an efficient photocatalyst minimizes recombination 
 of photoexcited 
 electron  hole pairs and maximizes electron and hole transfer 
 to the adsorbate upon consecutive surface reactions . in general , the 
 photocatalytic activity of a given particle system depends on multiple 
 factors . for a reliable photoactivity assessment ,
relative contributions 
 from the particles bulk and interfaces have to be sorted out , 
 and their impact to the overall performance requires a careful evaluation . a major limitation of any photocatalytic process originates from 
 charge carrier recombination .
an important approach to counteract 
 charge carrier recombination in photocatalytic particle systems is 
 based on the coupling of different semiconductor components with desirable 
 matching of their electronic band structures . in the case of composite nanoparticle systems , synergistic 
 properties 
 and
those that are enhanced with respect to the individual components 
 can only emerge if the particles are mixed intimately enough .
for that reason , researchers have explored efficient 
 and cost - effective methods for combining different metal oxide nanoparticles . 

chemical ways could offer great control in this respect . while a variety of experimental approaches leading to the coupling 
 of two semiconductors have been successfully employed for particles 
 that were grown in colloidal solutions ,
there are 
 very few reports about the charge separation properties of dry particle 
 systems that feature corresponding types of interfaces . with this 
 study
we present a simple as well as versatile approach that involves 
 surface charging of multiple particle systems inside the same aqueous 
 dispersion medium to achieve blended nanoparticle systems of superior 
 mixing quality and , thus , a maximum concentration of functional interfaces 
 between different particle types .
( a ) schematic showing the principle of 
 hydration - dehydration induced 
 particle aggregation and solid  solid interface formation .
( b ) 
 adjustment of opposite surface charges on multiple particle types 
 is expected to favor preferential attraction between different particles 
 and yields good mixing qualities .
we have shown in previous work that water - mediated 
 aggregation 
 of tio2 and zro2 nanoparticles and subsequent 
 dehydration procedures under high vacuum conditions are effective 
 in the generation of solid  solid interfaces .
the mechanistic steps of the underlying material transformation 
 process are as follows ( figure 1 ) : in aqueous 
 dispersion , mutual attraction and agglomeration of metal oxide nanoparticles 
 takes place . upon vacuum treatment dehydration / dehydroxylation processes 
 ( evaporation of liquid phase ) in conjunction with particle condensation 
 lead to the formation of chemical interparticle metal oxygen bonds . 

annealing - induced aggregation of particles during thermal treatment 
 results in interconnected particles but , according to n2 sorption measurements , does not reduce the specific surface area 
 in comparison to the unprocessed nanoparticle powders .
schematic energy diagram showing the positions of conduction and 
 valence band edges of tio2 and sno2 .
expected 
 vectorial charge transfer directions are indicated for electrons ( top 
 arrow ) and holes ( bottom arrow ) . for the exploration of surface charge - induced heteroaggregation , 
 we chose the tio2sno2 system .
both materials 
 are central to photocatalysis , solar water splitting , and dye sensitized 
 solar cells . as a result of composite formation upon generation of 
 heterointerfaces , light - induced charge separation and vectorial charge 
 transfer ( figure 2 )
the conduction band position 
 of sno2 is lower than that of tio2 and such 
 that it is incapable of reducing oxygen molecules to form superoxide 
 anions .
the band offsets between tio2 and sno2 will promote charge separation across the interfaces . using electron 
 paramagnetic resonance ( epr ) spectroscopy
, we address the vectorial 
 transfer of separated charges to generate paramagnetic o radicals as well as paramagnetic ti sites .
another 
 reason for selecting this system has been the exclusion of sn  ti  o 
 compound formation . for sno2tio2 core 
shell 
 nanostructures , it was found that the phase boundary between the two 
 oxides remains abrupt up to t = 1200 k without substantial 
 chemical transformations in this region .
thus , heteroaggregated sno2tio2 nanoparticle 
 networks represent an ideal model system to explore the potential 
 of surface charge - induced heteroaggregation with regard to the generation 
 of heterojunctions that enhance the separation of photogenerated charges .
tio2 and sno2 nanoparticles 
 were prepared by metal organic chemical vapor synthesis ( mo - cvs ) based 
 on the decomposition of either titanium(iv ) isopropoxide ( aldrich , 
 99.999% trace metals basis ) or tetra - n - butyltin ( aldrich , 
 technical grade , > 93% ) vapor at t = 1073 k in 
 a 
 flow reactor system .
for purification , the obtained powder samples 
 were subjected to thermal treatment under high vacuum conditions . 

