KNbO3

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12408−12418

pubs.acs.org/journal/ascecg

New Application and Excellent Performance of Ag/KNbO3 Nanocomposite in Photocatalytic NH3 Synthesis Pingxing Xing,† Shijie Wu,† Yijing Chen,‡ Pengfei Chen,‡ Xin Hu,‡ Hongjun Lin,§ Leihong Zhao,*,† and Yiming He*,†,‡ Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, ‡Department of Materials Science and Engineering, and §College of Geography and Environmental Sciences, Zhejiang Normal University, Yingbin Road 688, Jinhua, 321004, China

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S Supporting Information *

ABSTRACT: The present work was designed to synthesize Ag/KNbO3 nanocomposite for efficient photocatalytic conversion of N2 to NH3. KNbO3 was prepared in a low KOH concentration solution at 260 °C, while Ag nanoparticles (NPs) were loaded on KNbO3 by a photodeposition procedure. The as-synthesized nanocomposite presented excellent performance in photocatalytic N2 fixation. Under simulated sunlight, the NH3 generation rate reaches 385.0 μmol·L−1g−1·h−1, which is 4 times higher than that of pure KNbO3. Additionally, the Ag/KNbO3 composite also showed good photoactivity under visible light. Multiple techniques, including XRD, Raman, XPS, SEM, TEM, DRS, PL, EIS, PC, and LSV, were performed to reveal the origin of the high performance. The results indicated that a low KOH concentration is beneficial to formation of nanosize KNbO3, while a high hydrothermal temperature endowed KNbO3 high crystallinity which boosted the bulk charge separation. The Ag NPs’ decoration further increased the surface separation of charge carriers via trapping the electrons from KNbO3. The significantly elevated efficiency in charge separation is believed to be the origin of the high performance of the Ag/KNbO3 composite. Additionally, the surface plasmon resonance effect of Ag NPs also awarded the composite the capability in absorbing visible light, which resulted in its visible-light-driven photocatalytic activity. Interestingly, under simulated sunlight, a competition between the photosensitization mechanism and the electron-trapping mechanism existed, and the latter dominated the photocatalytic process on Ag/KNbO3. Besides the photocatalytic N2 fixation, the Ag/KNbO3 composite also performed well in photocatalytic H2 evolution and RhB degradation, indicating its great potential in practical photocatalytic application. KEYWORDS: KNbO3, Ag nanoparticles, Photocatalytic N2 fixation, H2 evolution

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and no pollution. In 1975, Schrauzer et al.3 first realized the conversion of nitrogen to ammonia under UV light illumination on TiO2 materials. Under the efforts of researchers, new photocatalysts have been gradually developed, and the corresponding catalytic efficiency is gradually improved. Nevertheless, it is still far from meeting the requirements of practical applications. The development of novel efficient materials for photocatalytic N2 fixation and an understanding of the photocatalytic mechanism are still the research front in the field of photocatalysis. The photocatalytic reduction of N2 to NH3 requires the strong reducibility of the photogenerated electrons,4 which means the photocatalyst should have a relative negative conduction band, such as TiO2, ZnO, and g-C3N4. KNbO3 is an ABO3-type metal oxide and shows good photocatalytic activity in H2 production.5 Meanwhile, it has a negative

INTRODUCTION As a clean energy source, hydrogen is recognized to be the most likely alternative to fossil energy. However, there is no economic and effective method for supply of pure hydrogen. Ammonia is also a hydrogen storage fuel, which is easy to liquefy, easy to store and transport, and has the advantage of high energy density. These merits make ammonia a more desirable energy carrier than hydrogen.1 At present, the Haber method is used in industry to synthesize ammonia, which requires high temperature and high pressure, consumes a lot of energy, and produces a large amount of greenhouse gas CO2. It is estimated that industrial ammonia synthesis consumes about 2% of the world’s energy and produces about 1% of global CO2 emissions.2 Therefore, in order to replace the current fossil fuel system with a hydrogen energy system based on ammonia, the real question is how to prepare ammonia cheaply, efficiently, and greenly. For this issue, the recognized synthetic ammonia route is to achieve the conversion from nitrogen to ammonia using renewable energy (solar, wind, etc.). Among them, photocatalytic nitrogen fixation has attracted scientists’ attention because of its low cost © 2019 American Chemical Society

Received: April 7, 2019 Revised: May 29, 2019 Published: June 17, 2019 12408

DOI: 10.1021/acssuschemeng.9b01938 ACS Sustainable Chem. Eng. 2019, 7, 12408−12418

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of KNbO3 prepared at different temperatures and KOH concentrations: (a) KNbO3-10-160, (b) KNbO3-8-260, (c) KNbO3-6260, and (d) KNbO3-4-260.

conduction band (CB) potential of −0.8 eV.6 Therefore, it shows good potential for applications in photocatalytic N2 fixation. However, the drawbacks of a wide band gap and fast charge recombination usually limit the photocatalytic activity, which triggers the development of modification work on KNbO3 materials. To date, some approaches including nanostructure fabrication,7−9 metal ion doping,10−12 nonmetal ion doping,13,14 noble metal loading,15,16 and semiconductor coupling17−21 have been reported. The method of nanostructure construction has attracted much attention because the nanomorphology can endow KNbO3 the advantages of higher surface area, more reactive sites, and fewer chances in charge recombination via decreasing the migration distance of electrons. To date, many works about the preparation of nano-KNbO3 with various morphologies including nanocubes, nanorods, nanotowers, nanowires, and nanoplates have been reported.7−9,22 Among them, Wang’s work is interesting because it provides a feasible way (changing the KOH concentration) to modulate the KNbO3 morphology.13 The low KOH content facilitates formation of nano-KNbO3 having a small particle size. The sample synthesized in 10 mol/L (M) KOH at 200 °C presented the smallest particle size and the highest photocatalytic activity in Rhodamine B (RhB) degradation. Nevertheless, they only investigated the KOH content in the range of 10−30 M. What would happen if the KOH concentration is lower than 10 M? Would the as-synthesized KNbO3 exhibit better photocatalytic performance? This topic is interesting and deserves to be studied. In addition to nanostructure fabrication, surface modification is another efficient way to improve the photocatalytic activity of KNbO3. Due to the high efficiency and ease of operation, noble metal loading is usually applied. For example, Lan et al. anchored

