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Tailoring Interfaces for Electrochemical Synthesis of Semiconductor Films: BiVO4, Bi2O3, or Composites Noseung Myung,† Sunyoung Ham,‡ Seungun Choi,† Yujin Chae,† Whan-Gi Kim,† Young Jin Jeon,† Ki-Jung Paeng,‡ Wilaiwan Chanmanee,§ Norma R. de Tacconi,*,§ and Krishnan Rajeshwar*,§ †
Department of Applied Chemistry, Konkuk University Chungju Campus, Chungju, Chungbuk 380-701, Republic of Korea Department of Chemistry, Yonsei University, Wonju Campus, Wonju, Kangwondo 220-710, Republic of Korea § Center for Renewable Energy Science & Technology (CREST), Department of Chemistry & Biochemistry, University of Texas at Arlington, Arlington, Texas 76109-0065, United States ‡
ABSTRACT: The mechanistic aspects of a two-step method for the electrodeposition of a BiVO4 semiconductor (previously developed in the Rajeshwar/Tacconi laboratory) were elaborated by the combined application of voltammetry and EQCM. The electrosynthesized films were also characterized ex situ using SEM, EDX, XRD, and XPS. Stripping of pre-electrodeposited bismuth films, followed by reaction either with VO43 (formed by hydrolysis from the initially added VO3 species) or with hydroxide ions, produced BiVO4 or Bi2O3 thin films in situ on the Pt electrode. The deposition potential, pH of the electrolyte, and choice of vanadium precursor were shown to be crucial variables in the composition of the electrodeposited film. When a more positive potential than 0.5 V (vs Ag/AgCl reference) was applied to the Bi-modified electrode in VO3-containing electrolyte, the content of Bi2O3 in the film increased instead of BiVO4. Stripping efficiency of the predeposited bismuth layer was increased at acidic electrolytes and resulted in higher BiVO4 content in electrodeposited films, whereas hydrolytic conversion of VO3 to VO43 was promoted in basic electrolytes. Formation of Bi2O3 was also favored by the use of alkaline electrolytes (e.g., pH 10) for the electrodeposition. Photoelectrochemical experiments showed the electrosynthesized BiVO4 to be an n-type semiconductor, and reproducible photocurrents were obtained using a Na2SO4 supporting electrolyte.
1. INTRODUCTION The electrode/electrolyte boundary offers considerable versatility and scope for tuning the interfacial chemistry for synthetic purposes. For one, the tremendous electric field strength that exists across this interface (nominally several million volts/ centimeter) translates to an ability to precisely control the fluxes of reaction precursors.1 This feature, when coupled with the usual variables that are used to control reactions in homogeneous counterpart scenarios (e.g., solution pH), considerably enhances the scope of semiconductor electrosynthesis procedures. This study describes results that are illustrative of these ideas as applied to the BiVO and BiO reaction systems. The present study also builds upon our earlier studies on the electrosynthesis of FeS,2 InS,3 CdX (X = S, Se, or Te),46 and BiX (X = Se or Te)7,8 films on polycrystalline Au substrate. All these cases represent variations on a theme of controlling the interfacial reactant fluxes by either anodic or cathodic stripping of a precursor layer supported on a noble metal (Au or Pt) surface (Figure 1). Thus, we show below that the availability of V oxyanionic species and/or hydroxyl ions for reaction with the anodically generated Bi3þ species at the interface (which is controlled by the stripping potential) dictates whether the deposited semiconductor film consists of BiVO4, Bi2O3, or a r 2011 American Chemical Society
composite of both. As in our earlier studies,28 the combination of voltammetry and electrochemical quartz crystal microgravimetry (EQCM) furnished an effective in situ probe of the deposition processes. Bismuth vanadate (BiVO4) is an n-type semiconductor that has received considerable attention in recent years.9 This material is interesting from a variety of perspectives, including ferroelastic behavior and acousto-optical and ion conductive properties. Yellow pigments based on BiVO4 are also environmentally attractive “green” substitutes for lead-, chromium-, and cadmium-based paints especially from an ecotoxicological perspective and high performance (good gloss and hiding power).10 Most relevant to this study, however, are the excellent photoelectrochemical and photocatalytic characteristics of BiVO4 rendering this material useful for applications related to solar water splitting and environmental remediation.9 Bismuth oxide (Bi2O3) has also been intensively studied because of its interesting thermal and electrical transport properties. For example, the δ phase (which is stable only at high Received: January 20, 2011 Revised: February 28, 2011 Published: March 24, 2011 7793
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Figure 1. Generalized two-step strategy for the anodic (a) and cathodic (b) electrosynthesis of compound semiconductor film. Figure 3. Linear sweep voltammmogram (—) and the corresponding EQCM frequency change (---) for a bismuth-modified Pt electrode in (A) 0.1 M KNO3 containing 20 mM NH4VO3 (pH = 7) and (B) 0.1 M KNO3 (pH = 7). The arrows designate the initial potential scan direction.
