Thin-Layer Indium Oxide and Cobalt Oxyhydroxide Cobalt-Modified

Article pubs.acs.org/JPCC

Thin-Layer Indium Oxide and Cobalt Oxyhydroxide Cobalt-Modified BiVO4 Photoanode for Solar-Assisted Water Electrolysis Weitao Qiu,† Yongchao Huang,† Songtao Tang, Hongbing Ji,* and Yexiang Tong* MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, Chemical Industry Research Institute, School of Chemistry, the Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Fabrication of high-performance tandem cell for solar-assisted water cleavage requires an efficient photoanode with excellent bulk charge separation and surface injection. In light of that, we developed a hybrid photoanode using visible light absorber as main scaffold, a thin layer In2O3 middle layer to enhance charge separation in bulk and finally an active CoOOH catalyst as outer decoration for better surface charge injection. Bulk separation was mainly augmented by In2O3 addition, while the addition of CoOOH largely advanced photocurrent onset and elevated injection efficiency. The resultant photoanode delivered a high current density at low applied bias, showing promising prospect for incorporation into tandem cell for solar-assisted water electrolysis.

light with a wide range of wavelength starting from ∼500 nm (Eg = ∼ 2.4 eV). On the basis of the theoretical capability of absorbing light, the maximum photocurrent generating from undoped BiVO4 is estimated to be 6.47 mA/cm2.6 However, the practical current density commonly reported is far lower than the estimated value, mainly due to the low bulk separation and poor surface kinetics which are respectively quantified by bulk separation efficiency (ηsep) and injection efficiency (ηinject). In order to further increase the photocurrent of BiVO4, both ηsep and ηinject should be improved. Nanosized material was proved capable to largely improve the separation of hole and electron by Choi et al.6,7 In their research, a new method was introduced to obtain nanoporous BiVO4 thin film with particles diameter less than 100 nm, the exact diffusion length of photogenerated holes in the single crystal counterpart according to Bard et al.8 Besides, utilizing the inner electric field in a heterojunction is another widely accepted way to augment ηsep in a photoanode. Through building a physical contact between two semiconductor materials with thermodynamically matched band structure, the equilibrium of charge carrier contributes to form an anisotropic field near the contacting interface which will boost the separation of electron−hole pairs and the flux of hole in the bulk of semiconductor. In light of that, In2O3 possess a favorable conduction band and valence band position, which are respectively situated at −0.62 and 2.18 V versus NHE and well matched with those of BiVO4.9,10

1. INTRODUCTION Studies in harvesting light and converting it into various forms of energy are drawing extensive attention with the growing demand of clean energy. In this field of research, solar energy could be successfully transformed into forms like electricity and chemical energy.1,2 One of the efficient media storing the energy collected from photoelectrochemical devices is hydrogen gas, a gas with high energy density considering its lightness and high atom utilization when participating in an energy releasing reaction. This naturally leads to a currently hot research field attempting to combine the solar energy conversion with water electrolysis, a reaction generating oxygen and hydrogen from water. In order to realize that, three configurations have been proposed:3 a photoelectrochemical cell comprising photoanode (n-type semiconductor), a cell comprising photocathode (p-type semiconductor), and a cell with both types of electrodes. Incorporating solar converting semiconductor into water electrolysis could induce internally “biased” photovoltage under irradiation and thus reduces the overpotential required to ignite water oxidation compared to conventional electrolysis cell. The extent to which a photoelectrode can promote the water electrolysis should largely depend on the light harvesting ability, charge separation, charge migration, and injection efficiency, properties and abilities eventually originating from the intrinsic bulk and surface properties of the semiconductor.3−5 Band gap energy (Eg), as one of the decisive factors affecting semiconductor light absorbing ability, is determined by the difference between conduction band bottom and valence band maximum, which is also the optical band gap most researchers are referring to. As a visible light absorbent, BiVO4 harvests © 2017 American Chemical Society

Received: June 29, 2017 Revised: July 22, 2017 Published: July 24, 2017 17150

DOI: 10.1021/acs.jpcc.7b06407 J. Phys. Chem. C 2017, 121, 17150−17159

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The Journal of Physical Chemistry C On the basis of the progresses already made by researchers, this work tentatively studied the BiVO4/In2O3 heterojunction for solar assisted electrolysis of water. Nanoporous BiVO4 is chosen due to its small crystal grain size. Evidence had proved that the formation of heterojunction structure resulted in no enhancement in light harvesting ability due to the wider band gap of In2O3 coating. The elevated photocurrent in the presence of hole scavenger indicates that the higher photoresponse in BiVO4/In2O3 composite is mainly originated from the synergistic effect from the thermodynamically favorable band alignment of the two semiconductors. Apart from the enhanced separation efficiency of electron−hole pairs, the surface kinetic for water oxidation are somehow improved, which is believed to be the contribution from In2O3 coating as well. This enhanced surface injection process could be further promoted by the addition of CoOOH layer by photoassisted deposition, leading to a photoanode with both improved bulk separation and surface injection.

