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Article

Preparation of Ag3PO4/TiO2(B) Heterojunction Nanobelt with Extended Light Response and Enhanced Photocatalytic Performance

by
Yong Li
1,2,3,†,
Yanfang Liu
1,2,†,
Mingqing Zhang
1,2,
Qianyu Zhou
1,2,
Xin Li
1,2,
Tianlu Chen
3,* and
Shifeng Wang
1,2,3,*
1
Department of Physics, and Innovation Center of Materials for Energy and Environment Technologies, College of Science, Tibet University, Lhasa 850000, China
2
Institute of Oxygen Supply, Center of Tibetan Studies (Everest Research Institute), Tibet University, Lhasa 850000, China
3
Key Laboratory of Cosmic Rays (Tibet University), Ministry of Education, Lhasa 850000, China
*
Authors to whom correspondence should be addressed.
†
Both authors contributed equally to this manuscript.
Molecules 2021, 26(22), 6987; https://doi.org/10.3390/molecules26226987
Submission received: 21 October 2021 / Revised: 16 November 2021 / Accepted: 17 November 2021 / Published: 19 November 2021
(This article belongs to the Special Issue Materials Chemistry in China)
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Abstract

:
Photocatalytic degradation, as an emerging method to control environmental pollution, is considered one of the most promising environmental purification technologies. As Tibet is a region with some of the strongest solar radiation in China and even in the world, it is extremely rich in solar energy resources, which is ideal for applying photocatalytic technology to its ecological environment protection and governance. In this study, Na2Ti3O7 nanobelts were prepared via a hydrothermal method and converted to TiO2∙xH2O ion exchange, which was followed by high-temperature calcination to prepare TiO2(B) nanobelts (“B” in TiO2(B) means “Bronze phase”). A simple in situ method was used to generate Ag3PO4 particles on the surface of the TiO2 nanobelts to construct a Ag3PO4/TiO2(B) heterojunction composite photocatalyst. By generating Ag3PO4 nanoparticles on the surface of the TiO2(B) nanobelts to construct heterojunctions, the light absorption range of the photocatalyst was successfully extended from UV (ultraviolet) to the visible region. Furthermore, the recombination of photogenerated electron–hole pairs in the catalyst was inhibited by the construction of the heterojunctions, thus greatly enhancing its light quantum efficiency. Therefore, the prepared Ag3PO4/TiO2(B) heterojunction composite photocatalyst greatly outperformed the TiO2(B) nanobelt in terms of photocatalytic degradation.

1. Introduction

The Qinghai–Tibet Plateau is the ecological security barrier of China and the origin of many rivers in China and South Asia. Therefore, it is of great significance to protect and manage its ecological environment [1,2,3]. As an emerging environmental pollution control technology, photocatalytic degradation is considered one of the most promising environmental purification technologies at present because of its advantages of mild reaction conditions, low secondary pollution, sustainability, and environmental friendliness [4,5,6,7]. Since Tibet is one of the regions with the strongest solar radiation in China and even the world, the abundant energy resource is ideal for applying photocatalytic technology to ecological environment protection and governance in Tibet [8,9,10]. TiO2 is recognized as one of the best photocatalyst materials with stable properties, low cost, ease of access, non-toxicity and safety, and good resistance to photocorrosion. However, TiO2 has a band gap as large as 3.2 eV and hence is photocatalytically active only under UV light, which accounts for merely approximately 5% of solar radiation energy. Therefore, TiO2 as a photocatalyst has a low utilization of solar radiation energy and insignificant photocatalytic performance [11,12,13]. However, combining TiO2 with other narrow-band-gap semiconductor materials to construct heterojunctions can effectively enhance the photocatalytic performance of TiO2. This is because, first, the introduction of narrow-band-gap semiconductors can extend the light-absorption range of TiO2 from the UV region to the visible region, greatly increasing the material’s absorption range of the solar spectrum; second, the construction of heterojunctions can effectively promote the separation of photogenerated electrons and holes, thereby improving the quantum yield of photogenerated carriers participating in photocatalytic redox reactions [13,14,15]. Current studies show that the use of Ag3PO4 compounded with TiO2 can effectively enhance TiO2 photocatalytic performance, but most studies use common anatase phase TiO2 [16,17,18,19], and monoclinic (space group C2/m) TiO2(B) has rarely been studied, while TiO2(B) has better electrical conductivity, a sparse porous structure, and stronger photogenerated hole oxidation, and these properties make it possible for TiO2(B) to obtain stronger photocatalytic performance by compounding [20,21,22,23]. In this study, Na2Ti3O7 nanobelts were successfully prepared via hydrothermal methods and further used as a precursor for ion exchange with H+ in 0.1 M dilute HCl to obtain TiO2 hydrate, which was dried and calcined to obtain TiO2(B) nanobelts. Throughout the preparation process, the morphology of the Na2Ti3O7 nanobelts was maintained. Na2Ti3O7 had good adsorption properties, as did TiO2(B) with the same morphology, which was beneficial to its absorption of sunlight and photocatalytic degradation performance. Finally, Ag3PO4 nanoparticles were generated on the surface of the TiO2(B) nanobelts via a simple in situ method, thereby successfully constructing Ag3PO4/TiO2(B) heterojunctions, which substantially enhanced the photocatalytic degradation performance of TiO2(B).

