Direct Liquid Injection Chemical Vapor Deposition of Molybdenum

Article pubs.acs.org/JPCC

Direct Liquid Injection Chemical Vapor Deposition of MolybdenumDoped Bismuth Vanadate Photoelectrodes for Efficient Solar Water Splitting Ashish Yengantiwar,†,‡ Soundarrajan Palanivel,† Panikar Sathyaseelan Archana,†,§ Yanxiao Ma,§ Shanlin Pan,§ and Arunava Gupta*,†,§ †

Center for Materials for Information Technology, University of Alabama, Box 870209, 2007 Bevill Building, Tuscaloosa, Alabama 35487, United States ‡ Department of Physics, Fergusson College, Savitribai Phule Pune University, Pune, India § Department of Chemistry, University of Alabama, Box 870336, 250 Hackberry Lane, Tuscaloosa, Alabama 35487-0336, United States S Supporting Information *

ABSTRACT: The direct liquid injection chemical vapor deposition (DLI-CVD) method is used to grow pristine and molybdenum (Mo)-doped monoclinic scheelite phase bismuth vanadate (BVO) photoelectrodes. Superior photoelectrochemical (PEC) performance is achieved with ∼200 ± 50 nm thick pristine and 8 at. % Mo-doped BVO films grown at 550 °C. Photocurrent densities as high as ∼1.65 and 3.25 mA/cm2 are obtained for pristine and optimum 8% Mo-doped BVO electrodes, respectively, at 1.23 V vs reversible hydrogen electrode (RHE) under visible light AM 1.5G (100 mW/cm2) in 0.5 M phosphate buffer electrolyte in the presence of 0.1 M Na2SO3 hole scavenger. Somewhat lower photocurrent densities of ∼1.5 and 2.4 mA/cm2 are obtained for pristine and optimum 8% Mo-doped BVO electrodes, respectively, in the absence of Na2SO3. Onset potential values as low as ∼0.1 and 0.3 V vs RHE are achieved with pristine and Mo-doped BVO films for sulfite and water oxidation, respectively. The increased photocurrent density with Mo doping is attributed to enhanced charge carrier density and film conductivity as confirmed by PEC and Mott−Schottky analyses. Because of the dense high quality polycrystalline structure, the DLI-CVD fabricated Mo-doped BVO electrodes exhibit substantial stability under water and sulfite oxidation conditions without any protective layer and/or oxygen evolution cocatalysts. Scanning electrochemical microscopy (SECM) studies confirm the low porosity of Mo:BVO films and production of oxygen in a local area of Mo:BVO electrode under light illumination.

1. INTRODUCTION Photoelectrochemical (PEC) systems for producing hydrogen under sunlight via direct water splitting hold the promise of helping to address future energy challenges.1 The general operation principle of a PEC system resembles that of photosynthesis.2 Briefly, a photocatalytic material in contact with aqueous solution absorbs specific wavelength of sunlight depending on its band gap and energy levels.3 The captured photons are subsequently used to generate free charge carriers (e.g. electrons and holes) to produce hydrogen and oxygen at solid−liquid interface by proton reduction half-reaction at cathode and oxygen evolution reaction at anode.4,5 While it was first reported back in 1998, presently bismuth vanadate (BVO) semiconductor, with a n-type monoclinic scheelite phase crystallographic structure system, is among the most intensively investigated photocatalytic materials.6,7 This is because of its optimal bandgap of ∼2.4 eV and high light absorptivity at visible wavelengths as well as suitable positioning of energy levels for water splitting.8 Additionally, BVO possesses relatively © 2017 American Chemical Society

low onset potential for water oxidation and long charge carrier lifetime (∼40 ns) along with high hole mobility (∼0.2 cm2/ (V s)), favorable for PEC energy conversion.7−9 BVO thin films have successfully been deposited by various deposition techniques,10 including chemical vapor deposition,11 pulsed laser deposition,12 electrodeposition,13 sputtering,14 and atomic layer deposition,15 spray pyrolysis, etc.16 Despite aforementioned distinctive properties, monoclinic scheelite phase BVO typically exhibits sluggish electron transport kinetics due to strong electron localization, which apparently opens a back door for recombination processes.17 Therefore, the reported PEC performance of BVO remains far below the theoretical limit of maximum photocurrent density ∼7.5 mA cm−2.18,19 Recent endeavors toward addressing the poor electron transport and related charge separation include Received: December 18, 2016 Revised: February 21, 2017 Published: February 21, 2017 5914

