New Insights into the Electronic Structure and Photoelectrochemical

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New Insights into Electronic Structure and Photoelectrochemical Property of Nitrogen-doped HNbO Behaviors via A Combined In-situ Experimental with DFT Investigation 3

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Xiaobin Liu, Zhenyu Wang, Peng Chen, Huanfu Zhou, Ling Bing Kong, Chunming Niu, and Wenxiu Que ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13704 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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ACS Applied Materials & Interfaces

New Insights into Electronic Structure and Photoelectrochemical Property of Nitrogen-doped HNb3O8 Behaviors via A Combined In-situ Experimental with DFT Investigation Xiaobin Liu 1, Zhenyu Wang 2, Peng Chen 1, Huanfu Zhou 3, Ling Bing Kong 4, Chunming Niu 2 and Wenxiu Que 1,∗∗ 1

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education &

International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China 2

Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and

Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China 3

Key laboratory of Nonferrous Materials and New Processing Technology-Ministry of Education, Guilin University of Technology, Guilin 541004, People’s Republic of China

4

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Abstract Nitrogen-doping approach has been intensively adopted to improve the various properties of metal

oxides—especially for adjusting energy band structure and extending photo-response range of oxide photocatalyst. However, nitrogen doped behavior is still unintelligible and complex due to the diversity of the compositions and crystal structures. In this work, new insights into electronic structure and photoelectrochemical property of nitrogen-doped HNb3O8 behaviors were presented. On the one hand, we utilized an in-situ experimental strategy to ascertain the effect of nitrogen-doping on the energy band and photoelectrochemical (PEC) property of HNb3O8 and nitrogen-doped HNb3O8 (N-HNb3O8). Their energy band level, donor densities and interfacial charge transfer properties were studied by Mott-Schottky plots and electrochemical impedance spectroscopy (EIS). After nitrogen-doping, the conduction band position is unusually descended by 0.23 eV and the valance band position is raised by 0.51 eV, the donor density (Nd) is increased from 3.71 × 10 21 to 6.46 ×1021 cm−3 and interfacial charge transfer efficiency is reduced yet. On the other hand, density functional theoretical (DFT) calculation

∗ Corresponding author: Tel.: +86-29-83395679; Fax: +86-29-83395679 Email address: [email protected] 1

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was also conducted, so as to understand their electronic structures of HNb3O8 and N-HNb3O8. After nitrogen-doping, the electronic structure is modified due to the up-shift valance band edge consisting of hybrid N 2p and O 2p orbitals and down-shift conduction band edge consisting of the H 1s and Nb 4d orbitals. Furthermore, these insights into nitrogen-doped semiconductor behavior has important guiding significance towards their potential applications. Keywords: In-Situ, Nitrogen-Doped, Layered Structure, Visible Light, and DFT Calculations.

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1.

INTRODUCTION Semiconductor-based photocatalysts are attracting increasing attention, due to their wide

applications in environmental pollutant cleaning, water splitting for hydrogen (H2) production and reduction of carbon dioxide (CO2) by utilizing abundant solar light.1-5 It is well known that solar light consists of only about 3-5% UV light (below 400 nm). However, the compositions of visible light (400-700 nm) and infrared light are up to 42-43% and 52-55%, respectively.6 Therefore, to better utilize solar light, the design and development of visible light responsive (even near infrared-responsive) photocatalysts are particularly important for practical applications.7-10 Currently, excellent visible light and UV light-responsive photocatalysts can be realized by using two approaches: (i) use of composition with visible light responsive materials (extrinsic) and (ii) element (metal and non-metal) doping (intrinsic). For element doping, non-metal dopants usually include C, N, S, P and so forth

11-14

, among which N-doping has been widely acknowledged to have favorable physicochemical

properties.15-19 Although significant efforts have been made to elucidate chemical characteristics and photocatalytic properties of N-doped photocatalysts, there are debated issues which need to be clarified, e.g., the effect of nitrogen on energy band structure and characteristics of the doped materials. 15-17 A large number of metal oxides based on titanium and niobium, such as TiO2 KNb3O8

