increasing in recent years; therefore, the interest in ZnO NW application for DSC is rapidly growing. In order to reach higher efficiency of ZnO NW DSCs, further surface modifications with an additional protec‐ tive layer of TiO2 or inert materials such as Al2 O3 are used. Other possibilities lie in tailor‐ ing the morphology of NWs, or employing hybrid structures of ZnO nanowires with other materials.

The application of ZnO NW for DSCs is an interesting topic for nanowire community and also to researchers from diverse scientific fields, since much effort was put in this topic, numer‐ ous approaches were tried and a definite improvement was reached. Still, there is a need to direct the effort in understanding more deeply the ZnO NW interaction with other materials, processes of light and charge transfer and master the synthesis methods in order to achieve the optimal structure for application in DSCs.

This chapter will provide a comprehensive review of the state‐of‐the‐art research activi‐ ties focused on the synthesis and application of ZnO nanowires for dye‐sensitized solar cells. The first section briefly overviews fundamentals of the DSC and introduces ZnO NW synthesis, divided into vapour and solution phase methods, demonstrating approaches to obtain different ZnO NW morphology. Next, the methods for ZnO NW surface modifica‐ tion are discussed, which include ZnO core‐shell structures with semiconductive layers or protective layers and their benefit for charge and light transport in DSCs. In the last part, we will review ZnO NW hybrid structures with other materials, such as nanoparticles, quantum dots or carbon nanomaterials. The chapter will then conclude with the perspec‐ tives and the outlook on the future developments in the ZnO nanowire application for DSCs.

## **2. Fundamentals of dye‐sensitized solar cells**

The concept of a dye‐sensitized solar cell was introduced in 1991, by O'Regan and Grätzel [3]. A schematic diagram showing the operation of a typical DSC is shown in **Figure 1**.

**Figure 1.** Schematic overview of the typical dye‐sensitized solar cell (DSC).

Dye‐sensitized solar cells (DSCs) have gained widespread attention because of the ease of fab‐ rication, low production costs and tuneable optical properties, such as colour and transpar‐ ency [1]. The most attractive properties of DSCs are their low cost and simple manufacturing processes together with their advantageous attributes such as lightweight, low toxicity and

ZnO attracted attention as an alternative photoelectrode material for DSCs due to multiple advantages, that is, excellent optoelectronic properties, low cost, easy synthesis, non‐toxicity and others. Moreover, ZnO has the diversity of one‐dimensional (1D) structures, which sug‐

ZnO nanowire (NW) application as DSC photoelectrode shows multiple advantages, such as higher electron mobility and increased optical way because of refraction; moreover, addi‐ tional functionalities such as flexible DSCs with the help of ZnO NW photoelectrode can also be obtained. The efficiency of ZnO nanowire DSCs is rapidly increasing in recent years; therefore, the interest in ZnO NW application for DSC is rapidly growing. In order to reach higher efficiency of ZnO NW DSCs, further surface modifications with an additional protec‐

O3

ing the morphology of NWs, or employing hybrid structures of ZnO nanowires with other

The application of ZnO NW for DSCs is an interesting topic for nanowire community and also to researchers from diverse scientific fields, since much effort was put in this topic, numer‐ ous approaches were tried and a definite improvement was reached. Still, there is a need to direct the effort in understanding more deeply the ZnO NW interaction with other materials, processes of light and charge transfer and master the synthesis methods in order to achieve

This chapter will provide a comprehensive review of the state‐of‐the‐art research activi‐ ties focused on the synthesis and application of ZnO nanowires for dye‐sensitized solar cells. The first section briefly overviews fundamentals of the DSC and introduces ZnO NW synthesis, divided into vapour and solution phase methods, demonstrating approaches to obtain different ZnO NW morphology. Next, the methods for ZnO NW surface modifica‐ tion are discussed, which include ZnO core‐shell structures with semiconductive layers or protective layers and their benefit for charge and light transport in DSCs. In the last part, we will review ZnO NW hybrid structures with other materials, such as nanoparticles, quantum dots or carbon nanomaterials. The chapter will then conclude with the perspec‐ tives and the outlook on the future developments in the ZnO nanowire application for

The concept of a dye‐sensitized solar cell was introduced in 1991, by O'Regan and Grätzel [3].

A schematic diagram showing the operation of a typical DSC is shown in **Figure 1**.

are used. Other possibilities lie in tailor‐

good performance in diverse light conditions [2].

the optimal structure for application in DSCs.

**2. Fundamentals of dye‐sensitized solar cells**

tive layer of TiO2

60 Nanowires - New Insights

materials.

