Fabrication of porous Cu-doped BiVO4 nanotubes as efficient oxygen

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Fabrication of porous Cu-doped BiVO4 nanotubes as efficient oxygen-evolving photocatalysts Bin He, Zhipeng Li, Dian Zhao, Huanhuan Liu, Yijun Zhong, Jiqiang Ning, Ziyang Zhang, Yongjiang Wang, and Yong Hu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00281 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Nano Materials

Fabrication of porous Cu-doped BiVO4 nanotubes as efficient oxygen-evolving photocatalysts Bin He,† Zhipeng Li,† Dian Zhao,† Huanhuan Liu,† Yijun Zhong*,†Jiqiang Ning*,‡ Ziyang Zhang,‡ Yongjiang Wang‡ and Yong Hu*,† †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials,

Department of Chemistry, Zhejiang Normal University, Jinhua 321004, China. ‡

Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and

Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China.

1

ACS Paragon Plus Environment

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

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ABSTRACT: We introduce a facile route to fabricate one-dimensional (1D) porous Cu2+-doped BiVO4 (Cu-BVO) nanotubes with uniform size distribution and high structural stability. Firstly, an electrospinning technique was developed to prepare the sacrificial template of polyacrylonitrile (PAN) nanofibers with a smooth surface. Secondly, a conformal layer of Cu-BVO nanoparticles was coated onto the PAN template through a solvothermal method to obtain the solid core-shell precursor which was finally converted into the product of porous Cu-BVO nanotubes with thermal treatment. Both experimental characterizations and theoretical calculations based on the density functional theory (DFT) calculations have revealed the crucial functionality of the appropriate band structure of the Cu2+-doped nanostructure and introduce beneficial defect states by Cu doping, which boosts light absorption and promotes charge migration and separation and therefore result in high-efficient photocatalytic O2 evolution with visible-light irradiation. As a result, the porous nanotube photocatalyst with an optimal Cu2+ doping of 5.0% exhibits an average O2 evolution rate of to 350.2 µmol h-1 g-1, about 2.4 times more than that of pristine BVO nanotubes. KEYWORDS:

Cu

doping,

BiVO4,

nanotubes,

density

functional

theory,

photocatalytic oxygen evolution

2


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■ INTRODUCTION Photocatalytic splitting of water into H2 and O2 has been regarded as one of the cleanest ways to solve the emerging energy problems, which converts solar energy directly into a renewable and storable energy source.1-3 In the reaction process of photocatalytic water splitting, O2 evolution is the crucial step because it is kinetically rather difficult due to the involvement of a four-electron and multiple-proton procedure.4,5 Thus, developing a suitable method to improve the water oxidation reaction efficiency, especially exploring robust and active photocatalysts, is highly desirable but rather challenging. Owing to a narrow band gap (2.4eV), BiVO4 (BVO) has been deemed as a photocatalytic candidate for visible-light-driven organic pollute degradation and O2 evolution.6-10 However, the weakness of charge migration and separation ability, resulting in a relatively poor photocatalytic activity for BVO.11 To obtain high-efficiency BVO-based photocatalysts, various methods have been developed, including noble metal deposition,12 co-catalysts loading,13,14 hetero-junction fabrication15,16 and metal ion doping,17,18 and so on. Metal ion doping has been proved to be an useful route which effectively extends the light absorption spectrum and improves the separation efficiency of photogenerated carriers.19,20 For instance, Jiang et al. reported that Ce-doped BVO exhibited significantly enhanced photocatalytic activities for O2 evolution in comparison with pristine BVO, and Ce3+ was proposed as hole traps to enhance interfacial electron transfer.21 Xu et al. prepared microspheres-like BVO with doped Ag via a hydrothermal method and a rate of about 76% for MB degradation was obtained under visible-light irradiation, and Ag doping was found to be crucial to reducing the band gaps and therefore enhancing visible light absorption.22 Li et al. found that N and Fe co-doped BVO photocatalyst 3