2.3. Characterizations. The structure and composition information were characterized by X-ray diffractometer (XRD, D8 ADVANCE), field emission scanning electron microscope (FE-SEM, JSM-6330F), transmission electron microscopy (TEM, Tecnai F30), Raman spectrum (Renishaw inVia), and X-ray photoelectron spectroscopy (XPS, ESCALab250). Besides this, optical properties are measured with UV−vis−NIR spectrophotometer (UV−vis−NIR, Shimadzu UV-2450). Electrochemical characterizations were conducted in a threeelectrode cell made of quartz. A saturated Ag/AgCl electrode and a platinum electrode were used as reference and counter electrode, respectively. 0.5 M phosphate buffer solution (PBS) with or without Na2SO3 was used as electrolyte. Electrode potential was controlled by a CHI 760E electrochemical workstation (CHI, Shanghai). All the potential values, if not otherwise mentioned, are expressed as values versus reversible hydrogen electrode (RHE), using the equation as follows:11

2. EXPERIMENTS 2.1. Fabrication of BiVO4/In2O3 Composite Photoanode. The BiVO 4/In2O 3 composite photoanode was prepared through three steps: synthesis of BiOI, turning BiOI to BiVO4, coating BiVO4 with In2O3. BiOI was electrodeposited onto F-doped SnO2 coated glasses (FTO, Nippon Sheet Glass Group), as described by previous reports elsewhere.6,7 Detailed experiment procedures with slight modifications are shown as follows. 20 mL absolute ethanol containing 0.23 M pbenzoquinone was mixed with 50 mL aqueous solution containing 0.04 M Bi(NO3)3 and 0.4 M KI to prepare electrodeposition bath. In the deposition process, Ag/AgCl (saturated KCl) electrode was used as reference electrode while Pt electrode as counter electrode. FTO glass, as working electrode, was polarized in as-prepared electrolyte at −0.1 V vs Ag/AgCl for 100s. Subsequently, a dark orange layer is deposited on the surface and washed with water and ethanol. To convert BiOI into BiVO4, 0.075 mL dimethyl sulfoxide (DMSO) solution containing 0.2 M vanadyl acetylacetonate (VO(acac)2) was dropped onto the predried BiOI surface and in following step the soaked BiOI electrode was annealed at 450 °C for 2 h (ramping rate 2 °C/min). Excessive V2O5 on the surface of converted BiVO4 was then dissolved by soaking it in 1 M NaOH for 20 min. For Indium oxide coating, the BiVO4 sample was immersed into 0.01 M In(NO3)3 bath, maintained at 80 °C for 1 h. At last, the surface was cleansed with water, dried in electric oven and then put into muffle oven for further annealing at 500 °C (5 °C/min, 2 h) to obtain BiVO-In. For clarity, pristine BiVO4 is designated as BVO while BiVO4/In2O3 composite photoanode as BVO-In. 2.2. Cobalt Oxyhydroxide Cocatalyst Loading. BVO-In was immersed in 0.01 M cobalt acetate aqueous solution and polarized at 0 V vs Ag/AgCl in dark using Pt mesh as counter electrode at first hand. Then it was illuminated with a sun light simulator with incident light power controlled to be 100 mW/ cm2 and maintained for only 20 to 45 s. When biased at 0 V vs Ag/AgCl in the dark, no perceivable current passing through the circuit was measured, while when illuminated with simulated sunlight the current boosted up to ∼0.5 mA/cm2 and gradually decreased over time. This indicated that the photoassisted oxidation was undergoing. Finally, the CoOOH loaded photoanode with optimized amount was termed as BVO-InCo.