22-23

20

, SrTiO3

21

and

, have been explored as UV photo-responsive photocatalysts, because they have unique

properties, including high chemical stability and non-toxicity in terms of light irradiation. Specifically, the layered KNb3O8 comprises stacked negatively charged slabs through corner and edge-sharing of NbO6 octahedra, whereas the K+ ions are found in between the Nb-O interlayers. In this case, the K+ ions can be substituted by protons to form protonated niobic acid that can be exfoliated and form the monolayer nanosheets based on the intercalation with certain organic cations.24 In addition, thanks to the layered structure with high protonic acidity and electronic conduction, various solid-acids photocatalysts have high UV-light photocatalytic activities, which are even better than those of their salt phases counterparts.25-27 The niobium-containing solid acid (HNb3O8), which is isostructural with KNb3O8, has been reported to exhibit excellent photocatalytic performance. Wu et al.

28-29

found that

ultrathin monolayer HNb3O8 can be used to selectively oxidize benzylic alcohols and high effectively produce hydrogen. Li, et al.

30

and Liu et al.

31

prepared HNb3O8 nanobelts and HNb3O8/graphene

hybrids, which CO2 could be reduced to renewable fuels (CH4 and CO), respectively. Exfoliated Pd/HNb3O8 nanosheet was reported to have highly efficient dual function of one-pot cascade 3



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deacetalization and hydrogenation.32 CdS sensitized SiO2-HNb3O8 and g-C3N4/SiO2-HNb3O8 composites could efficiently degrade rhodamine B under visible light.33-34 Furthermore, Hou et al.

35

deeply studied the polyaniline (PANI)/HNb3O8 layered nanocomposites by using density functional theory (DFT) calculation. In terms of element doping, Ye et al.

36-37

found that the nitrogen-doped

lamellar HNb3O8 exhibited high visible light activity, with performances even better than those of Degussa P25 and N-TiO2. However, up to now, the difference in photoelectrochemistry (PEC) performance and electronic structure between HNb3O8 and N-HNb3O8 has not been reported. Here, we report an in-situ study to clarify how the properties and energy band structure of HNb3O8 film have been influenced as a result of nitrogen-doping. Furthermore, the DFT calculation was applied to gain an in-depth understanding on the origin of the electronic structure variation from HNb3O8 to N-HNb3O8. 2. EXPERIMENTAL PROCEDURE 2.1. Material Preparation 2.1.1. In-Situ Preparation of N-HNb3O8 Porous Films All the chemical are of analytical grade and were used without purification. Deionized water (DI water) was used throughout the experiments. KNb3O8 nanosheet porous film were fabricated by using a two-step hydrothermal method, combined with a modified doctor-blade step, as reported previously. 23, 38

As shown in Figure 1, firstly, the KNb3O8 porous film was directly put into 3 M HNO3 solution for

ion exchanging, thus leading to H3ONb3O8 porous film. Secondly, the H3ONb3O8 porous film was put into saturated ethanol solution of urea for intercalation, because urea molecules entered the interlayers of the parent HNb3O8. The urea-HNb3O8 film was then annealed at 400°C for 2 h at heating rate of 2°C/min in an oxygen-free environment. Finally, the resultant N-HNb3O8 film was thoroughly washed with 3 M HNO3 solution and DI water to remove the residual alkaline items (such as ammonia and urea) on surface of the samples, followed by drying at 70°C. 2.1.2. N-HNb3O8 Film Photoanode Modified with Cobalt phosphate Cobalt phosphate (Co-Pi) was in-situ loaded onto the N-HNb3O8 film by using a photo-assistant electrodeposition method, with a three electrode system, in which a Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) was employed as the reference electrode. Cobalt nitrate (0.5×10−3 m) in potassium phosphate buffer solution at a concentration of 0.1 M with pH value of 7 was utilized as the electrolyte. Photo-assistant electrodeposition of Co–Pi was carried out at 0.4 V 4


