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Photoelectrochemical water splitting using lithium doped bismuth vanadate photoanode with near-complete bulk charge separation
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Photoelectrochemical water splitting using lithium doped bismuth vanadate photoanode with near-complete bulk charge separation Jyoti Prakash a, c, Umesh Prasad a, Xuan Shi a, Xihong Peng b, Bruno Azeredo a, Arunachala M. Kannan a, * a b c
The Polytechnic School, Ira A. Fulton Schools of Engineering, Arizona State University, Mesa, AZ, 85212, USA Science and Mathematics, College of Integrative Sciences and Arts, Arizona State University, Mesa, AZ, 85212, USA Materials Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Li:BiVO4 photoanode showed complete bulk charge separation efficiency. � Li doping showed an increase (>20 times) in the PEC water splitting. � Li:BiVO4 photoanode showed record PCD 7.3 � 0.36 mA cm 2 with hole scavenger. � DFT calculations showed the formation of inter-band with band gap reduction.
A R T I C L E I N F O
A B S T R A C T
Keywords: Bismuth vanadate Lithium doping Charge separation Water splitting
Photoelectrochemical performance of BiVO4 photoanode is limited by poor light absorption, charge separation and transfer efficiencies. For the first time in the literature, Li doped nano-porous BiVO4 photoanode showed complete bulk charge separation efficiency (~100%) at 1.23 V vs RHE along with enhanced light absorption for water splitting. Li doping showed an increase (>20 times) in the photoelectrochemical water splitting compared to pristine BiVO4 photoanode. In particular, oxygen evolution catalyst was also employed for further improving the photoelectrochemical performance (4.2 � 0.18 mA cm 2) of Li:BiVO4 photoanodes. The density functional theory calculations showing the formation of inter-band with band gap reduction due to interstitial Li doping in BiVO4 structure support enhancement in photoelectrochemical performance. In addition, Li doping in the BiVO4 lattice void positions led to a record photocurrent density of 7.3 � 0.36 mA cm 2 at 1.23 V vs RHE in the presence of hole scavenger under one sun illumination. Further, present study systematically demonstrates role of Li in BiVO4 host for water oxidation through a detailed characterization and study of optical and charge transport properties.
1. Introduction H2 is being considered as one of the most potential fuels in
transportation sector due to its highest gravimetric energy density among all the fuels [1,2]. Photoelectrochemical (PEC) [3] water split ting is one of the promising clean energy technologies for producing H2
* Corresponding author. E-mail address: [email protected] (A.M. Kannan). https://doi.org/10.1016/j.jpowsour.2019.227418 Received 4 October 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jyoti Prakash, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227418
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Fig. 1. (a) Schematic representation of thin film fabrication, and (b) and (c) SEM images of pristine and Li:BiVO4 samples.
and O2 gases [4]. However, commercialization of PEC devices is limited by the lack of well-developed anodes (O2 electrode) due to performance and durability issues. In the past, several wide-bandgap oxide anode materials have been evaluated for PEC technology based water splitting process in the literature [5,6]. In particular, BiVO4 is one of the well-studied materials towards PEC application due to its suitable
in-direct [7] bandgap (~2.4 eV) [8] and favorable band-edge position [9]. The maximum possible theoretical photo current density (PCD) value (Jmax) for BiVO4 based photoanodes [10] is ~7.4 mA cm 2 with a corresponding solar to H2 (STH) efficiency of ~9% under 1 sun illumi nation. The lower STH for the pristine BiVO4 photoanode is due to limited light absorption (ηabs), poor charge separation (ηsep) and lower 2
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band structure and charge transport of BiVO4 was elucidated by ab initio calculations and an inter-band with a band gap reduction of ~0.1 eV was observed. 2. Experimental section 2.1. Fabrication of BiVO4 based photoanodes The nano-porous pristine and Li doped BiVO4 photoanodes were fabricated as described earlier [20]. In brief, Ultrasonic spray coater (SonoTek Corporation, USA) was employed to coat the precursor solu tion of 3 mM NH4VO3 (Sigma-Aldrich; CAS No. 7803556) and 3 mM Bi (NO3)3⋅5H2O (Alfa Aesar; CAS No. 10035060) in HNO3 (Sigma-Aldrich; CAS No. 7697372) on 1 cm � 1 cm fluorine-doped tin oxide (FTO)- coated glass (Sigma-Aldrich; Product #: 735264). As given schemati cally in Fig. 1a, after first four layers of spraying with 0.1 ml min 1, the film was heated for 10 min in air and after the second set of four layers, the samples were heat treated at 500 � C for 2 h in air for forming the monoclinic phase. Various amounts of Li doping (1–6 wt %) were carried out by mixing required amounts of 3 mM LiNO3 (99%, Alfa Aesar; CAS No. 7790694) in the BiVO4 precursor solution. Optimum doping per centage of Li (4 wt %; denoted as Li:BiVO4) was selected based on the PEC performance (Supporting Fig. S15) of various Li doped BiVO4 photoanodes. The Li:BiVO4 photoanode material composition was also analyzed using ICPMS (Thermo Scientific ICAP-Q quadrupole) and the final composition was Li0.114BiV0.984O4. The surface catalyst (Fe:Ni (OH)2) and the Co-Pi layers on the photoanodes were deposited as re ported in the literature [10,21] and briefly described in the Supporting Information. 2.2. Physicochemical characterization The phase purity of the pristine and Li:BiVO4 samples were examined using XRD (PANalytical X’Pert PRO MRD; Cu Kα radiation) at 0.006� per sec and the surface morphology was examined using Zeiss sigma 300 FESEM-EDX spectral mapping analysis was performed under 25 k. XPS analysis was carried out using Thermo Scientific™ K-Alpha™ X-ray Photoelectron Spectrometer System; VG 220i-XL). The BE scale was calibrated with reference to Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, giving an accuracy of 0.1 eV in any peak energy posi tion determination. Steady-state UV–Visible measurements were carried out using Thermoscientific (Model: Genesys 150) for absorption/trans mittance and Agilent (Cary Series 7000 UV–Vis-NI) for diffuse reflec tance spectra (DRS) for estimating the bandgap values. Raman spectra of the pristine and Li:BiVO4 thin film samples were analyzed using a Raman Spectrometer (WITec Alpha 300R) at room temperature with Nd: YAG laser (532 nm) as excitation source.
Fig. 2. (a) XRD and (b) Raman spectra of pristine and Li:BiVO4 samples. (νs (V–O): symmetric stretching, νas(V–O): asymmetric stretching, νs (VO4)3-: symmetric deformation, and νas (VO4)3-: asymmetric deformation modes).
surface charge transfer (ηinj) efficiencies [11,12]. However, these effi ciencies can be improved by nanostructuring (in particular using 1-dimensional structure), metal doping, forming heterojunction [13] and adding surface catalysts [14]. The maximum ηsep values reported for H2 treated Mo–BiVO4 (87%) [15], Mo–BiVO4 nanocone (85%) [16], WO3/(W,Mo)BiVO4 helix (91%) [17], Sb/SnO2/BiVO4 (92%) [18] and WO3/BiVO4 nanorod (95%) [19] based photoanodes are summarized in Supporting Fig. S1a. It is worth mentioning that the complex nano structured/heterojunction photoanodes with mostly 1-dimensional morphology were employed for achieving maximum charge separation efficiency. However, this enhancement in ηsep still could not lead to achieving theoretical Jmax of BiVO4. The Supporting Fig. S1b shows the elements of interest in periodic table used in BiVO4 system, the present study is the first report with Li doping in BiVO4 system. The Li doped BiVO4 photoanodes fabricated by a simple ultrasonic spray technique exhibited a record high charge separation efficiency of ~100% with more than 20 times increase in PEC water splitting performance compared to pristine BiVO4. The Li doped BiVO4 photoanodes led to significant improvement PCD of 7.3 � 0.36 mA cm 2 at 1.23 V vs RHE in the presence of hole scavenger under one sun illumination. The theo retical PCD achieved in presence of hole scavenger (i.e. 100% ηinj) confirmed that Li doping lead to complete suppression of charge recombination in bulk of BiVO4. The effect of Li doping on the electronic
2.3. Photoelectrochemical measurements The PEC performance of the pristine and Li:BiVO4 photoanodes was evaluated in three electrode configuration using Pt wire and saturated calomel electrode (SCE) as counter and reference electrode, respec tively. The detailed photoanode fabrication method is provided in the Supporting Information. A solar simulator (Newport 67005) with one sun illumination (100 mW cm 2) along with a monochromator (New port CS-130) and PARSTAT-2273 galvanostat/potentiostat/impedance unit were employed for measuring PEC and impedance values [20]. The detailed description of the cell configuration along with the positioning of photoanode to achieve one sun illumination on surface is provided in the Supporting Information section 2. The PEC measurements were carried out at room temperature in 0.1 M K2HPO4 solution (pH 8.0) with and without hole scavenger (1 M Na2SO3) using linear sweep voltam metry (20 mV s 1) with the continuous and chopped light illumination (Frequency: 0.33 Hz). The voltage values measured against SCE were converted to RHE scale [20]. Before all electrochemical measurements, 3
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Fig. 3. XPS spectra of (a) Bi 4d, (b) V 2p, and (c) and (d) O1s with fitted peaks using a Shirley background across peak region for pristine and Li:BiVO4 samples, respectively.