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exhibited substantially improved photocatalytic activity for CO2 reduction under visible-light irradiation, and effective electron-hole separation caused by N and Fe co-doping was proposed.23 The doping method is a feasible strategy for effectively separating photogenerated charge carriers. Among various metal ion dopants investigated so far, BVO doping with Cu ion have recently found to be an useful method to enhance the photoactivity of semiconductor photocatalysts, which can change electronic behaviors effectively, extend the light absorption edge and expedite efficient separation of photogenerated charge carriers.24-26 Additionally, it is well known that the morphology and structure of photocatalysts also have important effects on photocatalytic activitiy.27 In particular, owing to the merits of high surface-to-volume ratio and hollow interior structures, rapid charge transfer, high light-harvesting efficiency and large varieties of available chemicals for photochemical reactions, one-dimensional (1D) tubular structures have demonstrated promising application potentials as photocatalysts.28-30 Herein, we introduce a facile two-step route to synthesize uniform 1D porous Cu2+-doped BiVO4 (Cu-BVO) nanotubes with high catalytic stability. As the sacrificial templates, polyacrylonitrile (PAN) nanofibers with a smooth surface were first prepared by an electrospinning technique. A conformal layer of Cu-BVO nanoparticles was coated onto the PAN nanofibers through a facile solvothermal method, and the solid core-shell precursors were converted into porous Cu-BVO nanotubes after removing the PAN core via a simple annealing treatment. The as-prepared 1D porous Cu-BVO nanotubes exhibit enhanced photocatalytic O2 evolution activity under visible-light irradiation (λ>420 nm). An optimal Cu2+ doping percentage of 5.0% was found to have the best photocatalytic performance, exhibiting an average O2 evolution rate of to 350.2 µmol h-1 g-1, about 2.4 times more than that of the pristine BVO nanotubes. Furthermore, 4


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the underlying mechanism of the improved photocatalytic activity was systematically investigated with the experimental methods of UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and Mott–Schottky plots as well as the theoretical method of density functional theory (DFT) calculations. ■ EXPERIMENTAL SECTION All the reagents were purchased from Shanghai Chemical Reagent Factory and utilized as received without further purification. Synthesis of PAN nanofibers. In a typical preparation process, PAN nanofibers were prepared using an electrospinning technique. 0.5 g of polyacrylonitrile (PAN) (Mw = 150000) powders was dissolved into 6 mL of N,N-dimethylformamide (DMF). The mixture was then vigorously stirred at room temperature for about 12 h to form a homogeneous precursor solution. The electrospinning was carried out at a positive voltage of 15 kV and negative voltage of -3 kV, and the aluminium-foil collector was placed at 15 cm from the needle tip. Synthesis of porous Cu-BVO nanotubes. 0.4 mmol Bi(NO3)3·5H2O was dissolved in 5 mL of ethylene glycol (EG) magnetically stirred for 10 min. Meanwhile, a certain amount of CuSO4·5H2O was added into the above solution to form solution A. 0.4 mmol NH4VO3 was added into 5 mL of ethylene glycol (EG) and magnetically stirred for 10 min to form solution B. 15 mg of the as-obtained PAN nanofibers and 30 mL of ethanol were then slowly added the mixed solution A and B under stirring for 30 min. The resulting mixture was transferred into the 50 mL autoclave and treated at 160 0C and maintained for 24 h. Finally, the products were washed with distilled water and ethanol for three times, respectively, and dried in a vacuum oven at 80 °C for 6 h. By heating the obtained products in air to 500 °C with a ramp rate of 2 °C min-1 for 2 h, the PAN template was removed and the porous Cu-BiVO4 5



