Trans. Nonferrous Met. Soc. China 27(2017) 1117-1126
Enhanced photocatalytic activity of TiO2 nanoparticles using WS2/g-C3N4 hybrid as co-catalyst
Li-li ZHENG, Xin-yan XIAO, Yang LI, Wei-ping ZHANG
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Received 14 March 2016; accepted 30 September 2016
Abstract:
TiO2/WS2/g-C3N4 composite photocatalysts were synthesized by a liquid-exfoliation-solvothermal method. In this process, the WS2/g-C3N4 nano-sheets were prepared by liquid-exfoliation method from the bulk WS2 and C3N4 in the alcohol system, and then the TiO2 nanoparticles (NPs) grew on the WS2/g-C3N4 nano-sheets by in-situ synthesized technique. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic degradation of methyl orange (MO). The results showed that the as-prepared samples exhibited higher photocatalytic activities as compared to the pure TiO2, g-C3N4 and TiO2/g-C3N4 composite. The enhanced photocatalytic activities of TiO2/WS2/g-C3N4 photocatalysts could be attributed to the synergistic effect of heterostructure between TiO2 NPs and WS2/g-C3N4 nano-sheets, which could efficiently improve the separation of photogenerated electron/hole pairs and utilization efficiency of photons. The quenching tests of radicals indicated that 
had crucial effect on degradation of MO, which demonstrated that was the main active radical in photocatalytic reaction process.
Key words:
g-C3N4; TiO2; WS2; photocatalysis; liquid-exfoliation; solvothermal method;
1 Introduction
TiO2 has been extensively and deeply investigated to decompose organic pollutants in environment purification for its high photocatalytic activity, excellent chemical stability and inexpensive property. However, pure TiO2 has low efficiency in utilization of solar energy and high recombination rate of photogenerated carriers. Currently, numerous methods have been attempted to enhance the photocatalytic activity of TiO2. Among these methods, TiO2 coupling with layered semiconductors, such as MoS2, WS2, and SnS2, has been attracted much attention to obtain the formation of heterostructure junction and novel TiO2-based photocatalysts with high photocatalytic activity for its simple preparation process. LENG et al [1] reported a TiO2-pristine-graphene hybrid via a continuous supercritical solvothermal technique that facilitated the complete degradation of methyl orange under UV light for 180 min. BASSAID et al [2] reported a simple sol-gel method to fabricate WS2/TiO2 composite photocatalyst and achieve intimate contact between TiO2 and WS2, which enhanced photocatalytic activity of WS2/TiO2 to decompose orange II under UV light. MIRANDA et al [3] reported a simple impregnation method to obtain g-C3N4/TiO2 composite, which can efficiently promote the separation of photogenerated electron-hole pairs and improve the photocatalytic activity of TiO2. The synergetic effect between catalysts significantly enhanced the photocatalytic performance in degrading phenol under UV irradiation.
Graphitic carbon nitride material (g-C3N4) has been studied a lot for the unique properties in heat endurance and chemical resistance, as well as the layered structure similar to those of graphene. It is also considered as the most stable allotrope of C-N composite for its primary building block of tri-s-triazine [4]. Since the researches about using bulk g-C3N4 as photocatalyst for water splitting were reported, massive efforts have been taken to study the combining bulk g-C3N4 with various semiconductors to obtain composite photocatalysts with excellent photocatalytic activity, such as g-C3N4-CdS [5,6], g-C3N4/Ag3PO4 [7], CuO@TiO2 [8], g-C3N4/SnO2 [9] and TiO2/g-C3N4 [10]. The combination of TiO2 and g-C3N4 has been attracted much more attention for their nontoxic resource and outstanding photocatalytic activity. FU et al [11] adopted a solid-state approach to synthesize TiO2/g-C3N4 nano-composites. The heterostructure formed between g-C3N4 and TiO2 could enhance the photocatalytic activity for degradation of methyl blue. 
et al [12] reported g-C3N4 modified TiO2-based photocatalysts to disintegrate toluene under UV and visible light irradiation. In order to obtain photocatalysts with high separation rate of photoinduced carriers, some researchers employed nonmetal-doping and g-C3N4 to co-modify TiO2. YANG et al [13] reported a N-doped TiO2/g-C3N4 composite, which was synthesized by heating the mixture of hydrolysis product of TiCl4 and C3N4, leading to decrease the recombination rate of photogenerated electron-hole pairs. PANY et al [14] reported a nano-composite of N, S-TiO2/g-C3N4 prepared via in situ thermal induced polymerization method. The synergistic effect of crystallite size, the crystalline anatase phase, enlarged specific surface area was closely related to enhancement of visible-light absorption property and optimization of photocatalytic hydrogen evolution under visible-light irradiation.
