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Low-Cost Oriented Hierarchical Growth of BiVO4/rGO/NiFe Nanoarrays Photoanode for Photoelectrochemical Water Splitting Xiao Han, Yankuan Wei, Jinzhan Su, and Yan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03259 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Sustainable Chemistry & Engineering

Low-Cost Oriented Hierarchical Growth of BiVO4/rGO/NiFe Nanoarrays Photoanode for Photoelectrochemical Water Splitting

Xiao Han,a,# Yankuan Wei,b,# Jinzhan Su,b,* Yan Zhaoa,*

a School

of Materials Science & Engineering, Beihang University, No. 37 Xueyuan Road,

Haidian District, Beijing, 100191, China b

International Research Center for Renewable Energy, State Key Laboratory for

Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi'an, Shaanxi, 710049, China # Authors

contributed equally

Email: [email protected] [email protected]

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Abstract A unique BiVO4/reduced graphene oxide (rGO)/NiFe hydroxide photoanode with oriented hierarchical nanostructure was fabricated via a facile and scalable solution route. Microstructure analysis shows that the ternary nanocomposite system presented a nanopyramid configuration with BiVO4 nanoarrays coated by sheet-like graphene interlayer and amorphous NiFe hydroxides. A maximum photocurrent density of 1.30 mA/cm2 at scan rate of 20 mV/s at 1.23 V vs. reversible hydrogen electrode was obtained for the BiVO4/rGO/NiFe photoelectrode, which was almost 3 times higher compared to that of pristine BiVO4. A boost of photocurrent density was achieved as a result of the synergistic effects between graphene ‘shuttle’ interlayer and NiFe-based nanoarrays cocatalyst outer layer. Herein rGO acts as an intermediate layer facilitating transfer of the photogenerated charge carriers, improving the adhesion of NiFe nanoarrays cocatalyst layer on the substrate and enhancing the stability and photoelectrochemical response of photoanode. This work demonstrates that tunable multicomponent design and rational hierarchical structure control are efficient approaches to enhance the photoelectrochemical water splitting performance of photoelectrode.

Key words: photoelectrochemical water splitting, bismuth vanadate, reduced graphene oxide, NiFe hydroxide, electrodeposition

Introduction 2


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With the concern about the global energy crisis and subsequent increased exploration of alternative renewable power sources, photoelectrochemical (PEC) water splitting for the production of hydrogen and oxygen under sunlight has attracted tremendous interest and effort in both academia and industry.1 PEC water splitting generally involves multiple processes: exciting electrons and holes by photon absorption in the semiconductor, separation of the generated electron-hole pairs and migration to the electrode/electrolyte interface, charge carrier injection into the reactants to drive the chemical reactions.2 Efficiency and stability are two key factors to evaluate the PEC performance,3 and a great amount of materials have been investigated and screened e.g. employing a combinatorial approach,4-6 and heterojunction approach.7-10 Yet their photoconversion efficiencies and long term durabilities are far from satisfactory due to poor charge carrier mobility, electron-hole pair recombination in the researched impure materials, and sluggish interfacial kinetics of water splitting. As the effective PEC water splitting process requires both efficient solar energy conversion and fast chemical reactions, a simple picture is that the semiconductor absorbs light and separates charge while the cocatalyst increases the rate of the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER).11 Nonetheless, the heterogeneous structure between semiconductor substrate and catalyst often hinders electron transport due to discrepant electronic configuration. Graphene, a newly emerged two-dimensional nanomaterial with excellent charge mobility,12 is an ideal candidate to solve above mentioned problem by acting as an electron mediator to shuttle photogenerated electrons 3