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nanotubes were obtained. The molar ratios of CuSO4·5H2O to BVO of 0, 3.0%, 5.0%, and 10.0% were prepared, which are depicted as Cu-BVO, Cu-BVO-3.0%, Cu-BVO-5.0%, Cu-BVO-10.0%, respectively. Characterizations. The phase purity of the as-obtained samples was characterized by powder X-ray diffraction (XRD) using Philips PW3040/60 X-ray diffractometer. Scanning electron microscopy (SEM) characterization was taken on Hitachi S-4800 scanning electron micro-analyzer. The microstructure of the as-prepared samples was further characterized with a JEM-2100F field emission transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) were measured using an ESCALab MKII X-ray photoelectron spectrometer. UV-Vis DRS were taken on a Thermo Nicolet Evolution 500 UV-Vis spectrophotometer with the wavelength range of 200-800 nm. N2 adsorption-desorption measurements were measured with Micrometrics JW-BK222 and the samples were outgassed in vacuum at 160 °C for 4 h. The photoluminescence (PL) spectra and the lifetimes of charge carriers were recorded using an FLS 980 fluorescence spectrophotometer with an excitation wavelength of 355 nm. The Mott-Schottky experiments were performed in 0.5 M Na2SO4 electrolyte with a frequency of 1 kHz and the amplitude of 10 mV on a Zennium E station (ZAHNER, Germany) in a three-electrode cell with Pt wire as the counter electrode and Hg/Hg2Cl2 as the reference electrode. The photocurrent was measured by electrochemical station (CHI 660E, China) with a conventional three electrode system. The working electrode is the sample coated on an FTO glass, a Ag/AgCl reference electrode and a Pt wire counter electrode. The electrolyte was 0.5 M Na2SO4 aqueous solution, and a 300 W xenon lamp was used as the visible light source. Electrochemical impedance spectroscopy (EIS) measurements were operated in a Zennium E station with the three-electrode system, using a 5 mM K3[Fe(CN)6] 6


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and 1 M KCl aqueous aqueous solution under visible-light illumination and applying an AC voltage of 10 mV with the frequency ranging from 0.1 Hz to 100 kHz. All the calculations were performed to study the band structures of the pristine BVO and Cu-BVO-5.0% samples using the Projector augmented wave (PAW) method,31 as implemented in the Vienna ab-initio simulation package (VASP).32 The exchange correlation function was processed by the Generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE).33 The conventional cell of pure BVO containing with 24-atom was relaxed using the cutoff energy at 500 eV for the plane-wave expansion, and a (9×9×5) Monkhorst-Pack k-point mesh was constructed (Figure S1a, see Supporting Information). The atomic force and energy for pure BVO were set less than 0.05 eV and 10-5 eV, respectively. The supercell of Cu-BVO containing with 96-atom was relaxed using the plane-wave cutoff energy at 400 eV and the k-point mesh of (5×5×5), and the atomic force and energy were set less than 0.05 eV and 10E-4 eV (Figure S1b), respectively. Photocatalytic O2 evolution tests. The photocatalytic O2 evolution experiments were analyzed by gas chromatograph (Agilent Technologies GC-7890B, thermal conductivity detector), employing high purity Ar as the carrier gas. 5% CoOx was loaded to the photocatalysts surface as the cocatalyst, and 20 mg photocatalyst was dispersed into 50 mL deionized water containing sacrificial reagent (0.02 M Na2S2O8 and 0.1 M NaOH). The reactant solution was irradiated by a 300W Xe lamp (MicroSolar 300, Perfect Light) equipped with a UV cuton filter at 420 nm, and the circulation water of 25 oC was employed to maintain the reaction temperature. Before the photocatalytic measurements, the reaction vessel was degassed for 60 min by Ar to remove the dissolved air. The apparent quantum efficiency (QE) of the Cu-BVO-5.0% sample was defined by the following Equation (1): 7



the number of reacted electrons ×100% the number of incident photos the number of envolved O 2 molecules × 4 = × 100% the number of incident photos = 4nO2 /n p ×100%

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QE[%] =

(1)

The number of incident photons was determinated by the following equation: np = IStλ / NAhct, where I is the light intensity (W cm-2), t is the irradiation time (s), λ is the irradiation wavelength number (nm), NA = 6.022 × 1023 mol-1 is the Avogadro constant, S means the irradiation area(cm2), h = 6.63×10-34 J·s is the Planck constant, and c = 3×108 m·s-1 is the speed of the light.