the electrolyte was thoroughly deaerated by purging with N2 gas. The PCD and separation efficiency values are the mean value using four different pristine and Li:BiVO4 photoanodes. Mott–Schottky measure ments were carried out in dark at 1 kHz and the impedance measure ment (100 mHz–100 kHz) was performed under light illumination with an AC amplitude of 20 mV at 1.23 V vs RHE. EC-Lab v11.20 software was used for curve fitting to develop equivalent circuits. The ECSA values were estimated by conducing cyclic voltammetry experiments at various scan rates in 0.1 M K2HPO4 electrolyte (pH 8.0) in dark [22]. The respective Jmax values for the pristine and Li:BiVO4 photoanodes were estimated based on literature [23] and detail provided in Supporting Information Section 2. The Jmax for the pristine and Li doped BiVO4 photoanodes were 5.9 and 7.4 mA cm 2, respectively. The photocurrent obtained for the water and sulfite oxidations were used to calculate the electron-hole separation efficiency (ηsep) using the procedure in our previous study [20] and brief detail provided in Sup porting Information. The durability of the pristine and Li:BiVO4 photo anodes was evaluated at 1.23 V vs RHE under one sun illumination in K2HPO4 electrolyte with and without hole scavenger.
wave (PAW) potentials [27,28] were employed. The Bi(5d, 6s, 6p), V (2p, 3d, 4s), O(2s, 2p) and Li(1s, 2s, 2p) electrons were treated as valence electrons [29]. The spin polarized wavefunctions and plane wave basis sets with an energy cutoff 550 eV were used. The reciprocal space was meshed at 5 � 5 � 7 using the Monkhorst-Pack method [11] and the line path L-M-A-Γ-Z-V was used to obtain the band structure [27] of pristine and Li doped BiVO4. The site projected partial density of states (PDOS) were also calculated for determining the contributions from various elements (Bi, V, O, Li) investigated. The energy conver gence criteria for electronic and ionic iterations were set to be 10 5 and 10 4 eV, respectively. The DFT þ U method [30] was used to improve the prediction of the highly localized d-electron correlation in the transition metal V with a Hubbard U value of 2.7 eV [11]. The detailed description of model provided in Supporting Information section 1. 3. Results and discussion Li doped BiVO4 with planar morphology based photoanodes were prepared for improving charge separation efficiency along with enhanced light absorption by controlling the photoanode thickness (>100 nm). As shown schematically in Fig. 1a, the pristine and Li doped BiVO4 layers were fabricated on the FTO glass using metal precursors by a simple ultrasonic spray coating method (Supporting Fig. S2 a-d). The scanning electron micrograph (Fig. 1b) of the pristine BiVO4 film shows formation of planar uniform nano-porous surface. The surface image (Supporting Fig. S3a) obtained with 30� tilting shows the uniform growth of nano-size grains on the surface and inset shows the presence
2.4. DFT calculation The first principles DFT calculations including structural optimiza tion and electronic properties were carried out using the Vienna Ab initio Simulation Package (VASP) with generalized gradient approximation (GGA) [24,25]. In particular, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [26] and the projector-augmented 4
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Fig. 4. (a) UV–visible absorption spectra (inset shows absorption efficiency derived from LHE plot; Supporting Information Section 2), (b) PEC performance in 0.1 M K2HPO4 electrolyte (chopped light), (c) PEC performance (chopped light), and (d) IPCE in 0.1 M K2HPO4 with 1 M Na2SO3 under 1 sun illumination.
of inherent cracks on the grain surface, which can help in increasing contact area between film surface and electrolyte. As seen in Fig. 1c, the Li doping in BiVO4 did not result in any significant difference in surface morphology. The cross sectional image shows (Supporting Fig. S3b) formation of connected nano-pores throughout thickness (~500 nm) of the BiVO4 layer. X-ray diffraction (XRD) data for the pristine and Li:BiVO4 layers shown in Fig. 2a confirmed the formation of single-phase monoclinic crystal structure (pdf # 14-0688) without any impurity phases. How ever, a slight shift of ( 112) and (040) hkl peaks towards lower angle was observed for the Li doped samples and the corresponding lattice parameters based on the Rietveld analysis are given in Supporting Table S1 and supporting Figs. S4a–c. As seen in Table S1, the cell pa rameters a, b and c increased slightly and the unit cell volume increase was ~0.3% for Li:BiVO4 compared to that of the pristine BiVO4. The volume increase could be due to the Li atoms doping at the interstitial positions of the BiVO4 crystal lattice and was verified by the ab initio density functional theory (DFT) calculations. The incorporation of Li into the BiVO4 lattice was also examined by Raman spectra and shown in Fig. 2b. The major peaks for pristine BiVO4 sample were observed at ~823, 330 and 364 cm 1, corresponding to the υs (V–O), δas (VO34 ) and δs (VO34 ) stretching mode vibrations, respectively. The Li doped sample showed shift of V–O stretching and VO34 deformation modes towards higher wave number. The Li present at the interstitial position in BiVO4 lattice could distort the vibrational modes of V–O bond and the stressed vibrational modes would show higher energy. As seen from the XPS data in Fig. 3a and b, the shifting of Bi4f and V2p peaks of the Li doped BiVO4
to lower binding energy could be due to the changes in local coordina tion environments of Bi and V ions [31]. It is very clear that the electron sphere around Li at interstitial position can exert electrostatic repulsive force, leading to decrease in (Bi–O and V–O) bond energy and an expansion in the unit cell volume, as observed in XRD (Fig. 2a). Fig. 3c and d shows the high-resolution XPS spectra for O1s for the pristine and Li doped BiVO4 samples (detailed analysis on the oxidation state is provided in Supporting Information, section 1, supporting Fig. S4d). The peaks at ~530.6, 533 and 529 eV can be assigned to surface lattice ox ygen (V–O), surface adsorbed oxygen ( OH) and for Li–O bonds [32], respectively. The peak area ratios of Li–O/ OH/V–O in pristine BiVO4 and Li:BiVO4 as shown in Fig. 4c and d confirm the presence of a weak Li–O bond [33]. The UV–Visible absorption spectra of the pristine and Li doped BiVO4 photoanodes are shown in Fig. 4a. As observed, the band edge position for the Li:BiVO4 photoanodes moved slightly towards higher wavelength (~505–~530 nm) and significant absorption occurred before absorption edge, suggesting formation of inter-band gap [5] level. As seen in Fig. 4a, the absorption was substantially higher for the Li:BiVO4 pho toanode compared to the pristine photoanode because of the Li doping in interstitial position of BiVO4 crystal lattice generating significant disorder in the atomic arrangement due to electrostatic interaction and oxygen deficiency. This has generated several active sites with the cre ation of inter-band states [34] in the band gap region leading to considerable lower light reflectance from Li:BiVO4 surface (Supporting Fig. S5). The calculated value of ηabs from LHE plot (Supporting Infor mation Section 2) for the pristine and Li:BiVO4 photoanodes were ~64 5
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Fig. 5. (a) Charge separation (ηabs) and transfer (ηinj) efficiencies for pristine and Li:BiVO4 photoanodes, (b) Nyquist data (100 mHz-100kHz) under 1 sun illumi nation at 1.23 V vs RHE in 0.1 M K2HPO4 with 1 M Na2SO3 electrolyte, (c) Resistance and capacitance values, and (d) Jstable/Jo plot for pristine and Li:BiVO4 photoanodes.