In this study, a liquid-exfoliation-solvothermal method was employed to prepare TiO2/WS2/g-C3N4 composite photocatalyst for enhancing photocatalytic activity. In the first step, bulk g-C3N4 and WS2 were treated by mixture solution of hydrochloric acid and ethanol to obtain WS2/g-C3N4 composite with rough surface and uniform pore structure. Then, TiO2 NPs grew on the surface of WS2/g-C3N4 hybrid, which increased the specific surface area and achieved intimate contact among TiO2, WS2 and g-C3N4. The photocatalytic activities of as-prepared photocatalysts were evaluated using degradation of MO under simulated sunlight irradiation. The photocatalytic degradation mechanism was also tentatively analyzed.
2 Experimental
2.1 Materials
All reagents used in the experiments were analytically pure and purchased from the company without further purification. Among these, thiourea and titanium tetrachloride were obtained from Fuchen Chemical Reagent Corporation in China. Dicyandiamide, tertbutyl alcohol (TBA) and disodium ethylene diamine tetraacetate (EDTA-2Na) were purchased from Shanghai Lingfeng Chemical Reagent Corporation in China. Besides, tungsten disulfide (WS2) and terephthalic acid (PTA) were purchased from Aladdin Reagent Corporation. Benzoquinone (BQ) was obtained from Sinopharm Chemical Reagent Corporation in China. Methyl orange (MO) was purchased from Nanhua Chemical Reagent Corporation in China.
2.2 Preparation and protonation of bulk g-C3N4
Bulk g-C3N4 was prepared by simple calcination [15] and protonation method [4]. Typically, 2.5 g thiourea and 2.5 g dicyandiamide were placed in a mortar and ground into a uniform powdered mixture. Then, the mixture was calcined at 500 °C for 1 h and further thermally treated at 550 °C for 2 h. And the bulk g-C3N4 was ground into powder at the room temperature. Finally, g-C3N4 was treated with 8% hydrochloric acid ethanol solution for 48 h to improve its dispersibility and surface activity, then washed with deionized water and anhydrous ethanol, and collected after drying at 80 °C over 3 h.
2.3 Preparation of TiO2/WS2/g-C3N4 composite photocatalysts
TiO2/WS2/g-C3N4 composite photocatalysts were prepared via a two-step solvothermal method. In the first step, 0.5 g bulk g-C3N4 and 0.02 g bulk WS2 were dispersed in 18 mL mixed solution of hydrochloric acid and anhydrous ethanol under ultrasonic treatment for 1 h. The obtained dispersion liquid was transferred to a 25 mL teflon-lined stainless steel autoclave and heated at 80 °C for 10 h. After the autoclave was naturally cooled down to room temperature, the samples were separated by centrifuge, washed with anhydrous ethanol and dried at 60 °C over 3 h to obtain WS2/g-C3N4 composite.
In the following step, the deposition of TiO2 NPs on WS2/g-C3N4 composite was carried out by solvothermal method similar to the previous report [16]. Firstly, 4 mL TiCl4 ethanol solution (2 mol/L) and certain amount of WS2/g-C3N4 composite were dispersed in 16 mL anhydrous ethanol under ultrasonic treatment for 1 h. Then, 5.6 mL mixed solution of sodium hexameta- phosphate, ethanol and deionized water was slowly dropped into mixture to promote the hydrolysis of TiCl4. Thirdly, the obtained TiO2/WS2/g-C3N4 precursor was transferred into a 25 mL Teflon-lined stainless steel autoclave and treated at 140 °C for 3 h. Finally, TiO2/WS2/g-C3N4 composite photocatalyst was received after being centrifugalized, washed with anhydrous ethanol and dried at 80 °C.
In order to simply signify the prepared samples, WS2/g-C3N4 composite was denoted as WG. Also, TiO2/WS2/g-C3N4 composite photocatalysts were marked as TiO2/WG-x, where x represented the mass fraction of WG composite to TiO2 NPs and was controlled to be 3%, 5%, 7% and 9%. Besides, for comparison, TiO2/g-C3N4 composite photocatalyst was also prepared via the similar two-step solvothermal process without the addition of WS2.
