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Smoothing Surface Trapping States in 3D Coral-Like CoOOHWrapped-BiVO for Efficient Photoelectrochemical Water Oxidation 4

Fumin Tang, Weiren Cheng, Hui Su, Xu Zhao, and Qinghua Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15674 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

Smoothing Surface Trapping States in 3D Coral-Like CoOOH-Wrapped-BiVO4 for Efficient Photoelectrochemical Water Oxidation Fumin Tang†, Weiren Cheng†, Hui Su, Xu Zhao, and Qinghua Liu* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China †These authors contributed equally to this work. *E-mail: [email protected] Abstract Highly-efficient oxygen evolution driven by the abundant sunlight is a key to realize overall water splitting for large-scale conversion of renewable energy. Here, we report a strategy for the interfacial atomic and electronic coupling of layered CoOOH and BiVO4 to deactivate the surface trapping states and suppress the charge carrier recombination for high photoelectrochemical (PEC) water oxidation activity. The successful synthesis of a 3D ultrathin-CoOOH-overlayer-coated coral-like BiVO4 photoanode effectively tailors the migration route of photocarriers on the semiconductor/liquid interface to realize a great increase of ~200% in the photovoltage relative to bare BiVO4, consequently decreasing the corresponding onset potential of PEC water splitting from 0.60 to 0.20 VRHE. As a result, the unique CoOOH/BiVO4 photoanode could efficiently perform PEC water oxidation in a neutral aqueous solution (pH=7) with a high photocurrent density of 4.0 mA/cm2 at 1.23 VRHE and a prominent quantum efficiency of 65% at 450 nm. Electronic structural characterizations and theoretical calculations reveal that the combination of layered CoOOH and BiVO4 forming interfacial oxo-bridge bonding could greatly eliminate surface trapping states and promote the direct transfer of photogenerated holes from the valence band to the surface water redox potential for water oxidation. Keywords: Photoelectrochemical water splitting; Surface trapping states; Interfacial coupling; Onset potential; Oxygen evolution

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Introduction Photoelectrochemical (PEC) water splitting using sunlight has been widely regarded as one of the most promising routes for worldwide renewable energy applications.1-3 Overall, PEC water splitting involves two concurrent catalytic half-cell reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).4,5 Compared to the HER process, attention has mainly focused on the OER process by photoanodes since the OER is a major reaction rate-limiting step for overall water splitting.6-8 As a result, tremendous research efforts have been put forth to develop a photoanode that has an efficient solar-to-hydrogen effect. Recently, bismuth vanadate (BiVO4) has emerged as a promising material for PEC water splitting, showing a high visible light photoactivity.3,9 Monoclinic scheelite BiVO4 with a direct band gap of ∼2.4 eV exhibits a higher solar absorption efficiency. Its lower band gap energy enables light absorption of up to 11% of the solar spectrum.3 Nevertheless, like most photoanode materials, bare BiVO4 is not catalytically active for OER and encounters a large onset potential, which is the main barrier for practical applications.10,11 To match the cathode and achieve the utilization of the entire solar spectrum, it is highly imperative to search for an effective route to reduce the onset potential and improve the OER kinetics of photoanode.12,13 To date, numerous attempts have been made to improve the OER catalytic activity of BiVO4 via element doping,14,15 morphology engineering,16-18 and heterojunction formation.19-21 Element doping has been used widely to tune the band structure for effective carrier transfer and extended absorption wavelengths. For example, Mo and W elements have been doped into BiVO4 to increase the photocurrent.22,23 However, improvement of the onset potential is limited (~0.60 VRHE) because metallic doping would introduce new defect states in the materials. Morphology engineering could enhance the photocurrent of BiVO4 by increasing the actual surface area of the reaction,24,25 which works well in high bias potential conditions but is not satisfactory in low bias potential conditions where thermodynamics dominate. To settle this issue, heterojunctions, especially 2