Linear sweep voltammetry was conducted at scan speed of 50 mV/s. The illumination source was a 100W Xe arc lamp (LCS100, ORIEL) directed at the quartz cell in a certain distance to ensure that the power density of incident light was 100 mW/ cm2. Applied bias photoconversion efficiency (ABPE) was calculated according to the following equation:12

E RHE (V) = EAg /AgCl (V) + 0.1976 + 0.059 pH

ABPE (%) = [J × (1.23 V − Eapp)/I ] × 100

Here, J is the photogenerated current density (mA/cm2), and I stands for power density of incident light, which is 100 mW/ cm2 in this case. Eapp equals the difference between measured potential and open circuit potential (OCP) at the same illumination intensity. Incident-photon-to-current-conversion efficiency (IPCE) measurements were conducted with a solar simulator (Newport 69911 300W xenon lamp), coupled to an aligned monochromator (Oriel Cornerstone 260 1/4m). IPCE can be expressed as13 IPCE (%) = (1240 × J(λ))/(λ × I ) × 100

where λ (nm) represents incident light wavelength, J(λ) (mA/ cm2) represents photocurrent at the corresponding wavelength, and I (mW/cm2) stands for the power density of incident light at wavelength of λ. For product quantification, a sealed electrolyzor was used, which is made of Teflon and a quartz window allowing light to pass through. The cell was connected with a sampling pump and gas chromatograph (GC) for product detection. In this setup, gas produced during reaction was able to circulate through the whole system and back to electrolyzor, to achieve uniform concentration. For hydrogen and oxygen measurement, a sampling loop connected with a six-port valve was used to collect sample. Calibration of this setup is conducted by using two Pt mesh electrodes to generate desired amount of gas in 1 M KOH under current density of 2 mA/cm2. Afterward, one of the Pt electrode was substituted with BVO electrodes and measurement was conducted in 0.5 M Na2SO4 at 1.23 V vs RHE.

3. RESULTS AND DISCUSSION 3.1. Formation of BiVO4/In2O3 Heterojunction. As a promising water oxidation photocatalyst BiVO4 possesses suitable valence band maximum (VBM) position (2.53 V vs NHE) which means a sufficient oxidative ability for oxygen 17151


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The Journal of Physical Chemistry C evolution.14 Meanwhile, its conduction band minimum (CBM) lies at potential (−0.02 V vs NHE) slightly lower than that of hydrogen evolution potential (0 V vs NHE).10 The band alignment renders BiVO4 a suitable band gap for visible light absorption and relatively lower overpotential for hydrogen evolution in a photoelectrochemical cell. On the other hand, In2O3, as was reported, has a conduction band bottom at −0.62 V vs NHE and valence band maximum situating at 2.18 V vs NHE. Its suitable band line-up with BiVO4 is shown in Figure 1, where the equilibrium of Fermi level is considered. In the

Figure 2. SEM images of (a) BVO and (b) BVO-In. (c) STEM image of a single BiVO4/In2O3 particle composited with In2O3 and corresponding element distribution of Bi, V, O, and In. (d) HRTEM of the selected area where the interface between BiVO4 and In2O3 was observed.

and the (222) planes of cubic In2O3, indicating the physical contact between the two component. The chemical composition of the composite photoanode is further revealed by Raman spectroscopy and X-ray diffraction meter (XRD). Results from Raman spectrum measurement in Figure 3a showed no differences between BVO and BVO-In, where all the characteristic vibrational modes were observed (210 cm−1, 325 cm−1, 367 cm−1, 710 and 825 cm−1).15,16 No other peaks correlating to In2O3 were discovered, indicating that the amount of In2O3 at the surface of BVO-In photoanode is lower than the detection limit of Raman spectrometer. Similar to the result of Raman spectroscopy, the XRD measurement results show almost identical diffraction patterns in the two samples. In Figure 3b, several major diffraction peaks centered at 18.5°, 19.0°, 28.7° and 30.5° can be precisely assigned to the (101), (011), (−121), and (004) planes of clinobisvanite BiVO4, accompanied by other minor diffraction peaks.17−19 The fact that no difference of the two samples is detected in both XRD and Raman analysis indicates that only a quite thin layer of In2O3 was coated on the surface of BiVO4, if we combined it with HRTEM and other test results. To overcome the challenge in detecting the low content of indium element coated on the surface of BiVO4 and its corresponding chemical surroundings, X-ray photoelectron spectroscopy (XPS) is exploited to explore the chemical information on the surface (Figure 3, parts c and d). Two prominent In 3d peaks were detected and deconvoluted into two components representing electronic states of In 3d3/2 and In 3d5/2 from the overall spectrum. Because of the presence of bismuth in BiVO4 substrate, the In 3d5/2 (at 444.6 eV) level partially overlapped with the Bi 4d5/2 (at 442.2 eV) level which is shown as a shoulder in Figure 3c. The energy of In 3d3/2 and In 3d5/2 states in BVO-In are coherent with that of In2O3, inferring that the In2O3 was successfully coated onto the BiVO4 photoanode.20 The combination of two semiconductors usually leads to the equilibrium of charge carrier in their interfaces, which changes their electronic structure.4,21−23 Previous studies