and 77% (inset in Fig. 4a). Thus, the Li:BiVO4 showed improved light harvesting with the film thickness more than the hole diffusion length (~100 nm). As seen in Fig. 4b, the Li:BiVO4 photoanode exhibited sig nificant enhancement (>20 times) in the PCD (2.35 � 0.12 and 6.2 � 0.30 mA cm 2 at 1.23 and 2.0 V vs RHE, respectively) compared to that of the pristine BiVO4 (0.11 � 0.01 and 1.04 � 0.05 mA cm 2 at 1.23 and 2.0 V, respectively) in K2HPO4 electrolyte. Whereas the PCD reaches 7.3 � 0.36 mA cm 2 (Jmax: 7.4 mA cm 2) at 1.23 V vs RHE in K2HPO4 with Na2SO3 electrolyte under 1 sun illumination (Fig. 4c). The record high PCD with surface transfer kinetics (ηinj) of almost 100% is the highest ever reported for the BiVO4 based photoanodes at 1.23 V vs RHE in the literature (PEC performance values are compared in Supporting Fig. S1). As shown in Fig. 4d, Li:BiVO4 exhibits a maximum IPCE value of ~95% (~4 times that of the pristine BiVO4) at 410 nm. This high IPCE in wavelength 400-450 nm region was further confirmed by low diffuse reflectance (supporting Fig. S5a) and negligible transmittance (sup porting Fig. S5b) of the Li:BiVO4 sample. The integrated current density [35] (shown on the secondary Y axis in Fig. 4d) corresponding to the IPCE matches well with experimental PCD value of 7.38 mA cm 2 at 1.23 V vs RHE. The absorbance (Fig. 4a) and the IPCE (Fig. 4d) values clearly demonstrate the extended wavelength range (400–550 nm) by the Li:BiVO4 photoanode towards PEC activity. The corresponding ηsep and ηinj values (detailed calculation method provided in Supporting Information Section 2) for the pristine and Li doped BiVO4 photoanodes shown in Fig. 5a. It was observed that the ηsep increased from 20 to 100% and the charge transfer efficiency increased from ~3 to 22% at 1.23 V vs RHE for Li:BiVO4 with respect to pristine photoanode. The band gap estimation through in Tauc plot (Supporting Fig. S6), showed that the bandgap value reduced from 2.45 to 2.36 eV due to Li doping in BiVO4 and the reduction in band gap was also
supported with slight negative shift in the onset of flat band potential (Mott-Schottky analysis; Supporting Fig. S7a), leading to enhanced light absorption. To determine onset voltage of the photocurrent, the photooxidation of sulfite was carried out, as photo oxidation of water suffers from slow oxidation kinetics leading to more error (Supporting Fig. S7b). The Nyquist plots and corresponding resistance and capaci tance values are given in Fig. 5b and c, respectively. The significantly smaller semicircle showed lower resistance values associated with Li: BiVO4 compared to that of the pristine photoanode (Fig. 5b). The resistance and capacitance values in bulk and surface for both the pristine and Li:BiVO4 photoanodes obtained using an equivalent circuit model (Supporting Fig. S8) for the impedance patterns are shown in Fig. 5c and the values are given in Supplementary Table S2. Evidently, the Li:BiVO4 showed extremely lower bulk resistance (Rb) compared to that of the pristine photoanode, leading exceptionally high ηsep (Fig. 5a). In addition, slight reduction in surface resistance (Rs) lead to increase in surface charge transfer efficiency (ηinj) (Fig. 5a) for the Li:BiVO4 pho toanode but not enough to significantly improve the PEC water oxida tion performance. The Bode measurements carried out at 1.23 V vs RHE (Supporting Fig. S9) supports that the ηinj increased with Li doping in BiVO4, as there is a significant reduction of the peak at 200 mHz, cor responding to the capacitance at photoanode surface and electrolyte interface. The increase in ηinj, was also confirmed by the CV measure ments for doped and undoped Li:BiVO4 photoanodes in dark in K2HPO4 electrolyte in a wider potential range and is given in Supporting Fig. S10. The cathodic peak in dark and light at ~1.4 V vs RHE is due to the 2þ recombination reaction (VOþ 2 /VO Þ with a lower intensity for Li:BiVO4 suggested reduction of trap states in BiVO4 with Li doping compared the pristine. In addition, the lower bulk (Cb) and higher surface (Cs) capacitance values of the Li:BiVO4 photoanode also improved the charge 6
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ηinj ~42% (Supporting Fig. S13). The stable high PCD shown by the Li:
BiVO4/Fe:Ni(OH)2/Co-Pi photoanode can be suitably used in standard PEC cell in the potential range of 1.6–2 V vs RHE to achieve optimum performance [4]. The performance stability of the Li:BiVO4/Fe:Ni (OH)2/Co-Pi photoanode for water oxidation at 1.23 V vs RHE was evaluated with light ON (11 h) and OFF (2 h) for three cycles (total duration: 33 h) and the overall performance retention observed was ~91% (Fig. 6b). In addition, the Li:BiVO4/Fe:Ni(OH)2/Co-Pi photo anode (Supporting Fig. S14) also exhibited excellent performance retention (96% of PCD) over 30 h of continuous operation in the pres ence of hole scavenger. Fig. 7a shows the monoclinic unit cell crystal structures for the pristine and Li doped BiVO4. Evidently the Li (4 wt %) doping did not change the space group of the crystal structure and about ~2% volume expansion was observed due to Li insertion at interstitial position, which is consistent with XRD and Raman results (Fig. 2a and b). Fig. 7b shows the simulated absorption spectrum (imaginary part of dielectric func tion) for the pristine and Li doped BiVO4. As seen, both the pristine and Li doped BiVO4 exhibited higher absorption >2 eV. However, the Li: BiVO4 showed an additional absorption peak ~1.2 eV, possibly due to reduction in band gap. This supports the experimental observation of increase in absorption at extended lower energy region due to Li doping (Fig. 4a). The simulated band structures in Fig. 7c shows, the presence of interband with reduction of bandgap (2.14–2.06 eV) [38,39] for the Li doped compared to the pristine BiVO4. With the Li doping at the inter stitial position in BiVO4, both the conduction and valence bands move downward leading to reduction in the bandgap by 0.08 eV, due to larger downward shift of the conduction band. This supports the experimental observation of reduction of band gap (Supporting Fig. S6) and the downward shift of conduction band edge in the Mott Schottkey plot (Supporting Fig. S7a) for the Li doped BiVO4. Fig. 7c also shows an interband formation at ~1.5 eV for the Li doped BiVO4, due to atomic distortedness generated in BiVO4 with Li doping. The crystal structure along with electron spin density contour for interband in Li:BiVO4 Fig. 7d showed the formation of possible small polarons localized pre dominantly at V atoms [11] leading to better charge mobility evidenced from lower bulk and surface charge transfer resistance values (Fig. 5c and Table S2). As evident from various characterizations including first principles DFT calculations, the Li:BiVO4 based electrode with the ex istence of interband along lower band gap combined with improved charge mobility, complete charge separation efficiency compared to the pristine electrodes, showed relatively higher PEC performance. The near complete charge separation efficiency observed in Li:BiVO4 based pho toanode with exceptionally higher performance could lead to commer cialization of photoelectrochemical water splitting system.
Fig. 6. (a) PEC performance in 0.1 M K2HPO4 electrolyte under 1 sun illumi nation (chopped light), and (b) PEC performance stability at 1.23 V vs RHE in 0.1 M K2HPO4 electrolyte for Li:BiVO4/Fe:Ni(OH)2/Co-Pi photoanodes with ON (11 h) and OFF (2 h).
separation and transfer in comparison with that of the pristine photo anode. The electron-hole recombination process was further elaborated by the transient photocurrent measurements at 1.23 V vs RHE without and with hole scavenger in potassium phosphate electrolyte (see Sup porting Fig. S11 a-b). Li:BiVO4 photoanode reached first to steady state Jstable after a certain period of illumination. The Jstable to Jo (Jo: initial photocurrent spike when the light is turned ON) ratio shown in Fig. 5d for the pristine and Li doped BiVO4 photoanodes clearly demonstrates that the electron-hole recombination is relatively lower with efficient charge separation [36] with Li doping. The Jstable to Jo ratio reached to unity in electrolytes with hole scavenger, confirmed that Li doping in BiVO4 lead to drastically improve the bulk charge separation. The in crease in the charge carrier density (ND) estimated from Mott-Schottky analysis (Supporting Fig. S7a) and electrochemically active surface area (ECSA) values (Supporting Table S2 and Figs. S12a–c) for the Li doped over the pristine BiVO4 photoanodes also support the perfect charge separation efficiency as well as the enhanced PEC performance. In addition, negative shift of the flatband potential (Supporting Fig. 7a) as well as photocurrent onset potential (Supporting Fig. S7b) is also evidenced by the carrier density increase for the Li doped BiVO4 n-type semiconductor [37]. In order to improve the PEC performance further, Li:BiVO4 photo anode was also coated with Fe:Ni(OH)2/Co-Pi surface catalyst for enhancing the charge transfer efficiency. The Li:BiVO4/Fe:Ni(OH)2/CoPi photoanode showed the impressive water oxidation PCD of 4.2 � 0.19 mA cm 2 at 1.23 V and 6.3 � 0.28 mA cm 2 at 2.0 V vs RHE (Fig. 6a). The PCD at 1.23 V vs RHE corresponds to the ηsep ~100% and
4. Conclusion The separation and transport of charge carriers in BiVO4 photo anodes are generally limited by diffusion length of holes (~100 nm) and could be managed through thin film photoanodes. However, thin film photoanodes result in poor light absorption leading to lower PEC per formance. In present study, a near complete bulk charge separation ef ficiency in ~500 nm thick film with improved absorption has been achieved in planar morphology BiVO4 system with lightest metal Li doping. The effect of Li doping in BiVO4 were thoroughly examined experimentally and computationally to provide new understanding of the electronic band structures and photo electrochemical properties of BiVO4. The Li:BiVO4 photo anode was prepared using simple ultrasonic spray coating technique and Li doping has not changed the space group of BiVO4 unit cell. The XRD and Raman results showed that with doping Li goes to interstitial position of BiVO4 unit cell. The Li doping has increased the light absorption, charge carrier density, mobility, elec trochemical active surface area and lower trap state formation in such a way that near complete bulk charge separation in PEC process can be achieved. The IPCE measurements confirmed the extended and 7
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Fig. 7. (a) The crystal structures of the pristine and Li doped BiVO4 predicted from DFT calculations (Bi: purple, V: blue, O: red and Li: green), (b) Calculated imaginary part of dielectric function for the pristine (black) and Li doped BiVO4 (red), (c) Band struc tures of pristine BiVO4 (blue dots) and Li doped BiVO4. The spin up and down are showed in red and black lines, respectively. The energies in the band structures and PDOS were aligned with the energies in 5s electrons of Bi atoms which are far away from the defect spot, and (d) Spin density map of Li doped BiVO4 with iso-surface at 0.02 e.bohr 3 at interband. (For interpreta tion of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
improved absorption in Li:BiVO4 photoanode that lead to achieve theoretical photo current density. The impedance results showed that the Li doping has drastically reduced the bulk resistance, creating better charge mobility environment. The Ab initio calculations showed that the 4 wt % Li doping in BiVO4 interstitial lattice position resulted in change of coordination environment for oxygen atom and lead to reduction in band gap as well as formation of inter-band between valence and con duction band. The experimental and theoretical studies reported herein have confirmed the strategic role played by Li in achieving exceptionally high charge separation efficiency.
