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CdS to enhance PEC water splitting efficiency
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Photoanode of LDH catalyst decorated semiconductor heterojunction of BiVO4/CdS to enhance PEC water splitting efficiency Shouli Bai a, Qiangqiang Li a, Jingyi Han a, Xiaojun Yang a, Xin Shu a,*, Jianhua Sun b,c, Lixia Sun b,***, Ruixian Luo a, Dianqing Li a,**, Aifan Chen a a
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China b Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China c Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China
highlights NiCo-LDH
decorated
graphical abstract BiVO4/CdS
heterojunction photoanode was fabricated successfully. PEC properties of triadic photoanode were significantly enhanced compared with BiVO4. The enhancement benefits from the combination of heterojunction and co-catalyst.
article info
abstract
Article history:
BiVO4 is an ideal photoanode material for solar-driven photoelectrochemical (PEC) water
Received 19 May 2019
splitting but it easily suffers from the recombination of photogenerated electrons and holes
Received in revised form
due to its low carrier mobility thus cause low efficiency of PEC water splitting. Herein, the
24 July 2019
BiVO4/CdS/NiCo-LDH photoanode was prepared by combining methods of metal organic
Accepted 27 July 2019
decomposition, chemical and electrodeposition. The photoanode photocurrent density
Available online 29 August 2019
reaches 2.72 mA cm2 at 1.23 V (vs. RHE), which is 3.6 folds of pure BiVO4 photoanode and onset potential shifts 450 mV toward cathodic. The incident photon-to-electron conversion
Keywords:
efficiency (IPCE) value is 2.86 folds of BiVO4, the calculated photonetoecurrent efficiency
Photoelectrochemical water
(ABPE) is 1.24% at 0.62 V (vs. RHE). The obtained results are higher than that of most BiVO4
splitting
based photoanodes published so far. The enhancement benefits from increase of visible
BiVO4
light absorption capacity, enhancement of separation efficiency of photoexcited electron-
CdS
hole and fast transfer of holes accumulated on electrode/electrolyte surface for water
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Shu), [email protected] (L. Sun), [email protected] (D. Li). https://doi.org/10.1016/j.ijhydene.2019.07.214 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Semiconductor heterojunction
oxidation, which has been confirmed by calculating carrier density and carrier transport
NiCo-LDH co-catalyst
rate. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to satisfy the growing energy demand for people to solve the environmental pollution caused by the combustion of fossil fuels, it is an effective strategy to develop the solar PEC water splitting that can convert solar energy to green chemical fuels [1e3]. Substantial attempts have been committed to fabricate photoanode with high efficiency and low cost to realize the solar conversion. The narrow band gap of metal oxide semiconductors including WO3, Fe2O3 and BiVO4 etc. as photoanode materials have been deeply investigated due to their rich sources, nontoxicity, PEC stability and cheaper synthesis routes [4e6]. N-type semiconductor of BiVO4 (band gap is 2.4eV) is seen as a good photoanode material [7,8], because the potential of valence band is apparently lower than the thermodynamic value of water-splitting oxygen, which has high theoretical conversion efficiency as high as 9.1%, and it can be modified by various methods. But single BiVO4 easily causes the recombination of charge carriers due to its low carrier mobility (0.044 cm2/VS) and poor water oxidation kinetics [9]. Thus, many different approaches were tried to increase the PEC properties of BiVO4 photoanode, including doping metal or non-metal elements, combining BiVO4 with other semiconductor metal oxides to produce composite or using rGO to decorate BiVO4 [10e13]. In particular, heterojunction architecture through coupling two energy matching semiconductors is recognized to be a highperformance method to enhance bulk charge separation of BiVO4 [14e17]. Herein, we selected the CdS as photo-sensitizer with BiVO4 to build the BiVO4/CdS heterojunction, because CdS (the band gap is 2.25 eV) has superior light-harvesting capability and its sufficiently negative conduction band edge makes the photoexcited electrons flowing toward BiVO4 to improve charge separation. Recently the CdS sensitized heterojunction photoanodes to drive the water oxidation under visible light illumination have been reported, such as ZnO/ CdS, TiO2/CdS and Fe2O3/CdS photoanodes showed superior PEC performances for PEC water oxidation [18e22]. However, anode water oxidation is still a problem for controlling the rate of solar PEC water splitting (rate-determining step) because the water oxidation rate at anode is slower than the water reduction rate at cathode for five orders of magnitudes [23]. In addition, the effect of semiconductor heterojunction is limited because the built-in electric field would reach saturation with charge accumulation in water oxidation procedure. Although Ir- and Ru- based catalysts are highly active to water oxidation, their scarcity significantly retard their wide application [24]. Layered double hydroxides (LDHs) are a kind of low cost and effective catalysts and have compatibility with semiconductors combination. According to literatures, the decorated photoanodes with LDHs including Fe, Ni, Co and Mn
exhibit evident effect for separating photogenerated charge carriers and accelerating water oxidation [21,25e28]. In this work, the NiCo-LDH nanosheets as the co-catalyst decorated on surface of the BiVO4/CdS heterojunction to construct a triadic photoanode for enhancement of PEC water oxidation efficiency. The as-prepared triadic photoanode exhibits higher PEC properties than most of BiVO4 based photoanode recently published [29,30]. The corresponding catalytic mechanism is also discussed in detail and confirmed by electrochemical impedance spectroscopy (EIS) and Intensity modulated photocurrent spectroscopy (IMPS) spectra.
Experimental section Fabrication of BiVO4 photoanode All chemical reagents we used in the work were analytical pure without further purification. The BiVO4 thin film was obtained by a metal organic decomposition method on fluo` /per square resisrine doped tin oxide substrates (FTO, 14 U tance and 2.2 mm thickness). In a typical synthetic procedure, the precursor solution of BiVO4 was fabricated by adding 50 mM bismuth nitrate pentahydrate (Bi(NO3)3$5H2O) in acetic acid (CH3COOH) and a solution of 50 mM vanadium acetylacetonate (C10H14O5V) in dimethyl sulfoxide (DMSO). Took 200 mL of precursor solutions spreading on FTO substrate and dried at 80 C followed by annealing at 450 C for 2 h in air.
Fabrication of BiVO4/CdS photoanode BiVO4/CdS heterojunction photoanode has been fabricated by a chemical bath deposition method. At first, CdS nanoparticles were prepared, took 0.05 g cadmium nitrate (Cd(NO3)2$4H2O), 0.05 g trisodium citrate (C6H5Na3O7) and 0.01 g thiourea (CH4N2S) dissolving in 70 mL of distilled water under sonication for 20 min to obtain a yellow precipitate. The solution pH value was adjusted to 11 by continuously adding ammonia solution. At last, CdS nanoparticles were successfully precipitated on BiVO4 by chemical bath deposition at 90 C for 30 min.