■ RESULTS AND DISCUSSION The fabrication process of porous Cu-BVO nanotubes is shown in Figure 1. PAN nanofibers were first prepared through the electrospinning technique, and a conformal layer of Cu-BVO nanoparticles was deposited onto the surface of PAN nanofibers through a facile solvothermal method, and finally the porous Cu-BVO nanotubes were obtained after removing the PAN core by a simple annealing treatment. The crystal properties of the as-prepared porous Cu-BVO nanotubes with different levels of Cu2+ doping were investigated by XRD measurements. Figure 2a reveals that all of the diffraction peaks can be well indexed to the monoclinic phase of BVO with lattice constants of a = 5.195 Å, b = 11.701 Å and c = 5.092 Å (JCPDS card no. 14-0688).34 No diffraction peaks corresponding to Cu2+ doping or other impurities can be observed, because the low Cu content does not develop new crystal orientations or change the crystalline structure of monoclinic BVO. The (121) diffraction peaks in range of 2θ = 28.6-29.2° are shown in Figure 2b, exhibiting a slight shift toward the lower angle compared with the undoped sample. This result can be explained by the difference of the ionic radius, Bi3+ (0.108 nm) > Cu2+ (0.072 nm) > V5+ (0.059 nm), 8


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which results in the shift of diffraction peaks toward lower angles because of the larger Cu2+ substituting V5+. 9 The morphology of the samples were inspected by SEM measurements. Figure 3a and b reveal that the PAN nanofibers exhibit a diameter of around 450 nm as well as a smooth surface and uniform length. The low-magnification SEM image (Figure 3c) reveals that, after solvothermal reaction, Cu-BVO nanoparticles were grown onto the PAN nanofibers, resulting in an average diameter ranging from 600 to 650 nm, and the high-magnification image (Figure 3d) further indicates a core–shell structure of the product. Figure 3e and f exhibit the SEM images of the as-prepared Cu-BVO-5.0% nanotube sample, revealing a porous hollow tubular structure. The hollow nanotube exhibits an averaged diameter of about 650 nm and wall thickness of the hollow structure is about 100 nm. Compared with other BVO samples doped with different Cu concentrations (the SEM images provided in Figure S2), the Cu2+ doping induces slight changes in size and morphology. The geometrical structure of the as-prepared Cu-BVO-5.0% nanotube sample is investigated by TEM. As can be seen from Figure 4a, the porous and hollow structure can be clearly observed, in good accord with the SEM results. Figure 4b shows the HRTEM image of Cu-BVO-5.0% nanotube, which further confirms that the Cu-BVO-5.0% nanotube is composed of a nanoparticle layer on a porous nanotube structure ( pores labeled with yellow circles), and the lattice fringe with a d spacing of 0.262 nm corresponding to the (200) plane of BVO is observed. The STEM image presented in Figure 4c and the elemental mapping (Figure 4d-g) clearly shows the uniform distribution of Cu, Bi, V and O elements

throughout

the

entire

Cu-BVO-5.0%

nanotubes

product.

The

energy-dispersive X-ray spectrometer (EDS) patterns are further presented in Figure

S3, where the existence of Cu element and different levels of Cu doping in Cu-BVO 9



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samples can be clearly confirmed. In addition, the atom percentages of O, V, Cu and Bi elements in different samples are also displayed in Table S1. The surface area and pore structure of the as-prepared pristine BVO and Cu-BVO-5.0% samples were studied by the Braunauer-Emmett-Teller (BET) method. The N2 adsorption-desorption isotherms and corresponding pore size distributions of the as-prepared pristine BVO and Cu-BVO-5.0% samples were investigated (Figure

