Photocatalytic technology is widely considered to be one of the most effective ways to manage pollution [1-5]. For many years, researchers have been looking for efficient photocatalysts. Many photocatalysts have been found, such as TiO2 [6, 7], CeO2 [8, 9], BiOX (X = Br, Cl, I) [10, 11], BiPO4 [12, 13] and ZnO [14, 15]. These photocatalysts have many advantages, such as high efficiency, low harm and low price [16, 17]. However, these photocatalysts can only absorb light in the ultraviolet (UV) region because of their relatively large band gap. To achieve high-efficiency photocatalysis, the use of semiconductors that are capable of absorbing a broad spectrum of solar light is desirable [18, 19]. Recently, various photocatalysts with visible light response have been developed, such as Ag3PO4 [20, 21], Bi2MoO6 [22, 23], SnNb2O6 [24, 25], g-C3N4 [26-28], BiVO4 [29-31], CdSe [32, 33] and CdS [34, 35]. These semiconductor photocatalysts have excellent photocatalytic activity in the visible light region. For example, Wang et al. [36] successfully prepared Ag3PO4 as a highly recyclable photocatalyst. Ying et al. [37] fabricated hierarchical Bi2MoO6 nanosheet-built frameworks with excellent photocatalytic properties. Jiang et al. [38] synthesized BiOCl single-crystalline nanosheets with facet-dependent photoreactivity.
Among these semiconducting materials, Ag3PO4 has attracted considerable attention because of its excellent photocatalytic activity [39-41]. Its efficiency is highly sought after by researchers. However, the photocatalytic performance of Ag3PO4 decreased significantly after several cycles, because the photo-generated electrons and holes are likely to recombine rapidly [42]. The instability of Ag3PO4 largely limits its practical application [43, 44]. Therefore, it is very urgent and essential to design some novel composite photocatalysts to inhibit the recombination of electron–hole pairs [45-47].
Z-scheme composites have many advantages in improving the photocatalytic activity [48, 49]. For example, Zhang et al. [50] found that the Z-scheme Bi12GeO20/g-C3N4 composite photocatalyst could efficiently degrade organic pollutants. Tang et al. [51] demonstrated Z-scheme C3N4-based systems for pure water splitting. The Z-scheme composite photocatalysts can combine the advantages of different photocatalysis and overcome the shortcomings of a single photocatalyst. In fact, besides the spatial separation of redox reactions, the other important advantage over single-catalyst systems is that Z-scheme systems replace a wide band gap photocatalyst by two narrow ones, thus harvesting more photons from light. Consequently Z-scheme systems have a theoretically higher efficiency than single-catalyst systems [52, 53]. In this way, the redox reactions take place separately, which can effectively prevent the recombination of electrons and holes [54, 55], so the performance of the composite photocatalyst is greatly improved. Moreover, the Z-scheme composites can give full play to the redox ability of single photocatalyst and further enhance the photocatalytic activity [56, 57]. Nevertheless, after comprehensive consideration of many factors, such as the band gap and band alignment, finding the right semiconductor is still a formidable challenge.
WO3(H2O)0.333 is a kind of semiconductor materials with a visible light responsive band gap (2.78 eV), which is similar to WO3 and has been widely applied in fields of photocatalysis, and the band gap is matched with Ag3PO4. Compared to previously reported [58], the traditional form of WO3 is lumpy or rod like with the size of 200–300 nm. WO3(H2O)0.333 shows a linear shape, its size is about 50 nm, and it can be fully combined by Ag3PO4. WO3(H2O)0.333 was successfully fabricated by a simple microwave hydrothermal method followed by annealing. Herein, we successfully prepared WO3(H2O)0.333/Ag3PO4 composites by in-situ synthesis. The constituent elements of the photocatalysts are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS). The morphology of photocatalyst was observed by SEM. The absorption light spectrum was determined by UV-Vis diffuse reflectance spectroscopy (DRS). From the photoluminescence (PL) measurements, we know that the separation efficiency of the WO3(H2O)0.333/Ag3PO4 composite is the highest. This indicates that the composites can largely reduce the recombination rates of electrons and holes. Through the degradation of methylene blue (MB), it is confirmed that the photocatalytic activity of WO3(H2O)0.333/Ag3PO4 was much higher than that of pure WO3(H2O)0.333 and Ag3PO4. In addition, after repeated testing, the 15%WO3(H2O)0.333/Ag3PO4 almost completely degraded organic pollutants in 4 min under visible light irradiation. After five cycles, the degradation ability of the composites has almost no decay, suggesting it has good stability. This work may provide the possibility of efficient and stable decomposition of organic pollutants in a practical environment.