Au nanoparticles on KNbO3 nanowires and studied its photocatalytic activity in RhB degradation under visible light.16 It was found that the sample with 10 nm Au particles presented the best performance. Zhang et al. reported that the deposition of Ag on KNbO3 improved the photoactivity in dye degradation under both UV and visible light irradiation.23 Chassé et al. realized the efficiently photocatalytic conversion of sec-phenethyl alcohol to acetophenone over Au/KNbO3 under visible light irradiation.24 The surface plasmon resonance (SPR) effect of noble metal is normally used to explain the improved photocatalytic activity under visible light. However, it is noted that these noble metals are also often considered as electron trappers to enhance the separation efficiency of electron−hole pairs and the photocatalytic activity.25−28 Thus, does this mechanism also work in the noble metal-loaded KNbO3? Does there exist a competition between the two different roles of noble metal? To our knowledge, this topic has not been investigated thoroughly. To resolve the aforementioned issues, we synthesized KNbO3 nanocubes in a relatively low KOH concentration (4−10 M) by a hydrothermal method. The hydrothermal temperature is raised to 260 °C to overcome the negative effect of the decreased KOH concentration on the solubility of Nb2O5. Ag nanoparticles (NPs) are chosen to further modify the optimal KNbO3, which is mainly due to their relatively low price. The photocatalytic test indicates that the as-synthesized Ag/KNbO3 presents excellent performance in photocatalytic N2 fixation. In order to gain a deeper understanding of the observed photocatalytic performance, the synthesized Ag/KNbO3 nanocomposite is characterized by multiple techniques. A thorough discussion based on the characterization result is also performed. 12409


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EXPERIMENTAL SECTION

particle size as KNbO3-10-160, which may be the reason why the two samples exhibit a similar specific surface area. Figure 2 shows the XRD patterns of KNbO3 prepared at different conditions. All of the synthesized samples present the

Catalysts Preparation. A hydrothermal method was applied to synthesize KNbO3 nanorods obtained in different KOH concentration solutions. The preparation procedure is listed as follows. A 1.222 g (4.6 mmol) amount of Nb2O5 was added to 70 mL of KOH solution and stirred for 1 h. Then the mixture was transferred into a 100 mL parapolyphenylene (PPL) reactor and hydrothermally treated at 160 or 260 °C for 18 h. After the reactor was naturally cooled to room temperature, a white precipitate was separated, washed with water and ethanol, and finally dried at 60 °C for 24 h. The hydrothermal temperature and the KOH were changed to modify the properties of KNbO3. At 160 °C, the KOH concentration was controlled to be 10 mol/L. The obtained KNbO3 is denoted as KNbO3-10-160. At 260 °C, three different KOH (4, 8, and 10 mol/L) contents were applied. The obtained samples are denoted as KNbO3-4-260, KNbO3-8-260, and KNbO3-10-260, respectively. Ag/KNbO3 catalyst was synthesized by the photodeposition method. First, 1.0 g of KNbO3-4-260 sample was finely dispersed in 50 mL of methanol−water (10−40 mL) solution via sonication. Then a certain amount (1, 2, 5, 7.5, and 10 mL) of AgNO3 solution (1 mg/mL) was added. After being bubbled with nitrogen for 30 min to remove oxygen, the suspension was irradiated under a 300 W xenon lamp for 20 min. Finally, the precipitate was separated, washed with water several times, and finally dried at 80 °C for 12 h to obtain the Ag/KNbO3 photocatalyst with different mole ratios of Ag to KNbO3 (0.1%, 0.2%, 0.5%, 0.75%, and 1%). Photocatalytic Experiments. The photocatalytic performance of the Ag/KNbO3 samples was evaluated by photocatalytic N2 fixation, H2 evolution, and RhB degradation. The photocatalytic N2 fixation was performed in a top-irradiation reactor under simulated sunlight and visible light illumination. The catalyst amount is 0.10 g, and 5 vol % ethanol was used as hole trapper. The formed NH3 was detected via Nessler’s reagent method. The photocatalytic H2 evolution was carried out in a glass sealed gas circulation system (Labsolar-IIIAI). Methanol was used as the holes trapper. The produced H2 was analyzed via online gas chromatography. The photocatalytic RhB degradation was performed in a self-build reactor. The RhB concentration is 10 ppm, and 0.1 g of catalyst was used during each test. For all of the photocatalytic tests, a 300 W Xe lamp was used as the light source. For the reaction under visible light and UV light, a UV cutoff filter (λ > 420 nm) and a UV pass-through filter were equipped, respectively. Detailed information about the photocatalytic test and characterization of Ag/ KNbO3 photocatalyst is shown in the Supporting Information.

Figure 2. XRD patterns of KNbO3 prepared at different conditions (a) and their photocatalytic performance in N2 fixation under simulated sunlight (b)

XRD patterns of pure KNbO3 with monoclinic phase (PDF 320822). However, the difference between them can still be observed. The peak intensity of KNbO3 synthesized at 160 °C is lower than that of KNbO3 prepared at 260 °C, indicating its lower crystallinity. Another signal about the higher crystallinity of KNbO3 obtained at 260 °C is that the XRD patterns clearly show the (220) and (002) planes at 44.9° and 45.6°, respectively. In the XRD patterns of KNbO3-10-160 sample, however, only one peak is observed. With the decrease of KOH concentration it can be observed that the splitting of the (220) and (002) planes is weakened. This change can be attributed to the fact that the decreased KOH concentration reduces the particle size of KNbO3, which brings more surface defects and decreases the crystallinity. Even so, the KNbO3-4-260 sample still presents much higher crystallinity than KNbO3-10-160. Actually, we also tried the preparation of KNbO3 in a lower KOH concentration (3 M). However, in addition to KNbO3, Nb2O5 impurity is also detected (Figure S1). Figure 2b demonstrates the performance of these KNbO3 samples in photocatalytic N2 fixation under simulated sunlight irradiation. The blank test suggests that no NH3 is generated without a photocatalyst. In the presence of KNbO3-10-160, NH3 is detected and the concentration of NH4+ is increased linearly with increasing irradiation time. The NH3 generation rate is determined to be 32.5 μmol·L−1g−1·h−1. The KNbO3 samples synthesized at 260 °C present higher photocatalytic activity than KNbO3-10-160, which can be ascribed to their higher crystallinity. Generally, a high crysallinity indicates that the

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RESULTS AND DISCUSSION Effect of Preparation Conditions on the Catalytic Performance of KNbO3. The SEM images of KNbO 3 synthesized at different temperatures and KOH concentrations are shown in Figure 1. It can be seen that KNbO3 synthesized at 160 °C with 10 M KOH presents nanorod morphology. The average size is about 400 × 200 nm, and the specific surface area is 7.66 m2/g. When the hydrothermal temperature is increased to 260 °C, the morphology of the synthesized KNbO3 is changed. The sample prepared in 8 M KOH exhibits the morphology of nanocubes. The size ranges from 0.5 to 1.3 μm. The increased particle size may be ascribed to the fact that the elevated hydrothermal temperature improves the chance of collision of KNbO3 nanocrystals, which favors the increase in the size of KNbO3 particles. Correspondingly, the BET surface area decreases to 3.05 m2/g. Besides the hydrothermal temperature, the KOH concentration is also crucial to the size of KNbO3 particle. As Figure 1 shows, a decreased KOH concentration results in a reduced particle size, which is consistent with Wang’s work.13 The BET surface areas of KNbO3-6-260 and KNbO3-4260 samples are 4.63 and 7.85 m2/g, respectively. The KNbO3 synthesized at 4 M KOH presents a similar morphology and 12410