DSCs.

gest attractive approach for photoelectrode scaffolding.

or inert materials such as Al2

Typical DSCs are composed of a transparent conducting oxide (such as fluorine‐doped tin oxide, FTO) on glass, a nanoparticle photoelectrode covered in a monolayer of sensitizing dye, a hole‐conducting electrolyte and a platinum‐coated, FTO‐coated glass back‐contact.

Nanoparticles of TiO2 (anatase) are mostly used as photoelectrode, although alternative wide‐band‐gap oxides such as ZnO and SnO2 have also been investigated. A monolayer of the sensitizer is attached to the surface of the nanoparticle photoelectrode. Under illumina‐ tion, sensitizer photo‐excitation results in the injection of an electron into the conduction band of the oxide. The dye is regenerated by electron donation from the electrolyte, mostly from a redox system (iodide/triiodide couple) in an organic solvent. The regeneration of the sensitizer by iodide intercepts the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated, in turn, by the reduction of triiodide at the coun‐ ter‐electrode, and the circuit is completed via electron migration through the external load. The voltage, which is obtained under illumination, corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte. Overall, there are no permanent chemical transformations involved in the generation of electric power from light.

The photoelectrode serves as a support for sensitizer loading and at the same time trans‐ port media of photo‐excited electrons from sensitizer to the external circuit. Hence, to ensure high dye loading, a large surface area is necessary. Moreover, a fast charge‐transport rate is required to ensure high electron collection efficiency. These two properties are the defining characteristics of an ideal photoelectrode [4].

Insight into the factors limiting DSC performance is gained by comparing theoretical cell efficiencies with those of current state‐of‐the‐art cells. The power conversion efficiency (PCE) of a solar cell is given as

$$\text{PCE} = \frac{J\_{\kappa} V\_{\text{av}} \text{FF}}{P\_{\text{in}}} \text{ \tag{1}$$

where *J* sc is the absolute value of the current density at short circuit, *V*oc is the photovoltage at open circuit, *FF* is the fill factor and *P*in is the incident light power density. In principle, the maximum *J* sc of a DSC is determined by how well the absorption window of its dye overlaps the solar spectrum. Much of the shortfall is due to the poor absorption of low‐energy photons by available dyes. Thickening of the photoelectrode to increase its optical density in order to improve the absorption of red and near‐infrared light is unsuccessful because the film thick‐ ness comes to exceed the electron diffusion length through the nanoparticle network.

The dynamic scale of the processes involved in light to electricity conversion (**Figure 2**) shows that the initial events of electron injection and dye regeneration leading to photoinduced charge separation occur on a femto‐ to nanosecond or microsecond time scale [6], while the electron transport across the photoelectrode takes place within milliseconds or even seconds [7]. However, for the efficient functioning of the DSC, the diffusion length of the electron should be greater than the thickness of the photoelectrode. Electron diffusion length *L*n can be expressed through electron lifetime *τ*n and electron diffusion coefficient *D*n as

**Figure 2.** Dynamics competition of the processes involved in the conversion of light to electric power by typical DSCs [5].

$$L\_u = \sqrt{D\_u \bullet \tau\_{u'}} \tag{2}$$

Disordered network of TiO2 nanoparticles with numerous grain boundaries weakens electron mobility and results in slow transport and recombination of photo‐excited electrons [8]. This greatly hinders the overall PCE of such devices. The mentioned problems associated with the standard architecture of the photoelectrode oblige a search for more effective nanostructured photoelectrode materials and morphologies [9–11].

## **3. ZnO nanowire synthesis and application in DSCs**

The relatively low transport resistance of transparent high‐mobility materials, such as ZnO compared to that of anatase TiO2 nanoparticles, is one of the major advantages for the application in charge injection and collection for DSCs [12–15]. ZnO, which has an energy bandgap similar to that of the ordinarily used TiO2 but possesses higher electron mobility, is an alternative photoelectrode material for DSCs [16]. Another advantage of ZnO is the diver‐ sity of 1D structures. Structurally, ZnO has several fast‐growth directions; therefore, various morphologies can be easily obtained.

ZnO nanowires were widely considered as an alternative photoelectrode material (**Figure 3**), since they address many of the mentioned problems; however, up to date the highest power conversion efficiency of ZnO‐based DSC reported [18] is still lower than TiO2 ‐based DSC. The main reason may be that best DSC dyes are designed for TiO2 photoelectrode, and since ZnO is less stable especially in acid, therefore there is still no efficient dye available for ZnO anode [19, 20]. It was found that electron injection from traditional Ru‐based dye to ZnO is much slower than to TiO2 [21]. However, much research is conducted in order to find modifications for ZnO photoelectrodes and further increase the overall PCE of the DSCs.

**Figure 3.** ZnO nanowire‐based DSC: (a) a schematic of the cell; (b) SEM image of a nanowire array. Scale bar, 5 µm [17].