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vs SCE for 5 min under the irradiation with a Xenon lamp (300 W). 2.2. Photoelectrochemical Performance Characterization Photoelectrochemical (PEC) performances of the HNb3O8 and N-HNb3O8 film photoelectrodes were characterized by using a three electrode system, where a Pt wire served as the counter electrode while SCE was used as the reference electrode. PEC data were collected by using an electrochemical analyzer (CHI600E, Shanghai China) at a scan rate of 10 mV/s. Xenon lamp (300 W), with or without a 400 nm cutoff filter (λ>400 nm), was employed as the light sources. Na2SO4 aqueous solution at 0.5 M and pH =7 was used as the electrolyte. RHE potential was derived from equation, E(vs RHE) = E(vs SCE) + 0.242 V + 0.059 × pH, where pH value is for electrolyte. 2.3. Computational Details Density functional theoretical (DFT) calculations were performed by using the Vienna ab initio

simulation package (VASP) 41-42

39-40

, combined with the projector-augmented-wave (PAW) approach

. All the computations were carried out with 480 eV cut-off plane-wave basis set and the

generalized gradient approximation (GGA) expressed with the Perdew-Burke-Ernzerhof (PBE) function

43

. The Monkhorst-Pack (MP) scheme k-points grid sampling was set to be 3×1×5 and

3×2×7 to conduct geometry optimization and obtain electronic properties of KNb3O8/HNb3O8 and the doped systems. A 3×1×3 MP-point meshes were applied for geometry optimization of the

corresponding 2×1×2 doped supercells. All ions were relaxed to a force tolerance of ≤0.02 eV/Å for the geometric optimization. 3. RESULTS AND DISCUSSION 3.1 In-situ Preparation and Structural Characterization of N-HNb3O8 Film Preparation mechanism diagram of the nitrogen doped HNb3O8 film on the FTO glass is shown in Figure 2 (a). All the preparation processes, including ion exchange reaction, intercalation reaction (acid-base interaction) and annealing treatment, are in-situ on the FTO glass, in order to accurately identify the effects of nitrogen-doping on the properties and energy band edges of the HNb3O8 film, by excluding other interference factors, such as film porosity, particle packing form, thickness and so on. It is necessary to mention here that KNb3O8 porous film must be obtained first, since H3ONb3O8 and HNb3O8 tends to decompose into H2O and Nb2O5 at a relatively low temperature of about 226 oC

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.

These involved chemical reactions can be summarized as follows : 44

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H+

aq ∆ ∆ → KNb3O8 ← H3 ONb3O8  → HNb3O8  → Nb 2 O5 +H 2 O + 80~150 o C 250 o C

K aq

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(1)

However, KNb3O8 is still stable at relatively high temperatures 22, because it can be synthesized by the conventional solid state reaction method at about 900 °C. Figure 2 (b) shows XRD patterns of the samples. The XRD pattern (i) shows a single phase KNb3O8 film, which is well matched with PDF #75-2182. As seen in the XRD pattern (ii), after the protonation process, the H3ONb3O8 phase can be obtained, which is well matched with PDF#44-0672. It can be obviously observed that the strongest peak (020) of the H3ONb3O8 film shifts to low diffraction angle due to the interlayer substitution of H3O+ ions for K+ ions. As shown in the XRD pattern (iii), after the planar urea molecules are intercalated into the H3ONb3O8 film, the diffraction peak (020) has a slight shift to high angle, illustrating that urea-HNb3O8 has a reduced interplanar spacing as compared to H3ONb3O8. However, the size of H3O+ is much smaller than the planar size of urea molecule (the details are shown in Figure S2 of supporting information), hence, the urea molecules are required to be arranged in approximate parallel. Furthermore, the diffraction peak (020) of the N-HNb3O8 film further shifted to high angle after annealing at 400 oC, while the original layered structure of the N-HNb3O8 film is not changed 37. In addition, XRD patterns of the urea-HNb3O8 film annealed at different temperatures are shown in Figure S1(a). With increasing annealing temperature, the diffraction peak (020) slightly shifted to high angle and the FWHM (Full width at half maximum) of the peak was changed, which could be attributed to the variation in degree of crystallinity. To investigate morphology and nitrogen element doping of the HNb3O8 film, the SEM and element mapping were conducted and shown in Figure S3. It can be observed that the HNb3O8 film is composited with the tiled bamboo-shaped nanosheet