2.4 Characterization
FT-IR spectra of TiO2/WS2/g-C3N4 composite photocatalyst were recorded on Bruker Tensor-27. XRD analysis was carried out to determine the crystalline structure of samples at room temperature with a Bruker D8 Advance X-diffractometer using Cu Ka radiation (λ=1.5406 
), operated at 40 kV and 40 mA, and a scanning speed of 5 (°)/min in the 2θ range from 5° to 80°. The specific surface area was performed at the temperature of liquid nitrogen using a Beishide 3H-2000PS1 analyzer. The images of surface morphologies were taken with Merlin scanning electron microscope (SEM). The composition and chemical state of the sample were analyzed by the ULVAC-PHI PHI X-tool X-ray photoelectron spectrometer, using Al Ka as X-ray source. Ultraviolet-visible diffraction spectra (UV-vis DRS) were carried out on a Hitachi U-3010 spectrophotometer with wavelength range from 800 to 200 nm, using BaSO4 as a reflectance standard. UV-vis patterns were recorded on a Shimadzu UV-2450 spectrophotometer. Photoluminescence (PL) spectra were collected on a JASCO FP-6500 type fluorescence spectrophotometer with 321 nm excitation source over a wavelength range from 300 to 800 nm. Additionally, the photocatalytic degradation of MO was carried out in a SGY-I multifunction photo-reactor apparatus (Nanjing Sidongke Co., China), using a 500 W xenon lamp as light source.
2.5 Evaluation of photocatalytic activity
Photocatalytic activity of TiO2/WS2/g-C3N4 composite photocatalysts was estimated by photocatalytic degradation of MO under the light irradiation in a photoreaction apparatus. In the experiments, a 500 W xenon lamp was employed as the light source to provide visible light. In a typical experiment, 50 mg photocatalyst was dispersed in 250 mL of 20 mg/L MO aqueous solution. Firstly, the suspension was magnetically stirred in the dark for 30 min to achieve adsorption–desorption equilibrium of MO on the surface of photocatalyst. Then, the suspension was exposed to simulated sunlight irradiation and 4 mL of sample was collected with each irradiation time interval of 10 min. After 60 min of irradiation, the collected samples were centrifugalized to remove photocatalyst and monitored using an UV-vis spectrophotometer at 464 nm with deionized water as a reference sample. The concentration of MO was directly calculated by its characteristic absorption peak at 464 nm. And the degradation rate (η) of MO under simulated sunlight was calculated by the following equation: η=Ct /C0×100%, where Ct and C0 are the concentrations of MO in solution at each given time and initial one.
3 Results and discussion
3.1 Characterization of TiO2/WS2/g-C3N4
X-ray diffraction patterns of TiO2/WG photo- catalysts, as well as TiO2/g-C3N4, WS2/g-C3N4 and pure g-C3N4 samples are shown in Fig. 1. All recognizable peaks of TiO2 corresponded to anatase TiO2 (JCPDS card No. 21-1272) [17-19]. Consistent with the previous report [11], the g-C3N4 sample had two characteristic diffraction peaks at 13.1° and 27.4°, respectively. The intense reflection peak at 27.4° indicated the existence of interlayer stacking of aromatic systems, corresponding to the (002) plane diffraction. The low-angle diffraction peak at 13.1° corresponded to the (100) crystal plane of g-C3N4. As for WS2/g-C3N4 composite, it can be observed that the characteristic diffraction peaks of WS2 (JCPDS card No. 08-0237) and g-C3N4 appeared in the spectra, illustrating that g-C3N4 and WS2 coexisted in the composite. Besides, the addition of WS2/g-C3N4 and g-C3N4 decreased the average grain size of TiO2 NPs, but had no obvious influence on the crystalline structure, suggesting that WS2/g-C3N4 and g-C3N4 suppressed the growth of TiO2 NPs to some extent. As shown in Table 1, the average size of TiO2 NPs in composites was less than 10 nm, which indicated that the size of TiO2 NPs was under control when coupled with WS2/g-C3N4 hybrid.