from semiconductor conduction band, minimizing the recombination of electrons and holes.13 Thus the amount of accumulated charges on the photoanode surface would decrease significantly and the carrier concentration in the vicinity of cocatalyst would therefore increase, leading to enhanced catalytic water splitting efficiency and photoanode stability. Another typical strategy to improve PEC efficiency is to tune the microstructure, namely morphology and architecture. Recently various metal oxide-based semiconductors with vertically aligned nanorod array structures (e.g., TiO2, Fe2O3, WO3, ZnO) have been reported to exhibit substantially enhanced PEC performance,14-17 attributed to their large surface area and lower recombination in a direct diffusion path perpendicular to the charge collecting substrate. Inspired by this phenomenon in oriented nanoarray architecture, BiVO4-based nanostructured pyramidal arrays are expected to present better light absorption and charge separation properties.18 In addition to the advantages previously reported in common nanoarray structures, nanopyramid geometry is particularly beneficial for the absorption of sunlight resulting from the sharp tip induced local strong electrical fields,19 making it a promising semiconductor scaffold for heterojunction construction. It seems ideal to combine a light-harvesting semiconductive layer, an electron-transferring conductive layer, and a reaction-accelerating cocatalyst layer into the photoelectrode to achieve the enhanced PEC water splitting performance.20-22 On the one hand, the efficient separation/migration of photogenerated charge carriers and fast surface reaction kinetics on the electrode/electrolyte interface are still highly desirable. More importantly, the 4


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understanding to the interactions between different compositions and synergistic effect mechanism behind this type of unique multi-component system is still a big challenge. Herein a novel BiVO4/reduced graphene oxide (rGO)/NiFe nanoarrays (NAs) photoanode, with unique oriented hierarchical nanostructure and enhanced PEC water splitting performance, was fabricated via spin coating of graphene oxide nanosheets on nanostructured pyramid BiVO4 substrate, followed by the subsequent electrodeposition of amorphous NiFe hydroxide NAs. The design of the hierarchical heterostructure mainly originates from the following considerations. First, graphene interlayer may act as a superfast charge tunnel to transmit photogenerated charge carriers thus to separate electronhole pairs effectively. Second, attributed to the synergistic effects between graphene shuttle and OER cocatalyst, the heterojunction structure could enhance PEC activity through faster water oxidation reaction. The comprehensive investigation on the microstructure and PEC performance of the BiVO4/rGO/NiFe NAs photoanode may shed some light on understanding the pyramid structure induced charge separation and graphene enhanced charge immigration. The results may also provide hints on design and fabrication of metal oxide based hierarchical photoanode with modified functionalities.

Experimental Section Synthesis of BiVO4/rGO/NiFe NAs The fabrication procedure of BiVO4/rGO/NiFe NAs is schematically illustrated in Figure 1(a). The BiVO4 nanopyramids were grown on the transparent conductive fluorine doped 5



tin oxide (FTO, MSE Supplies, 2.2 mm thickness, 7-8 ohm/sq) substrate according to our previously reported procedure.23 Briefly, Bi(NO3)3∙5H2O (383074, Sigma-Aldrich, ≥98.0%) and NH4VO3 (205559, Sigma-Aldrich, 99%) were dissolved in diluted HNO3 (438073, Sigma-Aldrich, 70%) aqueous solution containing polyvinyl alcohol (341584, Aldrich) and citric acid (251275, Sigma-Aldrich, ≥99.5%), and the above solution was spin coated on the surface of FTO to form a BiVO4 seed layer. Subsequently, BiVO4 crystals were grown on as-coated FTO in the solution bath containing Bi(NO3)3∙5H2O and NH4VO3 with pH adjusted to 6.5 by NaHCO3 (S6014, Sigma-Aldrich, ≥99.7%). As-grown BiVO4 substrates were stored in a vacuum desiccator for further modification. We precisely followed the modified Hummers’ method in reference to produce graphene oxide (GO) by oxidizing natural graphite flakes (496596, Aldrich, ＜45 μm, ≥99.99%).24 GO solution with a concentration of 0.5 mg/ml was drop cast on the surface of BiVO4/FTO and then spin coated at a rotation speed of 500 rpm for 30 s. Four different spin coating cycles, i.e., 1, 2, 4, 6 cycles were employed to adjust the GO content. As-coated BiVO4/GO sample was placed on the hot plate and baked at 60 °C for several minutes to evaporate the residual solvents before spin coating for the next cycle. To improve the conductivity of GO layer and the adhesion between GO and BiVO4/FTO substrate, BiVO4/GO was annealed at 350 °C under Argon atmosphere for 3 h. The sample was then immersed into ethanol solution and irradiated with a 150 W Xenon lamp for another 2 h to further transform GO to completely reduced graphene. For comparison, BiVO4/rGO samples via thermal annealing or photo reduction separately were also 6


