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AgBr heterojunction: The relevance of phase and electronic structure
Journal of Molecular Catalysis A: Chemical 426 (2017) 52–59
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Improved photocatalytic activity and durability of AgTaO3 /AgBr heterojunction: The relevance of phase and electronic structure Fang Wang 1 , Tingting Wang 1 , Junyu Lang, Yiguo Su ∗ , Xiaojing Wang ∗ College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, PR China
a r t i c l e
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Article history: Received 12 August 2016 Received in revised form 27 October 2016 Accepted 1 November 2016 Available online 3 November 2016 Keywords: Photocatalysis AgTaO3 Heterojunction Plasmonic enhancement Phase conversion
a b s t r a c t AgTaO3 /AgBr heterojunction was constructed for visible light driven photocatalytic purpose in order to investigate the relevance of phase conversion, electronic structure and photocatalytic properties. The result indicated that AgBr grafted on AgTaO3 to form AgTaO3 /AgBr heterojunction gave intense visible light absorption, which exhibits highly enhanced photocatalytic performance than their individual counterpart. Theoretical and experimental investigation showed that the matched electronic structure between AgTaO3 and AgBr induced an efficient transfer of photogenerated electrons from AgBr to AgTaO3 , leading to efficient charge separation and the subsequent improved photocatalytic activity. Partial AgBr converted to AgBr/Ag during the photocatalytic process, leading to the construction of ternary AgTaO3 /AgBr/Ag photocatalyst. Because of the surface plasmon resonance effect of Ag, the resulting AgTaO3 /AgBr/Ag exhibited wide range absorption and improved charge separation efficiency, which showed high durability and superior photocatalytic activity toward methyl orange degradation. On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated electrons, O2 −• , and OH• active species dominate the photodegradation of methyl orange. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis based on semiconductors harnesses considerable research attentions due to its potential applications in environmental remediation, hydrogen production by water splitting and chemical conversion for valuable products [1–5]. Till now, a variety of semiconductors for efficient photocatalytic purpose have been fully investigated. Among them, tantalate-based semiconductors are considered to be excellent photocatalysts due to their strong driving force for photoredox reduction, high chemical and physical stability, and superior photocatalytic activity [6–10]. However, stronger photoredox driving force is often accompanied with a wider band gap that considerably limits visible light absorption. In this regard, the improvement of solar light absorption of tantalate-based semiconductors for efficient visible light driven photocatalytic performance is still necessary. Numerous methodologies have been adopted to tailor the electronic structure for improving the visible light response of
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Su), wang xiao [email protected] (X. Wang). 1 Fang Wang and Tingting Wang are co-first authors. They contributed equally to this work. http://dx.doi.org/10.1016/j.molcata.2016.11.001 1381-1169/© 2016 Elsevier B.V. All rights reserved.
tantalate-based semiconductors by metal and/or nonmetal doping, heterojunction and surface/interface engineering [11–14]. In particular, the heterojunction a tantalate-based semiconductor with narrow band gap semiconductors can effectively improve the photocatalytic activity compared with the single component counterpart [15]. The enhancement of the photocatalytic activity is thought to be attributed to the efficient separation of photogenerated charge carriers as a consequence of the matching electronic structure such as band gap energy and the potential energy levels of valence band and conduction band. Recently, the enhanced visible light driven photocatalytic activity has been achieved in several heterojunction photocatalysts, including gC3 N4 /NaTaO3 , TaON/Bi2 O3 , carbon-Ta2 O5 , Sr2 Ta2 O7-x Nx /graphene and so on [16–19], in which tantalate-based semiconductors play critical roles. In order to develop tantalate-based heterojunction systems, it is still important to explore the phase, electronic structure and the underlying mechanism for efficient visible light absorption and photocatalytic activity. Silver tantalate (AgTaO3 ) exhibits distorted pseudocubic structure with a perovskite structure has started to obtain surged research attention due to its promising applications as solid lubricant, dielectric material and photocatalyst [20]. As previously reported, the band gap of AgTaO3 is determined to be about 3.4 eV [21], which is only reactive under ultraviolet (UV) light irradiation,
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limiting its solar energy applications. A systematic experimental identification of AgTaO3 and its relevant heterojunctions by modulating phase structure, chemical composition as well as interfacial contact is advantageous for tailoring the photophysical and photochemical properties of AgTaO3 as well as other isostructural semiconductors, such as NaTaO3 , AgNbO3 , CuTaO3 and so on. Most recently, much research attention has been focused on silver halide AgX (X = Cl, Br, I) because of their high photocatalytic performance toward the removal of deleterious contaminant [22–24]. Unfortunately, silver halide often suffers from chemical instability under visible light irradiation, being of a hindrance to its potential applications [25]. Recent studies suggests that heterojunction of AgBr can with other semiconductors can effectively improve the photocatalytic activity and stability of AgBr [26–28] Herein, with respect to the above-mentioned problems, we purposed to develop AgTaO3 /AgBr composite for efficient and renewable visible light photocatalyst by modulating the phase structure, chemical composition as well as the electronic structure, which is helpful to identify the underlying mechanism of the heterojunctions. 2. Experimental section 2.1. Procedure for the synthesis of the AgTaO3 /AgBr composite photocatalysts 2.1.1. Synthesis of AgTaO3 AgTaO3 sample was synthesized by a solid state reaction. Ta2 O5 was mixed with excess amounts (2–5 mol%) of Ag2 O in an agate mortar. The mixture was calcined in an alumina crucible at 1050 ◦ C for 5 h. The obtained sample was treated with certain amount of nitric acid to remove the excess silver and washed with distilled water for several times, and then dried in an oven at 80 ◦ C for 10 h. 2.1.2. Synthesis of AgBr Pure AgBr was prepared via a solution method. Briefly, AgNO3 and NaBr solutions with Ag/Br molar ratio of 1:1 were mixed to form a yellow suspension. After aged for 2 h, the suspension was filtered and washed several times using distilled water, then dried at 80 ◦ C for 10 h. 2.1.3. Synthesis of AgBr/AgTaO3 All the following processes were carried out in a dark situation. Briefly, 0.6 of AgTaO3 was added to 60 mL of distilled water to form a white suspension. The suspension was sonicated for 30 min at room temperature. 1.3 g of KBr was added to the above suspension with magnetically stirring for 2 h. Then, 0.6 g poly(ethyleneglycol)block-poly(ethylene glycol) (P123) was added to the suspension and stirred for 1 h. Subsequently, given amount of AgNO3 dissolved in 1.8 mL of NH3 ·H2 O (containing 25 wt% of NH3 ) was added to the above mixture. The initial weight ratio of AgBr/(AgTaO3 + AgBr) were modulated to be 30 wt%, 50 wt%, 70 wt%, 90 wt%, and defined as AA-30, AA-50, AA-70, AA-90, respectively. The resulting suspensions were stirred at room temperature for 12 h. The as-resulted products were filtered, washed and dried at 80 ◦ C over night.
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with a monochromatic Al K␣ and charge neutralizer. The surface photovoltage measurement system is constructed by a sample chamber, a lock-in amplifier with a light chopper and a source of monochromatic light (provided by a 500 W xenon lamp) and a monochromator. The monochromator and the lock-in amplifier were equipped with a computer. The analyzed product is assembled as a sandwich-like structure of ITO-product-ITO (ITO is an indium tin oxide electrode). The presence of hydroxyl radicals and superoxide radicals was measured by EPR spectra, which was performed on an ER200-SRC electron spin resonance spectrometer (Bruker, Germany) at 3186 G and 9056.895 MHz. 2.3. Photocatalytic activity tests The photocatalytic activity of the as-prepared samples was evaluated by using methyl orange as the probe molecule under visible light irradiation at room temperature. In brief, 50 mg of the sample was placed in a 100 mL beaker containing 50 mL 2 × 10−5 M methyl orange solution to form a suspension. Before illumination, the above suspension was stirred in dark in order to establish the absorption–desorption equilibrium of methyl orange on sample surfaces. Then, the suspension was irradiated by a 300 W Hg lamp using a filter ( ≥ 420 nm) as a cutoff, which is about 10 cm away from the beaker. Then, 5 mL of the suspension was extracted at given intervals and centrifuged at a rate of 8000 rpm for 10 min. UV–vis absorption spectra of the supernatant was measured using a Lambda 750 s Spectrometer. 3. Results and discussion XRD technique was adopted to investigate the crystallographic structure of the as-prepared samples. Fig. 1 shows the XRD patterns of AgTaO3 , AgBr and their heterojunctions. As for AgTaO3 , all diffraction peaks can be well indexed to perovskite-type phase of AgTaO3 (JCPDS card no. 72-1383). No trace of additional diffraction peaks was observed, suggesting a single phase perovskite structure is formed, being similar to Xu’s results [29]. By using the Retica Rietveld program based on a least-squares method, the lattice parameter of AgTaO3 was estimated to be a = 5.523 Å, and c = 13.623 Å, being close to the standard data of AgTaO3 . The narrowed diffraction peaks suggest high crystallinity and fine nature of AgTaO3 . Besides AgTaO3 , the XRD pattern of AgBr is also given for comparison. As shown in Fig. 1, all narrowed diffraction peaks of AgBr can be ascribed to the typical cubic structure, which is identical to the standard data of AgBr (JCPDS card no. 06-0438). As AgTaO3
2.2. Characterization of photocatalysts X-ray power diffraction (XRD) was used to characterize the phase structure of the as-prepared samples on a Rigaku DMAX2500 X-ray diffractometer using a copper target. Transmission electron microscopy (TEM) was performed on a JEM-2010 apparatus with an acceleration voltage of 200 kV. Energy-dispersive spectroscopy (EDX) data were obtained using scanning electron microscopy (SEM) on a S-4800 apparatus working at 10 kV. Optical diffuse reflectance spectrum was measured using a Lambda 750 s spectrometer. XPS analyses were performed on the ESCALab220i-XL
Fig 1. XRD patterns of the as-prepared samples. Vertical bars represent the standard diffraction data from the JCPDS files for AgTaO3 (72–1398, black line), AgBr (06-0438, red line) and Ag (87-0597, blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. TEM (a) and HRTEM (b) images of fresh AgTaO3 /AgBr sample. TEM (c) and HRTEM (d) images of used AgTaO3 /AgBr sample.