Fig. 1 XRD patterns of as-prepared TiO2/WS2/g-C3N4, TiO2/g-C3N4, TiO2, WS2/g-C3N4 and g-C3N4
Table 1 Surface morphological properties of prepared samples
In order to analyze the interactions among TiO2, WS2 and g-C3N4, FT-IR spectrometer was employed to characterize TiO2/WS2/g-C3N4, TiO2/g-C3N4, TiO2, WS2/g-C3N4 and g-C3N4, and the results are shown in Fig. 2. For the pure g-C3N4, three main absorption bands were observed around 3200 cm-1, 1200-1700 cm-1 and 808 cm-1, respectively. The broad band at 3200 cm-1 was attributed to the stretching vibration modes of N—H and O—H groups. The intense absorption region at 1200- 1700 cm-1 was composed of peaks at 1640, 1573, 1413, 1318 and 1240 cm-1, which were associated with the stretching modes of CN heterocycles. Additionally, the peak at 808 cm-1 can be ascribed to out of plane bending modes of tri-s-triazine ring which has been wildly accepted as the primary building block of g-C3N4 [20]. Due to the small amount of WS2 in composite, there was no obvious difference in FT-IR spectra between WS2/g-C3N4 composite and g-C3N4. In terms of TiO2, the broad peak at 400-700 cm-1 was characteristic of Ti—O stretching and Ti—O—Ti bridging stretching modes. And peaks observed at 3430 and 1630 cm-1 were attributed to the absorption of O—H stretching vibration and O—H bending mode, respectively [21]. Compared with TiO2, it is noted that major characteristic peaks of TiO2 existing in the TiO2/WG-5 and TiO2/g-C3N4 composite photocatalysts. Besides, it can be noted that there was an apparent redshift of the absorption of O—H stretching vibration at about 3300 cm-1, indicating the strong interaction between TiO2 and WS2/g-C3N4 composite. The interaction was conductive to the intimate contact among TiO2, WS2 and g-C3N4 and facilitated the separation of photogenerated electron/hole pairs.
Fig. 2 FT-IR spectra of TiO2/WS2/g-C3N4, TiO2/g-C3N4, TiO2, WS2/g-C3N4 and g-C3N4 samples
SEM images of bulk WS2, bulk g-C3N4, WS2/g-C3N4 hybrid and TiO2/WS2/g-C3N4 composite photocatalyst are shown in Fig. 3. From Figs. 3(a) and (b), it can be observed that bulk WS2 displayed a lamellar structure with a lamellar thickness of about 200 nm while g-C3N4 showed the destruction of the surface layered structure after g-C3N4 was treated by protonation, which were consistent with the previous research [4]. In terms of WS2/g-C3N4 composite (Fig. 3(c)), the surface morphology showed a breaking up of both laminated and stacking structure, as well as the development of pore structure among single layers. Besides, the formation of nanoparticles on the surface of the composite was in favor of the preparation of heterostructure between TiO2 and WS2/g-C3N4 hybrid. In TiO2/WS2/g-C3N4 photocatalyst (Fig. 3(d)), TiO2 NPs uniformly aggregated covering WS2/g-C3N4 composite to achieve a rough surface with large specific surface area (SBET) which was beneficial to improve the absorption of degradation products (Table 1) [3].
Fig. 3 SEM images of bulk g-C3N4 (a), bulk WS2 (b), WS2/g-C3N4 hybrid (c), and TiO2/WG-5 composite photocatalyst (d)
Fig. 4 XPS spectra of TiO2/WG-5 composite photocatalyst
In order to further investigate the components and chemical state of elements in TiO2/WG-5 composite photocatalyst, XPS was conducted to characterize the prepared photocatalyst and the results are shown in Fig. 4. C 1s (284.5 eV) peak was referred to the standard of all binding energies in XPS analysis. Two XPS signals can be observed at the binding energies of 285.2 and 288.2 eV (Fig. 4(a)), which were assigned to defect- containing sp2-hybridized carbon atoms present in graphitic domains and sp3-bonded C of N—C=N2, respectively [22]. The peak of N 1s was decomposed into three peaks at 399.0, 399.8 and 400.5 eV (Fig. 4(b)), which were ascribed to the groups of N (C=N—C), (N—(C)3 and C—N—H), respectively [3,23]. From Fig. 4(c), the peaks at the binding energies of 32.6 and 34.8 eV were observed, which were assigned to W 4f7/2 and W 4f5/2 of W4+, respectively [24]. The binding energies of 36.2 and 38.0 eV were ascribed to the peak of W6+, which was attributed to the oxidation of WS2 [25,26]. As shown in Fig. 4(d), the S 2p binding energies at 161.1 and 163.0 eV were observed, which indicated that the S2- existed in the composite photocatalyst [24]. It can be observed two XPS signals at the binding energies of 458.8 eV and 464.5 eV in Fig. 4(e), indicating the existence of Ti4+ in photocatalyst [27]. The O1s peaks were identified at around 530.1 and 531.9 eV in Fig. 4(f), which were attributed to the Ti—O in TiO2 and H—OH groups, respectively [28]. These results demonstrated that the prepared TiO2/WS2/g-C3N4 composite did not transform to other materials except a small amount of WS2 dissolved by hydrochloric acid solution.
