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Synthesis, Characterization, and Photocatalytic Activity of g-C3N4/KTaO3 Composites under Visible Light Irradiation
Novel graphitic carbon nitride/KTaO3 (g-C3N4/KTaO3) nanocomposite photocatalysts have been successfully synthesized via a facile and simple ultrasonic dispersion method. Compared to either g-C3N4 or KTaO3, the composite photocatalysts show significantly increased photocatalytic activity for degradation of Rhodamine B (RhB) under visible light irradiation. The increased photocatalytic performance of the composite could be attributed to the enhanced photogenerated charge carrier separation capacity. Moreover, it is observed that is the main active species in the photocatalytic degradation of RhB using the g-C3N4/KTaO3 composite photocatalysts.
Semiconductor photocatalysis has been regarded as an ideal green chemistry technology in dealing with the globally concerned energy shortage and environment pollution issues. In view of practical application, developing highly active photocatalysts has drawn great attention in recent years. Among the various photocatalytic materials, KTaO3 has been reported as a unique photocatalyst for hydrogen generation as well as organic pollutant degradation [1, 2]. However, its large band gap (3.6 eV) limits the photoactivity only to the UV light, which covers less than 4% of the solar spectrum.
Recently, Wang and coworkers discovered that the graphitic carbon nitride (g-C3N4), a conjugative structure material, is a metal-free visible-light-driven semiconductor . Due to its narrow band gap (2.7 eV) and high thermal and chemical stability, g-C3N4 has attracted extensive interest. However, the photocatalytic efficiency of bare g-C3N4 is greatly limited by the high recombination of photogenerated electron-hole pairs. It has been proved that the composite can promote the generation and separation of photoinduced electron-holes pairs. So far Bi2WO6/g-C3N4 , CdS/g-C3N4 [5, 6], DyVO4/g-C3N4 , WO3/g-C3N4 , TaON/g-C3N4 , SrTiO3/g-C3N4 , C60/g-C3N4 , Ag2O/g-C3N4 , NiS/g-C3N4 , and rGO/g-C3N4  have been demonstrated to exhibit better photocatalytic performance compared with pure g-C3N4.
Since the conduction band bottom of g-C3N4 (−1.13 eV)  is more negative than that of KTaO3 (−0.96 eV) [1, 2, 16]; g-C3N4 (CN) and KTaO3 (KTO) might be suitable candidates to form an ideal composite photocatalyst to show a high visible light activity. In this paper, we report for the first time that the g-C3N4/KTaO3 composite photocatalyst synthesized by a facile ultrasonic dispersion method has demonstrated a significantly improved photocatalytic performance for Rhodamine B degradation.
2. Experiment Section
2.1. Sample Preparation
The g-C3N4 powder sample was synthesized by directly heating melamine under 550°C according to the previously reported procedure . KTaO3 (KTO) was prepared by an improved polymerized complex (PC) method . In a typical run, 1 g TaCl5 was added into 15 mL 2-methoxyethanol and the solution was stirred for 30 min. Then, 0.20 g K2CO3, 12.0 g critic acid, 30 mL 2-methoxyethanol, and 2.0 mL of ethylene glycol were added. After it was stirred for 30 min, the mixture was heated to 120°C for 20 h in air and, finally, the polymer was oxidized in air at 600°C for 2 h.
The typical process for preparation of g-C3N4/KTaO3 (CN/KTO) composites was as follows: an appropriate amount of g-C3N4 was added into methanol and then ultrasonically treated in an ultrasonic bath for 30 min. After KTaO3 powder was added, the solution was stirred in the fume hood for 24 h. Finally the mixture was dried at 100°C overnight and then heated to 300°C for 2 h. The CN/KTO composites with different ratios of g-C3N4 to KTO were prepared and denoted as gcn30, gcn50, and gcn70, respectively, in which the number indicates percentage (mass%) of g-C3N4 in the composite.
Crystal structures of the synthesized samples were examined by a powder X-ray diffractometer (XRD, Rigaku D/MAX 2500 with Cu Kα1 radiation, λ = 0.154 nm). UV-Vis diffuse reflectance spectra (DRS) of the samples were measured in the range of 200–800 nm using a spectrophotometer (UV-2700, Shimadzu). Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 F20 with an accelerating voltage of 200 kV. The photoluminescence (PL) emission spectra were measured on a spectrofluorometer (Fluorolog-3 system, Horiba Jobin Yvon) using the Xenon lamp with a 325 nm as a source of excitation.