Fabrication of BiVO4/CdS/NiCo-LDH photoanode At first, 50 mL of distilled water was added in 1.784 g of cobaltous chloride hexahydrate (CoCl2$6H2O) and 2.181 g of nickel nitrate hexahydrate (Ni(NO3)2$6H2O) as the electrolyte solution,the solution was used for electrodeposition to prepare NiCo-LDH. After as-prepared BiVO4/CdS photoanode being inserted into above solution, the electrodeposition was implemented at a stable potential of about 1.0 V with respect to the Ag/AgCl electrode. Using distilled water
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washed the obtained BiVO4/CdS/NiCo-LDH photoanode and finally dried at 60 C for 2 h in air before being used. The schematic diagram of the synthetic triadic photoelectrode was shown in Fig. 1.
Results and discussion Phase structure and morphology of electrode materials The SEM equipped EDS for observing the morphologies of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH films were shown in Fig. 2(aec). The nanoparticles with a size of 100e300 nm were spread on pure BiVO4 (Fig. 2a) [31]. The morphology of BiVO4/CdS heterojunction basically same as BiVO4 after depositing CdS nanoparticles, some of CdS nanoparticles with 40 nm radius covered on BiVO4 (Fig. 2b). From Fig. 2c, the NiCoLDHs nanosheets have been uniformly electrodeposited on BiVO4/CdS heterojunction (0.05 C). NiCo-LDH exhibited a thin 2D structure, which is beneficial to reduce the holes accumulation during PEC process. HR-TEM images of BiVO4/CdS/ NiCo-LDH (0.05C) photoanode that recorded on JEOL JEM-2010 microscopy are shown in Fig. 2d,e. The distance of 0.25 nm represents the lattice space of the (020) plane of the BiVO4 phase. Moreover, the lattice spacing of 0.31 assigns as CdS's (101) planes, which confirms the formation of a BiVO4/CdS composite with nen heterojunction. Furthermore, the tight and continuous interface between BiVO4/CdS heterojunction and NiCo-LDH cocatalyst suggests the LDH successfully decorated on heterojunction, which is beneficial to improve PEC water splitting efficiency. Fig. 3beh shows the mapping images for elements of Bi, V, O, Cd, S, Ni, Co via the EDX spectrum and further supports the formation of a triadic photoanode. Moreover, the EDS spectrum (Fig. 3i) was also used to reveal the atomic ratio between elements that matched well with their added ratio in prepared process. Especially, the atomic ratio between Ni and Co in cocatalyst is approximately 1:2 [32].
The absorption spectra images of UV/vis were performed on a spectrometer (CRAIC 20/30 PV) for investigating the influence of heterojunction on the visible light harvesting. Fig. 4a demonstrates the red-shifting of absorption edge for heterojunction photoanode compared to pristine BiVO4, which implies that the BiVO4/CdS heterojunction photoanode has been successfully fabricated and expanded the absorption range of visible light compared to pristine BiVO4 photoanode [33]. A red-shifting of absorption edge also happened to the triadic photoanode. Moreover, the band gap can be calculated in term of the Tauc equation as shown in Fig. 4b [34]. From Fig. 4b, the obtained band gap energy of BiVO4 and CdS are 2.44 eV, 2.28 eV, respectively while the band gap energy of BiVO4/CdS heterojunction is 2.37 eV, which confirms the formation of heterojunction between two oxides for facilitating light absorption and photoexcited charge electron-hole separation [23]. The phase and structure were characterised by X-ray diffraction (XRD-600 diffractometer of Shimadzu). Fig. 5 shows the XRD patterns of BiVO4, BiVO4/CdS and BiVO4/ CdS/NiCo-LDH (0.05C) photoanodes. The characteristic peaks of pure BiVO4 in XRD patterns can correspond well to the standard data for monoclinic BiVO4 (JCPDS No.83-1700). For BiVO4/CdS photoanode, new peaks appearing at 24.8 , 26.5 , 28.2 correspond to the (100), (002) and (101) planes respectively, which confirms the diffraction peak position of hexagonal phase of CdS (JCPDS no.77-2306). The low intensity peak (003) that appeared in XRD patterns is the characteristic peak of NiCo-LDHs, which demonstrates the NiCo-LDHs has been successfully electrodeposited on the BiVO4/CdS heterojunctions. The valence states and chemical compositions of the BiVO4/CdS/NiCo-LDH(0.05C) photoanode were verified by Xray photoelectron spectrum (XPS, Thermo VG ESCALAB250 XPS). Fig. 6a shows the presence of elements such as Bi, V, O, Cd, S, Ni and Co from BiVO4/CdS/NiCo-LDH. The Bi 4f peaks that located at about 159.06 eV and 164.4 eV as shown in Fig. 6b assign to Bi3þ 4f 7/2 and 4f 5/2. In Fig. 6c, split peaks of V 2p
Fig. 1 e Graphic of BiVO4/CdS/NiCo-LDH photoanode fabricated process.
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Fig. 2 e SEM images of (a) BiVO4; (b) BiVO4/CdS; (c) BiVO4/CdS/NiCo-LDH(0.05C); (d) and (e) HRTEM images of BiVO4/CdS/NiCoLDH (0.05C).
corresponding to V 2p3/2 and V 2p1/2 presents at 516.6 and 524.2 eV, respectively [35,36]. The O 1s spectrum of BiVO4/CdS/ NiCo-LDH(0.05C) photoanode (Fig. 6d) can split into three peaks which is located at 529.8, 531.0, and 532.9 eV, respectively and correspond s to characteristic peaks of the Ob (O2 lattice), Oa (surface adsorbed oxygen) and Og (adsorbed molecular water) in electrode, respectively. Fig. 6e shows the Cd 3d XPS spectrum dividing into two peaks, one peak is at around 405.5 eV corresponding to 3d5/2, another is at 412.1 eV corresponding to 3d3/2, which are the typical position of Cd2þ. S 2p3/2 and S 2p1/2 have two distinct peaks locating at 161.8 and 163.0 eV, respectively, indicating the existence of S2þ as shown in Fig. 6f [37]. Fig. 6g,h shows the XPS spectra of the Co 2p and Ni 2p of BiVO4/CdS/NiCo-LDH(0.05C) photoanode. From Co 2p spectrum (Fig. 6g), two peaks of Co 2p at around 781.7 and 797.5 eV were observed corresponding to Co 2p3/2, Co 2p1/ 2, respectively. Co 2p3/2 spectrum split into two peaks at 781.5 and 783.0 eV where the first position can be assigned to Co3þ and the latter can be assigned to Co2þ oxidation state in the sample, respectively. The peaks of Ni 2p (Fig. 6h) were observed (the binding energy of 856.4 eV corresponding to Ni 2p3/2 and the binding energy of 874.1 eV corresponding to Ni 2p1/2), which indicates the existence of NiCo-LDH in the sample.