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prepared. The BiVO4/rGO/NiFe NAs were fabricated by the potentiostatic electrodeposition method. Typically, 5 mM Ni(NO3)2·6H2O (244074, Sigma-Aldrich) and 1 mM Fe(NO3)3·9H2O (254223, Aldrich, ≥99.95%) were dissolved in ultra-pure water (resistivity 18 MΩ·cm) from a Milli-Q water purifier to form a homogenous deposition bath solution. The electrodeposition process was conducted on a SP-300 electrochemical workstation with three electrode configuration. A platinum wire was used as a counter electrode and a saturated Ag/AgCl (in 3.4 M KCl) electrode was used as the reference electrode. The deposition potential was fixed at -1.0 V vs. Ag/AgCl. The deposition time was ranged from 60 s to 180 s. The total concentration of metal precursors was set constant at 6 mM, and the ratios between Ni and Fe precursors were varied among 1:0, 10:1, 5:1, 1:1, 1:5, 0:1. The samples were carefully rinsed with deionized water and ethanol, and dried under mild nitrogen stream after the electrodeposition.

Characterization techniques The microstructures of the BiVO4-based photoanodes were characterized by a FEI Quanta 400 field-emission scanning electron microscope (SEM). A PHI Quantera scanning X-ray microprobe (SXM) was utilized to conduct X-ray photoelectron spectroscopy (XPS). The survey spectrum from 0 to 1100 eV with a passing energy of 140 eV was obtained by irradiating with a monochromated Al Kα x-ray beam, and the elemental spectra were recorded with a passing energy of 26 eV. X-ray diffraction (XRD) was obtained with a 7



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Rigaku SmartLab X-ray diffractometer in the two theta range from 3 to 60 degrees and a step size of 0.01 degree (acquisition time of 3 s at each angle). Raman spectra were collected with a Renishaw Raman microscope exciting at 532 nm. All the data were measured directly on the as-prepared photoanodes.

Photoelectrochemical measurements The PEC water splitting performance was evaluated using a standard three electrode configuration with platinum wire as a counter electrode, a saturated Ag/AgCl (in 3.4 M KCl) electrode as reference electrode and a BioLogic SP-300 potentiostat. The potential, against the Ag/AgCl reference, was converted to the reversible hydrogen electrode (RHE) potential according to the Nernst equation: ERHE = EAg/AgCl + 0.059 × pH + 0.1976 V

(1)

where ERHE is a potential versus reversible hydrogen electrode, EAg/AgCl is the potential versus Ag/AgCl electrode, and pH is the pH value of the electrolyte. A 0.5 M Na2SO4 solution was utilized as the electrolyte and the pH of the aqueous solution was ~6.9. And a 150 W Xe lamp coupled with an AM 1.5 G filter was applied as a solar simulator. The illumination intensity was 100 mW/cm2. The photoanode holder contains an o-ring ensuring the contact area of the photoanode with the electrolyte was fixed to 0.5 cm2. The photocurrent-voltage (J-V) curve was measured by linear sweep voltammetry (LSV) recorded from 0 to 1.4 V vs. RHE at a scan rate of 50 mV/s. The chopped illumination was achieved by an electronic shutter with a light on and off cycle of 5 s, and the transient LSV 8


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measurements were conducted at a scan rate of 20 mV/s to match the chopped illumination cycles. The chronoamperograms (I-t) stability tests were conducted in 0.1 M KI solution at potential of 1 V vs. RHE. Mott-Schottky measurements were performed at a frequency of 1 kHz in the dark within the potential range of -0.1 V to 0.9 V vs. RHE. A Nyquist plot measured was performed in the frequency range of 1 Hz to 105 Hz at open circuit potential under illumination. The incident photon-to-current conversion efficiency (IPCE) was obtained by collecting the photocurrent density at an applied voltage of 1 V vs. RHE and measuring the illumination intensity at different wavelengths ranging from 350 nm to 500 nm in steps of 10 nm. The monochromatic light was obtained by a Newport monochromator 74126.