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co-catalyst/semiconductor heterojunctions of Co–Pi-, Co3O4 particles-, and double FeNi hydroxides-BiVO4, have been sought to improve the photocurrent of the photoanodes via reducing the dynamic barrier of OER.9,26-28 In this way, the interfacial dynamics are somewhat improved, but the onset potential for these materials is still relatively large, mainly attributed to the existence of severe trapping states in the surface of the photoanodes. In principle, the oxygen evolution reaction typically undergoes four proton adsorption and electron reduction steps. In these processes, the OER barrier is not only influenced by the electron transfer behavior of the materials but also intimately correlated with the surface states of the photoanode. One method to reduce the OER barrier and decrease the onset potential of the photoanode as much as possible would be to develop a new interfacial coupled BiVO4-based architecture by regulating the surface trapping states to modulate the interlayer electron behavior and OER dynamics of the material. In this work, to maximize the PEC efficiency of the BiVO4 photoanode, we have presented a design for interfacial atomic and electronic coupling for the first time by coating ultrathin CoOOH layers on a 3D coral-like BiVO4 photoanode to effectively promote charge separation and OER dynamics for an efficient PEC oxygen evolution reaction. Ultrathin layered CoOOH was chosen as the surface modifier due to its high surface energy and open surface atomic structure to realize an efficient interfacial covalent-bond and electronic field coupling between CoOOH and BiVO4. After interacting with several layers of CoOOH, the surface trapping states of BiVO4 can be removed by saturating the dangling bonds with Co(+3)–OH, resulting in a larger photovoltage of 0.28 V and a significant negative shift of the onset potential of photoelectrochemical water splitting from 0.60 to 0.25 VRHE for the CoOOH/BiVO4 photoanode. Furthermore, the great reduction in the number of surface trapping states could effectively facilitate the separation efficiency of photocarriers even at a low bias potential. As a result, an outstanding photocurrent density of 4.0 mA/cm2 was achieved for the CoOOH/BiVO4 photoanode at 1.23 VRHE in neutral aqueous solution (pH=7) under AM1.5 illumination, which was a 3-fold enhancement compared with that of the bare BiVO4 photoanode. 3



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Experimental section Materials. Bi(NO3)3·5H2O (99.0%), HNO3 (AR), CoCl2.6H2O (99.0%), Na2SO4 (99.0%), Na2SO3 (99.0%), KH2PO4 (99.0%), K2HPO4 (99.0%), dimethyl sulfoxide (DMSO, AR), ethyl alcohol (EtOH, AR), ethylene glycol (AR, Sinopharm), KI (99.0%), p-benzoquinone (98.0%), VO(acac)2 (AR, Sigma-Aldrich) were used as received without further purification. F-doped tin oxide (FTO) coated glass (~15Ω/cm2, KMT Corporation) was used as the substrate and cut into 1 cm × 2 cm pieces to prepare the photoanodes. Milli-Q deionized (DI) water was used to prepare aqueous solutions for material synthesis and electrochemistry experiments. Preparation of BiVO4 photoanodes. Bare BiVO4 photoanodes were prepared by a two-step electro-deposition route.9 Briefly, dilute HNO3 was dropped into a 50 mL KI aqueous solution (0.4 M) to achieve pH=1.7, and then 2 mmol Bi(NO3)3·5H2O was added. A 20 mL EtOH contained p-benzoquinone (0.23 M) was added to the above solution and stirred to obtain a crimson solution. Electrodeposition was taken in a typical three-electrode cell using the prepared electrolyte. An FTO film worked as the working electrode (WE), a platinum foil (1 × 1 cm2) worked as the counter electrode (CE),

and

saturated

Ag/AgCl

worked

as

the

reference

electrode

(RE).

Electro-deposition was done under a constant potential of −0.1 V vs Ag/AgCl for 4 mins to get a BiOI electrode. Afterwards, the FTO substrate was rinsed with deionized water and dried in ambient air. A drop of dimethyl sulfoxide (DMSO) solution containing 0.2 M vanadyl acetylacetonate (VO(acac)2) was placed on the BiOI electrode and then heated in a muffle furnace at 450 °C for 2 h. Excess V2O5 was removed by soaking them in 1 M NaOH solution for 30 mins with gentle stirring. The final bare BiVO4 electrodes were rinsed with DI water gently and dried at RT. Preparation of CoOOH/BiVO4 photoanodes. First, the initial α-Co(OH)2 nanosheet was obtained by a hydrothermal method.29 Typically, 0.097 g of CoCl2.6H2O was dissolved in a 40 mL mixed solution of ethylene glycol and deionized water. After bubbling with N2 for 30 min, 100 µL of ammonia was added, and then the solution was transferred into a 50 mL Teﬂon-lined stainless steel autoclave and maintained at 130 °C for 24 h. After that, the obtained products were collected by centrifugation, washed with deionized water three times, and then ultrasonically dispersed in a mixed solution of ethylene glycol and deionized water again for future oxidation treatment. 4