Photoelectrochemical properties of photoanode To evaluate the photoanode PEC characteristic, photocurrent density was measured in a three electrode system containing working electrode of fabricated photoanode, counter electrode of Pt wire and reference electrode of Ag/AgCl. Three electrodes were immersed in 0.5 M Na2SO3 (pH ¼ 7) phosphate buffer electrolyte solution at the scan rate of 20 mV/s. From Fig. 7a, the photocurrent density produced by BiVO4 photoanode is 0.76 mA cm2 at 1.23 V (vs. RHE) because of the electron-hole pairs bulk recombination and water oxidation sluggish kinetics. Photocurrent density of BiVO4/CdS reaches 2.19 mA cm2 which is 2.9 folds of pure BiVO4, indicating that the heterojunctions significantly enhanced PEC performance of BiVO4. The onset potential of the heterojunction photoanode shows an apparent cathodic shift of 334 mV under visible light illumination comparing to BiVO4 (733 mV) because of effective separation of charge carries which were caused by the directional flow of electrons in heterojunction (Fig. 7b) [38,39]. In addition, photocurrent density of triadic photoanode BiVO4/CdS/NiCo-LDH (0.05C) is 2.72 mA cm2, which is 1.2 and 3.6 folds higher than that of BiVO4/CdS and BiVO4, respectively (Fig. 7a), which definitely suggests the important role of NiCo-LDH in accelerating charge transfer.
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Fig. 3 e (a)e(h) Mappings including elements and (i) EDS spectrum of triadic photoanode of BiVO4/CdS/NiCo-LDH(0.05C).
Fig. 4 e (a) Absorption spectra of UVeVis for samples; (b) bandgap plots calculated heterojunction.
The onset potential of the triadic photoanode further shifted 450 mV towards cathode comparing to the heterojunction and BiVO4 photoanodes due to the improvement of water evolution oxygen kinetics upon the electrodeposition of NiCo-LDH as shown in Fig. 7b [40]. To further study the effect of NiCoLDH decoration, the effect of the deposition amount on photocurrent density was examined for the triadic photoanode. The result indicates that the photocurrent density exhibits the tendency firstly increasing then decreasing with the increase of NiCo-LDH deposition amount as shown in
Fig. 7a, because the first increase of photocurrent density is due to the increase of LDH catalytic activity, but excess LDH deposition would results in the increase in resistance and the decrease of light permeability. So, 0.05 C is the optimum deposited charge amount, which results in the best photocurrent performance. Thus, by introducing the appropriate amount of NiCo-LDH on BiVO4/CdS heterojunction, the PEC efficiency of water oxidation significantly will be enhanced [41]. Fig. 7c shows the corresponding photocurrent when the light during on-off cycles, a fast photocurrent was observed
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recombination of photoexcited electron-hole thus improves water oxidation PEC efficiency. Moreover, ND of photoanodes can be calculated according to equation (1) [22]: 1 ND ¼ 2=eεε0 A2 d 1 C2 dE
(1)
In which C, e, a˚, ε0 , symbolize the space charge layer capacitance(Fcm2), the elementary charge (1.602 1019 C), the relative dielectric constant and the vacuum permittivity(8.834 1014 F cm1), respectively. The carrier density (ND) was calculated in Table 1. The calculated value represents that the carrier density of BiVO4/CdS junction is significantly increased comparing with pristine BiVO4 and CdS because the energy band gap of heterojunction was reduced, leading to the increase of the absorption light range. Moreover, Fermi level of BiVO4/CdS heterojunction can be calculated according to equation (2) [42]: Fig. 5 e XRD patterns of different samples.
for the moment when the light turned on or off, respectively, which indicates these photoanode is sensitive to visible light. PEC stability of CdS electrode is recognized to be a significant parameter for PEC application. The stability of the triadic photoanode including CdS was tested at 1.23 V (vs. RHE) under a 4000 s continuous illumination. From Fig. 7d, the photocurrent density within 10 min appeared slight time decay then reached a steady value, which suggests that the chronoamperometry measurement as function of time for the photoanode shows better stability. In addition, Fig. 7d shows the transient photocurrent curves that were collected from three photoanodes under illumination of chopped light at 1.23 V (vs. RHE). The transient photocurrent curves of different photoanodes exhibited a rapid response when the light turned on, when the light turned off the response back to zero, indicating these photoanodes have a good sensibility to visible light. The photocurrent density produced by the BiVO4/CdS/ NiCo-LDH (0.05C) photoanode is higher than BiVO4/CdS and BiVO4 photoanodes, which further confirms effective separation and fast transfer of charge carries, as well as suppression of surface electron recombination due to modification of CdS and electrodeposition of NiCo-LDH. The MotteSchottky (MS) plots were implemented in phosphate buffer solution of 0.5 M Na2SO3 at a 1 kHz fixed frequency under the dark background to estimate carrier density (ND) and flat band potential (Efb). From Fig. 8a MS plots, the corresponding plots of BiVO4, CdS and their junction all exhibited positive slopes, which represents that these electrodes were made of n-type semiconductors. Moreover, by the linear part of MS plots extrapolating to x-intercept, the different Efb values corresponding to BiVO4, CdS, BiVO4/CdS and BiVO4/CdS/NiCo-LDH photoanodes were 0.41, 0.60, 0.50 and 0.57 V vs AgeAgCl, respectively because Efb value is important for deciding the electron flow direction between two semiconductors. Furthermore, the calculated results indicate that the CdS film exhibits a more negative Efb than the BiVO4 film. So the photoexcited electrons in CdS can transfer to conduction band of BiVO4, while the holes transferred in opposite direction as shown in Fig. 11. The directional flow of charge carriers in BiVO4/CdS heterojunctions inhibits the
Ef Ei ¼ kTlnðND =Ni Þ
(2)
Where Ei is the BiVO4 Fermi level (~0.35 eV), ND and Ni respectively are carrier densities of composite and BiVO4. The calculated result indicates that the Fermi level of BiVO4/CdS composite moves to 0.37 eV at the thermal equilibrium, which is in accordance with the negative migration of Efb, resulting in the reduction of potential barrier at interface in the heterojunction and excitation of more charge carriers in turn enhances the water oxidation efficiency. In order to explore the charge transport resistance, under a light illumination environment, EIS of all fabricated photoanodes were tested in 0.5 M Na2SO3 aqueous solution from 0.1 Hz to 100 kHz frequency. The resistance value was then obtained by fitting the tested impedance value to the appropriate equivalent circuit (ZView 3.2c software), as shown in Fig. 9a [43]. In the Nyquist plots the smaller radius reveals the lower charge-transfer resistance (Rct) and better charges migration for charge carriers. Fig. 9a shows that the electrode/ electrolyte interface in heterojunction has a smaller charge transfer resistance (426.8 Ώ) than that of pure BiVO4 (487.2 Ώ). Furthermore, the Rct of the BiVO4/CdS/NiCo-LDH(0.05C) photoanode is further decreased to 369.4 U, indicating the cocatalyst LDH reduces the charge transfer resistance, and the charge transfer kinetics is significantly enhanced for water oxidation by LDH relieving the photoexcited holes accumulated on interface of electrode/electrolyte. From EIS analysis data the effective electron lifetime (teff) can be estimated by equation (3) [44]: teff ¼
1 umax
(3)
The characteristic frequency, umax varies from 12.5 to 46.4 Hz from pristine BiVO4 to BiVO4/CdS/NiCo-LDH photoanodes as shown in Fig. 9a. teff is calculated to be 0.02 s for BiVO4/CdS/NiCo-LDH photoanode, which is 4.0 times smaller of pristine BiVO4 (0.08s), and further demonstrates that the NiCo-LDH cocatalyst rapidly transfers holes to suppress the recombination of electron-hole. Another effective method to evaluate the charge transfer performance of semiconductor is the IMPS. Fig. 9b shows IMPS spectra of BiVO4, BiVO4/CdS, and BiVO4/CdS/NiCo-LDH photoanodes. The response emerges in the fourth quadrant and
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Fig. 6 e (a) XPS spectra of (a) survey; (b) Bi 4f; (c) V 2p; (d) O 1s; (e) Cd 3d; (f) S 2p; (g) Co 2p and (h) Ni 2p of BiVO4/CdS/NiCoLDH(0.05C) photoanode.
