Solar light driven pure water splitting of B-doped BiVO4 synthesized via a sol–gel method

Journal of Alloys and Compounds 636 (2015) 131–137

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Solar light driven pure water splitting of B-doped BiVO4 synthesized via a sol–gel method Lian-wei Shan a,b,⇑, Gui-lin Wang a, Jagadeesh Suriyaprakash b, Dan Li a, Li-zhu Liu a, Li-min Dong a a b

College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Wenhua Road 72, 110016 Shenyang, China

a r t i c l e

i n f o

Article history: Received 2 November 2014 Received in revised form 14 January 2015 Accepted 15 February 2015 Available online 21 February 2015 Keywords: Photoelectrochemical BiVO4 XPS Boron doping

a b s t r a c t The most promising process to convert solar energy into chemical energy is photoelectrochemical (PEC) water splitting, which has received a significant attention in recent years. BiVO4 has been regarded as a promising material for photocatalytic water splitting process. Owing to its poor carrier transport properties, BiVO4 is not a high potential candidate for this process. In order to overcome the inadequacy, we have successfully prepared B-doped monoclinic scheelite BiVO4 by a sol–gel technique. Introduction of the 0.6 at% boron into the BiVO4 significantly improves the photocatalytic activities. Although there is almost no difference in the band gap energy for BiVO4 as changing the boron doping level, the onset potential was obviously reduced which contributes to the overall thermodynamic conversion efficiency. The enhanced PEC activity of BiVO4 is relative to interstitial boron doping among VO4 tetrahedrons, which improves the corresponding electron transport properties. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Photoelectrochemical (PEC) water splitting has great potential for renewable hydrogen production using solar energy [1,2]. Some potential metal oxides were widely reported in this field including titania (TiO2) [3,4], and tungsten trioxide (WO3) [5]. However, poor light absorption inhibits their photocatalytic applications because of wide band gap resulting into narrow range of solar irradiation. It is very imperative that we continue to develop photocatalysts that response over the wide range of radiation wavelength covered most of the solar light. The semiconductor BiVO4 possesses much attraction for utilization in solar energy collection and photocatalytic reactions from highlighted work in 1999 by Kudo et al. [6], due to its direct band gap of 2.4 eV, and favorably positioned band edges [7–10]. As a well-known there are three kind of crystalline phase for BiVO4 including monoclinic scheelite (ms), tetragonal scheelite (s-t) and tetragonal zircon (z-t). The ms-BiVO4 is found to exhibit visible-light-driven photocatalytic activity among these three structures [11–13]. Recent studies have shown that substitution of ions in BiVO4 has been a credible tactic to improve the photocatalytic activities [8,14,15]. ⇑ Corresponding author at: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Wenhua Road 72, 110016 Shenyang, China. E-mail address: [email protected] (L.-w. Shan). http://dx.doi.org/10.1016/j.jallcom.2015.02.113 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

It is worthwhile to note that the metal ions doping induces some impurity levels, thus causes thermal instability of the semiconductor. While doping with nonmetal elements generally induces new states in the band gap [16]. For example, N-doped BiVO4 demonstrated as superior candidate in photodegradation of methyl orange (MO) reaction [15]. Also, F-doped BiVO4 was reported to show excellent photocatalytic performance for the degradation of phenol [17]. Moreover, wang et al. reported that the boron and europium co-doped BiVO4 have high photodegradation of MO under visible light irradiation [18]. All of these research outcomes suggested that doping the nonmetal elements is of very incredibly supportive to enhance the photocatalytic performance. Narrowing an energy band gap and improving a separation efficiency of photon-generated electron–hole pairs usually were considered as key reasons to elucidate the sensitizer degradation efficiency. It is known that the VO4 tetrahedron in BiVO4 does not contact with subsequent one, which led to poor carrier transmission characteristic [19]. As a result, some works focused on reducing the impedance of BiVO4 to obtain high photocatalytic activities by doping Mo, W and P elements, and doping induced crystal distortion has also been proved by density functional theory (DFT) calculations, X-ray diffraction (XRD) and Raman shift [2,20]. As a widely utilized doping element, boron has been used to modify the transport properties [21,22]. However, there are few reports on PEC of BiVO4 which are affected by nonmetal elements to understand the enhancement of PEC activities, especially boron doping.