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ACS Sustainable Chemistry & Engineering sample has strong ability in separating charge carriers, which can result in the high photocatalytic activity. For the samples prepared at 260 °C, the photocatalytic activity is enhanced with decreasing KOH concentration. The KNbO3-4-260 sample exhibits the highest NH3 generation rate of 97.0 μmol·L−1g−1· h−1. The surface area is believed to play another crucial role. The sample with a large surface area would have more active sites to participate in the photocatalytic N2 fixation reaction. Therefore, via modification of the hydrothermal temperature and the KOH concentration, an optimal KNbO3 with high performance in photocatalytic N2 fixation is obtained. Characterizations of Ag/KNbO3 Composites. Since the KNbO3-4-260 photocatalyst presents the best potential in photocatalytic N2 fixation, a further modification of the sample was performed to boost its performance. Just as what has been said in the preface, Ag nanoparticles were loaded on the KNbO3 via a photodeposition method. Figure 3 exhibits the XRD

structure. However, the peak intensity is found to increase with increasing Ag concentration. This phenomenon can be ascribed to the surface plasmon resonance (SPR) effect of metal Ag nanoparticles, which has been reported by Zhang et al.23 Additionally, this result also indirectly proves the loading of Ag nanoparticles on the KNbO3 surface. XPS analysis on the Ag/KNbO3 composite provides direct proof about the existence of metal Ag. As shown in Figure 4d, Ag signal is observed in the XPS spectrum of 0.5% Ag/KNbO3 sample. The binding energies (BE) of Ag 3d5/2 and 3d3/2 peaks are located at 367.9 and 373.9 eV, respectively, which can be assigned to the metal Ag and confirms the loading of Ag nanoparticles on KNbO3 surface.31 Figure 4a−c shows the C 1s, K 2p, Nb 3d and O 1s XPS spectra of KNbO3 and 0.5%Ag/ KNbO3 photocatalysts. The C 1s peak appears at 284.6 eV, which can be assigned to the adventitious carbon.32 The BE of K 2p3/2 and 2p1/2 are close to 291.0 and 293.7 eV, respectively, corresponding to the K+ in KNbO3.33 Via the same way, the valence states of Nb and O are determined to be +5 and −2, respectively, based on previous work.34,35 Although the valence state of K, Nb, and O is the same in the two samples, it can be noted that the BE of Ag/KNbO3 is slightly positive than that of KNbO3. This phenomenon may be ascribed to the higher work function of metal Ag than KNbO3,36 which induces electron transfer from KNbO3 to Ag and decreased electron density on KNbO3. Correspondingly, the Schottky barrier in the Ag− KNbO3 contact region is formed, which can significantly benefit the charge separation in the composite. The morphology and microstructure of Ag/KNbO3 were investigated by SEM and TEM. Figure 5 shows the SEM images of KNbO3 and 0.5%Ag/KNbO3 composite. It can be seen that the Ag loading shows no effect on the appearance of KNbO3. Only KNbO3 nanorods are observed. EDS analysis indicates the existence of Ag. However, due to the weak signal, the concentration of Ag cannot be calculated. Compared with the SEM technique, TEM has higher resolution and thus provides a direct observation on Ag nanoparticles. As shown in Figure 6a− c, black nanoparticles are observed on the KNbO3 nanorods’ surface. The high-resolution TEM image (Figure 6d) indicates that KNbO3 presents a clear lattice fringe of 0.4058 nm, which can be assigned to the (110) face. The nanoparticles show the lattice fringe of 0.2098 nm, corresponding to the (200) plane of metal Ag. Combined with the Raman and XPS analysis, the result in Figure 6 definitely verifies that the metal Ag nanoparticles are closely adhered on the KNbO3 surface. Figure 7 demonstrates the DRS spectra of KNbO3 and Ag/ KNbO3 composite with different Ag content. Pristine KNbO3 can only absorb light with a wavelength lower than 400 nm (Figure 8a) and has a white color (Figure 8b). The band gap is estimated to be 3.13 eV via the K−M method, which is consistent with the previous result.29,30 Ag/KNbO3 sample presents nearly the same absorption threshold as KNbO3, indicating that the introduced Ag does not change the band structure. However, the absorption in the visible region is increased obviously, which can be ascribed to the SPR effect of Ag.16 With the increase of Ag contents, the SPR effect is more obvious. Correspondingly, the color of Ag/KNbO3 is getting darker and darker (Figure 8b). The color change suggests that the Ag/KNbO3 photocatalyst has better capability in absorbing visible light than KNbO3 and may have photocatalytic activity under visible light irradiation. In addition to the photoabsorption performance, the Ag loading also influences the migration and separation of the

Figure 3. XRD patterns (a) and Raman spectra (b) of Ag/KNbO3 composite.

patterns of the synthesized Ag/KNbO3 photocatalysts. All of the Ag/KNbO3 samples present nearly the same XRD patterns, indicating that the photodeposition of Ag nanoparticles shows no effect on the structure of KNbO3. No pattern corresponding to metal Ag is detected, which may be ascribed to the low Ag content. Actually, ICP-AES analysis indicates that the Ag contents in the 0.1% Ag/KNbO3, 0.2% Ag/KNbO3, 0.5% Ag/ KNbO3, 0.75% Ag/KNbO3, and 1% Ag/KNbO3 are determined to be 0.04%, 0.12%, 0.32%, 0.63%, and 0.85%, respectively. Such a low content of Ag is difficult to be detected by XRD. The Raman spectra of Ag/KNbO3 samples are presented in Figure 3b. Neat KNbO3 shows the Raman signals at 199, 284, 532, 602, and 845 cm−1, which originate from the NbO6 octahedral and accords well with the previous result.29,30 The Ag/KNbO3 samples do not show any new Raman peak, confirming that the Ag loading does not change the KNbO3 12411


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Figure 4. XPS spectra of KNbO3 and 0.5% Ag/KNbO3: (a) C 1s and K 2p; (b) Nb 3d; (c) O 1s; (d) Ag 3d.

