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A Novel and Highly Active Potassium Niobate-Based Photocatalyst for Dramatically Enhanced Hydrogen Production Kun Zhu, Xiangqing Li, Shi-Zhao Kang, Lixia Qin, and Guo-Dong Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00908 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Sustainable Chemistry & Engineering

A Novel and Highly Active Potassium NiobateBased Photocatalyst for Dramatically Enhanced Hydrogen Production Kun Zhu,† Shi-Zhao Kang,† Lixia Qin†, Sheng Han†*, Guodong Li‡ and Xiangqing Li†* †

School of Chemical and Environmental Engineering, Center of Graphene Research, Shanghai

Institute of Technology, 100 Haiquan Road, Shanghai 201418, China ‡

State Key Laboratory of Inorganic synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, China. * Xiangqing Li and Sheng Han. E-mail address: [email protected] (Xiangqing Li), [email protected] (Sheng Han) ABSTRACT: A novel and highly active photocatalytic materials, self-doped potassium niobate composite microflowers stimulated by noble-metal-free copper nanoparticles (Cu/K4Nb6O17), was achieved. Composition and structure of the composite microflowers were characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), and X-ray photoelectron spectroscopy (XPS). The results showed that Cu nanoparticles were evenly and closely loaded onto the flower slices of the composite microflowers. As testified by XPS, electrochemical impedance spectrum and fluorescence spectrum, the presence of Cu in K4Nb6O17 microflowers quickened the self-doping of Nb4+, enhanced light absorption and the unsaturated 1 ACS Paragon Plus Environment

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defects as active sites, and improved separation efficiency of electron/hole pairs, which led to excellent photocatalytic activity for hydrogen evolution over the composite microflowers. Subsequently, the optimal hydrogen generation rate for the composite microflowers was about 9 times higher than that of pure K4Nb6O17 microflowers under same conditions. Moreover, the composite photocatalyst was stable and easy to be recycled. The results demonstrated that the construction of the special heterojunction by facile interfacial modification is a promising strategy to efficiently enhance photocatalytic performance of semiconductor photocatalysts. KEYWORDS: Nanocomposite, Interfacial modification, Electron transfer, Photocatalytic Hydrogen production, Mechanism

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INTRODUCTION Transforming solar energy into hydrogen energy is a potential way to solve the problems of energy shortage and environmental pollution. In various technologies, photocatalytic hydrogen evolution is one of the most promising ways of achieving this transformation.1-3 In it, novel and highly active photocatalysts for hydrogen evolution are vital.4-6 An ideal photocatalyst should be highly efficient, stable and cheap. Due to stable structure, adjustable morphology and better photoactivity, potassium niobate based photocatalysts are paid more attentions.7-14 It is found that the structure and morphology of potassium niobates have important effects on their photocatalytic activity by the modifications of the surface area and hydrophilicity.15 Therefore, it is possible to adjust their photocatalytic activity by modulating the structures and morphologies of potassium niobate photocatalysts.16 Such as, a layered potassium niobate can usually realize the separation of photo-produced electrons and holes due to its special structure.7 Townsend et al. found that photocatalytic activity of hydrogen evolution over the exfoliated-scrolled K4Nb6O17 was obviously higher than that of the synthetic K4Nb6O17 crystals for UV light-driven hydrogen evolution.17 Zhou et al. prepared potassium niobate microspheres with high specific surface area by homogeneous precipitation method, which showed higher photocatalytic activity.18 However, the separation efficiency of electron and holes is low for pure potassium niobates. Moreover, pure potassium niobates are only UV response, which is disadvantageous to light absorption. It is found that it is an efficient way to improve the visible photocatalytic activity of K4Nb6O17 by coupling with some semiconductor with narrow band gap, such as coupling with Cu2S or CdS19,20. Additionally, it is also good method to improve the visible photocatalytic activity by reducing band gap of K4Nb6O17. Doping is the most commonly used method because it can 3 ACS Paragon Plus Environment


induce intrinsic electronic and band structure changes.21 However, doping of metal ions into the lattice of K4Nb6O17 could result in thermal or crystal instability and increased charge carrier recombination centers, and thus low photocatalytic activity. It is found that self-doping is an effective and promising way to extend the absorption of semiconductor photocatalysts.22 Generally, the surface species23-25 in photocatalysts, such as, noble metals (such as Pt or Au), NiO, CdS or Cu2S mainly act as cocatalysts or photosensizer, and are important because the photocatalytic reaction occurs primarily on the surface of photocatalysts. Fundamentally, the improvement is attributed to efficient charge separation at the interface of surface species/semiconductor or improved light adsorption. Recently, it is found that Cu nanoparticles are a cheap and efficient cocatalyst in some photocatalyst systems26. However, there is little report about Cu nanoparticles loaded potassium niobate microflowers and about the real role of Cu nanoparticles in the potassium niobate microflowers, which is important to explore the function of cocatalyst and photocatalytic mechanism. Herein, a facile strategy for the quick preparation of Nb4+ self-doped potassium niobate composite microflowers (Cu/K4Nb6O17) stimulated by noble-metal-free copper nanoparticles (Cu NPs) is developed. The structure and morphology of the Cu/K4Nb6O17 composite microflowers are studied. With the composite microflowers as the photocatalyst, photocatalytic hydrogen production from water reduction is performed in aqueous methanol solution. The relationship between the microstructure of K4Nb6O17 microflowers and its photocatalytic activity, the role of Cu NPs in the potassium niobate microflowers, and the mechanism of photo-produced electron transfer in the composite microflowers are also investigated in detail. EXPERIMENTAL SECTION