DOI: 10.1021/acs.jpcc.6b12710 J. Phys. Chem. C 2017, 121, 5914−5924

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The Journal of Physical Chemistry C doping (cationic and anodic sites),20 underlayer (forming heterostructures)21 and overlayer22 (acting as catalyst)23 for improving the charge carrier transport,24 charge separation,25 water oxidation,26 and stability enhancement.27,28 Among these, doping appears to be an effective approach to modify the electronic structure in order to address the charge transport and separation issues.29 Metal-ion-doped BVO thin films have been deposited using different deposition methods to achieve enhanced photocurrent density in a range of ∼1 and 3.5 mA cm−2 in various electrolytes.18,19 Doping metallic ion such as Mo6+ into BVO crystallographic lattice alters the bond lengths through slight expansion of lattice sites position, which enables tuning the optical and electrical properties.30 We report, for the first time, the growth of high-quality BVO nanocrystalline thin films using the direct liquid injection chemical vapor deposition (DLI-CVD) technique. This method provides uniform, well-adherent, and conformal coating of metal oxides with high deposition rate on desired substrates using moderate vacuum and temperature conditions.31 The primary advantage of this method is the ease of reproducible growth of multicomponent (binary, ternary, and quaternary) films using corresponding precursor source materials dissolved in a common solvent. The physicochemical properties of BVO films are tuned by mixing specific amounts of a source precursor in the solution for incorporating Mo dopant during growth. The effect of Mo dopant concentration on solar water splitting performance of BVO is investigated. As-grown pristine and Mo-doped BVO thin films reveal superior photocurrent characteristics without any additional cocatalyst layer and also demonstrate suitable stability for water and sulfite oxidation. The DLI-CVD grown crystalline films are electrochemically quite stable in neutral pH electrolytes and efficient for solar water splitting. Scanning electrochemical microscopy (SECM) characterization confirms the porous film structure and production of oxygen in local area of Mo-doped BVO substrates.

Bruker D8 Discover XRD with GADDS software, employing Co Kα radiation. The surface morphology and chemical composition of films were characterized using a JEOL 700 scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis (EDAX) detector. The surface electronic behavior and valence states of the films were investigated by high-resolution X-ray photoelectron spectroscopy (Kratos Axis 165 XPS/Auger). The absorption characteristics of the films were analyzed using a UV−vis spectrophotometer (Agilent, Cary 500, USA). All PEC measurements were performed with a potentiostat (Princeton EG&G Applied Research) in a threeelectrode configuration. A three-neck quartz photoreactor with a planar window was used to accommodate the BVO electrode, standard platinum coil counter electrode, and Ag/ AgCl (saturated with KCl) reference electrode. The PEC measurements were performed in the dark and under 1 sunlight using a Xe arc lamp with intensity of solar simulated light (100 mW/cm2) at AM 1.5G conditions. The action spectra of the electrodes were recorded in different electrolytes by measuring the current response while holding the potential constant at 0.6 V (vs Ag/AgCl) using a CHI 760 potentiostat (CH Instruments, Inc., Austin, TX). Monochromatic light from 400 to 600 nm was obtained using a monochromator (MD 1000, Optical Building Blocks) with white light input from a Xe arc lamp. Current versus time graphs were recorded with the potentiostat, and the time scale was then converted to wavelength scale to obtain the action spectrum characteristics. The action spectra were further converted into incident photon conversion efficiency (IPCE). Mott−Schottky (M−S) analyses with three-electrode configuration were recorded in the dark using a CHI760 bipotentiostat (CH Instruments Inc., Austin, TX) with respect to Ag/AgCl as a reference electrode. Scanning electrochemical microscopy (SECM) characterization of the samples was carried out using a CHI920D scanning electrochemical microscope (CH Instruments Inc., Austin, TX).

3. RESULTS AND DISCUSSION 3.1. Structural, Composition, and Morphological Studies. Figure 1 shows the XRD patterns of DLI-CVD grown pristine and Mo-doped BVO thin films on FTO substrates. The detected peaks at specific 2θ values match with the diffraction pattern for polycrystalline monoclinic scheelite phase BVO, which is known to exhibit the highest photocatalytic activity.11,12 All the diffraction peaks can be indexed to planes that match with information from standard JCPDS card pdf # 14-0688 and consistent with others reports.14−16

2. EXPERIMENTAL SECTION 2.1. Growth of Pristine and Mo-Doped BVO Thin Films. All chemicals of reagent grade were purchased and used as received without further purification. Vanadyl acetylacetonate (VO(C5H7O2)2) and triphenylbismuth (Bi(C6H5)3), Acros Organics, were chosen as precursor for V and Bi, respectively, because of their good solubility and stability in dimethylformamide (DMF) solvent. Triphenylbismuth (0.0125 M) and vanadyl acetylacetonate (0.025 M) were mixed in DMF and then sonicated for 20−30 min. This mixture was used as a precursor source solution for the deposition. Vanadyl acetylacetonate being partially soluble in DMF, half of the concentration of triphenylbismuth was utilized for solution preparation in order to achieve the desired film stoichiometry. For Mo doping, molybdenum hexacarbonyl (Mo(CO)6) was used as precursor, being substituted at the V site with varying atomic concentration. The stated atomic % Mo concentration in the doped films refers to that used in the precursor solutions. The film thickness on conductive fluorine-doped tin oxide substrates (FTO) was controlled by varying the deposition time of 15, 30, 45, and 60 min. The optimized synthetic parameter conditions in DLI-CVD technique are presented in Supporting Information Table S1. 2.2. Characterization and Photoelectrochemical Measurements. X-ray diffraction (XRD) patterns were recorded for as-grown pristine and Mo-doped BVO samples using a

