Fabrication of Z-scheme Ag3PO4/TiO2 Heterostructures for Enhancing Visible Photocatalytic Activity
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In this paper, a synthetical study of the composite Ag3PO4/TiO2 photocatalyst, synthesized by simple two-step method, is carried out. Supplementary characterization tools such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectroscopy were adopted in this research. The outcomes showed that highly crystalline and good morphology can be observed. In the experiment of photocatalytic performance, TiO2400/Ag3PO4 shows the best photocatalytic activity, and the photocatalytic degradation rate reached almost 100% after illuminating for 25 min. The reaction rate constant of TiO2400/Ag3PO4 is the largest, which is 0.02286 min−1, twice that of Ag3PO4 and 6.6 times that of the minimum value of TiO2400. The degradation effect of TiO2400/Ag3PO4 shows good stability after recycling the photocatalyst four times. Trapping experiments for the active catalytic species reveals that the main factors are holes (h+) and superoxide anions (O·− 2), while hydroxyl radical (·OH) plays partially degradation. On this basis, a Z-scheme reaction mechanism of Ag3PO4/TiO2 heterogeneous structure is put forward, and its degradation mechanism is expounded.
KeywordsComposite Heterostructures Superoxide anion Photocatalytic degradation
UV-vis diffuse reflectance spectroscopy
Energy dispersive X-ray spectrometer
High-resolution transmission electron microscopy
Scanning electron microscopy
Transmission electron microscopy
X-ray photoelectron spectroscopy
Semiconductor photocatalysts have attracted increasing interest due to extenstive use in organic pollutant degradation and solar cells [1, 2, 3, 4, 5, 6]. As the representative of semiconductor-based photocatalysts, TiO2 has been extensively investigated because of its excellent physical-chemical properties [7, 8]. However, the pure TiO2 photocatalyst has certain disadvantages in practical applications such as its wide band gap (3.2 eV for anatase and 3.0 eV for rutile), which leads to poor visible response.
A silver-based compound such as Ag2O, AgX (X = Cl, Br, I), Ag3PO4, Ag2CrO4, have been recently used for photocatalytic applications [9, 10, 11, 12]. Among others, silver orthophosphate (Ag3PO4) has already attracted attention from many researchers because Ag3PO4 has a band gap of 2.45 eV and strong absorption at less than 520 nm. The quantum yield of Ag3PO4 is over 90%. It is a good visible-light photocatalyst. However, due to the formation of Ag0 on the surface of the catalyst (4Ag3PO4 + 6H2O + 12h+ + 12e− → 12Ag0 + 4H3PO4 + 3O2) during the photocatalytic reaction, the reuse of Ag3PO4 is a major problem. Therefore, it is a common practice to reduce photocatalytic corrosion of Ag3PO4 and ensure good catalytic activity of Ag3PO4. Based on literature precedence, it is known that compounding can effectively improve the photocatalytic performance of both semiconductor materials. After compounding, the separation effect of photogenerated electrons and holes is strengthened, contributing to enhance the photocatalytic activity of composite materials. Numerous researchers have investigated heterojunctions such as Bi2O3-Bi2WO6, TiO2/Bi2WO6, ZnO/CdSe, and Ag3PO4/TiO2 [2, 13, 14, 15]. Compared with single-phase photocatalysts, heterojunction photocatalysts can expand the light response range by coupling matched electronic structure materials. And because of the synergistic effect between components, charge can be transferred through many ways to further improve heterojunction photocatalytic activity.
Based on the above analysis, Ag3PO4-based semiconductor composites with synergistic enhancement effect were designed to improve carrier recombination defects and Ag3PO4-based semiconductor composites catalytic performance. In this paper, nano-sized TiO2 was prepared by solvothermal method, and then the nanoparticles of TiO2400 were deposited on the surface of Ag3PO4 at room temperature to obtain TiO2/Ag3PO4 composites. The photocatalytic activity of TiO2/Ag3PO4 composite was tested using RhB dye (rhodamine B).
Hydrothermal Preparation of Nano-sized TiO2
0.4 g P123 was added to a mixed solution containing 7.6 mL absolute ethanol and 0.5 mL deionized water and stirred until P123 was completely dissolved. The clarified solution was labeled as A solution. Then a mixed solution containing 2.5 mL butyl titanate (TBOT) and 1.4 mL concentrated hydrochloric acid (12 mol/L) was prepared and labeled as B solution. The solution B was added to solution A by drop. After stirring for 30 min, 32 mL ethylene glycol (EG) was added to the solution and stirred for 30 min. Then, the solution was placed in oven, at 140 °C, high temperature, and high pressure for 24 h. Natural cooling, centrifugal washing, separation, collection of sediments, and drying at 80 °C oven for 8 h. The white precipitation was calcined in muffle furnace at different temperatures (300 °C, 400 °C, 500 °C) and marked as standby of TiO2300, TiO2400, and TiO2500, respectively.
Preparation of TiO2/Ag3PO4 Photocatalyst
The 0.1 g TiO2 powder was added to the 30-mL silver nitrate solution containing 0.612 g AgNO3 and then treated by ultrasound for 30 min to make TiO2 dispersed uniformly. We added 30-mL solution containing 0.43 g Na2HPO4.12H2O and stirred for 120 min at ambient temperature. By centrifugation, cleaning with deionized water and anhydrous ethanol, the precipitates were separated, collected, and dried at 60 °C. The products were named as TiO2300/Ag3PO4, TiO2400/Ag3PO4, and TiO2500/Ag3PO4, respectively. Ag3PO4 was prepared without adding TiO2 under the same conditions as the above process.