Results and discussion

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Figure 1. Fabrication and the structural morphology of BiVO4-based photoanodes. (a) Schematic illustration of the fabrication procedure. Lower magnification SEM images of (b1) BiVO4, (b2) BiVO4/NiFe NAs, (b3) BiVO4/rGO and (b4) BiVO4/rGO/NiFe NAs. The scale bars are 1 μm. Higher magnification SEM images of (c1) BiVO4, (c2) BiVO4/NiFe NAs, (c3) BiVO4/rGO and (c4) BiVO4/rGO/NiFe NAs. The scale bars are 300 nm.

As briefly illustrated in Figure 1(a), oriented BiVO4 pyramidal nanostructures were firstly grown on transparent conductive FTO substrate via BiVO4 seed layer assisted chemical bath deposition. From the SEM image shown in Figure 1(b1) we determined that the BiVO4 nanopyramids were approximately vertically aligned over the entire FTO surface with a lateral dimension of several hundred nanometers at the bottom and a length of 10


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approximately one to two micrometers. This preferentially oriented alignment was further confirmed by XRD patterns (Figure 2(a)). GO aqueous solution was then subsequently spin coated on the BiVO4/FTO surface, followed by thermal annealing and photo reduction. Thermal annealing of GO at 350 °C is not only beneficial for the reduction of GO and reconstruction of sp2 C conductive network, but also crucial for the stronger adhesion of intermediate rGO layer on the BiVO4 photoanode. This connection allows rGO to act as ‘glue’ to connect BiVO4 and NiFe hydroxides. Subsequent photo reduction of partially reduced GO was conducted to further enhance conductivity of the photoanode, as it was found during the measurement of PEC water splitting performance that photocurrent density increased gradually along with repeated testing, illustrating that GO could be further reduced and activated under light irradiation.25 As displayed in Figure 1(b3) and (c3), rGO nanosheets with crumpled layer structure were intertwined and randomly attached to the BiVO4 nanopyramids both on the tips and the sides, resulting in a much rougher surface compared to pristine BiVO4. Afterwards, intercrossed NiFe hydroxide nanoplatelets were deposited on FTO/BiVO4/rGO by potentiostatic electrodeposition using a bath solution containing Ni(II) and Fe(III) nitrate with a nickel to iron ratio of 5:1. It is worth noting that iron in the +3 oxidation state was used rather than in the +2 oxidation state, resulting in the formation of amorphous NiFe hydroxides instead of crystallized NiFe layered double hydroxides (LDH), although NiFe LDH materials have presented promising OER performance in many recently published papers.26 According to the measured photocurrent densities of BiVO4/NiFe NAs utilizing Fe(III) and Fe(II) as metal precursors 11



respectively, as presented in Figure S1, NiFe-based cocatalysts prepared from Fe(III) seem to exhibit larger photocurrents due to accelerated oxygen evolution. Moreover, since the deposition of Fe(II) has to be performed in the inert atmosphere to prevent its oxidation,26 the method proposed herein was proved to be more facile and effective. Distinct from the morphology of BiVO4/NiFe NAs in Figure 1(b2) and (c2) where NiFe hydroxide was deposited on the BiVO4 nanopyramids NAs, BiVO4/rGO/NiFe NAs presented a hierarchical nanostructure where the NiFe hydroxides grew on both the BiVO4 and rGO surfaces.

Figure 2. Structural characterizations of BiVO4-based photoanode. (a) XRD patterns. (b) Raman spectra with dashed area enlarged in (c).

The composition and structure of the as-prepared photoanodes were characterized by XRD, as shown in Figure 2(a). Apart from the crystalline phase originating from the conductive FTO substrate, the diffraction peaks labeled with black dots can be attributed to crystallites of BiVO4. Two dominant peaks located at 28.9° and 30.6° are attributed to the (112) and (004) planes of monoclinic crystalline BiVO4 respectively.27 However, the intensity ratios 12