was coupled with AgBr (AA-70 sample), the main characteristic diffraction peaks of AgTaO3 and AgBr showed no obvious variation. This result suggested the co-existence of AgTaO3 and AgBr in AgTaO3 /AgBr heterojunction. On the other hand, the purity of the as-prepared AgTaO3 , AgBr, and AgTaO3 /AgBr samples was also confirmed by EDX technique (Fig. S1), being in accordance with the XRD results. Moreover, it is noted that, after the photocatalytic activity test, additional weak diffraction peaks were observed in AgTaO3 /AgBr heterojunction. The additional diffraction peak at about 2 = 38.11◦ corresponds to the (111) plane of cubic Ag (JCPDS card no. 87-0597), which can be confirmed by transmission electron microscopy (TEM) observations, indicating the formation of the ternary AgTaO3 /AgBr/Ag heterostructure. Similar results have been reported in many AgBr based photocatalytic systems due to the photo-instability of AgBr nanoparticles [25,30]. SEM and TEM measurements were used to explore the morphologies and crystal structures of the as-prepared samples. SEM images and the corresponding EDX mapping results (Fig. S2 and S3) indicated the formation of AgTaO3 /AgBr heterojunction structure, which was further confirmed by TEM observation. As shown in Fig. 2a, for fresh AgTaO3 /AgBr heterojunction, two kinds of materials were observed in the AA-70 sample, which correspond to AgTaO3 and AgBr, respectively. HRTEM observation further confirmed the heterojunction feature of AgTaO3 /AgBr. As illustrated in Fig. 2b, the lattice plane space of the as-prepared sample was estimated to be 0.278 nm, which is similar to 0.279 nm of (104) plane for AgTaO3 . On the other hand, another lattice space of 0.205 nm is compatible with 0.204 nm of (220) plane for AgBr. As for the
used AgTaO3 /AgBr sample, it is seen that Ag nanoparticles were observed (Fig. 2c), which is identical to the XRD results. To further specify the presence of Ag nanoparticles in the AgTaO3 /AgBr heterojunction, HRTEM image is illustrated in Fig. 2d. From Fig. 2d, It is seen that the spacing between the adjacent lattice fringes was 0.236 nm, which is close to 0.237 nm for the (111) plane of Ag (JCPDS card no. 87-0597), suggesting the crystalline feature of Ag nanoparticles. Meanwhile, another space of lattice plane was determined to be 0.333 nm, which is identical to that of the (111) plane for the standard data of AgBr. This observation suggests AgTaO3 /AgBr heterojunction was in-situ converted to the ternary AgTaO3 /AgBr/Ag heterojunction during the photocatalytic process, where Ag nanoparticles anchored on the AgBr surfaces. XPS technique was further used to specify the chemical composition and charge state of AgTaO3 /AgBr heterojunction. The XPS survey spectra of fresh (AA-70 sample) and used AgTaO3 /AgBr heterojunctions is shown in Fig. 3a. From Fig. 3a, it is seen that all peaks in Fig. 3a can be recognized to Ag, Ta, Br, O, and C elements for both fresh and used AgTaO3 /AgBr heterojunctions. Fig. 3b–d shows the high-resolution scans of Ta 4f, O 1s, and Ag 3d. In Fig. 3b, the binding energies at 25.10 eV and 27.01 eV for both fresh and used AgTaO3 /AgBr heterojunctions, which can be recognized to the Ta 4f7/5 and Ta 4f5/2 of Ta5+ , respectively, being close to previous results [31]. As shown in Fig. 3c for O 1s XPS peak, it is seen that, for both fresh and used AgTaO3 /AgBr heterojunctions, the O 1 s signal can be fitted by three Lorentzian-Gaussian lines, which suggests that there exists three kinds of surface oxygen species in both samples. The XPS peak at ∼529.71 eV is related to the lattice
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Fig. 3. XPS spectra of fresh and used AgTaO3 /AgBr (AA-70 sample) heterojunction: whole scanning spectra (a) and the high-resolution regional spectra of Ta 4f (b), O 1 s (c), and Ag 3d (d). Fig. 4. UV–vis diffuse reflectance spectra of the as-prepared samples (a) and the optical band gap of bare AgTaO3 and AgBr (b).
oxygen atoms (Ob ). The XPS peak at ∼530.69 eV may belong to surface hydroxyl groups (Os ). And the binding energy at ∼532.11 eV is thought to be ascribed to the surface chemisorbed O2 or surface carboxylic group [32], which is originated from the surface defects including oxygen vacancies (Od ) [33,34]. It is seen that the Od content in used AgTaO3 /AgBr heterojunction is higher than that in fresh AgTaO3 /AgBr heterojunction, which may expect high photocatalytic performance. Fig. 3d illustrates Ag 3d XPS signal of both fresh and used AgTaO3 /AgBr heterojunctions. For fresh AgTaO3 /AgBr heterojunction, two strong XPS signals appeared at 367.30 eV and 373.31 eV is ascribed to Ag 3d5/2 and Ag 3d3/2 for ionic silver (Ag+ ), respectively [35]. However, for used AgTaO3 /AgBr heterojunction, two additional weak peaks located at about 367.95 eV and 374.03 eV were observed, which should be attributed to Ag 3d5/2 and Ag 3d3/2 of metallic silver (Ag0 ) [36]. This result implied that partial Ag+ ions in AgTaO3 /AgBr heterojunctions were reduced to metallic Ag nanoparticles to form the ternary AgTaO3 /AgBr/Ag heterojunctions during the photocatalytic process. Basically, the electronic structure and energy level of a semiconductor usually plays important roles in the modulation of their photocatalytic activity. Fig. 4a illustrates the UV–vis diffuse reflectance spectra of the samples. As shown in Fig. 4a, bare AgTaO3 showed intense UV absorption from 200 nm to 380 nm, which is assigned to the typical electronic transitions from Ag 4d + O 2p orbitals to Ta 5d orbitals [37]. For AgBr, the absorption edge appears at about 475 nm, indicating that AgBr can absorb solar light extending to 475 nm. Fresh AgTaO3 /AgBr exhibited a mixed absorption feature of both AgTaO3 and AgBr, indicating AgTaO3 /AgBr heterojunction should have visible light photocatalytic activity. Meanwhile, all as-prepared AgTaO3 /AgBr samples with different weight ratios showed visible light absorption (Fig. S4). Interestingly, it is noted that the used AgTaO3 /AgBr gave an obvious intense visible light absorption with the wavelength extending 800 nm. This absorption is attributed to the surface plasmon resonance effect (SPR) of
Ag nanoparticles [38]. The band gap energy of AgTaO3 and AgBr was also determined by the following equation
˛h = A h − Eg
n/2
where ␣, , n, A and Eg are the absorption coefficient, incident light frequency, an integer, constant and band gap, respectively. The value of n for AgTaO3 is 1 because AgTaO3 exhibits a direct optical transition type (Determined by the following DFT results). Whereas, the value of n for AgBr is 4 due to an indirect optical transition type of AgBr [39]. Finally, the electronic band gap values of AgTaO3 and AgBr were determined to be 3.40 eV and 2.52 eV, respectively. Since AgBr grafted on AgTaO3 can lead to the absorption in visible light range, it is expected that AgTaO3 /AgBr may exhibit visible light photocatalytic activity. The photocatalytic activity of AgTaO3 /AgBr heterojunctions was evaluated using methyl orange degradation as the model reaction. Preliminary studies indicated that AA-70 sample exhibited optimal photocatalytic activity (Fig. S5). Concentration changes of methyl orange were examined by using the variations in maximal adsorption at about 464 nm in the UV–vis absorption spectra. As shown in Fig. 5a, methyl orange concentration gradually decreases with an increase of visible light irradiation time. Fig. 5b shows the photocatalytic activity for all as-prepared samples. For comparison, the photodegradation of methyl orange in the absence of catalyst was also given in Fig. 5b. This observation shows that methyl orange solution is stable under visible light irradiation without any catalyst. It is clear that AgTaO3 showed no apparent photocatalytic activity toward methyl orange degradation under visible light irradiation. After the heterojunction of AgBr on AgTaO3 , the photocatalytic activity of AgTaO3 /AgBr greatly enhanced in comparison to their individual counterpart. This result may be ascribed to the enhanced interfacial charge separation efficiency between
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Fig. 5. UV–vis spectral changes of methyl orange in aqueous AgTaO3 /AgBr (AA-70 sample) dispersion as a function of visible light irradiation time (a), normalized concentration of methyl orange versus visible light irradiation time in the presence of various photocatalysts and cyclic degradation curve for AgTaO3 /AgBr (AA-70 sample).