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Fig. 7 e (a) Photocurrent densities of different photoanodes; (b) onset potentials of different photoanodes; (c) photocurrent densities of different photoanodes under chopped illumination; (d) Steady state photocurrent density curve at 1.23 V (vs. RHE) for triadic photoanode under light; Inset is transient photocurrent densities under chopped illumination for triadic photoanode.
as shown in Fig. 9b, the frequency of its apex is correlated with the time constant (tD) of charge transferring. tD can be calculated by the below equation (4) [45]:
tD ¼ 2pfmin
1
(4)
Where, fmin represents the typical minimum frequency, this estimated time constant of the BiVO4/CdS photoanode is 1.42 ms, which is smaller than pure BiVO4 (1.59 ms). The result indicates that the interface barrier of electron transfer was reduced by the formation of heterojunctions. The tD of the triadic photoanode is further decreased to 1 ms which is lower than that of pure BiVO4 photoanode, indicating the electron transport rate is further enhanced because of the catalytic action of NiCo-LDH to water evolution oxygen. The IPCE values were measured at different wavelengths to discover the photoelectric conversion efficiency under light. The IPCE value is defined as following equation (5) [46]:
Table 1 e Carrier density and flat band potentials of different photoanodes.
Fig. 8 e Mott-Schottky plots of BiVO4, CdS, BiVO4/CdS and BiVO4/CdS/NiCo-LDH (0.05C) photoanodes.
Samples
ND (cm3)
pristine BiVO4 CdS BiVO4/CdS BiVO4/CdS/NiCo-LDH
8.91 9.23 1.87 6.05
1019 1019 1020 1020
Efb (V) 0.19 0.60 0.50 0.57
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Fig. 9 e (a) Nyquist plots of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH (0.05C) photoanodes at 1.23 V (vs. RHE); (b) IMPS spectra of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH(0.05C) photoanodes.
IPCEð%Þ ¼
Jph ðmA=cm2 Þ 1239:8ðV,nmÞ Pmono ðmW=cm2 Þ lðnmÞ
(5)
In which, Jph, l and Pmono represent photocurrent density (mA$cm2), incident light wavelength (nm) and light power density (mW$cm2), respectively. The light intensity was measured with a radiometer (FZ-A). As shown in Fig. 10a, the IPCE value reaches about 30.25% at 420 nm for the BiVO4/CdS photoanode that is 2.4 folds higher than that of BiVO4 photoanode (12.56%). The result indicates that the heterojunction photoanode extends absorption region of visible light and exhibits high photoelectric transform efficiency due to decrease of band gap compared to single BiVO4. After the decoration of NiCo-LDH, the IPCE value is further enhanced to 35.89% at 420 nm, these IPCE values are apparently higher than that of mostly reported in literature for BiVO4 based photoanodes. To obtain the photoanode to solar conversion efficiency, ABPE value at the applied bias was estimated by below equation (6) [47]:
ABPE ¼
jjj,ð1:23 VA Þ PSun
(6)
In which j, VA, PSun represent photocurrent density (mA$cm2), applied potential (V) and power density of
monochromatic light at certain wavelength (mW$cm2), respectively. As previously reported, the ABPE can be calculated by the photocurrent curves [48]. The photoanodes ABPE were plotted in Fig. 10b. The photoanode of BiVO4/CdS/NiCoLDH shows a maximum ABPE value of 1.24% at 0.62 V (vs. RHE). The ABPE value is apparently higher than that of BiVO4/ CdS and pristine BiVO4 photoanodes. The results evidently confirmed that the efficient separation of charge carries, resulting in a good PEC water splitting performance.
Mechanism of enhancing PEC properties The mechanism enhancing PEC water splitting efficiency was discussed by the synergistic effect of semiconductor heterojunction and NiCo-LDH co-catalyst as shown in Fig. 11. Once the BiVO4/CdS photoanode was irradiated by visible light, the electrons can be excited respectively from their valence band to their conduction band. The single BiVO4 with low carrier mobility presents low electron-hole separation efficiency. After impregnating appropriate CdS nanoparticles on BiVO4, the n-n semiconductor heterojunctions was formed at the interface between BiVO4 and CdS, which is beneficial to reduce recombination of charge carries due to their directional flow of carries. However, holes gathered on CdS valence band will still recombine with the bulk of electrons due to the
Fig. 10 e (a) IPCE spectra of BiVO4, BiVO4/CdS, and BiVO4/CdS/NiCo-LDH(0.05C) photoanodes; (b) ABPE curves for different photoanodes.
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references
Fig. 11 e Schematic diagram of PEC water splitting for BiVO4/CdS/NiCo-LDH (0.05C) photoanode.
sluggish water oxidation kinetics. After the decoration of NiCo-LDH on the surface of BiVO4/CdS heterojunctions, NiCoLDH as hole collector can more efficiently transfer holes accumulated on the CdS/electrolyte interface for water evolution oxygen reaction under the applied potential, meanwhile, the rapid hole transfer rate at the interface can prevent CdS electrode photocorrosion. The low valence Co2þ ions in NiCo-LDH were oxidized by holes in CdS to the high valence Co3þ/Co4þ species, then Co3þ/Co4þ further oxidized water and released oxygen, Finally, the high valence Co3þ/Co4þ species were simultaneously reduced into low valence Co2þ ions. The catalysis cycle greatly promotes the holes transmission and effectively accelerates the charge separation, which in turn improves the efficiency of water splitting.
Conclusions A novel, low cost and effective triadic photoanode of BiVO4/ CdS/NiCo-LDH were successfully fabricated by combining three facile methods. The photoanode extended the response range of visible light and accelerated surface kinetics of water oxidation. Consequently, the photocurrent density reached the highest value of 2.72 mA cm2 at 1.23 V (vs. RHE) that was 3.6 folds of pure BiVO4, Moreover, the onset potential exhibited significant shift of 450 mV toward cathodic compared to BiVO4 photoanode. The IPCE value increased to 35.89% that was 2.86 times of BiVO4 at 420 nm, the ABPE reached 1.24%, which was higher than most of BiVO4 based photoanode that reported in literature. This work will produce profound impact to develop photoanode with high efficient, low-cost, and stable characteristics for solar-driven PEC water splitting.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 51772015), Fundamental Research Funds for the Central University (12060093063), Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University and Guangxi Natural Science Foundation (2018GXNSFAA294001); Major Program of Guangxi (AB18126008).