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between these two characteristic peaks were significantly different from that of scheelite BiVO4 (JCPDS: 01-075-1867). In particular, the peak at 28.9° corresponding to (112) plane of BiVO4 was largely suppressed, while the intensity of the (004) diffraction at 30.6° was greatly enhanced. Since scheelite BiVO4 presents layered structure with Bi-V-O stacking units along the c axis, the change in diffraction intensity ratio of the out-of-plane XRD suggests that the BiVO4 crystals were preferentially oriented perpendicular to the FTO substrate. Interestingly, the intensity ratio between these two peaks varied slightly across four different samples, which might be related to the capping effects from spin coated rGO intermediate layer and electrodeposited NiFe NAs cocatalyst layer. The absence of characteristic diffraction peaks of rGO is mainly due to its small loading amount and low crystallinity. What is more, no obvious diffraction signals from NiFe NAs were detected. As shown in Figure S2, XRD patterns without characteristic diffraction peaks of NiFe hydroxide were also obtained for BiVO4/NiFe NAs samples electrodeposited for 20 min, which confirms that undetectable XRD peaks of NiFe hydroxide nanoarrays is attributed to their amorphous structure other than small mass loading. Compared to the commonly formed NiFe-layered double hydroxides (LDH) crystalline structure,28 amorphous NiFe hydroxide nanoarrays with disordered structure would provide more specific area and electrochemical active sites, thus facilitating the water oxidation reaction on the photoelectrode/electrolyte interface. Raman spectra were also collected to investigate the chemical structure of BiVO4-based photoanodes, as shown in Figure 2(b). All the samples presented typical peaks of BiVO4 13



located at wave numbers of 124, 212, 328, 369, 714 and 824 cm-1, which could be assigned to external rotational and translational modes, antisymmetric and symmetric bending modes of VO43- units, symmetric and antisymmetric stretching modes of V-O, respectively.29 Similar to XRD results, Raman signal intensities from BiVO4 were reduced after coating with rGO and amorphous NiFe hydroxides due to their capping effects. Furthermore, Raman microscopy is an effective tool to provide a structural fingerprint of carbon materials. As shown in Figure 2(c), the deposition of rGO on the BiVO4 surface resulted in two additional peaks, the D band at 1358 cm-1, which originates from the disorder structure of rGO, and the G band located at 1601 cm-1, which stems from the stretching of C-C bonds in graphitic materials, suggesting the existence of rGO.30 Samples without rGO present a broad peak at around 1615 cm-1, which is correlated to the H-O bond deformation mode of water molecules,31 demonstrating that BiVO4-based photoanode is highly hydrophilic in nature and thus electrolyte can be easily accessible to the photoanode during PEC water splitting process.

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Figure 3. Compositional characterization of BiVO4-based photoanodes. XPS (a) Bi 4f, (b) V 2p, (c) Ni 2p and (d) Fe 2p spectra.

We analyzed the chemical elements and chemical bonds formed in BiVO4-based photoanodes using XPS technique. As shown in XPS survey spectrum (Figure S5), signals from Bi, V, C, O, Ni and Fe species were detected, demonstrating that BiVO4/rGO/NiFe NAs were successfully synthesized without any contaminations. Besides, high resolution XPS spectra in the Bi 4f, V 2p, Ni 2p and Fe 2p regions were also obtained. For pristine BiVO4 photoanode, binding energies of Bi 4f5/2 and Bi 4f7/2 are located at 164.1 and 158.8 eV, with a spin energy separation of 5.3 eV, in good agreement with previously reported data for Bi 4f states in BiVO4, suggesting that bismuth ions mainly exist in the form of Bi3+ oxidation state.32 After the incorporation of rGO, the peak for Bi 4f shifted slightly to

15



higher binding energy. This positive shift might be ascribed to the interactions between BiVO4 and rGO, indicating an efficient electron transfer from semiconductive BiVO4 light harvesting layer to conductive rGO intermediate layer. On the contrary, Bi 4f spectrum of BiVO4/NiFe NAs presented a negative shift compared to pristine BiVO4 photoanode, which suggests that electrons can be transferred from NiFe NAs to BiVO4 substrate. The combined effect of rGO and NiFe NAs gives rise to similar Bi 4f binding energies for BiVO4/rGO/NiFe NAs and BiVO4. XPS spectra in V 2p region presents a similar trend as for those in Bi 4f region, as shown in Figure 3(b). Moreover, we directly deposited NiFe NAs on FTO substrates to compare the oxidation states of Ni and Fe species among the different photoanodes. As shown in the black line for FTO/NiFe NAs in Figure 3(c), two spin-orbit peaks located at 855.0 and 872.3 eV, along with two shakeup satellites, on the high-resolution Ni 2p spectrum demonstrate the presence of nickel ions in the form of Ni2+. When employing BiVO4 as the substrate, the binding energies of Ni 2p shifted positively to 855.7 and 873.1 eV respectively, suggesting the interactions among electrodeposited NiFe NAs and BiVO4. There might be an induced electron transfer from NiFe NAs cocatalyst layer to BiVO4 inner layer. Similar trend could be found in the case of XPS Fe 2p spectra, indicating that iron ions in NiFe hydroxides predominantly exist in the Fe3+ oxidation state and spontaneous charge transfer behavior may exist in the system.33 Based on the above-discussed XPS results, it can be concluded that the charge carriers could be effectively separated and migrated in the ternary BiVO4/rGO/NiFe NAs photoanode, which makes it a promising candidate for photoelectrochemical water splitting. 16