[6] A. Eftekhari, V.J. Babu, S. Ramakrishna, Photoelectrode nanomaterials for photoelectrochemical water splitting, Int. J. Hydrogen Energy 42 (2017) 11078–11109. [7] J.K. Cooper, S. Gul, F.M. Toma, L. Chen, P.A. Glans, J. Guo, J.W. Ager, J. Yano, I. D. Sharp, Electronic structure of monoclinic BiVO4, Chem. Mater. 26 (2014) 5365–5373. [8] B. Lamm, B.J. Trze�sniewski, H. D€ oscher, W.A. Smith, M. Stefik, Emerging postsynthetic improvements of BiVO4 photoanodes for solar water splitting, ACS Energy Lett. 3 (2017) 112–124. [9] E.Y. Liu, J.E. Thorne, Y. He, D. Wang, Understanding photocharging effects on bismuth vanadate, ACS Appl. Mater. Interfaces 9 (2017) 22083–22087. [10] F.F. Abdi, N. Firet, R. vande Krol, Efficient BiVO4 thin film photoanodes modified with cobalt phosphate catalyst and W-doping, ChemCatChem 5 (2013) 490–496. [11] T.W. Kim, Y. Ping, G.A. Galli, K.S. Choi, Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting, Nat. Commun. 6 (2015) 1–10. [12] F.F. Abdi, L. Han, A.H.M. Smets, M. Zeman, B. Dam, R. Van De Krol, Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nat. Commun. 4 (2013) 1–7. [13] S.K. Cho, H.S. Park, H.C. Lee, K.M. Nam, A.J. Bard, Metal doping of BiVO4 by composite electrodeposition with improved photoelectrochemical water oxidation, J. Phys. Chem. C 117 (2013) 23048–23056. [14] J. Prakash, U. Prasad, R. Alexander, J. Bahadur, K. Dasgupta, A.M. Kannan, Photoelectrochemical Solar water splitting: the role of the carbon nano materials in bismuth vanadate composite photoanodes towards efficient charge separation and transport, Langmuir (2019), https://doi.org/10.1021/acs.langmuir.9b02782. [15] J.H. Kim, Y. Jo, J.H. Kim, J.W. Jang, H.J. Kang, Y.H. Lee, D.S. Kim, Y. Jun, J.S. Lee, Wireless solar water splitting device with robust cobalt-catalyzed, dual-doped BiVO4 photoanode and perovskite solar cell in tandem: a dual absorber artificial leaf, ACS Nano 9 (2015) 11820–11829. [16] Y. Qiu, W. Liu, W. Chen, G. Zhou, P.C. Hsu, R. Zhang, Z. Liang, S. Fan, Y. Zhang, Y. Cui, Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells, Sci. Adv. 2 (6) (2016) 1–9, e1501764. [17] X. Shi, I.Y. Choi, K. Zhang, J. Kwon, D.Y. Kim, J.K. Lee, S.H. Oh, J.K. Kim, J. H. Park, Efficient photoelectrochemical hydrogen production from bismuth vanadate-decorated tungsten trioxide helix nanostructures, Nat. Commun. 5 (2014) 1–8. [18] L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L.V. Titova, P.M. Rao, High light absorption and charge separation efficiency at low applied voltage from Sb-doped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano Lett. 16 (2016) 3463–3474. [19] Y. Pihosh, I. Turkevych, K. Mawatari, J. Uemura, Y. Kazoe, S. Kosar, K. Makita, T. Sugaya, T. Matsui, D. Fujita, M. Tosa, M. Kondo, T. Kitamori, Photocatalytic
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227418. References [1] I. Arto, I. Capell� an-P� erez, R. Lago, G. Bueno, R. Bermejo, The energy requirements of a developed world, Energy Sustain. Dev. 33 (2016) 1–13. [2] I. Dincer, C. Acar, Innovation in hydrogen production, Int. J. Hydrogen Energy 42 (2017) 14843–14864. [3] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Hydrogen from photocatalytic water splitting process: a review, Renew. Sustain. Energy Rev. 43 (2015) 599–610. [4] Hydrogen production: photoelectrochemical water splitting. https://www.energy. gov/eere/fuelcells/hydrogen-production-photoelectrochemical-water-splitting. [5] T. Yao, X. An, H. Han, J.Q. Chen, C. Li, Photoelectrocatalytic materials for solar water splitting, Adv. Energy Mater. 8 (2018) 1–36.
8
J. Prakash et al.
[20]
[21] [22] [23]
[24] [25] [26] [27] [28] [29]
Journal of Power Sources xxx (xxxx) xxx
generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency, Sci. Rep. 5 (2015) 1–2. U. Prasad, J. Prakash, S. Gupta, J. Zuniga, Y. Mao, B. Azeredo, A.N.M. Kannan, Enhanced photoelectrochemical water splitting with Er and W co-doped bismuth vanadate with WO3 heterojunction based 2-dimensional photoelectrode, ACS Appl. Mater. Interfaces 11 (2019) 19029–19039. L. Li, X. Yang, Y. Lei, H. Yu, Z. Yang, Z. Zheng, D. Wang, Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode, Chem. Sci. 9 (2018) 8860–8870. B.K. Kang, G.S. Han, J.H. Baek, D.G. Lee, Y.H. Song, S. Bin Kwon, I.S. Cho, H. S. Jung, D.H. Yoon, Nanodome structured BiVO4/GaOxN1 x photoanode for solar water oxidation, Adv. Mater. Interfaces. 4 (2017) 1–8. U. Prasad, J. Prakash, B. Azeredo, A.M. Kannan, Stoichiometric and nonstoichiometric tungsten doping effect in Bismuth Vanadate based photoactive material for Photoelectrochemical Water Splitting, Electrochim. Acta 299 (2019) 262–272. G. K, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (11169) (1996) 1–18. G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6 (1996) 15–50. J.P. Perdew, J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (3865) (1996) 1–4. P.E. Bl€ ochl, P.E. Bl€ ochl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 1–27, 17953. G. Kresse, G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59 (1758) (1999) 1–18. Z. Zhao, Z. Zou, Electronic Structure and Optical Properties of Monoclinic, 2011, pp. 4746–4753.
[30] S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Electronenergy-loss spectra and the structural stability of nickel oxide: an LSDAþ U study, Phys. Rev. B 57 (1505) (1998) 1–5. [31] A.M. Rassoolkhani, W. Cheng, J. Lee, A. Mckee, J. Koonce, J. Coffel, A.H. Ghanim, G.A. Aurand, C.S. Kim, W.I. Park, H. Jung, S. Mubeen, Nanostructured bismuth vanadate/tungsten oxide photoanode for chlorine production with hydrogen generation at the dark cathode, Commun. Chem. 5 (2015) 1–7. [32] K.P.C. Yao, D.G. Kwabi, R.A. Quinlan, A.N. Mansour, A. Grimaud, Y.L. Lee, Y.C. Lu, Y. Shao-Horn, Thermal stability of Li2O2 and Li2O for li-air batteries: in situ XRD and XPS studies, J. Electrochem. Soc. 160 (2013) 824–831. [33] T. Palaniselvam, L. Shi, G. Mettela, D.H. Anjum, R. Li, K.P. Katuri, P.E. Saikaly, P. Wang, Vastly Enhanced BiVO4 Photocatalytic OER Performance by NiCoO2 as Cocatalyst, 2017, pp. 1–10, 1700540. [34] X. Li, M. Han, X. Zhang, C. Shan, Z. Hu, Z. Zhu, J. Chu, Temperature-dependent band gap, interband transitions, and exciton formation in transparent p-type delafossite CuCr1 xMgxO2 films, Phys. Rev. B 90 (035308) (2014) 1–8. [35] T.G. Vo, Y. Tai, C.Y. Chiang, Novel hierarchical ferric phosphate/bismuth vanadate nanocactus for highly efficient and stable solar water splitting, Appl. Catal. B Environ. 243 (2019) 657–666. [36] Q. Shi, S. Murcia-L� opez, P. Tang, C. Flox, J.R. Morante, Z. Bian, H. Wang, T. Andreu, Role of tungsten doping on the surface states in BiVO4Photoanodes for water oxidation: tuning the electron trapping process, ACS Catal. 8 (2018) 3331–3342. [37] W.P. Gomes, F. Cardon, Electron energy levels in semiconductor electrochemistry, Prog. Surf. Sci. 12 (1982) 155–216. [38] Z. Zhao, Z. Li, Z. Zou, Electronic structure and optical properties of monoclinic clinobisvanite BiVO4, Phys. Chem. Chem. Phys. 13 (2011) 4746–4753. [39] Y. Yuan, Y. Huang, F. Ma, Z. Zhang, X. Wei, Effects of oxygen vacancy on the mechanical, electronic and optical properties of monoclinic BiVO4, J. Mater. Sci. 52 (2017) 8546–8555.