first , the powder sample was heated to t = 870 k 
 using a rate of r  5 k min .
subsequent oxidation with o2 at this temperature followed 
 by cooling in o2 atmosphere was successfully applied to 
 remove organic remnants originating from the precursor material , on 
 the one hand , and to guarantee the stoichiometric composition of the 
 oxide on the other .
the average particle size determined by transmission 
 electron microscopy ( tem ) and nitrogen sorption was 13 nm for tio2 and 10 nm for sno2 nanoparticles .
for the preparation of mixed ensembles of tio2 and sno2 nanoparticles , 250 mg of the powder samples 
 were dispersed in 100 ml of c = 10 moll formic acid solution for 30 min under 
 ultrasonication ( hielscher sonifier 200s ) and simultaneous agitation 
 by a magnetic stirrer . cooling with an ice 
centrifugation and subsequent 
 drying in vacuum at room temperature resulted in the formation of 
 monolithic pieces , which were transferred to a quartz glass cell and 
 annealed to t = 873 k at p < 10 mbar prior to 
 spectroscopic investigations . prior to liquid exposure
zeta potentials and agglomerate size 
 distributions of suspensions of 2.5 g oxide nanoparticlesl formic acid solution were derived from dynamic light 
 scattering measurements on a malvern zetasizer nano zs .
nitrogen sorption 
 isotherms were obtained at t = 77 k using nitrogen 
 adsorption / desorption ( quantachrome nova 4000e ) .
samples were degassed 
 for 2 h in the degas unit of the adsorption apparatus at t = 473 k under vacuum prior to analysis .
the bet surface area sbet was evaluated using adsorption data in a 
 relative pressure range p / p0 from 0.05 to 0.2 .
scanning electron microscopy ( sem ) measurements 
 were performed on a zeiss gemini ultra 55 microscope operating at 
 20 kv equipped with an energy dispersive x - ray emission ( edx ) detector . 
 to characterize the mixing quality inside the nanoparticle networks 
 we used edx to track compositional homogeneity changes in the range 
 of a few hundreds of nanometers .
the local resolution of chemical 
 information for edx analysis is limited to the penetration depth and 
 scattering of the primary electrons and therefore to the volume where 
 characteristic x - rays are emitted of the sample . with acceleration 
 voltages of 20 kv ,
the sampled volume in edx studies is on the order 
 of a few cubic micrometers for bulk tio2 or sno2 samples .
consequently , the determination of absolute ti and sn concentrations 
 via edx at the nanometer scale is not feasible .
nevertheless , by scanning 
 the aggregate with a linescan , the change in the ratio between ti 
 and sn can be tracked with a very high spatial resolution of approximately 100 nm .
for epr 
 measurements , the powder sample was contained within a 
 suprasil quartz glass tube connected to an appropriate high vacuum 
 pumping system with a base pressure p = 10 mbar .
the light beam was passed through a water filter 
 to exclude ir contributions from the excitation spectrum .
it was held constant 
 at pirr = 0.9 mwcm for the energy range 3.2 ev < e < 6.2 ev 
 throughout all experiments . during uv exposure , the samples were held 
 at a temperature between t = 90 k and t = 140 k in order to keep uv - induced heating 
 effects constant .
x - band epr measurements were performed on a bruker 
 emx micro spectrometer using a bruker er 4119 hs resonator . for measurements 
 in the temperature range between t = 90 k and t = 140 k , an er 4131 vt variable - temperature accessory 
 was used .
the procedure of particle aggregation 
 as outlined by figure 1 aims at the conversion 
 of aerosol metal oxide nanoparticle 
 powders , of high purity and well characterized particle surface properties , into binary metal oxide nanoparticle networks of high mixing quality 
 and therefore with high concentrations of solid  solid heterointerfaces 
 ( figure 1b ) .
prior to the 
 process , aggregation between particles must be avoided in the starting 
 mixture in order to achieve nanoparticle networks of different and 
 perfectly intermixed metal oxides .
dynamic light scattering experiments 
 on dispersions of vapor phase - grown tio2 nanoparticles 
 with an average particle size of 13 nm as determined by tem clearly demonstrate that metal oxide nanoparticles 
 that were grown by mo - cvs can be efficiently deagglomerated by ultrasonic 
 treatment in aqueous dispersion ( figure 3 ) . 
 as a consequence of ultrasonification , the agglomerate size distribution 
 maximum shifts from 100 nm ( figure 3a ) to 20 
 nm ( figure 3b ) .
after discontinuation of ultrasonic 
 treatment , particles reagglomerate , and the agglomerate size distribution 
 maximum shifts to approximately 40 nm ( figure 3c ) .
size distribution profiles determined by dynamic light scattering 
 experiments on dispersions of vapor phase - grown tio2 nanoparticles 
 with an average particle size of 13 nm in c = 10 moll formic acid .
ultrasonic 
 treatment breaks up particle agglomerates and leads to the shift of 
 the agglomerate size distribution maximum from ( a ) 100 nm to ( b ) approximately 
 20 nm .
after 15 min of reagglomeration , which occurs after discontinuation 
 of ultrasonic treatment , the maximum increases to approximately 40 
 nm ( c ) .
zeta potential measurements reveal that both oxides 
 carry a negative 
 surface charge in neutral aqueous dispersion ( figure 4 ) . as a result of particle aggregation , 
 tio2 and sno2 form a nonuniform network which 
 has a significant fraction of tio2tio2 and sno2sno2 interfaces .
self - assembly 
 of oppositely charged particles in colloidal dispersion and , in consequence , 
 the generation of desired tio2sno2 heterointerfaces 
 is achievable via control of the surface charge . in comparison 
 to inorganic acids , which potentially contaminate the particle surface 
 with ions such as cl , carboxylic acids are eligible 
 candidates for surface charge adjustment
since they can be efficiently 
 eliminated from the particle network via oxidation at elevated temperatures . 