2. Results and Discussion

Figure 1a,b present SEM (Scanning Electron Microscope) images of Na2Ti3O7 and the TiO2(B) nanobelt, respectively, which show that they had the same morphology and were both belt-like. Therefore, TiO2(B) prepared from Na2Ti3O7 using the experimental method in this study was able to well-maintain the morphology of Na2Ti3O7. Figure 1c shows EDS (Energy-Dispersive X-Ray Spectroscopy Spectra) of Na2Ti3O7; Na, O, and Ti appear in the spectra, which verifies the chemical composition of the prepared product. Figure 1d shows EDS spectra of TiO2(B); O and Ti appear in the spectra, which verifies the chemical composition of the prepared product. Figure 1e shows XRD (X-Ray Diffraction) patterns of TiO2, the analysis of which revealed that the TiO2(B) nanobelts prepared in this experiment did not belong to the rutile, anatase, or brookite phases common to TiO2 in nature, but they did belong to a monoclinic crystal system (space group C2/m).
Figure 2a shows TEM (Transmission Electron Microscopy) diffraction patterns of the TiO2(B) nanobelts, and Figure 2b shows the diffraction spots of crystal planes (0 2 0), (0 2 – 0), (6 0 3 – ), and ( 6 – 0 3) as calculated using the crystal structure data measured via XRD. Comparing the two images revealed that the measured and theoretically calculated results were in good agreement. In addition, the regular lattice pattern of the diffraction spots also indicated a high crystallinity of the prepared TiO2 nanobelts. This is also confirmed by Figure 2d, which clearly shows a uniform distribution of TiO2 grains. Figure 2c shows that the (0 2 0) crystal planes were uniformly arranged with a measured crystal plane spacing of 0.188 nm, which agreed very well with that measured via XRD. The above TEM test analysis further confirmed that the crystal structure of the TiO2(B) nanobelts did not belong to the rutile, anatase, or brookite phases of TiO2 that are common in nature but was a new phase of TiO2 that is not commonly found in nature.
Figure 3 shows XRD patterns of the samples. The phases of the TiO2(B) nanobelts and prepared Ag3PO4 are analyzed in the left panel of Figure 3, with the standard powder diffraction file (PDF) cards PDF#06-0505 and PDF#46-1237, corresponding to Ag3PO4 and TiO2(B), respectively. It can be clearly seen from the diffraction patterns that the crystallinity of Ag3PO4 was significantly better than that of the TiO2(B) nanobelts, and the diffraction peaks of Ag3PO4 corresponded very well to the standard PDF card. The right panel of Figure 3 shows XRD diffraction patterns of the composites of Ag3PO4 and TiO2(B) with different molar ratios. There were very obvious diffraction peaks of Ag3PO4 in the composites, and the relative intensity of the peaks did not change significantly with the increase in the molar ratio of Ag3PO4. Among the diffraction peaks of TiO2(B), only the strongest diffraction peak, corresponding to the (110) plane, was weakly reflected in the composite, and it weakened with the increase in the molar ratio of Ag3PO4.
Figure 4 shows SEM and mapping images of the Ag3PO4/TiO2(B) composite, where Figure 4a shows a SEM image when the molar ratio of Ag3PO4 to TiO2 was 0.4:1, and Figure 4b shows the SEM image when the molar ratio increased to 1.5:1. The Ag3PO4 particles were clearly attached to the surface of belt-like TiO2(B), and the number of Ag3PO4 particles increased as the molar ratio of Ag3PO4 increased. EDS mapping (Figure 4c–f) showed that the sample contained Ti, O, Ag, and P elements, which were uniformly distributed in the area where the composite was located, indicating that the composite contained Ag3PO4 and TiO2(B), and the two were distributed relatively uniformly.
