In Situ Formation of WO3-Based Heterojunction Photoanodes with

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In-situ Formation of WO3-based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method Faqi Zhan, Yang Liu, Keke Wang, Yisi Liu, Xuetao Yang, Yahui Yang, Xiaoqing Qiu, Wenzhang Li, and Jie Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21895 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Applied Materials & Interfaces

In-situ Formation of WO3-based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method Faqi Zhan a, b, Yang Liu b, Keke Wang b, Yisi Liu d, Xuetao Yang b, Yahui Yang c, Xiaoqing Qiu b, Wenzhang Li b*, Jie Li b* a

State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous

Metals, Lanzhou University of Technology, Lanzhou 730050, China b

School of Chemistry and Chemical Engineering, Central South University,

Changsha 410083 China c

College of Resources and Environment, Hunan Agricultural University,

Changsha 410128, China d

Institute of Advanced Materials, Hubei Normal University, Huangshi, 415000,

China

*Corresponding

author. Tel.: +86 731 8887 9616; fax: +86 731 8887 9616.

E-mail addresses: [email protected], [email protected]

Key words: Oxygen vacancies; heterojunction films; microbattery method; in situ; interfacial charge transfer; photoelectrochemical property

1

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Abstract Nonstoichiometric ratio semiconductor materials have exhibited excellent performance in energy conversion and storage fields. However, the hydrogen treatment method that is commonly used to introduce oxygen vacancies is too expensive and dangerous. In this paper, a novel microbattery method using Zn powder and Fe powder as reductant has been developed to synthesize the oxygen vacancies modified WO3-x films and oxygen-deficient heterojunction films (ZnWO4-x/WO3-x and Fe2O3-x/WO3-x) at room temperature. The as-prepared WO3-x and ZnWO4-x/WO3-x heterojunction films exhibit an improved photoelectrochemical performance. It is worth noting that this microbattery method can quickly introduce oxygen vacancies into semiconductor materials including powders and films at room temperature.

1. Introduction Nonstoichiometric ratio semiconductor materials have been widely studied and applied in photocatalysis 1-4, photoelectrochemical catalysis 5-6, electrocatalysis 7-8 and solar cell 9-12 . The introduction of defects into semiconductors has caused widespread concerns in the energy storage and conversion field. Specially, for the metal oxides semiconductors, oxygen vacancies play an important role in photoelectrochemical catalysis, which not only enhance the conductivity of the semiconductors, but also provide more reactive sites

13.

It is favor for the separation of photogenerated

electrons and holes when the semiconductors containing appropriate amount of oxygen vacancies, which can improve the efficiency of the photoelectrochemical catalytic system

14-16.

Li et al. has reported a series of semiconductors with 2


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incorporation of oxygen vacancies

14.

For examples, a oxygen defective TiO2

nanowires (H:TiO2) prepared by hydrogen treatment exhibited a 4-time higher photocurrent density than that of pristine TiO2 nanowires

17.

The hydrogen-treated

WO3 sample possessed a 10-fold enhanced photocurrent density compared to pristine WO3, at 1.2 V vs. Ag/AgCl

18.

The oxygen defective Fe2O3 nanowires thermal

annealed under N2/air atmosphere (N-hematite) presented a boosted PEC performance compared with the sample annealed under air atmosphere (A-hematite)

19.

The

hydrogenated ZnO nanorods (H:ZnO) showed a 5-fold increase in hydrogen evolution, compared to pristine ZnO nanorods 20. At present, the methods of synthesizing oxygen vacancies include solid phase reduction

21,

reduction

25-26

gas-phase reduction

22-23,

and plasma bombardment

liquid phase reduction 27-28.

24,

electrochemical

For the gas-phase reduction method, it

is mainly reduction by hydrogen at high temperature. However there are some security risks. The liquid phase reduction method is usually performed by using reductive solution, such as sodium borohydride (NaBH4) (N2H4·H2O)

30,

29

and hydrazine hydrate

but most of these solutions have certain toxicity and pollution. The

solid reduction reaction was carried out by solid substances with high reduction activity, and it usually requires high temperature and complex equipments as well as the plasma processing

27, 31-32.