## **3.1. ZnO nanowire DSC stability**

*PCE* = *J*

*Ln* = √

photoelectrode materials and morphologies [9–11].

ZnO compared to that of anatase TiO2

**3. ZnO nanowire synthesis and application in DSCs**

Disordered network of TiO2

where *J*

maximum *J*

62 Nanowires - New Insights

*sc Voc FF* \_\_\_\_\_\_ *Pin*

sc is the absolute value of the current density at short circuit, *V*oc is the photovoltage at open circuit, *FF* is the fill factor and *P*in is the incident light power density. In principle, the

the solar spectrum. Much of the shortfall is due to the poor absorption of low‐energy photons by available dyes. Thickening of the photoelectrode to increase its optical density in order to improve the absorption of red and near‐infrared light is unsuccessful because the film thick‐

The dynamic scale of the processes involved in light to electricity conversion (**Figure 2**) shows that the initial events of electron injection and dye regeneration leading to photoinduced charge separation occur on a femto‐ to nanosecond or microsecond time scale [6], while the electron transport across the photoelectrode takes place within milliseconds or even seconds [7]. However, for the efficient functioning of the DSC, the diffusion length of the electron should be greater than the thickness of the photoelectrode. Electron diffusion length *L*n can be

\_\_\_\_\_\_

mobility and results in slow transport and recombination of photo‐excited electrons [8]. This greatly hinders the overall PCE of such devices. The mentioned problems associated with the standard architecture of the photoelectrode oblige a search for more effective nanostructured

**Figure 2.** Dynamics competition of the processes involved in the conversion of light to electric power by typical DSCs [5].

The relatively low transport resistance of transparent high‐mobility materials, such as

application in charge injection and collection for DSCs [12–15]. ZnO, which has an energy

*Dn* ∙ *τn*, (2)

nanoparticles, is one of the major advantages for the

nanoparticles with numerous grain boundaries weakens electron

ness comes to exceed the electron diffusion length through the nanoparticle network.

expressed through electron lifetime *τ*n and electron diffusion coefficient *D*n as

sc of a DSC is determined by how well the absorption window of its dye overlaps

, (1)

ZnO has a low chemical stability in acidic environment. After the DSC assemble, ZnO pho‐ toelectrode degrades through the carboxylic groups of the acidic dye attached to it, and thus leads to the formation of Zn2+‐dye agglomerates.

On the other hand, the time of dye loading is crucial for the performance of an all‐ZnO‐based DSC, because longer dye‐loading time leads to the formation of more Zn2+‐dye agglomerates and shorter immersion time is insufficient for dye adsorption. For this reason, other than tra‐ ditional Ru‐based dyes could be used, such as porphyrin and indoline dyes [22]. Indoline dyes have been found to be a comparatively good match with ZnO because of its lower acidity and the lack of complexing agent. The CR147 dye‐sensitized ZnO film is almost free from Zn2+/dye agglomerations. A ZnO DSC with CR147 dye has high PCE of 6.89%, 40% higher than the cell with traditional N719 dye [23]. A metal‐free organic dye D149 allows to use lower‐sensitizing times, moreover a PCE of 5.14% was reported [24]. A mix of several specially engineered dyes could also be advantageous. Compared to only D149‐sensitized cell, the YD2‐o‐C8‐TBA and D149 co‐sensitized ZnO DSSC with a wavelength ranging from 475–700 nm exhibited improved photon‐to‐current conversion efficiencies with cell PCE of 5.6% [25]. Vegetable tannin and their Fe complexes could be used for their low cost and can be obtained from renewable sources, but the efficiencies are up to 0.99% [26].

Other stability problems are associated with a liquid electrolyte used, which tends to leak in time. Device instability and the need for good device packaging have become major problems for commercial application of DSCs. This problem is common to most DSCs; liquid electrolyte could be substituted with quasi‐solid‐state electrolytes [27]. Metal oxide nanoparticle gelators are applied [28], ZnO nanoparticles gel [29] shows PCE of 4.17%, and the optimized DSC shows a stability of 95% on the PCE value for 150 days. The hole transport is presumably done by Grotthuss‐type ion exchange mechanism.

## **3.2. ZnO nanowire synthesis**

As it was earlier stated, competition between the transport and the recombination of photo‐ excited carriers is one of the main obstacles for developing higher conversion efficiency pho‐ toelectrode. To date, numerous morphologies of low‐dimensional ZnO nanostructures such as nanowires, nanobelts, nanotubes and nanoflakes have been synthesized and are expected to improve the electron diffusion length in the photoelectrode.

This paragraph reviews the most important approaches of the ZnO NW synthesis with the aim of application in DSCs. In order to obtain an optimal structure for the photoelectrode of DSC, the growth of ZnO NWs should be carefully controlled. ZnO nanowires can be synthe‐ sized in a variety of methods; two main synthesis groups can be distinguished: vapour phase and solution phase synthesis.