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Preparation of K4Nb6O17 microflowers. K4Nb6O17 microflowers were prepared according to a modified process.17 In a typical synthesis process, 0.3 g Nb2O5 was dispersed into 30 mL of 3 mol L-1 KOH solution, and then reacted at 180 °C in a sealed Teflon stainless autoclave for about 10 h. After that, 9 mL clear solution was removed from the autoclave, and 2 g urea and 18 mL deionized water were added into it. Then, the mixture was loaded into a 50 mL Teflon stainless autoclave, and was heated at 220 °C for 24 h. The target product was collected by filtration, washed with deionized water and absolute ethanol for several times, respectively, and dried at ambient temperature. Preparation of Cu/K4Nb6O17 composite microflowers. A series of samples with various ratios of Cu to K4Nb6O17 microflowers were prepared. In a typical procedure, 100 mg of K4Nb6O17 microflowers was dispersed into 100 mL deionized water, 0.6 mL of 0.01 mol L-1 CuCl2 was added into the dispersion, and then stirred for 24 h at room temperature. Under nitrogen atmosphere, 0.5 mL of N2H4⋅2H2O was added slowly into the above mixture, and continued to react for 5 h at 90 °C. Then, the target product (0.4wt%Cu/K4Nb6O17) was obtained by filtration, rinsed with deionized water and ethanol several times, respectively. The solid obtained was dried for 12 h in a vacuum oven at 60 °C. For comparison, pure K4Nb6O17 microflowers, those loaded with various amounts of Cu, and that loaded with Pt were also prepared by the same procedures. Photocurrent measurement. The photoelectrochemical performance of the samples was measured by a CHI660E electrochemical system using a three-electrode cell (Shanghai Chenhua Instruments, China). Fluorine-doped tin oxide (FTO) glasses coated with the sample were utilized as working electrodes (FTO glasses were cleaned by sonication in ethanol, acetone, chloroform and double distilled water for 15 min, respectively, and then dried in the



atmosphere), which were prepared via impregnation and subsequent calcination. In brief, 2 mg of the sample was mixed with 4 mL of alcohol, and the obtained mixture was sonicated for 30 s. After that, the FTO glass (1 × 1.5 cm2) was soaked into the mixture for 5 min, and then was blow-dried. Repeated that for five times, and then heat-treated at 60 °C for 1 h. An Ag/AgCl electrode was used as the reference electrode, and a platinum wire as the counter electrode. A 300 W Xenon lamp was used as the light source. The distance between lamp and the FTO electrode was 7 cm. The electrolyte solution was 0.1 mol L-1 of Na2SO4 aqueous solution, and the air in the solution was removed by purging N2 for 15 min. In addition, the measurement was carried out at room temperature without any bias potential. Measurement of photocatalytic activity. The photocatalytic activity for hydrogen evolution over the samples was performed through a CEL-SP2 N water splitting system (Zhongjiao Jinyuan Instruments, China). For photocatalytic hydrogen evolution, 60 mg of photocatalyst was dispersed by a magnetic stirrer into an up-irradiated photocatalytic reactor containing methanol aqueous solution (60 mL, volume ratio of water to methanol is 4: 1). The reaction cell was connected to a gas circulation system, and the hydrogen evolved was analyzed by an online gas chromatograph (NaX zeolite column, high–purity N2 as carrier gas, thermal conductivity detector). The light source was a 500 W mercury lamp (UV light), 300 W Xe lamp with 380 nm filter (visible light), or 300 W Xe lamp without filter (UV-vis light) according to experimental demand. The reaction temperature was kept at about 8.5 °C by a circulating water jacket. The gas produced was automatically sampled and analyzed by the online gas chromatography. Before the photocatalytic reaction, the reactor was alternatively evacuated by a vacuum pump and flushed by high–purity N2 for several times to ensure complete removal of oxygen.