2.3. Photocatalytic Activity Evaluation
Photocatalytic activities of CN/KTO samples were evaluated by Rhodamine B (RhB) degradation in aqueous solution under visible light irradiation. 100 mL aqueous solution of RhB (4 mg/L) was put in a glass beaker, and 0.1 g photocatalyst was then added. In order to establish the adsorption-desorption equilibrium, the suspension was ultrasonically treated and stirred in the dark for 60 min, respectively. Photocatalytic activity was evaluated under irradiation from a 300 W Xenon lamp with a UV cutoff filter, which provides the visible light ranging from 420 to 700 nm. At each given irradiation time interval, 3 mL of the mixture was collected and then the slurry sample was centrifuged to separate the photocatalyst particles. The concentration of RhB was analyzed by measuring the maximum absorption at 553 nm using Shimadzu UV2700 spectrophotometer.
3. Results and Discussion
3.1. Characterization of the CN/KTO
The XRD patterns of KTO, g-C3N4, and their composites are shown in Figure 1. We can see that the pattern of KTO matches the standard data for a cubic structure (JCPDS Card number 38-1470). The formation of g-C3N4 can be confirmed by the strong characteristic peak at 2θ = 27.4° in its XRD pattern. For the composites, the XRD patterns can be indexed by two corresponding phases, that is, KTO and g-C3N4 (featured by the peak at 2θ = 27.4°), respectively.
From the UV-Vis diffuse reflectance spectra of KTO, g-C3N4, and their composites as shown in Figure 2, we can see clearly that KTO and g-C3N4 show absorption edges of 360 nm and 460 nm, respectively. For the composites, two absorption edges corresponding to KTO and g-C3N4 are observed. With the increase of g-C3N4 amount, the absorbance of the composite to visible light also increases.
The microstructures of KTO, g-C3N4, and their composites were observed on a transmission electron microscope (TEM). As shown in Figure 3(a), KTO is like small and irregular cube and the mean size is about 30 nm. The TEM image of g-C3N4 (Figure 3(b)) shows typical layered platelet-like morphology. From Figure 3(c) we can see clearly that the KTO nanoparticles are deposited on the surface of g-C3N4. In other words, KTO nanoparticles are wrapped well by g-C3N4.
3.2. Photocatalytic Activity
As shown in Figure 4, no noticeable degradation of RhB was observed with pure KTO photocatalyst or without photocatalyst. Pure g-C3N4 photocatalyst could degrade RhB by 65% in 90 min. All of the CN/KTO composites exhibit photocatalytic activities under visible light irradiation, indicating the success of hybrid. When the g-C3N4 amount was less than 50% of the total catalyst weight (such as gcn30, gcn50), the photocatalytic activities of gcn30 and gcn50 are inferior compared with g-C3N4. When the g-C3N4 amount was 70% of the total catalyst weight, the CN/KTO composite exhibited the best activity and nearly 90% RhB was photodegraded. The reason why gcn70 sample showed the best performance could be a result of competition between the following two facts, that is, the absorbance of CN/KTO composite to incident visible light and the effectiveness of photoexcited charge transfer from CN to KTO. When the g-C3N4 amount was small in the CN/KTO composite (such as gcn30 and gcn50), while the charge transfer from photoexcited CN to KTO could be secured, less visible light could be absorbed by CN because its surface was largely covered by the KTO nanocrystals. Therefore, the CN/KTO composite with too small amount of g-C3N4 did not show a higher activity than pure g-C3N4. On the contrary, when the amount of g-C3N4 was too high in the CN/KTO composite, although the absorbance of CN/KTO to visible light was sufficient, the charge transfer from photoexcited CN to KTO could not be secured due to the less amount of KTO. Thus, the CN/KTO composite with too large amount of g-C3N4 did not show a higher activity than pure g-C3N4. The competition and compromise between the absorbance to visible light and effectiveness of charge transfer give rise to the best performance of CN/KTO composite with an appropriate ratio of CN to KTO (such as the gcn70).