S4). The N2 adsorption isotherm of the samples display the type IV curve, implying the existence of mesoporous structures.35 According to the result obtained by the BJH method (in the inset of Figure S4), the pore size distribution of pristine BVO and Cu-BVO-5.0% samples is 9.8 nm and 4.1 nm, respectively. And the BET surface area increases from 10.2 m2/g to 26.5 m2/g with the doping of 5.0% Cu into the BVO nanotubes, indicating the increase of the specific surface area and the decrease of the average pore size caused by Cu doping. Pore size of BVO is reduced after doping of Cu, which may account for the enlarged surface area. The higher surface area is expected to provide more active sites and accelerate the transfer of the photogenerated carriers to the surface of the photocatalyst.36,37 In order to elucidate the the surface chemical states of the as-prepared bare BVO and Cu-BVO-5.0% samples, XPS analyses were performed. As seen in Figure S5, Bi, O, V and C elements are all observed in pure BVO, but Cu element is only detected in the Cu-BVO-5.0% sample. The C element should come from the instrumental environment. The XPS spectrum of the Bi 4f region (Figure 5a) displays the two peaks located at 159 and 164.4 eV correspond to the Bi 4f7/2 and Bi 4f5/2 of the Bi3+ state in BVO.38 The corresponding binding energies for the Cu-BVO-5.0% sample are 159.1 and 164.4 eV, respectively, similar to the binding energies obtained for Bi3+ in BiVO4. Figure 5b shows the XPS spectrum of Cu 2p, which exhibits two sets of 10


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Gaussian doublets located at 932.3/934.3 eV and 951.9/953.3 eV, corresponding to the Cu2p3/2 and Cu2p1/2, respectively.31,39 The shake-up satellite peak is the characteristic of Cu2p, which is related to O and diagnostic of an open 3d9 shell of Cu2+ in BVO.40 Considering the ionic radius of Bi3+ > Cu2+ > V5+, the doped Cu2+ may replace V5+ and results in the internal lattice structure of BiVO4 in the formation of Bi-Cu-O bond.41 The spectra of V 2p3/2 are presented in Figure 5c, which can be decomposed into two peaks at 516.7 and 517.1 eV for the BVO sample, and 516.6 and 517 eV for Cu-BiVO4-5.0% sample, which are assigned to the surface V4+ and V5+ species, respectively.18 The molar ratio V4+/V5+ for Cu-BiVO4-5.0% sample is 1.22, much higher than that of bare BVO sample (0.93). According to the electroneutrality principle, the reduction of the oxidation state of V from V5+ to V4+ can be explained by Cu2+ doping.42 Furthermore, the presence of more V4+ in the Cu-BVO-5.0% samples suggests that more active sites are present at the sample surface. The co-existence of V4+ and V5+ cations in the BVO and Cu-BVO-5.0% samples enables high electrical conductivity.43 Figure 5d displays the O1s XPS spectra of the BVO and Cu-BVO-5.0% samples. The asymmetric peak can be decomposed at two peaks at 529.5 and 530.3 eV, for the as-prepared bare BVO sample, and 529.8 and 530.9 eV for the as-prepared Cu-BVO-5.0% sample, respectively, which are ascribed to lattice oxygen in the as-prepared sample and the adsorbed H2O or surface hydroxyl group, respectively. Doped with Cu2+, V5+ is reduced to V4+, leads to extrinsic oxygen vacancies which could enhance separation and transfer of the photogenerated electrons during photocatalytic reactions. 44,45

Figure 6a shows the UV-vis DRS of as-prepared porous Cu-BVO nanotube products. Due to Cu2+ doping, the light absorption edge of BVO shifts to the longer wavelength, which is advantageous to the visible-light photoactivity by enlarge the 11



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light response range of BVO to visible-light region.46 The band gap energy of the samples were calculated from the UV-vis spectra by using the Tauc’s plot following the equation:47,48 αhν = A(hν-Eg)n/2

(2)

Where α, h, A and ν are the absorption coefficient, Planck’s constant, constant and light frequency, respectively. The coefficient n is determined by the optical transition type of the semiconductor, and for pure BVO n is 4.49 As a result, the Eg values calculated by using Eqn (1) are 2.30, 2.12, 2.07, and 2.02 eV for BVO, Cu-BVO-3.0%, Cu-BVO-5.0%, and Cu-BVO-10.0% nanotubes, respectively (shown