The left panel of Figure 5 shows the photocatalytic degradation curves of RhB solution by Ag3PO4/TiO2(B) composites with different molar ratios. It can be seen that the TiO2(B) nanobelts degraded RhB slowly, and the degradation rate was less than 20% after 30 min of degradation. After combining with Ag3PO4, the photocatalytic degradation of RhB by the composite product was improved, particularly when increasing the molar ratio of Ag3PO4. When the molar ratio of Ag3PO4 to TiO2(B) increased to 1.5:1, within 30 min, the composite photocatalyst achieved nearly 100% degradation of RhB, and the photocatalytic performance increased by nearly five times, which was much higher than the photocatalytic degradation performance of commercial P25. The right panel of Figure 5 shows actual photographs of the color change of RhB solution due to photocatalytic degradation by Ag3PO4/TiO2(B) composites with different molar ratios. These photographs visually demonstrate the rapid enhancement of the photocatalytic performance of the composite with the increase in the molar ratio of Ag3PO4. The Ag3PO4/TiO2(B) composite was obtained on the surface of the TiO2(B) nanobelts through the in situ growth of Ag3PO4 crystal grains. Compared with that of the TiO2(B) nanobelts, the photocatalytic performance of the composite increased rapidly with the increase in the molar ratio of Ag3PO4. When the molar ratio of Ag3PO4 to TiO2(B) reached 1.5:1, the photocatalytic performance of the composite far exceeded (was nearly five times) that of the pure TiO2(B) nanobelt. We believe that the enhancement of the photocatalytic ability of the composite was, first, due to the introduction of Ag3PO4, which greatly expanded the absorption range of the material within the solar spectrum and enhanced the absorption of visible light [24,25,26] and, second, because of the construction of heterojunctions, which promoted the separation of photogenerated electron–hole pairs and inhibited their recombination [27,28,29]. In addition, it can be seen from the mapping results that the two substances constructed heterojunctions that were highly uniformly distributed, which gave full play to their role in the composite.
Figure 6 shows the absorbance curves of the samples against a continuous spectrum. This test investigated the variation curves of absorbance of the TiO2(B) nanobelts and Ag3PO4 in the visible region (wavelength > 400 nm). It was obvious that the light-absorption ability of Ag3PO4 was significantly higher than that of the TiO2(B) nanobelts in the visible region. After the TiO2(B) nanobelts and Ag3PO4 formed a composite, its visible-light-absorption ability was enhanced to some extent compared with that of the TiO2(B) nanobelts, indicating that the introduction of Ag3PO4 enhanced the visible light absorption ability of the composite.
To investigate the recombination and migration of photogenerated electrons before and after the combination of Ag3PO4 and TiO2(B), we tested their PL (photoluminescence) spectra and photocurrent curves, respectively, as shown in Figure 7. As can be clearly seen from the PL spectrum in the left panel of Figure 7, the peaks of the PL lines of the TiO2(B) nanobelts were the strongest, while the peaks of the PL lines of the composite decreased rapidly with the increase in the molar ratio of the added Ag3PO4. Stronger fluorescence reflects an easier recombination of the photogenerated electron–hole pairs of the substance, making it difficult for the photogenerated electron–hole pairs to migrate to the surface of the catalyst to participate in the photocatalytic redox reaction, which is not conducive to the performance of photocatalysis [30,31,32]. The right panel of Figure 7 shows the photocurrent curves of the samples. The photocurrents of pure TiO2(B) nanobelts and Ag3PO4 nanoparticles were the lowest. In comparison, the photocurrent of Ag3PO4/TiO2(B) composite was enhanced and increased continuously with the increase in the molar ratio of Ag3PO4, reaching a maximum when the molar ratio of Ag3PO4 was increased to 1.5:1. These two sets of curves well explained the underlying reason for the variation pattern of photocatalytic performance in Figure 5 and also indirectly proved that Ag3PO4 nanoparticles grown on the surface of TiO2(B) nanobelts through our simple in situ method indeed successfully constructed a Ag3PO4/TiO2(B) heterojunction composite, effectively promoting the separation of photogenerated charges and thereby enhancing the photocatalytic performance of the composite catalyst.