Figure 1. Band alignment between BiVO4 and In2O3 from cited values of conduction band minimum and valence band maximum.

composite photoanode itself, the band alignment results in a Zscheme structure where electrons, in thermodynamic considerations, tends to travel from In2O3 to BiVO4 and eventually move toward the Pt counter electrode. In contrary, holes travel toward the outer coating In2O3 layer. Under open circuit potential, holes prefer to accumulate at surface while electrons incline to travel inward. The equilibrium requires the whole system (including Pt electrode) to achieve the same Fermi level, which is higher than the required potential (lower in energy) to initiate water reduction in counter electrode. Thus, an external bias is still required for water oxidation even under illumination. In order to create the aforementioned heterojunction with proper energetic for carrier transfer, In2O3 was grown as a coating layer on BiVO4 photoanode with wet chemistry process at low temperature (Experiment). Scanning electron microscopy (SEM) images in Figure 2a show that pristine BiVO4 photoanode is consisting of numerous nanoworms like particles with narrowest dimension less than 200 nm. After In2O3 coating (Figure 2b), the surface roughness was increased, possibly caused by the introduction of In2O3 on the surface. Elemental distribution information in a single particle was investigated by energy-dispersive X-ray spectroscopy (EDX) technique in Figure S1, where the existence of In was confirmed. As shown in Figure 2c, the scanning transmission electron microscopy (STEM) image and element distribution of Bi, V, O, and In reveal that the BiVO4 nanoparticle was covered with In containing oxide layer on the surface (marked in dashed circles). The HRTEM images further shows the interface of BiVO4 and In2O3 from the same particles (Figure 2d). Crystal lattices with interplanar spacing of 0.463 and 0.290 nm were discovered near the interface. They should be respectively assigned to (011) planes of clinobisvanite BiVO4 17152


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Figure 3. (a) Raman spectra of BVO and BVO-In. (b) XRD diffraction patterns of BVO and BVO-In, where hollow squares indicate FTO signal. (c) XPS spectrum of In 3d and Bi 4d level in BVO-In. (d) XPS spectrum of Bi 4d level in BVO photoanode. (e) Valence band maxima of BVO-In and BVO.

Figure 4. (a) I−V curves of BVO and BVO-In at scan rate of 50 mV/s. (b) Photoconversion efficiency calculated from corresponding I−V curves.

holes would be more favorable in transporting across from BiVO4 to In2O3 than moving through pristine BiVO4 alone. Since the valence band potential of In2O3 is still more positive than that for water oxidation, the hole accumulated at the surface coating can transfer into electrolyte to participate in oxygen evolution reaction (OER); thereby the flux of holes, which is also named current density, will significantly increase. As for the photoconversion efficiency shown in Figure 4b, the calculated maximal value is around 0.28% for BVO and 0.78% for BVO-In. An interesting finding here is that the required applied potential to acquire maximal conversion efficiency also showed a cathodic shift, which means the photoelectrochemical cell using BVO-In as photoanode requires less energy to achieve a higher conversion efficiency maximum, which means its preferable capability in a solar energy assisted electrolysis cell for hydrogen generation. This phenomenon could be explained with the enhanced injection efficiency induced by indium coating, which will be involved in later discussions. According to many previous works concerning the fabrication of heterojunctions to promote photoelectrochemical performance of semiconductor photoelectrode, the optical band gap of both components in the junction might have a great influence on the final photoresponse output. For instance, building a heterojunction with large Eg host and narrow Eg material with thermodynamically favorable band configurations as light absorber has been proved as an efficient strategy to