Figure 1. X-ray diffraction (XRD) patterns of pristine and Mo-doped BVO thin films. Peaks marked with an asterisk correspond to the FTO substrate. 5915


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The Journal of Physical Chemistry C

Figure 2. XPS spectra of pristine and 8% Mo:BVO samples: (a) Overall survey spectra, (b) Mo 3d high-resolution XPS spectra; (c) Bi 4f high-resolution XPS spectra; (d) V 2p high-resolution XPS spectra.

Figure 3. FE-SEM image of (a, b) Pristine BVO and (c, d) 8% Mo:BVO nanocrystalline thin film. Inset shows cross-section SEM image of 30 min deposited 8% Mo:BVO film.

No significant shift of diffraction peaks is observed in the case of Mo-doped samples of different concentrations with respect to undoped BVO due to the small difference in the ionic radii between Mo6+ (59 pm) and V5+ (54 pm).14

The intensity of the diffracted peaks of the different percentage doped samples of thickness ∼250 ± 50 nm is compared with thicker BVO sample (∼800 nm) to depict clearly the presence of crystalline XRD peaks. The intensity of the 5916


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The Journal of Physical Chemistry C peaks is observed to decrease for 10% Mo doping, beyond which MoO3 phase is observed in the XRD pattern mixed with the monoclinic scheelite phase. The 8% Mo-doped BVO samples are not only suitable for optimal doping but also show highest water splitting activity as compared to all other atomic percentage of Mo-doped samples studied. Therefore, 8% Mo-doped BVO film is considered as an optimized doping composition for our DLI-CVD growth conditions for investigating water oxidation reaction. The elemental compositions of the pristine and 8% Mo-doped BVO films have been determined using EDAX. The recorded spectra and composition values are tabulated in Figure S1. The presence of host elements such as Bi and V is confirmed in pristine BVO film, along with the observation of systematic shifts of Bi and V spectra and presence of Mo in the pattern of the doped sample with respect to undoped one. From EDAX quantitative composition calculations, the atomic percentage ratio of Mo:V is estimated to be ∼2.75%, which is low as compared to 8% Mo substitutional doping used in original precursor solution. The difference in atomic concentration may arise due to inhomogeneous distribution of Mo as substitutional dopants at V sites in the BVO structure, which is also observed in Mo-doped and other metal-doped bismuth vanadate thin films.32−34 The presence of host and dopant ions in the BVO lattice is also confirmed from XPS measurements. The XPS spectra of the pristine and 8% Mo:BVO films are presented in Figure 2a. From Figure 2b, the satellite peaks centered at 232.5 and 235.3 eV are attributed to the binding energy of hexavalent Mo (Mo6+) in the Mo:BVO system.33 Figure 2c,d shows the slight shifting of peaks of the host cations (Bi3+ and V5+) in Mo-doped BVO toward higher binding energy values as compared to pristine BVO, which may be due to the relatively higher electronegativities of the doping impurities (Mo6+ > V5+).34,35 The quantitative analysis of the elemental compositions in the doped sample shown in Table S2 is acquired from area under fitted XPS peaks, and the atomic ratio of Mo:V is estimated to be ∼4.6%.35 The surface morphological changes with respect to Mo doping of BVO are characterized using FE-SEM technique. Figure 3a,b shows low- and high-magnification morphological SEM images of the BVO film, while Figure 3c,d shows images for the 8% Mo-doped BVO system. Both these films are grown at 550 °C on FTO by DLI-CVD and are well adherent. The 200−300 nm size grains in the case of the later are uniformly distributed and interconnected with some voids. The change of the inhomogeneous grain size surface to spherical grains of relatively uniform morphology clearly indicates the consequence of Mo doping BVO. The cross-section SEM images of 8% Mo-doped BVO is shown as inset in Figure 3c. The thickness of the pristine and 8% Mo-doped BVO films are 200 and 250 ± 30 nm, respectively. 3.2. Optical Studies. Light absorption in the visible region is an essential characteristic for a photocatalytic material, which determines the overall performance of PEC cell. The ultraviolet− visible (UV−vis) absorption spectra (Figure 4) show that light absorption for the 8% Mo-doped BVO thin film is significantly higher than that of pristine BVO. As compared with pristine BVO film, the absorption edge of 8% Mo-doped BVO is redshifted and can be attributed to changes in the forbidden gap.36 Further, no sharp excitonic peak is observed for Mo-doped BVO due to the presence of intrinsic defects. The absorption edge for doped sample is at ∼520 nm as compared to pristine BVO at ∼480 nm, which is consistent with previously reported