Figure 2. Comparison of band edges for Bi2O3 and BiVO4 at pH 7.
temperatures) exhibits an extraordinarily high anionic conductivity of ∼100 S m1.11 As with BiVO4, this compound also exists in a variety of polymorphs. Unlike BiVO4 (where only n-type behavior is known), Bi2O3 can exhibit either n-type or p-type semiconductor behavior.12 Composites of BiVO4 and Bi2O3 would be of interest from an application perspective. Thus, the position of the conduction band edge in BiVO4 is too low (i.e., lies at too positive a potential) to be useful to drive the photoreduction of either protons (to H2) or CO2 (Figure 2). In fact, water splitting into H2 and O2 did not proceed without an external bias to compensate for the rather positive location of the BiVO4 conduction band.13,14 On the other hand, Bi2O3 does not suffer from this handicap (Figure 2).15 There is also literature precedence for the
enhanced photocatalytic attributes of composites, such as BiVO4/V2O5,16 BiVO4/WO3,17 or BiVO4/Co3O4,18 for photodriving useful reactions, such as water oxidation or organic compound (e.g., dye, phenol) decomposition. Very recently, the controlled synthesis of coreshell BiVO4@Bi2O3 microspheres was reported; these materials were claimed to show enhanced visible-light-responsive photocatalytic properties.19 Our electrosynthesis procedure can be simply tuned, as elaborated below, for preparing BiVO4, Bi2O3, or the composite depending on the targeted need. Although there is precedence for the anodic electrosynthesis of Bi2O3,12,2022 we are not aware of previous reports on the electrosynthesis of BiVO4 other than our own companion communication on this topic.23
2. EXPERIMENTAL SECTION Details of the electrochemical instrumentation and the EQCM setup along with its calibration are given elsewhere.7 An EG&G Princeton Applied Research (PAR) 263A instrument equipped with model M250/270 electrochemistry software was used for voltammetry and film deposition. For EQCM, a Seiko EG&G model QCA 917 instrument consisting of an oscillator module (QCA 917-11), a 9 MHz AT-cut Pt-coated quartz crystal 7794
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Figure 4. Ex situ XRD patterns for thin films electrodeposited on a bismuth-modified Pt electrode in 0.1 M KNO3 electrolyte containing (a) 20 mM NH4VO3 at 1.0 V for 600 s, (b) 20 mM NH4VO3 at 0.5 V for 600 s, and (c) no NH4VO3 at 1.0 V for 600 s.
(geometric area, 0.2 cm2) working electrode, a Pt counterelectrode, and a Ag/AgCl/3 M NaCl reference electrode was used. All potentials below are quoted with respect to this reference electrode. All chemicals were from commercial sources: bismuth chloride (purity, þ99.99%), potassium bromide (purity, þ99%), sodium sulfate (purity, þ99%), potassium nitrate (purity, þ99%), ammonium metavanadate (purity, þ99%), sodium hydroxide (purity, þ99.99%), sodium hydrogen carbonate (purity, þ99%), conc. nitric acid (70% v/v), and conc. hydrochloric acid (37% v/ v) were from Aldrich. All chemicals were used as received. A M€uller Elektronik-Optik tungsten halogen lamp was used as the light source. The light intensity measured on the electrode surface with a Newport model 70260 radiant power meter combined with a model 70268 probe was ∼100 mW/cm2 in all the experiments described below. Film morphology and composition were obtained on a field emission scanning electron
microscope (JEOL model 6700F) equipped with an energydispersive X-ray analysis (EDX) probe. X-ray diffraction (XRD) patterns were recorded on a Philips XPERT-MPD diffractometer with a Cu KR radiation source. X-ray photoelectron spectroscopy (XPS) was performed using instrumentation and procedures detailed elsewhere.24
3. RESULTS AND DISCUSSION 3.1. BiVO4 versus Bi2O3 Deposition: Effect of Deposition Potential and Electrolyte Composition. Shown in Figure 3A is
a linear sweep voltammogram (solid line) accompanied by the corresponding frequency change (as observed by EQCM, dashed line) for a Bi-modified Pt electrode in 0.1 M KNO3 containing 20 mM NH4VO3. The pH of the electrolyte was adjusted to 7 with 0.1 M NaOH solution, and the Bi thin film was previously prepared on a polycrystalline Pt electrode at 0.2 V for 10 s in 1 7795
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Figure 5. Chargemass plot obtained from coulometryEQCM data for a bismuth-modified Pt electrode subjected to an anodic scan in 0.1 M KNO3 (pH = 7) from (a) 0.38 to 0.52 V and (b) 0.55 to 0.76 V. Potential scan rate = 10 mV/s.