Figure 5. SEM images of KNbO3 (a), 0.5% Ag/KNbO3 (b), and 1.0% Ag/KNbO3 (c), and EDS result of 0.5% Ag/KNbO3 (d).

than pure KNbO3, indicating the lower interface resistance of the composite.37−39 Thus, the Ag/KNbO3 composite demonstrates a higher migration rate of electrons, which results in the

photogeneration electrons and holes in the KNbO3 photocatalyst. Figure 8a displays the EIS spectra of KNbO3 and 0.5% Ag/KNbO3. Ag-loaded KNbO3 demonstrates a smaller arc size 12412


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Figure 6. TEM (a−c) and HRTEM (d) pictures of 0.5% Ag/KNbO3 composite.

separation efficiency of photoexcited charge carriers.40−42 Figure 8c displays the linear sweep voltammetry (LSV) curves of KNbO3 and 0.5% Ag/KNbO3 composite in 0.5 M Na2SO4 electrolyte. The increased current density of KNbO3 is observed after the photodeposition of Ag, suggesting that Ag NPs rapidly captures electrons from the KNbO3 surface.43,44 An overpotential of −0.23 V (vs Ag/AgCl) was observed for 0.5% Ag/ KNbO3 sample, which is lower than that of KNbO3 (−0.54 V vs Ag/AgCl), confirming the fast electron migration from KNbO3 to the active sites (Ag NPs) to perform the photocatalytic reaction.45 In order to further understand the charge migration between KNbO3 and Ag, the corresponding Tafel plots were constructed (Figure 8d). The 0.5%Ag/KNbO3 sample presents a smaller Tafel slope (144 mV/dec) than KNbO3 (213 mV/ dec), indicating its faster photocatalytic reduction rate.46,47 In other words, the Ag/KNbO3 sample may have exceptional photocatalytic performance due to its lower overpotential and smaller Tafel slope. Photocatalytic Performance of Ag/KNbO3 Composites. The photocatalytic performance of the synthesized Ag/ KNbO3 was evaluated via photocatalytic N2 fixation, H2 generation, and RhB degradation. Figure 9a shows the performance of Ag/KNbO3 composite in photocatalytic N2 fixation under simulated sunlight. It is obvious that the loading of Ag nanoparticles significantly improves the catalytic performance of KNbO3. With the increase of Ag content, the NH3 generation rate enhances first and then reduces. When the Ag content is 0.5%, the Ag/KNbO3 sample presents the highest performance with a NH3 generation rate of 385.0 μmol·L−1g−1· h−1, which reaches about 4 times that of pure KNbO3. Many photocatalysts,48−58 including C-WO3·H2O,49 Zn0.1Sn0.1Cd0.8S,51 MoS2/C-ZnO,56 and C-BiOI,58 have been

Figure 7. DRS spectra of KNbO3 and Ag/KNbO3 composite.

improved separation efficiency of charge carriers. A similar result is also obtained in the PC analysis. As Figure 8b shows, under simulated sunlight irradiation, Ag/KNbO3 sample has much higher photocurrent than pure KNbO3, suggesting its better 12413


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Figure 8. EIS (a), transient photocurrent response (b), LSV curves (c), and Tafel slopes (d) of KNbO3 and 0.5% Ag/KNbO3 samples.

Figure 9. Photocatalytic N2 fixation of Ag/KNbO3 composite under simulated sunlight (a) and visible light (b), and cycle test of 0.5% Ag/KNbO3 (c). Performance of different photocatalysts under simulated sunlight (d).

applied in the photocatalytic N2 fixation (Figure 9d). Compared with these reported samples, it is obvious that the Ag/KNbO3 is an efficient photocatalyst. The cycling test further indicates that the synthesized Ag/KNbO3 composite has high photocatalytic

stability (Figure 9c). After cycling six times, the NH3 generation rate of the 0.5% Ag/KNbO3 still keeps stable. Figure 9b shows the photocatalytic activity of Ag/KNbO3 under visible light irradiation. Pure KNbO3 cannot be excited by visible light. 12414


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ACS Sustainable Chemistry & Engineering Nevertheless, it presents a NH3 generation rate of 4.6 μmol· L−1g−1·h−1, which may be ascribed that some UV light passes through the UV cutoff filter and subsequently drives the photocatalytic N2 fixation over KNbO3. Ag/KNbO3 photocatalysts demonstrate much higher performance. The NH3 generation rate varies with the Ag concentration. A 0.5% Ag/ KNbO3 sample still displays the highest performance with a NH3 generation rate of 95 μmol·L−1g−1·h−1, which is 20 times higher than that of KNbO3. Considering this promotion effect is much higher than that under simulated sunlight, the enhanced performance should be mainly ascribed to the visible light response of the composite. It is clear that the loaded Ag NPs endow KNbO3 the ability in photocatalytic conversion of N2 to NH3 under visible light irradiation. The performance of Ag/KNbO 3 in photocatalytic H2 evolution under simulated sunlight is shown in Figure S2a. Just as what happens in photocatalytic N2 fixation, the loading of Ag nanoparticles significantly improves the photocatalytic H2 evolution of KNbO3, and the 0.5% Ag/KNbO3 is still the best sample. Pure KNbO3 only presents a H2 generation rate of 7.8 μmol·g−1·h−1. For 0.5% Ag/KNbO3 sample, the H2 evolution rate is 138 μmol·g−1·h−1, which is 17 times faster than that of KNbO3. Metal Pt was usually used as a cocatalyst to hasten the photocatalytic H2 production due to its contribution in charge separation. Therefore, the H2-generation performance of 0.5% Ag/KNbO3 sample in the presence of 0.37 wt % metal Pt was also investigated. The result in Figure S2b indicates that the loaded Pt can greatly increase the H2 evolution rate to 2116 μmol·g−1·h−1. Compared with the values of the reported photocatalysts,14,19,30,59−61 such as MoS2/C-KNbO3,19 C/ KTa0.75Nb0.25O3,30 and KTa0.75Nb0.25O3/g-C3N4,59 the performance of Ag/KNbO3 is impressive. A similar result is also obtained in the photocatalytic RhB degradation. As Figure S2c shows, the photocatalytic activity of KNbO3 is greatly enhanced via the loading of Ag nanoparticle. The optimal 0.5% Ag/ KNbO3 sample displays a degradation rate of 0.0174 min−1, which is 6.2 times higher than that of pure KNbO3 (Figure S2d). Obviously, the fabricated Ag/KNbO3 is an efficient photocatalyst for applications in photocatalytic N2 fixation, H2 evolution, and RhB degradation. Explanation. The catalytic test has verified that the synthesized Ag/KNbO3 catalyst presents excellent photocatalytic performance which is obtained via optimization of the preparation condition of KNbO3 and subsequent loading of Ag nanoparticles. For KNbO3 phase, the sample synthesized at 260 °C in 4 M KOH solution presents similar morphology and surface area with KNbO3-10-160, while the crystallinity and the photocatalytic activity are much higher. Thus, it is obvious that the enhanced NH3 generation rate should be mainly ascribed to the increased crystallinity which can decrease the defects concentration and boost the separation efficiency of electron− hole pairs. The loading of Ag nanoparticles is another important procedure to promote the photocatalytic activity of KNbO3. Actually, some noble metal-loaded KNbO3 photocatalysts have been reported.16,23,24 The deposited noble metal can retard the charge recombination or increase the photoabsorption performance and thus hastens the photocatalytic activity. In the current case, the promotion effect of Ag nanoparticles in charge separation and optical property is also observed, which results in the excellent photocatalytic activity of Ag/KNbO3. Nevertheless, the data in Figure 9 indicates that the promotion effect of Ag under simulated sunlight and visible light is about 4 and 20 times, respectively. The varied improvement effect suggests that