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Electrodeposition is a facile and efficient method to produce active materials especially with film-like structure,34 and can be easily scaled up for industrial applications. Inspired by pioneer work showing that hierarchically structured NiFe-based electrodes could be applied as anodic catalyst for efficient oxygen evolution at high current densities, NiFe hydroxides were employed as cocatalyst for PEC water splitting based on BiVO4 photoanodes. Because of the distinct difference between binding capacities of ferric ions and nickel ions with hydroxide ions, as well as significantly different Ksp of Fe(OH)3 and Ni(OH)2,35 it could be inferred that the composition and catalytic performance of NiFe hydroxides could be tuned simply by adjusting the Ni to Fe precursor ratios. Based on the PEC water splitting performance of a series of NiFe NAs modified BiVO4 photoanodes in Figure S3(c), BiVO4/NiFe NAs sample with Ni to Fe ratio of 5 seems to possess the largest photocurrent in the full potential range. Apart from hydroxide compositions, the mass loading is another critical factor affecting the photoelectrocatalytic performance of the photoanodes.36 As shown in Figure S3(d), appropriate deposition time of 120 s gives the largest photocurrent of 0.97 mA/cm2 at 1.23 V vs. RHE. Since rGO nanosheets act as intermediate layer in BiVO4/rGO/NiFe NAs ternary system, its loading amount, physical and chemical properties would have significant influence on the photoelectrocatalytic performance of the photoanodes. According to the PEC measurement results presented in Figure S4(a), BiVO4/rGO sample spin coated for 2 cycles possessed the best performance. In terms of the reduction method of GO, the advantage of thermal annealing is to achieve recovered conductivity and compacted adhesion at the 17



meantime (confirmed by the color change of BiVO4-based photoanode from bright yellow to dark brown),37 while photo reduction is simple to perform and more energy-efficient through ethanol consuming photogenerated holes under irradiation while leaving negative electrons to be injected to GO nanosheets. Combined technique endows BiVO4/rGO sample greatest photocurrent of 0.81 mA/cm2 at 1.23 V vs. RHE.

Figure 4. PEC water splitting performance of BiVO4-based photoanodes. (a) Polarization curve of PEC response. (b) Calculated photoconversion efficiency as a function of applied voltage. (c) Transient PEC response under chopped illumination. (d) Calculated IPCE values. (e) I-t stability curve. (f) SEM image of BiVO4/rGO/NiFe NAs photoelectrode after long-term stability test. The scale bar is 1 μm.

Figure 4 depicts and compares the PEC response of BiVO4, BiVO4/NiFe NAs, BiVO4/rGO and BiVO4/rGO/NiFe NAs photoelectrodes. As previously described, NiFe NAs and rGO 18


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can both promote photocurrent density to 0.97 mA/cm2 and 0.81 mA/cm2 at 1.23 V vs. RHE, respectively. However, as the simultaneous introduction of rGO nanosheets and NiFe hydroxides NAs with hierarchical nanostructures, the photocurrent jumped to 1.33 mA/cm2 from 0.46 mA/cm2 at scan rate of 50 mV/s for pristine BiVO4 photoanode with an enhancement factor of 2.9. This could be attributed to the synergistic effects achieved by the rGO intermediate layer and NiFe cocatalyst layer. At the meantime, the dark current of BiVO4/rGO/NiFe NAs did not change too much, displaying an almost negligible current density and manifesting the largely enhanced photocurrent density was derived from the effective photoelectrical response rather than absolute electrochemical kinetics. Moreover, to quantitatively evaluate the efficiency of the as-prepared photoanodes, applied bias photon-to-current efficiency (ABPE, termed as 𝜂) was calculated via the following equation: 𝜂=

