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Electrochimica Acta xxx (2014) xxx–xxx

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Electrochimica Acta

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hotoelectrochemical water oxidation at electrophoreticallyeposited WO3 films as a function of crystal structure and morphology

anuel Rodríguez-Péreza, Cecilia Chacónb, Eduardo Palacios-Gonzálezc,eonel Rodríguez-Gattornoa,∗, Gerko Oskama,∗

Department of Applied Physics, CINVESTAV-IPN, Antigua Carretera a Progreso km 6, Mérida, Yucatán, 97310, MéxicoDepartment of Nanomaterials, CICATA-IPN, Legaria 694, Col. Irrigación, Miguel Hidalgo, México, D.F. 11500, MéxicoDepartment of Microscopy, Instituto Mexicano del Petróleo (IMP), Eje Central Lázaro Cárdenas 152, Col. San Bartolo Atepehuacan México,.F. 07730, México

r t i c l e i n f o

rticle history:eceived 5 February 2014eceived in revised form 1 March 2014ccepted 5 March 2014vailable online xxx

eywords:ynthesis and characterization of WO3

lectrophoretic depositionhotoelectrochemical water oxidation WO3

rystal structures and morphologies

a b s t r a c t

Phase-pure WO3 materials with different crystal structures and morphologies are prepared using alcohol-ysis and partial hydrolysis under solvothermal conditions, and their promise for photoelectrochemicalwater oxidation is evaluated under simulated sunlight. The materials are obtained in the hexagonal,orthorhombic and monoclinic structures. Electrophoretic deposition under ambient conditions is usedto prepare mechanically stable and well-adhered films on transparent conducting glass substrates. Pho-toelectrochemical characterization of the films illustrates that sufficient electrical interconnectivity isachieved; however, a heat treatment at 500 ◦C dramatically improves the photocurrent, which is relatedto the passivation of defects and traps of the WO3 materials. The orthorhombic and monoclinic materials,the latter depending on the morphology and texture, achieve the highest photocurrents correspondingto water oxidation, while the hexagonal material has the lowest photocurrents. The novel orthorhombicmaterial prepared in this work shows good promise for water oxidation, also because of the low density ofelectron traps as inferred from experiments under modulated illumination using a chopper. The proper-ties of the monoclinic material depend on the morphology, in particular the texture. The electrochemicalproperties for a rod morphology with texture defined by the (002) or (020) reflections are characterized
by lower photocurrents and a higher trap density, while a square platelets type of morphology with atexture characterized by the (002) reflection results in the highest photocurrents. The results indicatethat electrophoretically deposited films of WO3 materials with shapes corresponding to two or threedimensions have better photoelectrochemical properties than one dimensional shapes such as rods andwires.
© 2014 Elsevier Ltd. All rights reserved.

. Introduction

Global climate change and environmental problems related tohe overexploitation of fossil fuels have resulted in an urgenteed for the development of sustainable energy technologies. Solarnergy is the most abundant resource, and the conversion ofunlight to electricity in photovoltaic devices is a fast-growing

Please cite this article in press as: M. Rodríguez-Pérez, et al., Photoelectfilms as a function of crystal structure and morphology, Electrochim. A

echnology [1–3]. On the other hand, clean fuels produced fromenewable energy sources are needed for off-grid energy needs,ncluding transportation. Hydrogen may be the most promising

∗ Corresponding authors. Fax: +52 999 9812917.E-mail addresses: [email protected] (G. Rodríguez-Gattorno),

[email protected] (G. Oskam).

ttp://dx.doi.org/10.1016/j.electacta.2014.03.022013-4686/© 2014 Elsevier Ltd. All rights reserved.

clean fuel, however, efficient and low-cost hydrogen generationtechnologies based on solar energy are still elusive [1,4–6].

The concept of water splitting by photoelectrolysis was firstdemonstrated by Fujishima and Honda in an electrochemical cell,where titanium dioxide (TiO2) in the rutile phase was used to splitwater under sunlight illumination [7]. Although the efficiency wasrather low and a pH bias was used to assist the process, the ben-efits of having a material that was able to perform direct waterphotoelectrolysis mobilized the scientific community, providingthe starting point for a strong research effort on photoelectro-chemical cells [8]. Work in this area has been primarily focused

rochemical water oxidation at electrophoretically deposited WO3cta (2014), http://dx.doi.org/10.1016/j.electacta.2014.03.022

on the utilization of TiO2, related to its easy synthesis and excellentelectrochemical stability [9]. The function of the semiconductingmaterial is to absorb sunlight, resulting in the creation of an elec-tron in the conduction band with sufficient energy to reduce water

dx.doi.org/10.1016/j.electacta.2014.03.022


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nd a hole in the valence band with sufficient energy to oxidizeater. The thermodynamic minimum energy needed for water

plitting is 1.23 eV, however, for each reaction an overpotential iseeded in order to achieve efficient water splitting. The optimumand gap for a water splitting semiconductor can be estimated toe about 1.75 eV, assuming that the sum of the overpotentials forater reduction and oxidation at a sufficiently high current den-