Figure 6b). This result indicates that Cu2+ doping has a strong influence on the bandgap structure and the light absorption edge of the as-prepared porous Cu-BVO nanotubes, which improve the utilized rate of the visible light energy. The Mott-Schottky analyses with different frequencies (1, 2, 5 kHz) for the as-prepared porous Cu-BVO nanotubes were also performed. As seen in Figure 7, the CB potentials of the BVO, Cu-BVO-3.0%, Cu-BVO-5.0%, and Cu-BVO-10.0% sample are estimated to be -0.19, 0.36, 0.33, and 0.42 V versus RHE, respectively.50 By using the equation Eg = Evb - Ecb,51 the VB potentials of the as-prepared BVO, Cu-BVO-3.0%, Cu-BVO-5.0%, and Cu-BVO-10.0% samples are calculated at 2.11, 2.48, 2.4, and 2.44 eV, respectively. The VB and CB values of all the samples are also listed in Table S2. Compared with bare BVO, it is very obvious that Cu2+ doping into BVO can simultaneously induce a downward shift in the CB and VB potentials. The photocatalytic water oxidation activities of the as-prepared porous Cu-BVO nanotubes with different concentrations of Cu2+ doping were evaluated by the reduction reaction experiment in 50 mL of distilled water using persulfate (Na2S2O8) as a sacrifice oxidant and irradiated with visible light ( λ > 420 nm). CoOx was loaded 12


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to the photocatalyst surface as the cocatalyst to promote the water oxidation reactions.52,53 Figure 8a shows the amount of O2 production of different samples, it is discovered that the O2 evolution activities for all Cu2+ doping samples are significantly improved compared with the pure BVO. Among the doped samples, the as-prepared Cu-BVO samples with 5.0% Cu doping exhibits the highest O2 production activity. The photocatalytic activity is ordered as BVO < Cu-BVO-10.0% < Cu-BVO-3.0% < Cu-BVO-5.0%. From Figure 8b, it can be seen that the sample with the optimal Cu2+ doping of 5.0% exhibits an average O2 evolution rate reach 350.2 µmol h-1 g-1, about 2.4 times higher than that of the pristine BVO nanotubes.

Figure S6 gives the O2 evolution curves of the as-prepared samples without the co-catalyst, it can see that the sample with the optimal Cu2+ doping of 5.0% exhibits an average O2 evolution rate of up to 122 µmol h-1 g-1, about 2.5 times higher than that of the pristine BVO. Obviously, the CoOx loaded photocatalyst dramatically improves the visible-light photocatalytic performance. Additionally, the apparent quantum yield (AQY) test was further performed similar to the photocatalytic O2 evolution test, and a 300w Xe lamp with 420, 450 or 500nm band pass filter was used as the monochromatic light source. The measured QE of the Cu-BVO-5.0% sample are 2.63%, 1.81% and 0.73% at 420 nm, 450 nm and 500 nm, respectively. And the measured QE of the as-prepared BVO sample at 420 nm is only measured to be 0.55%. The doping of Cu2+ can modulate the electronic conduction band to some extent by occupying the position of V5+ with V4+, and the V4+ ions act as electron-trapping centers to enhance the lifetimes of photoexcited carriers.54 Additionally, the hollow structures can make better use of the light source and influence the electron transfer to improved photocatalytic activity.55,56 Table S3 shows the O2 evolution activities of the as-obtained Cu-BVO nanotubes in this work 13



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compared with previously reported photocatalysts. To further establish the performance stability of the Cu-BVO-5.0% photocatalyst, three recycling tests (Figure S7a) were conducted under the same reaction conditions, and no noticeable degradation of the photocatalytic performance was detected after three cycles. Through further examined by the XRD and SEM results of the Cu-BVO-5.0% sample before and after the cyclic O2 production test under visible-light irradiation (Figures

S7b and S7c,d), and no obvious crystalline and morphological changes can be detected as compared to the initial sample, indicating that the as-prepared Cu-BVO-5.0% photocatalyst has good durability for photocatalytic O2 evolution. The calculated band structure of BVO and Cu-BVO-5.0% samples were compared in Figure 9. The Eg of pure BVO sample is 2.16 eV, which are consistent with previous calculations, but smaller than the measured value (2.29 eV) (Figure 9a).24 For Cu-BVO-5.0% sample, impurity levels above the VB decrease the band gap to 1.92 eV from 2.16 eV, which leads to the broader optical absorption range and enhanced utilization of the solar energy, consistent with the experimental observations (Figure 9b).57,58 The total density of state (TDOS) and partial density of state (PDOS) for the BVO and Cu-BVO-5.0% samples were presented in Figure 10. As shown in