𝐽 × (1.23 ― 𝐸𝑅𝐻𝐸) 𝑃𝑙𝑖𝑔ℎ𝑡

× 100%

(2)

where J is the photocurrent density, ERHE is the potential versus RHE, and Plight is the illumination intensity.38 As clearly seen from the Figure 4(b), the efficiencies for all the four photoanodes work as a function of the applied potential, with efficiency reaching maximum in the vicinity of 0.8 V to 0.9 V. And BiVO4/rGO/NiFe NAs ternary photoanode displays the highest photoconversion efficiency over the whole potential range. The enhancement of binary BiVO4/NiFe NAs was not satisfactory probably due to the poor charge mobility of transitional metal hydroxides,39 while there might be restrained electrochemical catalytic dynamics and limited electrochemical active sites to water 19



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oxidation reactions for binary BiVO4/rGO system. To further investigate the transient PEC response of BiVO4-based photoanodes, polarization curves under chopped illumination with a light on and off duration of 5 s were recorded. As illustrated in Figure 4(c), all the photoanodes display quick and transient response within the illumination on and off cycles, illustrating a fast irradiation driven electron-hole pair generation process. It is noticed that photocurrent densities were slightly lower compared to the values in Figure 4(a), because the tests were conducted at a scan speed of 20 mV/s rather than 50 mV/s to match the chopped illumination cycles. Normally, faster scan speed would result in larger current density.40 Besides, the incident photon-to-current conversion efficiency (IPCE) of BiVO4, BiVO4/NiFe NAs, BiVO4/rGO and BiVO4/rGO/NiFe NAs photoanodes were calculated adopting the following equation: IPCE =

1240 λ

×

𝐽𝑙𝑖𝑔ℎ𝑡 𝑃λ

× 100%

(3)

where λ and Pλ are wavelength and irradiance intensity of the incident light, and Jlight is the current densities measured upon irradiation at a specified wavelength.41 The results are compared and presented in Figure 4(d). Apparently, the IPCE values of all these four BiVO4-based photoanodes dropped to close to zero in the wavelength range beyond 500 nm as a result of lacking efficient light absorption.42 Significantly, in consistent with the LSV results, BiVO4/rGO/NiFe NAs exhibit the highest IPEC value in the UV region. Apart from photocurrent densities, long-term durability is another essential parameter to evaluate the photoanodes in their practical applications.43 The BiVO4/rGO/NiFe NAs 20


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photoanode exhibited great stability. The photocurrent kept stable at ~0.7 mA/cm2 even after 10 h continuous measurement, as shown in the chronoamperograms (I-t) curve in Figure 4d. Besides, SEM image of BiVO4/rGO/NiFe NAs photoanode after long-term stability test was also obtained to evaluate the structure evolution accompanied with the photoelectrochemical process. As confirmed in Figure 4f, the microstructure of this unique ternary system was well retained, showing its great potential as an efficient and stable photoelectrochemical water splitting photoelectrode.

Figure 5. (a) Nyquist plots. (b) Mott-Schottky plots. (c) Schematic of PEC water oxidation on the BiVO4/rGO/NiFe NAs photoanode.

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To deeply understand the enhanced photocurrent density and improved stability of BiVO4/rGO/NiFe NAs photoanode, electrochemical impedance technique was utilized to investigate the charge transfer behavior and impedance of solid and liquid interface. The results are shown in Figure 5(a) with the form of Nyquist plot. Based on the electrochemical simulation circuit, the minor semicircle part at high frequency is closely related to the charge transfer resistance (Rct) and the major straight line at low frequency is relevant to the mass transfer resistance (Rm). Normally, smaller semicircle radius represents better charge transfer ability. Similarly, the higher the slope of straight line, the smaller the mass transfer resistance. The BiVO4/rGO/NiFe NAs displays the smallest Rct and Rm values across four BiVO4-based photoanodes, indicating that efficient charge carrier transportation and rapid surface oxidation kinetics occurred on the photoanode and electrolyte interface,44 in which case recombination of electron-hole pairs was largely suppressed and excellent photochemical performance was consequently achieved.45 Besides, based on the Mott-Schottky measurement results in Figure 5(b), all the four samples present positive slopes, indicating n-type semiconductor characteristic of BiVO4.46 The charge carrier density could be estimated based on Mott-Schottky equation: 1 𝐶2