the zeta potential dependence of tio2 and sno2 nanoparticles on the formic acid concentration ( figure 4 ) prompted us to choose a formic acid concentration of c = 10 moll ( indicated by gray arrow ) where the zeta potential of tio2 and sno2 particles is + 24 mv and 9 mv , respectively .
zeta potentials 
 of tio2 and sno2 nanoparticle 
 samples in aqueous dispersion as a function of formic acid concentration . 

to 
 zeta potential measurements , all metal oxide samples were subjected 
 to vacuum annealing and subsequent oxidation treatment at elevated 
 temperatures to eliminate unintended effects that could originate 
 from surface impurities .
surface charge adjustment for dispersed particles 
 in combination 
 with control over the ph in solution shows a complex functional dependence 
 on the formic acid concentration .
this is due to the formate adsorption 
 equilibria at the metal oxide particle surfaces which , in turn , are 
 subject to particle concentration in the dispersion and on the concentration 
 of the acid and , concomitantly , on the ph . ( for further details please see supporting information , figure s1 . ) dehydration and dehydroxylation as well as oxygen 
 treatment of 
 the obtained particle networks at elevated temperatures were applied 
 in order to eliminate organic surface groups that previously lead 
 to surface charging and heteroaggregation .
x - ray diffraction , tem , and n2 sorption measurements 
 were carried out to identify potential process - induced structural 
 changes that may result from hydration- and annealing procedures . 