The X-ray diffraction (XRD) patterns of the resulted samples were performed on a D/MaxRB X-ray diffractometer (Japan), which has a 35 kV Cu-Ka with a scanning rate of 0.02° s−1, ranging from 10 to 80°. Scanning electron microscopy (SEM), JEOL, JSM-6510, and JSM-2100 transmission electron microscopy (TEM) assembly with energy dispersive X-ray spectroscopy (EDX) were used to study its morphology at 10-kV acceleration voltage. X-ray photoelectron spectroscopy (XPS) information were collected by using an ESCALAB 250 electron spectrometer under 300-W Cu Kα radiation. The basic pressure was about 3 × 10−9 mbar, Combine to refer to the C1s line at amorphous carbon 284.6 eV.
Photocatalytic Activity Measure
The photocatalytic performance of TiO2/Ag3PO4 catalysts was tested by using the photodegradation of RhB in aqueous solution as the research object. Fifty milligrams of the photocatalyst was mixed with 50 mL of RhB aqueous solution (10 mg L−1) and stirred in darkness for a certain time before illumination to ensure adsorption balance. In the reaction process, cooling water is used to keep the system temperature constant at room temperature. A 1000-W Xenon lamp provides illumination to simulate visible light. LAMBDA35 UV/Vis spectrophotometer was used to characterize the concentration (C) change of RhB solution at λ = 553 nm. The decolorization rate is indicated as a function of time vs Ct/C0. Where C0 is the concentration before illumination, and Ct is the concentration after illumination. Used catalysts were recollected to detect the cycle stability of the catalysts. The experiment was repeated four times.
Results and Discussion
where A, hv, c, and Eg are the absorption coefficient, incident photon energy, absorption constant, and band gap energy, respectively. The value of n for direct semiconductor is 1/2, and that for indirect semiconductor is 2. Anatase TiO2 and Ag3PO4 are indirect semiconductors, so n takes 2.
The plots depicting (αhv)1/2 versus incident photon energy (hv) from Fig. 4b indicates the band gap energy diagrams (Eg) of Ag3PO4, TiO2400, and TiO2400/Ag3PO4 catalysts are 2.45 eV, 3.1 eV, and 2.75 eV, respectively. This further proves that TiO2400/Ag3PO4 is a good visible-light photocatalyst with suitable band gap width and visible light capture ability.
Photo degradation rate constants and linear regression coefficients of different catalysts from equation − ln(C/C0) = kt.
− ln(C/C0) = 0.02286x + 0.21496
R2 = 0.68755
− ln(C/C0) = 0.01513x + 0.15984
R2 = 0.753
− ln(C/C0) = 0.01148x + 0.1079
R2 = 0.71128
− ln(C/C0) = 0.00525x + 0.06354
R2 = 0.82635
− ln(C/C0) = 0.00345x + 0.0383
R2 = 0.78461
Figure 5c is the stability test result of four times of degradation of RhB solution by recycling of TiO2400/Ag3PO4. The degradation effect of TiO2400/Ag3PO4 shows good stability in four times of recycling, and in the fourth cycle experiment, the degradation effect of TiO2400/Ag3PO4 was slightly higher than that of the third cycle. This may be due to the formation of composite material between Ag3PO4 and TiO2 to accelerate photogenerated electron-hole pair transfer and in situ formation of a small amount of Ag in Ag3PO4 during photocatalysis to inhibit further photo-corrosion.
The results of TiO2/Ag3PO4 capture factors are shown in Fig. 5d. After the addition of trapping agent IPA, the degradation activity decreased partially. When BQ and TEOA were added, the degradation degree of RhB decreased significantly, even close to 0. Therefore, we can infer that the main factors are holes (h+) and superoxide anions (O·− 2), while hydroxyl radical (·OH) plays partially degradation.
Basing on the above discussion, the degradation reaction of TiO2/Ag3PO4 is expressed by the chemical equation as follows:
In summary, a comprehensive investigation of the composite Ag3PO4/TiO2 photocatalyst, prepared by a simple two-step method is presented. Complementary characterization tools such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (DRS) were utilized in this study. The results showed that the composite Ag3PO4/TiO2 photocatalyst is highly crystalline and has good morphology. For Ag3PO4/TiO2 degradation of RhB, TiO2400/Ag3PO4 shows the highest photocatalytic activity. After 25 min of reaction, the photocatalytic degradation rate reached almost 100%. The reaction rate constant of TiO2400/Ag3PO4 is 0.02286 min−1, which is twice that of Ag3PO4 and 6.6 times that of the minimum value of TiO2400. The TiO2400/Ag3PO4 also exhibits good stability after recycling four times. The main active catalytic species are holes (h+) and superoxide anions (O·− 2), while hydroxyl radical (·OH) plays partially degradation from trapping experiments. In addition, a Z-scheme reaction mechanism of Ag3PO4/TiO2 heterogeneous structure is proposed to explain the RhB degradation mechanism. The accumulation of photogenerated electrons on Ag3PO4 conductive band causes photoetching of Ag3PO4 photocatalyst to form a small amount of Ag nanoparticles, consequently, accelerating photogenerated electron transfer in the Ag3PO4 conduction band, thus preventing further Ag3PO4 photocatalyst corrosion.
This work is greatly indebted to professors Shuangqi Hu for his meticulous instruction and Lishuang Hu who helped us with great encouragement.
This work presented here was performed in collaboration of all the authors. All authors read and approved the final manuscript.
No funding support.
The authors declare that they have no competing interests.
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