2

(

= 𝜀𝜀 𝐴2𝑒𝑁 𝑉 ― 𝑉𝐹𝐵 ― 0

𝐷

𝑘𝐵𝑇 𝑒

)

(4)

where C is capacitance, ε is dielectric constant of the semiconductor, εr is permittivity of free space, A is electrode area, e is elementary charge, ND is charge carrier density, V is applied potential, VFB is flat band potential, kB is Boltzmann constant, and T is absolute temperature.47 Smaller slope of linear fitting plot in Figure 5(b) for BiVO4/rGO/NiFe NAs 22


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sample signifies higher charge carrier density. As schematically illustrated in Figure 5(c), electrons and holes were generated within BiVO4 under illumination. Due to the high work function and excellent conductivity of graphene nanosheets,48 the photogenerated charge carrier recombination was largely suppressed, and thus charges were favorably transported to the photoelectrode/electrolyte interface. With the assistance of NiFe hydroxides that act as the catalyst, the water oxidation reaction was successfully accelerated, leading to enhanced photocurrent density. Apart from the synergistic effects of BiVO4/rGO/NiFe ternary system, the improvement may also benefit from their unique hierarchical nanostructure. On the one hand, the nanopyramidal structure is favorable for separation of the photogenerated charge carriers due to short hole diffusion path and better permeation of the electrolyte with increased solid/liquid contacting areas. On the other hand, three dimensional interconnected NiFe NAs with amorphous structure also provides more electrochemical catalytic active sites for water oxidation. Therefore, this work demonstrates that the outstanding PEC water splitting performance of photoelectrodes could be accomplished by tunable multicomponent design and rational hierarchical structure control.

In summary, a novel BiVO4/rGO/NiFe hydroxide photoanode with enhanced PEC water splitting performance was fabricated via a facile and scalable solution route. XRD results illustrated a crystalline scheelite BiVO4 and amorphous NiFe hydroxide structure. And morphology characterization showed a nanopyramid configuration of BiVO4 covered by 23



sheet-like graphene interlayer and amorphous NiFe hydroxides NAs. The photocurrent density of ternary BiVO4/rGO/NiFe NAs photoanode was enhanced to 1.30 mA/cm2 at scan rate of 20 mV/s along with great stability. Based on Nyquist and Mott-Schottky results, such enhancement was mainly attributed to the smaller charge transfer resistance and higher charge carrier density, which was originated from the superior charge transfer ability of graphene shuttle layer and accelerated water oxidation kinetics of NiFe cocatalyst layer. The unique ternary system with hierarchical structure can provide some hints on design, control and fabrication of metal oxide based heterostructure photoanode with modified functionalities.

Acknowledgement X.H. and Y.W. thank the China Scholarship Council Postgraduate Scholarship Program provided by the Ministry of Education, China for its support for their stay at Rice University as visiting PhD students.

Supporting information The Supporting Information is available free of charge on the ACS Publications website. TEM images of BiVO4-based photoanodes (S1), J-V curves of BiVO4/NiFe NAs (S2), XRD pattern of BiVO4/NiFe NAs (S3), XRD pattern of BiVO4/NiFe LDH (S4), XPS survey spectrum of BiVO4/rGO/NiFe NAs (S5), I-t, Q-t, J-V curves of BiVO4/NiFe NAs 24


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(S6), J-V curves of BiVO4/rGO (S7), schematic of electrochemical simulation circuit (S8), PL spectra of BiVO4-based photoanodes (S9), PEC water splitting performance of some BiVO4-based photoelectrodes (Table S1).

25



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TOC

A nanopyramid BiVO4-based hierarchical structure can be achieved via a green solution route and be engineered into photoelectrochemical electrodes.

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NiFe

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