*Vapour phase ZnO NW synthesis* involves temperatures typically 400–1100°C, needed to vapourize and transport the precursors. A typical reaction involves ZnO NW growth by vapour‐liquid‐solid mechanism (VLS) [30, 31] using catalyst particles such as Au, Pt or Ag, or by vapour solid mechanism (VS) [32] without the use of catalysts. Zn vapour is obtained by heating the mixture of ZnO and C [33] or directly from Zn powder [34]. A direct growth on Zn metallic substrate is also interesting for its simplicity [35–37]. ZnO NW growth by vapour phase synthesis typically involves a direct Zn oxidation:

$$\text{Zn} + \text{O}\_2 \rightarrow \text{ZnO} \tag{3}$$

*Solution phase ZnO NW synthesis* is especially interesting for its low‐temperature approach and high‐output possibilities. Since the typical synthesis conditions apply temperatures lower than 200°C, the variety of materials can be used as substrates. In the solution phase synthesis, the ZnO NW growth process is carried out in aqueous or organic solvents from zinc salts. Normally, aqueous solutions are used and the methods are often referred to as hydrothermal growth [38]. However, other solutions such as ethanol [39] are also used. A typical ZnO NW solution phase growth involves the following reaction:

$$\text{Zn}^{2+} + 2\text{OH}^{-} \rightarrow \text{Zn(OH)}\_{2} \rightarrow \text{ZnO} + \text{H}\_{2}\text{O} \tag{4}$$

where hexamethylenetetramine (HMTA) [40], NH4 OH [41] or NaOH [42] may be used as the hydroxide source. High aspect ratio can be achieved by controlling the preferential growth with catalytic Au nanoparticles [43] or polymers such as polyethylenimine (PEI) [44], or polyethylene glycol (PEG) [45]. ZnO NW aspect ratio can be further improved by sequential growth [46].

Comparing the vapour and liquid phase synthesis, it is important to note that the vapour phase growth of ZnO NWs uses higher temperatures; therefore for low‐melting substrates an efficient post synthesis, transferring and attachment methods should be developed, but none‐ theless high crystallinity can be obtained without further annealing. On the contrary, most of the solution growth methods use low temperatures, therefore NWs can be directly grown on a variety of low‐melting materials and surfaces; however, the synthesis of high aspect ratio structures with high crystallinity is demanding.

Applying the mentioned methods, ZnO NWs could be synthesized in complex forms (**Figure 4**) such as nanoforest [47], tetrapods [49, 50], hierarchical nanowires [48], coral‐shaped nanostructures [51], nanocactus [24], flower‐like [52] and many others [53–55] by simply con‐ trolling the crystal grow direction.

**Figure 4.** Complex structures of ZnO nanowires: (a) nanoforest [47]; (b) hierarchical nanowires (inset shows SEM figure) [48]; (c) tetrapods [49].

## **4. ZnO NW interface modifications**

and their Fe complexes could be used for their low cost and can be obtained from renewable

Other stability problems are associated with a liquid electrolyte used, which tends to leak in time. Device instability and the need for good device packaging have become major problems for commercial application of DSCs. This problem is common to most DSCs; liquid electrolyte could be substituted with quasi‐solid‐state electrolytes [27]. Metal oxide nanoparticle gelators are applied [28], ZnO nanoparticles gel [29] shows PCE of 4.17%, and the optimized DSC shows a stability of 95% on the PCE value for 150 days. The hole transport is presumably done

As it was earlier stated, competition between the transport and the recombination of photo‐ excited carriers is one of the main obstacles for developing higher conversion efficiency pho‐ toelectrode. To date, numerous morphologies of low‐dimensional ZnO nanostructures such as nanowires, nanobelts, nanotubes and nanoflakes have been synthesized and are expected

This paragraph reviews the most important approaches of the ZnO NW synthesis with the aim of application in DSCs. In order to obtain an optimal structure for the photoelectrode of DSC, the growth of ZnO NWs should be carefully controlled. ZnO nanowires can be synthe‐ sized in a variety of methods; two main synthesis groups can be distinguished: vapour phase

*Vapour phase ZnO NW synthesis* involves temperatures typically 400–1100°C, needed to vapourize and transport the precursors. A typical reaction involves ZnO NW growth by vapour‐liquid‐solid mechanism (VLS) [30, 31] using catalyst particles such as Au, Pt or Ag, or by vapour solid mechanism (VS) [32] without the use of catalysts. Zn vapour is obtained by heating the mixture of ZnO and C [33] or directly from Zn powder [34]. A direct growth on Zn metallic substrate is also interesting for its simplicity [35–37]. ZnO NW growth by vapour