M HCl containing 0.02 M BiCl3 and 0.5 M KBr.23 The EQCM frequency decrease started at ∼0.5 V and continued until 2.0 V during the positive-going potentiodynamic scan of the Bi-modified electrode. This is diagnostic of mass gain and the electrodeposition of a film (identified below) on the Bi-modified Pt electrode. The anodic peak at 0.6 V on the voltammogram arises from bismuth oxidation to Bi3þ.12,2022 Contrasting with the monotonic frequency decrease in 0.1 M KNO3 electrolyte containing 20 mM NH4VO3, an anodic scan of the Bi-modified electrode in 0.1 M KNO3 blank electrolyte showed an initial frequency increase (mass loss) instead (Figure 3B). This mass loss coincides with the leading edge of the main voltammetric wave for bismuth oxidation, now appearing at 0.5 V, and may be assigned to the oxidative (anodic) stripping of Bi from the electrode surface. Beyond 0.55 V, the frequency reverses its tendency and continues to decrease (after the initial increase) while the potential is scanned down to ∼2.0 V where, ultimately, it tails off (and even reverses its trend). Therefore, both the electrochemical waves at 0.5 and 0.9 V are accompanied by a net frequency decrease (mass gain) on the EQCM trace, diagnosing that film deposition has occurred. To probe the composition of the films deposited under the conditions corresponding to Figure 3A,B, ex situ XRD was
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performed on specimens derived from a Bi-modified Pt electrode in 0.1 M KNO3 electrolyte containing (a) 20 mM NH4VO3 and held at 1.0 V for 600 s, (b) 20 mM NH4VO3 but held now at 0.5 V for 600 s, and (c) no NH4VO3 and held at 1.0 V for 600 s. The corresponding data are contained in Figure 4ac, respectively. Clearly, under the conditions corresponding to those in Figure 3A, both BiVO4 and Bi2O3 are electrodeposited at relative amounts depending on the deposition potential. Thus, if the potential is limited to 0.5 V, the film is exclusively BiVO4, as indicated by the diffraction lines at 15.2, 28.8, 46.5, and 47.2° (Figure 3A) (Powder Diffraction File: JCPDS 14-0688), and no oxide contamination is observed (Figure 4b). On the other hand, at 1.0 V, both compounds are electrodeposited on the substrate surface (Figure 4a) and XRD lines at 29.0, 31.0, and 54.5° point to R-Bi2O3 formation (JCPDS 76-1730). However, if no NH4VO3 is present in the electrolyte, only Bi2O3 is formed (Figure 4c). In all the cases in Figure 4, the XRD data also show the inevitable signals from the Pt substrate. 3.2. Mechanistic Aspects of BiVO4 and Bi2O3 Deposition. An electron stoichiometry of 3 is anticipated for the initial oxidation of Bi (to Bi3þ); as seen in Figure 5, this initial step is common to both compound deposition routes. Whether BiVO4, Bi2O3, or both are formed depends on (a) the availability of V oxyanionic species for precipitation with the anodically generated Bi3þ and (b) the availability of hydroxide anions for precipitation with the anodically generated Bi3þ. Thus, when the potential is very positive (e.g., 1.0 V, Figure 4a), more Bi3þ is interfacially electrogenerated than can be accommodated by the available vanadium species so that Bi2O3 is coprecipitated (reaction 4 below) along with BiVO4. On the other hand, a deposition potential of 0.5 V matches the interfacial fluxes of Bi3þ and the vanadium species so that BiVO4 deposition is optimized (Figure 4b). When the vanadium species are completely absent in the supporting electrolyte, the only possible reaction is the precipitation of Bi3þ with the hydroxide species (Figure 4c). Note that subsequent Bi2O3 formation is necessarily accompanied by the loss of water from the precipitated hydroxide compound. Note also that precipitation of BiVO4 (reaction 3 below) requires the orthovanadate (VO43) species to be generated via hydrolysis (reaction 1): 3 þ VO 3 þ H2 O S VO4 þ 2H
ð1Þ
EQCM offered useful mechanistic insights via computation of the electron stoichiometry number, n, or the number of electrons transferred per mole of species deposited. To this end, coulometry and EQCM can be combined:25,26 Q ¼ ðnFk=MÞΔf