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Characterization. The surface morphology of the sample was observed by a NOVA Nano SEM450 ultrahigh resolution field emission scanning electron microscope (FESEM) operated at an accelerating voltage of 10 kV (Japan). Crystal structure identification of the samples was carried out using a PAN analytical Xpert Pro MRD X-ray diffractometer (XRD) using Cu Ka radiation (λ = 0.154056 nm) (Netherlands). The samples for XRD were supported on glass substrates. The element maps were taken with Hitachi JOEL 2100F field emission high resolution transmission electron microscope operated at an accelerating voltage of 200 kV (Japan). The samples for TEM images were prepared by placing drops of the sample dispersion on a carbon-coated copper grid or molybdenum grid (maps) and dried at room temperature. Xray photoelectron spectra (XPS) were carried out on a Thermo ESCALAB 250 X-RAY photo electron spectrometer with a monochromatic X-ray source (Al Ka hv = 1486.6 eV) (USA). The energy scale of the spectrometer was calibrated using C1s 284.5 eV peak position. Solid diffuse reflectance UV-vis spectra were recorded at room temperature using a SHIMADZU 3600 spectrophotometer (Japan). The fluorescence spectra of the solid samples were measured without pretreatment, and it was carried out at room temperature using a HITACHI F-4600 spectrophotometer (Japan). The exited wavelength (λex) was 270 nm. Nitrogen adsorption– desorption isotherm was obtained at 77 K by using Quantachrome Instruments QUADRASORB SI volumetric adsorption analyzer (USA). The sample was pre-outgassed at 150 °C for 4 h under vacuum. RESULTS AND DISCUSSION The characterizations of structure, morphology and composition of the potassium niobate composite microflowers



As shown in Figure 1a, XRD peaks of pure potassium niobate microflowers are indexed to K4Nb6O17 (JCPDS card 53-0780). Compared with the standard XRD pattern of K4Nb6O17, some XRD peaks for the K4Nb6O17 microflowers slightly shift and broaden, and the relative intensity of some peaks also changes. According to the results reported27, it could be attributed the different size, morphology and microstructure of the K4Nb6O17 microflowers with the standard K4Nb6O17 (JCPDS card 53-0780). As can be seen in Figure 1b and Figure 1c, diffraction peaks of the 0.4%Cu/K4Nb6O17 microflowers and the 3%Cu/K4Nb6O17 microflowers are similar to those of pure K4Nb6O17 (Figure 1a). It is demonstrated that the introduction of Cu has little influence on the structure of K4Nb6O17. Differently, the peak at 2θ = 28.4° is hardly observed in the XRD patterns of Cu/K4Nb6O17 composites, which could be due to that the crystal plane was partially covered by Cu NPs. The diffraction peaks of the Cu are not noticeable in XRD patterns of composite microflowers. It could be due to small particle size and good dispersion of Cu NPs in the composite microflowers. The presence of Cu NPs in the composite microflowers will be further confirmed by the following TEM images, maps, and XPS spectra.

Figure 1

Furthermore, the morphology of the samples was investigated by SEM and TEM. It can be seen in Figure 2, the average diameter of the microflowers is ~3 µm (Figure 2a). After being loaded with Cu NPs, no significant morphological change for the microflowers is observed (Figure 2b). It indicates that the loading of Cu NPs has little effect on the structure and the morphology of K4Nb6O17. As can be seen in Figure 2c, some black particles are homogeneously loaded onto the flower slices of K4Nb6O17 microflowers, and the size of nanoparticles ranges 8 ACS Paragon Plus Environment

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from 2 to 11 nm, and is mostly 4-5 nm, as illustrated in the inset of Figure 2c. Moreover, no isolation or aggregation of nanoparticles is observed in the image, which demonstrates that the aggregation of Cu NPs can be effectively avoided by loading onto the K4Nb6O17 microflowers. The target product is further characterized by HRTEM. As is shown in Figure 2d, the lattice fringe with a spacing of 0.209 nm is clearly observed, which is indexed as the (111) crystal plane of cubic Cu.28 It is demonstrated that cubic Cu NPs are loaded onto the surface of K4Nb6O17 microflowers. Moreover, the Cu NPs are closely embedded into the flower slices of K4Nb6O17 microflowers. The strong interaction coming from the close contact is very profitable for interfacial electron transfer between K4Nb6O17 and Cu NPs, which is a vital factor to improve photocatalytic activity of semiconductors.

Figure 2

To further clarify the component of the composite microflowers and the distribution of Cu NPs, energy filtered TEM of the composite microflowers is taken. Figure 3 shows the element maps of composite microflowers, which are measured according to the region shown in Figure 3a (white square). It can be seen that there exist K, Nb, O and Cu elements in the composite microflowers. Moreover, the maps of K, Nb, O and C shown in Figure 3b, 3c, 3d and 3e, respectively, are corresponding to the result shown in Figure 3a. Namely, the grey-black particles on the microflowers are Cu NPs.