Zn + O2 → ZnO (3)

*Solution phase ZnO NW synthesis* is especially interesting for its low‐temperature approach and high‐output possibilities. Since the typical synthesis conditions apply temperatures lower than 200°C, the variety of materials can be used as substrates. In the solution phase synthesis, the ZnO NW growth process is carried out in aqueous or organic solvents from zinc salts. Normally, aqueous solutions are used and the methods are often referred to as hydrothermal growth [38]. However, other solutions such as ethanol [39] are also used. A typical ZnO NW

hydroxide source. High aspect ratio can be achieved by controlling the preferential growth

2 → ZnO + H2 O (4)

OH [41] or NaOH [42] may be used as the

sources, but the efficiencies are up to 0.99% [26].

by Grotthuss‐type ion exchange mechanism.

to improve the electron diffusion length in the photoelectrode.

phase synthesis typically involves a direct Zn oxidation:

solution phase growth involves the following reaction:

Zn2+ + 2 OH− → Zn (OH)

where hexamethylenetetramine (HMTA) [40], NH4

**3.2. ZnO nanowire synthesis**

64 Nanowires - New Insights

and solution phase synthesis.

The advantage of fast electron transfer is often counterbalanced by faster electron recombina‐ tion dynamics with the oxidized electrolyte via intrabandgap surface states which ultimately limits the PCE [15]. In order to slow down the recombination and to increase the photovoltage and photocurrent, the ZnO NW interfaces can be covered by a thin layer of various capping materials (Al2 O3 , TiO2 , ZrO2 , etc.). Alternatively, hole transfer can be modified by forming ZnO NW hybrid structures with conductive polymers, or various nanostructures such as quantum dots (QDs), nanocrystals (NCs) and carbon nanomaterials.

## **4.1. ZnO NW core‐shell structure in DSC**

The purpose of NW coating with an insulating or semiconducting oxide layer is to diminish the recombination by forming a potential barrier on NW surface, to physically separate elec‐ trons from ions and to increase the dye adsorption on the surface. Moreover, band edge can be shifted by using higher bandgap layer to increase the open‐circuit voltage. In addition to acting as an effective protection layer for the chemically unstable ZnO against the acidic dye solution, the complete coverage of the NWs with a dense layer can passivate surface traps [12]. ZnO NW core‐shell structures with different metal oxides can be obtained with atomic layer deposition (ALD) technique [56, 57]. The coverage of ZnO NWs with an insulating mate‐ rials results in a low‐efficiency DSCs [58–60]; by contrast, ZnO NW core‐shell structures with semiconductors have several advantages, especially interesting is TiO2 shell.

There are several motives why the ZnO‐TiO2 core‐shell structure is attractive. TiO2 is chemi‐ cally more stable compared to ZnO in acidic dye solutions [61]. Thus, the presence of TiO2 shell prevents ZnO surface from being dissolved and the formation of Zn2+/dye agglomer‐ ates. The TiO2 shell can also increase the injected electrons and more dye absorption, which lead to a higher light‐harvesting efficiency. Moreover, the shell reduces recombination by forming a tunnelling barrier to confine the photoinjected electrons within the core, and by passivating the recombination centres on the core surface [62]. The charge transfer is sig‐ nificantly improved since ZnO has much higher electron mobility compared to TiO2 . DFT ab initio study of ZnO‐TiO2 core‐shell structure [63] shows that TiO2 coating induces changes in surface states and shifts the conduction and valence band edges to higher energies; therefore, an increase in open‐circuit voltage and a decrease in short‐circuit current are expected.

TiO2 shell on ZnO NWs can be formed by dip coating in Ti precursors, such as tetrabutyl titanate (TBOT) [64, 65], titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl4 ). ZnO NW coated with a thin shell layer of TiO2 (**Figure 5**) showed an increased PCE up to 6% [66]. A comparative study [67] showed that at the same conditions TBOT‐treated ZnO DSC showed the best performance with the highest PCE and short‐circuit current density of 4.92% and 12.49 mA/cm2 , respectively.

In order to increase the surface area of the photoelectrode and without losing the charge‐ transport properties, hierarchical structures could be used. A hierarchical structure of ZnO NWs could also increase the efficiency of DSCs by providing a high area for dye absorption

**Figure 5.** ZnO‐TiO2 core‐shell formed by low‐temperature TiCl4 treatment [68].

and also effective light scattering. ZnO DSC efficiency can be doubled by decorating ZnO NW photoelectrodes with different nanoparticles of SnO2 [69], TiO2 [70, 71] or in situ‐synthesized ZnO nanoparticles [72]. Even though the large surface area of nanoparticles empowers a high dye‐loading capacity, yet the disordered network with numerous grain boundaries weak‐ ens electron mobility and results in slow transport and recombination of photo‐excited elec‐ trons [8]. The application of ZnO NW with semiconductive nanoparticles has advantages of both high surface area and fast charge transport. A complex structure of hierarchical NWs improves the PCE up to five times compared to the initial NW, the reported efficiencies for nanotrees 2.63% [73], coral‐shaped nanostructure gives 4.58% [51], nanocactus 5.14% [24] are encouraging.