ð2Þ
In eq 2, Q is the charge consumed (as assayed by coulometry), F is the Faraday constant, Δf is the frequency change (mass change) (as measured by EQCM), k is the Sauerbrey constant,25,26 and M is the molar mass of the deposit. Thus, n can be determined from the slope of a Q versus Δf plot. Such chargefrequency plots are contained in Figure 5 for anodic scans (at 10 mV/s) from 0.38 to 0.52 V (Figure 5a) and from 0.55 to 0.76 V (Figure 5b). From the slopes of the straightline plots, the n values were determined to be 2.81 (Figure 5a) and 5.5 (Figure 5b). Thus, an electron stoichiometry close to 3 signals an electrochemicalchemical process involving the initial electrogeneration of Bi3þ species from the preformed Bi layer. This step is then followed by the in situ precipitation of Bi3þ with 7796
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Figure 6. Linear sweep voltammogram (—) and the corresponding EQCM frequency change (---) for a bismuth-modified Pt electrode in 0.1 M KNO3 at (A) pH 5 and (B) pH 3. The pH was adjusted by adding 1 M HNO3 solution dropwise. The arrows designate the initial potential scan direction.
VO43 species generated from reaction 1: Bi3þ þ VO4 3 f BiVO4 ðsÞ
ð3Þ
On the other hand, Bi2O3 deposition entails an n value of 6 for the reaction:27 2BiðsÞ þ 6OH f Bi2 O3 ðsÞ þ 3H2 O þ 6e
ð4Þ
Indeed, the data in Figure 5b are consistent with a situation where the film deposition leads predominantly to Bi2O3 formation, in agreement with the XRD data in Figure 4c. 3.3. Effect of Solution pH and Vanadium Precursor. Clearly, solution pH would influence film deposition and compound formation according to reactions 1, 3, and 4 above. Further, the initial Bi stripping step is crucially dependent on solution pH because of solubility considerations. Figure 6 contains voltammetryEQCM data bearing on this aspect. For example, the amount of bismuth oxidized is appreciably increased as the pH of the electrolyte is decreased (cf. panel A vs panel B in Figure 6), which would have an effect on the amount of
Figure 7. Linear sweep voltammogram (—) and the corresponding EQCM frequency change (---) for a bismuth-modified Pt electrode in 0.1 M KNO3 containing 20 mM NH4VO3 at (A) pH 5 and (B) pH 3. The arrows designate the initial potential scan direction.
BiVO4 electrodeposited. Bismuth stripping is facilitated in the more acidic electrolyte, consistent with the higher solubility of bismuth in acidic solutions compared with neutral ones. Concomitantly, BiVO4 formation should also be favored by a lower pH. Thus, when the pH of 0.1 M KNO3 electrolyte containing 20 mM NH4VO3 is decreased to 5, the amount of BiVO4 electrodeposited is increased, as seen from a comparison of the EQCM frequency changes with the corresponding amount electrodeposited in an electrolyte of pH 7 (cf. Figures 3A vs 7A). However, this trend fails to hold when the pH is further decreased to 3 (Figure 7B); the amount of BiVO4 electrodeposited is now less than that at pH 5. Despite the stripping step being more effective at this pH, subsequent compound formation has clearly been compromised relative to the situation at pH 5. A clue to this anomaly may be sought in the confounding influence of the hydrolysis step, reaction 1. The orthovanadate species would be suppressed in favor of the corresponding metavanadate (VO3) species at lower pH, adversely affecting BiVO4 formation. In fact, the NH4VO3 is known to contain infinitely long chains of corner-sharing VO4 tetrahedra with the four oxygen atoms coordinated around the vanadium atoms. The VO4 chains are mutually bonded by means of electrostatic bonds with the NH4þ ions.28 The splitting of the chains by hydrolysis in an aqueous solution generates orthovanadate species and liberates 7797
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Figure 8. High-resolution X-ray photoelectron spectra in the Bi 4f (left), V 2p (center), and O 1s (right) binding energy regimes for BiVO4 thin film prepared from BiCl3/NH4VO3, pH 7 (a); BiCl3/Na3VO4, pH 10 (b); and BiCl3/V2O5, pH 10 (c), respectively. Data on a film prepared from an authentic sample of BiVO4 is presented for comparison in Figure 8d. 7798