Figure 10a, the DOS result shows that the VB and CB for pure BVO sample are consistent with previous calculations.24 According to the PDOS of the Cu-BVO-5.0% sample shown in Figure 10b, the existence of the impurity energy levels mainly comes from the localized Cu 3d states which is hybridized with O 2p states around the Fermi level and it can reduce the carrier transition energy, beneficial to the limit the recombination of photoinduced carriers and the improvement of the photocatalytic activity of pure BVO. The shallow acceptor energy levels due to the Cu2+ substituting V5+ in BVO may act as electron traps which suppress the recombination rate of the 14


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photogenerated electron-hole pairs. Thus, the doping of Cu atom into BVO not only enhance the absorption of visible light by extending the absorption range to the longer wavelength but also enhances the separation efficiency of photogenerated electron-hole pairs, which therefore significantly improves the photocatalytic O2 evolution activity of the pure BVO.59,60 PL spectra and PL decay spectra are usually used to study the transfer and separation efficiency of photoinduced carriers. The PL spectra of BVO and Cu-BVO-5.0% are shown in Figure S8a. It is revealed that the pure BVO exhibits strong emission peaks around 510 nm, which is similar to a previous report.61 Obviously, the Cu-BVO-5.0% sample possesses the lowest PL intensity and no new PL signals can be observed, which means the reduced recombination of photoinduced carriers and the successful doping of Cu in the BVO sample. Furthermore, the PL decay spectra in Figure S8b exhibit that the average decay lifetime of carriers in the sample Cu-BVO-5.0% is about 0.62 ns, whereas the pure BVO is only 0.50 ns. These results revealed that the BVO doping with Cu2+ could increase the lifetime of charge carriers, indicating effectively separate and transfer the electrons and holes. The photocurrent response is a facile technique for studying the generation, separation and transfer processes of photo-generated electrons and holes in the photocatalysts.62,63 The transient photocurrent responses of the as-prepared porous Cu-BVO nanotubes with different concentrations of Cu2+ doping are presented in Figure 11a. Obviously, all Cu2+ doping samples exhibit a higher photocurrent than that of pure BiVO4, implying that the formation of the Cu-BVO samples by Cu2+ doping can impove the separation of photogenerated electrons and holes.64 Specially, the Cu-BVO-5.0% sample shows the highest photocurrent density, which is in good agreement with the best photoactivity O2 evolution activity. EIS is a useful tool to analyze the charge 15



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transfer and recombination processes of photocatalysts.16,65,66 The Nyquist plot can be explained by the equivalent circuit shown in the inset (Figure 11b). In the equivalent Randle circuit, Rct is the charge transfer at the electrode interface, Rs is the solution resistance, CPE is the constant phase element for the semiconductor/electrolyte interface.67,68 The Cu-BVO-5.0% sample shows a smaller circular radius than the other samples, which means a fastest charge transfer process. It can be therefore concluded that the Cu2+ doping can promote charge separation and further improve the efficiency of photocatalytic oxygen production. To examine the effect of impurity CuO in the photocatalytic system, bare CuO sample was synthesized by grinding 0.5g of Cu(NO3)2·3H2O in a mortar for 20 min and then annealed in an alumina crucible in air to 500 0C maintained for 2 h at a rate of 2 0C min-1. Figure S10 shows the XRD pattern of the as-prepared bare CuO, all of the diffraction peaks can be well indexed to the monoclinic phase of CuO with lattice constants of a = 4.688 Å, b = 3.423 Å and c = 5.132 Å (JCPDS card no. 48-1548), indicating that the bare CuO was successfully synthesized. Figure S11 depicts the O2 evolution curves of the samples under visible-light irradiation, the activity of the bare CuO is nearly negligeable compared with Cu-BVO-5.0% photocatalyst. These results demonstrate that the CuO impurities have no contribute to the catalytic activity of the as-prepared Cu-BVO samples. Based on the results discussed above, Figure S9 illustrates the mechanism for the enhanced O2 evolution activity water oxidation efficiency of the Cu-BVO-5.0% nanotube. Here, Na2S2O8 is as an electron scavenger and the holes generated in Cu-BVO-5.0% nanotube can be easily captured and transferred to the surface of CoOx nanoparticles, leading to an efficient charge-separation. The doping of Cu2+ induces impurity states located above the VB maximum, which act as electron traps and extend the visible-light absorption range and therefore further enhance the 16


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photocatalytic O2 evolution efficiency of the BVO catalyst.