ZnO tetrapods used with SnO2 nanoparticles provided large roughness factors, good charge collection and tunable light‐scattering properties. High PCE of 6.31% was attributed to NH3 treatment, which was believed to create ZnO shell on SnO2 nanoparticles [74]. A hierarchical core‐shell ZnO NW with TiO2 nanosheets resulted in an outstanding performance with a solid‐state electrolyte, showing the conversion efficiencies of up to 7.46% [75].

The prepared ZnO photoelectrode can be further treated to improve the conductance and porosity parameters. Hot‐press treatment of the ZnO photoelectrode was demonstrated to improve PCE of the DSC by 45% [76]. Another powerful technique, the room‐temperature chemical bath deposition, was used to increase the surface area of the ZnO photoelectrode, a PCE of 5.24% was obtained [77].

## **4.2. ZnO NW hybrid solar‐cell structures**

be shifted by using higher bandgap layer to increase the open‐circuit voltage. In addition to acting as an effective protection layer for the chemically unstable ZnO against the acidic dye solution, the complete coverage of the NWs with a dense layer can passivate surface traps [12]. ZnO NW core‐shell structures with different metal oxides can be obtained with atomic layer deposition (ALD) technique [56, 57]. The coverage of ZnO NWs with an insulating mate‐ rials results in a low‐efficiency DSCs [58–60]; by contrast, ZnO NW core‐shell structures with

cally more stable compared to ZnO in acidic dye solutions [61]. Thus, the presence of TiO2 shell prevents ZnO surface from being dissolved and the formation of Zn2+/dye agglomer‐

lead to a higher light‐harvesting efficiency. Moreover, the shell reduces recombination by forming a tunnelling barrier to confine the photoinjected electrons within the core, and by passivating the recombination centres on the core surface [62]. The charge transfer is sig‐

nificantly improved since ZnO has much higher electron mobility compared to TiO2

titanate (TBOT) [64, 65], titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl4

core‐shell structure [63] shows that TiO2

surface states and shifts the conduction and valence band edges to higher energies; therefore, an increase in open‐circuit voltage and a decrease in short‐circuit current are expected.

comparative study [67] showed that at the same conditions TBOT‐treated ZnO DSC showed the best performance with the highest PCE and short‐circuit current density of 4.92% and

In order to increase the surface area of the photoelectrode and without losing the charge‐ transport properties, hierarchical structures could be used. A hierarchical structure of ZnO NWs could also increase the efficiency of DSCs by providing a high area for dye absorption

treatment [68].

shell on ZnO NWs can be formed by dip coating in Ti precursors, such as tetrabutyl

shell can also increase the injected electrons and more dye absorption, which

shell.

is chemi‐

. DFT ab

). ZnO

coating induces changes in

core‐shell structure is attractive. TiO2

(**Figure 5**) showed an increased PCE up to 6% [66]. A

semiconductors have several advantages, especially interesting is TiO2

There are several motives why the ZnO‐TiO2

NW coated with a thin shell layer of TiO2

, respectively.

core‐shell formed by low‐temperature TiCl4

ates. The TiO2

66 Nanowires - New Insights

TiO2

12.49 mA/cm2

**Figure 5.** ZnO‐TiO2

initio study of ZnO‐TiO2

Hybrid polymer solar cells composed of metal oxide nanostructures and polymers have attracted great interest mostly due to the good physical and chemical stability. In such solar cells, p‐type donor polymers are combined with n‐type acceptor ZnO nanostructures [78] among other oxides. Nanowires are one of the most attractive forms for such solar cells because they provide a direct path for charge transport, high carrier motilities and a high elec‐ tron affinity necessary for charge injection from the complementary organic donor material [79]. Mostly used conducting polymer poly(3‐hexylthiophene) (P3HT) composite with ZnO nanowire shows hybrid solar‐cell PCE in the range of 0.2–0.5% [80–82] and can be further increased to 2% by adding dye [83]. Such low‐power conversion efficiencies are associated with poor polymer infiltration and therefore low‐active interfacial area between polymer and ZnO NWs. An interfacial layer of carbon nanoparticles (CNPs) can be employed to enhance the charge‐transfer properties in hybrid ZnO NW solar cells. The energy‐level diagram of the solar cells is shown in **Figure 6b**. The LUMO level of the CNPs (‐4.00 ± 0.02 eV) is perfectly aligned with both the LUMO level of P3HT (‐3.2 eV) and the conduction band edge of TiO2 (‐4.2 eV). This ensures both efficient exciton dissociation at the P3HT/CNP interface and effi‐ cient electron extraction. In such a device, ZnO‐TiO2 core‐shell works as electron collector and transporter, P3HT acts as electron donor, and the CNP layer acts as electron acceptor [84].