In contrast, electrochemical method is based on

electrochemical reduction reaction to prepare oxygen vacancies, which is simple and controllable

33.

The traditional electrochemical method refers to the electrolytic cell,

which requires a certain amount of electrical energy. Based on this, we envisage 3



exploiting the principle of the primary battery for the electrochemical reduction process without external voltage, so as to prepare oxygen vacancies. Seen from the sequence of metal activity 34: Al > Ti > Zn > Fe > W, metals Zn, Fe, Ti and Al can reduce WO3 to form oxygen vacancies (Ov), but the activities of metal Al and Ti are too high to control the quantity of oxygen vacancies. Expectedly, the reduction of WO3 by Zn and Fe is not only thermodynamically feasible but also mild in kinetics, which is beneficial to the regulation of oxygen vacancies. So we choose Zn and Fe to prepare oxygen vacancies through the Zn- or Fe-WO3 primary battery. Then, the tungsten oxide with oxygen vacancies (WO3-x) can be obtained by removing the active metal Zn or Fe and its oxides by using dilute acid; while in-situ formed Zn or Fe oxides-WO3-x heterojunction photoanodes can be obtained directly. The prepared nonstoichiometric tungsten oxide and oxygen-deficient heterojunction photoanodes exhibited excellent photoelectrochemical performance.

2. Experimental Section 2.1 Synthesis of photoanodes films WO3 plate-like films: WO3 plate-like arrays films grown on FTO substrates were prepared by a typical hydrothermal method, according to our previous literature 35. Oxygen-defective and oxygen-deficient heterojunction films: In a typical preparation, 20 mg/mL of Zn or Fe powder was ultrasonically dispersed into deionized (DI) water. Then, the as-prepared WO3 film was immersed into the above solution for different time, followed by rinsing and dry. The WO3-x films were obtained by removing the residual Zn or Fe with dilute hydrochloric acid (HCl) and 4


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calcining at 450 oC for 0.5 h under Ar atmosphere. The digital photographs were shown in figure S1. While the ZnWO4-x/WO3-x and Fe2O3-x/WO3-x films were obtained without HCl treatment. The ZnWO4/WO3 and Fe2O3/WO3 films can be obtained by calcinations in air. The specific preparation processes are shown in Scheme 1.

Scheme 1. The preparation schematic diagram of composite films 2.2 Characterization The microscopic morphologies were examined by field-emission scanning electron microscope (FESEM, Nova NanoSEM 230) and high resolution transmission electron microscope (HRTEM, G2 F20). The surface chemical compositions and states of the films were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS system). The crystal structures of all films were analysed by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan) with Cu Kα (λ=0.15406 nm) radiation, and the UV-vis spectra were recorded by a diffused reflectance spectrophotometer with an integrating sphere (DR-UVS, Shimadzu 2450 spectrophotometer). 5



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2.3 Photoelectrochemical measurements

Photoelectrochemical (PEC) performances of the as-prepared films were examined using an electrochemical analyzer (Zennium, Zahner) in a standard three-electrode system with the samples as the working electrode, Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. A solution of 0.2 M Na2SO4 (pH=7) was used as the electrolyte. Simulated 1 sun light irradiation condition (100 mW/cm2) was provided by a 500 W Xenon lamp with AM 1.5 G filters. All photoelectrodes were illuminated from the back side. The scan rate of the photocurrent-potential curve (J-V) was 20 mV/s. The Mott-Schottky measurements were conducted at the AC frequency of 1 kHz, and the electrochemical impedance spectroscopy (EIS) was performed at 1.0 V (vs. Ag/AgCl) with an AC frequency range of 10 kHz~100 mHz. The EIS spectra were analyzed by Z-View program (Scribner Associates Inc.). The incident photon-to-current conversion efficiency (IPCE) was measured using a Xenon lamp (150 W, Oriel) equipped with a monochromator at 1.0 V (vs. Ag/AgCl). The intensity modulated photocurrent spectroscopy (IMPS) were conducted with a Zahner CIMPS-2 system. A blue light-emitting diode (LED) lamp providing both DC and AC components of illumination was used as the light source. A 10% AC component of the current was superimposed on the DC light intensity. The measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation 36:

ERHE  E Ag / AgCl  0.059 pH  E  Ag / AgCl 6


(1)

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Where ERHE is the converted potential vs. RHE, EΘAg/AgCl=0.1976 V at 25 ℃, and EAg/AgCl is the experimentally measured potential against an Ag/AgCl reference. The photoelectrochemical measurements were carried out in 0.2 M Na2SO4 (pH=7) at room temperature; therefore, ERHE  E Ag / AgCl  0.6V

(2)

3. Results and discussion 3.1 Characterization of the synthesized photoanodes First, the oxygen vacancies in as-prepared films were characterized. The XPS analysis of oxygen-defective WO3-x films was shown in figure 1. It is found that the W4f and O1s characteristic peaks are shifted to the lower energy with the increasing treatment time, indicating that the existence of oxygen vacancies. The oxygen vacancies appear together with the W5+ and other low-valence metal cations to keep charge balance, resulting in increased electron cloud density, so the electron binding energy obtained by XPS is reduced

37.

Importantly, the Zn or its compounds are not

detected after HCl treatment, indicating that no other impurities have been introduced (figure 1c). In addition, the XPS valence band spectra indicates that the formation of oxygen vacancies (Ov) leads to a negative shift in the valence band position of the semiconductors (figure 1d). The reason is that the valence band of transition metal oxides is mainly composed of the non-metallic O2p orbital, and the conduct band is mainly composed of the metal d orbital, so the position of valence band will change after O missing. This is consistent with the published literature 37. 7



To determine the chemical composition of the oxygen-deficient heterojunction films, the XPS spectra were conducted as presented in figure 2. The XPS results confirm the existence of Zn, Fe, W and O elements. The W 4f5/2 and W 4f7/2 are located at 37.6 and 35.5 eV, corresponding to W6+ oxides. Importantly, there is a little amount of W5+ (~35.2 eV) and O- (~531 eV) characteristic peaks, indicating the existence of oxygen vacancies (Ov) in the heterojunction films.

Figure 1. XPS spectra of as-prepared Zn-WO3-x films: a-0 s, b-5 s, c-10 s, d-20 s, e-40 s, f-60 s

8


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Figure 2. XPS spectra of (a-c) ZnWO4-x/WO3-x, (d-f) Fe2O3-x/WO3-x films Electron paramagnetic resonance (EPR) spectrometer is usually used to detect the non-paired electrons in semiconductor materials. When the oxygen vacancies is introduced into the metal oxides semiconductors, the metal atoms will possess the non-paired electrons, resulting corresponding characteristic peaks in the EPR spectra. The EPR spectra of as-prepared films are shown in figure 3a. For the initial WO3 film, there is no characteristic peak, indicating the absence of oxygen vacancies. Nevertheless, the EPR characteristic peak of oxygen vacancies appears in the WO3-x films after Zn-reduction and diluted acid treatment. In addition, there are two EPR 9



characteristic peaks in the ZnWO4-x/WO3-x film without diluted acid treatment, corresponding to the oxygen vacancies in the W-O and Zn-O bonds, respectively. These results indicate that the oxygen-deficient heterojunction films were successfully prepared.

Figure 3. (a) EPR spectra and (b) Raman spectra of as-prepared films: a-WO3, b-WO3-x, c-ZnWO4-x/WO3-x, d-ZnWO4/WO3 It is well known that the appearance or disappearance of Raman characteristic peaks, including the variations of peaks intensity and position in Raman spectra, indicate the changes of chemical bonds in crystals. In order to further analyze the oxygen vacancies from the bond structures, the as-prepared oxygen-deficient films were analyzed by Raman spectroscopy. As shown in figure 3b, the characteristic peaks located at 272 and 325 cm-1 represent the bending vibrations of WO3 bridge oxygen in O-W-O crystals, while the peaks at 715 and 808 cm-1 are attributed to the stretching vibrations

38.