Figure 4. Absorption spectra with inset showing Tauc’s plot of pristine and Mo-doped BVO thin films.

absorption spectra of BVO films.14 Additionally, the absorption tail extends to a longer wavelength of 750 nm, which could be due to more scattering effects in the Mo-doped BVO film.18,19 The optical band gap have been determined from the Tauc’s plot equation (αhυ)n = A(hυ − Eg), with the plot shown as an inset in Figure 4, where A is a constant, hυ is light energy, Eg is optical band gap energy, α is the measured absorption coefficient, and n = 2 for direct band gap nature of bismuth vanadate semiconductor. The estimated optical band gaps are ∼2.4 and ∼2.15 eV for pristine BVO and 8% Mo-doped BVO film, respectively, The decrease in Eg value with Mo doping is in good agreement with the reported literature.18,19 Mo-doped BVO exhibit strong optical behavior and scatters more light above 500 nm as indicated from UV−vis absorption spectra. 3.3. Photoelectrochemistry of Pristine and Mo-Doped BVO Films. The dark and photoresponse of pristine and different Mo concentration doped BVO electrodes in a PEC cell have been evaluated, and the photoelectrochemical performances of pristine and 8% Mo-doped BVO films for sulfite and water oxidation are shown in Figures 5a and 5c, respectively. The photoactivity of each photoanode is determined by measuring the photocurrent density generated during water and sulfite oxidation when the electrode is irradiated from the front (semiconductor to substrate) or back (substrate to semiconductor) side of the film immersed in electrolyte solutions. Table 1 lists the observed photocurrent density values and compares them with those reported in previous studies. Our Mo-doped BiVO4 thin films exhibit photocurrent density values as high as ∼2.4 and ∼3.2 mA/cm2 for water and sulfite oxidation, respectively. These numbers are significantly higher than previously reported values as listed in the Table 1. 3.3.1. PEC Sulfite Oxidation. To obtain optimum efficiency for the water-splitting reaction under light illumination, PEC performance of the pristine and a series of increasing atomic percentage Mo-doped BVO samples have been examined under front and back side illumination of light in a three-electrode configuration. Sodium sulfite is used as a sacrificial hole scavenger for BVO since it is known to undergo fast oxidization by photogenerated holes at the surface of BVO with essentially 100% efficiency, thereby enabling examination of its photoelectrochemical properties without any complication from slow water oxidation kinetics. Therefore, by using sulfite electrolyte, it is possible to compare our results with the larger body of previous work.7,37−40 For sulfite oxidation the PEC characteristics of pristine and Mo-doped samples are carried out in a solution of 0.5 M phosphate buffer along with addition of 0.1 M Na2SO3 electrolyte (at pH 7). 5917


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Figure 5. (a) J−E curves of the pristine and 8% Mo:BVO on FTO-coated glass measured in phosphate buffer solution containing 0.1 M Na2SO3. (b) IPCE spectra of pristine and Mo-doped BVO film tested in phosphate buffer solution containing 0.1 M Na2SO3 solution. (c) J−E curves of the pristine and Mo:BVO on FTO-coated glass measured in phosphate buffer solution. (d) IPCE spectra of pristine and Mo-doped BVO film scanned in phosphate buffer electrolyte.

Table 1. Comparison of Present Photocurrent Results for Mo-Doped BVO with Literature Dataa no.

synthesis/deposition method

photoanode film

1

direct liquid injection chemical vapor deposition

8% Mo:BVO/FTO

2

10% Mo:BVO/FTO

3

reactive magnetron cosputtering reactive cosputtering

4

dip-coating technique

1.8% Mo:BVO/FTO

5 6

spin-coating sol gel process

7

sonication and spin-coating

10% Mo:BVO/FTO 3% Mo:BiVO4/Si substrate textured 2% Mo:BVO/ FTO

8

electrospinning technique

BiV0.98O4Mo0.02/FTO

9

modified metal−organic decomposition

3% Mo:BVO/FTO

3% Mo:BVO

electrolyte solution 0.5 M phosphate buffer + 0.1 M Na2SO3 (pH = 7) 0.5 M phosphate buffer 0.5 M Na2SO4 in 0.1 M phosphate buffer (pH = 6.8) 1 M phosphate buffer + 0.1 M Na2SO3) (pH = 6.8) 1 M phosphate buffer 0.1 M phosphate buffer + 0.1 M Na2SO3 (pH = 7) 0.1 M Na2SO4 (pH = 6.5) 0.5 M phosphate buffer (pH = 6.8) 0.5 M phosphate buffer + 0.1 M Na2SO3) (pH = 6.8) 0.5 M phosphate buffer 0.5 M phosphate buffer + 0.5 M Na2SO3 (pH = 7) 0.5 M phosphate buffer seawater 0.5 M K2SO4 (pH = 6.5)