the Ag/KNbO3 composite may work in different ways under simulated sunlight and visible light. A thorough discussion on how the Ag NPs hasten the photocatalytic performance of KNbO3 is thus performed. Just as what has been said in the preface, many Ag-loaded semiconductors have been reported.23,25,26 The loaded Ag usually has two kinds of role. One role is the sensitizer.23 The SPR effect makes Ag NPs have the ability to absorb visible light and generate hot electrons which can move to the adjacent semiconductor. Another role is the electron trapper.25,26 Noble Ag has a relatively high work function, which usually leads to formation of a Schottky barrier between Ag and the contacted semiconductor. Thus, Ag can trap the photogenerated electrons from semiconductor to increase the charge separation. Herein, for the photocatalytic reaction occurring under visible light, it is believed that the photosensitization mechanism works in the Ag/KNbO3 composite because KNbO3 has no capability in absorbing visible light. Under visible light irradiation, hot electrons are generated in the metal Ag via the SPR effect and then migrate to the CB of KNbO3 to perform the photocatalytic N2 fixation reaction. For the photocatalytic reaction under simulated sunlight, however, the circumstance becomes complex because both mechanisms may work. Meanwhile, it can be noted that the electron transfer direction is opposite for the two processes. Thus, which one would dominate the photocatalytic reaction over Ag/KNbO3 composite under simulated sunlight irradiation? In order to resolve the question, the photocatalytic N2 fixation of 0.5% Ag/KNbO3 and KNbO3 under UV light was performed, and the result is shown in Figure 10. The photoactivity of the two samples under simulated sunlight (UV−vis) is also added as reference. It can be observed that the NH3 generation rate of KNbO3 under UV light is lower than that under simulated

Figure 10. Performance of KNbO3 (a) and 0.5% Ag/KNbO3 (b) in photocatalytic N2 fixation under different light irradiation. 12415


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Figure 11. Possible mechanisms in Ag/KNbO3 composite under simulated sunlight and visible light.

sunlight, which can be ascribed to the decreased light intensity. As Figure 10b shows, the light intensity decreases from 191 to 162 mW·cm−2 due to the equipped UV pass-through filter. When a UV cutoff filter is equipped, the light intensity presents the lowest value of 88 mW·cm−2. Although the light intensity decreases only about 50%, the photocatalytic ability of KNbO3 under visible light reduces to nearly zero, which is due to its poor visible light response and has been discussed before. For the Ag/ KNbO3 sample, however, a different behavior is observed. It exhibits good performance under visible light due to the photosensitization effect of Ag NPs (Figure 11). Under UV light irradiation, Ag/KNbO3 sample has a much higher NH3 generation rate than that under UV−vis light, although the light intensity is smaller. This result indicates that the electrontrapping mechanism dominates the Ag/KNbO3. The photosensitization procedure also works. Nevertheless, it is harmful to the charge separation because some photoexcited electrons are moved back to the KNbO3 via this process (Figure 11). Under UV light, the photosensitization electron−migration procedure is prevented, which elevates the amount of free electrons and thus leads to the enhanced performance in photocatalytic N2 fixation. The transient photocurrent response of KNbO3 and Ag/ KNbO3 under different light irradiation is carried out to confirm the above explanation. As Figure 12 shows, KNbO3 shows ignorable photocurrent response under visible light due to its wide band gap. Under simulated sunlight, the generated photocurrent is 0.7 μA/cm2, which is higher than that under UV light. For Ag/KNbO3 composite it generates a photocurrent of 1.5 μA/cm2 under visible light, which confirms the existence of the photosensitization effect of Ag NPs. The highest photocurrent is obtained under UV light irradiation, indicating its best efficiency in charge separation.62−67 Meanwhile, it confirms the negative effect of the photosensitization effect. The result in Figure 12 agrees well with the data in Figure 10, confirming the rationality of the proposed mechanism in Figure 11.

Figure 12. Transient photocurrent of KNbO3 (a) and 0.5% Ag/KNbO3 (b) under different light irradiation.

visible light, only the photosensitization mechanism worked. Under simulated sunlight, the electron-trapping mechanism dominated the photocatalytic reaction. The different mechanisms may result in the varied promotion effect of Ag under simulated sunlight and visible light. The cycle test showed that the Ag/KNbO3 demonstrated high stability. Considering the high activity and stability, the Ag/KNbO3 may have potential applications in photocatalytic N2 fixation and H2 production.

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CONCLUSIONS To summarize, Ag NPs-loaded KNbO 3 nanorods were synthesized via the combination of hydrothermal and photodeposition methods. Optimization of the hydrothermal condition improved the bulk charge separation of KNbO3, while the Ag NPs decoration further increases the surface separation of charge carriers. Therefore, the synthesized Ag/ KNbO3 presented high performance in photocatalytic N2 fixation under simulated sunlight and visible light. Additionally, the composite also worked well in photocatalytic H2 evolution and RhB degradation. The detailed investigation indicated that two mechanisms exist in the Ag/KNbO3 composite. Under

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01938. Detailed information about the photocatalytic test, characterization of the synthesized Ag/KNbO3 composites, XRD patterns of KNbO3-4-260 and KNbO3-3-260 photocatalysts, photocatalytic H2 evolution and RhB 12416