all methods unambiguously reveal that primary particle properties 
 size , structure , and morphology have been retained ( supporting information , figure s2 ) .
changes in the adsorption 
 desorption hysteresis of nitrogen sorption 
 experiments ( left panel of figure 5 ) clearly 
 demonstrate that loose nanoparticles transform into a mesoporous network , which are made up from interconnected particles 
 and show a distinct pore size distribution ( right panel of figure 5b ) .
sorption isotherms ( left ) and bjh pore size distributions 
 ( right ) 
 of sno2tio2 nanoparticle ensembles before 
 ( a ) and after formation of heterointerfaces
the sorption isotherms and pore size distributions 
 are shifted on the ordinate scale for 300 cmg and 3 cmgnm , respectively , for clarity .
we employed sem to characterize the composition 
 and mixing quality 
 inside the nanoparticle networks and used edx to track compositional 
 homogeneity changes in the range of few hundreds of nanometers .
a typical edx line scan on nanoparticle networks 
 obtained from surface charge - induced heteroaggregation in aqueous 
 formic acid solution ( c = 10 moll ) is shown in figure 6a and 
 reveals that the ratio of the two elements should remain constant 
 across the scanned distance ( gray line ) . the mixing quality and thus 
 the concentration of sno2tio2 heterojunctions 