Figure 3



XPS is employed to study the element constitution and the chemical binding state of the potassium niobate composite microflowers. As shown in Figure 4A, XPS peaks of K, Nb, O and Cu can be observed. The K, Nb and O come from the potassium niobate, and the Cu comes from Cu NPs. In order to confirm the valence state of Cu in the composite, the high-resolution XPS of Cu is measured. As shown in Figure 4B, two typical Cu 2p3/2 and 2p1/2 peaks centered at 932.5 eV and 952.4 eV can be observed, respectively, which is corresponding to metallic copper. The result of XPS demonstrates that metallic copper was loaded onto the K4Nb6O17 microflowers.

Figure 4

Photocatalytic activity for hydrogen evolution The photocatalytic performance for hydrogen evolution over the as-obtained pure K4Nb6O17 microflowers and Cu/K4Nb6O17 composite microflowers are investigated using methanol as the sacrificial agent. It is shown that the catalytic activity without the sacrificial agent is low over Cu/K4Nb6O17 composite microflowers (Figure 5A), which indicates the low electron/hole separation efficiency in the composite. However, in the presence of methanol, the catalytic activity is dramatically enhanced, which indicates that methanol is the suitable sacrificial agent for the Cu/K4Nb6O17 microflowers photocatalyst system. Figure 5A shows the time courses for hydrogen evolution photo catalyzed by various samples. After introducing Cu NPs into K4Nb6O17 microflowers, the photocatalytic activity is significantly enhanced. Furthermore, as shown in Figure 5B, when only 0.4% of Cu is introduced and irradiated for 6 h, the hydrogen evolution amount for the composite microflowers is about 11.6 mmol g-1, about 9 times as that over pure K4Nb6O17 microflowers. With monochromatic light irradiation (λ = 325 nm), the


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quantum yield (QE) of hydrogen evolved over the 0.4%Cu/K4Nb6O17 microflowers is about 3% (Figure S5, QE is calculated by the method shown in Supporting information). In addition, it is observed that there exists an inducing period within about 1 h. After the inducing period is over, the rate of the photocatalytic reaction is quickened. Especially, the samples loaded with Cu NPs show higher activity. It is deduced that the valence state of niobium could be transformed in the inducing period, which is achieved efficiently in the presence of Cu NPs. In addition, electron transfer efficiency in K4Nb6O17 microflowers is enhanced after introducing Cu NPs (It will be confirmed by the result in Figure S1 and Figure 6), which is profitable for improving the photocatalytic activity of photocatalysts.29 An optimum loading amount for the Cu is about 0.4%, which is 3.4 times higher than that of 0.4%Pt/K4Nb6O17 microflowers under the same conditions. In general, novel metals (such as Pt) are good cocatalysts, and its activity is high for photocatalytic hydrogen evolution. However, in the Cu/K4Nb6O17 composite microflowers, the Cu NPs acted not only as the cocatalyst and active centers for photocatalytic hydrogen evolution, but also acted as the catalyst for the transformation of Nb5+ to Nb4+ (Figure S1 and following Figure 7). In addition, the surface area for the Cu/K4Nb6O17 composite microflowers (111 cm2 g-1) is bigger than that of pure K4Nb6O17 microflowers (78 cm2 g-1). The bigger surface area for the composite microflowers will provide more active sites and be profitable for improving photocatalytic

activity.

Subsequently,

the

novel

self-doped

Cu/K4Nb6O17

composite

microflowers stimulated by noble-metal-free Cu NPs show higher activity. When the content of Cu is beyond 0.4%, the activity is decreased. This result can be explained that a certain amount of Cu NPs deposited on the K4Nb6O17 microflowers can facilitate photoelectron transfer to the catalyst surface and decreases the recombination of electrons and holes.30,31 However, when



excessive Cu NPs is loaded onto K4Nb6O17 microflowers, the Cu NPs could become combination centers of electrons and holes.32

Figure 5

As can be seen in Table 16,10,11,19,20,23, the deposition of noble metals Pt on KNb3O8 is an efficient approach for enhancing the photocatalytic activity. But noble-metals are expensive, which is disadvantageous to its application. In addition, photocatalytic activity of the K4Nb6O17 can be greatly improved by coupling with Cu2S or CdS. Compared with these photocatalysts, the Cu/K4Nb6O17 composite microflower also shows high photocatalytic activity for hydrogen evolution, even is comparable to the Pt loaded KNb3O824 and the Pt loaded K4Nb6O17 microflowers (Figure 5). Importantly, the loading amount of Cu NPs in the composite is obviously lower than that of the Pt loaded KNb3O8. Though the photocatalytic activity of Cu/K4Nb6O17 composite microflowers is slightly lower than that of the Cu2S/K4Nb6O17, only 0.4% Cu is introduced in our work. Moreover, the Cu NPs in the K4Nb6O17 composite microflowers not only acted as the catalyst for the transformation of Nb5+ to Nb4+, but also acted as cocatalyst and active centers for photocatalytic hydrogen evolution (proved by the following Figure 6-Figure 8). Subsequently, the preparation and photocatalytic mechanism of the selfdoped potassium niobate composite microflowers stimulated by noble-metal-free Cu NPs will provide some basis for designing a novel photocatalyst and exploring the function of cocatalyst and photocatalytic mechanism.