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Photoanode of LDH catalyst decorated semiconductor heterojunction of BiVO4/CdS to enhance PEC water splitting efficiency Shouli Bai a, Qiangqiang Li a, Jingyi Han a, Xiaojun Yang a, Xin Shu a,*, Jianhua Sun b,c, Lixia Sun b,***, Ruixian Luo a, Dianqing Li a,**, Aifan Chen a a
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China b Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China c Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China
highlights NiCo-LDH
decorated
graphical abstract BiVO4/CdS
heterojunction photoanode was fabricated successfully. PEC properties of triadic photoanode were significantly enhanced compared with BiVO4. The enhancement benefits from the combination of heterojunction and co-catalyst.
article info
abstract
Article history:
BiVO4 is an ideal photoanode material for solar-driven photoelectrochemical (PEC) water
Received 19 May 2019
splitting but it easily suffers from the recombination of photogenerated electrons and holes
Received in revised form
due to its low carrier mobility thus cause low efficiency of PEC water splitting. Herein, the
24 July 2019
BiVO4/CdS/NiCo-LDH photoanode was prepared by combining methods of metal organic
Accepted 27 July 2019
decomposition, chemical and electrodeposition. The photoanode photocurrent density
Available online 29 August 2019
reaches 2.72 mA cm2 at 1.23 V (vs. RHE), which is 3.6 folds of pure BiVO4 photoanode and onset potential shifts 450 mV toward cathodic. The incident photon-to-electron conversion
Keywords:
efficiency (IPCE) value is 2.86 folds of BiVO4, the calculated photonetoecurrent efficiency
Photoelectrochemical water
(ABPE) is 1.24% at 0.62 V (vs. RHE). The obtained results are higher than that of most BiVO4
splitting
based photoanodes published so far. The enhancement benefits from increase of visible
BiVO4
light absorption capacity, enhancement of separation efficiency of photoexcited electron-
CdS
hole and fast transfer of holes accumulated on electrode/electrolyte surface for water
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Shu), [email protected] (L. Sun), [email protected] (D. Li). https://doi.org/10.1016/j.ijhydene.2019.07.214 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Semiconductor heterojunction
oxidation, which has been confirmed by calculating carrier density and carrier transport
NiCo-LDH co-catalyst
rate. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to satisfy the growing energy demand for people to solve the environmental pollution caused by the combustion of fossil fuels, it is an effective strategy to develop the solar PEC water splitting that can convert solar energy to green chemical fuels [1e3]. Substantial attempts have been committed to fabricate photoanode with high efficiency and low cost to realize the solar conversion. The narrow band gap of metal oxide semiconductors including WO3, Fe2O3 and BiVO4 etc. as photoanode materials have been deeply investigated due to their rich sources, nontoxicity, PEC stability and cheaper synthesis routes [4e6]. N-type semiconductor of BiVO4 (band gap is 2.4eV) is seen as a good photoanode material [7,8], because the potential of valence band is apparently lower than the thermodynamic value of water-splitting oxygen, which has high theoretical conversion efficiency as high as 9.1%, and it can be modified by various methods. But single BiVO4 easily causes the recombination of charge carriers due to its low carrier mobility (0.044 cm2/VS) and poor water oxidation kinetics [9]. Thus, many different approaches were tried to increase the PEC properties of BiVO4 photoanode, including doping metal or non-metal elements, combining BiVO4 with other semiconductor metal oxides to produce composite or using rGO to decorate BiVO4 [10e13]. In particular, heterojunction architecture through coupling two energy matching semiconductors is recognized to be a highperformance method to enhance bulk charge separation of BiVO4 [14e17]. Herein, we selected the CdS as photo-sensitizer with BiVO4 to build the BiVO4/CdS heterojunction, because CdS (the band gap is 2.25 eV) has superior light-harvesting capability and its sufficiently negative conduction band edge makes the photoexcited electrons flowing toward BiVO4 to improve charge separation. Recently the CdS sensitized heterojunction photoanodes to drive the water oxidation under visible light illumination have been reported, such as ZnO/ CdS, TiO2/CdS and Fe2O3/CdS photoanodes showed superior PEC performances for PEC water oxidation [18e22]. However, anode water oxidation is still a problem for controlling the rate of solar PEC water splitting (rate-determining step) because the water oxidation rate at anode is slower than the water reduction rate at cathode for five orders of magnitudes [23]. In addition, the effect of semiconductor heterojunction is limited because the built-in electric field would reach saturation with charge accumulation in water oxidation procedure. Although Ir- and Ru- based catalysts are highly active to water oxidation, their scarcity significantly retard their wide application [24]. Layered double hydroxides (LDHs) are a kind of low cost and effective catalysts and have compatibility with semiconductors combination. According to literatures, the decorated photoanodes with LDHs including Fe, Ni, Co and Mn
exhibit evident effect for separating photogenerated charge carriers and accelerating water oxidation [21,25e28]. In this work, the NiCo-LDH nanosheets as the co-catalyst decorated on surface of the BiVO4/CdS heterojunction to construct a triadic photoanode for enhancement of PEC water oxidation efficiency. The as-prepared triadic photoanode exhibits higher PEC properties than most of BiVO4 based photoanode recently published [29,30]. The corresponding catalytic mechanism is also discussed in detail and confirmed by electrochemical impedance spectroscopy (EIS) and Intensity modulated photocurrent spectroscopy (IMPS) spectra.
Experimental section Fabrication of BiVO4 photoanode All chemical reagents we used in the work were analytical pure without further purification. The BiVO4 thin film was obtained by a metal organic decomposition method on fluo` /per square resisrine doped tin oxide substrates (FTO, 14 U tance and 2.2 mm thickness). In a typical synthetic procedure, the precursor solution of BiVO4 was fabricated by adding 50 mM bismuth nitrate pentahydrate (Bi(NO3)3$5H2O) in acetic acid (CH3COOH) and a solution of 50 mM vanadium acetylacetonate (C10H14O5V) in dimethyl sulfoxide (DMSO). Took 200 mL of precursor solutions spreading on FTO substrate and dried at 80 C followed by annealing at 450 C for 2 h in air.
Fabrication of BiVO4/CdS photoanode BiVO4/CdS heterojunction photoanode has been fabricated by a chemical bath deposition method. At first, CdS nanoparticles were prepared, took 0.05 g cadmium nitrate (Cd(NO3)2$4H2O), 0.05 g trisodium citrate (C6H5Na3O7) and 0.01 g thiourea (CH4N2S) dissolving in 70 mL of distilled water under sonication for 20 min to obtain a yellow precipitate. The solution pH value was adjusted to 11 by continuously adding ammonia solution. At last, CdS nanoparticles were successfully precipitated on BiVO4 by chemical bath deposition at 90 C for 30 min.