2. Experimental 2.1. Fabrication of boron-doped BiVO4

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(132) (240) (042)

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(211)

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(002) (220)

(040)

In this paper, we reported the preparation of pure BiVO4 and boron-doped BiVO4 photocatalysts via the sol–gel technique, characterization of these samples (XRD, XPS, TEM, DRS), and photocatalytic activities for PEC water splitting under visible-light illumination.

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(d) (c) (b)

In accordance with 1:1 molar ratio (Bi in excess of 5% compensating for losses in the annealing), raw materials (5.145 g Bi(NO3)35H2O and 1.182 g NH4VO3 at 1:1 molar ratio) were weighed. 3.843 g C6H8O7 were added to the Bi(NO3)3 solution prepared in 20 ml HNO3 (1 mol L1) in advance, then 80 ml deionized water added. After that, pH was adjusted with aqueous ammonia (25% mass concentration) till it reaches neutral value (pH = 7), and this was labeled as solution A. NH4VO3 and 3.843 g C6H8O7 were dissolved in 100 ml deionized water to acquire solution B with the concentration of 0.1 mol L1. 100 ml solution A was added slowly into the solution B of equal volume, and then pH adjusted until to 7. After 6 h in water bath, the precursor powder was obtained by various subsequent process such as drying, ashing, grinding. The precursor powders of the bismuth vanadate were placed in the crucible, and a certain amount of boric acid solution was mixed with the precursor. After drying, grinding, then they were calcined at 375 °C for 4 h under the ambience of atmosphere. The sample was labeled as B0, B03, B06, B12 and B30, respectively. They represent the initial molar ratio of B to V (between 0% and 3.0%).

2.2. Characterization The crystallization behavior of the synthesized material was analyzed by X-ray diffraction (XRD; Model D/MAX-3B, Tokyo, Japan) excited by Cu Ka radiation, a sampling interval of 0.02°, and a scan speed of 4° min1. Transmission electron microscopy (TEM, JEM-2010) was used to characterize the microstructure of BiVO4. The surface XPS analysis was performed with a Thermo Escalab 250 using a spherical capacitance analyzer, and all spectra were referenced to the adventitious C1s peak at 284.6 eV. Diffuse reflectance spectra were obtained on a Shimadzu UV2401PC UV/vis scanning spectrophotometer equipped with a diffuse reflectance accessory using BaSO4 as standard. The effectively separation of photogenerated electron–hole pairs were analyzed by a fluorescence spectrophotometer (Shimadzu, model RF-5301 PC).

2.3. Photocatalytic evaluation The photoelectrochemical properties were investigated in a conventional threeelectrode cell arrangement using an electrochemical analyzer (Autolab, PGSTAT 302N). The corresponding powder samples were coated onto the fluorine-doped tin oxide (FTO) glass by using electrophoretic deposition (EPD) technique. The light source was a 100 mW cm2 solar light simulator. They were irradiated from the FTO glass side (back light illumination). A cut-off filter (420 nm) was used to remove the UV light component. The prepared samples were used as the working electrode, and a Pt foil was used as the counter electrode. Potentials were applied vs. the Ag/AgCl reference electrode. Na2SO4 aqueous solution (0.5 mol L1, pH = 6.6) was used as the electrolyte. The measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst relation ERHE = EAg/AgCl + 0.0591 pH + 0.1976 V.