All the Raman characteristic peaks correspond to the

monoclinic WO3 phase, which is consistent with the XRD results. It is noteworthy that the Raman peak at ~610 cm-1 is related to the surface oxygen vacancies of WO3. For the WO3-x film, the Raman peak at ~610 cm-1 disappears, the intensity of WO3 10


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main peak decreases, and the half-peak width increases, indicating the formation of oxygen vacancies in WO3-x 37. In addition, there is no peaks belong to Zn compounds. For the ZnWO4-x/WO3-x film, on one hand, the peak intensity of surface W-O bond is also weakened; on the other hand, there appears a Zn-O bond, confirming the formation of oxygen-deficient heterojunction film. After heat treatment under air, the Raman peak intensity of surface W-O bond increases due to the disappearance of surface oxygen vacancies. Figure S2 shows the SEM images of oxygen-defective WO3-x films obtained from different reduction time of Zn powder. It can be seen that all the films exhibit the plate-like structure, indicating that there is no changes in the microstructure of WO3 film after introducing oxygen vacancies. The HRTEM images in figure 4b show that there are some crystal defects (bumpy and blurred lattice fringes) in the high-resolution lattice of WO3-x film, attributing to the formation of oxygen vacancies (Ov). While the initial WO3 film shows a clear and regular lattice fringes (Figure 4a).

Figure 4. HRTEM images of (a) WO3, (b) WO3-x (Zn-5 s), (c) ZnWO4-x/WO3-x and (d) 11



Fe2O3-x/WO3-x films. The insets are the lattice fringes of the corresponding section. For the samples reduced by Zn or Fe and directly annealed under Ar atmosphere without HCl treatment, the oxygen-deficient heterojunction films will be formed. The SEM images of oxygen-deficient heterojunction films are shown in figure S3. With the formation of heterojunction, the thickness of WO3 plates increases, and the surface becomes rougher, which is conducive to the catalytic reaction. The Figure S4 shows the SEM images of traditional heterojunction annealed under air atmosphere. There exist no differences in morphology between oxygen-deficient and traditional heterojunction films.

The HRTEM images in figure 4c present the micro-bonding

mode of in-situ formed heterojunction. For the ZnWO4-x/WO3-x heterojunction film, a layer of ZnWO4-x is formed on WO3-x surface. A clear crystal lattice fringe with d-spacing of 0.293 nm belongs to ZnWO4 (111) crystal plane. For the Fe2O3-x/WO3-x film, some Fe2O3 nanoparticles are attached onto the WO3 plate (Figure 4d). A clear crystal lattice fringe with d-spacing of 0.270 nm belongs to Fe2O3 (104) crystal plane. The different morphologies may result from the different reduction rate of WO3 between Zn and Fe, thus resulting in different formation rate of ZnWO4 and Fe2O3. Zn-reduction rate is more rapid, which is preferential to form a coating-layer; while Fe-reduction is more likely to form nanoparticles. The elemental-mapping in figure S5 also confirms the uniform distribution of ZnWO4-x on the WO3-x surface. The crystalline phase characterization of oxygen-defective tungsten oxide films prepared by Zn reduction was carried out. In the XRD pattern of Figure 5, the characteristic peaks at 26.4°, 37.7°, and 51.4° are attributed to FTO conductive glass 12


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(PDF#46-1088), while the other marked characteristic peaks are attributed to monoclinic phase WO3 (PDF#43-1035). The XRD results show that the oxygen-defective tungsten oxide (WO3-x) films obtained by Zn reduction have no change in the crystal phase after Zn and ZnO removal by dilute HCl, remaining the original monoclinic phase, and the peak intensity does not change significantly (Figure 5a). This reveals the reduction process did not affect the crystallinity of the WO3 film. However, the WO3 reduced by Zn or Fe powders without further HCl treatment but calcinated under Ar or air atmosphere shows the phase of ZnWO4 or Fe2O3 in XRD patterns (Figure 5b-c), indicating the formation of in-situ heterojunction. There exist no characteristics of oxygen vacancies and crystalline phase such as WO2 or W18O49, probably due to the low oxygen vacancies content.