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protons, as indicated in reaction 1.29 Therefore, a pH value higher than 7 is optimal for the reaction to proceed. The data in Figures 6 and 7 underline that there is an optimum pH (close to neutral) for the electrosynthesis of BiVO4 thin films in terms of balancing the Bi stripping efficiency and VO43 formation. However, this only applies to NH4VO3 as the V precursor. Next, the influence of the V precursor was probed. The rich speciation of vanadium solution chemistry, particularly for V5þ with VO2þ, VO3 (metavanadate), and VO43 (orthovanadate) species (not counting other numerous protonated complexes), prompted us to use NH4VO3, V2O5, or Na3VO4 salts as the V5þ precursor and vary the solution pH in the range from 7 to 14. The acidic pH range was avoided because of the factors outlined above and also because the derived films had rather low photoactivity, as elaborated later (but see Figure 9). For Na3VO4 solutions, the use of a basic pH (e.g., 10) minimizes generation of soluble H2VO4 species that are then able to form dimers30 2 2H2 VO 4 T H2 V 2 O 7 þ H2 O
ð5Þ
thereby decreasing the availability of VO43 species for reaction with the electrogenerated Bi3þ on the electrode surface. Similarly, when V2O5 is used as the V5þ precursor, the solution pH must be basic to dissolve the oxide as orthovanadate species: þ V 2 O5 þ 3OH f 2VO3 4 þ 3H
ð6Þ
High-resolution XPS spectra are contained in Figure 8 for the three precursors. The data for the film derived from NH4VO3 (pH 7) are reproduced here from our companion study23 for comparison purposes. As in ref 23, XPS data are also shown for an authentic sample of BiVO4 for comparison. All four samples in Figure 8 show the spinorbit splitting of Bi 4f7/2 and Bi 4f5/2 signals (Figure 8, left), V 2p1/2 and V 2p3/2 signals (Figure 8, center), and the O 1s peak (Figure 8, right); all these peaks are characteristic of BiVO4, as reported by previous authors.31,32 They are also broadly in good agreement with the corresponding profiles for the authentic sample (Figure 8d), although there are important differences in the O 1s regime (see below). Focusing on the Bi binding energy regime, it must be noted that the Bi 4f7/2 and Bi 4f5/2 signals in Figure 8 are at far higher energies than the characteristic peaks for metallic Bi located at 156.8 and 162.2 eV, respectively.33 The Bi 4f7/2 peak for BiVO4 was reported at values ranging from 158.8 to 159.5 eV31,32 depending on the method of preparation and post-treatment with light. The present XPS data in Figure 8 show the peak in the reported range for the films derived from NH4VO3 and Na3VO4 and for the benchmark BiVO4 sample. However, the film derived from V2O5 (at pH 10) shows the Bi 4f7/2 peak at 158.6 eV pointing to a surface contamination with Bi2O3.34 Indeed, films electrodeposited either at basic pHs or at high positive potentials in neutral media showed the Bi 4f7/2 peak in the range from 158.6 to 158.5 eV, independent of the Vþ5 precursor, leading to the conclusion that electrodeposition of Bi2O3 is favored under these conditions. This conclusion is also corroborated by the EQCM and XRD data (Figures 3 and 4). The location of the V 2p3/2 peak, at 516.4 eV in the benchmark sample, and slightly shifted to higher energies on the three types of electrodeposited films (Figure 8), is rather ambiguous as the reported position for V2O5 is 516.6 eV.35 Therefore, recourse to the O 1s binding energy region is necessary to ascertain the surface chemical composition of the electrodeposited films.
Figure 9. Comparison of photoresponse for the various BiVO films prepared by the new electrosynthesis method in bar plot format. The measured anodic photocurrent at 0.8 V is shown for the films derived from the three V5þ precursors at different pH values (1, 4, 7, 10, and 14) (refer to the text).