23

and the thickness is about 3 µm. The

morphology of the N-HNb3O8 film is consistent with that of the HNb3O8 film. In addition, the element mapping of the N-HNb3O8 film illustrates that the doped nitrogen element is evenly distributed in the porous film. 3.2 Photo Absorption and PEC Performance of the HNb3O8 and N-HNb3O8 Films Photo-absorption curves of the HNb3O8 and N-HNb3O8 films are shown in Figure 2 (c). The HNb3O8 and N-HNb3O8 films have photo-absorption edges at 339 nm and at 425 nm, respectively, corresponding of band gap values of 3.66 eV and 2.92 eV, according to the equation of Egap=1240/λedge. Therefore, photo-absorption of the N-HNb3O8 film has been extended to visible light region. In addition, Figure S1(b) shows photographs of the HNb3O8 film and N-HNb3O8 film (the same sample 6


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before and after nitrogen-doping). The color of the N-HNb3O8 film is yellow-green, which is consistent of the photo-absorption behavior. In addition, the photoelectrochemical performances of the HNb3O8 and N-HNb3O8 film photoanodes are shown in Figure 2(d). The HNb3O8 film electrode exhibited a negligible dark current at over a broad potential range (-0.28 to 1.22 V vs SCE). Production potentials of hydrogen and oxygen of the HNb3O8 film electrode are about -0.29 and 1.22 V vs SCE, respectively. The N-HNb3O8 film electrode had an almost same dark current as that of the HNb3O8 film electrode in a narrow potential range (-0.1 to 0.45 V vs SCE). The N-HNb3O8 film electrode in dark had lower potentials for hydrogen and oxygen production, which were about -0.1 and 0.45 V vs SCE, respectively. In addition, the positive photocurrent was increased with increasing applied bias voltage, indicating typical n-type semiconductor behavior of the sample. The N-HNb3O8 film photoanode possessed much higher photocurrent under irradiation of full range of light than the HNb3O8 counterpart. For example, at 0.4 V bias vs SCE, photocurrent of the N-HNb3O8 film photoanode was increased by about 6 times, as compared with that of the HNb3O8 photoanode. Furthermore, the N-HNb3O8 film photoanode also exhibited a considerable photocurrent under the visible light irradiation at >400 nm. This performance is even higher than that of the HNb3O8 film photoanode with full light-irradiation. Therefore, the N-HNb3O8 film photoanode showed excellent photoelectronchemical performances, including high photocurrent, visible light response and low applied production potentials of oxygen and hydrogen. 3.3 Mott-Schottky Plots of the HNb3O8 and N-HNb3O8 Film Electrodes The flat band potentials of HNb3O8 and N-HNb3O8 were derived from the Mott-Schottky curves, which were presented by using the apparent capacitances measured at different potentials, based on the following equation 45-47:

1 2 kT = ( E − E fb − ) 2 Csc eεε 0 N e

(2)

where Csc, ε0, ε, e, N, k, Efb, E and T stand for capacitance of the space charge region, permittivity of free space (8.85×10−14 F cm−1), permittivity of the semiconductor, the electron charge (1.602×10−19 C), the donor density (hole concentration for p-type semiconductor or electron concentration for n-type semiconductor), the Boltzmann constant (1.38×10−23 J K−1), the flat band potential, the applied potential and temperature of the absolute, respectively. Figure 3 shows plots of Mott-Schottky data of the HNb3O8 and N-HNb3O8 film electrodes. As expected, the positive slopes of the Mott−Schottky curves imply n-type semiconductor behavior of the