Figure 5. The XRD patterns of prepared films: (a) WO3-x films, (b) ZnWO4-x/WO3-x, Fe2O3-x/WO3-x, and (c) ZnWO4/WO3, Fe2O3/WO3 films The optical absorption performances of the oxygen defective WO3-x films and 13



oxygen-deficient heterojunction films are shown in figure S6. The WO3-x films exhibit optical absorption edge around 460 nm, consistent with its intrinsic bandgap (~2.7 eV). Importantly, the visible light absorption intensity in 460-600 nm is gradually enhanced with the increase in oxygen vacancies content. This is consistent with the color variation of the films, which may be due to the change of electron structures caused by oxygen vacancies 14. The construction of heterojunction results in a slight red-shift of the optical absorption edge, and the light absorption intensities in visible range of oxygen-deficient heterojunction films are also enhanced as shown in figure S6b. 3.2 Photoelectrochemical performances To investigate the photoelectrochemical properties of as-prepared films, the photocurrent density and monochromatic incident photon-to-electron conversion efficiency (IPCE) were detected by an electrochemical workstation. As presented in figure S7a, after reducing by Zn for 5 s, the WO3-x (Zn-5 s) film exhibits a photocurrent density of 0.37 mA/cm2 at 1.23 V vs. RHE，which is 1.3 times higher than the bare WO3 (0.50 mA/cm2). And the maximum value of 0.72 mA/cm2 is obtained at 1.6 V vs. RHE. When extending the reduction time, the oxygen vacancies content increase, and the photocurrent density decreases. The WO3-x (Zn-60 s) film shows a photocurrent density of 0.30 mA/cm2 at 1.6 V vs. RHE. After heat treatment in the air to form a heterojunction, the photocurrent density of ZnWO4/WO3 film increases to 0.96 mA/cm2, and that of Fe2O3/WO3 film is 0.90 mA/cm2 (Figure S7b-c). While after heat treatment in the Ar atmosphere, the photocurrent densitis of 14


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oxygen-deficient heterojunction ZnWO4-x/WO3-x film and Fe2O3-x/WO3-x film are improved to 0.78 mA/cm2 and 0.63 mA/cm2 at 1.23 V vs. RHE, corresponding to a 2.6- and 2.1-fold enhancement, respectively. Also, the photocurrent densities of both two samples further increase to 1.17 and 1.10 mA/cm2 at 1.6 V vs. RHE, respectively (Figure 6a). All the as-prepared photoanodes exhibit no current in dark conditions. The corresponding IPCE values are also increased from 26% (355 nm) of bare WO3 film to 32% (355 nm) of WO3-x film, which is further increased to about 52% (383 nm) for the oxygen-deficient heterojunction films (Figure 6b). As seen from IPCE curves, the introduction of oxygen vacancies on the basis of heterojunction allows the maximum IPCE values locations of photoanodes shift from 355 to 383 nm. And the maximum photoelectric response wavelength extends from 460 to 490 nm. The results of photocurrent and IPCE indicate that a certain amount of oxygen vacancies will improve the separation efficiency of photogenerated charges and further enhance the photoelectrochemical performance; but the excess oxygen vacancies will act as the electron-hole recombination center in turn, thus reducing the PEC activity. The synergistic effect of oxygen vacancies and heterojunction makes the photocurrent density of ZnWO4-x/WO3-x film increase by two times, as well as the incident photon-to-electron conversion efficiency (IPCE).

15



Figure 6. (a) Photocurrent densities and (b) IPCE curves of as-prepared films

To study the effect of oxygen vacancies and heterojunction on the electron transmission performance of photoanodes films, figures 7a and d show the electrochemical impedance spectra (EIS) under irradiation condition. All the EIS plots are semicircular, indicating that the electrochemical reaction on the electrode surface is the rate-controlling step

39.