AgTaO3 and AgBr because the physical mixture of AgTaO3 and AgBr showed much lower photocatalytic activity than that of AgTaO3 /AgBr heterojunction. Since AgBr is highly sensitive to visible light, the AgTaO3 /AgBr heterojunction could in-situ convert to the ternary AgTaO3 /AgBr/Ag heterojunction. This conversion has been observed in many AgBr-based heterojunction photocatalysts [40,41]. Then, it is necessary to investigate the photocatalytic activity of AgTaO3 /AgBr/Ag. Interestingly, as shown in Fig. 5b, the photocatalytic activity of AgTaO3 /AgBr/Ag is compatible with that of AgTaO3 /AgBr and more active than AgBr/Ag. Basically, heterojunction photocatalysts can enhance the photocatalytic performance due to the variation of electronic energy levels that can promote the charge separation between the semiconductors. On the basis of previous literatures, the conduction band edge potential of AgBr is determined to be −1.04 V versus NHE [42,43], whereas the conduction band edge potential of AgTaO3 locates at about −0.9 V versus NHE [37]. This result gives an evidence that the photogenerated electrons may migrate from the conduction band of AgBr to the conduction band of AgTaO3 , thus enhancing the charge separation efficiency. This conception can be further verified by density functional theory (DFT) results. On the basis of TEM results and previous literatures of the AgTaO3 /AgBr heterojunction interfaces, a theoretical model of the interface, corresponding to the AgTaO3 (104) surface interfacing with stable (AgBr)6 cluster [44], was used (Fig. 6). Due to large lattice mismatch between AgTaO3 and AgBr, the AgTaO3 /AgBr interface is not stable and structural optimization and the convergence is inaccessible. Consequently, a cluster/surface model was adopted to reflect the interaction between AgBr and AgTaO3 [45]. After geometry optimizations, the AgTaO3 (104) surface and AgBr show obvious distortion, indicating strong interfacial interaction of AgBr and AgTaO3 occurs. The atomic population analysis indicates that a net charge of ∼−1.69 e on AgTaO3 is observed, and meanwhile, a net charge of ∼1.68 e on AgBr occurs (Table S1). This observation clearly suggests that the electrons prefers to
Fig. 6. Models for simulating the heterojunction of AgTaO3 /AgBr before geometry optimization (a and b) and after geometry optimization (c and d). The green, blue, red, and brown represent silver, tantalum, oxygen, and bromine, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
migrate from the conduction band of AgBr to the conduction band of AgTaO3 [46]. In order to shed light on the mechanism of the interaction of the AgBr/AgTaO3 interface, the interface electronic structures was investigated by partial density of states (DOS) data. For AgTaO3 (104) surface, the valence band maximum consists of the O 2p and Ag 4d states, and the conduction band minimum mainly originates from the hybridization of Ta 5d and O 2p orbitals. And the band gap energy is determined to be ∼2.47 eV, which is smaller than the experimental data. As for (AgBr)6 cluster, various hybridized orbitals are observed through molecular orbital analysis. The calculated HOMO consists of Ag 4d, Ag 5 s and Br 4p orbitals, whereas the LUMO is mainly related to the hybridization of Ag 5 s and Br 4p orbitals. Meanwhile, the energy gap was also calculated to be ∼2.81 eV. Moreover, the DOS of AgBr/AgTaO3 is also given in Fig. 7. From Fig. 7, it is clearly seen that there exists a significant modification of the conduction band edge. While no obvious variation of the valence band of AgTaO3 is observed via (AgBr)6 cluster modification. In particular, the states extend downwards in the energy of the conduction band edge, which leads to the energy gap of ∼1.08 eV, being attributed to the hybridization of the LUMO of (AgBr)6 cluster. From this aspect, it is clear that photogenerated electrons would readily transfer from the conduction band of AgBr to that of AgTaO3 and therefore enhance the photocatalytic activity. To explain the notable enhancement in charge separation efficiency of the heterojunction from the theoretical calculations, we comparatively investigated the transfer properties of photogenerated charges at the surface or interface of AgTaO3 , AgBr and their relevant heterojunctions by surface photovoltage (SPV) technique. Since only the charge generation and separation process under light irradiation can produce the SPV signal, the intensity of SPV signal is closely related to the charge separation efficiency. Basically, a higher SPV signal intensity usually means a higher charge separation efficiency [47]. As shown in Fig. 8a, pristine AgTaO3 and AgBr samples, the SPV signal appeared in the wavelength range of 300–380 nm and 300–470 nm, respectively, which were related to the typical band to band electronic transition, being in accordance with the UV–vis spectra. No obvious SPV response
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Fig. 7. Density of states (DOS) of AgTaO3 (104) surface, (AgBr)6 cluster and AgTaO3 /AgBr heterojunction.
for AgTaO3 was observed in the visible light region. However, fresh AgTaO3 /AgBr exhibited a similar SPV signal to that of AgBr, indicating that the photovoltage response of the heterojunctionin the visible light region mainly originated from AgBr. Notably, the SPV signal intensity of fresh AgTaO3 /AgBr is much higher than that of pristine AgTaO3 and AgBr, suggesting an efficient charge separation occurred between AgTaO3 and AgBr. This result led us to speculate that there exists an interfacial electric field in AgTaO3 /AgBr heterojunction, which dominates the photogenerated charge separation[48]. Moreover, the SPV signal intensity of used AgTaO3 /AgBr is close to that of fresh AgTaO3 /AgBr, which may predict similar photocatalytic performance of fresh and used AgTaO3 /AgBr, as confirmed by Fig. 5b. An intense shoulder peak at about 470 nm was also observed. Because AgBr (AgBr = 5.3 eV) has a higher work function than Ag (Ag = 4.25 eV), the Fermi energy level of AgBr is lower than that of Ag. It is possible that direct electron prefer to transfer from Ag nanoparticle to the conduction band of AgBr[23]. Therefore, the shoulder absorption may be related the SPR of Ag nanoparticles. Fig. 8b shows the field-induced surface photovoltage spectroscopy of fresh AgTaO3 /AgBr under different external electric fields. According to previous literature, a positive SPV signal implies that the positive charges accumulate at the surface of the photocatalyst [49]. In the AgTaO3 /AgBr heterojunction, large amounts of photogenerated electrons were injected into AgTaO3 leaving plenty of holes in AgBr, which produced a positive SPV signal. As shown in Fig. 8b, under negative bias, the SPV signal intensity increased with an increase of negative bias intensity. Nevertheless, when positive bias was applied in the experiment, the SPV signal weakened with the increase of bias intensity. This result suggests that the interface states could be fully filled with electrons under a strong positive bias, indicating the n-type conductor character of the heterojunction [50]. This observation demonstrates that photogenerated electrons played dominant roles in the photocatalytic degradation on methyl orange. To specify the roles of photogenerated carriers and the primary radical species over AgTaO3 /AgBr heterojunction during the
Fig. 8. Surface photovoltage spectroscopy of AgTaO3 , AgBr, fresh AgTaO3 /AgBr, and used AgTaO3 /AgBr (a). Field-induced surface photovoltage spectroscopy of fresh AgTaO3 /AgBr under different external electric fields (b).
photocatalytic process, certain types of active species scavengers were added by repeating the methyl orange photodegradation process. In brief, tert-butyl alcohol (TBA), ammonium oxalate (AO) benzoquinone (BQ), and K2 Cr2 O7 were added as hydroxyl radical scavenger (OH• ), hole (h+ ) scavenger, superoxide radical scavenger, and electron (e− ) scavenger, respectively [51,52]. As illustrated in Fig. 9a, it is clearly that methyl orange photodegradation was greatly inhibited by adding BQ and K2 Cr2 O7 under visible light irradiation, which implies that O2 −• and photogenerated electrons played important roles in the photodegradation process. However, the addition of AO and TBA has little consequence on the photocatalytic process. This means that the photogenerated holes and OH• played minor roles during the photodegradation of methyl orange. To further verify the actives involved in the photocatalytic process, electron paramagnetic resonance (EPR) technique was conducted. Briefly, DMPO was used as a spin trap to capture O2 −• and OH• active species. As shown in Fig. 9b, with an increase of visible light irradiation time, the characteristic sextet peaks of DMPO-O2 −• EPR signal was observed. Meanwhile, EPR signal of DMPO-O2 −• gradually increased with an increase of the irradiation time. Moreover, as shown in Fig. 9b, it can be seen that the characteristic EPR signal of DMPO OH• was also detected with prolonged visible light irradiation time. However, it is noted that the valence band edge potential of AgBr locates at about 1.48 V versus NHE, which is more negative than those of OH• /OH− (1.9 V versus NHE) and OH• /H2 O (2.73 V versus NHE) [8], suggesting OH• radical species is not involved in the photocatalytic process. This seems to be contrary to the above results. The OH• species are likely to be generated from the reaction of O2 −• , electrons and hydrogen ions[53]. Having these results in mind, it is suspected that photogenerated electrons, O2 −• and
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togenerated electrons occurred from Ag nanoparticles into AgBr conduction band, thus promoting the charge separation efficiency. Consequently, though the decrement of partial active AgBr component occurred during the photocatalytic process, the photocatalytic activity of AgTaO3 /AgBr/Ag was retained and was compatible with that of AgTaO3 /AgBr due to the SPR effect of Ag nanoparticles. For present photocatalytic heterojunction, the photogenerated electrons in AgTaO3 conduction band can react with electron acceptors including surface adsorbed O2 species and lead to the formation of O2 −• active species and the subsequent degradation of methyl orange. 4. Conclusions
Fig. 9. Effects of different scavengers on methyl orange degradation in the presence of AgTaO3 /AgBr under visible light irradiation (a). EPR spectra obtained from AgTaO3 /AgBr heterojunction containing 0.22 M DMPO and 4.0 mg catalyst with total volume of 90% methanol/10% water (b) and 2 mL water (c) under different visible light irradiation time.