Table 1


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The investigation of photocatalysis mechanism The investigation on the mechanism of photocatalysts is not only of scientific importance, but also can offer rational guidance to optimization of materials’ performances. The photocatalytic mechanism is first analyzed by the optical absorption of the samples before and after photo irradiation. Solid UV-vis diffuse reflectance spectra of the samples are shown in Figure S1. According to the formula of band gap (Eg = 1240/λ), the band gap for K4Nb6O17 microflowers is calculated to be 3.54 eV. It can be seen that a slight red-shift of absorption edge is observed in the spectrum of Cu/K4Nb6O17 microflowers, which indicates that the band gap of K4Nb6O17 microflowers is decreased after the introduction of Cu NPs, and the band gap of the samples is about 3.44 eV on the basis of the optical absorption threshold, respectively. It is indicated that the samples mainly response to UV light. Compared with that of the K4Nb6O17 microflowers before irradiation, the absorption edge of the K4Nb6O17 microflowers is red-shifted 5 nm after UV-vis light irradiation, and followed with an about 0.10 eV decrease in the band gap and an enhanced visible response from 400 nm-800 nm, which could be due to the formation of Nb4+ by electron trapping at Nb5+ centers under UV-vis light irradiation (It can be further confirmed by the result in Figure 6). Furthermore, after 0.4%Cu is introduced, the absorbance of K4Nb6O17 composite microflowers in the visible region is enhanced significantly, and no drastic change in the absorption edge is observed (Figure S1d). However, in the spectrum of the 0.4%Cu/K4Nb6O17 microflowers irradiated by visible light (Figure S1e), no red-shifted band edge and enhanced visible response are observed. It suggests an increment33 of surface electric charge in the K4Nb6O17 microflowers combined with efficient valence transformation of Nb5+ (evidenced by the following Figure 6 and Figure 7) due to the introduction of Cu under UV-vis



irradiation. Namely, there exists strong electronic interaction between Cu NPs and the K4Nb6O17 microflowers, which is favorable to photo absorption, photogenerated electron transfer and the enhancement of the photocatalytic activity of K4Nb6O17 microflowers. Moreover, it was observed that the color of the K4Nb6O17 microflowers suspension changed from white to light gray after irradiated for 6 h. Under the same conditions, the color of the Cu/K4Nb6O17 microflowers darkens. What’s the reason of color change? It could be the main reason of the enhanced photocatalytic activity after introducing Cu NPs. First, the composition and valence states of Nb and Cu are analyzed by high resolution XPS spectra of Nb 3d and Cu 2p. As show in Figure 6A, the binding energy of Nb 3d3/2 and Nb 3d5/2 for the K4Nb6O17 microflowers before photo irradiation appear at 206.8 eV and 209.5 eV, respectively. For the K4Nb6O17 microflowers subjected to photo irradiation, the peaks for Nb 3d5/2 and Nb 3d3/2 are shifted to lower binding energy at 206.7 eV and 209.4 eV, respectively. The similar result is observed in Figure 6B: in the Nb 3d XPS spectrum of the 0.4% Cu/K4Nb6O17 microflowers, two typical Nb 3d5/2 and Nb 3d3/2 peaks at 207.1 eV and 209.9 eV, respectively, are observed. After it is subjected to photo irradiation, Nb 3d5/2 and Nb 3d3/2 peaks are slightly shifted to 206.9 eV and 209.7 eV, respectively. In addition, high resolution XPS spectrum of Cu 2p is measured. Cu 2p3/2 and 2p1/2 peaks centered at 932.5 eV and 952.4 eV attributed to metallic Cu are observed, respectively. The two peaks have no significant change after irradiation (Figure S2). Combined with the result in Figure S1, it is concluded that the changes of color and absorption are attributed to the valence change of niobium not copper,9 and the Cu NPs loaded onto the K4Nb6O17 microflowers can accelerate the change of Nb5+ to Nb4+ by electron trapping at Nb5+ centers, which could be the main reason for the changed color after irradiation, and enhanced photocatalytic activity for Cu/K4Nb6O17.