of valence band spectra of semiconductor composite have proved that a stepwise deposition of one semiconductor on the other might induce a gradual shift in its valence band measurement value in XPS inspection.24 On the basis of the surface sensitive nature of XPS, a very thin layer might cause a significant shift in valence band spectrum, which is commonly recognized as the contribution from the outer layer metal oxide. When the thickness reached a critical value, the band structure measured in XPS should close in that of the coating layer. The valence band spectra of the two samples are displayed in Figure 3e, where the VBM in BVO-In shifted toward the Fermi-level by around 0.6 eV. The change in VBM was believed to be caused by the deposition of In2O3 thin layer. In this context, the interactions between the two semiconductors are successfully established, as evidenced by the apparent shifting in VBM. 3.2. Photoresponse and Light Harvesting Ability of BVO and BVO-In. The impact of the In2O3 coating was discovered to be profound in the I−V curves of the two samples. In Figure 4a, both samples show an onset potential around 0.5 V vs RHE. Current density at 1.23 V vs RHE is about 0.8 mA/cm2 for pristine BiVO4 while the thin layer In2O3 coating augmented the photoresponse up to ∼1.8 mA/cm2, which is more than twice that of the pristine one. On the basis of the suitable band alignment between In2O3 and BiVO4 mentioned before where the valence band of In2O3 situates slightly more positive than that of BiVO4, the photogenerated 17153


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Figure 5. (a) Diffuse reflect UV−vis spectrum of BiVO4. (b) UV−vis spectrum of In2O3 powder collected from autoclave. (c) Incident photon to electron efficiency (IPCE) in AM 1.5G spectrum from 300 to 600 nm regime. Insets are the correspondingly converted Tauc plot.

augment the light harvesting ability.25−27 Liu et al. discovered that coating FeTi2O5 absorber on a wider band gap TiO2 photoanode could efficiently enhance its photocurrent under illumination.22 In their case, the IPCE was in agreement with the broadened absorption range in UV−vis spectrum, which means the visible light absorber not only took in light but also effectively convert them into holes and electrons pairs for water cleavage. Contrarily, in BVO-In composite photoanode the spectral distribution of IPCE remains the same as that of BVO sample, delivering no photon conversion at around 500 nm (Figure 5a). According to the UV−vis spectra of BiVO4 and In2O3 (Figure 5b and Figure 5c), the absorption edge of BiVO4 is ∼502 and ∼450 nm for In2O3. In line with the absorption edge, the band gaps of BiVO4 and In2O3 obtained from their corresponding Tauc plots are around 2.46 and 2.80 eV, respectively. This infers that the outer In2O3 layer possesses a wider band gap proving its no influence on enhancing the light harvesting ability. Therefore, the augmented photocurrent are mainly caused by the space charge layer due to the formation of heterojunction rather than a broadened absorption spectrum. As a result of the coating with wider band gap semiconductor, the UV−vis spectrum and IPCE of BVO-In showed the same absorption and “conversion edge” as those of BVO. 3.3. Bulk Separation and Injection Efficiency. As a widely accepted method to probe the enhanced separation in the bulk of the material, hole scavenger is added into the electrolyte when testing photoelectrochemical performance. The effect of hole scavenger is to eliminate kinetically unfavorable factors. The photoresponse of electrode materials in the existence of hole scavenger manifest the photocurrent solely generated from the bulk, without interference of surface recombination which happens at the electrode−electrolyte interface. Therefore, the enhanced photocurrent in Figure 6a of BVO-In compared to BVO is due to the promoting effect of In2O3 layer. It was also reported that the onset potential in hole scavenger electrolyte is close to the flat-band potential (Vfb) of the electrode materials.14 In Figure 6a the onset potential is shifted toward a lower value, indicating a cathodic shift of Vfb.