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samples. By extrapolating the graph of (1/C2) versus E, the value of Efb can be obtained, as the x-axis intercept (E0), given by:

E fb = E0 −

kT e

(3)

Therefore, flat band potentials of the HNb3O8 and N-HNb3O8 film electrodes are -0.76 V and -0.52 V vs SCE at pH=7, or -0.11 and 0.13 V vs RHE at pH=0, respectively. At 1 kHz, donor densities (Nd) of the HNb3O8 and N-HNb3O8 film electrodes were about 3.71 × 1021 and 6.46 × 1021 cm−3, respectively, which were obtained from the slope of the Mott-Schottky curves (the calculation process was stated in the supporting materials). The N-HNb3O8 film photoanode has higher carrier density by about two times after nitrogen-doping, as compared with the HNb3O8 counterpart. Due to the increased carrier density, the band bending would be further enhanced, thus shifting to the Fermi level, so that the charge separation would be facilitated.48 In terms of n-type semiconductor, their conduction band position (Ec) can be determined from the Efb through the following equation 49:

EC = E fb − kT ln(

Nd ) NV

(4)

where NV stands for the effective density of states (DOS) at the valence band edge (~1019). Hence, the conduction band edge positions (Ec) of the HNb3O8 and N-HNb3O8 film electrodes are -0.26 V and -0.03 V vs RHE at pH=0, respectively. In addition, the valance band position (Ev) can be derived from the band gap (Eg) through the following equation:

EV = Eg + EC

(5)

Hence, the valance band edge positions (Ev) of the HNb3O8 and N-HNb3O8 film electrodes are 3.40 V and 2.89 V vs RHE at pH=0, respectively. It has been shown that the narrowed band gap of HNb3O8 after nitrogen-doping is attributed to the down-shift (~0.23 V) of the conduction band edge (Ec) and the up-shift (~0.51 V) of the valance band edge (Ev), as shown schematically in Figure 7. 3.4 Chemical State and Composition of the N-HNb3O8 Film XPS spectra of the N-HNb3O8 sample were collected to clarify characteristics of the doped nitrogen items. Figure 4(a) presents fine XPS spectrum (N 1s) of the N-HNb3O8 film over 395-402 eV. With curve fitting, the two N 1s peaks of N-HNb3O8 are located at about 395.1 and 401.0 eV, respectively. Because the signals of the chemisorbed N2 and NOx are centered at 399-402 eV50-51, the peak with low binding energy centered at about 395.1 eV can therefore be assigned to the substituting 8


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nitrogen in the sample.52 Recently, it was confirmed that the two peaks at 396 eV and 399.6 eV are ascribed to the β-type nitrogen to replace oxygen in the lattice of metal oxides and anionic nitrogen (γ-type nitrogen) as O-M-N in the metal oxides, respectively.37, 53-54 It is thus concluded that nitrogen atoms have successfully introduced and they have substituted oxygen in the HNb3O8 lattice. Furthermore, the oxygen-nitrogen atomic ratios could be derived approximately from the sensitivity factor and peak areas of O 1s (532-528 eV) and N 1s (390-400 eV) in the XPS spectra of the N-HNb3O8 sample.