A novel heterostructuredphotocatalystAgTaO3 /AgBr was prepared with enhanced visible light driven photocatalytic activity toward methyl orange degradation. Detailed study indicatedAgTaO3 /AgBr was in-situ converted into ternary AgTaO3 /AgBr/Ag during the photocatalytic process, giving rise to enhanced visible light absorption that showed high durability and superior photocatalytic activity. Moreover, it’s found that AgBr grafted on AgTaO3 to form AgTaO3 /AgBr heterojunction showed intense visible light absorption, which exhibits highly enhanced photocatalytic performance than their individual counterpart. Theoretical and experimental investigation predicted that the matching of the electronic structure between AgTaO3 andAgBr induced an efficient transfer of photogenerated electrons from AgBr conduction band to AgTaO3 conduction band, leading to efficient charge separation and the subsequent enhancement of photocatalytic activity. On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated electrons, O2 −• , and OH• active species dominate the photodegradation of methyl orange. This work provides some hints for the fabrication of efficient, stable, and recyclable visible light active photocatalysts for environmental applications. Conflict of interest The authors declare no conflict of interest. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grants 21267041, 21367018, 21563021), the Inner Mongolia Natural Science Foundation (grant no. 2016JQ01) and the Project of Scientific and Technological Innovation Team of Inner Mongolia University (12110614). Appendix A. Supplementary data
Fig. 10. Possible mechanism illustration for methyl orange degradation over AgTaO3 /AgBr photocatalytic system under visible light irradiation.
OH• active species may play important role in the photocatalytic process. On the basis of the above mentioned results, a plausible explanation for the enhanced photocatalytic performance of AgTaO3 /AgBr heterojunction is illustrated in Fig. 10. Under visible light irradiation, the photogenerated electrons are injected into the conduction band of AgTaO3 , which could greatly enhance the photogenerated charge separation efficiency. Meanwhile, partial Ag+ ions were in-situ photo-reduced to Ag nanoparticles to produce a ternary AgTaO3 /AgBr/Ag heterojunction. Ag nanoparticles can absorb the incident photons, which was separated into electrons and holes due to local electromagnetic field by SPR effect. The injection of pho-
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata.2016.11. 001. References [1] Y. Zhou, Y. Zhang, M. Lin, J. Long, Z. Zhang, H. Lin, J.C.S. Wu, X. Wang, Nat. Commun. 6 (2015) 8340–8348. [2] G. Zhang, S. Zang, X. Wang, ACS Catal. 5 (2015) 941–947. [3] A. Manzi, T. Simon, C. Sonnleitner, M. Döblinger, R. Wyrwich, O. Stern, J.K. Stolarczyk, J. Am. Chem. Soc. 137 (2015) 14007–14010. [4] H. Chen, Y. Xie, X. Sun, M. Lv, F. Wu, L. Zhang, L. Li, X. Xu, Dalton Trans. 44 (2015) 13030–13039. [5] Y. Xie, Y. Wang, Z. Chen, X. Xu, ChemSusChem 9 (2016) 1403–1412. [6] F.F. Li, D.R. Liu, G.M. Gao, B. Xue, Y.S. Jiang, Appl. Catal. B: Environ. 166–167 (2015) 104–111. [7] J. Soldat, R. Marschall, M. Wark, Chem. Sci. 5 (2014) 3746–3752.
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 426 (2017) 52–59 [8] Y. Su, L. Peng, J. Guo, S. Huang, L. Lv, X. Wang, J. Phys. Chem. C 118 (2014) 10728–10739. [9] M. Lv, S. Ni, Z. Wang, T. Cao, X. Xu, Int. J. Hydrogen Energy 41 (2016) 1550–1558. [10] M. Lv, Y. Wang, L. Lu, R. Wang, S. Ni, G. Liu, X. Xu, Phys. Chem. Chem. Phys. 18 (2016) 21491–21499. [11] H. Husin, H.M. Chen, W.N. Su, C.J. Pan, W.T. Chuang, H.S. Sheu, B.J. Hwang, Appl. Catal. B: Environ. 102 (2011) 343–351. [12] Y. Su, S. Wang, Y. Meng, H. Han, X. Wang, RSC Adv. 2 (2012) 12932–12939. [13] B. Wang, P.D. Kanhere, Z. Chen, J. Nisar, B. Pathak, R. Ahuja, J. Phys. Chem. C 117 (2013) 22518–22524. [14] S.W. Seo, S. Park, H.Y. Jeong, S.H. Kim, U. Sim, C.W. Lee, K.T. Nam, K.S. Hong, Chem. Commun. 48 (2012) 10452–10454. [15] L. Xu, X. Sun, H. Tu, Q. Jia, H. Guo, J. Guan, Appl. Catal. B: Environ. 184 (2016) 309–319. [16] S. Kumar, B. Kumar, T. Surendar, V. Shanker, Mater. Res. Bull. 49 (2014) 310–318. [17] S.P. Adhikari, Z.D. Hood, K.L. More, I. Ivanov, L. Zhang, M. Gross, A. Lachgar, RSC Adv. 5 (2015) 54998–55005. [18] A.S. Cherevan, P. Gebhardt, C.J. Shearer, M. Matsukawa, K. Domen, D. Eder, Energy. Environ. Sci. 7 (2014) 791–796. [19] A. Mukherji, B. Seger, G.Q. (Max)Lu, L. Wang, ACS Nano 5 (2011) 3483–3492. [20] M. Li, J. Zhang, W. Dang, S.K. Cushing, D. Guo, N. Wu, P. Yin, Phys. Chem. Chem. Phys. 15 (2013) 16220–16226. [21] H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 106 (2002) 12441–12447. [22] J. Jiang, H. Li, L. Zhang, Chem. Eur. J. 18 (2012) 6360–6369. [23] W.S. Wang, H. Du, R.X. Wang, T. Wen, A.W. Xu, Nanoscale 5 (2013) 3315–3321. [24] M. Zhu, P. Chen, M. Liu, ACS Nano 5 (2011) 4529–4536. [25] L. Kong, Z. Jiang, H.H. Lai, R.J. Nicholls, T. Xiao, M.O. Jones, P.P. Edwards, J. Catal. 293 (2012) 116–125. [26] M. Shekofteh-Gohari, A. Habibi-Yangjeh, RSC Adv. 6 (2016) 2402–2413. [27] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Ceram. Int. 42 (2016) 15224–15234. [28] M. Shekofteh-Gohari, A. Habibi-Yangjeh, J. Colloid Interface Sci. 471 (2016) 103–113. [29] X. Xu, G. Liu, A.K. Azad, Int. J. Hydrogen Energy 40 (2015) 3672–3678. [30] J. Cao, B. Luo, H. Lin, S. Chen, J. Hazard. Mater. 190 (2011) 700–706.
59
[31] X. Zong, C. Sun, Z. Chen, A. Mukherji, H. Wu, J. Zou, S.C. Smith, G.Q. Lu, L. Wang, Chem. Commun. 47 (2011) 6293–6295. [32] M. Lv, Y. Xie, Y. Wang, X. Sun, F. Wu, H. Chen, S. Wang, C. Shen, Z. Chen, S. Ni, G. Liu, X. Xu, Phys. Chem. Chem. Phys. 17 (2015) 26320–26329. [33] J. Lim, D. Monllor-Satoca, J.S. Jang, S. Lee, W. Choi, Appl. Catal. B: Environ. 152–153 (2014) 233–240. [34] Y. Su, S. Huang, T. Wang, L. Peng, X. Wang, J. Hazard. Mater. 295 (2015) 119–126. [35] P. Wang, Y. Tang, Z. Dong, Z. Chen, T.T. Lim, J. Mater. Chem.A 1 (2013) 4718–4727. [36] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, Appl. Catal. B: Environ. 106 (2011) 445–452. [37] L. Ni, M. Tanabe, H. Irie, Chem. Commun. 49 (2013) 10094–10096. [38] X. Bai, R. Zong, C. Li, D. Liu, Y. Liu, Y. Zhu, Appl. Catal. B: Environ. 147 (2014) 82–91. [39] J. Cao, B. Luo, H. Lin, B. Xu, S. Chen, J. Hazard. Mater. 217-218 (2012) 107–115. [40] Y. Sang, L. Kuai, C. Chen, Z. Fang, B. Geng, ACS Appl. Mater. Interfaces 6 (2014) 5061–5068. [41] W.S. Wang, H. Du, R.X. Wang, T. Wen, A.W. Xu, Nanoscale 5 (2013) 3315–3321. [42] X. Li, D. Tang, F. Tang, Y. Zhu, C. He, M. Liu, C. Lin, Y. Liu, Mater. Res. Bull. 56 (2014) 125–133. [43] J. Belloni, M. Treguer, H. Remita, K.R. De, Nature 402 (1999) 865–867. [44] Y.H. Yin, H.S. Chen, Y. Song, J. Mol. Struc.: Theochem. 959 (2010) 30–34. [45] S.J.A. Moniz, S.A. Shevlin, X. An, Z.X. Guo, J. Tang, Chem. Eur. J. 20 (2014) 15571–15579. [46] E. Finazzi, C.D. Valentin, G. Pacchioni, J. Phys. Chem. C 113 (2009) 220–228. [47] H. Fan, H. Li, B. Liu, Y. Lu, T. Xie, D. Wang, ACS Appl. Mater. Interfaces 4 (2012) 4853–4857. [48] W.H. Lin, J.J. Wu, M.M.C. Chou, Y.M. Chang, M. Yoshimura, J. Phys. Chem. C 118 (2014) 19814–19821. [49] T. Jiang, T. Xie, L. Chen, Z. Fu, D. Wang, Nanoscale 5 (2013) 2938–2944. [50] Y. Lin, D. Wang, Q. Zhao, M. Yang, Q. Zhang, J. Phys. Chem. B 108 (2004) 3202–3206. [51] X. Zheng, D. Li, X. Li, L. Yu, P. Wang, X. Zhang, J. Fang, Y. Shao, Y. Zheng, Phys. Chem. Chem. Phys. 16 (2014) 15299–15306. [52] C. Zhang, L. Ai, L. Li, J. Jiang, J. Alloys Compd. 582 (2014) 576–582. [53] W. Liu, M. Wang, C. Xu, S. Chen, Chem. Eng. J. 209 (2012) 386–393.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Editor’s choice paper
Improved photocatalytic activity and durability of AgTaO3 /AgBr heterojunction: The relevance of phase and electronic structure Fang Wang 1 , Tingting Wang 1 , Junyu Lang, Yiguo Su ∗ , Xiaojing Wang ∗ College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, PR China
a r t i c l e
i n f o
Article history: Received 12 August 2016 Received in revised form 27 October 2016 Accepted 1 November 2016 Available online 3 November 2016 Keywords: Photocatalysis AgTaO3 Heterojunction Plasmonic enhancement Phase conversion
a b s t r a c t AgTaO3 /AgBr heterojunction was constructed for visible light driven photocatalytic purpose in order to investigate the relevance of phase conversion, electronic structure and photocatalytic properties. The result indicated that AgBr grafted on AgTaO3 to form AgTaO3 /AgBr heterojunction gave intense visible light absorption, which exhibits highly enhanced photocatalytic performance than their individual counterpart. Theoretical and experimental investigation showed that the matched electronic structure between AgTaO3 and AgBr induced an efficient transfer of photogenerated electrons from AgBr to AgTaO3 , leading to efficient charge separation and the subsequent improved photocatalytic activity. Partial AgBr converted to AgBr/Ag during the photocatalytic process, leading to the construction of ternary AgTaO3 /AgBr/Ag photocatalyst. Because of the surface plasmon resonance effect of Ag, the resulting AgTaO3 /AgBr/Ag exhibited wide range absorption and improved charge separation efficiency, which showed high durability and superior photocatalytic activity toward methyl orange degradation. On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated electrons, O2 −• , and OH• active species dominate the photodegradation of methyl orange. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis based on semiconductors harnesses considerable research attentions due to its potential applications in environmental remediation, hydrogen production by water splitting and chemical conversion for valuable products [1–5]. Till now, a variety of semiconductors for efficient photocatalytic purpose have been fully investigated. Among them, tantalate-based semiconductors are considered to be excellent photocatalysts due to their strong driving force for photoredox reduction, high chemical and physical stability, and superior photocatalytic activity [6–10]. However, stronger photoredox driving force is often accompanied with a wider band gap that considerably limits visible light absorption. In this regard, the improvement of solar light absorption of tantalate-based semiconductors for efficient visible light driven photocatalytic performance is still necessary. Numerous methodologies have been adopted to tailor the electronic structure for improving the visible light response of
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Su), wang xiao [email protected] (X. Wang). 1 Fang Wang and Tingting Wang are co-first authors. They contributed equally to this work. http://dx.doi.org/10.1016/j.molcata.2016.11.001 1381-1169/© 2016 Elsevier B.V. All rights reserved.