This change in Vfb indicates an augmented band bending effect in the vicinity of electrode−electrolyte interface, facilitating the migration of holes from bulk to interface. The extent of band bending is usually quantified by Vfb, where the contribution of potential drop (Gouy layer, Helmholz layer and space charge layer) is included.28 Gouy layer and Helmholtz layer are of several nanometers in width and the potential drop in these regimes is slight enough to be neglected. Hence, the cathodic shift in Vfb of BVO-In should be mainly caused by the In2O3 thin layer, which enhanced the potential drop in space charge layer that drives holes out of the bulk of electrode. Apart from bulk migration, dividing photocurrent (Jscav) value measured in hole scavenger electrolyte by the photocurrent measured in PBS (Jph) gives the injection efficiency, which provides insight into the reaction kinetics at the surface (Figure 6b). Interestingly, we discovered that In2O3 coating drastically increased the injection efficiency of BiVO4 at 1.23 V vs RHE from 30% to 57%, indicating the kinetics of water oxidation near surface is promoted by In2O3 as well. In accordance to the enhanced injection efficiency, transient spike, which is indicative of the recombination occur at the surface, was also measured, in order to further confirm the reduced recombination in the photoanode as well as to calculate the steady-state current density. As shown in parts c and d of Figure 6, spikes in photocurrent transient measured with illumination period of 2 s and sampling interval of 1 ms at various potentials present a decreasing tendency with increasing potential. Comparing the transient profile of BVO-In to that of BVO, it was found that the spikes are apparently reduced. The interfacial recombination was undermined as a result of the presence of In2O3, in agreement with the injection efficiency calculations. Generally speaking, in each illuminated period, the photocurrent record first showed a spike at initial stage and then instantly reduced to a stable value, namely steady-state current density. (Figure 6e) The exponential decay of current density has been attributed to surface charging effect29 or hole accumulation due to slow injection kinetics.30 The current generated at this spot is solely dedicated for water oxidation. 17154


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Figure 6. (a) I−V curves of BVO and BVO-In in the presence of 1 M Na2SO3 hole scavenger in 0.5 M PBS solution. (b) Injection efficiency of BVO and BVO-In. (c) Photocurrent transients measured at different potentials of BVO. (d) Photocurrent transients for BVO-In. (e) Steady-state current density of BVO (blue) and BVO-In (red). (f) EIS of BVO and BVO-In at 1.23 V vs RHE under 1 sun illumination.

voltage region is critically demanded to reach as high as possible. In ideal consideration, the working potential and current density of a zero-biased solar assisted water electrolysis cell depends on the intersect point of linear sweep voltammetry profiles of two photoelectrodes or that of a photoelectrode with a photovoltaic device.33−36 This means the onset potential of a photoanode should be positioned at a lower potential to achieve high current density in a nonbiased system. According to plenty of previous works relevant to cocatalyst incorporation with photoanodes like Fe2O3, WO3 and BiVO4, onset potential and injection efficiency could be enhanced with the assistance of a catalytic layer.4,37,38 Herein, we introduced a layer of OER electrocatalyst as well to facilitate surface reaction kinetics. CoOOH was deposited on BiVO4/In2O3 composite electrode via a photoassisted electrodeposition technique, as described in Experiment. The illumination period was controlled within 45 s and color of the sample turned slightly darker after deposition. Figure S2 showed that BVO-InCo was coated with an uniform layer of CoOOH (roughly 30 nm in thickness) by the photooxidation of Co2+ in the deposition bath solution. XPS provided a substantial evidence for the existence of high-valence cobalt species. O 1s spectrum revealed a large proportion of hydroxyl (531.5 eV) at the surface of BVO-InCo and Co 2p spectrum of

The steady-state current densities from chop light measurement manifested that the photocurrent of BVO-In is still higher than twice that of BVO in steady state. As a tool for charge transfer resistance characterization, electrochemical impedance spectrum (EIS) was used. The equivalent charge transfer impedance was measured under AM 1.5G illumination. In Figure 6f, the intercepts of the semicircles with real axis represents the total series resistance of the system, Rct, which comprises of bulk resistance in space charge layer and surface charge transfer resistance like resistances in Helmholtz layerand surface states.30−32 The Rct of BVO-In was measured to be ∼300 Ω, smaller than ∼520 Ω for BVO, coherent to the fact that both bulk migration and surface reaction kinetics were improved by the introduction of indium oxide layer. 3.4. Effect of Cocatalyst on BVO-In Composite Photoanode. It was found that the surface kinetic was improved after the loading of a thin In2O3 layer. The reason is unknown and out of the scope of the paper. However, the injection efficiency of the resultant composited photoanode, remain as low as ∼60% at 1.23 V vs RHE. In order to be able to incorporate this photoanode into a tandem cell biased by photovoltaic device, or with a p-type photocathode to achieve a spontaneous solar assisted electrolysis, the photocurrent at low 17155