55

It was found that the N-HNb3O8 had the O/N ratio of ~5.5, corresponding to a

concentration of the substituted nitrogen to be 15.4%. In addition, the XPS spectra of C 1s for the HNb3O8 and N-HNb3O8 samples are shown in Figure S4(a), where no new bonded carbon (such as C-N, C-Nb, etc.) was detected. In Figure S4(b), the valance band position of the N-HNb3O8 sample has an up-shift by about 0.36 eV (vs Ef) as compared with the HNb3O8 sample. 3.5 DFT Calculation of HNb3O8 and N-HNb3O8 Figure 4(b) shows crystal structure image of the layered [+]Nb3O8 (Interlayer cations are not shown), where O2 and O5 represent two types of terminated oxygen atoms. As mentioned above, the urea molecule entered the interlayer of HNb3O8, so as to enhance the stability of the layered structure at temperatures of up to 400 oC 36. At highly reactive sites, the oxygen species, O2 and O5, would be first substituted by nitrogen during the nitridation reaction. In this case, based on the doped concentration estimated from the XPS spectra, four models as shown in Figure S5 with different substitution positions and nitrogen concentrations (1.56% and 12.5%) were implemented to shed the light on the improvement of visible absorption due to the nitrogen-doping. Although the H4Nb12O28N4-1 and H4Nb12O28N4-2 models have the same doped concentration, only the former is reasonable in the calculation process, which was described in the supporting materials. Figure 5 shows the optimized crystal structures of KNb3O8, HNb3O8 and H4Nb12O28N4-1. Interestingly, compared with HNb3O8, the proton is closer to the nitrogen atom in the H4Nb12O28N4-1, due to electrostatic attraction. Electronic properties of HNb3O8 and H4Nb12O28N4-1, such as the energy band structures, the projected density of states (PDOS) and the charge density isosurfaces, were also studied, which are shown in Figure 6. Based on the energy band structures, both HNb3O8 and H4Nb12O28N4-1 are indirect semiconductors, with indirect and direct band gaps to be 0.62, 2.07 and 0.45, 1.76 eV, respectively. We also observed electron localization at protons, as described in Ref. 35. Therefore, the separated states of protons can be treated as doped states. In this case, the calculated band gap value of H4Nb12O28N4-1 is 9



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smaller than that of HNb3O8, which is band narrowing caused by the nitrogen-doping, in a good agreement with the photo-absorption and Mott-Schottky graphs discussed above. PDOSs of HNb3O8 and H4Nb12O28N4-1, with respective charge density isosurfaces of valence band maximum and conduction band minimum, i.e., VBM and CBM, respectively, are shown in Figure 6. For HNb3O8, the states at VBM consist of O 2p orbitals, while Nb 4d orbits contribute mostly to the CBM, separately. However, for H4Nb12O28N4-1, N 2p and O 2p orbitals dominate the VBM, while orbitals from Nb 4d and H 1s dominate the CBM, which was also confirmed by the charge density isosurfaces of the VBM and CBM. It is found that the conduction band states are mainly concentrated at Nb d(π) and H 1s orbitals, whereas the valance band states are concentrated at N 2p and O 2p orbitals. As demonstrated in Figure 7, the band gap value is reduced as a result of the nitrogen-doping, where the CBM of N-HNb3O8 was lowered by 0.23 eV and the VBM was raised by 0.53 eV, as compared with those of HNb3O8. Therefore, after the nitrogen-doping, the electrons localized around Nb and O ions are transferred partly to N and H ions, which widens the conduction band (CB) and the valence band (VB), thus leading to the decrease in the band gap. 3.6 Interfacial Charge Transfer Efficiency of the HNb3O8 and N-HNb3O8 film Photoanodes Figure 8(a) shows EIS of the HNb3O8 film photoanode at the open circuit voltage (0.288 V vs SCE) and the N-HNb3O8 film photoanode at bias 0.288 V vs SCE, with the inset showing the equivalent circuit diagram and data. As illustrated in the inset of Figure 8(a), the uncompensated resistance of the bulk electrolyte solution (Rs), which is also named as the equivalent series resistance (ESR), can be derived from the intercept in real axis at high frequencies. Charge transfer resistance of the semiconductor-electrolyte (Rct) can be obtained according to the EIS semicircle.

56-57

As shown in

Figure 8(a), the N-HNb3O8 electrode has a larger Rct value (52.71 kΩ) than that of HNb3O8 electrode (35.34 kΩ), which corresponds to a lower interfacial charge transfer efficiency. Furthermore, the interfacial charge transfer efficiency (ηint) can be linked to the recombination of electron and hole on the surface, which can be characterized quantitatively by using the holes that were scavenged into the electrolyte.