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The NaClO solution (5.2 wt%) was dropped into the dispersion in a 25 °C water bath under vigorous stirring. The final product was collected by centrifugation, washed with deionized water three times, and then dispersed in ethyl alcohol and sonicated to form a homogenous ink. CoOOH ink was then spin-coated onto a BiVO4 electrode at 2000 rpm for 1 minute and dried at 70 °C to obtain a CoOOH/BiVO4 electrode. Morphological characterization. The field emission scanning electron microscopy (FE-SEM) images were taken on an FEI Sirion-200 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were performed by using a JEOL-2010 TEM with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg Kα (hυ = 1253.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging S3 by referencing C 1s to 284.5 eV. X-ray Diffraction (XRD) patterns were recorded by using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The UV-vis absorption spectra of the photoanodes were obtained using a UV-vis spectrophotometer (SOLID3700) in the range of 300-800 nm. Photoelectrochemical measurements. All PEC and electrochemical impedance spectroscopy (EIS) measurements were performed with a CHI 760D potentiostat in a three-electrode photoelectrochemical cell with the photoanode as the working electrode, Ag/AgCl as the RE, and a platinum wire mesh as the CE. Photoelectrochemical measurements were conducted under a 300-W Xenon arc lamp with an AM1.5 filter to gain 1 Sun simulated sunlight, and the illuminated region of the electrode was controlled to 100 mW/cm2. Photoelectrochemical experiments were performed in a 0.2 M potassium phosphate buffer solution (pH=7) contained 0.5 M Na2SO4 for water oxidation or 0.5 M Na2SO3 for sodium sulfite oxidation. Potentials are reported as measured vs Ag/AgCl, and adjusted to vs RHE using the Nernstian relation VRHE = VAg/AgCl +0.0591pH + 0.197 V. For the dark and photocurrent measurements, linear sweep voltammetry (LSV) was used, and the scan rate was maintained at 20 mV/s without correcting iR losses unless otherwise noted. The open-circuit potential was tested before enough time (more than 0.5 h) to get steady data under dark. EISs were done at an oscillation voltage of 10 mV from 100000 Hz to 0.5 Hz under different bias voltages with illumination. 5



Results and discussion

Figure 1. Top SEM images of bare BiVO4 (a) and CoOOH/BiVO4 (b) photoanode. The insets show the cross-sectional SEM images of the photoelectrode. (c) HRTEM image of CoOOH/BiVO4. (d) XRD patterns of bare (black) and hybrid (red) BiVO4 photoelectrode. The scale bars in (a) and (b) are 500 nm, and in (c) 5 nm.

Due to the open surface structure of colloidal CoOOH nanosheets and the elaborate spin-coating process, a CoOOH layer was successfully coated over BiVO4. The successful synthesis of a 3D coral-like CoOOH/BiVO4 photoanode was confirmed by field emission scanning electron microscopy (FE-SEM), high-resolution TEM (HRTEM), and X-ray diffraction (XRD). Figure 1a shows a typical SEM image of the bare BiVO4 photoanode, displaying a 3D coral-like structure with a dendritic diameter of 100 nm.9,27,30 Compared with the bare BiVO4 photoanode, the CoOOH nanosheets (Figure S1) are wrapped around BiVO4 as an ultrathin overlayer in the CoOOH/BiVO4 composite (Figure 1b). As seen from the inset of Figure 1b, the optimized thickness of the 3D coral-like CoOOH/BiVO4 photoanode is approximately 1 µm and the surface coating of CoOOH does not change the typical geometry of 6


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BiVO4. Furthermore, the HRTEM image in Figure 1c presents a distinct boundary and clearly reveals a heterojunction interface of the structure. The lattice interplanar spacing of 0.31 nm and 0.23 nm can be well ascribed to the (-121) planes of monoclinic scheelite BiVO4 (Figure S2) and the (012) face of CoOOH (Figure S3), respectively, which are consistent with prior works.29 Furthermore, the determination of the interface structure can be confirmed by the FFT of the local area (Figure S4). The crystal phases of the as-prepared CoOOH/BiVO4 photoanode are further revealed by the X-ray diffraction patterns (Figure 1d). Peaks located at 18.9°, 28.8°, and 30.5° can be indexed to monoclinic scheelite BiVO4 (JCPDS 14-0688). Because of the ultrathin nature and poor crystallinity of the CoOOH layers (Figure S5), no obvious peaks corresponding to CoOOH were obtained in the XRD patterns. These above results

undoubtedly

confirm

the

successful

synthesis

of

an

ultrathin

CoOOH-coated-BiVO4 photoelectrode.