3. Results and discussion XRD study was carried out to investigate the changes of the BiVO4 phase structure after boron doping. Fig. 1 shows the XRD patterns of the as-fabricated samples. It was found that all diffraction peaks can be perfectly indexed as monoclinic BiVO4 (JCPDS cards No. 1400688). The diffraction peaks of all samples were sharp and intense, indicating the highly crystalline character of the powders. Furthermore, no obvious peaks other than monoclinic BiVO4 were detected. The results demonstrated that the calcination temperature (375 °C) is appropriate for formation of single-phase monoclinic crystal structure. After the doping of boron element, there is no significance changes in the (1 2 1) diffraction peak such as slight shift to higher or lower diffraction angle. One factor is that the doping scale is too low to cause the tiny change in the lattice size even the boron ion radius (3+, 0.27 Å) is evidently smaller than that of any elements in the BiVO4 (V5+ 0.54 Å, O2 1.4 Å, Bi3+

(a)

20

30

40

50

60

2 theta (degree) Fig. 1. XRD patterns of different BiVO4 samples for (a) B0; (b) B03; (c) B06; (d) B12; (e) B30. The generated peaks are well matched corresponding stand monoclinic scheelite BiVO4.

1.03 Å). Another probable cause is due to boron atoms prefer to occupy the interstice positions of BiVO4 as reported in ZnO [23,24]. Fig. 2 represents the typical transmission electron microscopy (TEM) images of the m-BiVO4 materials, which shows that the materials are composed of particle nanoparticles. Further analysis showed that the nonmetal boron element caused an increase of crystallinity compared with the bare BiVO4. The increasing grain size agrees well with the reduced maximum FWHM of (1 1 2) diffraction peak that corresponding XRD spectra. Polycrystalline pure BiVO4 particles were basically created through a sol–gel reaction. Selected area electron diffraction (SAED) ring patterns (Fig. 2b) originated from polycrystalline m-BiVO4 (0 1 1), (1 2 1), (2 3 1) and other planes. The diffraction spots of the corresponding (SAED) pattern can be indexed as the (0 0 2) and (1 3 0) plane (inset in Fig. 2d), being very matching with the m-BiVO4 phase structure, indicating the single-crystal nature of the nanoparticle. This SAED pattern was taken along [0 0 1] direction of m-BiVO4. The UV–vis diffuse reflectance spectra of pure BiVO4 and Bdoped BiVO4 samples are shown in Fig. 3. Both pure BiVO4 and B-doped BiVO4 samples show strong absorption in the visible light region as well as in the UV region. It is worth noting that the absorption edge of B-doped BiVO4 shifts to slightly higher wavelength with the substantially long band tailing which is different from the result reported by Wang et al. They found that the absorption edge values of BiVO4 photocatalysts varied from 545 to 610 nm as boron doping content increasing from 0 to 0.1 at% [25]. The boron doping characters were widely reported in the titanium dioxide, and there are different views on the substitution position of boron in the TiO2 lattice. For instance, substitutional B (B–Ti–O structure) was reported in the case for TiO2 structure which was accompanied by obvious red shifts for absorption edge compared to the bare TiO2 [26], whereas some works attributed it to boron knitting into the interstitial sites (Ti–O–B) of the TiO2 lattice [27,28]. For the latter, B-doped titanium dioxides have almost unchanged absorption edges even B doping content increased to 5 at%. Compared with the previous work, this difference is also caused by different behavior of boron element substituted in the BiVO4 lattice. The band gap energy (Eg) values (inset in Fig. 3) for the different samples were calculated from the UV–vis spectra using the Tauc formula as follows [29]: ahm = A(hm  Eg)n/2, where a is the absorption coefficient, hv is the photon energy, A is a constant, and Eg is the band-gap energy. In the above equation, n is a constant that depends on the semiconductor type. The n is equal to 1 and 4 for the direct and indirect band-gaps, respectively. BiVO4 is found to be a direct band gap semiconductor [30]. The absorption onsets

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(011) (-121) (231)

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130

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002

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Fig. 2. (a) TEM image the pure BiVO4 and (b) SAED patterns of from the white circle region mentioned in (a), (c–f) TEM images of B03, B06, B12 and B30 samples respectively. The inset in (d) is the SAED pattern of B06 sample which is obtained from the circle area.