The other possibility is to use quantum dots (QDs) for hole transfer. The use of PbS quantum dots (QDs) is promising because of the tunable, size‐dependent bandgap from 0.7 to 1.3 eV [85].

**Figure 6.** Hybrid solar cell with ZnO‐TiO2 core‐shell, carbon nanoparticles (CNP) and conductive polymer P3HT. (a) Device structure; (b) energy‐level diagram; (c) absorption spectra of the active films on ITO glass substrates; (d) J‐V characteristics (empty: dark; filled: illuminated) of the solar cells with and without CNP [84].

Pairing ZnO with PbS QDs for photovoltaic applications shows high stability in ambient atmosphere. An advantage of using ZnO NWs instead of ZnO planar or nanoparticle layer is demonstrated [86]. Vertically oriented ZnO NW arrays are fully infiltrated with QDs, increas‐ ing considerably the light absorption and carrier collection. Ordered interface architecture of ZnO NW arrays can decouple absorption from collection, extending the effective depletion width throughout a thick QD film.

The ZnO NW performance was tested in both earlier mentioned hole conductor configura‐ tions: conducting polymer P3HT and PbS QDs [82]. As it is shown in **Figure 7b**, band align‐ ment is similar; however, the PCE of 4.2% and 0.5% for QDs and P3HT, respectively, was obtained. It is worth noting that graphene was used as the conductive electrode instead of the traditional ITO, which demonstrates the flexibility of application for various substrates.

ZnO NW solar‐cell absorption in the near‐infrared region can be enhanced by using plasmonic nanocrystals (NCs). Ag NC enhances ZnO NW solar‐cell (**Figure 8**) PCE from 4.5 to 6.0% [87].

A plasmonic NC traps a photon on the basis of localized surface plasmon resonance, and gen‐ erates a strong oscillating electric field (i.e. optical near field) that is localized in the vicinity of the NC. The optical near field excites a dye molecule or semiconductor more efficiently than incident far‐field light, and therefore photocurrents are enhanced.

**Figure 7.** Hybrid ZnO NW solar cells. (a) Schematic diagram of ZnO NWs (400 nm) infiltrated and covered with PbS QDs (300 nm) or P3HT (700 nm); (b) flat‐band energy‐level diagram of the solar cell [82].

**Figure 8.** PbS QD/ZnO NW solar cell with embedded Ag NCs. (a) Schematic; (b) J‐V characteristics without and with Ag NCs [87].

## **5. Conclusions**

Pairing ZnO with PbS QDs for photovoltaic applications shows high stability in ambient atmosphere. An advantage of using ZnO NWs instead of ZnO planar or nanoparticle layer is demonstrated [86]. Vertically oriented ZnO NW arrays are fully infiltrated with QDs, increas‐ ing considerably the light absorption and carrier collection. Ordered interface architecture of ZnO NW arrays can decouple absorption from collection, extending the effective depletion

(a) Device structure; (b) energy‐level diagram; (c) absorption spectra of the active films on ITO glass substrates; (d) J‐V

characteristics (empty: dark; filled: illuminated) of the solar cells with and without CNP [84].

core‐shell, carbon nanoparticles (CNP) and conductive polymer P3HT.

The ZnO NW performance was tested in both earlier mentioned hole conductor configura‐ tions: conducting polymer P3HT and PbS QDs [82]. As it is shown in **Figure 7b**, band align‐ ment is similar; however, the PCE of 4.2% and 0.5% for QDs and P3HT, respectively, was obtained. It is worth noting that graphene was used as the conductive electrode instead of the traditional ITO, which demonstrates the flexibility of application for various substrates.

ZnO NW solar‐cell absorption in the near‐infrared region can be enhanced by using plasmonic nanocrystals (NCs). Ag NC enhances ZnO NW solar‐cell (**Figure 8**) PCE from 4.5 to 6.0% [87]. A plasmonic NC traps a photon on the basis of localized surface plasmon resonance, and gen‐ erates a strong oscillating electric field (i.e. optical near field) that is localized in the vicinity of the NC. The optical near field excites a dye molecule or semiconductor more efficiently than

incident far‐field light, and therefore photocurrents are enhanced.

width throughout a thick QD film.