ity is about 0.52 eV. TiO2 has a band gap of 3.2 eV [10] and onlybsorbs the UV portion of the solar spectrum (about 4%), whichignificantly limits the attainable water splitting efficiency. Hence,

strong effort has been devoted to finding an ideal semiconductorhat meets all the requirements: visible light harvesting, ade-uate band edge positions, fast water oxidation and/or reductioninetics, and photoelectrochemical stability [11]. Several materi-ls have shown certain promise for water decomposition underisible light, including SrTiO3 [12], Fe2O3 [13,14], WO3 [15–19],uNb3O8 [20,21], Cu5Ta11O30 [22], CuBi2O4 [23], as well as com-inations of these materials, such as Fe2O3/TiO2, Fe2O3/WO3 [8],uBi2O4/TiO2 [24], CuBi2O4/WO3 [25], among others [1,26–29].ue to the large number of possible materials, the use of combina-

orial methods to scan more complex oxides and other materials forheir water splitting promise has gained momentum in recent years11,23,30,31]. In addition, electrocatalysts can be used to speed uphe charge transfer kinetics thus minimizing reaction overpoten-ials and increasing the conversion efficiency [2,32].

In addition to the search for the perfect material for water split-ing, it has been shown that the combination of two light absorbing

aterials in a tandem-cell scheme may be a good option in ordero achieve efficient water splitting [3,19]. By a judicial choice of theand gap energies of both materials the conversion efficiency cane larger than for a single junction system, and the reduction andxidation processes can be optimized separately, allowing for moreexibility in materials design. In general, the aim is to use blue toreen light absorption in one material, coupled with red to infraredight absorption in the second material: the different absorptionnergies allow the materials to be stacked, as long as reflectancend scattering losses in the top layer are minimized.

Tungsten trioxide (WO3) is an n-type material with a band gapf 2.5-2.7 eV, which shows photocatalytic activity in acidic condi-ions with excellent resistance to photocorrosion [17]. The band gapepends on a variety of parameters, including the crystal structurend water content, and has been found to depend on the synthesisethod. The conduction band position is generally found to be at

lightly positive potentials with respect to the hydrogen evolutioneaction, however, the valence band edge potential is generally atore than 1 V more positive than the water oxidation potential.ence, WO3 is an excellent photoanode material to oxidize water

o O2 [1] and the reaction kinetics can be improved with the appro-riate catalyst [32]. The material also has good charge transportharacteristics, with a Hall electron mobility of ∼12 cm−2 V−1 s−1

t room temperature [33].Different synthesis methods have been reported for WO3 micro

nd nanoparticles, including thermal evaporation [34], reverseicelle routes [35], chemical and physical vapor deposition (CVD

nd PVD) [36–38], solvothermal and hydrothermal reactions39–42], electrochemical techniques [43–45], solution-based col-oidal approaches, template-directed synthesis [46,47], and sol-gel

ethods in general [48–52]. The solvothermal process is an eco-omical, attractive method that allows for a good control over theorphological properties of nanostructured materials. In general,

he synthesized materials need to be immobilized for application in water splitting system and there are many different techniques

Please cite this article in press as: M. Rodríguez-Pérez, et al., Photoelectfilms as a function of crystal structure and morphology, Electrochim.

o produce WO3 films, such as CVD and PVD, spin coating, sprayyrolysis, and colloidal nanoparticle approaches, including screenrinting [53]. Electrophoretic deposition (EPD) is an importantechnology for colloidal coating processes where charged colloids

PRESSmica Acta xxx (2014) xxx–xxx

or particles suspended in a fluid are moved towards an electrodeunder the influence of an electric field, forming a deposit uponreaching the electrode. Positively charged particles will migratetowards the negative electrode (cathodic EPD), in contrast, neg-atively charged particles will be attracted by the positive electrode(anodic EPD) [54,55]. EPD is an easy, reliable deposition method,and the deposition WO3 onto fluor-doped tin oxide (FTO) and tin-doped indium oxide (ITO) conductive glass substrates has beenreported for electrochromic applications [16,56]. The deposition ofWO3 using low AC-EPD on alumina for gas sensing applicationshas also been described [57]. It has been reported that prepa-ration of WO3 films is generally achieved by anodic EPD usingsub-stoichiometric WO3 nanomaterials (blue powder), while theuse of stoichiometric WO3 (yellow powder) did not result in filmformation [16].

In spite of the extensive research on WO3, the influence ofcrystal structure and morphology on the photoelectrochemicalactivity of WO3 has not been systematically studied. In the presentwork, we present a simple methodology for the synthesis of WO3based on alcoholysis and partial hydrolysis of WCl6 in ethanolassisted by solvothermal conditions. By tuning the experimentalparameters, WO3 was crystallized in its three main crystal struc-tures, monoclinic, orthorhombic, and hexagonal, also controllingthe morphology. Constant voltage EPD under ambient temper-ature conditions was used to prepare photoelectrodes of eachphase on transparent conductive oxide substrates (FTO), in orderto avoid phase transformations during the film deposition process.The films obtained were photoelectrochemically characterized bycyclic voltammetry and chronoamperometric techniques in orderto establish the influence of the crystal phase and morphology onthe oxygen evolution reaction.