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Figure 6

In order to further investigate the relationship between efficient transformation of valence states of Nb and the enhancement of photocatalytic activity and light resource, next, using a mercury lamp, a Xe lamp with 380 nm filter or a Xe lamp without 380 nm filter as the light source, respectively, the photocatalytic performance for hydrogen evolution over the as-prepared samples is investigated under the same conditions, and the results are presented in Figure 7. It is observed that the amount of hydrogen evolved is in the order of UV light > UV-vis light > visible light, no matter what K4Nb6O17 microflowers or 0.4%Cu/K4Nb6O17 microflowers are used as the photocatalyst. In addition, the hydrogen production over the 0.4%Cu/K4Nb6O17 microflowers under UV light is 3 times higher as that under UV-vis light. Moreover, under visible light irradiation, the activity of hydrogen production over the 0.4%Cu/K4Nb6O17 microflowers is neglectable (It is consistent with the result in Figure 5). It is indicated that the valence transformation of Nb5+ to Nb4+ cannot proceed without the assistance of UV light. Moreover, under same light source, the activity of 0.4%Cu/K4Nb6O17 microflowers is greatly higher than that of K4Nb6O17 microflowers. It is deduced that the formation of Nb4+ in the Nb5+ centers is facilitated in the presence of Cu NPs, and the introduction of Cu NPs accelerates the electron transfer. Therefore, the photocatalytic activity of K4Nb6O17 microflowers is increased significantly by means of Cu NPs under UV light or UV-vis light.

Figure 7



The fluorescence spectrum is used to investigate the separation of photo-induced carriers in the composite microflowers, which is important for activity investigation of photocatalysts34. Figure 8A shows fluorescence spectra of two samples. The pure K4Nb6O17 microflowers exhibit a strong and wide band centered at around 420 nm, which indicates that electron/hole pairs are generated after the K4Nb6O17 microflowers are irradiated. After loading a certain amount of Cu NPs, the intensity of the band declines drastically. It is demonstrated that the Cu NPs loaded on the K4Nb6O17 microflowers facilitate the separation of electrons and holes.35 Subsequently, the recombination of electron/holes pairs in K4Nb6O17 microflowers is prohibited, which is profitable for enhancing the photocatalytic activity of the composite microflowers. The photocatalytic activity for a photocatalyst has a close relationship with its interfacial resistance and electron transfer.36 Furthermore, as shown in Figure 8B, the semicircle becomes small after introducing Cu NPs in K4Nb6O17 microflowers, which indicates a lower interfacial resistance in the Cu/K4Nb6O17 microflowers. It is profitable for the transfer of photo-produced electrons and the enhancement of hydrogen evolution.37 it is known that the photocatalytic activity is related to the electron transfer efficiency. According to Lindquist’s theoretical models about current-voltage characteristics in photoelectrochemical cells38, it is valid to evaluate the separation efficiency of electrons and holes by the derived current-voltage characteristics. Therefore, the transient photocurrent response of the samples was measured. As shown in Figure 8C, after introducing Cu NPs, the K4Nb6O17 composite microflowers electrode displays higher photocurrent. It is demonstrated that increased electron transfer efficiency is achieved39. Namely, the separation and migration of photoinduced charge carrier are facilitated in the heterostructured K4Nb6O17 composite microflowers. The results in Figure 8 provide the additional reasons for higher photocatalytic activity over the Cu/K4Nb6O17 composite microflowers. The composition,


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structure (Figure S2-Figure S4) and activity of the Cu/K4Nb6O17 composite catalyst are stable. Therefore, the specific decay during the illumination period (Figure 8C) is attributed to the photogenerated charge carriers accumulated before the light turns on40,41. In practical terms, the photogenerated charge carriers were concentrated during the continued light illumination.

Figure 8

Based on the above results, a mechanism for photocatalytic hydrogen production over the Cu/K4Nb6O17 composite microflowers is proposed. According to the literatures reported19,42 and our result (Figure S1), as for the Cu-simulated self-doped K4Nb6O17, the conduction band (CB) position is about −0.21 eV vs NHE and the valence band (VB) position is about +3.23 eV vs NHE. Based on the positions of the CB and VB, as shown in Scheme 1, K4Nb6O17 microflowers were excited to produce electron/hole pairs under UV/UV-vis light irradiation. Then, the photoelectrons were transferred to the CB, during this process, some electrons were captured by Nb5+, and transformed quickly into Nb4+ in the presence of Cu NPs (Figure 6), which makes the separation of electron/hole pairs be more effective, as shown in Figure S1 and Figure 8. Hence, the Cu NPs in the composite microflowers is vital, which act dual roles: (i) acts as the catalyst for the transformation of Nb5+ to Nb4+; and (ii) acts as cocatalyst and active centers for photocatalytic hydrogen evolution. After that, H2O was reduced by the photo-produced electrons transferred to the active centers. The photo-induced holes accumulated on the VB of K4Nb6O17 microflowers were consumed by the sacrificial agent (methanol). Therefore, benefiting from the decoration of Cu NPs, the photo-produced electrons are thermodynamically favored to migrate from the K4Nb6O17 photocatalyst to the Cu NPs, which also provide active sites for adsorbing



protons in the solution. Particularly, strong interfacial contact (Figure 2) between Cu NPs and the K4Nb6O17 microflowers ensures the crucial role in directing the electron transfer flow, remarkably prolonging the lifetime of photo-triggered charge carriers. As a result, the photocatalytic activity for hydrogen evolution is improved.