Silver nitrate (AgNO3), ammonium hydroxide (NH3·H2O), dibasic sodium phosphate (Na2HPO4), nitric acid (HNO3), sodium tungstate dihydrate (Na2WO4·H2O), sodium chloride (NaCl), and hydrogen chloride (HCl) were bought from the Shanghai Chemical Reagent Co. Ltd. (China). L-ascorbic acid, isopropanol (IPA), triethanolamine (TEOA), and MB were bought from Sinopharm Chemical Reagent Corp. (China).
Na2WO4·H2O (0.28 g) and NaCl (0.04 g) were mixed in solution of 30 mL deionized water (DW) and stirred for 30 min. Then, HCl was added to the mixed solution to adjust pH = 2 and stirred for another 2 h. The mixture was transferred into a reaction kettle and the autoclave reacts at 453 K for 30 min in the microwave digestion apparatus and then cooled to room temperature and then washed with DW repeatedly. The product was finally obtained by centrifugation and dried in a freeze dryer for 12 h to obtain the WO3(H2O)0.333 nanowires.
A certain amount of AgNO3 was added in 20 mL DW and stirred for 10 min. Then, NH3·H2O of pH = 10 was added dropwise to the AgNO3 solution, which first became turbid and then clear. After that, a certain amount of WO3(H2O)0.333 was added and stirred for 5 min, and the mixed solution was sonicated for 10 min. Then, an appropriate amount of Na2HPO4 solution was added dropwise to the mixed solution and stirred for 10 min. Then, 1.0 mol/L HNO3 was dropped into the mixed solution until the pH of the solution was 7. The mixed solution was washed several times with 18.2 MΩ DW, and the washed precipitation was frozen in a refrigerator for several hours. Finally, the precipitation was placed in the freeze dryer for 12 h, and the WO3(H2O)0.333/Ag3PO4 complex was obtained. The preparation process of the WO3(H2O)0.333/Ag3PO4 composites is shown in Fig. 1.
XRD patterns of WO3(H2O)0.333, Ag3PO4, and WO3(H2O)0.333/Ag3PO4 composites were recorded on a DX-2000 diffractometer with the 2θ range from 10° to 80°. SEM (FE-SEM Hitachi S5500) was implemented to examine the granularity and structure of the samples. XPS was performed on a Thermo ESCALAB 250 spectrometer. The UV-Vis DRS measurements were surveyed with a PerkinElmer Lambda 950 UV-Vis spectrophotometer. The specific surface area of the samples was measured by N2 adsorption using the multipoint Brunauer-Emmett-Teller (BET) method (ASAP 2010). The photoelectrochemical measurements were carried out on a Shanghai Chenhua CHI-660D electrochemical system. The electrolyte solution was 1.0 mol/L Na2SO4. A 0.05 g catalyst sample was mixed with 50 μL 5% nafion and 0.5 mL ethyl alcohol absolute to make slurry. The slurry was then injected onto a 1.0 cm2 ITO conductive glass electrode and these electrolytes were dried at 333 K for 30 min.