58-60

Because sulfite has a high oxidation kinetic rate, sodium sulfite was employed as hole

scavenger in this work. With the addition of Na2SO3, the photogenerated holes that arrived at electrode-electrolyte interface of the electrodes of HNb3O8 or N-HNb3O8 can be responsible for the current-generation, rather than lost to the recombination of surface electron and hole. Therefore, ηint is given by: 56 10


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ηint =

J Na2 SO4

(6)

J Na2 SO4 + Na2 SO3

Interfacial charge transfer efficiencies (ηint) of the HNb3O8 and N-HNb3O8 film photoanodes are shown in Figure 8(b). Dark/photo-currents ( J Na2 SO4 + Na2 SO3 ) of the HNb3O8 and N-HNb3O8 film electrodes in the presence of Na2SO3 are shown in Figure S11. Figure 8(b) indicates that the N-HNb3O8 film photoanode possesses lower ηint value, as compared to the HNb3O8 counterpart, which is consistent of the EIS result as stated above. At 0.4 V bias vs SCE, the value of ηint of the N-HNb3O8 film photoanode is only 31.4%, while that of the HNb3O8 sample is 45.3%. It is thus concluded that the interfacial charge transfer efficiency of HNb3O8 is reduced due to the nitrogen-doping. 3.7 N-HNb3O8 film Photoanode with Co–Pi Modification As mentioned above, the interfacial charge transfer efficiency of the HNb3O8 film was reduced due to the nitrogen-doping. As reported in previous references

61-63

, the Co-Pi loading can specifically

reduce the recombination of electron and hole on the surface. Hence, if the photocurrent of N-HNb3O8 film photoanode can be improved after the Co-Pi loading, it can illustrate that the N-HNb3O8 has low interfacial charge transfer efficiency. For this purpose, the Co-Pi was deposited onto the N-HNb3O8 film photoanode by using a photo-assistant electrodeposition method. Figure 9 shows time dependent photocurrent curves of the bare N-HNb3O8 and Co-Pi loaded N-HNb3O8 film photoanodes in 0.5 M Na2SO4 aqueous solution at 0.4 V vs SCE. Different irradiation time durations were used, i.e., 0-40 s for full light, 40-60 s for visible light and >60 s for dark. It can be seen that the photocurrent of the Co-Pi loaded N-HNb3O8 film photoanode is higher than that of the bare N-HNb3O8 film photoanode by about two times, regardless of the light irradiation condition, which means that the interfacial charge transfer efficiency (ηint) of the N-HNb3O8 film photoanode is increased due to the Co-Pi loading. 3.8 The effects of the N-doping on HNb3O8 Based on above results and discussion, it can be concluded that the effects of the N-doping on HNb3O8 have two aspects: modification of energy band and photoelectrochemical property. On the one hand, the N-doping can introduce the N 2p orbital, which is higher than level of O 2p orbital. Hence, the N-doping can up-shift the location of valance band and thus decrease the bandgap value of HNb3O8. Thus, the N-doping can make the UV type HNb3O8 harvest visible light. Besides, the N-doping can also down-shift the location of conduction band of HNb3O8. DFT calculations reveal that the N-doping of HNb3O8 induces the H 1s orbital takes part in the contribution of conduction band consisting of Nb 11



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4d orbital. In conclusion, the N-doping can effectively adjust the energy band structure of HNb3O8 and obtain the ability to absorb and respond visible light. On the another hand, the N-doping can also enhance photoelectrochemical property of HNb3O8. 4.

CONCLUSIONS In our work, new insights into electronic structure and photoelectrochemical property of the

nitrogen-doped HNb3O8 behaviors were presented. The conduction band position was descended by 0.23 eV and the valance band position was raised by 0.51 eV, the donor density (Nd) was increased from 3.71 × 10 21 to 6.46 × 1021 cm−3. The photocurrent of the N-HNb3O8 film photoanode was increased by about 6 times, as compared with that of the HNb3O8 counterpart, although the nitrogen-doping resulted in an increase in surface electron-hole recombination. Accordingly, with Co-Pi loading, the surface electron-hole recombination rate of the N-HNb3O8 film photoanode was effectively reduced. Thus, a two times increase in photocurrent was observed as compared with bare N-HNb3O8 counterpart. According to density functional theoretical (DFT) calculation results, it has been demonstrated that the change in electronic structure is attributed to the up-shift valance band consisting of the N 2p and O 2p orbitals and the down-shift conduction band consisting of the H 1s and Nb 4d orbitals.