of the samples were estimated by linear extrapolation of a plot of the absorbance squared vs. energy. The band gaps are 2.55, 2.55, 2.51, 2.52 and 2.53 eV for BiVO4, B03, B06, B12 and B30 respectively, which presents a tendency with slight reduction compared to bare BiVO4. It was reported that the band gap is related to the crystallite size of BiVO4 samples [31,32]. Thus the slight difference in the band gap energy of these samples originated from the B doping and increased grain size (can be seen in Fig. 2). In order to certify the boron doping, XPS was performed to study the chemical and bonding environment of the Bi 4f, V 2p, O 1s and B 1s for B0 and B30 samples. Fig. 4a revealed that the binding energies are 159.5 and 164.8 eV for Bi 4f7/2 and Bi 4f5/2, respectively. The Bi 4f 7/2 peak for BiVO4 was reported at values ranged from 158.8 to 159.9 eV which depended on the method of preparation and effects of doping [25,33–35]. Focusing on the V binding energy regime, the peaks at a binding energy of 524.3 (V 2p1/2) and 516.7 eV (V 2p3/2) were the split signal of V 2p, and the spin–orbit splitting was 7.6 eV. In the pure BiVO4, the shape

of a wide and asymmetric peak of the O1s spectrum indicated that there could be more than one chemical state according to the binding energy (Fig. 4c). The O 1s region is displayed with the characteristic peaks at 529.3 and 531.6 eV, including crystal lattice oxygen and chemiadsorbed oxygen (hydroxyl) with increasing binding energy. Thus, the O1s XPS spectrum is fitted to two kinds of chemical states in terms of Gaussian–Lorentzian function (Shirley background). The binding energy located at 531.6 eV is closely related to the hydroxyl groups resulting mainly from the chemisorbed water, which are considered to favor photocatalytic reactions reported in research articles [36,37]. After the doping with 3.0 at% B, the peak located at 531.6 eV becomes much stronger than that of pure BiVO4. The high resolution XPS spectra of the B1s region around 192.3 eV is shown in Fig. 4d. Boron elements exist in precursor in the form of boric acid, which is trivalent valence. Thus, it is difficult to form –Bi–B or –V–B chemical bonds between Bi3+ or V5+ and B3+. In the previous reference, the B1s binding energy of VB2 centered at 188 eV [38] which is distinctly

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B0-2.55

Absorbance (a.u.)

(αhv)2 (eV)2

B03-2.55

B03 B12 B0 B06 B30

300

400

B12-2.52 B06-2.51

2.4

2.5

B30-2.53

2.6

2.7

2.8

2.9

3.0

hv (eV)

500

600

700

800

Wavelength (nm) Fig. 3. UV–Vis spectra of different samples calcined at 375 °C. These samples have strong light absorption from UV region to 510 nm. Inset is the plots of (ahm)2 vs. energy (hm) for pure BiVO4 and B-doped BiVO4 samples.

lower than that of observed experimental data. The binding energy of greater than 193 eV was generally identified to arise from the B2O3 or H3BO3 [28,39,40]. As a result, based on diffuse reflectance spectra and the core level of boron, the observed B1s peak located at 192.3 eV is probably due to the interstitial B in the BiVO4 lattice. In addition, B doping of 3.0 at% results in a negative shift of Bi 4f 7/ 2 peak and a positive shift of O 1s. It originates from the interstitial doping of B ions with positive charge in the BiVO4. The doping is often performed to further improve the photoelectrocatalytic performance of semiconductors [41,42]. As shown in Fig. 5, the photoanodes were evaluated for their PEC water oxidation activity under visible-light irradiation (k > 420 nm). The generated photocurrents are due to transfer and collection of conductive band electrons to the back ohmic contact during the PEC water oxidation reaction caused by the valence band holes [34]. It was observed that the activity of the BiVO4 photoanodes depended on the B doping levels. From the linear sweep voltammograms spectra, the photocurrent density of BiVO4 photoanode (17.8 lA cm2, 1.23 V vs. RHE) very closely equals to the photocurrent density of 0.3 at% doped BiVO4 photoanode (24.5 lA cm2, 1.23 V vs. RHE). Compared to the bare BiVO4 photoanode, it was observed a dramatic increasing in the photocurrent density in such 0.6 at% B-doped BiVO4 photoanode (212.4 lA cm2, 1.23 V vs. RHE). This current represents ca. 11 times increase relative to that

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Fig. 4. XPS spectra of (a) Bi 4f, (b) V 2p, (c) O 1s and (d) B 1s for the B0 and B30. In the (a, b and c), the upper panel is corresponding to B0, the low panel is B30. The last curve is XPS spectrum of B 1s in the B30.