Generally, the smaller the arc radius presents, the

smaller resistance the films possess and the better electron transmission are obtained. The inset in Figure 7d shows the equivalent circuit of films. The physical resistance R1, charge transfer resistance R2 and constant-phase element CPE of different films are obtained by fitting EIS plots through the Z-view software 40, as shown in table 1. The results show that the introduction of oxygen vacancies and construction of heterojunction reduces the charge transfer resistance R2 of WO3 films, which also reflects the more effective separation of photo-generated charge. The bode plots corresponding to EIS nyquist are shown in figure 8b and e. The frequency of characteristic peak for WO3 film in the bode plot is 24.75 Hz, and it is 10.03 and 7.15 Hz for heterojunction films before and after introducing oxygen vacancies, respectively. The lifetime of electrons (τe) can be determined from the maximum of frequency (fp) in bode plots, given by the formula 41: 16


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τe=1/(2πfp)

(3)

According to the bode plots, the lifetime (τe) of photogenerated electrons are calculated in table 1. The introduction of appropriate amount of oxygen vacancies increases the electron lifetime of WO3 films by nearly 2 times. The construction of oxygen-deficient heterojunction further extends the electron life to 22.3 ms. In a word, on one hand, it is attributed to the construction of internal electric field within heterojunction, which benefits the separation of photogenerated charge and increases the electrons lifetime

42.

On the other hand, the moderate amounts of oxygen

vacancies create new intermediate energy level, which makes a well energy alignment between the heterojunction and faster charge transfer ability

43-44.

In addition,

moderate amounts of oxygen vacancies can improve the electronic delocalization of semiconductor materials, increase the conductivity, and further improve the photoelectrochemical performances 45-46.

17



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Figure 7. (a, d) EIS Nyquist, (b, e) Bode plots and (c, f) I-t curves of photoanodes films

Table 1 The resistances (R), electrons life time (τe) and transfer time (τd) of as-prepared photoanodes Samples

R1 (Ω)

R2 (Ω)

CPE

τe (ms)

τd (ms)

WO3

31.4

925

867

6.4

3.8

WO3-x

24.4

751

742

11.5

2.9

ZnWO4/WO3

19.1

766

701

15.9

1.9

Fe2O3/WO3

20.6

826

707

15.9

1.9

ZnWO4-x/WO3-x

11.3

576

521

22.3

1.2

Fe2O3-x/WO3-x

27.1

625

571

22.3

1.5

18


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The stability of the photoelectrodes plays a very important role in the practical application, and figure 7c and f give the photostability curves of the photoelectrodes under 1.2 V vs. Ag/AgCl. Figure 7c shows that the photocurrent density of the pure WO3 film decreases by 4.5% after the 3000 s illumination, while the WO3-x film decreases by 3.2%, indicating that the photostability of WO3 film was improved after introducing oxygen vacancies. It may be due to that a certain amount of oxygen vacancies on the surface can increase the covalent bonding between the electrode surface and the electrolyte water molecule. This can rapidly transfer the hole to participate in the catalytic oxidation reaction, avoiding the photocorrosion from the photo-induced hole accumulation and the etching of the WO3 film by the peroxide species

1, 18.

As the case of oxygen-deficient heterojunction ZnWO4-x/WO3-x film, the

photocurrent density decreases from 1.1 mA/cm2 to 0.92 mA/cm2 after illumination for 2 h, and that of the Fe2O3-x/WO3-x film decreases by about 7.8%. Especially, the ZnWO4-x/WO3-x film is less stable than Fe2O3-x/WO3-x film, which may be due to that the stability of Zn compounds in solution is worse than that of Fe compounds. The Mott-Schottky measurement (M-S) is also used to study the electrochemical properties of photoanode films, as shown in figure 8a. The positive slopes of the M-S curves show that the prepared films are n-type semiconductor materials, wherein the electron is the majority carrier. The photogenerated carrier concentrations of different films are calculated by the M-S formula 42: N d  (2 /  0 q )  d 1/ C 2  / dV 

1

(4)