tantalate-based semiconductors by metal and/or nonmetal doping, heterojunction and surface/interface engineering [11–14]. In particular, the heterojunction a tantalate-based semiconductor with narrow band gap semiconductors can effectively improve the photocatalytic activity compared with the single component counterpart [15]. The enhancement of the photocatalytic activity is thought to be attributed to the efficient separation of photogenerated charge carriers as a consequence of the matching electronic structure such as band gap energy and the potential energy levels of valence band and conduction band. Recently, the enhanced visible light driven photocatalytic activity has been achieved in several heterojunction photocatalysts, including gC3 N4 /NaTaO3 , TaON/Bi2 O3 , carbon-Ta2 O5 , Sr2 Ta2 O7-x Nx /graphene and so on [16–19], in which tantalate-based semiconductors play critical roles. In order to develop tantalate-based heterojunction systems, it is still important to explore the phase, electronic structure and the underlying mechanism for efficient visible light absorption and photocatalytic activity. Silver tantalate (AgTaO3 ) exhibits distorted pseudocubic structure with a perovskite structure has started to obtain surged research attention due to its promising applications as solid lubricant, dielectric material and photocatalyst [20]. As previously reported, the band gap of AgTaO3 is determined to be about 3.4 eV [21], which is only reactive under ultraviolet (UV) light irradiation,
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 426 (2017) 52–59
limiting its solar energy applications. A systematic experimental identification of AgTaO3 and its relevant heterojunctions by modulating phase structure, chemical composition as well as interfacial contact is advantageous for tailoring the photophysical and photochemical properties of AgTaO3 as well as other isostructural semiconductors, such as NaTaO3 , AgNbO3 , CuTaO3 and so on. Most recently, much research attention has been focused on silver halide AgX (X = Cl, Br, I) because of their high photocatalytic performance toward the removal of deleterious contaminant [22–24]. Unfortunately, silver halide often suffers from chemical instability under visible light irradiation, being of a hindrance to its potential applications [25]. Recent studies suggests that heterojunction of AgBr can with other semiconductors can effectively improve the photocatalytic activity and stability of AgBr [26–28] Herein, with respect to the above-mentioned problems, we purposed to develop AgTaO3 /AgBr composite for efficient and renewable visible light photocatalyst by modulating the phase structure, chemical composition as well as the electronic structure, which is helpful to identify the underlying mechanism of the heterojunctions. 2. Experimental section 2.1. Procedure for the synthesis of the AgTaO3 /AgBr composite photocatalysts 2.1.1. Synthesis of AgTaO3 AgTaO3 sample was synthesized by a solid state reaction. Ta2 O5 was mixed with excess amounts (2–5 mol%) of Ag2 O in an agate mortar. The mixture was calcined in an alumina crucible at 1050 ◦ C for 5 h. The obtained sample was treated with certain amount of nitric acid to remove the excess silver and washed with distilled water for several times, and then dried in an oven at 80 ◦ C for 10 h. 2.1.2. Synthesis of AgBr Pure AgBr was prepared via a solution method. Briefly, AgNO3 and NaBr solutions with Ag/Br molar ratio of 1:1 were mixed to form a yellow suspension. After aged for 2 h, the suspension was filtered and washed several times using distilled water, then dried at 80 ◦ C for 10 h. 2.1.3. Synthesis of AgBr/AgTaO3 All the following processes were carried out in a dark situation. Briefly, 0.6 of AgTaO3 was added to 60 mL of distilled water to form a white suspension. The suspension was sonicated for 30 min at room temperature. 1.3 g of KBr was added to the above suspension with magnetically stirring for 2 h. Then, 0.6 g poly(ethyleneglycol)block-poly(ethylene glycol) (P123) was added to the suspension and stirred for 1 h. Subsequently, given amount of AgNO3 dissolved in 1.8 mL of NH3 ·H2 O (containing 25 wt% of NH3 ) was added to the above mixture. The initial weight ratio of AgBr/(AgTaO3 + AgBr) were modulated to be 30 wt%, 50 wt%, 70 wt%, 90 wt%, and defined as AA-30, AA-50, AA-70, AA-90, respectively. The resulting suspensions were stirred at room temperature for 12 h. The as-resulted products were filtered, washed and dried at 80 ◦ C over night.
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with a monochromatic Al K␣ and charge neutralizer. The surface photovoltage measurement system is constructed by a sample chamber, a lock-in amplifier with a light chopper and a source of monochromatic light (provided by a 500 W xenon lamp) and a monochromator. The monochromator and the lock-in amplifier were equipped with a computer. The analyzed product is assembled as a sandwich-like structure of ITO-product-ITO (ITO is an indium tin oxide electrode). The presence of hydroxyl radicals and superoxide radicals was measured by EPR spectra, which was performed on an ER200-SRC electron spin resonance spectrometer (Bruker, Germany) at 3186 G and 9056.895 MHz. 2.3. Photocatalytic activity tests The photocatalytic activity of the as-prepared samples was evaluated by using methyl orange as the probe molecule under visible light irradiation at room temperature. In brief, 50 mg of the sample was placed in a 100 mL beaker containing 50 mL 2 × 10−5 M methyl orange solution to form a suspension. Before illumination, the above suspension was stirred in dark in order to establish the absorption–desorption equilibrium of methyl orange on sample surfaces. Then, the suspension was irradiated by a 300 W Hg lamp using a filter ( ≥ 420 nm) as a cutoff, which is about 10 cm away from the beaker. Then, 5 mL of the suspension was extracted at given intervals and centrifuged at a rate of 8000 rpm for 10 min. UV–vis absorption spectra of the supernatant was measured using a Lambda 750 s Spectrometer. 3. Results and discussion XRD technique was adopted to investigate the crystallographic structure of the as-prepared samples. Fig. 1 shows the XRD patterns of AgTaO3 , AgBr and their heterojunctions. As for AgTaO3 , all diffraction peaks can be well indexed to perovskite-type phase of AgTaO3 (JCPDS card no. 72-1383). No trace of additional diffraction peaks was observed, suggesting a single phase perovskite structure is formed, being similar to Xu’s results [29]. By using the Retica Rietveld program based on a least-squares method, the lattice parameter of AgTaO3 was estimated to be a = 5.523 Å, and c = 13.623 Å, being close to the standard data of AgTaO3 . The narrowed diffraction peaks suggest high crystallinity and fine nature of AgTaO3 . Besides AgTaO3 , the XRD pattern of AgBr is also given for comparison. As shown in Fig. 1, all narrowed diffraction peaks of AgBr can be ascribed to the typical cubic structure, which is identical to the standard data of AgBr (JCPDS card no. 06-0438). As AgTaO3
2.2. Characterization of photocatalysts X-ray power diffraction (XRD) was used to characterize the phase structure of the as-prepared samples on a Rigaku DMAX2500 X-ray diffractometer using a copper target. Transmission electron microscopy (TEM) was performed on a JEM-2010 apparatus with an acceleration voltage of 200 kV. Energy-dispersive spectroscopy (EDX) data were obtained using scanning electron microscopy (SEM) on a S-4800 apparatus working at 10 kV. Optical diffuse reflectance spectrum was measured using a Lambda 750 s spectrometer. XPS analyses were performed on the ESCALab220i-XL
Fig 1. XRD patterns of the as-prepared samples. Vertical bars represent the standard diffraction data from the JCPDS files for AgTaO3 (72–1398, black line), AgBr (06-0438, red line) and Ag (87-0597, blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. TEM (a) and HRTEM (b) images of fresh AgTaO3 /AgBr sample. TEM (c) and HRTEM (d) images of used AgTaO3 /AgBr sample.