is expected to be significantly higher than in the water processed 
 networks . here ,
deviations of the desired ti to sn ratio point to 
 a nonuniform distribution of the two elements ( gray line in figure 6b ) and therefore to the low mixing quality of the 
 network .
characteristic edx linescans of tio2sno2 networks obtained by heteroaggregation in ( a ) aqueous formic 
 acid solution ( c = 10 moll ) and ( b ) water . scanned 
 paths are shown in the sem insets .
the black and red lines show the 
 distribution of ti and sn within the scanned distance .
the here presented procedure provides a very good 
 measure of the 
 local mixing quality within the nanoparticle networks . to obtain statistical 
 meaningful information throughout different nanoparticle network samples
, 
 we evaluated the deviation of the desired ratio of ti / sn for approximately 
 600 sample spots ( figure 7 ) .
deviations of 
 0% indicate that the chosen value of one tio2 particle 
 per sno2 particle has been achieved .
it can be shown that 
 the nanoparticle network that originates from processing in pure water 
 displays a very broad distribution of ti / sn deviation , while the network 
 which was prepared in aqueous formic acid exhibits excellent mixing 
 quality , i.e. , relatively small deviations from desired ti / sn ratio .
deviations of the desired ti / sn ratios within line scans 
 of several micrometers across the particle networks are plotted in 
 the diagram .
the samples were prepared in c = 10 moll aqueous formic acid 
 solution ( hcooh / high mixing quality ) and water ( h2o / low 
 mixing quality ) .
a good mixing quality of sno2 and tio2 nanoparticle 
 mixtures represents a necessary requirement for a high concentration 
 of heterojunctions inside the resulting particle network . to verify 
 that these solid  solid interfaces are truly suitable for interfacial 
 charge transfer
, we spectroscopically probed the photoelectronic materials 
 properties . for this purpose we utilized epr to track the photogeneration 
 of surface
trapped hole centers o , unpaired electrons 
 in shallow trap states ti or adsorbed o2 ions ( supporting informations ) .
as demonstrated by previous studies 
 this technique provides site specific information about charge trapping 
 sites .
band gap and conduction band edge energies 
 determine the pathway of electrons or holes from one oxide to the 
 other ( figure 2 ) .
the relative positions of 
 the conduction band minima favor electron transfer from tio2 to sno2 . on the other hand , the valence band positions 
 of the two oxides are such that photogenerated hole centers become 
 accumulated on tio2 nanoparticles .
epr spectra of metal 
 oxide samples after 30 min uv / vis light exposure 
 ( pirr = 0.9 mwcm for the energy range 3.2 ev < e < 6.2 ev ) . 
 ( a ) simulated epr spectrum of o and ti centers on tio2 nanoparticles .
( b ) experimental epr spectrum 
 of o and ti centers on tio2 nanoparticles .
all spectra 
 were acquired at t = 90 k and p < 
 10 mbar using a microwave power of pmw = 6.32 mw . in figure 8a
, a simulated 
 epr spectrum of 
 o and ti centers is shown in comparison 
 to an experimental spectrum ( b ) , which was acquired on dehydroxylated 
 tio2 nanoparticles after 30 min of uv / vis exposure under 
 high vacuum conditions .
the separation of photoexcited states ( eq 1 ) is followed by persistent trapping of electrons 
 and holes upon formation of ti and o centers , respectively ( eqs 2 and 3).123 the corresponding epr 
 spectrum of sno2 nanoparticles 
 shows no significant signals after irradiation and is therefore not 
 presented here .
after irradiation of a nanoparticle network 
 of low mixing quality , 
 the corresponding epr spectrum reveals the presence of o and ti stabilized on tio2 nanoparticles 
 ( figure 8c ) . however , the network prepared 
 from surface charge induced heteroaggregation in aqueous formic acid 
 solution shows a strong signal for o stabilized 
 on tio2 nanoparticles but no ti centers ( figure 8d ) .
their absence points to an efficient interfacial 
 electron transfer from tio2 particles to sno2 particles ( figure 2 ) upon formation of epr 
 silent states .
for corroboration of this assumption , we exposed the 
 samples after uv exposure to molecular oxygen ( p = 
 10 mbar ) . in the case of tio2 nanoparticles ,
photogenerated 
 electrons transfer to o2 and form paramagnetic superoxide 
 anions ( o2 ) ( eqs 4 and 5 ) , which remain stabilized on the particle 
 surface and give rise to characteristic epr signal signatures ( table 1 , figure 9a).45 epr spectra of preirradiated metal oxide samples 
 after oxygen exposure 
 ( p(o2 ) = 10 mbar ) .
( a ) epr spectrum of 
 o and o2 species 
 stabilized on mo - cvs tio2 nanoparticles .
the spectra were acquired at t = 90 k and p < 
 10 mbar using a microwave power of pmw = 0.2 mw . for better comparison
the redox potential of o2/o2 ( figure 2 gray dotted 
 line , e = 4.22 ev ) is slightly above 
 the conduction band minimum of sno2 ( ecb = 4.5 ev ) , which 
 inhibits the transfer of sno2 related conduction band electrons 
 to molecular oxygen and , thus , the formation of surface adsorbed o2 ions .
figure 9 shows epr spectra of the preirradiated metal oxide 
 samples after 15 min oxygen exposure at t = 90 k 
 and subsequent pumping to p < 10 mbar . in the case of tio2 nanoparticles ( a ) ,
additional epr resonances are observed on the network 
 of low mixing 
 quality ( b ) and attributed to o2 radicals , 
 which are stabilized on sno2 surfaces ( table 1 ) . for the network of high mixing quality , 

no o2 stabilized on tio2 are 
 observed in the epr spectrum ( c ) . from the absence of o2 ions we infer that all photogenerated electrons 
 are drained into sno2 ( figure 2 ) . 