The photocatalyst (30 mg) was mixed with MB solution (30 mL, 20 mg/L). Light-emitting diode (LED) arrays (active power 50 W, λ = 410 nm) were used as a solar model. The irradiation time intervals were set at 2 min and the process was repeated five times. After each light irradiation, the absorbance of the MB solution was determined using the clear liquid obtained after centrifuging the mixture. The dominant reactive species affecting MB photodegradation were identified using IPA, L-ascorbic acid, and TEOA as scavengers for ·OH, ·O2−, and holes, respectively. In a typical process, either L-ascorbic acid (150 mg), IPA (20 mmol/L, 5.4 mL), or TEOA (10 mmol/L, 6.1 mL) was added to a mixture of the photocatalyst and MB solution. Radical-trapping experiments were performed using the same method as for the photodegradation experiments.
Fig. 2 shows the XRD patterns of the samples. The strong diffraction peaks centered at 2θ = 14.17°, 18.18°, 23.05°, 24.28°, 27.12°, 28.23°, 33.72°, 36.68°, 50.25°, and 55.68° match the (020), (111), (002), (200), (022), (220), (202), (222), (260), and (224) crystal planes of cubic WO3(H2O)0.333 (PDF No. 87-1203), respectively. The strong diffraction peaks located at 2θ = 20.99°, 29.82°, 33.46°, 36.73°, 47.96°, 52.84°, 55.18°, and 57.47° are assigned to the (110), (200), (210), (211), (310), (222), (320), and (321) crystal planes of orthorhombic Ag3PO4 (PDF No. 84-0510), respectively. All strong diffraction peaks in 5%, 15% and 25%WO3(H2O)0.333/Ag3PO4 corresponded to Ag3PO4, indicating that Ag3PO4 is present in the composites. There is no peak relative to WO3(H2O)0.333 in 5%WO3(H2O)0.333/Ag3PO4, and the WO3(H2O)0.333 content may be too small to be detected. The XRD pattern of 15%WO3(H2O)0.333/Ag3PO4 shows that a small peak corresponds to the (220) crystal face of WO3(H2O)0.333, suggesting that the composite contains WO3(H2O)0.333. The XRD pattern of 25%WO3(H2O)0.333/Ag3PO4 shows two small peaks corresponding to the WO3(H2O)0.333 (002) and (220) planes, indicating that the composite contains WO3(H2O)0.333.
As shown in Fig. 3a, WO3(H2O)0.333 presents a linear shape with the size of about 50 nm. In Fig. 3b, Ag3PO4 exhibits a cubic shape with the size of about 1 μm. The WO3(H2O)0.333/Ag3PO4 composite retains the shape of Ag3PO4, and a small amount of WO3(H2O)0.333 is deposited on Ag3PO4. Fig. 3d–3f show that all the elements in WO3(H2O)0.333, Ag3PO4, and the WO3(H2O)0.333/Ag3PO4 composites can be detected. The origin of the C element can be ascribed to the conductive adhesive material, and no other impurities were observed, indicating that the samples are pure. The elemental mapping shows homogeneous distribution of Ag, P, O, and W in the as-prepared WO3(H2O)0.333/Ag3PO4 composites (Fig. 3g).
XPS was used to study the elemental composition and chemical state of each catalyst. From Fig. 4a, it can be seen that the 15%WO3(H2O)0.333/Ag3PO4 composite contains all elements of pure WO3(H2O)0.333 and Ag3PO4, which is consistent with the EDS results in Fig. 3. Fig. 4b shows the binding energy of O 1s in WO3(H2O)0.333, with 532.92 and 530.63 eV corresponding to hydroxyl oxygen and lattice oxygen [47, 59], respectively. The hydroxyl oxygen and lattice oxygen in Ag3PO4 are 532.54 and 530.80 eV, respectively. The hydroxyl oxygen and lattice oxygen of 15%WO3(H2O)0.333/Ag3PO4 are 532.25 and 530.86 eV, which are between WO3(H2O)0.333 and Ag3PO4, indicating that the composites contain WO3(H2O)0.333 and Ag3PO4. In Fig. 4c, the binding energies of W 4f5/2 and W 5f7/2 in WO3(H2O)0.333 correspond to 37.92 and 35.83 eV [60, 61], respectively. In 15% WO3(H2O)0.333/Ag3PO4, the binding energies of W 4f5/2 and W 5f7/2 are 38.29 and 36.21 eV [62, 63], respectively. Compared to WO3(H2O)0.333, the peak value of W 4f in 15%WO3(H2O)0.333/Ag3PO4 shifts 0.38 eV to a high position. This may be due to the fact that the reconstitution of atoms between WO3(H2O)0.333 and Ag3PO4 has resulted in mutual interaction in the process of combination with each other. It is further explained that WO3(H2O)0.333 and Ag3PO4 are composited rather than simply mixed. In Fig. 4d, the binding energies of Ag 3d3/2 and Ag 3d5/2 of Ag3PO4 are 374.17 and 368.16 eV [64, 65], respectively. The binding energies of Ag 3d3/2 and Ag 3d5/2 in 15%WO3(H2O)0.333/Ag3PO4 are 374.20 and 368.18 eV, respectively, and there is almost no change in binding energy with respect to WO3(H2O)0.333. In Fig. 4e, the P 2p binding energies of Ag3PO4 and 15%WO3(H2O)0.333/Ag3PO4 are 133.03 and 133.13 eV, respectively.