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Photoelectrochemical water splitting using lithium doped bismuth vanadate photoanode with near-complete bulk charge separation Jyoti Prakash a, c, Umesh Prasad a, Xuan Shi a, Xihong Peng b, Bruno Azeredo a, Arunachala M. Kannan a, * a b c
The Polytechnic School, Ira A. Fulton Schools of Engineering, Arizona State University, Mesa, AZ, 85212, USA Science and Mathematics, College of Integrative Sciences and Arts, Arizona State University, Mesa, AZ, 85212, USA Materials Group, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Li:BiVO4 photoanode showed complete bulk charge separation efficiency. � Li doping showed an increase (>20 times) in the PEC water splitting. � Li:BiVO4 photoanode showed record PCD 7.3 � 0.36 mA cm 2 with hole scavenger. � DFT calculations showed the formation of inter-band with band gap reduction.
A R T I C L E I N F O
A B S T R A C T
Keywords: Bismuth vanadate Lithium doping Charge separation Water splitting
Photoelectrochemical performance of BiVO4 photoanode is limited by poor light absorption, charge separation and transfer efficiencies. For the first time in the literature, Li doped nano-porous BiVO4 photoanode showed complete bulk charge separation efficiency (~100%) at 1.23 V vs RHE along with enhanced light absorption for water splitting. Li doping showed an increase (>20 times) in the photoelectrochemical water splitting compared to pristine BiVO4 photoanode. In particular, oxygen evolution catalyst was also employed for further improving the photoelectrochemical performance (4.2 � 0.18 mA cm 2) of Li:BiVO4 photoanodes. The density functional theory calculations showing the formation of inter-band with band gap reduction due to interstitial Li doping in BiVO4 structure support enhancement in photoelectrochemical performance. In addition, Li doping in the BiVO4 lattice void positions led to a record photocurrent density of 7.3 � 0.36 mA cm 2 at 1.23 V vs RHE in the presence of hole scavenger under one sun illumination. Further, present study systematically demonstrates role of Li in BiVO4 host for water oxidation through a detailed characterization and study of optical and charge transport properties.
1. Introduction H2 is being considered as one of the most potential fuels in
transportation sector due to its highest gravimetric energy density among all the fuels [1,2]. Photoelectrochemical (PEC) [3] water split ting is one of the promising clean energy technologies for producing H2
* Corresponding author. E-mail address: [email protected] (A.M. Kannan). https://doi.org/10.1016/j.jpowsour.2019.227418 Received 4 October 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jyoti Prakash, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227418
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Fig. 1. (a) Schematic representation of thin film fabrication, and (b) and (c) SEM images of pristine and Li:BiVO4 samples.
and O2 gases [4]. However, commercialization of PEC devices is limited by the lack of well-developed anodes (O2 electrode) due to performance and durability issues. In the past, several wide-bandgap oxide anode materials have been evaluated for PEC technology based water splitting process in the literature [5,6]. In particular, BiVO4 is one of the well-studied materials towards PEC application due to its suitable
in-direct [7] bandgap (~2.4 eV) [8] and favorable band-edge position [9]. The maximum possible theoretical photo current density (PCD) value (Jmax) for BiVO4 based photoanodes [10] is ~7.4 mA cm 2 with a corresponding solar to H2 (STH) efficiency of ~9% under 1 sun illumi nation. The lower STH for the pristine BiVO4 photoanode is due to limited light absorption (ηabs), poor charge separation (ηsep) and lower 2
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band structure and charge transport of BiVO4 was elucidated by ab initio calculations and an inter-band with a band gap reduction of ~0.1 eV was observed. 2. Experimental section 2.1. Fabrication of BiVO4 based photoanodes The nano-porous pristine and Li doped BiVO4 photoanodes were fabricated as described earlier [20]. In brief, Ultrasonic spray coater (SonoTek Corporation, USA) was employed to coat the precursor solu tion of 3 mM NH4VO3 (Sigma-Aldrich; CAS No. 7803556) and 3 mM Bi (NO3)3⋅5H2O (Alfa Aesar; CAS No. 10035060) in HNO3 (Sigma-Aldrich; CAS No. 7697372) on 1 cm � 1 cm fluorine-doped tin oxide (FTO)- coated glass (Sigma-Aldrich; Product #: 735264). As given schemati cally in Fig. 1a, after first four layers of spraying with 0.1 ml min 1, the film was heated for 10 min in air and after the second set of four layers, the samples were heat treated at 500 � C for 2 h in air for forming the monoclinic phase. Various amounts of Li doping (1–6 wt %) were carried out by mixing required amounts of 3 mM LiNO3 (99%, Alfa Aesar; CAS No. 7790694) in the BiVO4 precursor solution. Optimum doping per centage of Li (4 wt %; denoted as Li:BiVO4) was selected based on the PEC performance (Supporting Fig. S15) of various Li doped BiVO4 photoanodes. The Li:BiVO4 photoanode material composition was also analyzed using ICPMS (Thermo Scientific ICAP-Q quadrupole) and the final composition was Li0.114BiV0.984O4. The surface catalyst (Fe:Ni (OH)2) and the Co-Pi layers on the photoanodes were deposited as re ported in the literature [10,21] and briefly described in the Supporting Information. 2.2. Physicochemical characterization The phase purity of the pristine and Li:BiVO4 samples were examined using XRD (PANalytical X’Pert PRO MRD; Cu Kα radiation) at 0.006� per sec and the surface morphology was examined using Zeiss sigma 300 FESEM-EDX spectral mapping analysis was performed under 25 k. XPS analysis was carried out using Thermo Scientific™ K-Alpha™ X-ray Photoelectron Spectrometer System; VG 220i-XL). The BE scale was calibrated with reference to Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, giving an accuracy of 0.1 eV in any peak energy posi tion determination. Steady-state UV–Visible measurements were carried out using Thermoscientific (Model: Genesys 150) for absorption/trans mittance and Agilent (Cary Series 7000 UV–Vis-NI) for diffuse reflec tance spectra (DRS) for estimating the bandgap values. Raman spectra of the pristine and Li:BiVO4 thin film samples were analyzed using a Raman Spectrometer (WITec Alpha 300R) at room temperature with Nd: YAG laser (532 nm) as excitation source.
Fig. 2. (a) XRD and (b) Raman spectra of pristine and Li:BiVO4 samples. (νs (V–O): symmetric stretching, νas(V–O): asymmetric stretching, νs (VO4)3-: symmetric deformation, and νas (VO4)3-: asymmetric deformation modes).
surface charge transfer (ηinj) efficiencies [11,12]. However, these effi ciencies can be improved by nanostructuring (in particular using 1-dimensional structure), metal doping, forming heterojunction [13] and adding surface catalysts [14]. The maximum ηsep values reported for H2 treated Mo–BiVO4 (87%) [15], Mo–BiVO4 nanocone (85%) [16], WO3/(W,Mo)BiVO4 helix (91%) [17], Sb/SnO2/BiVO4 (92%) [18] and WO3/BiVO4 nanorod (95%) [19] based photoanodes are summarized in Supporting Fig. S1a. It is worth mentioning that the complex nano structured/heterojunction photoanodes with mostly 1-dimensional morphology were employed for achieving maximum charge separation efficiency. However, this enhancement in ηsep still could not lead to achieving theoretical Jmax of BiVO4. The Supporting Fig. S1b shows the elements of interest in periodic table used in BiVO4 system, the present study is the first report with Li doping in BiVO4 system. The Li doped BiVO4 photoanodes fabricated by a simple ultrasonic spray technique exhibited a record high charge separation efficiency of ~100% with more than 20 times increase in PEC water splitting performance compared to pristine BiVO4. The Li doped BiVO4 photoanodes led to significant improvement PCD of 7.3 � 0.36 mA cm 2 at 1.23 V vs RHE in the presence of hole scavenger under one sun illumination. The theo retical PCD achieved in presence of hole scavenger (i.e. 100% ηinj) confirmed that Li doping lead to complete suppression of charge recombination in bulk of BiVO4. The effect of Li doping on the electronic
2.3. Photoelectrochemical measurements The PEC performance of the pristine and Li:BiVO4 photoanodes was evaluated in three electrode configuration using Pt wire and saturated calomel electrode (SCE) as counter and reference electrode, respec tively. The detailed photoanode fabrication method is provided in the Supporting Information. A solar simulator (Newport 67005) with one sun illumination (100 mW cm 2) along with a monochromator (New port CS-130) and PARSTAT-2273 galvanostat/potentiostat/impedance unit were employed for measuring PEC and impedance values [20]. The detailed description of the cell configuration along with the positioning of photoanode to achieve one sun illumination on surface is provided in the Supporting Information section 2. The PEC measurements were carried out at room temperature in 0.1 M K2HPO4 solution (pH 8.0) with and without hole scavenger (1 M Na2SO3) using linear sweep voltam metry (20 mV s 1) with the continuous and chopped light illumination (Frequency: 0.33 Hz). The voltage values measured against SCE were converted to RHE scale [20]. Before all electrochemical measurements, 3
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Fig. 3. XPS spectra of (a) Bi 4d, (b) V 2p, and (c) and (d) O1s with fitted peaks using a Shirley background across peak region for pristine and Li:BiVO4 samples, respectively.