It is known that tert-butyl alcohol (TBA) can easily react with hydroxyl radicals () for a dehydrogenation reaction [18, 19]: + (CH3)3COH →H2O + C(CH3)2OH. As an effective electron donor, triethanolamine (TEOA) can supply electrons to combine with photogenerated holes . As for p-benzoquinone (p-benz), it can react with and possibly form phenol, implying that p-benz is an effective trapper [21, 22]. Considering the above-mentioned facts, TBA, TEOA, and p-benz could be employed as the scavengers for hydroxyl radicals (), photogenerated holes (h+), and superoxide radicals (), respectively. Corresponding control reactions were carried out with the purpose of clarifying the main active species in the photocatalytic process of RhB degradation.
As shown in Figure 5, when TBA was added, the photocatalytic degradation of RhB was decreased slightly. With the introduction of TEOA, the photocatalytic activity also decreased with the addition of hole-scavenger. At last, the introduction of scavenger for (p-benz, 0.5 mM) resulted in a remarkable deactivation. These results clearly indicate that superoxide radical plays the main role in the photocatalytic performance.
Repeatability of the photocatalytic activity was tested by running several cycles of photocatalytic degradation for RhB over gcn70. Each cycle ran for the same time of 90 min, and the photocatalyst was filtered to use for next cycle. As shown in Figure 6(a), gcn70 could still degrade RhB by nearly 90% after running 5 cycles, indicating quite good stability for photocatalytic degradation for RhB. Moreover, the XRD patterns spectra (Figure 6(b)) of gcn70 sample are almost the same before and after running for 5 cycles of photocatalytic degradation for RhB, which further proved the stability of the composite photocatalyst.
Figure 7 presents the PL spectra of the samples. Obviously, a KTO modification leads to a significant fluorescence quenching of the gcn70 composite because it prevents the recombination of photogenerated charge pairs. In order to rule out the possibility that the PL intensity decreased when the g-C3N4 was diluted with KTO in the composite, the intensity was normalized to the mass of g-C3N4.
As mentioned above, the composites CN/KTO have shown much better photocatalytic activity than either KTO or g-C3N4 under visible light irradiation. It is known that under visible light irradiation, g-C3N4 can be excited due to the appropriate band gap (2.7 eV), whereas KTO is inert owing to its wide band gap. Since the conduction band bottom of g-C3N4 (−1.13 eV)  is more negative than that of KTO (−0.96 eV) [1, 2, 16], the photoexcited electrons on g-C3N4 could be directly injected into the CB of KTO through the well-developed interfaces. The charge transfer from g-C3N4 to KTO was also confirmed by the significantly decreased photoluminescence intensity of gcn70 in comparison with that of g-C3N4 (see Figure 7). It is believed that the effective charge transfer can inhibit the recombination of photogenerated electrons and holes and thus enhance the photocatalytic activity. The photoinduced electrons diffuse to the surface and reacted with the oxygen molecule (electron acceptor), generating superoxide radicals . The active superoxide radicals are highly oxidative to decompose RhB effectively. On the basis of experimental results and theoretical analysis, a possible synergistic mechanism for the charge transfer between g-C3N4 and KTO and for the enhanced photocatalytic activity of RhB degradation was proposed as illustrated in Figure 8.
The novel g-C3N4/KTaO3 composites were successfully prepared by a facile and simple ultrasonic dispersion method. The resulting g-C3N4/KTaO3 composites showed an enhanced photocatalytic activity for degradation of RhB under visible light irradiation, and the optimal mass ratio of g-C3N4/KTaO3 was 70/30. Owing to the well-aligned energy band edges and interface between g-C3N4 and KTaO3, effective photogenerated charge carrier transfer and separation was evidenced by photoluminescence, which suppressed the recombination of electrons and holes. As a consequence, the photocatalytic activity of the g-C3N4/KTaO3 composite was significantly improved.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Basic Research Program of China (973 Program, Contract no. 2014CB239301). Zhiqing Yong is grateful to Dr. Naoto Umezawa and Professor Jinhua Ye (NIMS) for hosting his internship visit to NIMS. Dr. Hua Tong, Dr. Haiying Jiang (NIMS), and Dr. Xianguang Meng (NIMS) are appreciated for their valuable discussion and/or assistance in photocatalytic evaluation.
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