was coupled with AgBr (AA-70 sample), the main characteristic diffraction peaks of AgTaO3 and AgBr showed no obvious variation. This result suggested the co-existence of AgTaO3 and AgBr in AgTaO3 /AgBr heterojunction. On the other hand, the purity of the as-prepared AgTaO3 , AgBr, and AgTaO3 /AgBr samples was also confirmed by EDX technique (Fig. S1), being in accordance with the XRD results. Moreover, it is noted that, after the photocatalytic activity test, additional weak diffraction peaks were observed in AgTaO3 /AgBr heterojunction. The additional diffraction peak at about 2 = 38.11◦ corresponds to the (111) plane of cubic Ag (JCPDS card no. 87-0597), which can be confirmed by transmission electron microscopy (TEM) observations, indicating the formation of the ternary AgTaO3 /AgBr/Ag heterostructure. Similar results have been reported in many AgBr based photocatalytic systems due to the photo-instability of AgBr nanoparticles [25,30]. SEM and TEM measurements were used to explore the morphologies and crystal structures of the as-prepared samples. SEM images and the corresponding EDX mapping results (Fig. S2 and S3) indicated the formation of AgTaO3 /AgBr heterojunction structure, which was further confirmed by TEM observation. As shown in Fig. 2a, for fresh AgTaO3 /AgBr heterojunction, two kinds of materials were observed in the AA-70 sample, which correspond to AgTaO3 and AgBr, respectively. HRTEM observation further confirmed the heterojunction feature of AgTaO3 /AgBr. As illustrated in Fig. 2b, the lattice plane space of the as-prepared sample was estimated to be 0.278 nm, which is similar to 0.279 nm of (104) plane for AgTaO3 . On the other hand, another lattice space of 0.205 nm is compatible with 0.204 nm of (220) plane for AgBr. As for the
used AgTaO3 /AgBr sample, it is seen that Ag nanoparticles were observed (Fig. 2c), which is identical to the XRD results. To further specify the presence of Ag nanoparticles in the AgTaO3 /AgBr heterojunction, HRTEM image is illustrated in Fig. 2d. From Fig. 2d, It is seen that the spacing between the adjacent lattice fringes was 0.236 nm, which is close to 0.237 nm for the (111) plane of Ag (JCPDS card no. 87-0597), suggesting the crystalline feature of Ag nanoparticles. Meanwhile, another space of lattice plane was determined to be 0.333 nm, which is identical to that of the (111) plane for the standard data of AgBr. This observation suggests AgTaO3 /AgBr heterojunction was in-situ converted to the ternary AgTaO3 /AgBr/Ag heterojunction during the photocatalytic process, where Ag nanoparticles anchored on the AgBr surfaces. XPS technique was further used to specify the chemical composition and charge state of AgTaO3 /AgBr heterojunction. The XPS survey spectra of fresh (AA-70 sample) and used AgTaO3 /AgBr heterojunctions is shown in Fig. 3a. From Fig. 3a, it is seen that all peaks in Fig. 3a can be recognized to Ag, Ta, Br, O, and C elements for both fresh and used AgTaO3 /AgBr heterojunctions. Fig. 3b–d shows the high-resolution scans of Ta 4f, O 1s, and Ag 3d. In Fig. 3b, the binding energies at 25.10 eV and 27.01 eV for both fresh and used AgTaO3 /AgBr heterojunctions, which can be recognized to the Ta 4f7/5 and Ta 4f5/2 of Ta5+ , respectively, being close to previous results [31]. As shown in Fig. 3c for O 1s XPS peak, it is seen that, for both fresh and used AgTaO3 /AgBr heterojunctions, the O 1 s signal can be fitted by three Lorentzian-Gaussian lines, which suggests that there exists three kinds of surface oxygen species in both samples. The XPS peak at ∼529.71 eV is related to the lattice
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Fig. 3. XPS spectra of fresh and used AgTaO3 /AgBr (AA-70 sample) heterojunction: whole scanning spectra (a) and the high-resolution regional spectra of Ta 4f (b), O 1 s (c), and Ag 3d (d). Fig. 4. UV–vis diffuse reflectance spectra of the as-prepared samples (a) and the optical band gap of bare AgTaO3 and AgBr (b).
oxygen atoms (Ob ). The XPS peak at ∼530.69 eV may belong to surface hydroxyl groups (Os ). And the binding energy at ∼532.11 eV is thought to be ascribed to the surface chemisorbed O2 or surface carboxylic group [32], which is originated from the surface defects including oxygen vacancies (Od ) [33,34]. It is seen that the Od content in used AgTaO3 /AgBr heterojunction is higher than that in fresh AgTaO3 /AgBr heterojunction, which may expect high photocatalytic performance. Fig. 3d illustrates Ag 3d XPS signal of both fresh and used AgTaO3 /AgBr heterojunctions. For fresh AgTaO3 /AgBr heterojunction, two strong XPS signals appeared at 367.30 eV and 373.31 eV is ascribed to Ag 3d5/2 and Ag 3d3/2 for ionic silver (Ag+ ), respectively [35]. However, for used AgTaO3 /AgBr heterojunction, two additional weak peaks located at about 367.95 eV and 374.03 eV were observed, which should be attributed to Ag 3d5/2 and Ag 3d3/2 of metallic silver (Ag0 ) [36]. This result implied that partial Ag+ ions in AgTaO3 /AgBr heterojunctions were reduced to metallic Ag nanoparticles to form the ternary AgTaO3 /AgBr/Ag heterojunctions during the photocatalytic process. Basically, the electronic structure and energy level of a semiconductor usually plays important roles in the modulation of their photocatalytic activity. Fig. 4a illustrates the UV–vis diffuse reflectance spectra of the samples. As shown in Fig. 4a, bare AgTaO3 showed intense UV absorption from 200 nm to 380 nm, which is assigned to the typical electronic transitions from Ag 4d + O 2p orbitals to Ta 5d orbitals [37]. For AgBr, the absorption edge appears at about 475 nm, indicating that AgBr can absorb solar light extending to 475 nm. Fresh AgTaO3 /AgBr exhibited a mixed absorption feature of both AgTaO3 and AgBr, indicating AgTaO3 /AgBr heterojunction should have visible light photocatalytic activity. Meanwhile, all as-prepared AgTaO3 /AgBr samples with different weight ratios showed visible light absorption (Fig. S4). Interestingly, it is noted that the used AgTaO3 /AgBr gave an obvious intense visible light absorption with the wavelength extending 800 nm. This absorption is attributed to the surface plasmon resonance effect (SPR) of
Ag nanoparticles [38]. The band gap energy of AgTaO3 and AgBr was also determined by the following equation
˛h = A h − Eg
n/2
where ␣, , n, A and Eg are the absorption coefficient, incident light frequency, an integer, constant and band gap, respectively. The value of n for AgTaO3 is 1 because AgTaO3 exhibits a direct optical transition type (Determined by the following DFT results). Whereas, the value of n for AgBr is 4 due to an indirect optical transition type of AgBr [39]. Finally, the electronic band gap values of AgTaO3 and AgBr were determined to be 3.40 eV and 2.52 eV, respectively. Since AgBr grafted on AgTaO3 can lead to the absorption in visible light range, it is expected that AgTaO3 /AgBr may exhibit visible light photocatalytic activity. The photocatalytic activity of AgTaO3 /AgBr heterojunctions was evaluated using methyl orange degradation as the model reaction. Preliminary studies indicated that AA-70 sample exhibited optimal photocatalytic activity (Fig. S5). Concentration changes of methyl orange were examined by using the variations in maximal adsorption at about 464 nm in the UV–vis absorption spectra. As shown in Fig. 5a, methyl orange concentration gradually decreases with an increase of visible light irradiation time. Fig. 5b shows the photocatalytic activity for all as-prepared samples. For comparison, the photodegradation of methyl orange in the absence of catalyst was also given in Fig. 5b. This observation shows that methyl orange solution is stable under visible light irradiation without any catalyst. It is clear that AgTaO3 showed no apparent photocatalytic activity toward methyl orange degradation under visible light irradiation. After the heterojunction of AgBr on AgTaO3 , the photocatalytic activity of AgTaO3 /AgBr greatly enhanced in comparison to their individual counterpart. This result may be ascribed to the enhanced interfacial charge separation efficiency between
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Fig. 5. UV–vis spectral changes of methyl orange in aqueous AgTaO3 /AgBr (AA-70 sample) dispersion as a function of visible light irradiation time (a), normalized concentration of methyl orange versus visible light irradiation time in the presence of various photocatalysts and cyclic degradation curve for AgTaO3 /AgBr (AA-70 sample).