this is different from tio2 , where photogenerated electrons , 
 irrespective of whether they localize in shallow trap states ( ti ) or whether they remain in the conduction band , readily transfer to molecular oxygen . for particle systems that were only in contact with pure water 
 and
obviously , related networks contain too many 
 regions where tio2 or sno2 particles are aggregated 
 in such a way that interfaces between identical metal oxide particles 
 types prevail .
corresponding solid  solid interfaces were found 
 to actually enhance the recombination of photogenerated charge carriers .
the quantitative analysis of persistently trapped 
 photogenerated charges on the three different powder samples shows 
 no concentration increase of stabilized hole centers ( o radicals ) for the water prepared network with low mixing quality 
 in comparison to the pure mo - cvs tio2 nanoparticles ( figure 10 ) .
however , a strong enhancement of the concentration 
 of o radicals was observed for the sample of high 
 mixing quality .
the metal oxide samples were irradiated 
 for 15 min ( pirr = 0.9 mwcm for the energy range 
 3.2 ev 
 < e < 6.2 ev ) at t = 90 k 
 and p < 10 mbar .
the quantitative analysis of the yield of photogenerated 
 charges 
 clearly shows that the adjustment of surface charge during particle 
 network formation allows for the achievement of high mixing qualities 
 and enables the realization of a high concentration of heterojunctions 
 that are vital for the separation of photogenerated electrons and 
 holes .
the here presented results clearly underline the necessity 
 to characterize and annihilate charge recombination on photoactive 
 materials .
surface charge - directed aggregation of different 
 types of particles 
 and the subsequent introduction of functional interfaces is a cheap 
 and versatile particle engineering approach to generate high concentrations 
 of heterojunctions inside particle - based devices for photochemical 
 application and energy production .
we intentionally introduced functional 
 particle interfaces between 
 two types of metal oxide nanoparticles and quantitatively determined 
 the yield of photogenerated and trapped charges . to yield blended nanoparticle systems hosting high concentrations 
 of heterointerfaces ,
it is vital to prepare nanoparticles that subsequently 
 can be easily deagglomerated in colloidal dispersion and to control 
 the process of interface formation via surface charge induced heteroaggregation . 

as shown in this study , mo - cvs - grown particles can be simply dispersed 
 by use of ultrasonic treatment to the single particle level .
controlled 
 interface formation was achieved via heteroaggregation , i.e. , via 
 the self - assembly of oppositely charged particles in colloidal dispersion 
 using formic acid for the adjustment of surface charge . in comparison 
 to mixed particle systems prepared in pure water via surface charge - directed 
 aggregation - prepared mixed particle systems show a substantially increased 
 yield for photogenerated hole centers .
the extremely high concentration 
 of sno2tio2 heterojunctions actually 
 gives rise to an enhanced cross section for the separation of photogenerated 
 charges , which results from an effective interfacial charge transfer 
 across the particle  particle interfaces .
 we explored the impact of interfacial property changes 
 on aggregation 
 behavior and photoinduced charge separation in mixed metal oxide nanoparticle 
 ensembles . 
 tio2 and sno2 nanoparticles were 
 synthesized by metal organic chemical vapor synthesis and subsequently 
 transformed into aqueous colloidal dispersions using formic acid for 
 adjustment of the particles surface charge . 
 surface charge - induced 
 heteroaggregation was found to yield blended nanoparticle systems 
 of exceptionally high mixing quality and , after vacuum annealing , 
 to extremely high concentrations of heterojunctions between tio2 and sno2 nanoparticles with dehydroxylated surfaces . 
 for tracking charge transfer processes across heterojunctions , 
 the 
 photogeneration of trapped charge carriers was measured with electron 
 paramagnetic resonance ( epr ) spectroscopy . on blended nanoparticles 
 systems with high concentrations of sno2tio2 heterojunctions , we observed an enhanced cross section for 
 interparticular charge separation . 
 this results from an effective 
 interfacial charge transfer across the interfaces and gives rise to 
 substantially increased concentrations of electrons and hole centers . 
 
 the here presented insights are key to the rational design of particle - based 
 heterojunctions and mesoporous nanoparticle networks and help to engineer 
 composite nanomaterials for photocatalysis and solar energy conversion . 
Introduction
Experimental Section
Results and Discussion
Conclusions