2. Experimental Section

2.1. Synthesis of WO3 phases

The tungstite WO3•H2O precursor material was obtainedfrom tungsten hexachloride (WCl6, Aldrich ≥ 99.9%) and absoluteethanol (CH3CH2OH, J.T. Baker ≥ 99.7%). The ethanol was dried overcalcium oxide (CaO, Sigma–Aldrich ≥ 99.9%) before each exper-iment. A series of experiments was performed in which theconcentration of WCl6 in ethanol, the amount of water in theethanol–water mixture, and the solvothermal treatment time weresystematically varied. In a typical synthesis, a stock solution of 0.2 MWCl6 was prepared in 50 mL of absolute ethanol with moderatestirring at room temperature. After 5 minutes, the desired quantityof the starting solution was diluted into 50 mL ethanol–water solu-tion at the desired water content with vigorous stirring, resultingin the quick formation of a dark blue colloid. The colloidal solutionwas kept stirring for 2 hours, after which 15 mL was loaded intoa Teflon–lined autoclave and heated to 180 ◦C in a furnace, withvarying treatment times from 48 to 72 hours. The final concentra-tion of WCl6 in the ethanol–water mixture was varied from 0.05 M,0.025 M to 0.0125 M following the above mentioned synthesisprocedure. The reaction conditions of selected samples are summa-rized in Table 1. When the solvothermal treatment was completed,the material was separated from the solution by centrifugation, andwashed three times with water-ethanol (1:1) solution to removeions and organic residues. Finally, the products were dried at 60 ◦Cfor 6 h.

rochemical water oxidation at electrophoretically deposited WO3Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.03.022

2.2. Electrophoretic deposition (EPD)

For the EPD process, 6 mg of WO3 powder was dispersed in50 mL of 30 vol.% water in isopropanol (H2O, 18 M�•cm resistivity,


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Table 1Different WO3 crystal structures and morphologies, obtained by alcoholysis and partial hydrolysis of WCl6 in ethanol assisted by solvothermal conditions.

[WCl6] (M) in ethanol Time (hrs) Ethanol/Water (% v/v)] Morphology JCPDS # crystal structure

0.1 48 10/90 Rods 43-1035 monoclinic72 Elonged sheets 43-1035 monoclinic

0.0125 48 Square platelets 43-1035 monoclinic0.025 72 Mixed square platelets and rods 43-1035 monoclinic

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H3CHOHCH3 J.T. Baker ≥ 99.88%) using a titanium tip sonicatort 50% of its total power (130 W) for 15 min. Both the anode andhe cathode consisted of SnO2:F-coated glass substrates (FTO, TEC5 �/sq, Pilkington), and the electrodes were separated by 5 mm.O3 films were obtained by applying 300 V for 1 h. A magnetic

tirrer was used during the process to prevent sedimentation ofhe WO3 material. The circular deposition area with a diameter of

cm was delimited with Teflon tape on both electrodes.

.3. Characterization techniques

An Ocean Optics (USB 2000) fiber optics spectrophotometer wassed for UV–Vis reflectance measurements. The reflectance spec-ra were analyzed using the Kubelka–Munk formalism to converteflectance into the equivalent absorption coefficient, �K-M, usinghe following expression: �K-M = (1-R)2/2R, where R is the mea-ured diffuse reflectance and the scattering coefficient is assumedo be equal to 1 [58]. The crystal phase and texture of WO3 materialsere analyzed with a Siemens D-5000 diffractometer with Cu-K�

adiation at 34 kV and 25 mA; the diffraction patterns were col-ected from 10◦ to 70◦ (2�) with a 0.02◦ step size and 2 s integrationime, with the films at a 3◦ inclination. SEM measurements werearried out using a scanning electron microscope (JEOL JSM-7600F)perated at an accelerating voltage of 5 kV and using the lower and


pper secondary electron image (LEI and SEI, respectively) as imag-ng modes. For high-resolution transmission electron microscopyHRTEM) studies the samples were dispersed in ethanol, and a dropf the suspension was placed on a holey carbon covered Cu grid.

ig. 1. Flow chart illustrating the main steps in the synthesis of WO3 particles with darameters.

Irregular platelets 35-0270 orthorhombicSpheres 43-1035 monoclinicRods 85-2460 hexagonal

The observations were made in a JEOL 2010F Transmission ElectronMicroscope using an accelerating voltage of 200 kV. The thicknessof the electrophoretically deposited films was determined usinga KLA Tencor D-120 profilometer. Photoelectrochemical experi-ments were performed using front side illumination and wereconducted in a single compartment cell adapted with a quartzwindow, using the three electrode configuration: the WO3 film onFTO as working electrode, a Ag/AgCl/3 M NaCl reference electrode,and a platinum wire as counter electrode. Cyclic voltammetry andchronoamperometry were performed in a 1 M HClO4 electrolytesolution adjusted to pH 5 with NaOH, using an Autolab Potentiostat(PGSTAT302 N). The photocurrent potential curves were recordedusing simulated sunlight obtained from a 400 W ozone-free Xe arclamp (Newport 66921) and an AM 1.5G filter. The light intensity atthe sample position was adjusted to 100 mW cm−2.