(a)

CoOOH/BiVO4 BiVO4

60

2

50

1.0 Abs

2

j (mA/cm )

3

(b) 70

CoOOH/BiVO4 BiVO4

IPCE (%)

4

40 30

0.5 0.0 300 450 600 750 Wavelength (nm)

20

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10 0.4

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0 420

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Potential (V vs RHE)

500

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Wavelength (nm)

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(c) CoOOH/BiVO4

CoOOH/BiVO4

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BiVO4

j (mA/cm2)

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60 40

BiVO4

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0 0.2

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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0 0

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Time (s)


Figure 2. PEC test in a 0.5 M Na2SO4 aqueous solution containing 0.2 M KPi buffer solution (pH=7.0) if without mentioned. (a) Linear sweep voltammogram (LSV) of bare (black) and hybrid (red) BiVO4 for water oxidation under AM1.5 illumination and under dark (black and dash). (b) Incident photon-to-electron conversion efficiency (IPCE) of bare BiVO4 (black) and 7



CoOOH/BiVO4 (red) at 1.23 VRHE. The inset shows the UV-vis absorption spectra of the electrodes. (c) Efficiency values of charge carrier separation in the bulk (ηbulk, solid) and on the surface (ηsurface, dash) of BiVO4 and CoOOH/BiVO4. (d) Chopped-light AM1.5 chronoamperometry plots of bare (black) and hybrid (red) photoelectrodes at 1.0 VRHE. The inset shows details of the decay at the moment of shading.

To examine the PEC activity, the photocurrent density of all the prepared BiVO4-based photoelectrodes were measured in a neutral buffer electrolyte by front-side illumination under standard reaction and measurement conditions.31,32 Noticeably, the optimized CoOOH/BiVO4 electrode had a thickness of ~15 nm of the CoOOH layer to obtain the best PEC properties (Figures S6 and S7). Figure 2a shows the typical PEC water oxidation J–V curves of the bare and CoOOH-coated BiVO4 photoelectrodes. A large photocurrent density of 4.0 mA/cm2 at 1.23 VRHE was achieved for CoOOH/BiVO4, in contrast to 1.3 mA/cm2 at 1.23 VRHE for bare BiVO4. Furthermore, the onset potential of photoelectrochemical water oxidation for the CoOOH-coated BiVO4 photoanode showed a significant negative shift of ~0.4 V relative to bare BiVO4, reaching 0.25 V vs RHE. In particular, the IPCE of the CoOOH-coated BiVO4 photoanode at 1.23 VRHE showed a universal 3-fold increase and reaches a maximum of 65% at the wavelength of 450 nm, as shown in Figure 2b, which is much higher than that of bare BiVO4. Notably, additional light absorption over the wavelength range of 500−750 nm was observed for the CoOOH-coated BiVO4 photoanode due to strong interfacial coupling between CoOOH and BiVO4, as shown in the inset of Figure 2b. To clarify the contributions of the enhanced photocurrent density, the photocarrier separation efficiency in the bulk and at the semiconductor/liquid interface was quantified via comparing the photocurrent density of electrodes in a 0.5 M NaSO4 aqueous solution with and without sodium sulfite (see the part 3 in Supporting Information). As seen from Figure 3c, the efficiencies of the photocarrier separation in the bulk (ηbulk) and over the semiconductor/liquid interface (ηsurface) for CoOOH/BiVO4 are both much higher than that of bare BiVO4. The ηbulk was increased by 35% and the ηsurface was increased by 65% for the total enhancement of the photocurrent after eliminating the surface trapping states when the bias potential is larger than 1.0 VRHE (Figure S14). Notably, the ηsurface of CoOOH/BiVO4 at a low bias potential of less than 0.7 V is above 20%, while that of bare BiVO4 is nearly zero, indicating more holes at the surface had been used for water oxidation after coating with CoOOH.33 Moreover, the chopped-light chronoamperometry 8