photocurrent density (mA/cm2) at 1.23 V vs RHE vis light (100 mW/cm2) 3.2 ± 0.2

ref no. present work

2.4 ± 0.1 1.1

14

3.5

16

2 1.2

26

1.2 2.10 ± 0.14

34 37

3.1

38

1.7 2.4

41

0.77 2.16

42

The photocurrent densities are compared based on the literature data of Mo-containing BVO films (without any underlayer and overlayer coating(s)). a

Figure 5a shows the photocurrent density curves of pristine and optimum 8% Mo:BVO films. The photocurrent increases with increasing applied anodic potential representing a typical n-type semiconductor behavior. The applied potential shown in

the bottom scale of the graph (vs Ag/AgCl) is converted into the top scale (vs RHE) using the Nernst equation: E (vs RHE) = E (vs Ag/AgCl) + 0.1976 V + 0.059pH 5918


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where J is the photocurrent density (mA cm−2), Eapp represents the magnitude of applied potential in V versus Ag/AgCl, and Ilight is incident power (mW cm−2). These results are plotted in Figure S5. Interestingly, the maximum in the photoconversion efficiency for the case of 8% Mo:BVO is ∼1.5% at maximum ∼0.6 V vs RHE as compared to pristine BVO ∼ 0.64%. This represents the favorable modification of electronic states in Mo-doped BVO electrode for photoconversion. 3.3.2. PEC Water Oxidation. In addition to sulfite, the pristine and all atomic % concentrations of Mo-doped BVO films have been examined for their ability to oxidize water, as shown in Figure S6. Figure 5c,d shows J−V and IPCE characteristics measurements of pristine and 8% Mo:BVO photoelectrode using only 0.5 M phosphate buffer (pH 7) for water oxidation, respectively. As in the case of sulfite oxidation, 8% Mo:BVO electrodes of the same thickness ∼250 nm demonstrate the highest photocurrent, reaching J ∼ 2.4 mA cm−2 as compared to pristine BVO showing J ∼ 1.5 mA cm−2 at 1.23 V vs RHE under front illumination. The PEC performance is better with front side illumination indicating that Mo doping significantly improves the majority carrier transport within the films. The progressive and significant enhancement in photocurrent on Mo doping is due to its enhanced carrier density, improved mobility in transport of charge carriers, and reduction in back carrier recombination.18,19 One possibility is that Mo dopant passivates the intrinsic bulk defects in the BVO lattice by increasing the electron density. Also, there may be prospect of increasing the diffusion length of charge carriers due to Mo doping with optimum thickness of the deposited film. The more anodic shift of onset potential ∼0.3 V in the absence of hole scavenger but slightly toward cathodic ∼0.1 V vs RHE in the case of sulfite oxidation. This is mainly due to slower kinetics of water oxidation reaction of Mo-doped BVO film.7 Overall, the best PEC performance in the case of 8% Mo-doped BVO film is due to the combined effect of its polycrystallinity and highly dense coating, which offers better semiconductor/electrolyte contact and faster electron−hole separation transfer and effectively suppresses bulk carrier recombination. The water oxidation kinetics is slower in the absence of hole scavenger sulfite electrolyte and exhibits a low photocurrent, which is further confirmed from the oxygen evolution reaction under back illumination. The charge injection efficiency of Mo-doped BVO photoanode film is estimated by taking the ratio of observed photocurrent densities of J(H2O) to J(Na2SO3) that comes out to be ∼75%, which clearly provides evidence of better transport of charge carriers during water oxidation. Also, to confirm whether the water oxidation performance is increased or decreased by varying the thickness of dopant film, 8% Mo-doped BVO photoanode films have been grown on FTO for a longer duration of 60 min of CVD deposition under as mentioned optimized conditions. The cross-section FE-SEM image depicted in Figure S7 shows the average thickness of the film is ∼650 nm. Figures S8a and S8b show the LSV scan and chopping of light of same photoanode film, respectively. It is observed that the PEC performance of the sample is decreased on increasing the thickness of the photoanode film. The thick films have lower photocurrent density because the optimum electron diffusion length is only about 300 nm. The best photochemical performance of our 30 min deposited Mo-doped BVO film of thickness ∼250 nm results from trade-off between absorbance and transport