■ CONCLUSIONS In summary, we have developed a facile two-step strategy, which combines the technique of electrospinning and the solvothermal method, to synthesize porous 1D Cu-BVO nanotubes. Both experimental results and the DFT calculations reveal that Cu doping can reduce the band structure of the BVO and introduce beneficial defect states, which effectively impove the separation and transfer efficiency of the photogenerated charge carriers and therefore boost the photocatalytic O2 evolution efficiency under visible-light irradiation. For an optimal Cu2+ doping of 5.0%, an average O2 evolution rate reach 350.2 µmol h-1 g-1 is obtained, which is about 2.4 times higher than that of the pristine BVO nanotubes. This study provides a strategy for the development of 1D hollow metal-doped semiconductor photocatalysts for applications of photocatalytic O2 evolution.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org. Details

of

structural

model,

SEM

images,

EDS

patterns,

nitrogen

adsorption-desorption isotherms, full XPS spectra, VB and CB value of the photocatalysts, catalytic performance of the samples, PL spectra, Schematic diagram (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected],

17



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[email protected]

ORCID Yong Hu: 0000-0003-3777-167X

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y. Hu acknowledges financial support from the Natural Science Foundation of China (21671173, 61674166). J. Q. Ning acknowledges the financial support from National Key Technologies R&D Program of China (2016YFA0201101), Key Frontier Scientific Research Program of the Chinese Academy of Sciences (QYZDB-SSW-JSC014), and Hundred Talents Program of Chinese Academy of Sciences and the NANO-X Workstation of SINANO, CAS. And Z. Y. Zhang acknowledges the financial support from The Thousand Youth Talents Plan.

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Figure 1. Schematic illustration of the fabrication of Cu-BVO-5.0% nantube.

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Figure 2. (a) XRD patterns of the as-prepared different samples. (b) XRD patterns of the as-prepared different samples in the range of 28.6~29.2°.

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Figure 3. SEM images of as-prepared samples: (a,b) PAN nanofibers, (c,d) PAN/Cu-BVO-5.0% nanofibers, and (e,f) Cu-BVO-5.0% nantube.

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Figure 4. (a) TEM image and (b) HRTEM image of the as-prepared Cu-BVO-5.0% nantube. (c) STEM image and (e-g) elemental mapping images of Cu, Bi, V, and O, respectively.

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Figure 5. XPS spectra of the Cu-BVO-5.0% sample: (a) Bi 4f, (b) Cu 2p, (c) V 2p and (d) O 1s.

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Figure 6. (a) UV-Vis DRS and (b) Tauc plots of the as-prepared different samples.

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Figure 7. Mott–Schottky plots of the as-prepared samples with different frequencies: (a) BVO , (b) Cu-BVO-3.0% , (c) Cu-BVO-5.0% , (d) Cu-BVO-10.0%.

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Figure 8. (a) Time course photocatalytic O2 evolution and (b) O2 evolution rate for different concentrations of Cu2+ doping samples under visible-light irradiation (λ > 420 nm).

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Figure 9. The band structures for the (a) pure BVO and (b) Cu-BVO-5.0%.

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Figure 10. The total and partial density of states for (a) pure BVO and (b) Cu-BVO-5.0% samples.

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Figure 11. (a) Photocurrent response of the different samples under visible-light illumination (b) Electrochemical impedance spectra of the different samples under visible light illumination. The inset shows an ideal equivalent circuit for the working photoelectrode.

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Table of Contents graphics

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Fabrication of porous Cu-doped BiVO4 nanotubes as efficient oxygen

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