Turning to the O 1s XPS data, the dominant O 1s peak at 529.6 eV and assignable to oxygen in the BiVO4 lattice31 is clearly seen in all our XPS spectra, thus indicating that the three vanadium precursors are generating mainly a BiVO4 film. This signal is always accompanied by a shoulder at ∼531.7 eV corresponding to OH groups, and a clear peak at ∼533.0 eV is associated with oxygen in water,36 indicating that the electrodeposited films contain adsorbed water on the surface. This behavior is to be expected considering that BiVO4 is hygroscopic. Coincidentally, SEM images showed electrosynthesized BiVO4 with a cotton-like morphology interpreted as being due to its high hygroscopic property.23 It is worth noting that the authentic (dry) sample of BiVO4 is free of these contributions from adsorbed water (Figure 8d, bottom right-hand corner panel). Given that the O 1s peak at 529.6 eV can be also assigned to oxygen in a V2O5 lattice,36 the presence of surface V2O5 is possible, according to the XPS data. However, this possibility may be ruled out on the basis of the following: (a) As pointed out earlier, the XPS signals for the three component elements (Bi, V, and O) are located at the correct energies for BiVO4 in Figure 8. (b) XPS data show the surface atomic ratio of V to Bi to be lower than the stoichiometric ratio in all the cases, including the benchmark BiVO4 where the V/Bi ratio is only 0.6. The V/Bi ratio in the electrodeposited BiVO4 films was found in the 0.60.2 range and notably affected by the pH used in the film preparation with the highest values for films prepared at pH 7 and lowest at pH 14 (independent of the V5þ precursor used), thus pointing to a contribution from Bi2O3 in the film. In the presence of surface V2O5, a V/Bi ratio > 1 should have been assayed, and thus, V2O5 can be ruled out as a surface component in the films. Contrarily, if any oxide can be formed during the stripping of Bi0 to Bi3þ in the vanadium precursor solution, it is Bi2O3 whose formation is enhanced at the most basic pH because of competition of OH solution species with the V5þ precursor species to react with the interfacially electrogenerated Bi3þ. 3.4. Photoelectrochemical Behavior. As detailed in our companion communication,23 photoelectrochemical experiments in 7799
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The Journal of Physical Chemistry C 0.1 M Na2SO4 (pH ∼ 7) revealed anodic photocurrents on reverse bias of the film diagnostic of n-type semiconductor behavior. Photoaction spectroscopy yielded an energy band gap for this material of 2.14 eV.23 Further data are shown in bar plot format in Figure 9 for films electrosynthesized from the three V precursors and at variant pHs and subsequently measured in Na2SO4 electrolyte (pH ∼7). All the photocurrents in Figure 9 refer to a fixed (reverse bias) potential of 0.8 V. For films derived from NH4VO3, the pH 7 synthesis electrolyte yielded the most optimal photoresponse. On the other hand, when either V2O5 or Na3VO4 was used, pH 10 proved to be most optimal for reasons outlined in the preceding section. Particularly, for films formed from V2O5 as the vanadium precursor and at variant pHs, the initial dissolution of V2O5 was performed at basic pHs (10 or higher), and then the resulting solutions were brought to lower pHs (7, 4, and 1) by progressive acidification with H2SO4 under stirring. The resulting solutions remained clear with a yellow hue and were used as prepared for the BiVO4 film preparation.
4. CONCLUSIONS This study has built upon the proof-of-concept data presented in our companion communication23 to glean further mechanistic insights into the film deposition process. In particular, voltammetry and EQCM were shown to be an effective combination for this purpose. Ex situ corroboration from XRD, EDX, and XPS facilitates the development of a complete picture on the interfacial chemistry and electrochemistry underlying our film electrosynthesis approach. The deposition potential along with electrolyte composition (i.e., whether V precursor species are present or not) as well as the solution pH and the particular V precursor were all shown to be critical variables in whether BiVO4, Bi2O3, or a composite of both is electrodeposited. The quality of the ultimate photoresponse obtained in inert electrolytes (e.g., 0.1 M Na2SO4, pH ∼ 7) is also dictated by the above variables. ’ AUTHOR INFORMATION Corresponding Author
*Tel: 817 272 5421. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by the following agencies: the U. S. Department of Energy (Hydrogen Program); the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0074367); and the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-C1AAA001-0093168). We thank the two anonymous reviewers for comments on an earlier version of this manuscript. ’ REFERENCES
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dx.doi.org/10.1021/jp200632f |J. Phys. Chem. C 2011, 115, 7793–7800