The photocurrent of pristine BVO under back illumination (substrate to semiconductor) shows slightly higher photocurrent density of ∼1.62 mA/cm2 than for front illumination of photocurrent ∼1.5 mA/cm2 at 0.6 V (vs Ag/AgCl), equivalent to 1.23 V (vs RHE) scale. Considerably higher photocurrent is observed with back illumination as compared to front illumination, mainly due to better transport of hole carriers in the presence of acceptor electrolyte near the FTO substrate.7,43 However, the PEC performance of optimized 8% Mo:BVO photoanode film under front illumination gives a value of ∼3.25 mA/cm2 at 1.23 V vs RHE. This oxidation reaction photocurrent in the presence of hole acceptor electrolyte is among the best reported for Mo-doped BVO photoanodes without any underlayer/overlayer or catalysts modification as compared in Table 1. As examined and shown in Figure S2, as the dopant concentration increases from 2% to 10%, there is consistent increase in the photocurrent, but interestingly, photocurrent density decreases for 10% Mo-doped film. The highest photocurrent density of ∼3.25 mA/cm2 (at 1.23 V vs RHE) is observed for 8% Mo-doped BVO film among all the samples. The improved film conductivity with Mo doping is evidenced by the enhanced charge carrier density. Also, optimum Mo-doped BVO electrode can possibly accelerate the catalytic role in enhancing oxygen evolution.38 Figure S3 shows the PEC performance of pristine and Mo-doped electrode thin films using pure sacrificial hole acceptor electrolyte Na2SO3 solution (pH 9) under front and back side illumination. Also, corresponding chopping of illumination during linear sweep voltammetry (LSV) scan is shown in Figure S4. In order to make a quantitative correlation between the maximum photocurrent density and incident photon to current conversion efficiency (IPCE) values, IPCE measurements of pristine and optimum 8% Mo-doped BVO photoanode films have been carried out using a three-electrode configuration as shown in Figure 5b. The light intensity of the monochromatic light for the IPCE is measured using a calibrated Si photodiode detector. The % IPCE or quantum efficiency is then calculated from the equation IPCE (%) =

1240 × J × 100 Iinc × λ

where J is the measured photocurrent density in mA/cm2, Iinc is the light intensity in mW/cm2, and λ is incident photon wavelength in nm. Most importantly, in the case of sulfite oxidation, % IPCE at 1.23 V vs RHE of pristine BVO is more than 40% at 400 nm, and it can also be seen from trend of UV−vis absorption spectra that indicates that BVO absorbs well up to ∼480 nm. The absorbed light in BVO is efficiently converted into photocurrent via back illumination due to effective hole acceptance by sulfite electrolyte near to the substrate.43 However, in the case of 8% Mo-doped BVO photoanode, there is approximately 2× increase in quantum efficiency, signifying efficient photon absorption due to a slight nanoporous morphology, effective excitonic separation, and improved charge transport. The photoconversion efficiency (η) of light energy to chemical energy in the presence of an external applied potential Eapp is calculated from the equation η (%) =

J(1.23 − Eapp) Ilight

× 100 5919


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Figure 6. (a) Mott−Schottky plots of pristine BVO and 8% Mo:BVO samples measured in 0.5 M phosphate buffer (pH 7) in dark. (b) Chopping of illumination with linear sweep voltammetry measurements for pristine and 8% Mo:BVO in sulfite electrolyte solution (pH 9).

Figure 7. (a) Schematic of scanning electrochemical microscope study of local redox activities of Mo:BVO; (b) CV of a 10 μm (in diameter) Pt ultramicroelectrode (UME) in 10 mM K3Fe(CN)6 of 0.1 M Na2SO4; (c) Approaching curve of the 10 μm Pt tip toward a planar Mo-BVO film at tip potential of −0.2 V, and substrate potential of 0.5 V without light illumination; inset is the tip current response to ∼50 mW/cm2 visible light (> 400 nm) illumination at tip potential of −0.2 V and substrate potential of 0.5 V; the tip−substrate distance is approximately around 30 μm. (d) Cyclic voltammetry of Mo:BVO in 10 mM K3Fe(CN)6 of 0.1 M Na2SO4 in the absence of light illumination.

collection. The light absorption at wavelengths beyond 520 nm could be due to scattering effect, which does not seem to directly contribute to photocurrent generation.37,38 The maximum external quantum efficiency values reaches about 50% for Mo-doped BVO thin film of thickness ∼250 nm at 1.23 V vs RHE. Most importantly, in the case of water oxidation, the IPCE of pristine BVO is more than 30% at 400 nm. However, in the case of 8% Mo-doped BVO photoanode, it can be seen that there is 1.5× increases in % IPCE; hence, the photon to

distance for the photogenerated charge carriers through the photoanode layer to substrate. Figure 5d displays the IPCE curves of the pristine and 8% Mo:BVO films at 1.23 V vs RHE bias potential. For all the films, the IPCE values drops off rapidly for wavelengths higher than 400 nm and approaches zero at 520 nm, as indeed expected for a material with a band gap of 2.4 eV (∼517 nm). IPCE spectra exhibit photocurrent which is not only determined by light absorption but also charge separation and 5920