the electrolyte was thoroughly deaerated by purging with N2 gas. The PCD and separation efficiency values are the mean value using four different pristine and Li:BiVO4 photoanodes. Mott–Schottky measure ments were carried out in dark at 1 kHz and the impedance measure ment (100 mHz–100 kHz) was performed under light illumination with an AC amplitude of 20 mV at 1.23 V vs RHE. EC-Lab v11.20 software was used for curve fitting to develop equivalent circuits. The ECSA values were estimated by conducing cyclic voltammetry experiments at various scan rates in 0.1 M K2HPO4 electrolyte (pH 8.0) in dark [22]. The respective Jmax values for the pristine and Li:BiVO4 photoanodes were estimated based on literature [23] and detail provided in Supporting Information Section 2. The Jmax for the pristine and Li doped BiVO4 photoanodes were 5.9 and 7.4 mA cm 2, respectively. The photocurrent obtained for the water and sulfite oxidations were used to calculate the electron-hole separation efficiency (ηsep) using the procedure in our previous study [20] and brief detail provided in Sup porting Information. The durability of the pristine and Li:BiVO4 photo anodes was evaluated at 1.23 V vs RHE under one sun illumination in K2HPO4 electrolyte with and without hole scavenger.
wave (PAW) potentials [27,28] were employed. The Bi(5d, 6s, 6p), V (2p, 3d, 4s), O(2s, 2p) and Li(1s, 2s, 2p) electrons were treated as valence electrons [29]. The spin polarized wavefunctions and plane wave basis sets with an energy cutoff 550 eV were used. The reciprocal space was meshed at 5 � 5 � 7 using the Monkhorst-Pack method [11] and the line path L-M-A-Γ-Z-V was used to obtain the band structure [27] of pristine and Li doped BiVO4. The site projected partial density of states (PDOS) were also calculated for determining the contributions from various elements (Bi, V, O, Li) investigated. The energy conver gence criteria for electronic and ionic iterations were set to be 10 5 and 10 4 eV, respectively. The DFT þ U method [30] was used to improve the prediction of the highly localized d-electron correlation in the transition metal V with a Hubbard U value of 2.7 eV [11]. The detailed description of model provided in Supporting Information section 1. 3. Results and discussion Li doped BiVO4 with planar morphology based photoanodes were prepared for improving charge separation efficiency along with enhanced light absorption by controlling the photoanode thickness (>100 nm). As shown schematically in Fig. 1a, the pristine and Li doped BiVO4 layers were fabricated on the FTO glass using metal precursors by a simple ultrasonic spray coating method (Supporting Fig. S2 a-d). The scanning electron micrograph (Fig. 1b) of the pristine BiVO4 film shows formation of planar uniform nano-porous surface. The surface image (Supporting Fig. S3a) obtained with 30� tilting shows the uniform growth of nano-size grains on the surface and inset shows the presence
2.4. DFT calculation The first principles DFT calculations including structural optimiza tion and electronic properties were carried out using the Vienna Ab initio Simulation Package (VASP) with generalized gradient approximation (GGA) [24,25]. In particular, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [26] and the projector-augmented 4
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Fig. 4. (a) UV–visible absorption spectra (inset shows absorption efficiency derived from LHE plot; Supporting Information Section 2), (b) PEC performance in 0.1 M K2HPO4 electrolyte (chopped light), (c) PEC performance (chopped light), and (d) IPCE in 0.1 M K2HPO4 with 1 M Na2SO3 under 1 sun illumination.
of inherent cracks on the grain surface, which can help in increasing contact area between film surface and electrolyte. As seen in Fig. 1c, the Li doping in BiVO4 did not result in any significant difference in surface morphology. The cross sectional image shows (Supporting Fig. S3b) formation of connected nano-pores throughout thickness (~500 nm) of the BiVO4 layer. X-ray diffraction (XRD) data for the pristine and Li:BiVO4 layers shown in Fig. 2a confirmed the formation of single-phase monoclinic crystal structure (pdf # 14-0688) without any impurity phases. How ever, a slight shift of ( 112) and (040) hkl peaks towards lower angle was observed for the Li doped samples and the corresponding lattice parameters based on the Rietveld analysis are given in Supporting Table S1 and supporting Figs. S4a–c. As seen in Table S1, the cell pa rameters a, b and c increased slightly and the unit cell volume increase was ~0.3% for Li:BiVO4 compared to that of the pristine BiVO4. The volume increase could be due to the Li atoms doping at the interstitial positions of the BiVO4 crystal lattice and was verified by the ab initio density functional theory (DFT) calculations. The incorporation of Li into the BiVO4 lattice was also examined by Raman spectra and shown in Fig. 2b. The major peaks for pristine BiVO4 sample were observed at ~823, 330 and 364 cm 1, corresponding to the υs (V–O), δas (VO34 ) and δs (VO34 ) stretching mode vibrations, respectively. The Li doped sample showed shift of V–O stretching and VO34 deformation modes towards higher wave number. The Li present at the interstitial position in BiVO4 lattice could distort the vibrational modes of V–O bond and the stressed vibrational modes would show higher energy. As seen from the XPS data in Fig. 3a and b, the shifting of Bi4f and V2p peaks of the Li doped BiVO4
to lower binding energy could be due to the changes in local coordina tion environments of Bi and V ions [31]. It is very clear that the electron sphere around Li at interstitial position can exert electrostatic repulsive force, leading to decrease in (Bi–O and V–O) bond energy and an expansion in the unit cell volume, as observed in XRD (Fig. 2a). Fig. 3c and d shows the high-resolution XPS spectra for O1s for the pristine and Li doped BiVO4 samples (detailed analysis on the oxidation state is provided in Supporting Information, section 1, supporting Fig. S4d). The peaks at ~530.6, 533 and 529 eV can be assigned to surface lattice ox ygen (V–O), surface adsorbed oxygen ( OH) and for Li–O bonds [32], respectively. The peak area ratios of Li–O/ OH/V–O in pristine BiVO4 and Li:BiVO4 as shown in Fig. 4c and d confirm the presence of a weak Li–O bond [33]. The UV–Visible absorption spectra of the pristine and Li doped BiVO4 photoanodes are shown in Fig. 4a. As observed, the band edge position for the Li:BiVO4 photoanodes moved slightly towards higher wavelength (~505–~530 nm) and significant absorption occurred before absorption edge, suggesting formation of inter-band gap [5] level. As seen in Fig. 4a, the absorption was substantially higher for the Li:BiVO4 pho toanode compared to the pristine photoanode because of the Li doping in interstitial position of BiVO4 crystal lattice generating significant disorder in the atomic arrangement due to electrostatic interaction and oxygen deficiency. This has generated several active sites with the cre ation of inter-band states [34] in the band gap region leading to considerable lower light reflectance from Li:BiVO4 surface (Supporting Fig. S5). The calculated value of ηabs from LHE plot (Supporting Infor mation Section 2) for the pristine and Li:BiVO4 photoanodes were ~64 5
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Fig. 5. (a) Charge separation (ηabs) and transfer (ηinj) efficiencies for pristine and Li:BiVO4 photoanodes, (b) Nyquist data (100 mHz-100kHz) under 1 sun illumi nation at 1.23 V vs RHE in 0.1 M K2HPO4 with 1 M Na2SO3 electrolyte, (c) Resistance and capacitance values, and (d) Jstable/Jo plot for pristine and Li:BiVO4 photoanodes.