ASSOCIATED CONTENT Supporting Information Equipment information, Conversion of the potentials (vs SCE), Donor density (Nd) calculation, Supplement of DFT calculations, Supplementary XRD patterns, Structural image of H3 O+ and urea molecule, Photography, SEM and element mapping of the HNb3O8 and N-HNb3O8 films, XPS spectrum of C 1s and VB of HNb3O8 and N-HNb3O8 samples, Calculation models, Energy band and PDOS of KNb3O8, H8Nb24O63N-1, H8Nb24O63N-2 and H4Nb12O28N4-2, PEC and EIS results of the Co-Pi loaded N-HNb3O8 electrode, and LSV of HNb3O8 and N-HNb3O8 electrodes in 0.5 M Na2SO3+Na2SO4 aqueous solution under dark and light conditions. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No.61774122) and the Science and Technology Developing Project of Shaanxi Province (No. 2015KW-001).

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Figure Captions

Figure 1 In-situ preparation flow diagram of the N-HNb3O8 film on FTO glass. Figure 2 (a) Mechanism diagram for synthesizing N-HNb3O8 film sample. (b) XRD patterns of the in-situ prepared samples: (i) KNb3O8 film, (ii) protonated HNb3O8 film, (iii) urea intercalated HNb3O8 film and (iv) nitrogen doped HNb3O8 film. (c) Photo-absorption spectra of the HNb3O8 and N-HNb3O8 films. (d) Linear sweep voltammetry (LSV) of working HNb3O8 and N-HNb3O8 electrodes in 0.5 M Na2SO4 aqueous solution under dark and light conditions.

Figure 3 Plots of Mott-Schottky data of the electrodes together with their corresponding flat band potentials: (a) HNb3O8 and (b) N-HNb3O8.

Figure 4 (a) Fine XPS spectra (N 1s) of the N-HNb3O8 film. (b) Crystal structure image of the layered [+]Nb3O8 (Interlayer cation not given), with O2 and O5 representing two types of oxygen sites near to interlayer position.

Figure 5 Optimized crystal structure of (a) KNb3O8, (b) HNb3O8 and (c) H4Nb12O28N4-1 (with the smallest white atom to be H atom).

Figure 6 Calculated energy band and DOS of (a1) HNb3O8 and (b1) N-HNb3O8 (H4Nb12O28N4-1). Electron density plot of conduction band states within 0.5 eV of the band edge of (a2) HNb3O8 and (b2) N-HNb3O8 (H4Nb12O28N4-1). Electron density plot of valance band states within 0.5 eV of the band edge of (a3) HNb3O8 and (b3) N-HNb3O8 (H4Nb12O28N4-1).

Figure 7 Energy band structures of HNb3O8 and N-HNb3O8. Figure 8 (a) Electrochemical impedance spectra (EIS) of the HNb3O8 film photoanode at the open circuit voltage (0.288 V vs SCE) and the N-HNb3O8 film photoanode at the same bias voltage. The inset is the equivalent circuit diagram and data. (b) Interfacial charge transfer efficiencies (ηint) of the HNb3O8 and N-HNb3O8 film photoanodes at various potentials.

Figure 9 Time dependent photocurrent curves of the N-HNb3O8 and Co-Pi loaded N-HNb3O8 film photoanodes in 0.5 M Na2SO4 aqueous solution at 0.4 V vs SCE.

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Figure 1

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Figure 2

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Figure 3

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Figure 5

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Figure 7

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Figure 9

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