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Figure 8. (a) Cyclic voltammetry of 10 μm tip in 0.1 M NaOH with a tip−substrate distance of 30 μm and substrate potential at 0.5 V under dark and light illumination. (b) The tip current response to ∼50 mW/cm2 visible light (> 400 nm) illumination at tip potential of −0.2 V and substrate potential of 0.5 V; the tip−substrate distance is approximately 30 μm.

current conversion is better as compared to pristine BVO film. The photocurrent response edge of Mo-doped BVO extends to ∼515 nm, which is in good agreement with the band gap of BVO and as well as reported work.14,18,19 3.4. Mott−Schottky Analysis. The most commonly used method for confirming the increase in carrier density in dopant semiconductor studies is the analysis of the Mott−Schottky plots based on the equation

characterize the Mo-BVO semiconductor electrode by approaching a 10 um Pt tip close to its surface while probing its PEC response (cf. Figure 7a). The 10 μm Pt tip provides about 15 nA limiting current in 10 mM K3Fe(CN)6 solution (panel b of Figure 7). The tip is then advanced toward the Mo:BVO film whose potential is set at 0.5 V as shown by the approaching curve in panel c of Figure 7. A positive feedback is received indicating that the substrate FTO is not quite fully coated by Mo:BVO. This is further evidenced by the CV response of the substrate Mo:BVO in K3Fe(CN)6 (panel d of Figure 7). Enhanced reduction current at tip is obtained when Mo:BVO substrate is under light illumination (inset of panel c of Figure 7) because of the oxidation of K4Fe(CN)6 produced near the Pt tip. SECM image with the 10 μm Pt tip does not provide much heterogeneities in the spatial distribution of the tip current because the pinholes of uncovered sites of FTO substrate are estimated to be under the micrometer scale. This is consistent with the SEM images (Figure 3). To further provide evidence of oxygen production by Mo:BVO photoanode under light illumination, the abovementioned SECM experiment has been reproduced with 0.1 M NaOH by placing the Pt tip around 30 μm above Mo:BVO photoanode. CVs of tip are shown in Figure 8a with substrate potential hold at 0.5 V in order to oxidize OH− into oxygen under light illumination. A significant reduction current of 12 nA can be obtained at tip potential around −0.4 V vs Ag/AgCl when the Mo:BVO film is under light illumination. The dark current of tip shows a quasi-reversible reduction peak near about −0.1 V. We attribute this peak to redox activity of released vanadium from Mo:BVO sample, as has been suggested in the literature that photocorrosion takes place at the surface of BVO in strong alkaline solution.47−49 Time evolution of the tip current for reducing oxygen is shown in panel b of Figure 8. Also, in comparison, we observe that Mo-doped BVO electrode exhibits better photoresponse as compared to pristine substrate at same tip potential of −0.3 V under chopping of ∼50 mW/cm2 > 400 nm visible light illumination (Figure S9). 3.6. Stability of Photoanode. The photostability of the photoanode is an important performance parameter of the PEC water-splitting process. Therefore, chronoamperometry measurement of pristine and 8% Mo:BVO photoanode films have been carried out in extreme electrochemical environment by choosing a high concentration (1 M) of the phosphate buffer electrolyte (pH 7) solution. The J−t characteristics are recorded at constant potential 1.23 V vs RHE under continuous front

⎡ 1 kT ⎤ = 2/eεε0Nd⎢(Va − Vfb) − ⎥ 2 ⎣ e ⎦ Csc

where Csc is the space charge capacitance in F, e is the electronic charge in C, ε is the dielectric constant of the semiconductor (for BVO ∼ 68), ε0 is the permittivity of free space, Nd is the carrier density in cm3, A is the surface area of the electrode in cm2, Va is the applied potential in V, Vfb is the flat band potential in V, k is the Boltzmann constant, and T represents the temperature in K. Mott−Schottky analyses were undertaken in order to determine the nature of conductivity and flat band potential Vfb of pristine and 8% Mo: BVO. Mott−Schottky plots were scanned in the dark with three electrode assembly and perturbation amplitude of 5 mV in 0.5 M phosphate buffer aqueous electrolyte. Figure 6a shows the Mott−Schottky plots for the pristine BVO and 8% Mo: BVO samples. The flatband potential is obtained from the x-intercept on the potential axis and the slope of the Mott−Schottky plot can be used to calculate the magnitude of carrier density and also to compare the majority charge carrier densities before and after doping. Positive slope of BVO and 8% Mo:BVO Mott− Schottky plots imply n-type conductivity in both cases. Extrapolating the linear region of Mott−Schottky plots provides the flatband potential of 8% Mo:BVO and pristine close to ∼0.1 V vs RHE scale. The slopes obtained at 1 kHz frequency from pristine and 8% Mo:BVO samples indicate that there is an increase of an order of magnitude in the carrier density (Nd ∼ 1.52 × 1020/cm3) for Mo-doped BVO electrode serving as electron donor as compared to pristine BVO (Nd ∼ 1.02 × 1019/cm3).26 Figure 6b presents the accurate onset potential, with chopping of light during sulfite oxidation reaction (using Na2SO3 electrolyte, pH 9), which enables a more precise estimation of onset potential ∼0.1 V vs RHE which is closes to flatband potential. 3.5. SECM Investigation of Mo-Doped BVO. Scanning electrochemical microscopy (SECM)44−46 has been used to 5921