3. Results and discussion

3.1. Synthesis and characterization of WO3

Fig. 1 illustrates the solvothermal method developed for thesynthesis of WO3 with control over the crystal structure andmorphology of the product. In general terms, WCl6 was dis-solved in ethanol, and an orthorhombic hydrated tungsten trioxide


(WO3•H2O tungstite) material was obtained, which was subse-quently redispersed in a water/ethanol mixture and submitted toa solvothermal treatment at 180 ◦C in a Teflon-lined autoclave.Depending on the water content and treatment duration, different

ifferent crystalline phases, and the relation between crystal phase and synthesis



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Fig. 2. Representative TEM images of each WO pure phase. The (a) column shows a low magnification micrograph of the corresponding phase, (b) a high magnification TEMm

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icrograph, and (c) calculated FFT from a high resolution TEM image.

rystal structures and morphologies were obtained. The systematicodification of the synthesis parameters resulted in many differentixtures of structures and morphologies; only phase-pure samples

ccording to X-ray diffraction (XRD) measurements were selectedor the present work. Additionally, a variety of morphologies werebtained for the materials that crystallized in the monoclinic phase,llowing for the exploration of the dependence of the photore-ponse on morphological features. The most important samples andhe experimental conditions under which they were obtained areummarized in Table 1.

As can be observed from Table 1, when the water content is highhe products tend to crystallize in the monoclinic structure. Forhe monoclinic phase, a variety of morphological features can bebtained depending on concentration and solvothermal treatmentime: in this case, particles are generally obtained as small rods thatith time tend to convert in elongated sheets and square platelets.

In Fig. 2, high-resolution transmission electron microscopyHRTEM) images are shown of each of the WO3 phases at differ-nt magnifications, as well as the fast-Fourier transform (FFT) of theigh-resolution image. A detailed study of the FFT in several micro-


raphs for each phase allowed establishing the preferential growthirection of the particles and the predominant surfaces for thehree crystal phases. As can be observed in the low-magnification

icrographs, the particles tend to be elongated and the preferential

growth directions are [010] for the monoclinic phase, [001] for theorthorhombic structure, and [110] for the hexagonal phase. Theseresults are corroborated by X-ray diffraction.

Fig. 3 shows the results from X-ray diffraction, UV-Vis spec-trophotometry and a photograph of the three phase-pure crystalstructures obtained using the solvothermal synthesis method.The monoclinic material was prepared from 0.1 M WCl6 in 90/10water/ethanol solution, and treated at 180 ◦C for 48 hrs. Theorthorhombic material was synthesized using 0.05 M WCl6 in 10/90water/ethanol solution, and treated at 180 ◦C for 72 hrs. A verylow intensity reflection at 23.5◦ appears to be related with smallquantity of monoclinic phase within the orthorhombic structure.The hexagonal material was prepared from 0.05 M WCl6 in 100%ethanol solution, treated at 180 ◦C for 48 hrs.

Two main features can be distinguished in the UV-Visabsorbance spectra of the three phases, which were obtaineddirectly from experimentally measured reflectance spectra. Theabsorbance band in the visible part of the spectrum can beattributed to the presence of W5+ or W4+ sites related to an oxy-gen deficiency or to ion intercalation in WO3 [59–62], while the


band in the UV region is related to the electronic transition fromthe valence band to the conduction band. Due to the superpo-sition of these two absorbance bands, it is difficult to estimatethe band gap of the materials. In an attempt to separate both


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Fig. 3. (a) X-ray diffraction patterns of the phase-pure WO3 materials prepared: monoclinic phase (space group P21/n); orthorhombic phase (space group F/mm2); and hexagonal phase (space group P63/mcm). (b) UV-Vis diffusereflectance spectra for each crystal phase; the shadowed area corresponds to the inter-band electronic transitions and the gray lines correspond to the deconvoluted contributions. c) Photographs of the synthesis products inpowder form.




Fig. 4. SEM micrographs of electrophoretically deposited WO3 films with different crystal structures: (a) monoclinic, (b) orthorhombic, (c) hexagonal. On the right side ofe at 500fi

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lms of each phase before and after heat treatment at 500 ◦C.

ontributions, a deconvolution was performed using Gaussianunctions, the results of which are shown as gray lines in Fig. 3b.he shadowed areas roughly correspond to the inter-band transi-ions and the estimated apparent optical gaps for the three phasesre 2.71 eV for the monoclinic phase, 2.42 eV for the orthorhom-ic material and 2.72 eV for the hexagonal phase. The absorbanceands in the visible are responsible for the strong brown and blueoloration of the synthesis products, as illustrated in the pho-ographs in Fig. 3.