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0.8 240

1.23 V

B06 B12 B30 B03 B0

ABPE (%)

160

B06 B12 B30 B03 B0

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2

J (μA cm )

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Potential (V vs. RHE) Fig. 5. The linear sweep voltammograms of bare and doped BiVO4 electrodes under different potential vs. RHE (upon irradiation with visible light, k > 420 nm). The obtained experimental potentials (vs. RHE) were converted to the RHE scale according to the Nernst relation.

of bare BiVO4. It was found that higher doping levels of boron reduce the performance slightly compared with the BiVO4 photoanode with doping content of 0.6 at%. The large overpotential is typically required to drive water oxidation reaction electrochemically [43] due to water splitting reaction is a multistep, four-electron, four-proton process [44,45]. Thus, it is very meaningful to reduce the onset potential to improve the utilization efficiency of solar photon. BiVO4 photoanode showed an onset potential of 0.441 V for water oxidation in according to the linear sweep voltammograms (LSV). The 0.6 at% B-doped BiVO4 photoanode yields a remarkable 65 mV cathodic shift in the onset potential for water oxidation relative to the parent BiVO4 photoanode. This onset potential is only 66 mV higher than that obtained with the Co-Pi/W:BiVO4 photoanodes (310 mV) [44]. In the case of Kim et al., by different oxygen evolution catalyst (OEC) layers including FeOOH, NiOOH and the composite of FeOOH and NiOOH, the onset potential was reduced when compared to the bare BiVO4 [46], which is related to surface hydroxylation due to the hydroxy radical being reagent in the oxygen evolution reaction (OER) using the following relation:

4OH ! 2H2 O þ O2 þ 4e

þ

ð2Þ

50.0k 45.0k

RS

ð4Þ

The other photoanodes exhibit very similar onset potentials which are slightly smaller than that of pure BiVO4 photoanode. It is obvious that the low onset potential would contribute to the overall thermodynamic solar-to-hydrogen conversion efficiency. The applied bias photon-to-current efficiency (ABPE) was calculated from the JV curve shown in Fig. 6 assuming 100% Faradaic efficiency using the following equation

  JðmA=cm2 Þ  ð1:23  V bias ÞðVÞ  100 ABPEð%Þ ¼ Pin ðmW=cm2 Þ

B0 B03 B06 B12 B30

RCT

40.0k

ð3Þ

CPE

30.0k

Zim(Ω)

H2 O þ h ! OH þ Hþ

1.2

maximum ABPEs for B06, B12, and B0 are 0.52% (at 0.55 V vs. RHE), 0.37% (at 0.83 V vs. RHE), and 0.07% (0.54 V vs. RHE), respectively. There exists an inadequate amount of photocurrent is generated at low input biases to be efficient, while higher biases negate the advantage of using light altogether since the limiting voltage for water splitting is 1.23 V. An optimum point results, with the B06 exhibiting a peak at lower biases than the other samples since they are able to generate more photocurrent. In the course of semiconductors generating photocurrents, the impedance response is directly related to the physical processes, which is responsible for the photocurrent generation. Thus, Electrochemical Impedance Spectroscopy (EIS) is widely used electrochemical technique, which can provide insight into electronic behavior during water oxidation. In the Nyquist plot the real part of impedance (Zre) is located on the X-axis (Ohm unit), the imaginary part of impedance (Zim) is located on the Y-axis (Ohm unit). The EIS measurements are presented as Nyquist plots in Fig. 7 under the same conditions as that of LSV spectra. Since the water splitting processes occurring at the interface between photoanodes and electrolytes can be interpreted by equivalent circuit models [34], A simple Randles–Ershler (R–E) circuit model was adopted [48], where the resistance of electrolyte is RS, a charge-transfer

35.0k þ

1.0

Fig. 6. ABPE (applied bias photon-to-current efficiency) obtained using a threeelectrode system. The maximum ABPE is 0.52% at 0.55 V vs. Pt counter electrode.