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Figure 7. (a) Linear sweep voltammetry of BVO, BVO-InCo with 20 and 45 s photoassisted deposition period. (b) Linear sweep voltammetry profiles compared with sulfite oxidation sweep profile of BVO-InCo and BVO-In (dashed lines). (c) Water oxidation performance of BVO, BVO-In and BVO-InCo in dark, indicating the electrocatalytic effect of CoOOH for oxygen evolution. (d) Photocurrent density upon illumination at 0.6 V vs RHE.

potential was largely advanced and current density improved after CoOOH deposition, indicative of the excellent OER activity of the photoanode surface modified with CoOOH. It is also worth noting that the BVO-In possess larger current density compared to pristine sample. In accordance to the elevated surface injection efficiency of BVO-In observed in previous section, this results in water oxidation in dark condition suggests that In2O3 might possess a better surface kinetics toward water oxidation. Finally, the photocurrent transient spikes were investigated (Figure 7d). Compared to BVO, the spike in BVO-In and BVO-InCo were less obvious owing to faster charge injection and thereby less hole accumulation upon illumination. 3.5. Band Alignment Estimations. According to Figure 3e, a shift in VBM was observed. This shift was supposed to be caused by the combination of BiVO4 and In2O3. In order to further confirm the suitable band alignment, estimation can be made from two widely accepted equations that are used to calculate the VBM and CBM of two semiconductor:44,45

BVO-InCo showed only a broad peak centered at 780.4 eV with no shakeup satellite peaks (Figure S3), which is characteristic of Co3+ in CoOOH.39−41 Besides, we managed to deposited a thick film CoOOH on FTO by electrodeposition and two peaks in the Raman spectrum of the sample were consistent to previous reports pertaining the material (Figure S4).42,43 It was found that the optimal photocurrent can be achieved at deposition period of 20 s. Exceeding that duration, the current density started to drop and shape of the J-V curve changed. As shown in Figure 7a, the onset potential apparently shifted from 0.35 V to a lower value of 0.21 V, suggesting a cathodic shift of 140 mV after CoOOH deposition. At 1.23 V, the current density reached 3.4 mA/cm2 after photodeposition for 20 s. When the deposition duration prolonged to 45 s, photocurrent at 1.23 V deteriorated to 2.9 mA/cm2, probably due to the unused light absorption when CoOOH grew too thick. Photocurrent improvement owing to the cocatalyst is stable for up to 1 h, as shown in Figure S5 where the current profile of BVO-In and BVO-InCo are demonstrated. During stability test, gas evolution (H2 and O2) was detected by GC and measured gas quantity well matched with expected value that was calculated from accumulated charge (Figure S6, parts a and b). For BVO-InCo with geometric area of 1 cm2, 29.6 μmol O2 was produced after 3600 s, reaching a faradic efficiency of 101.1%. The addition of CoOOH did not introducing any sidereaction undermining efficiency of the electrode. Although current density of BVO-InCo for sulfite (Na2SO3) oxidation was slightly higher than that of BVO-In suggesting a probably enhanced bulk charge separation (dashed lines in Figure 7b), most drastically changed is the injection efficiency: it was augmented from 64% of BVO-In to 90% of BVO-InCo at 1.23 V. This could be attributed to the enhanced surface kinetics, as shown in Figure 7c where the water oxidation performance in dark condition was demonstrated. The onset

E VBM = X − Ee + 0.5Eg ECBM = E VB − Eg

where X stands for the absolute electronegativity of the semiconductor, while Ee means the energy of free electron on the hydrogen scale (4.5 eV). Eg is the band gap energy of the semiconductor. For BiVO4 and In2O3, the X values are 6.035 and 5.270 eV, respectively. The VBM of BiVO4 and In2O3 are accordingly calculated to be 2.765 and 2.170 V. Surprisingly, the difference between the two values (∼0.6 eV) is close to the VBM shift observed in XPS measurement, suggesting a good agreement of those experimental results. On the basis of all the above discussion, a diagram from band position estimations are shown as a diagram in Figure 8, where the CBM values are also labeled on the basis of measured band gap energy and 17156



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (H.J.). ORCID

Hongbing Ji: 0000-0003-1684-9925 Yexiang Tong: 0000-0003-4344-443X Author Contributions †

These authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was preliminarily supported by the National Science Fund for Distinguished Young Scholars (21425627), the Natural Science Foundation of China (21461162003 and 21476271), National Natural Science Foundation of ChinaSINOPEC Joint fund (U1663220), and Natural Science Foundation (2014KTSCX004, 2014A030308012 and S2013030013474) of Guangdong Province, Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC019), Nanjing University, and was also supported by the Fundamental Research Funds for the Central Universities (17lgpy74).