AgTaO3 and AgBr because the physical mixture of AgTaO3 and AgBr showed much lower photocatalytic activity than that of AgTaO3 /AgBr heterojunction. Since AgBr is highly sensitive to visible light, the AgTaO3 /AgBr heterojunction could in-situ convert to the ternary AgTaO3 /AgBr/Ag heterojunction. This conversion has been observed in many AgBr-based heterojunction photocatalysts [40,41]. Then, it is necessary to investigate the photocatalytic activity of AgTaO3 /AgBr/Ag. Interestingly, as shown in Fig. 5b, the photocatalytic activity of AgTaO3 /AgBr/Ag is compatible with that of AgTaO3 /AgBr and more active than AgBr/Ag. Basically, heterojunction photocatalysts can enhance the photocatalytic performance due to the variation of electronic energy levels that can promote the charge separation between the semiconductors. On the basis of previous literatures, the conduction band edge potential of AgBr is determined to be −1.04 V versus NHE [42,43], whereas the conduction band edge potential of AgTaO3 locates at about −0.9 V versus NHE [37]. This result gives an evidence that the photogenerated electrons may migrate from the conduction band of AgBr to the conduction band of AgTaO3 , thus enhancing the charge separation efficiency. This conception can be further verified by density functional theory (DFT) results. On the basis of TEM results and previous literatures of the AgTaO3 /AgBr heterojunction interfaces, a theoretical model of the interface, corresponding to the AgTaO3 (104) surface interfacing with stable (AgBr)6 cluster [44], was used (Fig. 6). Due to large lattice mismatch between AgTaO3 and AgBr, the AgTaO3 /AgBr interface is not stable and structural optimization and the convergence is inaccessible. Consequently, a cluster/surface model was adopted to reflect the interaction between AgBr and AgTaO3 [45]. After geometry optimizations, the AgTaO3 (104) surface and AgBr show obvious distortion, indicating strong interfacial interaction of AgBr and AgTaO3 occurs. The atomic population analysis indicates that a net charge of ∼−1.69 e on AgTaO3 is observed, and meanwhile, a net charge of ∼1.68 e on AgBr occurs (Table S1). This observation clearly suggests that the electrons prefers to
Fig. 6. Models for simulating the heterojunction of AgTaO3 /AgBr before geometry optimization (a and b) and after geometry optimization (c and d). The green, blue, red, and brown represent silver, tantalum, oxygen, and bromine, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
migrate from the conduction band of AgBr to the conduction band of AgTaO3 [46]. In order to shed light on the mechanism of the interaction of the AgBr/AgTaO3 interface, the interface electronic structures was investigated by partial density of states (DOS) data. For AgTaO3 (104) surface, the valence band maximum consists of the O 2p and Ag 4d states, and the conduction band minimum mainly originates from the hybridization of Ta 5d and O 2p orbitals. And the band gap energy is determined to be ∼2.47 eV, which is smaller than the experimental data. As for (AgBr)6 cluster, various hybridized orbitals are observed through molecular orbital analysis. The calculated HOMO consists of Ag 4d, Ag 5 s and Br 4p orbitals, whereas the LUMO is mainly related to the hybridization of Ag 5 s and Br 4p orbitals. Meanwhile, the energy gap was also calculated to be ∼2.81 eV. Moreover, the DOS of AgBr/AgTaO3 is also given in Fig. 7. From Fig. 7, it is clearly seen that there exists a significant modification of the conduction band edge. While no obvious variation of the valence band of AgTaO3 is observed via (AgBr)6 cluster modification. In particular, the states extend downwards in the energy of the conduction band edge, which leads to the energy gap of ∼1.08 eV, being attributed to the hybridization of the LUMO of (AgBr)6 cluster. From this aspect, it is clear that photogenerated electrons would readily transfer from the conduction band of AgBr to that of AgTaO3 and therefore enhance the photocatalytic activity. To explain the notable enhancement in charge separation efficiency of the heterojunction from the theoretical calculations, we comparatively investigated the transfer properties of photogenerated charges at the surface or interface of AgTaO3 , AgBr and their relevant heterojunctions by surface photovoltage (SPV) technique. Since only the charge generation and separation process under light irradiation can produce the SPV signal, the intensity of SPV signal is closely related to the charge separation efficiency. Basically, a higher SPV signal intensity usually means a higher charge separation efficiency [47]. As shown in Fig. 8a, pristine AgTaO3 and AgBr samples, the SPV signal appeared in the wavelength range of 300–380 nm and 300–470 nm, respectively, which were related to the typical band to band electronic transition, being in accordance with the UV–vis spectra. No obvious SPV response
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Fig. 7. Density of states (DOS) of AgTaO3 (104) surface, (AgBr)6 cluster and AgTaO3 /AgBr heterojunction.
for AgTaO3 was observed in the visible light region. However, fresh AgTaO3 /AgBr exhibited a similar SPV signal to that of AgBr, indicating that the photovoltage response of the heterojunctionin the visible light region mainly originated from AgBr. Notably, the SPV signal intensity of fresh AgTaO3 /AgBr is much higher than that of pristine AgTaO3 and AgBr, suggesting an efficient charge separation occurred between AgTaO3 and AgBr. This result led us to speculate that there exists an interfacial electric field in AgTaO3 /AgBr heterojunction, which dominates the photogenerated charge separation[48]. Moreover, the SPV signal intensity of used AgTaO3 /AgBr is close to that of fresh AgTaO3 /AgBr, which may predict similar photocatalytic performance of fresh and used AgTaO3 /AgBr, as confirmed by Fig. 5b. An intense shoulder peak at about 470 nm was also observed. Because AgBr (AgBr = 5.3 eV) has a higher work function than Ag (Ag = 4.25 eV), the Fermi energy level of AgBr is lower than that of Ag. It is possible that direct electron prefer to transfer from Ag nanoparticle to the conduction band of AgBr[23]. Therefore, the shoulder absorption may be related the SPR of Ag nanoparticles. Fig. 8b shows the field-induced surface photovoltage spectroscopy of fresh AgTaO3 /AgBr under different external electric fields. According to previous literature, a positive SPV signal implies that the positive charges accumulate at the surface of the photocatalyst [49]. In the AgTaO3 /AgBr heterojunction, large amounts of photogenerated electrons were injected into AgTaO3 leaving plenty of holes in AgBr, which produced a positive SPV signal. As shown in Fig. 8b, under negative bias, the SPV signal intensity increased with an increase of negative bias intensity. Nevertheless, when positive bias was applied in the experiment, the SPV signal weakened with the increase of bias intensity. This result suggests that the interface states could be fully filled with electrons under a strong positive bias, indicating the n-type conductor character of the heterojunction [50]. This observation demonstrates that photogenerated electrons played dominant roles in the photocatalytic degradation on methyl orange. To specify the roles of photogenerated carriers and the primary radical species over AgTaO3 /AgBr heterojunction during the
Fig. 8. Surface photovoltage spectroscopy of AgTaO3 , AgBr, fresh AgTaO3 /AgBr, and used AgTaO3 /AgBr (a). Field-induced surface photovoltage spectroscopy of fresh AgTaO3 /AgBr under different external electric fields (b).
photocatalytic process, certain types of active species scavengers were added by repeating the methyl orange photodegradation process. In brief, tert-butyl alcohol (TBA), ammonium oxalate (AO) benzoquinone (BQ), and K2 Cr2 O7 were added as hydroxyl radical scavenger (OH• ), hole (h+ ) scavenger, superoxide radical scavenger, and electron (e− ) scavenger, respectively [51,52]. As illustrated in Fig. 9a, it is clearly that methyl orange photodegradation was greatly inhibited by adding BQ and K2 Cr2 O7 under visible light irradiation, which implies that O2 −• and photogenerated electrons played important roles in the photodegradation process. However, the addition of AO and TBA has little consequence on the photocatalytic process. This means that the photogenerated holes and OH• played minor roles during the photodegradation of methyl orange. To further verify the actives involved in the photocatalytic process, electron paramagnetic resonance (EPR) technique was conducted. Briefly, DMPO was used as a spin trap to capture O2 −• and OH• active species. As shown in Fig. 9b, with an increase of visible light irradiation time, the characteristic sextet peaks of DMPO-O2 −• EPR signal was observed. Meanwhile, EPR signal of DMPO-O2 −• gradually increased with an increase of the irradiation time. Moreover, as shown in Fig. 9b, it can be seen that the characteristic EPR signal of DMPO OH• was also detected with prolonged visible light irradiation time. However, it is noted that the valence band edge potential of AgBr locates at about 1.48 V versus NHE, which is more negative than those of OH• /OH− (1.9 V versus NHE) and OH• /H2 O (2.73 V versus NHE) [8], suggesting OH• radical species is not involved in the photocatalytic process. This seems to be contrary to the above results. The OH• species are likely to be generated from the reaction of O2 −• , electrons and hydrogen ions[53]. Having these results in mind, it is suspected that photogenerated electrons, O2 −• and
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togenerated electrons occurred from Ag nanoparticles into AgBr conduction band, thus promoting the charge separation efficiency. Consequently, though the decrement of partial active AgBr component occurred during the photocatalytic process, the photocatalytic activity of AgTaO3 /AgBr/Ag was retained and was compatible with that of AgTaO3 /AgBr due to the SPR effect of Ag nanoparticles. For present photocatalytic heterojunction, the photogenerated electrons in AgTaO3 conduction band can react with electron acceptors including surface adsorbed O2 species and lead to the formation of O2 −• active species and the subsequent degradation of methyl orange. 4. Conclusions
Fig. 9. Effects of different scavengers on methyl orange degradation in the presence of AgTaO3 /AgBr under visible light irradiation (a). EPR spectra obtained from AgTaO3 /AgBr heterojunction containing 0.22 M DMPO and 4.0 mg catalyst with total volume of 90% methanol/10% water (b) and 2 mL water (c) under different visible light irradiation time.