.2. Electrophoretic film deposition and characterization

In order to be able to relate the electrochemical propertiesf the three phase-pure WO3 materials to the crystal struc-ure, electrophoretic deposition from a dispersion of WO3 inater-isopropanol at room temperature was used to prepareell-adhered films. Ultrasonic treatment of the dispersion waserformed before deposition, which resulted in the break-upf larger agglomerates into smaller units; however, the overallorphology of the materials was not affected by the ultra-

ound procedure. The deposition of uniform sub-stoichiometricblue, brown) and stoichiometric (yellow) films was achievedy an anodic EPD process, while amorphous WO3 was simul-aneously deposited on the cathode. The amount of amorphous

O3 deposited appeared to depend on the coloration of theowder used for the dispersion, resulting in more material forhe blue powders than for the yellow and brown powders;his interesting observation could be related to the larger frac-


ion of tungsten bronzes incorporated in the blue powder asompared to the yellow and brown powders. SEM images atower magnification are shown in Figure S1 in the Support-ng information illustrating the smoothness and homogeneity

◦C. The plots on the right show the corresponding X-ray diffraction patterns of the

of the films. One of the possible drawbacks of EPD is that theelectronic interconnectivity of the film is not optimal, related tothe low temperature of the process, which could significantly affectthe electrochemical properties of the photoelectrodes. On the otherhand, a heat treatment is often applied in order to passivate defectssuch as oxygen vacancies and electron traps. Hence, the EPD filmswere evaluated both as-prepared and after sintering the films at500 ◦C.

Fig. 4 shows scanning electron microscopy (SEM) images, pho-tographs and X-ray diffraction patterns of the electrophoreticallydeposited films of the three phases. The X-ray diffraction spectrashow that the deposition process conditions do not affect the crys-tal structure of the WO3 material. The morphology of the WO3particles also did not change significantly upon deposition: themonoclinic material consisted of rods, the orthorhombic materialof platelets, and the hexagonal material of small rods. In the lat-ter case, the sonication process used to disperse the powder inthe water-isopropanol mixture resulted in the break-up of largerrods of the synthesis product into smaller rods. The X-ray diffrac-tion patterns in Fig. 4 illustrate the effects of sintering at 500 ◦Con the crystal structure and morphology of the films. The sinteringprocess in some cases generated a better definition of the crys-tal phases, for example, for the monoclinic films an increase inthe peak intensity corresponding the (020) and (200) planes wasobserved. For the orthorhombic phase, the signal at 23.1◦ ascribedto the (002) plane presented a small shoulder, possibly due thestart of a transition to the monoclinic structure at this tempera-ture. In the hexagonal phase the relative intensities of the signals at


13.9◦ and 28.1◦ decrease, with the formation of new peaks at 23.1◦,23.6◦, 24.4◦, 33.4◦, 33.6◦ and 34.2◦, which can be assigned to the(001), (020), (200), (021) and (220) planes of the monoclinic phase.Hence, it is concluded that the hexagonal structure is not stable at


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rthorhombic, (c) hexagonal, the inset in each plot shows the Tauc plots used tonalyze the absorption edge for indirect and direct transitions; the estimated valuesor the direct and indirect band gap are indicated (Egd and Egi, respectively).

he sintering temperature of 500 ◦C, and that a partial transition toonoclinic structure take place creating a mixed-phase material,hich is denominated as h-monoclinic.

The UV-visible absorbance spectra of the samples after theeat treatment in Fig. 5 show that inter-band electronic transi-ions are better separated from contributions in the visible regime,s compared to before heat treatment (Fig. 3b). This behavior islso corroborated by simple observation of coloration changes inhe films as shown in Fig. 4: upon sintering all films turn yellow,orresponding to stoichiometric WO3. Hence, after sintering, theefects responsible for the blue-brown coloration of the materialsave been mostly removed [63], which allows for a more preciseetermination of the band gap values from Tauc-plots using theubelka-Munk formalism as shown in the insets in each plot for theifferent phases in Fig. 5. The optical response of the monoclinichase is the best known among the tungsten oxide polymorphs,otwithstanding the reported band gap values appear to be scat-ered over a wide range of energies ranging from 2.5 - 3.2 eV,hich probably results from the extensive variety of parameters

hat can affect the optical response of the material. The monoclinichase has been experimentally classified as an indirect semicon-uctor [17,18,33,34,52,61]. On the other hand, theoretical quantumredictions are somewhat contradictory regarding to the directr indirect nature prevalence at the absorption onset (h� > Eg)60,64–67]. It has also been argued that there is mixed character


t the absorption onset, related to the very small estimated energyifference between the direct � - � and indirect Z - � transitions≈ 0.01 eV) [68]. In Fig. 5, the Tauc plots for indirect and directransitions show that linear behavior is observed for both types

PRESSmica Acta xxx (2014) xxx–xxx 7

of transition; the measured transition energies are 2.60 eV and2.81 eV for indirect and direct transitions, respectively. The opticalproperties of hexagonal phase after heat treatment are essentiallythe same as those of the monoclinic material related to its par-tial phase transformation. The estimated gaps from Tauc plots are2.65 and 2.85 eV for indirect and direct transitions, respectively.The orthorhombic material obtained in this work corresponds bestto WO3•0.33H2O and has not been reported much in the litera-ture; hence, as far as we are aware, the experimental or theoreticalband gap related with this specific orthorhombic phase have notbeen reported previously. Notice that in this case the dependenceof the absorption coefficient with energy follows a linear behavioronly for the analysis of direct optical transitions; the band gap inthis case is 2.78 eV, which is slightly smaller than that obtained formonoclinic phase.