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(Figure 2d) was taken to further identify the transfer kinetics of the photocarriers. Although there was some fast decay of the photocurrent after sudden illumination due to competition between the fast carrier generation and slow surface oxidation dynamics, the decay of CoOOH/BiVO4 is obviously smaller than bare BiVO4. Moreover, the steady-state photocurrent of CoOOH/BiVO4 at 1.0 V vs RHE is 5 times larger than BiVO4, which is consistent with the LSV shown in Figure 2a. It can be seen that a cathodic peak current was observed for both CoOOH/BiVO4 and bare BiVO4 when removing the illumination, which can be attributed to accumulated charges at the surface due to surface trapping states.34 As estimated from the integration of the corresponding damped-current versus time, as shown in the inset of Figure 2d, the accumulated charges of CoOOH/BiVO4 are only 0.019 mC, which is one order of magnitude smaller than that of bare BiVO4 (0.183 mC), inferring significant deactivation of the surface trapping states for CoOOH/BiVO4 photoanodes. To verify the stability of the CoOOH/BiVO4 photoanode, a long-term i-t measurement was performed, and the result shows a minor decay of ~ 3% after 4 hours (Figure S8). The results of SEM, XRD, and XPS show a reliable stability of CoOOH/BiVO4 after long-term reaction process (Figure S9 and S10). A positive shift of ~0.1 eV of Co 2p indicates the possibility of forming Co(IV) species as active oxidation sites (Figure S10b).35 The above results clearly demonstrate CoOOH/BiVO4 as a promising photoanode with a low onset potential and high photocurrent for photoelectrochemical water oxidation. To gain an in-depth understanding of the photogenerated carrier transfer kinetics in the CoOOH/BiVO4 photoanode, electrochemical impedance spectroscopy (EIS) analyses were carefully performed. Since the photocarrier transfer behavior is highly sensitive to the bias potential, the typical Nyquist plots of bare BiVO4 and CoOOH/BiVO4 under 0.6 VRHE are shown in Figure 3a, respectively, for comparison. It can be readily seen that the charge transfer impedance of CoOOH/BiVO4 is much lower than that of bare BiVO4. To provide quantitative analysis of the photocarrier transfer kinetics, the EIS plots were fitted by employing a reasonable equivalent circuit model, as shown in the inset of Figure 3a, where Rs, Rtrap, and RSC correspond to the contact ohm resistance,36,37 photocarrier trapping resistance, and transfer resistance, respectively, and Ctrap and CSC are ascribed to the photocarrier trapping 9



capacitance and interfacial double-layer capacitance, respectively.38 In terms of the fitting results shown in Figures S12 and S13, CoOOH/BiVO4 exhibited a much lower Rtrap of 0.9 k ohm relative to bare BiVO4 (Rtrap 3 k ohm), indicating reduced surface trapping states after coating with CoOOH, and the same variation of CSC with potential for both electrodes indicates that the CoOOH nanosheets did not alter the BiVO4 bulk. Moreover, as shown in Figure 3b, it can be seen that the CTrap of CoOOH/BiVO4 is clearly lower than that of bare BiVO4 at the same potential and, it declines with increasing bias voltage. Interestingly, for both CoOOH/BiVO4 and bare BiVO4, the turning point of the CTrap–V curve coincides well with that of the J–V curves, corresponding exactly to the onset potential of photoelectrochemical water oxidation and suggesting an intimate and direct relationship between the surface trapping state and the onset potential. Based on the above analysis, the coating of ultrathin CoOOH layers could evidently reduce the surface trapping states of the CoOOH/BiVO4 photoanode toward higher PEC water oxidation activity.

CoOOH/BiVO4

60

BiVO4

Ctrap (µF)

Z imag (Ω)

2000

2.0

(b) 70

CoOOH/BiVO4

1500 1000

BiVO4

1.5

50 40

1.0

30 20

500

j (mA/cm2)

(a) 2500

0.5

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0

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Zreal (Ω)

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(d) UOC (V)

(c) 0.4 0.3

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10 20 30 40 Time (s)

CoOOH/BiVO4

Eredox

0.16V

BiVO4

BiVO4 0.1

0.2

0.3

0.4

0.5

0.6

EFn

Vph

0.28V

EFp EFn

CoOOH

0.2

Trap

Voc,L Voc,D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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BiVO4

0.7

Uoc (V vs RHE)

Figure 3. (a) Nyquist plots of bare (black) and hybrid (red) BiVO4 under 0.6 V vs RHE, and the inset shows the equivalent circuit model. (b) Ctrap–potential plots and J–V curves of bare (black) and hybrid (red) photoelectrodes. (c) Open-circuit voltage of bare BiVO4 (black) and CoOOH/BiVO4 (red) under dark (solid) and illumination (hollow), and the inset shows the 10


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transient photovoltage response without immediate illumination. (d) Schematic diagram of the photovoltaic response on photoanodes under different conditions: dark (black lines) and light (gray lines).