Fig. 5 shows the N2 adsorption-desorption isotherms and the BET surface area of different samples. The SBET values of Ag3PO4 and WO3(H2O)0.333 are 0.63 and 16.71 m2 g–1, respectively. When Ag3PO4 and WO3(H2O)0.333 were composited, the SBET of WO3(H2O)0.333/Ag3PO4 composites grows with increasing content of WO3(H2O)0.333.
Fig. 6a shows UV-Vis DR spectra of WO3(H2O)0.333, Ag3PO4, and the WO3(H2O)0.333/Ag3PO4 composites. WO3(H2O)0.333 and Ag3PO4 have absorption edges at 473 and 540 nm, respectively. The absorption edge of the WO3(H2O)0.333/Ag3PO4 composites is 553 nm. There is a significant blue shift in the absorption edge in the composites, and the blue shift increases with the increase of WO3(H2O)0.333 ratios. Therefore, the WO3(H2O)0.333/Ag3PO4 composites exhibit better capture of visible light than WO3(H2O)0.333 and Ag3PO4, which can be ascribed to the synergistic reaction of WO3(H2O)0.333 and Ag3PO4. This result indicates that the composites have a potential application in visible light photocatalysis. In Fig. 6b, the energy band gaps (Eg) of WO3(H2O)0.333 and Ag3PO4 are 2.78 and 2.38 eV, respectively. Eg is calculated as follows [66]:
where α and hv represent the absorbance and energy of radiation, respectively. Valence band (VB) energy (EVB) and conduction band (CB) energy (ECB) of photocatalyst are determined using the following formula [67]:
where X is the electronegativity of semiconductor, and the X values of WO3(H2O)0.333 and Ag3PO4 are 6.49 and 5.96 eV, respectively. Ec is 4.5 eV. Therefore, EVB of WO3(H2O)0.333 and Ag3PO4 is 3.38 and 2.65 eV, respectively, and ECB of WO3(H2O)0.333 and Ag3PO4 is 0.6 and 0.27 eV, respectively.
In order to further explore the separation efficiency of photogenerated electrons and holes, the transient photocurrent-time curves of different photocatalysts are measured. As shown in Fig. 7, the photocurrent densities of WO3(H2O)0.333/Ag3PO4 composites are higher than that of pure WO3(H2O)0.333 and Ag3PO4. Among them, 15%WO3(H2O)0.333/Ag3PO4 has the highest photocurrent density. This result indicates that addition of 15%WO3(H2O)0.333 can greatly enhance the separation efficiency of photogenerated electrons and holes in Ag3PO4. This result further suggests that the heterojunction formed between WO3(H2O)0.333 and Ag3PO4 is the key to promote the separation of photogenerated electrons and holes.