3.3. WO3 photoelectrochemistry

Fig. 6 shows the current density - potential (j,V) curves of theelectrophoretically deposited WO3 films with different phases in1 M HClO4 adjusted to pH 5 using NaOH. In the left panel of Fig-ure 6, the (j,V) curves of the as-deposited films prior to sinteringin the dark and under illumination are shown, while the middlepanel shows the curves after the heat treatment at 500oC. In thedark, the (j,V) curves in Fig. 6a present a cathodic current at poten-tials negative of about 0.2 V (vs. Ag/AgCl) for the monoclinic phase,which can be attributed to tungsten bronze formation by hydro-gen intercalation [69]. This process is highly reversible, and on thereturn scan a large oxidation peak is observed. On the other hand,the onset potential for the reduction process for the orthorhombicmaterial is significantly more negative at -0.2 V (vs. Ag/AgCl) and, inaddition, the current hysteresis was observed to be much smalleror absent. Similarly, the results for the h-monoclinic material indi-cate a more negative onset potential and smaller hysteresis thanfor the monoclinic material. The latter observation suggests thatthe morphology of the monoclinic material affect the electrochem-ical properties, as the material in Fig. 6a consists of long rods withpreferential orientation of [020], while the h-monoclinic materialconsists of much smaller rods, and much less-defined preferentialorientation (see Fig. 4).

These results clearly illustrate the influence of phase and mor-phology on the electrochemical properties; further research is inprogress to better understand and inter-relate these phenomena.The anodic current in the dark is small, as expected for an n-typesemiconducting material. At about 1.4 V (vs. Ag/AgCl) the anodiccurrent sharply increases related to water oxidation; the anodicdark current onset is essentially the same for all materials, and isalso observed on bare FTO; however, the current is significantlylarger for the WO3 materials than for bare FTO indicating thatthe larger surface area of the materials allow for a larger oxida-tion current. These results, together with the observation that thedark (j,V) curves for the as-deposited and sintered films are verysimilar, indicate that the electrical connectivity of the films is suf-ficiently established with the EPD process at room temperature.The films were mechanically stable and produce repeatable resultsupon extensive, repeated electrochemical cycling. Under illumina-tion the (j,V) curves of the as-deposited films essentially overlay thecurves obtained in the dark, hence, a photocurrent is not observed.

For the sintered films, the cathodic regime is generally unaf-fected under illumination, while a photocurrent is observed atpositive potentials. Fig. 6a shows that the photocurrent onset forthe monoclinic phase cannot be determined, as the photocurrent


overlaps the dark re-oxidation current, while for the orthorhom-bic (Fig. 6b) and h-monoclinic materials (Fig. 6c), the photocurrentonset potential is at around 0.2 to 0.3 V (vs. Ag/AgCl). The photocur-rent corresponds to the oxidation of water to oxygen, which at pH




-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

μ

μ

μ

μ

μ

μ

Fig. 6. Current density - potential (j,V) curves for the electrophoretically deposited films of the three WO3 phases, before (left panel) and after (middle panel) sinteringa handc correA y elec

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t 500 ◦C, in the dark and under illumination, at a scan rate of 5 mV s−1. The righthopper at 1.0 V (vs Ag/AgCl), for both as-deposited and sintered films. The figuresll measurements were performed in 1 M HClO4 with pH adjusted to 5, at stationar

has a Nernst equilibrium potential of 0.73 V (vs. Ag/AgCl). Hence,he photocurrent onset is about 0.5 V negative with respect to thequilibrium potential, illustrating the excellent photo-oxidationroperties of the WO3 materials. In the case of the h-monoclinicaterial, and in a lesser proportion the monoclinic material, an

nodic photocurrent maximum is observed, which is interpreteds inhibition related to peroxide species absorbed at the WO3urface [70]. Interestingly, this effect was not observed with therthorhombic films. For all materials, an intensity-limited pho-ocurrent plateau is observed between about 0.6 V (vs. Ag/AgCl)nd 1.4 V (vs. Ag/AgCl). It is important to stress that the illumina-ion used for these experiments corresponds to the solar spectrumt 100 mW cm−2 (1 sun) with AM 1.5G spectrum, in order toetermine the suitability of the materials for solar water splittingystems. From the optical properties of the sintered films, it is clearhat essentially only light with energy equal or larger than the bandap of about 2.7 eV (corresponding to wavelengths smaller thanbout 460 nm) contribute to the photocurrent observed in theseystems, in agreement with previous reports [17,18,71]

The panel on the right hand side in Fig. 6 shows the currentensity at 1 V (vs. Ag/AgCl) under illumination modulated with ahopper, for both as-deposited and sintered EPD films. A small,ut notable photoresponse was observed for the as-deposited filmsefore sintering (blue lines), which is shown in more detail inigure S2 in the Supporting Information. The photocurrent dramat-cally increases after the films have been sintered at 500 ◦C. Theseesults contrast the dark (j,V) curves, where the current densitiesere large, and essentially independent of sintering. Hence, we can

nfer that there is sufficient electrical interconnectivity for the dark


urrent, but that the photocurrent for as-deposited WO3 materi-ls is very low. Since the as-deposited films consist of generallyub-stoichiometric WO3 materials that transform to stoichiometricaterials upon sintering, it can be concluded that the high density

panel shows photocurrent measurements under modulated illumination using aspond to: (a) monoclinic (b) orthorhombic and (c) h-monoclinic crystal structures.trodes, under 100 mW cm−2 illumination with an AM 1.5G filter.

of oxygen vacancies is most likely related to increased recombina-tion, thus suppressing the photocurrent.