Fabrication of BiVO4/CdS/NiCo-LDH photoanode At first, 50 mL of distilled water was added in 1.784 g of cobaltous chloride hexahydrate (CoCl2$6H2O) and 2.181 g of nickel nitrate hexahydrate (Ni(NO3)2$6H2O) as the electrolyte solution,the solution was used for electrodeposition to prepare NiCo-LDH. After as-prepared BiVO4/CdS photoanode being inserted into above solution, the electrodeposition was implemented at a stable potential of about 1.0 V with respect to the Ag/AgCl electrode. Using distilled water
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washed the obtained BiVO4/CdS/NiCo-LDH photoanode and finally dried at 60 C for 2 h in air before being used. The schematic diagram of the synthetic triadic photoelectrode was shown in Fig. 1.
Results and discussion Phase structure and morphology of electrode materials The SEM equipped EDS for observing the morphologies of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH films were shown in Fig. 2(aec). The nanoparticles with a size of 100e300 nm were spread on pure BiVO4 (Fig. 2a) [31]. The morphology of BiVO4/CdS heterojunction basically same as BiVO4 after depositing CdS nanoparticles, some of CdS nanoparticles with 40 nm radius covered on BiVO4 (Fig. 2b). From Fig. 2c, the NiCoLDHs nanosheets have been uniformly electrodeposited on BiVO4/CdS heterojunction (0.05 C). NiCo-LDH exhibited a thin 2D structure, which is beneficial to reduce the holes accumulation during PEC process. HR-TEM images of BiVO4/CdS/ NiCo-LDH (0.05C) photoanode that recorded on JEOL JEM-2010 microscopy are shown in Fig. 2d,e. The distance of 0.25 nm represents the lattice space of the (020) plane of the BiVO4 phase. Moreover, the lattice spacing of 0.31 assigns as CdS's (101) planes, which confirms the formation of a BiVO4/CdS composite with nen heterojunction. Furthermore, the tight and continuous interface between BiVO4/CdS heterojunction and NiCo-LDH cocatalyst suggests the LDH successfully decorated on heterojunction, which is beneficial to improve PEC water splitting efficiency. Fig. 3beh shows the mapping images for elements of Bi, V, O, Cd, S, Ni, Co via the EDX spectrum and further supports the formation of a triadic photoanode. Moreover, the EDS spectrum (Fig. 3i) was also used to reveal the atomic ratio between elements that matched well with their added ratio in prepared process. Especially, the atomic ratio between Ni and Co in cocatalyst is approximately 1:2 [32].
The absorption spectra images of UV/vis were performed on a spectrometer (CRAIC 20/30 PV) for investigating the influence of heterojunction on the visible light harvesting. Fig. 4a demonstrates the red-shifting of absorption edge for heterojunction photoanode compared to pristine BiVO4, which implies that the BiVO4/CdS heterojunction photoanode has been successfully fabricated and expanded the absorption range of visible light compared to pristine BiVO4 photoanode [33]. A red-shifting of absorption edge also happened to the triadic photoanode. Moreover, the band gap can be calculated in term of the Tauc equation as shown in Fig. 4b [34]. From Fig. 4b, the obtained band gap energy of BiVO4 and CdS are 2.44 eV, 2.28 eV, respectively while the band gap energy of BiVO4/CdS heterojunction is 2.37 eV, which confirms the formation of heterojunction between two oxides for facilitating light absorption and photoexcited charge electron-hole separation [23]. The phase and structure were characterised by X-ray diffraction (XRD-600 diffractometer of Shimadzu). Fig. 5 shows the XRD patterns of BiVO4, BiVO4/CdS and BiVO4/ CdS/NiCo-LDH (0.05C) photoanodes. The characteristic peaks of pure BiVO4 in XRD patterns can correspond well to the standard data for monoclinic BiVO4 (JCPDS No.83-1700). For BiVO4/CdS photoanode, new peaks appearing at 24.8 , 26.5 , 28.2 correspond to the (100), (002) and (101) planes respectively, which confirms the diffraction peak position of hexagonal phase of CdS (JCPDS no.77-2306). The low intensity peak (003) that appeared in XRD patterns is the characteristic peak of NiCo-LDHs, which demonstrates the NiCo-LDHs has been successfully electrodeposited on the BiVO4/CdS heterojunctions. The valence states and chemical compositions of the BiVO4/CdS/NiCo-LDH(0.05C) photoanode were verified by Xray photoelectron spectrum (XPS, Thermo VG ESCALAB250 XPS). Fig. 6a shows the presence of elements such as Bi, V, O, Cd, S, Ni and Co from BiVO4/CdS/NiCo-LDH. The Bi 4f peaks that located at about 159.06 eV and 164.4 eV as shown in Fig. 6b assign to Bi3þ 4f 7/2 and 4f 5/2. In Fig. 6c, split peaks of V 2p
Fig. 1 e Graphic of BiVO4/CdS/NiCo-LDH photoanode fabricated process.
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Fig. 2 e SEM images of (a) BiVO4; (b) BiVO4/CdS; (c) BiVO4/CdS/NiCo-LDH(0.05C); (d) and (e) HRTEM images of BiVO4/CdS/NiCoLDH (0.05C).
corresponding to V 2p3/2 and V 2p1/2 presents at 516.6 and 524.2 eV, respectively [35,36]. The O 1s spectrum of BiVO4/CdS/ NiCo-LDH(0.05C) photoanode (Fig. 6d) can split into three peaks which is located at 529.8, 531.0, and 532.9 eV, respectively and correspond s to characteristic peaks of the Ob (O2 lattice), Oa (surface adsorbed oxygen) and Og (adsorbed molecular water) in electrode, respectively. Fig. 6e shows the Cd 3d XPS spectrum dividing into two peaks, one peak is at around 405.5 eV corresponding to 3d5/2, another is at 412.1 eV corresponding to 3d3/2, which are the typical position of Cd2þ. S 2p3/2 and S 2p1/2 have two distinct peaks locating at 161.8 and 163.0 eV, respectively, indicating the existence of S2þ as shown in Fig. 6f [37]. Fig. 6g,h shows the XPS spectra of the Co 2p and Ni 2p of BiVO4/CdS/NiCo-LDH(0.05C) photoanode. From Co 2p spectrum (Fig. 6g), two peaks of Co 2p at around 781.7 and 797.5 eV were observed corresponding to Co 2p3/2, Co 2p1/ 2, respectively. Co 2p3/2 spectrum split into two peaks at 781.5 and 783.0 eV where the first position can be assigned to Co3þ and the latter can be assigned to Co2þ oxidation state in the sample, respectively. The peaks of Ni 2p (Fig. 6h) were observed (the binding energy of 856.4 eV corresponding to Ni 2p3/2 and the binding energy of 874.1 eV corresponding to Ni 2p1/2), which indicates the existence of NiCo-LDH in the sample.