Scheme 1

Long-term stability is an important factor to evaluate the performance of a photocatalyst. Hydrogen evolution over the recycled 0.4%Cu/K4Nb6O17 composite microflowers is investigated under same conditions. The composite microflowers photocatalyst is recycled for 5 times, each time interval is 6 h, and the reaction system is evacuated before irradiation. The each time course for photocatalytic hydrogen evolution over the 0.4%Cu/K4Nb6O17 is shown in Figure 9. It can be seen that, no noticeable decrease in photocatalytic H2 production rate is observed after being recycled for 5 times, which indicates that the composite microflowers photocatalyst is highly active and stable for hydrogen evolution. To better demonstrate the stability of the catalyst, XRD, SEM and XPS for the used catalyst after photocatalysis were measured. The results of XRD and XPS for the as-prepared catalyst and the used catalyst were almost same (Figure S2Figure S3). Namely, there is little alternation in the composition and the structure of the catalyst before and after the reaction. It is observed that the morphology of the used catalyst is slightly changed (Figure S4). However, it has little effect on the photocatalytic activity (Figure 9). The results indicate that the Cu/K4Nb6O17 composite photocatalyst is stable. About the additional control experiments such as scavengers study, amount of catalyst and volume of optimized scavenger will be reported in our subsequent work.


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Figure 9

CONCLUSIONS A novel potassium niobate-based composite photocatalyst was prepared by facilely modifying the interface of K4Nb6O17 microflowers with Cu NPs. Compared to pure K4Nb6O17 microflowers, the photocatalytic activity for hydrogen evolution over the Cu/K4Nb6O17 microflowers was improved remarkably. It was found that the Cu NPs in the composite microflowers acted as the catalyst for the transformation of Nb5+ to Nb4+, and also acted as cocatalyst and active centers for photocatalytic hydrogen evolution. Under the same conditions, Cu NPs was better than that of Pt supported K4Nb6O17 microflowers. Moreover, the Cu/K4Nb6O17 microflowers showed a stable photocatalytic activity. It is expected that current work might provide a facile strategy for exploring superior photocatalysts to be used in photocatalytic clean energy production. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: DOI: 10.1021/acssuschemeng. AUTHOR INFORMATION Corresponding Author * Xiangqing Li and Sheng Han. E-mail address: [email protected] (Xiangqing Li), [email protected] (Sheng Han).



ACKNOWLEDGMENTs This work was financially supported by the National Natural Science Foundation of China (No. 21771125, 21301118 and 21305092) and Innovation Fund of China Petroleum Science & Technology (No. 2107D-5007-0207). REFERENCES (1) Luo, B.; Song, R.; Jing, D. Significantly Enhanced Photocatalytic Hydrogen Generation over Graphitic Carbon Nitride with Carefully Modified Intralayer Structures. Chem. Eng. J. 2018, 332, 499-507. (2) Zhang, G.G.; Zhang, M. W.; Ye, X. X.; Qiu, X. Q.; Lin, S.; Wang, X. C. Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv. Mater. 2014, 26, 805–809. (3) Wei, Y.; Cheng, G.; Xiong, J.; Xu, F.; Chen, R. Positive Ni(HCO3)2 as a Novel Cocatalyst for Boosting the Photocatalytic Hydrogen Evolution Capability of Mesoporous TiO2 Nanocrystals. ACS Sustainable Chem. Eng. 2017, 5, 5027-5038. (4) Peng, J. W.; Xu, J. L.; Wang, Z. Y.; Ding, Z. X.; Wang, S. B. Developing an Efficient NiCo2S4 Cocatalyst for Improving the Visible Light H2 Evolution, Performance of CdS Nanoparticles. Phys. Chem. Chem. Phys., 2017, 19, 25919-25926. (5) Tian, B.; Li, Z.; Zhen, W.; Lu, G. Uniformly Sized (112) Facet Co2P on Graphene for Highly Effective Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2016, 120, 6409-6415.


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Figure captions

Figure 1. XRD patterns of (a) K4Nb6O17 microflowers, (b) 0.4%Cu/K4Nb6O17 microflowers and (c) 3%Cu/K4Nb6O17 microflowers.

Figure 2. SEM images of (a) the K4Nb6O17 microflowers, (b) the as-prepared Cu/K4Nb6O17 composite microflowers, and TEM images of (c, d) as-prepared Cu/K4Nb6O17 composite microflowers. Inset: the distribution diagram of Cu NPs.

Figure 3. Energy filtered TEM images of the 0.4%Cu/K4Nb6O17 composite microflowers. (a) TEM image; (b) K map; (c) Nb map; (d) O map; (e) Cu map; (f) zero loss map of K, Nb, O and Cu.