Figure 8. Band diagram of BVO-In photoanode for water oxidation, based on calculated values of VBM and CBM, without consideration of CoOOH layer.

estimated VBM value. The effect of CoOOH layer is not considered in the figure, since the detailed mechanism of OER catalyst operating at semiconductor surface is still unclear. There have been several works suggesting the contribution of electrocatalyst lies in the effects including: (a) flat band potential tuning (band bending effect),6 (b) hole trapping that induces electron gradient in bulk,46 and (c) fast kinetics with the assistance of multivalence metal ions (e.g., transformation from Co2+/Co3+ to Co4+ for water oxidation) .46−48 However, we are not able to classify the effect of CoOOH layer as any of the three possibilities based on our limited evidence.

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(1) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−320. (2) Xie, S.; Li, M.; Wei, W.; Zhai, T.; Fang, P.; Qiu, R.; Lu, X.; Tong, Y. Gold Nanoparticles Inducing Surface Disorders of Titanium Dioxide Photoanode for Efficient Water Splitting. Nano Energy 2014, 10, 313−321. (3) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520−35. (4) Chang, X.; Wang, T.; Zhang, P.; Zhang, J.; Li, A.; Gong, J. Enhanced Surface Reaction Kinetics and Charge Separation of p-n Heterojunction Co3O4/BiVO4 Photoanodes. J. Am. Chem. Soc. 2015, 137, 8356−9. (5) Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.; Minegishi, T.; Nakabayashi, M.; et al. Surface Modification of CoOx Loaded BiVO4 Photoanodes with Ultrathin pType NiO Layers for Improved Solar Water Oxidation. J. Am. Chem. Soc. 2015, 137, 5053−60. (6) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (7) McDonald, K. J.; Choi, K.-S. A New Electrochemical Synthesis Route for a BiOI Electrode and Its Conversion to a Highly Efficient Porous BiVO4 Photoanode for Solar Water Oxidation. Energy Environ. Sci. 2012, 5, 8553−8557. (8) Rettie, A. J.; Lee, H. C.; Marshall, L. G.; Lin, J. F.; Capan, C.; Lindemuth, J.; McCloy, J. S.; Zhou, J.; Bard, A. J.; Mullins, C. B. Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide. J. Am. Chem. Soc. 2013, 135, 11389−11396. (9) Wang, Z.; Huang, B.; Dai, Y.; Qin, X.; Zhang, X.; Wang, P.; Liu, H.; Yu, J. Highly Photocatalytic ZnO/In2O3 Heteronanostructures Synthesized by a Coprecipitation Method. J. Phys. Chem. C 2009, 113, 4612−4617. (10) Yin, J. Z.; Huang, S. B.; Jian, Z. C.; Pan, M. L.; Zhang, Y. Q.; Fei, Z. B.; Xu, X. R. Enhancement of the Visible Light Photocatalytic Activity of Heterojunction In2O3/BiVO4 Composites. Appl. Phys. A: Mater. Sci. Process. 2015, 120, 1529−1535.

4. CONCLUSION In conclusions, we successfully fabricated a composite photoanode based on BiVO4 light absorber and In2O3 thin layer coating, where the formation of BiVO4/In2O3 heterojunction was confirmed through multiple characterization techniques. Not only physical contact of the two semiconductors was directly observed, the change in the electronic structure was also discovered. Because of the wide band gap nature of coated In2O3, the light harvesting capability of the electrode was not appreciably elevated and the spectral range of converted photon, as evidenced by IPCE measurement, was also not broadened by In2O3. However, both separation efficiency in bulk and injection efficiency at the electrode surface were enhanced in this case, leading to easier charge transfer both in bulk and through the electrode−electrolyte interface. This charge injection process can be further augmented by adding an electrocatalytically active CoOOH layer on surface by photoassisted electrodeposition, which resulted in a high-performance photoanode with low onset potential and high current density.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06407. EDX elemental analysis and TEM image of BVO-In, Co 2p XPS spectrum of BVO-InCo, Raman spectrum, current−time profiles, and hydrogen and oxygen production (PDF) 17157


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Thin-Layer Indium Oxide and Cobalt Oxyhydroxide Cobalt-Modified

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