Photoelectrochemical properties of photoanode To evaluate the photoanode PEC characteristic, photocurrent density was measured in a three electrode system containing working electrode of fabricated photoanode, counter electrode of Pt wire and reference electrode of Ag/AgCl. Three electrodes were immersed in 0.5 M Na2SO3 (pH ¼ 7) phosphate buffer electrolyte solution at the scan rate of 20 mV/s. From Fig. 7a, the photocurrent density produced by BiVO4 photoanode is 0.76 mA cm2 at 1.23 V (vs. RHE) because of the electron-hole pairs bulk recombination and water oxidation sluggish kinetics. Photocurrent density of BiVO4/CdS reaches 2.19 mA cm2 which is 2.9 folds of pure BiVO4, indicating that the heterojunctions significantly enhanced PEC performance of BiVO4. The onset potential of the heterojunction photoanode shows an apparent cathodic shift of 334 mV under visible light illumination comparing to BiVO4 (733 mV) because of effective separation of charge carries which were caused by the directional flow of electrons in heterojunction (Fig. 7b) [38,39]. In addition, photocurrent density of triadic photoanode BiVO4/CdS/NiCo-LDH (0.05C) is 2.72 mA cm2, which is 1.2 and 3.6 folds higher than that of BiVO4/CdS and BiVO4, respectively (Fig. 7a), which definitely suggests the important role of NiCo-LDH in accelerating charge transfer.
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Fig. 3 e (a)e(h) Mappings including elements and (i) EDS spectrum of triadic photoanode of BiVO4/CdS/NiCo-LDH(0.05C).
Fig. 4 e (a) Absorption spectra of UVeVis for samples; (b) bandgap plots calculated heterojunction.
The onset potential of the triadic photoanode further shifted 450 mV towards cathode comparing to the heterojunction and BiVO4 photoanodes due to the improvement of water evolution oxygen kinetics upon the electrodeposition of NiCo-LDH as shown in Fig. 7b [40]. To further study the effect of NiCoLDH decoration, the effect of the deposition amount on photocurrent density was examined for the triadic photoanode. The result indicates that the photocurrent density exhibits the tendency firstly increasing then decreasing with the increase of NiCo-LDH deposition amount as shown in
Fig. 7a, because the first increase of photocurrent density is due to the increase of LDH catalytic activity, but excess LDH deposition would results in the increase in resistance and the decrease of light permeability. So, 0.05 C is the optimum deposited charge amount, which results in the best photocurrent performance. Thus, by introducing the appropriate amount of NiCo-LDH on BiVO4/CdS heterojunction, the PEC efficiency of water oxidation significantly will be enhanced [41]. Fig. 7c shows the corresponding photocurrent when the light during on-off cycles, a fast photocurrent was observed
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recombination of photoexcited electron-hole thus improves water oxidation PEC efficiency. Moreover, ND of photoanodes can be calculated according to equation (1) [22]: 1 ND ¼ 2=eεε0 A2 d 1 C2 dE
(1)
In which C, e, a˚, ε0 , symbolize the space charge layer capacitance(Fcm2), the elementary charge (1.602 1019 C), the relative dielectric constant and the vacuum permittivity(8.834 1014 F cm1), respectively. The carrier density (ND) was calculated in Table 1. The calculated value represents that the carrier density of BiVO4/CdS junction is significantly increased comparing with pristine BiVO4 and CdS because the energy band gap of heterojunction was reduced, leading to the increase of the absorption light range. Moreover, Fermi level of BiVO4/CdS heterojunction can be calculated according to equation (2) [42]: Fig. 5 e XRD patterns of different samples.
for the moment when the light turned on or off, respectively, which indicates these photoanode is sensitive to visible light. PEC stability of CdS electrode is recognized to be a significant parameter for PEC application. The stability of the triadic photoanode including CdS was tested at 1.23 V (vs. RHE) under a 4000 s continuous illumination. From Fig. 7d, the photocurrent density within 10 min appeared slight time decay then reached a steady value, which suggests that the chronoamperometry measurement as function of time for the photoanode shows better stability. In addition, Fig. 7d shows the transient photocurrent curves that were collected from three photoanodes under illumination of chopped light at 1.23 V (vs. RHE). The transient photocurrent curves of different photoanodes exhibited a rapid response when the light turned on, when the light turned off the response back to zero, indicating these photoanodes have a good sensibility to visible light. The photocurrent density produced by the BiVO4/CdS/ NiCo-LDH (0.05C) photoanode is higher than BiVO4/CdS and BiVO4 photoanodes, which further confirms effective separation and fast transfer of charge carries, as well as suppression of surface electron recombination due to modification of CdS and electrodeposition of NiCo-LDH. The MotteSchottky (MS) plots were implemented in phosphate buffer solution of 0.5 M Na2SO3 at a 1 kHz fixed frequency under the dark background to estimate carrier density (ND) and flat band potential (Efb). From Fig. 8a MS plots, the corresponding plots of BiVO4, CdS and their junction all exhibited positive slopes, which represents that these electrodes were made of n-type semiconductors. Moreover, by the linear part of MS plots extrapolating to x-intercept, the different Efb values corresponding to BiVO4, CdS, BiVO4/CdS and BiVO4/CdS/NiCo-LDH photoanodes were 0.41, 0.60, 0.50 and 0.57 V vs AgeAgCl, respectively because Efb value is important for deciding the electron flow direction between two semiconductors. Furthermore, the calculated results indicate that the CdS film exhibits a more negative Efb than the BiVO4 film. So the photoexcited electrons in CdS can transfer to conduction band of BiVO4, while the holes transferred in opposite direction as shown in Fig. 11. The directional flow of charge carriers in BiVO4/CdS heterojunctions inhibits the
Ef Ei ¼ kTlnðND =Ni Þ
(2)
Where Ei is the BiVO4 Fermi level (~0.35 eV), ND and Ni respectively are carrier densities of composite and BiVO4. The calculated result indicates that the Fermi level of BiVO4/CdS composite moves to 0.37 eV at the thermal equilibrium, which is in accordance with the negative migration of Efb, resulting in the reduction of potential barrier at interface in the heterojunction and excitation of more charge carriers in turn enhances the water oxidation efficiency. In order to explore the charge transport resistance, under a light illumination environment, EIS of all fabricated photoanodes were tested in 0.5 M Na2SO3 aqueous solution from 0.1 Hz to 100 kHz frequency. The resistance value was then obtained by fitting the tested impedance value to the appropriate equivalent circuit (ZView 3.2c software), as shown in Fig. 9a [43]. In the Nyquist plots the smaller radius reveals the lower charge-transfer resistance (Rct) and better charges migration for charge carriers. Fig. 9a shows that the electrode/ electrolyte interface in heterojunction has a smaller charge transfer resistance (426.8 Ώ) than that of pure BiVO4 (487.2 Ώ). Furthermore, the Rct of the BiVO4/CdS/NiCo-LDH(0.05C) photoanode is further decreased to 369.4 U, indicating the cocatalyst LDH reduces the charge transfer resistance, and the charge transfer kinetics is significantly enhanced for water oxidation by LDH relieving the photoexcited holes accumulated on interface of electrode/electrolyte. From EIS analysis data the effective electron lifetime (teff) can be estimated by equation (3) [44]: teff ¼
1 umax
(3)
The characteristic frequency, umax varies from 12.5 to 46.4 Hz from pristine BiVO4 to BiVO4/CdS/NiCo-LDH photoanodes as shown in Fig. 9a. teff is calculated to be 0.02 s for BiVO4/CdS/NiCo-LDH photoanode, which is 4.0 times smaller of pristine BiVO4 (0.08s), and further demonstrates that the NiCo-LDH cocatalyst rapidly transfers holes to suppress the recombination of electron-hole. Another effective method to evaluate the charge transfer performance of semiconductor is the IMPS. Fig. 9b shows IMPS spectra of BiVO4, BiVO4/CdS, and BiVO4/CdS/NiCo-LDH photoanodes. The response emerges in the fourth quadrant and
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Fig. 6 e (a) XPS spectra of (a) survey; (b) Bi 4f; (c) V 2p; (d) O 1s; (e) Cd 3d; (f) S 2p; (g) Co 2p and (h) Ni 2p of BiVO4/CdS/NiCoLDH(0.05C) photoanode.