Figure 4. (A) XPS spectrum of 0.4%Cu/K4Nb6O17 composite microflowers and (B) high resolution XPS spectrum of Cu 2p.

Figure 5. (A) The time courses for hydrogen evolution and (B) the amount of hydrogen evolved over K4Nb6O17 samples with various loadings under UV-vis light irradiation for 6 h.

Table 1. The comparison of our results with previous work.

Figure 6. (A) High resolution XPS of Nb 3d of K4Nb6O17 microflowers (a) before and (b) after UV-vis irradiation for 6 h, and the inset is the corresponding digital photo before (left) and after


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(right) irradiation. (B) High resolution XPS of Nb 3d of 0.4%Cu/K4Nb6O17 microflowers (a) before and (b) after irradiation for 6 h, and the inset is the corresponding digital photo before (left) and after (right) UV-vis irradiation.

Figure 7. The amount of hydrogen evolved over K4Nb6O17 microflowers and 0.4%Cu/K4Nb6O17 microflowers under various light irradiation for 6 h, respectively.

Figure 8. (A) Fluorescence spectra, (B) Nyquist plots of EIS and (C) photocurrent response. (a) K4Nb6O17 microflowers and (b) 0.4%Cu/K4Nb6O17 composite microflowers. λex = 270 nm. Scanning rate: 50 mV s-1; supporting electrolyte: 0.1 mol L-1 Na2SO4 aqueous solution.

Scheme 1. Schematic illustration for separation and transfer of photo-produced charges in the Cu/K4Nb6O17 microflowers.

Figure 9. Hydrogen evolution over the recycled 0.4%Cu/K4Nb6O17 composite microflowers under UV-vis irradiation.



Figure 1

c

Intensity (a. u.)

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

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b

a PDF#53-0780

10

20

30

40

50

2 Theta (degree)


60

70

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Figure 2



Figure 3


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Page 31 of 39

O 1s

Figure 4

932.5 eV

Intensity (a.u.)

B

Cu 2p3/2

Nb 3p3/2 Nb 3p1/2

Nb C 1s K 2p

Intensity(a.u.)

A

Cu 2p1/2

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


200 400 600 800 1000 1200 Binding energy (eV)

952.4 eV

930 936 942 948 954 960 Binding energy (eV)



12 A 10 8 6

-1 Hydrogen evolution ( mmol g )

Figure 5

-1 H2 evolution ( mmol g )

No loading 0.2 %Cu 0.4 %Cu 0.8%Cu 3 %Cu 0.4 %Pt Withour sacrificial agent

4

0 0

1

2

3 4 Time (h)

5

10 8 6 4 2 0

lo ad in g 0. 4% 0. Pt 2% C 0. u 4% C 0. u 6% 0. Cu 8% Cu 1% Cu 3% Cu

2

12 B

6

No

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


Page 32 of 39

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Table 1 Loading amount of

Amount of hydrogen

(wt.%)

evolved (umol h-1 g-1)

2.0

973.1

Ni/Nb ratio range of 0.2%

144

Catalysts

Ref. 6 Ref. 23

Cu/ZnS microspheres NiO/K4Nb6O17

Light source

300W Xe lamp 500W lamp

halogen

Ref. 11

K4Nb6O17 nanosheets

0

198.3

Ref. 10

RGO/K4Nb6O17 microspheres

5.0

1530

Ref. 24

Pt/KNb3O8

1.0

1930

300W Xe lamp

Ref.19

Cu2S/K4Nb6O17

20.0

2450

300W Xe lamp

Ref. 20

CdS/K4Nb6O17

4.8

1893

500 W Xe lamp

Our result

Cu/K4Nb6O17 microspheres

0.4

1921.7

300W Xe lamp


300W Xe lamp 300W Xe lamp


Figure 6

A Intensity(a.u.)

B

Intensity(a.u.)

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

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b

b

a 202 204 206

a

208 210 212 214 216

Binding energy (eV)

202

204 206

208 210 212 214 216

Binding energy (eV)


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Figure 7

35

H evolution ( mmol g-1) 2

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


30 25

UV-vis light UV light ※ Visible light

20 15 10 5

※

0

※ 0.4% Cu/K4Nb6O17

K4Nb6O17



Figure 8

10000 B

A

a

a

8000

-Z"/ohm

2000

1500

b

6000

b

4000

1000

2000 500 300

350

400

450

0 0

500

500 1000 1500 2000 2500 3000

Wavelength (nm)

Z'/ohm

C Photocurrent (A)

Intensity

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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b a

50

100

150

200

250

300

Time (s)


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Scheme 1



Figure 9

H2 evolution ( mmol g-1)

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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10

5

0 0

6

12

18

24

Time (h)


30

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For Table of Contents Use Only

Novel self-doping potassium niobate composite microflowers with excellent photocatalytic activity for hydrogen evolution were achieved by smart interface modification.


A Novel and Highly Active Potassium Niobate - ACS Publications

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