**Figure 6.** Hybrid solar cell with ZnO‐TiO2

68 Nanowires - New Insights

Several conclusions could be made with the proposal for the future trends:

with good crystallinity. Nevertheless, these methods are unbeatable for high‐yield depo‐ sition. A variety of substrates can be used for hydrothermal ZnO NW growth because of the low temperatures used. By contrast, vapour phase growth of ZnO NWs uses higher temperatures, therefore for a low‐melting substrates an efficient post‐synthesis transfer‐ ring and attachment methods should be developed; however, high crystallinity can be obtained without further annealing.

## **Acknowledgements**

The work has received funding from the European Union's Seventh Framework programme for research and innovation under the Marie Sklodowska‐Curie grant agreement No 609402 – 2020 researchers: Train to Move (T2M).

## **Author details**

Simas Rackauskas\*, Nadia Barbero, Claudia Barolo and Guido Viscardi

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Chemistry, NIS Interdepartmental Centre and INSTM Reference Centre, University of Turin, Turin, Italy

## **References**

[3] O'regan B, Grätzel M. A low‐cost, high‐efficiency solar cell based on dye‐sensitized col‐ loidal TiO2 films. Nature. Nature Publishing Group; 1991;353:737‐40.

with good crystallinity. Nevertheless, these methods are unbeatable for high‐yield depo‐ sition. A variety of substrates can be used for hydrothermal ZnO NW growth because of the low temperatures used. By contrast, vapour phase growth of ZnO NWs uses higher temperatures, therefore for a low‐melting substrates an efficient post‐synthesis transfer‐ ring and attachment methods should be developed; however, high crystallinity can be

3. ZnO NW surface is decomposed in acidic environment of best‐to‐date dyes, therefore special dyes for ZnO should be developed or alternatively surface modification of ZnO is

semiconductor layer. Alternatively forming a protective layer from dielectric materials

4. Hybrid structures of ZnO NWs are primarily interesting for their high physical and chemical stability, but can also boost other properties such as charge transfer or light ab‐ sorption; however, more work should be done to understand the underlying mechanisms

The work has received funding from the European Union's Seventh Framework programme for research and innovation under the Marie Sklodowska‐Curie grant agreement No 609402 –

Department of Chemistry, NIS Interdepartmental Centre and INSTM Reference Centre,

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**4. Conclusions**

In this work, we systematically investigated the influence of different carrier gases (O2, N2, and air) on the grown film quality on c-plane sapphire substrates by using the mist-CVD method. Although XRD and Raman measurements show that the pure corundum-structured α-Ga2O3 was successfully obtained on the c-plane sapphire substrates with single (0006) plane orientation for all three different carrier gases, the crystal quality could be greatly affected by the carrier gas. When O2 was used as the carrier gas, the sample showed the smallest FWHM of about 72 arcsec obtained from the XRD rocking curves. The UV-VIS measurement also showed that the sample grown with O2 as the carrier gas showed a very sharp absorption cutoff edge. It is proposed that the oxygen content in carrier gas should be responsible for all of these results. More oxygen in the carrier gas can greatly increase growth rate, and at the same time guarantee the film quality. This is further verified by the smoother surface and rapid growth rate (10.3 nm/min) for O2 as the carrier gas compared to air and N2 as the carrier gases (5.3 nm/min and 2.4 nm/min respectively). XPS analysis confirmed that more oxygen element can be included in epitaxial film and thus help improve the crystal quality. The proper carrier gas is essential for high quality α-Ga2O3 growth.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/22/3670/s1, Figure S1: TEM images of the sample grown with air as the carrier gas. (1) Cross-sectional α-Ga2O3/α-Al2O3 interface, (2) diffraction spots of α-Ga2O3/α-Al2O3, Figure S2: TEM images of the sample grown with N2 as the carrier gas. (1) Cross-sectional α-Ga2O3/α-Al2O3 interface, (2) diffraction spots of α-Ga2O3/α-Al2O3, Figure S3: X-ray photoelectron wide spectra for the α-Ga2O3 sample grown with air as the carrier gas, Figure S4: X-ray photoelectron wide spectra for the α-Ga2O3 sample grown with N2 as the carrier gas.

**Author Contributions:** Conceptualization, C.Z.; methodology, C.Z. and Y.X.; validation, Y.C. (Yaolin Cheng), Z.L., Y.C. (Ya'nan Cheng); formal analysis, Y.X. and Y.C. (Yaolin Cheng); investigation, C.Z. and J.Z.; resources, Y.H.; data curation, D.C.; writing—original draft preparation, Y.X.; writing—review and editing, C.Z.; visualization, Q.F.; project administration, C.Z.; funding acquisition, Y.H.

**Funding:** This research was funded by National Key R&D Program of China, grant number 2018YFB0406504.

**Conflicts of Interest:** The authors declare no conflict of interest.