The film thickness obtained under the same electrophoresisparameters was found to depend on the crystal phase. In addi-tion, the thickness decreased upon sintering, especially for themonoclinic materials; in the following sections, the film thick-ness determined after sintering is reported. For the monoclinicphase a film thickness of 4.1 �m was obtained, and a maximumphotocurrent under modulated illumination using a chopper at1 V (vs. Ag/AgCl) of 0.44 �A cm−2 and 10.3 �A cm−2 was observedbefore and after sintering, respectively. The orthorhombic filmshad a thickness of around 3.3 �m, and the photocurrent was0.19 �A cm−2 and 9.8 �A cm−2 before and after sintering. Thehexagonal films were obtained with a larger thickness than theother crystal phases at 6.3 �m, but the photocurrent before sinter-ing was lower than for the others crystal phases at 0.03 �Acm−2,in agreement with a recent report [71]. Unfortunately, the hexag-onal phase was not stable under the sintering conditions, and thephotoresponse of the h-monoclinic material is similar to that ofthe monoclinic films. The shape of the current rise and decay ofthe photoresponse are markedly different for the monoclinic andh-monoclinic materials as compared to the orthorhombic material.The slow rise and decay for the monoclinic materials can be inter-preted to be due to a large density of trap states, which are filledwhen the light is switched on, and slowly emptied upon switch-ing off the light. For the orthorhombic material, the photoresponseshape is much more square, indicating a lower density of trap states.Also, it should be noted that a current spike corresponding to fastrecombination after an initial large current is not observed for these


WO3 materials.The morphology dependence of the photoelectrochemical prop-

erties of monoclinic WO3 was investigated by controlling thesynthesis parameters such as concentration, water content and



M. Rodríguez-Pérez et al. / Electrochimica Acta xxx (2014) xxx–xxx 9

F WO3

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ig. 7. SEM images, color photographs and X-ray diffraction pattern, of monoclinicarge square platelets, (e) square platelets.

ime, as detailed in Table 1. Fig. 7 shows SEM images, color pho-ographs and X-ray diffraction pattern of electrodes prepared usingPD of 5 different monoclinic materials that present differenthapes and preferential orientations. As these X-ray diffraction pat-erns correspond to materials deposited on FTO substrates, theylso carry information on the texture of the films; hence, X-rayiffraction patterns of the same materials in powder form wereeasured and are shown in Figure S3 in the Supporting Infor-ation. In Fig. 7a, the SEM image shows that the material has aorphology of spheres with an 800 nm diameter, which consist

f small, randomly oriented rods of about 15 nm wide. Accordingo the powder X-ray diffraction pattern (Figure S3), this sampleoes not show any preferential orientation, however, the film pat-ern in Fig. 7 shows texturing that favors the (002) reflection.ig. 7b shows that the material consists of large rods, composed bymaller sub-units of the same morphology. The X-ray diffractionatterns for both the powder and the film show that this material

s highly anisotropic with a preferential orientation along [020].


n Fig. 7c, the X-ray diffraction pattern of the film shows someexturing, favoring the (020) and (200) planes, but the powder-ray diffraction pattern corresponds to a material without anyreferential orientation. The monoclinic material in Fig. 7d, on

with different morphologies: (a) large spheres, (b) large rods, (c) smaller rods, (d)

the other hand, consists of large, square platelets of about2 �m x 2 �m and about 150 nm thin, and both powder andfilm X-ray diffraction patterns indicate that the main orienta-tion is [002], hence, in this case the orientation agrees withtexture of the material. Finally, the monoclinic material inFig. 7e also has a square platelet type of morphology, althoughwith thicker plates, and the powder XRD pattern shows thesame orientation along [200] but with higher intensity in (002)reflection, however, after deposition the texture favors the (200)reflection. It should be noted that these details are important sincepreferential orientation is related to the distribution of crystalplanes exposed to the solution, while texturing upon deposition ona substrate could modulate device properties such as grain contactsin the film.

The photoelectrochemical properties of the sintered, elec-trophoretically deposited, monoclinic films of different morpholo-gies were determined using cyclic voltammetry and chromoam-perometry at 1.0 V (vs. Ag/AgCl). The cyclic voltammograms are


shown in Figure S4 in the Supporting Information, and the cleardifference in cathodic current onset potentials, reoxidation currentpeaks, and anodic photocurrent onset potentials can be observed.The chronoamperometry results presented in Fig. 8 clearly show a




Fig. 8. Photocurrent measurements of monoclinic WO3 electrodes with differentmorphologies: (a) large spheres, (b) large rods, (c) smaller rods, (d) large squareptc

spmrwrrtaecigrac(mrob1ta

cmt

Fig. 9. Chronoamperometric measurements of monoclinic WO3 films with prefer-

latelets, (e) square platelets, at 1.0 V (vs Ag/AgCl) in 1 M HClO4 with pH adjustedo 5, under 100 mW cm−2 illumination with an AM 1.5G filter modulated using ahopper.