3. Materials and Experiment

3.1. Materials

Regarding the chemicals used in the experiment, NaOH and nano-TiO2 were purchased from Aladdin Industries, Inc.(Shanghai, China), Na2HPO4∙12H2O from Chengdu Jinshan Chemical Reagent Co., Ltd. (Chengdu, China), AgNO3 from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and Rhodamine B (RhB, C28H31CIN2O3) from Beijing Solarbio Technology Co., Ltd. (Beijing, China). All the reagents were analytically pure and used without further purification.

3.2. Preparation of TiO2(B) Nanobelt, Ag3PO4, and Ag3PO4/TiO2(B)

Nano-TiO2 (1 g) was added to 100 mL of 10 M NaOH aqueous solution, stirred evenly, poured into a 150 mL hydrothermal reactor, heated to 200 °C, and held for 24 h before cooling naturally. Then, the reaction product was taken out of the reactor and rinsed with a large amount of deionized (DI) water to a nearly neutral pH, which was followed by vacuum filtration and drying to obtain the Na2Ti3O7 nanobelts. After that, Na2Ti3O7 was soaked in 0.1 M dilute HCl for 72 h to allow ion exchange between H+ and Na+ in Na2Ti3O7 to obtain TiO2∙xH2O, which was rinsed with a large amount of DI water until the pH of the solution was nearly neutral, vacuum filtered, dried at 60 °C in a blast drying oven, calcined at 500 °C in a muffle furnace with a heating rate of 1 °C/min for 2 h, and cooled naturally to obtain the TiO2(B) nanobelts. To prepare Ag3PO4/TiO2(B), 100 mg of the prepared TiO2(B) nanobelts was weighed first, and corresponding masses of Ag3PO4 were calculated according to molar ratios of (1:0.4), (1:0.8), and (1:1.5), respectively, as were the corresponding masses of AgNO3 and Na2HPO4∙12H2O required for the preparation of Ag3PO4. Then, 100 mg of the TiO2 nanobelts and the corresponding mass of Na2HPO4∙12H2O were added into 100 mL of DI water, which was followed by ultrasonication for 30 min. Next, the corresponding mass of AgNO3 was dissolved in 50 mL of DI water and then slowly added into the sonicated mixture with constant stirring. Finally, the mixture was vacuum-filtrated and dried to obtain composites with different molar ratios, which were labeled as Ag3PO4/TiO2(B) (0.4:1), Ag3PO4/TiO2(B) (0.8:1), and Ag3PO4/TiO2(B) (1.5:1). Ag3PO4 was directly obtained by mixing a certain proportion of AgNO3 and Na2HPO4∙12H2O solution, which was followed by vacuum filtration and drying.