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nanowires by Au nanoparticles under ultraviolet and visible-light. Nanoscale 2011, 3, 5161−5167. (17) Choi, J.; Ryu, S. Y.; Balcerski, W.; Lee, T. K.; Hoffmann, M. R. Photocatalytic production of hydrogen on Ni/NiO/KNbO3/CdS nanocomposites using visible light. J. Mater. Chem. 2008, 18, 2371− 2378. (18) Wang, J.; Wang, X.; Cui, Z.; Liu, B.; Cao, M. One-pot synthesis and Nb4N5 surface modification of Nb4+ self-doped KNbO3 nanorods for enhanced visible-light-driven hydrogen production. Phys. Chem. Chem. Phys. 2015, 17, 14185−14192. (19) Yu, J. X.; Chen, Z. Q.; Chen, Q. Q.; Wang, Y.; Lin, H. J.; Hu, X.; Zhao, L. H.; He, Y. M. Giant enhancement of photocatalytic H2 production over KNbO3 photocatalyst obtained via carbon doping and MoS2 decoration. Int. J. Hydrogen Energy 2018, 43, 4347−4354. (20) Yu, J. X.; Chen, Z. Q.; Wang, Y.; Ma, Y. Y.; Feng, Z.; Lin, H. J.; Wu, Y.; Zhao, L. H.; He, Y. M. Synthesis of KNbO3/g-C3N4 composite and its new application in photocatalytic H2 generation under visible light irradiation. J. Mater. Sci. 2018, 53, 7453−7465. (21) Qu, Z. P.; Wang, J.; Tang, J. H.; Shu, X. Q.; Liu, X. D.; Zhang, Z. H.; Wang, J. Carbon quantum dots/KNbO3 hybrid composites with enhanced visible-light driven photocatalytic activity toward dye wastewater degradation and hydrogen production. Mol. Catal. 2018, 445, 1− 11. (22) Sun, S. Y.; Ge, Y. Y.; Zhao, Y. J.; Yuan, X. Y.; Zhao, Y. Z.; Zhou, H. P. Up-conversion luminescence behaviors in Er3+ doped single crystal KNbO3 nanosheets. RSC Adv. 2016, 6, 113038−113044. (23) Zhang, T. T.; Lei, W. Y.; Liu, P.; Rodriguez, J. A.; Yu, J. G.; Qi, Y.; Liu, G.; Liu, M. H. Organic pollutant photodecomposition by Ag/ KNbO3 nanocomposites: A combined experimental and theoretical study. J. Phys. Chem. C 2016, 120, 2777−2786. (24) Chassé, M.; Hallett-Tapley, G. L. Gold nanoparticle-functionalized niobium oxide perovskites as photocatalysts for visible lightinduced aromatic alcohol oxidations. Can. J. Chem. 2018, 96, 664−671. (25) Ge, L.; Han, C.; Liu, J.; Li, Y. Enhanced visible light photocatalytic activity of novel polymeric g-C3N4 loaded with Ag nanoparticles. Appl. Catal. A: Gen. 2011, 409−410, 215−222. (26) Ren, J.; Wang, W. Z.; Sun, S. M.; Zhang, L.; Chang, J. Enhanced photocatalytic activity of Bi2WO6 loaded with Ag nanoparticles under visible light irradiation. Appl. Catal., B 2009, 92, 50−55. (27) Cheng, N. Y.; Tian, J. Q.; Liu, Q.; Ge, C. J.; Qusti, A. H.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. P. Au-nanoparticle-loaded graphitic carbon nitride nanosheets: Green photocatalytic synthesis and application toward the degradation of organic pollutants. ACS Appl. Mater. Interfaces 2013, 5, 6815−6819. (28) Wu, J.-J.; Tseng, C.-H. Photocatalytic properties of nc-Au/ZnO nanorod composites. Appl. Catal. B: Environ. 2006, 66, 51−57. (29) Hu, Y. M.; Gu, H. S.; Zhou, D.; Wang, Z.; Chan, H. L.; Wang, Y. Orientation-control synthesis of KTa0.25Nb0.75O3 nanorods. J. Am. Ceram. Soc. 2010, 93, 609−13. (30) Chen, Z. Q.; Chen, P. F.; Xing, P. X.; Hu, X.; Lin, H. J.; Wu, Y.; Zhao, L. H.; He, Y. M. Novel carbon modified KTa0.75Nb0.25O3 nanocubes with excellent efficiency in photocatalytic H2 evolution. Fuel 2018, 233, 486−496. (31) Cui, M.; Yu, J. X.; Lin, H. J.; Wu, Y.; Zhao, L. H.; He, Y. M. In-situ preparation of Z-scheme AgI/Bi5O7I hybrid and its excellent photocatalytic activity. Appl. Surf. Sci. 2016, 387, 912−920. (32) Hontoria-Lucas, C.; López-Peinado, A. J.; López-González, J. D.; Rojas-Cervantes, M. L.; Martín-Aranda, R. M. Study of oxygencontaining groups in a series of graphite oxides: Physical and chemical characterization. Carbon 1995, 33, 1585−1592. (33) Winiarski, A.; Neumann, T.; Mayer, B.; Borstel, G.; Neumann, M. XPS study of KTaO3, KTa0.69Nb0.31O3 and KNbO3 single crystals. Phys. Status Solidi (B) 1994, 183, 475−480. (34) Lu, C. J.; Kuang, A. X.; Huang, G. Y.; Wang, S. M. XPS study on composition and structure of epitaxial KTa1‑xNbxO3 (KTN) thin films prepared by the sol-gel process. J. Mater. Sci. 1996, 31, 3081−3085. (35) Patnaik, S.; Swain, G.; Parida, K. M. Highly efficient charge transfer through a double Z-scheme mechanism by a Cu-promoted

degradation of Ag/KNbO3 under simulated sunlight (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: +86-0579-82291500; E-mail: zhaoleihong@163. com. *Tel/Fax: +86-0579-82291500; E-mail: [email protected]. ORCID