ignificant dependence of the photocurrent response on the mor-hology of the monoclinic material. In general, the square plateletorphology generated a higher photocurrent than the various

od morphologies, while the lowest photocurrent was obtainedith the mixed spheres and rods morphology. In the case of the

od morphology with texturing favoring the (020) reflection, aeduction in the film thickness generates an increase in the pho-ocurrent density from 1.14 �A cm−2 to 3.59 �A cm−2, for 5.7 �mnd 4.9 �m, respectively; a film thickness of 4.1 �m resulted in anven higher photocurrent, however, the current transients indi-ate a large density of trap states for this morphology, as shownn Fig. 6a. It is known that the WO3 rod morphology has a largerain boundary area, which may result in an increase in theecombination rate between photogenerated electrons and holest the interface [71]. For the square platelets morphology, wean directly compare the photoelectrochemical properties of the002) and (200) texture reflections, and it is observed that the

aterial with the preferential orientation that favors the (002)eflection generates a larger photocurrent than the (200) reflection,f 9.33 �A cm−2 versus 2.69 �A cm−2, respectively. In general, it cane concluded that 2D materials have a better performance thanD materials, which highlights the importance of the detrimen-al effects related to grain boundaries and larger exposed surfacerea.


With the results presented in Fig. 8, it is also possible toompare the orthorhombic and monoclinic phases with the sameorphologies, same texturing properties and almost the same

hickness: square-platelets, 3.3 �m thick film, and (002) preferred

ential texture reflection at (200) and a square platelets morphology, as a functionof the film thickness. The inset shows the photocurrent density versus the filmthickness.

reflection (Fig. 6). In this case, the orthorhombic material generatesa slightly larger photocurrent density than the monoclinic mate-rial, of 9.8 �A cm−2 as compared to 9.3 �A cm−2, with good currenttransient characteristics in both cases related to the morphology.An increase in the thickness of the orthorhombic film to 4.0 �mled to an increase of the photocurrent to 10.9 �A cm−2, while areduction in the monoclinic film thickness to 2.9 �m resulted ina photocurrent of 12 �A cm−2, respectively. In general, it can beconcluded that both the orthorhombic and monoclinic materialshave good photoelectrochemical properties; the monoclinic phasehas been studied previously reported [72], however, the novelorthorhombic phase presented here presents very interestingcharacteristics for photoelectrochemical water oxidation, andfurther studies are in progress.

Fig. 9 shows the effect of film thickness on the photocurrent den-sity for the monoclinic films with a texturing that favors the (200)reflection and having a square platelet morphology. An optimumfilm thickness of 4.0 �m was obtained, illustrating the complexinterplay between increased light absorption and limited carrierdiffusion length. In addition, the optimum thickness is dependenton both the crystal structure and morphology of the material,illustrating the challenges and possibilities for the preparation ofmaterials with optimized photoelectrochemical properties.

4. Conclusions

Phase-pure WO3 materials with monoclinic, orthorhombic andhexagonal crystal structures were prepared from the alcoholysisand partial hydrolysis of WCl6 in ethanol, assisted by solvothermalconditions. A careful control over the synthesis parameters alsoallowed the synthesis of the monoclinic material with a varietyof morphologies. WO3 films were prepared by electrophoreticdeposition, and the influence of crystal structure, morphologyand preferential texture reflections of the materials on the pho-toelectrochemical properties were investigated. It was found thatEPD results in mechanically stable, well-adhered and electricallyinterconnected films, as evidenced by the dark (j,V) curves, butthat a heat treatment is essential in order to obtain photoelectro-chemical oxidation of water, which is related to the conversionof the sub-stoichiometric, high defect density WO3 materials to


stoichiometric WO3. The results show that a previously unstudiedWO3 phase with the orthorhombic crystal structure is a promisingmaterial for photoelectrochemical water oxidation. The results forthe monoclinic materials can be similarly promising, depending


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M. Rodríguez-Pérez et al. / Elec

n the morphology, in particular the texture. It was found thathe electrochemical properties for a rod morphology with textureefined by the (002) or (020) reflections are characterized by

ower photocurrents and higher trap density, while a squarelatelets type of morphology with a texture characterized by (002)eflections resulted in the highest photocurrents, indicating thedvantages of materials with shapes corresponding to two or threeimensions, as opposed to rods and wires.

cknowledgements

X-ray diffraction and SEM measurements were performed athe National Laboratory for the Study of Nano and Biomaterialst Cinvestav-Mérida. We would like to thank Daniel Aguilar, Dorauerta, José Bante and Beatriz Heredia for their technical help. Theuthors gratefully acknowledge funding of this work under grantsOMIX-Yucatán 170120 and Conacyt 193850.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.electacta.2014.3.022.

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http://dx.doi.org/10.1016/j.electacta.2014.03.022

http://dx.doi.org/10.1016/j.electacta.2014.03.022

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photoelectrochemical water oxidation at electrophoretically deposited wo3 films as a function of...

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