3.3. Analysis and Testing

Field emission scanning electron microscopy (FE-SEM, Gemini SEM 300, Manufactured by Zeiss, Oberkochen, Germany) and field emission transmission electron microscopy (FE-TEM, FEI Talos F200X, Manufactured by FEI, Hillsboro, OR, USA) were used to observe and analyze the morphology of the samples as well as test and analyze the elemental composition and distribution of the samples. FE-TEM (FEI Talos F200X, Manufactured by FEI, USA) and X-ray powder diffractometry (XRD, Bruker D8 Advance, Manufactured by Bruker, Bremen, Germany) were used to test and analyze the crystal structure and phase of the samples. A fluorescence spectrometer (PL FLS 1000/FS5, Manufactured by Edinburgh, UK) was used to test the fluorescence emission spectra of the samples. An electrochemical workstation (CHI-760E, Manufactured by Chenhua, Shanghai, China) was used to test the photocurrent of the samples. A UV-Vis spectrophotometer (UV-1200, Manufactured by Macy, Shanghai, China) was used to test and analyze changes in photocatalytic degradation performance, as well as obtain absorbance curves of the samples for RhB. In addition, a muffle furnace (KSL-1700X, Manufactured by Kejing, Hefei, China), a blast drying oven (DHG-9246A, Manufactured by Kejing, Hefei, China), and a benchtop high-speed centrifuge (LC-LX-H185C, Manufactured by Lichen, Shanghai, China) were also used to prepare the experimental materials.

3.4. Photocatalytic Performance Test

Each sample (20 mg) was added to 100 mL of 20 mg/L RhB solution and sonicated for 30 min. The mixture was placed under a light source simulated by a solar simulator (Solar-500Q, Manufactured by Newbit, Beijing, China), with the intensity of the light source adjusted to obtain a light intensity of 600 W/m2 at the liquid surface. Approximately 6 mL of the experimental liquid was taken every 10 min and placed into the centrifuge for 10 min at 10,000 r/min. The supernatant was taken and tested by the UV-Vis spectrophotometer for absorbance An (n = experimental serial number, with 1 for the first sample taken). Since the absorbance is proportional to the concentration of the solution (Cn), then there is a solution concentration ratio Cn/C0 = An/A0 (A0 = absorbance of the 20 mg/L of RhB solution, C0 = concentration of the 20 mg/L of RhB solution). The absorbance of each sample was measured at 10-min intervals for 60 min according to the above method, and the corresponding degradation rates were calculated to plot the photocatalytic degradation curve.

4. Conclusions

In this study, Na2Ti3O7 nanobelts were successfully prepared via the hydrothermal method, and then, TiO2∙xH2O was obtained via ion exchange, which was followed by calcinating TiO2∙xH2O in a muffle furnace at 500 °C for 2 h to obtain TiO2(B) nanobelts. The belt-like morphology of Na2Ti3O7 was maintained throughout the preparation process, which made the TiO2(B) have good adsorption performance and enhanced the photocatalytic performance of the TiO2(B) photocatalyst. Using AgNO3 and Na2HPO4∙12H2O as reactants, Ag3PO4 nanoparticles were generated on the surface of the TiO2(B) nanobelts by a simple in situ method, thereby successfully preparing Ag3PO4/TiO2(B) heterojunction composites. The pure TiO2(B) nanobelt degraded the simulated pollutant RhB slowly, with a degradation rate lower than 20% after 30 min of degradation. By combining the TiO2(B) nanobelts with Ag3PO4, the performance of the composite photocatalyst was rapidly improved with the increase in the molar ratio of Ag3PO4, and the composite was able to degrade nearly 100% of RhB in 30 min when the molar ratio of Ag3PO4 to TiO2 was increased to 1.5:1. The improvement of the photocatalytic performance of the Ag3PO4/TiO2(B) composite was mainly attributed to the successful construction of heterojunctions between Ag3PO4 and TiO2(B), thereby greatly inhibiting the recombination of photogenerated electron–hole pairs and enhancing the light quantum yield of the photocatalyst. In addition, the introduction of Ag3PO4 in the composite successfully extended the light absorption range of the photocatalyst into the visible region, thus improving the utilization of simulated sunlight. Based on the above two reasons, the photocatalytic performance of the Ag3PO4/TiO2(B) composite was significantly improved.