3.2 Photocatalytic performance of TiO2/WS2/g-C3N4
UV-vis diffuse reflection spectra of as-prepared TiO2/g-C3N4 and TiO2/WG-5 composite photocatalysts, as well as TiO2, g-C3N4 and WS2/g-C3N4 composite are shown in Fig. 5. It can be observed that g-C3N4 exhibited intense optical absorption in the region of wavelength less than 450 nm while had little absorption in the region of wavelength more than 450 nm, which was attributed to its narrow band gap of 2.7 eV [5]. Bulk WS2 had excellent absorption capacity in ultraviolet and visible light region for its indirect band gap of around 1.4 eV and a direct band gap of 2.01 eV [29], as well as 1.72 eV of nano WS2 [2]. Accordingly, WS2/g-C3N4 composite achieved stronger visible-light absorption than pure g-C3N4. Afterwards, compared with pure TiO2, TiO2/WS2/g-C3N4 photocatalyst had stronger absorption in visible-light region with its absorption band edge red shift. Therefore, the results indicated that the prepared TiO2/WS2/g-C3N4 composite photocatalyst had excellent optical absorption in the ultraviolet and visible regions.
Fig. 5 UV-vis diffuse reflection spectra of TiO2, g-C3N4, WS2/g-C3N4 composite and TiO2/WS2/g-C3N4 composite photocatalysts
The photocatalytic activity of the prepared samples was evaluated by degradation of MO under simulated sunlight irradiation and the results are shown in Fig. 6. In the absence of photocatalyst, there was no noticeable curve descending of Ct/C0 in MO solution, ruling out the direct photodegradation of MO under illumination. For comparison, pure TiO2 was conducted to photocatalytic degradation of MO under the same conditions, showing lower photocatalytic activity. Because the heterostructure junction was produced between TiO2 NPs and bulk g-C3N4 [11,12,30], TiO2/g-C3N4 photocatalyst achieved enhanced the photocatalytic performance. As for the prepared TiO2/WG-x (x=3, 5, 7 and 9, respectively) composite photocatalysts, all of them exhibited excellent photocatalytic degradation with MO degradation rate of 84.7%, 95.8%, 94.8% and 81.2% under simulated- sunlight irradiation for 60 min, respectively. And the optimum mass ratio of WG to TiO2 was 0.05:1, which showed the best photocatalytic behavior. Additionally, the physical mixtures of TiO2-WS2 and TiO2-g-C3N4 were prepared by blending TiO2/WS2-0.2 and TiO2/g-C3N4-4.8 according to the mass ratio of 1:24. The photocatalytic performance of the prepared mixture was obviously inferior to that of TiO2/WG composite photocatalysts, illustrating that a unique heterostructure was formed among TiO2 NPs, bulk WS2 and bulk g-C3N4. In other words, the synergistic effect of bulk WS2 and g-C3N4 promoted the separation of photogenerated hole-electron pairs in TiO2/WG composite photocatalysts and improved their photocatalytic activity [31].
Fig. 6 Photocatalytic activities of TiO2, TiO2/g-C3N4, TiO2/WS2+TiO2/g-C3N4 and TiO2/WG-x (x=3, 5, 7 and 9) photocatalysts for MO degradation under simulated sunlight
3.3 Photocatalytic mechanism of TiO2/WS2/g-C3N4
Fluorescence technique was an important method to evaluate the migration, transfer and recombination of photogenerated hole/electron pairs in photocatalysts [5]. Figure 7 showed the fluorescence spectra of pure TiO2, WS2/g-C3N4, TiO2/g-C3N4 and TiO2/WS2/g-C3N4 samples with excitation wavelength of 321 nm. It can be observed that pure g-C3N4 had a strong fluorescence emission peak at about 441 nm due to its high recombination rate of photoinduced holes and electrons. In terms of WS2/g-C3N4, TiO2/g-C3N4 and TiO2/WS2/g- C3N4 photocatalysts, the PL intensities significantly decreased due to the hetero-structures, reducing the recombination of photoexcited electron/hole pairs. Moreover, 398 nm was the quenching of intrinsic fluorescence of TiO2, and the PL intensities of TiO2/g-C3N4 and ternary TiO2/WS2/g-C3N4 composites at 398 nm are lower than that of pure TiO2, suggesting a lower recombination of photo-generated e/h+ pairs [32].