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that the morphological and PEC properties of BVO are significantly improved by Mo doping. Overall, the DLI-CVD method is an efficient technique to deposit pristine and Mo-doped BVO with reduced surface defects, which yield better PEC and onset potential values. The synthetic approach discussed here provides a new method of deposition of pristine and Mo-doped BVO dense polycrystalline thin films using DLI-CVD. The results indicate that optimum Mo doping favors significant enhancement in photocurrent density likely due to increased electrical conductivity resulting in improved charge carrier separation efficiency for oxygen evolution reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. PEC stability versus time profiles measured for Mo-doped BVO thin film in 0.5 M phosphate buffer solution containing 0.1 M Na2SO3 at pH 7. Inset: chronoamperometry characteristic for pristine and 8% Mo:BVO thin films in concentrated 1 M phosphate buffer solution at pH 7 (without sulfite electrolyte) at a fixed applied bias of 1.23 V vs RHE.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12710. DLI-CVD growth conditions (Table S1), EDAX spectra and XPS composition calculations of pristine and 8% Mo-doped films (Figure S1 and Table S2), water and sulfite oxidation of different percentage of Mo-doped BVO films (Figures S2, S3, S4, and S6), photoconversion efficiency plot (Figure S5), cross-section SEM image and photoelectrochemical characteristics of thick Mo-doped BVO (Figures S7 and S8), chopping of illumination plots of pristine and Mo-doped BVO during SECM experiments (Figure S9), and water oxidation action and top view SEM images of pristine and Mo-doped BVO after photocorrosion test (Figures S10 and S11) (PDF)

illumination of simulated 1 SUN light (AM 1.5) of intensity 100 mW/cm2 (Figure 9). In this study, it is observed that the photocurrents from the both photoanode films decay slowly due to photocorrosion in the first few minutes with the photocurrent of pure BVO deteriorating slightly faster. The photocurrent continues to decrease slowly over the next several hours. With an illumination time of 3 h, the photocurrents of BVO and 8% Mo:BVO photoanodes decrease to approximately 30% and 40% of the original, respectively. Thus, the photostability of BVO is somewhat improved by Mo doping as shown from the Mo-doped electrode that was illuminated 1 h more with respect to pristine one. This suggests that the presence of Mo dopant active sites in the lattice act as chemically passivating surface traps, further reducing surface recombination by preventing back electron transfer that leads to reasonable stability under illumination and electrical bias. However, as reported earlier,38 the photocurrent for sulfite oxidation for Mo-doped BVO samples is stable for 12 h, and no significant decay is observed that signifies the sulfite oxidation reactions are very stable. LSV characteristics after degradation/photocorrosion test of water oxidation which confirms that the stability of Mo-doped samples is to some extent better than pristine one (Figure S10). However, the inset of Figure 9 shows that both films are unstable under extreme electrochemical conditions of high alkaline electrolyte solution as supported by SECM results and stability testing, but Mo-doped film is slightly more stable over long-term than undoped one. Figure S11 shows the top view SEM images of pristine and 8% Mo-doped BVO samples after continuous front illumination in concentrated 1 M phosphate buffer (pH 7) electrolyte solution. No significant morphological change can be detected for bare and Mo-doped film after longer photocorrosion test, suggesting that the Mo-doped DLI-CVD BiVO4 films can be potentially utilized for the improved solar water-splitting system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.G.). ORCID

Ashish Yengantiwar: 0000-0001-6156-8351 Panikar Sathyaseelan Archana: 0000-0003-0554-4130 Shanlin Pan: 0000-0003-2226-9687 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.Y. is thankful to UGC (India) for providing one year Raman fellowship. Y.M. and S.P. acknowledge NSF for supporting the SECM work under Award OIA-1539035 and 1508192. A.Y. acknowledges the assistant of Zhichao Shan for IPCE measurements.

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REFERENCES

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4. CONCLUSIONS In summary, Mo-doped BVO films have been successfully grown by DLI-CVD and used as photocatalytic electrode for water-splitting application. The optimum atomic percentage of Mo doping into BVO crystalline lattice by DLI-CVD for solar water splitting is outlined in this work. Optical absorbance, IPCE, Mott−Schottky plots, and SECM studies clearly reveal 5922


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