Author Contributions

Conceptualization, methodology, S.W. and T.C.; software, Y.L. (Yong Li); validation, Y.L. (Yong Li) and Y.L. (Yanfang Liu); formal analysis, Y.L. (Yong Li) and Y.L. (Yanfang Liu); investigation, Y.L. (Yong Li), Y.L. (Yanfang Liu) and M.Z.; resources, S.W. and T.C.; data curation, S.W.; writing—original draft preparation, Y.L. (Yong Li) and Y.L. (Yanfang Liu); writing—review and editing, Y.L. (Yong Li) and Y.L. (Yanfang Liu); visualization, Q.Z. and X.L.; project administration, Y.L. (Yong Li); funding acquisition, S.W. and Y.L. (Yong Li). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Tibet Autonomous Region (Grant No.: XZ202101ZR0121G), Central Government Funds for the Reform and Development of Local Colleges and Universities (Grant No.: ZCKJ2020-11), The National Natural Science Foundation of China (Grant Nos: 52062045 and 12047575), Central Government Funds for Local Scientific and Technological Development (Grant No.: XZ202101YD0019C), High level talent training program of Tibet University (2019-GSP-B001), and Everest Discipline Construction Project of Tibet University (Grant No.: ZF21000002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples are available from Yong Li on valid request.

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Molecules 26 06987 g001 550
Figure 1. (a) SEM image of Na2TiO7 nanobelt; (b) SEM image of TiO2 nanobelt; (c,d) EDS spectra of TiO2 and Na2Ti3O7 nanobelt; (e) XRD patterns of TiO2. EDS, energy-dispersive X-ray spectroscopy.
Figure 1. (a) SEM image of Na2TiO7 nanobelt; (b) SEM image of TiO2 nanobelt; (c,d) EDS spectra of TiO2 and Na2Ti3O7 nanobelt; (e) XRD patterns of TiO2. EDS, energy-dispersive X-ray spectroscopy.
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Figure 2. (a,b) Diffraction patterns of TiO2(B) nanobelt; (c,d) HRTEM (High-Resolution Transmission Electron Microscopy) images of TiO2(B) nanobelt.
Figure 2. (a,b) Diffraction patterns of TiO2(B) nanobelt; (c,d) HRTEM (High-Resolution Transmission Electron Microscopy) images of TiO2(B) nanobelt.
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Figure 3. (a) XRD patterns of Ag3PO4 and TiO2(B); (b) XRD patterns of Ag3PO4/TiO2(B) with different molar ratios.
Figure 3. (a) XRD patterns of Ag3PO4 and TiO2(B); (b) XRD patterns of Ag3PO4/TiO2(B) with different molar ratios.
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Figure 4. (a,b) SEM images of Ag3PO4/TiO2 composite; (c) The SEM image corresponding to map; (d) Map of Ti; (e) Map of O; (f) Map of Ag; (g) Map of P.
Figure 4. (a,b) SEM images of Ag3PO4/TiO2 composite; (c) The SEM image corresponding to map; (d) Map of Ti; (e) Map of O; (f) Map of Ag; (g) Map of P.
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Figure 5. Photocatalytic degradation curves and images of Ag3PO4/TiO2 composite.
Figure 5. Photocatalytic degradation curves and images of Ag3PO4/TiO2 composite.
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Figure 6. Continuous spectra of absorbance of synthesized samples.
Figure 6. Continuous spectra of absorbance of synthesized samples.
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Figure 7. PL and photocurrent spectra of synthesized samples.
Figure 7. PL and photocurrent spectra of synthesized samples.
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Li, Y.; Liu, Y.; Zhang, M.; Zhou, Q.; Li, X.; Chen, T.; Wang, S. Preparation of Ag3PO4/TiO2(B) Heterojunction Nanobelt with Extended Light Response and Enhanced Photocatalytic Performance. Molecules 2021, 26, 6987. https://doi.org/10.3390/molecules26226987

AMA Style

Li Y, Liu Y, Zhang M, Zhou Q, Li X, Chen T, Wang S. Preparation of Ag3PO4/TiO2(B) Heterojunction Nanobelt with Extended Light Response and Enhanced Photocatalytic Performance. Molecules. 2021; 26(22):6987. https://doi.org/10.3390/molecules26226987

Chicago/Turabian Style

Li, Yong, Yanfang Liu, Mingqing Zhang, Qianyu Zhou, Xin Li, Tianlu Chen, and Shifeng Wang. 2021. "Preparation of Ag3PO4/TiO2(B) Heterojunction Nanobelt with Extended Light Response and Enhanced Photocatalytic Performance" Molecules 26, no. 22: 6987. https://doi.org/10.3390/molecules26226987

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