To further clarify the role of trapping states on the PEC water oxidation performance, photovoltaic response tests were performed. According to previous work,39 the simple integration of a co-catalyst on the surface of the electrode could reduce the potential loss corresponding to the reaction kinetics in the surface Helmholtz layer, but nominally changes the difference in photovoltage between the dark and illuminated conditions. As seen from Figure 3c, the photovoltage of CoOOH/BiVO4 is 0.28 V, which is ~100% enhancement relative to bare BiVO4 (0.16 V), indicating that the deactivation of the surface trapping states by coating CoOOH leads to an increase in the photovoltage for CoOOH/BiVO4. As shown in Figure 3d, when the semiconductor electrode is immersed in electrolyte, a sharp downward bending of the energy occurs at the liquid/semiconductor interface due to the Fermi level balance between the semiconductor and electrolyte, and in this case, the value of the open-circuit voltage under the dark condition (Voc,D) only depended on the redox potential of the electrolyte and the carrier density of the semiconductor. When under illumination, photoexcited carriers were generated and transferred to the semiconductor surface, changing the density of electrons on the surface and creating a new quasi-fermi level, which depends on the surface carriers density of the semiconductor.40,41 If there were surface trapping states in the semiconductor, it would trap partial photocarriers and cause a great loss of the effective surface carrier density leading to a decrease in the photovoltage. Due to the same light illumination condition, the improved photovoltage of CoOOH/BiVO4 relative to bare BiVO4 should originate from the reduced density of the surface trapping states after coating with CoOOH, as shown in Figure 3d. Taken together, all above results conclusively demonstrate that the surface coating of the ultrathin CoOOH layer endows CoOOH/BiVO4 with minimal trapping states, which could significantly reduce the water oxidation onset potential and enhance the PEC performance.

11



(a)

(c)

Intensity (a.u.)

V 2p1/2

V 2p3/2

CoOOH/BiVO4

BiVO4 528

526

524

522

520

518

Ef -6

516

-4

(b)

(d) Co 2p1/2

CoOOH 805 524 800 522 795

520 790

0

2

518 785

Surface saturated

4

6

Total V

Density of States (a.u.)

0.3 eV

CoOOH/BiVO4

810

-2

Energy (eV)

Binding Energy (eV)

Co 2p1/2

Total V

Surface defected

Density of States (a.u.)

0.3 eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ef -6

516 780

-4

-2

0

2

4

6

Energy (eV)

Binding Energy (eV)

Figure 4. XPS and calculated DOS for both electrodes. XPS patterns of CoOOH, BiVO4, and CoOOH/BiVO4: (a) V 2p and (b) Co 2p. The calculated DOS of the defected surface (c) and the saturated surface (d) of BiVO4.

To clarify the influence of the coated CoOOH overlayer on the electronic structure of CoOOH/BiVO4, X-ray photoelectron spectroscopy (XPS) and first-principles calculations at the density functional theory (DFT) level were performed (Figure 4). For CoOOH/BiVO4, a negative shift of V 2p3/2 from 517.0 eV to 516.7 eV relative to BiVO4 was observed (Figure 4a), inferring an increase in the electron density around the V nucleus after coating with CoOOH. Meanwhile, a positive shift to higher energy of 0.3 eV for Co 2p3/2 was obtained for CoOOH/BiVO4 (Figure 4b), implying electron donation from the Co species to the V species after coating with CoOOH. This strong electron exchange effect between Co and V indicates that there are many covalent Co-O-V bonds at the CoOOH/BiVO4 interface because of the substitution reaction of abundant hydroxyl groups on the surface of the CoOOH nanosheets and wet BiVO4. A higher binding energy of the V nucleus in the bare BiVO4 surface compared to the bulk means a deficiency of coordinated oxygen atoms around the V atom on the surface compared to the tetrahedral coordination of V 12