Fig. 7 PL spectra of pure TiO2, WS2/g-C3N4, TiO2/g-C3N4 and TiO2/WS2/g-C3N4 samples
During the photocatalytic degradation process, it was known that main reactive species were generated to oxidize organic pollutants [33,34]. To elucidate the photocatalytic reaction mechanism of TiO2/WG photocatalyst, TBA, EDTA-2Na and BQ were considered as scavengers of hydroxyl radicals (·OH), holes (h+) and superoxide radical ions (
) [35], respectively. Figure 8 showed that the photocatalytic degradation efficiency of MO decreased significantly in the presence of TBA and BQ compared with that of TiO2/WG-5 photocatalytic degradation system without addition of capture agent. On the other hand, the appearance of EDTA-2Na in photocatalysts system promoted the photocatalytic degradation of MO. Therefore, it was concluded that ·OH and played an important role in the photocatalytic degradation of MO while h+ recombining with e led to low efficiency in photocatalytic degradation of MO.
Fig. 8 Photocatalytic degradation of MO over TiO2/WG-5 in the presence of TBA, EDTA-2Na and BQ as capture agent
To further investigate the photocatalytic reaction mechanism, hydroxyl radicals (·OH) were detected in the TiO2/WG-5 photocatalytic system under simulated sunlight irradiation by fluorescence technique. Terephthalic acid (PTA) was employed as probe molecule to react with hydroxyl radicals on the surface of photocatalyst and produce hydroxyl terephthalic acid (PTAOH). Figure 9 showed that the fluorescence spectra excited at 321 nm in the TiO2/WG-5 photocatalyst system with the addition of different capture agents. Compared with TiO2/WG-5 photocatalytic system without addition of capture agents, the presence of TBA could slightly reduce the fluorescence intensity of hydroxyl radicals while BQ suppressed the formation of hydroxyl radicals severely. According to the entrapping experimental, removing the h+ could obviously enhance the photocatalytic activity of composite photocatalyst, thus, the PTA could be degraded by the photocatalyst when adding the EDTA-2Na into the reaction system, leading to the decrease of PL intensity.
Fig. 9 Fluorescence intensity at 450 nm against irradiation time in TiO2/WG-5 photocatalyst system with different capture agents
According to the results of carrier trapping experiments and fluorescence spectra, the ·OH was mainly produced by the transform of . It was universally accepted that hydroxyl radicals (·OH) in photocatalytic system were mainly generated by following processes (Eqs. (1) and (2)): reactive electrons reduced oxygen absorbed on the surface of photocatalyst to superoxide radical ions (), then the formed combined with an electron to finally produce ·OH [33,35]. Meanwhile, g-C3N4, as well as TiO2 exhibited adequate reduction potential to achieve the formation of ·OH via a two-electron reduction reaction process [36]. At the same time, EDTA-2Na reacting with h+ improved the separation rate of photoinduced carriers and promoted the photocatalytic degradation of MO. The separated electrons reduced oxygen to and then the formed degraded MO under irradiation, which corresponded to the results of carrier trapping experiments. Therefore, we can draw a conclusion from the analysis above. intermediate species mainly took the responsibility to degrade the MO while ·OH played the minor role in degradation of MO in photocatalytic system under simulated sunlight.
(1)
(2)
A probable reaction mechanism of photocatalytic degradation of MO was proposed in Fig. 10. When the photocatalytic system of TiO2/WG-5 was exposed to the illumination, the electrons on the valence band of TiO2, g-C3N4 and WS2 were excited to their conduction bands and generated electron-hole pairs (Eq. (3)). On one hand, because the position of conduction band of g-C3N4 and WS2 were lower than that of TiO2, photoexcited electrons were transformed from conduction band of g-C3N4 and WS2 to that of TiO2 simultaneously. The h+ on the valence band of g-C3N4 and WS2 cannot oxidize H2O into ·OH owing to the lower oxidation potential. Moreover, the aggregated electrons on the conduction band of TiO2 moved to the surface of photocatalyst and reduced absorbed O2 to (Eq. (4)). Certain amount of further reacted with electrons to form ·OH (Eq. (5)). On the other hand, some of photoinduced holes in the valance band of TiO2 directly oxidized H2O to ·OH while others injected into that of g-C3N4 and WS2 as a result of electric potential of valence band of TiO2 superior to others. Finally, these formed actives species (, ·OH) completely degraded MO under irradiation (Eq. (6)). Thus, the synergetic effect of g-C3N4 and WS2, enlargement of the specific surface area and high separation of photoinduced carriers, significantly improved the photocatalytic activity of as-prepared photocatalysts.