A novel heterostructuredphotocatalystAgTaO3 /AgBr was prepared with enhanced visible light driven photocatalytic activity toward methyl orange degradation. Detailed study indicatedAgTaO3 /AgBr was in-situ converted into ternary AgTaO3 /AgBr/Ag during the photocatalytic process, giving rise to enhanced visible light absorption that showed high durability and superior photocatalytic activity. Moreover, it’s found that AgBr grafted on AgTaO3 to form AgTaO3 /AgBr heterojunction showed intense visible light absorption, which exhibits highly enhanced photocatalytic performance than their individual counterpart. Theoretical and experimental investigation predicted that the matching of the electronic structure between AgTaO3 andAgBr induced an efficient transfer of photogenerated electrons from AgBr conduction band to AgTaO3 conduction band, leading to efficient charge separation and the subsequent enhancement of photocatalytic activity. On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated electrons, O2 −• , and OH• active species dominate the photodegradation of methyl orange. This work provides some hints for the fabrication of efficient, stable, and recyclable visible light active photocatalysts for environmental applications. Conflict of interest The authors declare no conflict of interest. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grants 21267041, 21367018, 21563021), the Inner Mongolia Natural Science Foundation (grant no. 2016JQ01) and the Project of Scientific and Technological Innovation Team of Inner Mongolia University (12110614). Appendix A. Supplementary data
Fig. 10. Possible mechanism illustration for methyl orange degradation over AgTaO3 /AgBr photocatalytic system under visible light irradiation.
OH• active species may play important role in the photocatalytic process. On the basis of the above mentioned results, a plausible explanation for the enhanced photocatalytic performance of AgTaO3 /AgBr heterojunction is illustrated in Fig. 10. Under visible light irradiation, the photogenerated electrons are injected into the conduction band of AgTaO3 , which could greatly enhance the photogenerated charge separation efficiency. Meanwhile, partial Ag+ ions were in-situ photo-reduced to Ag nanoparticles to produce a ternary AgTaO3 /AgBr/Ag heterojunction. Ag nanoparticles can absorb the incident photons, which was separated into electrons and holes due to local electromagnetic field by SPR effect. The injection of pho-
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata.2016.11. 001. References [1] Y. Zhou, Y. Zhang, M. Lin, J. Long, Z. Zhang, H. Lin, J.C.S. Wu, X. Wang, Nat. Commun. 6 (2015) 8340–8348. [2] G. Zhang, S. Zang, X. Wang, ACS Catal. 5 (2015) 941–947. [3] A. Manzi, T. Simon, C. Sonnleitner, M. Döblinger, R. Wyrwich, O. Stern, J.K. Stolarczyk, J. Am. Chem. Soc. 137 (2015) 14007–14010. [4] H. Chen, Y. Xie, X. Sun, M. Lv, F. Wu, L. Zhang, L. Li, X. Xu, Dalton Trans. 44 (2015) 13030–13039. [5] Y. Xie, Y. Wang, Z. Chen, X. Xu, ChemSusChem 9 (2016) 1403–1412. [6] F.F. Li, D.R. Liu, G.M. Gao, B. Xue, Y.S. Jiang, Appl. Catal. B: Environ. 166–167 (2015) 104–111. [7] J. Soldat, R. Marschall, M. Wark, Chem. Sci. 5 (2014) 3746–3752.
F. Wang et al. / Journal of Molecular Catalysis A: Chemical 426 (2017) 52–59 [8] Y. Su, L. Peng, J. Guo, S. Huang, L. Lv, X. Wang, J. Phys. Chem. C 118 (2014) 10728–10739. [9] M. Lv, S. Ni, Z. Wang, T. Cao, X. Xu, Int. J. Hydrogen Energy 41 (2016) 1550–1558. [10] M. Lv, Y. Wang, L. Lu, R. Wang, S. Ni, G. Liu, X. Xu, Phys. Chem. Chem. Phys. 18 (2016) 21491–21499. [11] H. Husin, H.M. Chen, W.N. Su, C.J. Pan, W.T. Chuang, H.S. Sheu, B.J. Hwang, Appl. Catal. B: Environ. 102 (2011) 343–351. [12] Y. Su, S. Wang, Y. Meng, H. Han, X. Wang, RSC Adv. 2 (2012) 12932–12939. [13] B. Wang, P.D. Kanhere, Z. Chen, J. Nisar, B. Pathak, R. Ahuja, J. Phys. Chem. C 117 (2013) 22518–22524. [14] S.W. Seo, S. Park, H.Y. Jeong, S.H. Kim, U. Sim, C.W. Lee, K.T. Nam, K.S. Hong, Chem. Commun. 48 (2012) 10452–10454. [15] L. Xu, X. Sun, H. Tu, Q. Jia, H. Guo, J. Guan, Appl. Catal. B: Environ. 184 (2016) 309–319. [16] S. Kumar, B. Kumar, T. Surendar, V. Shanker, Mater. Res. Bull. 49 (2014) 310–318. [17] S.P. Adhikari, Z.D. Hood, K.L. More, I. Ivanov, L. Zhang, M. Gross, A. Lachgar, RSC Adv. 5 (2015) 54998–55005. [18] A.S. Cherevan, P. Gebhardt, C.J. Shearer, M. Matsukawa, K. Domen, D. Eder, Energy. Environ. Sci. 7 (2014) 791–796. [19] A. Mukherji, B. Seger, G.Q. (Max)Lu, L. Wang, ACS Nano 5 (2011) 3483–3492. [20] M. Li, J. Zhang, W. Dang, S.K. Cushing, D. Guo, N. Wu, P. Yin, Phys. Chem. Chem. Phys. 15 (2013) 16220–16226. [21] H. Kato, H. Kobayashi, A. Kudo, J. Phys. Chem. B 106 (2002) 12441–12447. [22] J. Jiang, H. Li, L. Zhang, Chem. Eur. J. 18 (2012) 6360–6369. [23] W.S. Wang, H. Du, R.X. Wang, T. Wen, A.W. Xu, Nanoscale 5 (2013) 3315–3321. [24] M. Zhu, P. Chen, M. Liu, ACS Nano 5 (2011) 4529–4536. [25] L. Kong, Z. Jiang, H.H. Lai, R.J. Nicholls, T. Xiao, M.O. Jones, P.P. Edwards, J. Catal. 293 (2012) 116–125. [26] M. Shekofteh-Gohari, A. Habibi-Yangjeh, RSC Adv. 6 (2016) 2402–2413. [27] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Ceram. Int. 42 (2016) 15224–15234. [28] M. Shekofteh-Gohari, A. Habibi-Yangjeh, J. Colloid Interface Sci. 471 (2016) 103–113. [29] X. Xu, G. Liu, A.K. Azad, Int. J. Hydrogen Energy 40 (2015) 3672–3678. [30] J. Cao, B. Luo, H. Lin, S. Chen, J. Hazard. Mater. 190 (2011) 700–706.
59
[31] X. Zong, C. Sun, Z. Chen, A. Mukherji, H. Wu, J. Zou, S.C. Smith, G.Q. Lu, L. Wang, Chem. Commun. 47 (2011) 6293–6295. [32] M. Lv, Y. Xie, Y. Wang, X. Sun, F. Wu, H. Chen, S. Wang, C. Shen, Z. Chen, S. Ni, G. Liu, X. Xu, Phys. Chem. Chem. Phys. 17 (2015) 26320–26329. [33] J. Lim, D. Monllor-Satoca, J.S. Jang, S. Lee, W. Choi, Appl. Catal. B: Environ. 152–153 (2014) 233–240. [34] Y. Su, S. Huang, T. Wang, L. Peng, X. Wang, J. Hazard. Mater. 295 (2015) 119–126. [35] P. Wang, Y. Tang, Z. Dong, Z. Chen, T.T. Lim, J. Mater. Chem.A 1 (2013) 4718–4727. [36] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, Appl. Catal. B: Environ. 106 (2011) 445–452. [37] L. Ni, M. Tanabe, H. Irie, Chem. Commun. 49 (2013) 10094–10096. [38] X. Bai, R. Zong, C. Li, D. Liu, Y. Liu, Y. Zhu, Appl. Catal. B: Environ. 147 (2014) 82–91. [39] J. Cao, B. Luo, H. Lin, B. Xu, S. Chen, J. Hazard. Mater. 217-218 (2012) 107–115. [40] Y. Sang, L. Kuai, C. Chen, Z. Fang, B. Geng, ACS Appl. Mater. Interfaces 6 (2014) 5061–5068. [41] W.S. Wang, H. Du, R.X. Wang, T. Wen, A.W. Xu, Nanoscale 5 (2013) 3315–3321. [42] X. Li, D. Tang, F. Tang, Y. Zhu, C. He, M. Liu, C. Lin, Y. Liu, Mater. Res. Bull. 56 (2014) 125–133. [43] J. Belloni, M. Treguer, H. Remita, K.R. De, Nature 402 (1999) 865–867. [44] Y.H. Yin, H.S. Chen, Y. Song, J. Mol. Struc.: Theochem. 959 (2010) 30–34. [45] S.J.A. Moniz, S.A. Shevlin, X. An, Z.X. Guo, J. Tang, Chem. Eur. J. 20 (2014) 15571–15579. [46] E. Finazzi, C.D. Valentin, G. Pacchioni, J. Phys. Chem. C 113 (2009) 220–228. [47] H. Fan, H. Li, B. Liu, Y. Lu, T. Xie, D. Wang, ACS Appl. Mater. Interfaces 4 (2012) 4853–4857. [48] W.H. Lin, J.J. Wu, M.M.C. Chou, Y.M. Chang, M. Yoshimura, J. Phys. Chem. C 118 (2014) 19814–19821. [49] T. Jiang, T. Xie, L. Chen, Z. Fu, D. Wang, Nanoscale 5 (2013) 2938–2944. [50] Y. Lin, D. Wang, Q. Zhao, M. Yang, Q. Zhang, J. Phys. Chem. B 108 (2004) 3202–3206. [51] X. Zheng, D. Li, X. Li, L. Yu, P. Wang, X. Zhang, J. Fang, Y. Shao, Y. Zheng, Phys. Chem. Chem. Phys. 16 (2014) 15299–15306. [52] C. Zhang, L. Ai, L. Li, J. Jiang, J. Alloys Compd. 582 (2014) 576–582. [53] W. Liu, M. Wang, C. Xu, S. Chen, Chem. Eng. J. 209 (2012) 386–393.