Where ε0 and ε are the vacuum permittivity and the dielectric constant of the 19



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semiconductor, Nd is the carrier density, q is the electron charge, and V is the electrode potential, respectively. The photogenerated carrier density of WO3 film is 9.1×1020 cm-3, and 15.2×1020 cm-3 for WO3-x film, 34.8×1020 cm-3 for ZnWO4/WO3 film and 80.1×1020 cm-3 for ZnWO4-x/WO3-x heterojunction film. The introduction of oxygen vacancies and the construction of the heterojunction increase the carrier concentration of the films by an order of magnitude, due to the effective separation of the charge and the increase of the internal delocalized electrons 47-48. The slope of the fitting line in M-S plots is the flat-band potential of the film electrode. The introduction of oxygen vacancies has little effect on the flat-band potential. But forming heterojunctions with the ZnWO4 and Fe2O3 which possess more negative Fermi levels cause negative shift of the flat-band potential of the WO3 film, consistent with the results of the literatures 49-50. In order to further explore the dynamic process of photogenerated charge transfer in composite films, the intensity modulated photocurrent spectroscopy (IMPS) of photoelectrodes are given in figure 8b. The IMPS plots are located in the fourth quadrant of the planar complex diagram, indicating that the transfer and recombination of charge is the main rate-control step 51. The characteristic peak frequency fIMPS in the IMPS plots corresponds to the time τd of the photo-induced charge transfer, i.e. the average time of the photo-induced electrons transmission from the semiconductor surface to the conductive substrate and the average time of the photo-induced holes reaching to the electrode surface. The average charge transfer time of different films is calculated by the formula 52: τd=1/(2πfIMPS) 20


(5)

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Where fIMPS is the frequency at the imaginary minimum. The results as shown in table 1 indicate that a certain amount of oxygen vacancies and heterojunction accelerates the transmission of photogenerated charges in the bulk phase of photoanodes, and the average charge transfer time is shortened to nearly one third of the bare WO3.

Figure 8. (a) Mott-Schottky plots, (b) IMPS plots of photoanodes films: a-WO3, b-WO3-x, c-Fe2O3/WO3, d-ZnWO4/WO3, e-Fe2O3-x/WO3-x, f-ZnWO4-x/WO3-x; (c) charge separation efficiency and (d) charge injection efficiency of photoanodes.

The charge transport kinetics in the bulk and surface of photoanodes films is ultimately reflected in the charge separation efficiency (ηsep) and charge injection efficiency (ηinj). Both of them affect the photocurrent density of the photoanodes, and can be calculated by formula 53:

 sep  J Na SO J abs 2

(6)

3

inj  J H O J Na SO 2

2

3

(7)

First, Jabs was determined to be 2.66 mA/cm2 (Figure S8a) by integrating the UV-vis 21



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absorption spectra of photoanodes films with respect to the AM 1.5 G solar light spectrum

54.

The photocurrent densities of photoanodes tested with the hole capture

agent-sodium sulfite are shown in figure S8b. According to the above formulas, the charge separation efficiency (ηsep) and charge injection efficiency (ηinj) were calculated as presented in Figure 8c and d. The results indicate that the introduction of oxygen vacancies and the construction of heterojunction make the charge separation efficiency of WO3 film increase over the entire potential range. As shown in Fig. 8c, the ηsep of the ZnWO4-x/WO3-x film at 1.23 V vs. RHE is calculated to be 45.2%, about 2 fold than that of bare WO3 (22.5%). And the ηsep of WO3-x film further increases from 45% to 52%, while that of further increases to 67% at 1.6 V vs. RHE. Similarly, the oxygen vacancies and heterojunction also improve the surface charge injection efficiency and accelerate the transfer of photogenerated charge at the interface between the semiconductor and electrolyte 55. The oxygen vacancies in the metal oxides belong to the positive center, and the electrons which are bound by the positive center oxygen vacancies are easily stimulated to form free electrons. So the oxygen vacancies plays the role of the donor energy level, which can produce more free electrons under the light condition, and increases the optical carrier concentration of the semiconductor film, consistent with the Mott-Schottky (M-S) test results. In addition, the total band bending (∆ϕsc) within the semiconductor space charge region is proportional to the photogenerated carrier density (Nd), according to the formula 56: sc =  eN d r 2   3 0  22


(8)