The photocatalytic activity of the photocatalysts was evaluated by the degradation of MB solution. As shown in Fig. 8a, we first added various photocatalysts into the MB solution and stirred it under dark conditions, and tested every 60 min. After three tests, the pollutants were hardly degraded. However, under visible light, the degradation ability of the composites is significantly enhanced relative to pure WO3(H2O)0.333 and Ag3PO4. The photocatalytic ability of 15%WO3(H2O)0.333/Ag3PO4 was much higher than that of pure WO3(H2O)0.333, Ag3PO4, and a physical mixture of 15%WO3(H2O)0.333 + 85%Ag3PO4. The MB solution was almost completely degraded in 4 min. Fig. 8b shows that there is a linear relationship between LnC0/C and t, and the photodegradation reaction conforms to the pseudo-first-order kinetic equation. The equation is shown as follows:
where C0 is the absorbance of the initial MB solution, C is the absorbance of the dye solution after t min light irradiation time, and kapp is a rate constant of pseudo-first-order. According to Eq. (4) and Fig. 8b, the Fig. 8c shows the kapp constants of different photocatalysts. The kapp of 15%WO3(H2O)0.333/Ag3PO4 is 0.969 min−1, which is 161.5 and 2.9 times higher than that of WO3(H2O)0.333 (0.006 min−1) and Ag3PO4 (0.329 min−1), respectively.
The stability of a catalyst determines whether it can be applied in practice. In order to further study the stability of the 15%WO3(H2O)0.333/Ag3PO4 composite, five cycle experiments on composites and pure Ag3PO4 were carried out under the same conditions. As shown in Fig. 9, the degradation efficiency of the composites can still retain 88.2% after five cycles, but the degradation efficiency of pure Ag3PO4 is only 20.2% after five cycles. This result indicates that the addition of WO3(H2O)0.333 greatly enhances the stability of Ag3PO4. The higher stability enables 15%WO3(H2O)0.333/Ag3PO4 have a wide range of practical applications.
Fig. 10 shows the Z-scheme photocatalytic mechanism. From Fig. 6, we know that the CB and VB values of WO3(H2O)0.333 are 0.6 and 3.38 eV, respectively. Ag3PO4 has CB and VB values of 0.27 and 2.65 eV, respectively. Under the visible light irradiation, electron-hole pairs are generated on the surfaces of Ag3PO4 and WO3(H2O)0.333. The photogenerated electrons of WO3(H2O)0.333 first transfer to the CB of WO3(H2O)0.333 and then migrate to the VB of Ag3PO4 to combine with holes. Therefore, the photo-induced electrons and holes of Ag3PO4 are separated effectively, and the photoelectrons are continuously transferred to the CB interface of Ag3PO4. In this way, a large amount of electrons are accumulated on the CB interface of Ag3PO4, and a large number of holes are accumulated in the VB interface of WO3(H2O)0.333. Under the action of holes, –OH reacts with h+ to produce ·OH, and ·OH reacts with contaminants to form CO2 and H2O. Meantime, a great deal of H+ and O2 react with electrons to generate H2O2 at the conduction band interface of Ag3PO4. After that, H2O2 reacts with electrons to generate ·OH, and ·OH reacts with MB to form CO2 and H2O [59, 68]. In this way, the photogenerated electrons and holes are separated continuously, which largely increases photocatalytic reaction rate speed [69], and finally the photocatalytic activity of the catalyst is greatly improved.
We have successfully prepared a novel Z-scheme WO3(H2O)0.333/Ag3PO4 composite photocatalysts, which exhibits excellent photocatalytic activity and good stability under visible light irradiation. The 15%WO3(H2O)0.333/Ag3PO4 composite has the best ability to degrade pollutants and can almost completely degrade methylene blue within 4 min, which is 161.5 and 2.9 times higher than that of WO3(H2O)0.333 and Ag3PO4, respectively. Moreover, after five cycles, the photodegradation ability of the 15%WO3(H2O)0.333/Ag3PO4 composite hardly changed. Therefore, is novel WO3(H2O)0.333/Ag3PO4 composite might provide an effective and feasible route for environmental pollution control.
2019,
Vol. 40 Issue (3): 326-334 
PDF
2019,
Vol. 40 Issue (3): 326-334 PDF