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in the bulk.42,43 Seen from Figure 4c, the electron density of states (DOS) for the BiVO4 with surface oxygen defects exhibits an isolated middle energy level across the Fermi level, corresponding to surface trapping states. Notably, as shown in Figure 4d, this new-forming surface trapping energy level completely disappears after saturating the defected surface with the O-Co groups. The results of the DFT analysis agree well with the electrochemical analysis (Figure 3d) and XPS results. Based on the above results, the surface trapping states of BiVO4 originate from surface oxygen vacancies and can be removed by the formation of an oxo-bridge oxygen bond between BiVO4 and CoOOH. Summarizing all the above results, the hybrid CoOOH/BiVO4 structure of the photoelectrode shows evident deactivation of the density of surface trapping states and OER catalytic barrier, improving the PEC water oxidation performance. Thereby, the CoOOH/BiVO4 photoelectrode exhibits a lower onset potential of 0.20 VRHE and a significantly increased photocurrent density of 4.0 mA/cm2 at 1.23 VRHE under 1 sun AM1.5 illumination, which is three times greater than that of bare BiVO4. EIS and chopped-light chronoamperometry display that unsaturated bonds on the surface of bare BiVO4 cause trapping states, which consume additional photogenerated carriers (Figure 2d). A CoOOH overlayer was carefully loaded on the surface of BiVO4 to form a coupled structure, forming [CoO6] clusters on CoOOH, which link well with unsaturated bonds on the surface of BiVO4 by oxo-bridge oxygen bonds. This structure introduces a strong interfacial coupling effect between CoOOH/BiVO4, which significantly modifies the electronic properties of BiVO4 for highly efficient PEC water splitting.4,44 Furthermore, the negative charges from the Co(+3) to V(+5) element on the surface of BiVO4 remove the deep energy level in the forbidden band to reduce the density of surface trapping states by an order of magnitude and enhance the photovoltage of the hybrid electrode from 0.16 V to 0.28 V. Moreover, benefited from both the improvement in photovoltage and the decrease in the OER catalytic barriers, the efficiency of the carrier transfer in the bulk (ηbulk) and hole injection efficiency on the surface (ηsurface) of CoOOH/BiVO4 are higher than those of the bare electrode, especially for the ηsurface (Figure 2c). Therefore, the CoOOH/BIVO4 photoanode achieves a lower onset potential of 0.20 VRHE and a higher IPCE of 65% at 450 nm (Figure 2b). 13



Conclusion In summary, we have designed interfacial atomic and electronic coupling between ultrathin layered CoOOH and coral-like BiVO4 to greatly smooth the surface trapping states and promote the OER kinetics for efficient PEC water splitting. As a result, the 3D coral-like CoOOH-coated-BiVO4 photoanode exhibits a low onset potential of 0.20 VRHE and a large photocurrent density of 4.0 mA/cm2 at 1.23 VRHE under AM1.5 illumination in neutral aqueous solution with a high IPCE of 65% at 450 nm. Electrochemical impedance spectroscopy and photocurrent measurements demonstrate that the coating of layered CoOOH on coral-like BiVO4 effectively deactivates the surface trapping states of BiVO4 and promotes the photocarrier transfer over the semiconductor/liquid interface, achieving a higher photovoltage of 0.28 V. The electronic structure characterizations and first-principles calculations reveal that the interfacial charge exchange from Co(+3) to V(+5) via oxo-bridge bonding has effectively smoothed the surface trapping states for high-efficiency charge transfer in the CoOOH/BiVO4 photoanode. Our findings may offer new understanding of the mechanism of an interfacial coupling heterojunction structure for improving solar energy conversion applications.

Supporting Information Characterizations of the CoOOH nanosheets, bare BiVO4, and hybrid CoOOH/BiVO4 photoelectrodes samples, details of the photoelectrochemical and electrochemical measurements, and the theoretical calculations of maximum Jabs.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (WK2310000054), the National Key Research and Development Program of China (2017YFA0402800), and the National Natural Science Foundation of China (Grants No. U1532265, 11605204, 21603207, 21533007, and 11621063).

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Table of Contents Graphic

EFn

Vph Eredox BiVO4

EFp EFn

BiVO4

20


CoOOH

Trap

Voc,L Voc,D

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Smoothing Surface Trapping States in 3D Coral ... - ACS Publications

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