Xin Hu: 0000-0002-9851-1049 Hongjun Lin: 0000-0002-1960-5208 Yiming He: 0000-0002-1919-0719 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Service, R. F. Liquid sunshine. Science 2018, 361, 120−123. (2) Medford, A. J.; Hatzell, M. C. Photon-driven nitrogen fixation: Current progress, thermodynamic considerations, and future outlook. ACS Catal. 2017, 7, 2624−2643. (3) Schrauzer, G. N.; Guth, T. D. Photolysis of water and photoreduction of nitrogen on titanium dioxide. J. Am. Chem. Soc. 1977, 99, 7189−7193. (4) Chen, X. Z.; Li, N.; Kong, Z. Z.; Ong, W. J.; Zhao, X. J. Photocatalytic fixation of nitrogen to ammonia: State-of-the-art advancement and future prospects. Mater. Horiz. 2018, 5, 9−27. (5) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Hydrothermal synthesis and photocatalytic properties of ATaO3 and ANbO3 (A = Na and K). Int. J. Hydrogen Energy 2007, 32, 2269−2272. (6) Shi, H. F.; Zhang, C. L.; Zhou, C. P.; Chen, G. Q. Conversion of CO2 into renewable fuel over Pt−g-C3N4/KNbO3 composite photocatalyst. RSC Adv. 2015, 5, 93615−93622. (7) Ding, Q.; Yuan, Y.; Xiong, X.; Li, R.; Huang, H.; Li, Z.; Yu, T.; Zou, Z.; Yang, S. Enhanced photocatalytic water splitting properties of KNbO3 nanowires synthesized through hydrothermal method. J. Phys. Chem. C 2008, 112, 18846−18848. (8) Wang, G.; Selbach, S. M.; Yu, Y.; Zhang, X.; Grande, T.; Einarsrud, M. Hydrothermal synthesis and characterization of KNbO3 nanorods. CrystEngComm 2009, 11, 1958−1963. (9) Jiang, L. Q.; Qiu, Y.; Yi, Z. G. Potassium niobate nanostructures: Controllable morphology, growth mechanism, and photocatalytic activity. J. Mater. Chem. A 2013, 1, 2878−2885. (10) Yin, J.; Zou, Z. G.; Ye, J. H. Possible role of lattice dynamics in the photocatalytic activity of BaM1/3N2/3O3 (M= Ni, Zn; N= Nb, Ta). J. Phys. Chem. B 2004, 108, 8888−8893. (11) Yin, J.; Zou, Z. G.; Ye, J. H. A Novel series of the new visible-lightdriven photocatalysts MCo1/3Nb2/3O3 (M = Ca, Sr, and Ba) with special electronic structures. J. Phys. Chem. B 2003, 107, 4936−4941. (12) Yin, J.; Zou, Z. G.; Ye, J. H. Photophysical and photocatalytic properties of MIn0.5Nb0.5O3 (M = Ca, Sr, and Ba). J. Phys. Chem. B 2003, 107, 61−65. (13) Wang, R. W.; Zhu, Y. F.; Qiu, Y. F.; Leung, C. F.; He, J.; Liu, G. J.; Lau, T. C. Synthesis of nitrogen-doped KNbO3 nanocubes with high photocatalytic activity for water splitting and degradation of organic pollutants under visible light. Chem. Eng. J. 2013, 226, 123−130. (14) Yu, J. X.; Chen, Z. Q.; Zeng, L.; Ma, Y. Y.; Feng, Z.; Wu, Y.; Lin, H. J.; Zhao, L. H.; He, Y. M. Synthesis of carbon-doped KNbO3 photocatalyst with excellent performance for photocatalytic hydrogen production. Sol. Energy Mater. Sol. Cells 2018, 179, 45−56. (15) Guo, Y. W.; Li, Y.; Li, S. G.; Zhang, L.; Li, Y.; Wang, J. Enhancement of visible-light photocatalytic activity of Pt supported potassium niobate (Pt-KNbO3) by up-conversion luminescence agent (Er3+:Y3Al5O12) for hydrogen evolution from aqueous methanol solution. Energy 2015, 82, 72−79. (16) Lan, J. Y.; Zhou, X. M.; Liu, G.; Yu, J. G.; Zhang, J. C.; Zhi, L. J.; Nie, G. J. Enhancing photocatalytic activity of one-dimensional KNbO3 12417


Research Article

ACS Sustainable Chemistry & Engineering MoO3/g-C3N4 hybrid nanocomposite with superior electrochemical and photocatalytic performance. Nanoscale 2018, 10, 5950−5964. (36) Chen, F.; Wu, T. H.; Zhou, X. P. The photodegradation of acetone over VOx/MgF2 catalysts. Catal. Commun. 2008, 9, 1698− 1703. (37) Ding, P. H.; Di, J.; Chen, X. L.; Ji, M. X.; Gu, K. Z.; Yin, S.; Liu, G. P.; Zhang, F.; Xia, J. X.; Li, H.M. S. N codoped graphene quantum dots embedded in (BiO)2CO3: Incorporating enzymatic-like catalysis in photocatalysis. ACS Sustainable Chem. Eng. 2018, 6, 10229−10240. (38) Chen, Q. Q.; Zhao, C. R.; Wang, Y.; Chen, Y. J.; Ma, Y. Y.; Chen, Z. Q.; Yu, J. X.; Wu, Y.; He, Y. M. Synthesis of MoS2/YVO4 composite and its high photocatalytic performance in methyl orange degradation and H2 evolution. Sol. Energy 2018, 171, 426−434. (39) Reddy, K. H.; Martha, S.; Parida, K. M. Erratic charge transfer dynamics of Au/ZnTiO3 nanocomposites under UV and visible light irradiation and their related photocatalytic activities. Nanoscale 2018, 10, 18540−18554. (40) Jiang, J.; Zhang, L. Z.; Li, H.; He, W. W.; Yin, J. J. Self-doping and surface plasmon modification induced visible light photocatalysis of BiOCl. Nanoscale 2013, 5, 10573−10581. (41) Huang, H. W.; He, Y.; Du, X.; Chu, P. K.; Zhang, Y. H. A general and facile approach to heterostructured core/shell BiVO4/BiOI p−n junction: Room-temperature in situ assembly and highly boosted visible-light photocatalysis. ACS Sustainable Chem. Eng. 2015, 3 (12), 3262−3273. (42) Jing, L. Q.; Xu, Y. G.; Xie, M.; Liu, J.; Deng, J. J.; Huang, L. Y.; Xu, H.; Li, H. M. Three dimensional polyaniline/MgIn2S4 nanoflower photocatalysts accelerated interfacial charge transfer for the photoreduction of Cr(VI), photodegradation of organic pollution and photocatalytic H2 production. Chem. Eng. J. 2019, 360, 1601−1612. (43) Chen, F.; Yang, H.; Luo, W.; Wang, P.; Yu, H. Selective adsorption of thiocyanate anions on Ag-modified g-C3N4 for enhanced photocatalytic hydrogen evolution. Chin. J. Catal. 2017, 38, 1990− 1998. (44) Yu, H. J.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I. N.; Zhao, Y. F.; Zhou, C.; Wu, L. Z.; Tung, C. H.; Zhang, T. Nitrogen-doped porous carbon nanosheets templated from g-C3N4 as metal-free electrocatalysts for efficient oxygen reduction reaction. Adv. Mater. 2016, 28, 5080−5086. (45) Hao, X. Q.; Zhou, J.; Cui, Z. W.; Wang, Y. C.; Wang, Y.; Zou, Z. G. Zn-vacancy mediated electron-hole separation in ZnS/g-C3N4 heterojunction for efficient visible-light photocatalytic hydrogen production. Appl. Catal., B 2018, 229, 41−51. (46) Wang, H. F.; Tang, C.; Wang, B.; Li, B. Q.; Zhang, Q. Bifunctional transition metal hydroxysulfides: Room-temperature sulfurization and their applications in Zn-air batteries. Adv. Mater. 2017, 29, 1702327. (47) Tomon, C.; Sarawutanukul, S.; Duangdangchote, S.; Krittayavathananon, A.; Sawangphruk, M. Photoactive Zn−air batteries using spinel-type cobalt oxide as a bifunctional photocatalyst at the air cathode. Chem. Commun. 2019, 55, 5855−5858. (48) Sun, S.; An, Q.; Wang, W.; Zhang, L.; Liu, J.; Goddard, W. A., Iii Efficient photocatalytic reduction of dinitrogen to ammonia on bismuth monoxide quantum dots. J. Mater. Chem. A 2017, 5, 201−209. (49) Li, X.; Wang, W.; Jiang, D.; Sun, S.; Zhang, L.; Sun, X. Efficient solar−driven nitrogen fixation over carbon-tungstic-acid hybrids. Chem. - Eur. J. 2016, 22, 13819−13822. (50) Li, H.; Shang, J.; Shi, J.; Zhao, K.; Zhang, L. Facet-dependent solar ammonia synthesis of BiOCl nanosheets via a proton-assisted electron transfer pathway. Nanoscale 2016, 8, 1986−1993. (51) Hu, S.; Chen, X.; Li, Q.; Zhao, Y.; Mao, W. Effect of sulfur vacancies on the nitrogen photofixation performance of ternary metal sulfide photocatalysts. Catal. Sci. Technol. 2016, 6, 5884−5890. (52) Cao, S.; Zhou, N.; Gao, F.; Chen, H.; Jiang, F. All-solid-state Zscheme 3,4-dihydroxybenzaldehyde- functionalized Ga2O3/graphitic carbon nitride photocatalyst with aromatic rings as electron mediators for visible-light photocatalytic nitrogen fixation. Appl. Catal., B 2017, 218, 600−610.