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Fig. 7 e (a) Photocurrent densities of different photoanodes; (b) onset potentials of different photoanodes; (c) photocurrent densities of different photoanodes under chopped illumination; (d) Steady state photocurrent density curve at 1.23 V (vs. RHE) for triadic photoanode under light; Inset is transient photocurrent densities under chopped illumination for triadic photoanode.
as shown in Fig. 9b, the frequency of its apex is correlated with the time constant (tD) of charge transferring. tD can be calculated by the below equation (4) [45]:
tD ¼ 2pfmin
1
(4)
Where, fmin represents the typical minimum frequency, this estimated time constant of the BiVO4/CdS photoanode is 1.42 ms, which is smaller than pure BiVO4 (1.59 ms). The result indicates that the interface barrier of electron transfer was reduced by the formation of heterojunctions. The tD of the triadic photoanode is further decreased to 1 ms which is lower than that of pure BiVO4 photoanode, indicating the electron transport rate is further enhanced because of the catalytic action of NiCo-LDH to water evolution oxygen. The IPCE values were measured at different wavelengths to discover the photoelectric conversion efficiency under light. The IPCE value is defined as following equation (5) [46]:
Table 1 e Carrier density and flat band potentials of different photoanodes.
Fig. 8 e Mott-Schottky plots of BiVO4, CdS, BiVO4/CdS and BiVO4/CdS/NiCo-LDH (0.05C) photoanodes.
Samples
ND (cm3)
pristine BiVO4 CdS BiVO4/CdS BiVO4/CdS/NiCo-LDH
8.91 9.23 1.87 6.05
1019 1019 1020 1020
Efb (V) 0.19 0.60 0.50 0.57
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Fig. 9 e (a) Nyquist plots of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH (0.05C) photoanodes at 1.23 V (vs. RHE); (b) IMPS spectra of BiVO4, BiVO4/CdS and BiVO4/CdS/NiCo-LDH(0.05C) photoanodes.
IPCEð%Þ ¼
Jph ðmA=cm2 Þ 1239:8ðV,nmÞ Pmono ðmW=cm2 Þ lðnmÞ
(5)
In which, Jph, l and Pmono represent photocurrent density (mA$cm2), incident light wavelength (nm) and light power density (mW$cm2), respectively. The light intensity was measured with a radiometer (FZ-A). As shown in Fig. 10a, the IPCE value reaches about 30.25% at 420 nm for the BiVO4/CdS photoanode that is 2.4 folds higher than that of BiVO4 photoanode (12.56%). The result indicates that the heterojunction photoanode extends absorption region of visible light and exhibits high photoelectric transform efficiency due to decrease of band gap compared to single BiVO4. After the decoration of NiCo-LDH, the IPCE value is further enhanced to 35.89% at 420 nm, these IPCE values are apparently higher than that of mostly reported in literature for BiVO4 based photoanodes. To obtain the photoanode to solar conversion efficiency, ABPE value at the applied bias was estimated by below equation (6) [47]:
ABPE ¼
jjj,ð1:23 VA Þ PSun
(6)
In which j, VA, PSun represent photocurrent density (mA$cm2), applied potential (V) and power density of
monochromatic light at certain wavelength (mW$cm2), respectively. As previously reported, the ABPE can be calculated by the photocurrent curves [48]. The photoanodes ABPE were plotted in Fig. 10b. The photoanode of BiVO4/CdS/NiCoLDH shows a maximum ABPE value of 1.24% at 0.62 V (vs. RHE). The ABPE value is apparently higher than that of BiVO4/ CdS and pristine BiVO4 photoanodes. The results evidently confirmed that the efficient separation of charge carries, resulting in a good PEC water splitting performance.
Mechanism of enhancing PEC properties The mechanism enhancing PEC water splitting efficiency was discussed by the synergistic effect of semiconductor heterojunction and NiCo-LDH co-catalyst as shown in Fig. 11. Once the BiVO4/CdS photoanode was irradiated by visible light, the electrons can be excited respectively from their valence band to their conduction band. The single BiVO4 with low carrier mobility presents low electron-hole separation efficiency. After impregnating appropriate CdS nanoparticles on BiVO4, the n-n semiconductor heterojunctions was formed at the interface between BiVO4 and CdS, which is beneficial to reduce recombination of charge carries due to their directional flow of carries. However, holes gathered on CdS valence band will still recombine with the bulk of electrons due to the
Fig. 10 e (a) IPCE spectra of BiVO4, BiVO4/CdS, and BiVO4/CdS/NiCo-LDH(0.05C) photoanodes; (b) ABPE curves for different photoanodes.
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Fig. 11 e Schematic diagram of PEC water splitting for BiVO4/CdS/NiCo-LDH (0.05C) photoanode.
sluggish water oxidation kinetics. After the decoration of NiCo-LDH on the surface of BiVO4/CdS heterojunctions, NiCoLDH as hole collector can more efficiently transfer holes accumulated on the CdS/electrolyte interface for water evolution oxygen reaction under the applied potential, meanwhile, the rapid hole transfer rate at the interface can prevent CdS electrode photocorrosion. The low valence Co2þ ions in NiCo-LDH were oxidized by holes in CdS to the high valence Co3þ/Co4þ species, then Co3þ/Co4þ further oxidized water and released oxygen, Finally, the high valence Co3þ/Co4þ species were simultaneously reduced into low valence Co2þ ions. The catalysis cycle greatly promotes the holes transmission and effectively accelerates the charge separation, which in turn improves the efficiency of water splitting.
Conclusions A novel, low cost and effective triadic photoanode of BiVO4/ CdS/NiCo-LDH were successfully fabricated by combining three facile methods. The photoanode extended the response range of visible light and accelerated surface kinetics of water oxidation. Consequently, the photocurrent density reached the highest value of 2.72 mA cm2 at 1.23 V (vs. RHE) that was 3.6 folds of pure BiVO4, Moreover, the onset potential exhibited significant shift of 450 mV toward cathodic compared to BiVO4 photoanode. The IPCE value increased to 35.89% that was 2.86 times of BiVO4 at 420 nm, the ABPE reached 1.24%, which was higher than most of BiVO4 based photoanode that reported in literature. This work will produce profound impact to develop photoanode with high efficient, low-cost, and stable characteristics for solar-driven PEC water splitting.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 51772015), Fundamental Research Funds for the Central University (12060093063), Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University and Guangxi Natural Science Foundation (2018GXNSFAA294001); Major Program of Guangxi (AB18126008).
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