In the result of XPS (Fig. 4c), the peak located at 531.6 eV could be attribute to the chemiadsorbed hydroxyl. It could act as holes captured agent in the photocatalytic process, producing OH as the following reactions [47]:

OH þ h ! OH

0.8

Potential (V vs. RHE)

25.0k 20.0k 15.0k 10.0k 5.0k 0.0

ð5Þ

where J is the photocurrent density, Vbias is the applied bias between working and Pt counter electrodes, and Pin is the incident illumination power density (100 mW/cm2). As shown in Fig. 6, the

0.0

2.0k

4.0k

6.0k

8.0k

10.0k 12.0k 14.0k 16.0k 18.0k

Zre(Ω) Fig. 7. Nyquist plots measured at 1.23 V (vs. RHE) in 0.5 M Na2SO4 solution. They were irradiated with visible light (k > 420 nm) from back side of FTO. The frequency is in the frequency domain 0.1 MHz to 0.01 Hz.

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(a)

(b)

(c)

(d)

Mainly V2p

Bi B 2s Mainly O2p

V

B 2s

O B

Bi 6s

Bi 6s

Bi 6s

Fig. 8. (a) Schematic energy diagram for BiVO4 indicating the energetic positions for the V 2p and O 2p, (b and c) In case of substitutional B entering into cation and anion sites of BiVO4, (d) schematic of monoclinic BiVO4 structure, showing unconnected VO4 (red) tetrahedron units. The interstitial boron enters into space among VO4 tetrahedrons. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

resistance of photoanode || electrolyte interface is RCT, and a capacitance phase element is CPE. The RCT is relative to the charge transport characteristics for carrier separation of water oxidation reactions [20]. Scrutinization of these data revealed that the charge transfer resistance on the photoanode surface was 34.2, 29.4, 10.8, 19.5 and 33.1 kO for the samples containing 0, 0.3, 0.6, 1.2 and 3.0 mol% of B, respectively. The smaller arc of B06 sample represents more favorable carrier transport than that of other samples in PEC reactions. Fig. 8a is the valence and conductive band structure for BiVO4 photocatalysis. The lower part of VBs mainly consists of O 2p orbitals and the hybridizations between O 2p and V 2p. The CBs are predominantly composed of V 2p orbitals and significant contribution from Bi 6p [49]. The second case (Fig. 8b) is the substitutional B entering into Bi or V sites in the BiVO4 lattice. The third case (Fig. 8c) is the substitutional B entering into O sites in the BiVO4 lattice. By the incident photon-to-electron conversion efficiency measurements, it indicates that electron transport is slower than hole transport for the BiVO4 films in the case of Liang et al. [19]. As shown in Fig. 8a, the conductive band is made up primarily of V 2p orbitals. The photogenerated electrons in the conductive band have to stride across unconnected VO4 tetrahedron (Fig. 8d). As for the reported cases, the substitutional B entering into corresponding lattice sites results into reduced band gap width. According to our experimental results, at different doping content, there is almost no variation in the energy gap width, at the same time the boron doping reduces obviously the electrochemical impedance and results a remarkable increasing of photocurrent for BiVO4 photoanode. As displayed in Fig. 8d, some weak chemical bonds form between the doped boron ions and corners of VO4 tetrahedrons. Thus the interstitial boron doping plays a key role in improving the poor electron transport properties to obtain desire photocurrent. Furthermore, we suggest that the other small ions (e.g. Li, Be ions) could be considered as candidates which enter the interstitial space among VO4 tetrahedrons, and it would be another effective way to enhance the photocatalytic performance of BiVO4 except for the aforementioned high valence ions (Mo, W and P) doping. 4. Conclusions In summary, we have successfully fabricated BiVO4 photocatalysts as an advanced photocatalyst. It is demonstrated that in this system, the introduction of boron with different doping level significantly improved crystallinity of BiVO4. Maximum

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