(53) Dong, G.; Ho, W.; Wang, C. Selective photocatalytic N2 fixation dependent on g-C3N4 induced by nitrogen vacancies. J. Mater. Chem. A 2015, 3, 23435−23441. (54) Bai, Y.; Ye, L.; Chen, T.; Wang, L.; Shi, X.; Zhang, X.; Chen, D. Facet-dependent photocatalytic N2 gixation of bismuth-rich Bi5O7I nanosheets. ACS Appl. Mater. Interfaces 2016, 8, 27661−27668. (55) Ye, L. Q.; Han, C. Q.; Ma, Z. Y.; Leng, Y. M.; Li, J.; Ji, X. X.; Bi, D. Q.; Xie, H. Q.; Huang, Z. X. Ni2P loading on Cd0.5Zn0.5S solid solution for exceptional photocatalytic nitrogen fixation under visible light. Chem. Eng. J. 2017, 307, 311−318. (56) Xing, P. X.; Chen, P. F.; Chen, Z. Q.; Hu, X.; Lin, H. J.; Wu, Y.; Zhao, L. H.; He, Y. M. Novel ternary MoS2/C-ZnO composite with efficient performance in photocatalytic NH3 synthesis under simulated sunlight. ACS Sustainable Chem. Eng. 2018, 6, 14866−14879. (57) Wang, Y.; Wei, W.; Li, M.; Hu, S.; Zhang, J.; Feng, R. In situ construction of Z-scheme g-C3N4/Mg1.1Al0.3Fe0.2O1.7 nanorod heterostructures with high N2 photofixation ability under visible light. RSC Adv. 2017, 7, 18099−18107. (58) Zeng, L.; Zhe, F.; Wang, Y.; Zhang, Q. L.; Zhao, X. Y.; Hu, X.; Wu, Y.; He, Y. M. Preparation of interstitial carbon doped BiOI for enhanced performance in photocatalytic nitrogen fixation and methyl orange degradation. J. Colloid Interface Sci. 2019, 539, 563−574. (59) Chen, Z. Q.; Chen, P. F.; Xing, P. X.; Hu, X.; Lin, H. J.; Zhao, L. H.; Wu, Y.; He, Y. M. Rapid fabrication of KTa0.75Nb0.25/g-C3N4 composite via microwave heating for efficient photocatalytic H2 evolution. Fuel 2019, 241, 1−11. (60) Chen, P. F.; Xing, P. X.; Chen, Z. Q.; Lin, H. J.; He, Y. M. Rapid and energy-efficient preparation of boron doped g-C3N4 with excellent performance in photocatalytic H2 evolution. Int. J. Hydrogen Energy 2018, 43, 19984−19989. (61) Chen, Z. Q.; Xing, P. X.; Chen, P. F.; Chen, Q. Q.; Wang, Y.; Yu, J. X.; He, Y. M. Synthesis of carbon doped KTaO3 and its enhanced performance in photocatalytic H2-generation. Catal. Commun. 2018, 109, 6−9. (62) Liu, X. Z.; Jiang, X. L.; Chen, Z. Q.; Yu, J. X.; He, Y. M. Preparation of Bi3O4Br/BiOCl composite via ion-etching method and its excellent photocatalytic activity. Mater. Lett. 2018, 210, 194−198. (63) Qian, J. J.; Yuan, A. L.; Yao, C. K.; Liu, J. Y.; Li, B. X.; Xi, F. N.; Dong, X. P. Highly efficient photo-reduction of p-nitrophenol by protonated graphitic carbon nitride nanosheets. ChemCatChem 2018, 10, 4747. (64) Han, X. X.; Yuan, A. L.; Yao, C. K.; Xi, F. N.; Liu, J. Y.; Dong, X. P. Synergistic effects of phosphorous/sulfur co-doping and morphological regulation for enhanced photocatalytic performance of graphitic carbon nitride nanosheets. J. Mater. Sci. 2019, 54, 1593−1605. (65) Qian, J. J.; Yan, J.; Shen, C.; Xi, F. N.; Dong, X. P.; Liu, J. Y. Graphene quantum dots-assisted exfoliation of graphitic carbon nitride to prepare metal-free zero-dimensional/two-dimensional composite photocatalysts. J. Mater. Sci. 2018, 53, 12103−12114. (66) Qian, J. J.; Shen, C.; Yan, J.; Xi, F. N.; Dong, X. P.; Liu, J. Y. Tailoring the electronic properties of graphene quantum dots by P doping and their enhanced performance in metal-free composite photocatalyst. J. Phys. Chem. C 2018, 122, 349−358. (67) Han, X. X.; Yao, C. K.; Yuan, A. L.; Xi, F. N.; Dong, X. P.; Liu, J. Y. Enhanced charge separation ability and visible light photocatalytic performance of graphitic carbon nitride by binary S, B co-doping. Mater. Res. Bull. 2018, 107, 477−483.

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