and 77% (inset in Fig. 4a). Thus, the Li:BiVO4 showed improved light harvesting with the film thickness more than the hole diffusion length (~100 nm). As seen in Fig. 4b, the Li:BiVO4 photoanode exhibited sig nificant enhancement (>20 times) in the PCD (2.35 � 0.12 and 6.2 � 0.30 mA cm 2 at 1.23 and 2.0 V vs RHE, respectively) compared to that of the pristine BiVO4 (0.11 � 0.01 and 1.04 � 0.05 mA cm 2 at 1.23 and 2.0 V, respectively) in K2HPO4 electrolyte. Whereas the PCD reaches 7.3 � 0.36 mA cm 2 (Jmax: 7.4 mA cm 2) at 1.23 V vs RHE in K2HPO4 with Na2SO3 electrolyte under 1 sun illumination (Fig. 4c). The record high PCD with surface transfer kinetics (ηinj) of almost 100% is the highest ever reported for the BiVO4 based photoanodes at 1.23 V vs RHE in the literature (PEC performance values are compared in Supporting Fig. S1). As shown in Fig. 4d, Li:BiVO4 exhibits a maximum IPCE value of ~95% (~4 times that of the pristine BiVO4) at 410 nm. This high IPCE in wavelength 400-450 nm region was further confirmed by low diffuse reflectance (supporting Fig. S5a) and negligible transmittance (sup porting Fig. S5b) of the Li:BiVO4 sample. The integrated current density [35] (shown on the secondary Y axis in Fig. 4d) corresponding to the IPCE matches well with experimental PCD value of 7.38 mA cm 2 at 1.23 V vs RHE. The absorbance (Fig. 4a) and the IPCE (Fig. 4d) values clearly demonstrate the extended wavelength range (400–550 nm) by the Li:BiVO4 photoanode towards PEC activity. The corresponding ηsep and ηinj values (detailed calculation method provided in Supporting Information Section 2) for the pristine and Li doped BiVO4 photoanodes shown in Fig. 5a. It was observed that the ηsep increased from 20 to 100% and the charge transfer efficiency increased from ~3 to 22% at 1.23 V vs RHE for Li:BiVO4 with respect to pristine photoanode. The band gap estimation through in Tauc plot (Supporting Fig. S6), showed that the bandgap value reduced from 2.45 to 2.36 eV due to Li doping in BiVO4 and the reduction in band gap was also
supported with slight negative shift in the onset of flat band potential (Mott-Schottky analysis; Supporting Fig. S7a), leading to enhanced light absorption. To determine onset voltage of the photocurrent, the photooxidation of sulfite was carried out, as photo oxidation of water suffers from slow oxidation kinetics leading to more error (Supporting Fig. S7b). The Nyquist plots and corresponding resistance and capaci tance values are given in Fig. 5b and c, respectively. The significantly smaller semicircle showed lower resistance values associated with Li: BiVO4 compared to that of the pristine photoanode (Fig. 5b). The resistance and capacitance values in bulk and surface for both the pristine and Li:BiVO4 photoanodes obtained using an equivalent circuit model (Supporting Fig. S8) for the impedance patterns are shown in Fig. 5c and the values are given in Supplementary Table S2. Evidently, the Li:BiVO4 showed extremely lower bulk resistance (Rb) compared to that of the pristine photoanode, leading exceptionally high ηsep (Fig. 5a). In addition, slight reduction in surface resistance (Rs) lead to increase in surface charge transfer efficiency (ηinj) (Fig. 5a) for the Li:BiVO4 pho toanode but not enough to significantly improve the PEC water oxida tion performance. The Bode measurements carried out at 1.23 V vs RHE (Supporting Fig. S9) supports that the ηinj increased with Li doping in BiVO4, as there is a significant reduction of the peak at 200 mHz, cor responding to the capacitance at photoanode surface and electrolyte interface. The increase in ηinj, was also confirmed by the CV measure ments for doped and undoped Li:BiVO4 photoanodes in dark in K2HPO4 electrolyte in a wider potential range and is given in Supporting Fig. S10. The cathodic peak in dark and light at ~1.4 V vs RHE is due to the 2þ recombination reaction (VOþ 2 /VO Þ with a lower intensity for Li:BiVO4 suggested reduction of trap states in BiVO4 with Li doping compared the pristine. In addition, the lower bulk (Cb) and higher surface (Cs) capacitance values of the Li:BiVO4 photoanode also improved the charge 6
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ηinj ~42% (Supporting Fig. S13). The stable high PCD shown by the Li:
BiVO4/Fe:Ni(OH)2/Co-Pi photoanode can be suitably used in standard PEC cell in the potential range of 1.6–2 V vs RHE to achieve optimum performance [4]. The performance stability of the Li:BiVO4/Fe:Ni (OH)2/Co-Pi photoanode for water oxidation at 1.23 V vs RHE was evaluated with light ON (11 h) and OFF (2 h) for three cycles (total duration: 33 h) and the overall performance retention observed was ~91% (Fig. 6b). In addition, the Li:BiVO4/Fe:Ni(OH)2/Co-Pi photo anode (Supporting Fig. S14) also exhibited excellent performance retention (96% of PCD) over 30 h of continuous operation in the pres ence of hole scavenger. Fig. 7a shows the monoclinic unit cell crystal structures for the pristine and Li doped BiVO4. Evidently the Li (4 wt %) doping did not change the space group of the crystal structure and about ~2% volume expansion was observed due to Li insertion at interstitial position, which is consistent with XRD and Raman results (Fig. 2a and b). Fig. 7b shows the simulated absorption spectrum (imaginary part of dielectric func tion) for the pristine and Li doped BiVO4. As seen, both the pristine and Li doped BiVO4 exhibited higher absorption >2 eV. However, the Li: BiVO4 showed an additional absorption peak ~1.2 eV, possibly due to reduction in band gap. This supports the experimental observation of increase in absorption at extended lower energy region due to Li doping (Fig. 4a). The simulated band structures in Fig. 7c shows, the presence of interband with reduction of bandgap (2.14–2.06 eV) [38,39] for the Li doped compared to the pristine BiVO4. With the Li doping at the inter stitial position in BiVO4, both the conduction and valence bands move downward leading to reduction in the bandgap by 0.08 eV, due to larger downward shift of the conduction band. This supports the experimental observation of reduction of band gap (Supporting Fig. S6) and the downward shift of conduction band edge in the Mott Schottkey plot (Supporting Fig. S7a) for the Li doped BiVO4. Fig. 7c also shows an interband formation at ~1.5 eV for the Li doped BiVO4, due to atomic distortedness generated in BiVO4 with Li doping. The crystal structure along with electron spin density contour for interband in Li:BiVO4 Fig. 7d showed the formation of possible small polarons localized pre dominantly at V atoms [11] leading to better charge mobility evidenced from lower bulk and surface charge transfer resistance values (Fig. 5c and Table S2). As evident from various characterizations including first principles DFT calculations, the Li:BiVO4 based electrode with the ex istence of interband along lower band gap combined with improved charge mobility, complete charge separation efficiency compared to the pristine electrodes, showed relatively higher PEC performance. The near complete charge separation efficiency observed in Li:BiVO4 based pho toanode with exceptionally higher performance could lead to commer cialization of photoelectrochemical water splitting system.
Fig. 6. (a) PEC performance in 0.1 M K2HPO4 electrolyte under 1 sun illumi nation (chopped light), and (b) PEC performance stability at 1.23 V vs RHE in 0.1 M K2HPO4 electrolyte for Li:BiVO4/Fe:Ni(OH)2/Co-Pi photoanodes with ON (11 h) and OFF (2 h).
separation and transfer in comparison with that of the pristine photo anode. The electron-hole recombination process was further elaborated by the transient photocurrent measurements at 1.23 V vs RHE without and with hole scavenger in potassium phosphate electrolyte (see Sup porting Fig. S11 a-b). Li:BiVO4 photoanode reached first to steady state Jstable after a certain period of illumination. The Jstable to Jo (Jo: initial photocurrent spike when the light is turned ON) ratio shown in Fig. 5d for the pristine and Li doped BiVO4 photoanodes clearly demonstrates that the electron-hole recombination is relatively lower with efficient charge separation [36] with Li doping. The Jstable to Jo ratio reached to unity in electrolytes with hole scavenger, confirmed that Li doping in BiVO4 lead to drastically improve the bulk charge separation. The in crease in the charge carrier density (ND) estimated from Mott-Schottky analysis (Supporting Fig. S7a) and electrochemically active surface area (ECSA) values (Supporting Table S2 and Figs. S12a–c) for the Li doped over the pristine BiVO4 photoanodes also support the perfect charge separation efficiency as well as the enhanced PEC performance. In addition, negative shift of the flatband potential (Supporting Fig. 7a) as well as photocurrent onset potential (Supporting Fig. S7b) is also evidenced by the carrier density increase for the Li doped BiVO4 n-type semiconductor [37]. In order to improve the PEC performance further, Li:BiVO4 photo anode was also coated with Fe:Ni(OH)2/Co-Pi surface catalyst for enhancing the charge transfer efficiency. The Li:BiVO4/Fe:Ni(OH)2/CoPi photoanode showed the impressive water oxidation PCD of 4.2 � 0.19 mA cm 2 at 1.23 V and 6.3 � 0.28 mA cm 2 at 2.0 V vs RHE (Fig. 6a). The PCD at 1.23 V vs RHE corresponds to the ηsep ~100% and
4. Conclusion The separation and transport of charge carriers in BiVO4 photo anodes are generally limited by diffusion length of holes (~100 nm) and could be managed through thin film photoanodes. However, thin film photoanodes result in poor light absorption leading to lower PEC per formance. In present study, a near complete bulk charge separation ef ficiency in ~500 nm thick film with improved absorption has been achieved in planar morphology BiVO4 system with lightest metal Li doping. The effect of Li doping in BiVO4 were thoroughly examined experimentally and computationally to provide new understanding of the electronic band structures and photo electrochemical properties of BiVO4. The Li:BiVO4 photo anode was prepared using simple ultrasonic spray coating technique and Li doping has not changed the space group of BiVO4 unit cell. The XRD and Raman results showed that with doping Li goes to interstitial position of BiVO4 unit cell. The Li doping has increased the light absorption, charge carrier density, mobility, elec trochemical active surface area and lower trap state formation in such a way that near complete bulk charge separation in PEC process can be achieved. The IPCE measurements confirmed the extended and 7
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Fig. 7. (a) The crystal structures of the pristine and Li doped BiVO4 predicted from DFT calculations (Bi: purple, V: blue, O: red and Li: green), (b) Calculated imaginary part of dielectric function for the pristine (black) and Li doped BiVO4 (red), (c) Band struc tures of pristine BiVO4 (blue dots) and Li doped BiVO4. The spin up and down are showed in red and black lines, respectively. The energies in the band structures and PDOS were aligned with the energies in 5s electrons of Bi atoms which are far away from the defect spot, and (d) Spin density map of Li doped BiVO4 with iso-surface at 0.02 e.bohr 3 at interband. (For interpreta tion of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
improved absorption in Li:BiVO4 photoanode that lead to achieve theoretical photo current density. The impedance results showed that the Li doping has drastically reduced the bulk resistance, creating better charge mobility environment. The Ab initio calculations showed that the 4 wt % Li doping in BiVO4 interstitial lattice position resulted in change of coordination environment for oxygen atom and lead to reduction in band gap as well as formation of inter-band between valence and con duction band. The experimental and theoretical studies reported herein have confirmed the strategic role played by Li in achieving exceptionally high charge separation efficiency.