Fig. 10 Photocatalytic mechanism for MO degradation in TiO2/WG photocatalytic system under simulated sunlight
(3)
(4)
(5)
(6)
4 Conclusions
TiO2/WS2/g-C3N4 composite photocatalysts were successfully synthesized by a two-step solvothermal treatment method. In the first step, solvothermal treatment, WS2/g-C3N4 composite was prepared with rough surface and uniform pore structure. In the following process, the prepared WS2/g-C3N4 composite was successfully coated by TiO2 nanoparticle, which achieved the intimate contact among TiO2, bulk WS2 and g-C3N4. The characterization results indicated that as-prepared photocatalysts had good optical absorption property in ultraviolet and visible light regions and low recombination rate of photogenerated electron-hole pairs compared with pure TiO2 and TiO2/g-C3N4 photocatalysts. Moreover, TiO2/WS2/g-C3N4 composite photocatalysts exhibited excellent photocatalytic activity for degradation of MO with degradation rate of more than 95% under irradiation for 60 min. The results of carrier trapping experiments and fluorescence spectra illustrated that rather than ·OH and h+ played the key role in photocatalytic degradation of MO.
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以WS2/g-C3N4杂化复合物为共催化剂提高TiO2光催化活性
郑莉莉，肖新颜，李 阳，张卫平
华南理工大学 化学与化工学院，广州 510640
摘 要：采用液相剥离和溶剂热法制备TiO2/WS2/g-C3N4复合光催化剂。通过液相剥离的方法在乙醇体系中将块状WS2和C3N4剥离得到相应的纳米片；利用热处理法使TiO2纳米粒子原位生长并固定于WS2/g-C3N4纳米片上。采用光催化降解甲基橙(MO)来评价TiO2/WS2/g-C3N4的光催化活性。结果表明，TiO2/WS2/g-C3N4复合光催化剂的光催化活性远高于纯态TiO2，g-C3N4 及TiO2/g-C3N4 复合物，这主要归因于WS2/g-C3N4杂化复合物与TiO2纳米粒子之间的协同作用，有效促进了复合光催化剂中光生电子/空穴对的分离，提高了光子的利用率。活性自由基的捕获实验表明，对MO的降解起着决定性的作用，这说明是光催化反应过程中主要的活性自由基。
关键词：g-C3N4；TiO2；WS2；光催化；液相剥离；溶剂热法
(Edited by Xiang-qun LI)
Foundation item: Projects (21376099, 21546002) supported by the National Natural Science Foundation of China
Corresponding author: Xin-yan XIAO; Tel: +86-13668995789; E-mail: cexyxiao@scut.edu.cn
DOI: 10.1016/S1003-6326(17)60130-4
Abstract: TiO2/WS2/g-C3N4 composite photocatalysts were synthesized by a liquid-exfoliation-solvothermal method. In this process, the WS2/g-C3N4 nano-sheets were prepared by liquid-exfoliation method from the bulk WS2 and C3N4 in the alcohol system, and then the TiO2 nanoparticles (NPs) grew on the WS2/g-C3N4 nano-sheets by in-situ synthesized technique. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic degradation of methyl orange (MO). The results showed that the as-prepared samples exhibited higher photocatalytic activities as compared to the pure TiO2, g-C3N4 and TiO2/g-C3N4 composite. The enhanced photocatalytic activities of TiO2/WS2/g-C3N4 photocatalysts could be attributed to the synergistic effect of heterostructure between TiO2 NPs and WS2/g-C3N4 nano-sheets, which could efficiently improve the separation of photogenerated electron/hole pairs and utilization efficiency of photons. The quenching tests of radicals indicated that had crucial effect on degradation of MO, which demonstrated that was the main active radical in photocatalytic reaction process.
