Regular Article
Improving photocatalytic activity by construction of immobilized Z-scheme CdS/Au/TiO2 nanobelt photocatalyst for eliminating norfloxacin from water

https://doi.org/10.1016/j.jcis.2020.10.088Get rights and content

Abstract

To improve the photocatalytic activity of TiO2 NBs under irradiation of solar light, an immobilized Z-scheme composite photocatalyst CdS/Au/TiO2 NBs has been constructed. For the unique architectures, the TiO2 NBs provide more absorption and reaction sites, the CdS nanoparticles enhance overall light harvesting, and Au acts as the electron transfer mediator, promoting the interfacial charge transfer and efficient separation of electrons and holes. The morphology, elements, crystal structure, optical and photoelectrochemical properties, and photocatalytic activity of CdS/Au/TiO2 NBs were characterized. Results showed that CdS/Au/TiO2 NBs possesses higher photocatalytic activity toward the degradation of antibiotic norfloxacin under irradiation of simulated sunlight, which is attributed to the synergetic interaction of increased light absorption and separation of photogenerated electrons and holes. Besides, the degradation of norfloxacin was promoted by HCO3, but inhibited by NO3 and Cl. The radicals trapping experiments proved that superoxide radicals (radical dotO2) was the dominating active species during the photocatalysis process. The photocatalytic degradation products of norfloxacin was analyzed, and nine intermediates were identified. Moreover, the photocatalytic degradation mechanism and photostability of CdS/Au/TiO2 NBs were analyzed in detail. The matched energy levels and unique ternary Z-scheme design are the key for improved photocatalytic activity. The deactivation of CdS/Au/TiO2 NBs after recycles mainly due to the release of CdS by photocorrosion and the loss of deposited Au.

Introduction

Recently, the pollution of surface water leading to deficiency of clean water is becoming a great problem for the healthy living of human beings. Especially, antibiotics have been frequently detected in aqueous environments. Antibiotics have high toxicity, high solubility, and low biodegradability, which are considered potentially threat to human health and ecological safety. Amongst them, norfloxacin has a stable chemical structure and is recalcitrant to biodegradation. Thus, it is critical to find an efficient and economical technique to eliminate norfloxacin pollution.

Recently, different remediation techniques, such as adsorption [1], microwave catalysis [2], and photocatalysis [3], have been used to remove organic pollutants from water. As an advanced oxidation process (AOP), semiconductor photocatalysis has been regarded as a promising and green technology for degradation decontamination of antibiotics from water. TiO2 is a traditional photocatalyst, which can degrade organic contaminants into micromolecular compounds or CO2 and H2O under the assistance of light energy [4], [5]. Nevertheless, the inefficient utilization of visible light and high recombination rate of electron-hole pairs make it has certain limitations in the process of practical application [6], [7], [8]. Therefore, designing a high performance photocatalyst, which possesses high transfer and utilization efficiency of photogenerated charge carriers and high absorption and utilization efficiency of light, is urgently needed for the photocatalytic degradation of antibiotics.

Because of the appropriate band gap, appreciable visible light performance, and more negative conduction band edges, CdS could be one of the most suitable materials to overcome the disadvantages of pure TiO2 [9], [10], [11], [12]. Unfortunately, some disadvantages of CdS limit its efficiency of photocatalysis, such as rapid recombination of electron-hole pairs. However, constructing Z-scheme energy band structure and electron transfer route is an important strategy to solve these problems. In Z-scheme photocatalytic system, appropriate band positions can promote efficient charge separation by recombining the weaker oxidative holes with the weaker reductive electrons [13]. The net result is that the uncombined electrons and hole are located on separate semiconductors, which have stronger redox ability [14], [15], [16].

In this work, an immobilized Z-scheme CdS/Au/TiO2 NBs photocatalyst was designed and fabricated. In this ternary architecture, CdS and TiO2 acts as photosystem I and photosystem II, respectively. Especially, Au serves as electron transfer mediator to promote interfacial charge transfer and separation efficiency of photogenerated carriers. The morphology, elements, crystal structure, and optical properties of CdS/Au/TiO2 NBs were characterized. The CdS/Au/TiO2 NBs photocatalyst was used to degrade antibiotic norfloxacin under simulated sunlight irradiation. It was found that the CdS/Au/TiO2 NBs showed higher photocatalytic activity compared with non-incorporated photocatalysts and biodegradation. Moreover, the influence on photocatalytic degradation by inorganic ions and possible degradation products and pathways of norfloxacin were explored. The photocatalytic degradation mechanism of CdS/Au/TiO2 NBs was discussed in detail.

Section snippets

Fabrication of CdS/Au/TiO2 NBs

TiO2 NBs were fabricated by using the method of electrochemical anodization [17], [18]. Before anodization, putting the Ti sheets (specification: 90 × 10 × 0.5 mm) into acetone and alcohol to ultrasonic clean for 30 mins. Then the Ti sheets were socked in an ethylene glycol solution which containing NH4F (0.5 wt%) and water (5 vol%). The anodization experiment was conducted for 2.5 h at the condition of 25 ℃ and 60 V. The as-prepared TiO2 NBs were washed by deionized water and dried out

Morphology and composition analysis

Fig. 2(a) exhibits the top-view SEM image of TiO2 NBs, which shows a belt-like shape with a clean and smooth surface. The width and length are about 50–100 nm and 3 μm, respectively. Fig. 2(b) shows the SEM image of CdS/Au/TiO2 NBs, clearly, it can be observed that a large number of CdS/Au nanoparticles are anchored on the surface of TiO2 NBs and the surface of TiO2 NBs becomes rough. Furtherly, High-resolution TEM (HRTEM) was used to ascertain the coupling of CdS, Au, and TiO2 NBs. As seen

Conclusions

In this work, an immobilized Z-scheme composite photocatalyst CdS/Au/TiO2 NBs has been prepared. The CdS/Au/TiO2 NBs photocatalyst has the following advantages: (1) enhanced light absorption capacity; (2) effective separation of photogenerated electron-hole pairs; (3) higher photocatalytic activity; (4) easy recycle compared with powder-like photocatalysts. Besides, the photocatalytic degradation of norfloxacin is promoted by HCO3, but inhibited by NO3 and Cl. The radical dotO2, radical dotOH, and h+ are the

CRediT authorship contribution statement

Jingying Li: Investigation, Data curation, Writing - original draft. Zhi Xia: Methodology, Software. Dong Ma: Methodology. Guocheng Liu: Software. Ningning Song: Methodology. Dan Xiang: Validation. Yanjun Xin: Writing - review & editing. Guodong Zhang: Visualization. Qinghua Chen: Conceptualization, Methodology, Supervision, Project administration, Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Support Plan on Youth Innovation Science and Technology for Higher Education of Shandong Province (No. 2019KJD014), Natural Science Foundation of Shandong Province (No. ZR2019MD012, ZR2017MEE026), the Project of Shandong Province Higher Educational Science and Technology Program (No. J17KA110), and Natural Science Foundation of Heilongjiang Province, China (B2015024). The authors sincerely thank the Central Laboratory of Qingdao Agricultural University for

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