[6] A. Eftekhari, V.J. Babu, S. Ramakrishna, Photoelectrode nanomaterials for photoelectrochemical water splitting, Int. J. Hydrogen Energy 42 (2017) 11078–11109. [7] J.K. Cooper, S. Gul, F.M. Toma, L. Chen, P.A. Glans, J. Guo, J.W. Ager, J. Yano, I. D. Sharp, Electronic structure of monoclinic BiVO4, Chem. Mater. 26 (2014) 5365–5373. [8] B. Lamm, B.J. Trze�sniewski, H. D€ oscher, W.A. Smith, M. Stefik, Emerging postsynthetic improvements of BiVO4 photoanodes for solar water splitting, ACS Energy Lett. 3 (2017) 112–124. [9] E.Y. Liu, J.E. Thorne, Y. He, D. Wang, Understanding photocharging effects on bismuth vanadate, ACS Appl. Mater. Interfaces 9 (2017) 22083–22087. [10] F.F. Abdi, N. Firet, R. vande Krol, Efficient BiVO4 thin film photoanodes modified with cobalt phosphate catalyst and W-doping, ChemCatChem 5 (2013) 490–496. [11] T.W. Kim, Y. Ping, G.A. Galli, K.S. Choi, Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting, Nat. Commun. 6 (2015) 1–10. [12] F.F. Abdi, L. Han, A.H.M. Smets, M. Zeman, B. Dam, R. Van De Krol, Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nat. Commun. 4 (2013) 1–7. [13] S.K. Cho, H.S. Park, H.C. Lee, K.M. Nam, A.J. Bard, Metal doping of BiVO4 by composite electrodeposition with improved photoelectrochemical water oxidation, J. Phys. Chem. C 117 (2013) 23048–23056. [14] J. Prakash, U. Prasad, R. Alexander, J. Bahadur, K. Dasgupta, A.M. Kannan, Photoelectrochemical Solar water splitting: the role of the carbon nano materials in bismuth vanadate composite photoanodes towards efficient charge separation and transport, Langmuir (2019), https://doi.org/10.1021/acs.langmuir.9b02782. [15] J.H. Kim, Y. Jo, J.H. Kim, J.W. Jang, H.J. Kang, Y.H. Lee, D.S. Kim, Y. Jun, J.S. Lee, Wireless solar water splitting device with robust cobalt-catalyzed, dual-doped BiVO4 photoanode and perovskite solar cell in tandem: a dual absorber artificial leaf, ACS Nano 9 (2015) 11820–11829. [16] Y. Qiu, W. Liu, W. Chen, G. Zhou, P.C. Hsu, R. Zhang, Z. Liang, S. Fan, Y. Zhang, Y. Cui, Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells, Sci. Adv. 2 (6) (2016) 1–9, e1501764. [17] X. Shi, I.Y. Choi, K. Zhang, J. Kwon, D.Y. Kim, J.K. Lee, S.H. Oh, J.K. Kim, J. H. Park, Efficient photoelectrochemical hydrogen production from bismuth vanadate-decorated tungsten trioxide helix nanostructures, Nat. Commun. 5 (2014) 1–8. [18] L. Zhou, C. Zhao, B. Giri, P. Allen, X. Xu, H. Joshi, Y. Fan, L.V. Titova, P.M. Rao, High light absorption and charge separation efficiency at low applied voltage from Sb-doped SnO2/BiVO4 core/shell nanorod-array photoanodes, Nano Lett. 16 (2016) 3463–3474. [19] Y. Pihosh, I. Turkevych, K. Mawatari, J. Uemura, Y. Kazoe, S. Kosar, K. Makita, T. Sugaya, T. Matsui, D. Fujita, M. Tosa, M. Kondo, T. Kitamori, Photocatalytic
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227418. References [1] I. Arto, I. Capell� an-P� erez, R. Lago, G. Bueno, R. Bermejo, The energy requirements of a developed world, Energy Sustain. Dev. 33 (2016) 1–13. [2] I. Dincer, C. Acar, Innovation in hydrogen production, Int. J. Hydrogen Energy 42 (2017) 14843–14864. [3] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Hydrogen from photocatalytic water splitting process: a review, Renew. Sustain. Energy Rev. 43 (2015) 599–610. [4] Hydrogen production: photoelectrochemical water splitting. https://www.energy. gov/eere/fuelcells/hydrogen-production-photoelectrochemical-water-splitting. [5] T. Yao, X. An, H. Han, J.Q. Chen, C. Li, Photoelectrocatalytic materials for solar water splitting, Adv. Energy Mater. 8 (2018) 1–36.
8
J. Prakash et al.
[20]
[21] [22] [23]
[24] [25] [26] [27] [28] [29]
Journal of Power Sources xxx (xxxx) xxx
generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency, Sci. Rep. 5 (2015) 1–2. U. Prasad, J. Prakash, S. Gupta, J. Zuniga, Y. Mao, B. Azeredo, A.N.M. Kannan, Enhanced photoelectrochemical water splitting with Er and W co-doped bismuth vanadate with WO3 heterojunction based 2-dimensional photoelectrode, ACS Appl. Mater. Interfaces 11 (2019) 19029–19039. L. Li, X. Yang, Y. Lei, H. Yu, Z. Yang, Z. Zheng, D. Wang, Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode, Chem. Sci. 9 (2018) 8860–8870. B.K. Kang, G.S. Han, J.H. Baek, D.G. Lee, Y.H. Song, S. Bin Kwon, I.S. Cho, H. S. Jung, D.H. Yoon, Nanodome structured BiVO4/GaOxN1 x photoanode for solar water oxidation, Adv. Mater. Interfaces. 4 (2017) 1–8. U. Prasad, J. Prakash, B. Azeredo, A.M. Kannan, Stoichiometric and nonstoichiometric tungsten doping effect in Bismuth Vanadate based photoactive material for Photoelectrochemical Water Splitting, Electrochim. Acta 299 (2019) 262–272. G. K, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54 (11169) (1996) 1–18. G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6 (1996) 15–50. J.P. Perdew, J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (3865) (1996) 1–4. P.E. Bl€ ochl, P.E. Bl€ ochl, Projector augmented-wave method, Phys. Rev. B 50 (1994) 1–27, 17953. G. Kresse, G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B 59 (1758) (1999) 1–18. Z. Zhao, Z. Zou, Electronic Structure and Optical Properties of Monoclinic, 2011, pp. 4746–4753.
[30] S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Electronenergy-loss spectra and the structural stability of nickel oxide: an LSDAþ U study, Phys. Rev. B 57 (1505) (1998) 1–5. [31] A.M. Rassoolkhani, W. Cheng, J. Lee, A. Mckee, J. Koonce, J. Coffel, A.H. Ghanim, G.A. Aurand, C.S. Kim, W.I. Park, H. Jung, S. Mubeen, Nanostructured bismuth vanadate/tungsten oxide photoanode for chlorine production with hydrogen generation at the dark cathode, Commun. Chem. 5 (2015) 1–7. [32] K.P.C. Yao, D.G. Kwabi, R.A. Quinlan, A.N. Mansour, A. Grimaud, Y.L. Lee, Y.C. Lu, Y. Shao-Horn, Thermal stability of Li2O2 and Li2O for li-air batteries: in situ XRD and XPS studies, J. Electrochem. Soc. 160 (2013) 824–831. [33] T. Palaniselvam, L. Shi, G. Mettela, D.H. Anjum, R. Li, K.P. Katuri, P.E. Saikaly, P. Wang, Vastly Enhanced BiVO4 Photocatalytic OER Performance by NiCoO2 as Cocatalyst, 2017, pp. 1–10, 1700540. [34] X. Li, M. Han, X. Zhang, C. Shan, Z. Hu, Z. Zhu, J. Chu, Temperature-dependent band gap, interband transitions, and exciton formation in transparent p-type delafossite CuCr1 xMgxO2 films, Phys. Rev. B 90 (035308) (2014) 1–8. [35] T.G. Vo, Y. Tai, C.Y. Chiang, Novel hierarchical ferric phosphate/bismuth vanadate nanocactus for highly efficient and stable solar water splitting, Appl. Catal. B Environ. 243 (2019) 657–666. [36] Q. Shi, S. Murcia-L� opez, P. Tang, C. Flox, J.R. Morante, Z. Bian, H. Wang, T. Andreu, Role of tungsten doping on the surface states in BiVO4Photoanodes for water oxidation: tuning the electron trapping process, ACS Catal. 8 (2018) 3331–3342. [37] W.P. Gomes, F. Cardon, Electron energy levels in semiconductor electrochemistry, Prog. Surf. Sci. 12 (1982) 155–216. [38] Z. Zhao, Z. Li, Z. Zou, Electronic structure and optical properties of monoclinic clinobisvanite BiVO4, Phys. Chem. Chem. Phys. 13 (2011) 4746–4753. [39] Y. Yuan, Y. Huang, F. Ma, Z. Zhang, X. Wei, Effects of oxygen vacancy on the mechanical, electronic and optical properties of monoclinic BiVO4, J. Mater. Sci. 52 (2017) 8546–8555.
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