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So that the oxygen vacancies on the surface causes the WO3 to bend more strongly on the surface, forming a stronger electric field in the space charge layer and greater electron transfer driving force. This enables the rapid transfer of the photo-induced electrons to the conductive substrate, which improves the photoelectrochemical properties of the semiconductor film, as shown in figure 9a. For oxygen-deficient ZnWO4-x/WO3-x heterojunction film, on the one hand, the donor energy level of oxygen vacancies increases the density of the photogenerated carriers; on the other hand, because the oxygen vacancies energy level is usually located near the conduct band bottom of the semiconductor

57,

thus, the difference between the conduct band

edge of ZnWO4 and WO3 becomes larger, as presented in figure 9b. As a result, the driving force of the optical electron transfer increases, which leads to the more effective separation of the photogenerated carriers and a much better PEC performance.

Figure 9. The energy band structures and charge transport processes in (a) WO3-x and (b) ZnWO4-x/WO3-x films After investigating the photoelectrochemical properties of oxygen-deficient heterojunction films, the formation mechanism of oxygen vacancies is further explored. When the Zn and Fe residues were removed by diluted HCl, it is found that 23



the blue color of thin films deepened rapidly once immersed in dilute acid. This phenomenon indicated that the reduction process accelerated under acidic conditions, and the main reduction process should be electrochemical reaction. The Zn/Fe and WO3 films form a local primary cell, and Zn/Fe have negative electrode potentials, which causes the electrons to run toward the WO3 film to produce a reduction reaction as shown in Figure 10b. The potential-pH diagram (Figure S9) shows that the balanced potential of WO3/WO2 is more positive and more easily to be reduced under acidic conditions, so the color is deepened rapidly under acidic conditions. When the WO3 film and metal Zn plate were immersed in acidic solution, the WO3 film did not change color in the absence of conductive adhesive connection, reflecting that no reduction reaction occurred. While the color of WO3 thin film quickly changed into a dark blue when they were connected by the conductive adhesive, indicating the occurrence of the reduction process. At the same time, the open circuit voltage of the original battery was measured to be -0.78 V, as shown in Figure 10a. In the whole reaction process, Zn or Fe powder acts as the anode of the primary battery and performs the oxidation reaction, while the WO3 film acts as a cathode and performs the reduction reaction to form an oxygen defective WO3-x film.

24


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Figure 10. (a) The diagram of Zn-WO3 battery and (b) formation mechanism of oxygen vacancies in WO3 films

4. Conclusion In conclusion, a novel and convenient method to introduce oxygen vacancies into semiconductors materials at room temperature was developed. The oxygen defective WO3-x films and oxygen-deficient heterojunction ZnWO4-x/WO3-x, Fe2O3-x/WO3-x films were synthesized using Zn or Fe powder as reductants at room temperature rapidly. Compared to the original WO3 film, the photocurrent density of the oxygen-defective WO3-x film was increased by 1.3 times, and that of the oxygen-deficient heterojunction ZnWO4-x/WO3-x film was increased by 2.6 times at 1.23

V

vs.

RHE.

Through

the

electrochemical

analysis,

the

improved

photoelectrochemical performance is mainly due to that the new created defect-levels of oxygen vacancies provide a fast transfer channel for photogenerated charges, 25



increasing the concentration and lifetime of photogenerated electrons, and shortening the transfer time of charges in the films. In addition, the construction of oxygen-deficient heterojunction promotes efficient separation of photogenerated charges. Importantly, this method has certain universality and can introduce oxygen vacancies into semiconductors materials (including films and powders) at room temperature.

Associated content Supporting Information The SEM images, optical absorption spectra, spatial chemical composition of ZnWO4-x/WO3-x heterojunction films; photocurrent densities of photoanodes measured with Na2SO3; Potential-pH diagrams of metals and so on. Notes The authors declare no competing financial interest.

Acknowledgements This study was supported by the National Nature Science Foundation of China (21471054).

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35



TOC Graphic

Integration of heterojunction and interfacial oxygen vacancies of WO3 photoanodes by a novel microbattery method

36


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In Situ Formation of WO3-Based Heterojunction Photoanodes with

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