Original Research PaperControllable preparation and efficient visible-light-driven photocatalytic removal of Cr(VI) using optimized Cd0.5Zn0.5S nanoparticles decorated H-Titanate nanotubes
Graphical abstract
The optimized Cd0.5Zn0.5S nanoparticles decorated H-Titanate nanotubes (Cd0.5Zn0.5S/TNTs) with highly efficient visible-light-induced photocatalytic degradation of organic pollutants and removal of the dichromium ions were prepared by the in-situ deposition method.
Introduction
Industrial wastewater pollution is a growing concern during rapid urbanization and industrialization. Generally, fresh groundwater or river water receive effluents discharged directly from the industrial wastewater which contains a wide spectrum of organic and heavy metal pollutants, and it is difficult to remove such water pollution by natural means [1], [2], [3]. Particularly, the toxic elements enter the human body through the food chains and cause poisoning to some extent. Furthermore, the high concentration of heavy metal pollutants will radiate the human body and cause cancer sometimes. The heavy metal ions in the water could directly destruct the land ecosystem, as well as influence the physicochemical properties of the soil and result in poor harvesting conditions.
At present, there are many methods to control heavy metal pollution, including the precipitation method of flocculation [4], [5], [6], adsorption method [7], [8], [9], and bioremediation method [10], [11], [12], etc. Nevertheless, these methods have expensive price, associated with high energy consumption, and production of secondary pollution readily, thus cannot be widely implemented. Compared with the methods above, the photocatalysis has been proved as a potential alternative in many fields, especially in environmental purification. The heavy metal ions and organic molecules can be reduced or decomposed into non-toxic substances through a series of redox reactions [13], [14], [15], [16]. Among many photocatalysts, TiO2-based nanotubes, as a promising photocatalyst, received significant research attention owing to low cost, non-toxicity, eco-friendly nature, and high stability, etc [17], [18]. The TiO2-based nanotubes possess unique morphological structure with an inner diameter and outer diameter about 5–6 nm and 8–10 nm respectively [19], [20], [21], and thus exhibit large surface area, bulky pore volume, and abundant surface negative charges. Therefore, the TiO2-based nanotubes, especially H-titanate nanotubes (H-TNTs), can serve as an excellent charge carrier comparing with bulk TiO2 [22]. The H-TNTs are usually prepared by the hydrothermal method [23], [24], [25]. Unlike the sol–gel method [26], [27] and anodic oxidation [28], [29], it not only can adjust the conditions of the reaction process, but also circumvents the aggregation of the reactants even at high temperatures [30], [31]. Consequently, the obtained products possess high purity, good crystallinity, uniform dispersion, and controllable morphology, etc. [32], [33]. However, the energy bandgap of the H-TNTs is high (~3.3 eV), and hence band-to-band transitions of electrons begin with ultraviolet light (λ ~ 387.5 nm), which is a significant drawback as well as low separation efficiency of photogenerated electron-hole pairs that limit its further applications [34], [35], [36].
To extend the absorption of light for the H-TNTs into visible range and enhance the efficiency of charge separation, the metal sulfides semiconductor coupling is an excellent choice due to its narrow bandgap and suitable conduction band (CB). The TiO2 nanotubes attached with metal sulfides, usually are used for photocatalytic hydrogen production and employed in sterilization. Wu et al. deposited the Bi2S3 nanoparticles on the surface of the TiO2 nanotubes (Bi2S3-TNTAs) using electrochemical anodization and chemical bath deposition [37]. Their samples showed better visible light response ability and excellent performance in hydrogen production. The optimized Bi2S3-TNTAs possess the best hydrogen production efficiency (28.58 μmol/cm2 h), which was 14.5-fold than pure TNTAs (1.96 μmol/cm2 h). Yan et al. combined MoS2 and TiO2 nanotubes (MoS2/TiO2 NTAs) by the hydrothermal method and anodic-oxidation methods [38]. The prepared MoS2/TiO2 NTAs binary composites exhibited excellent antivirus effect (MASR inactivation efficiency is as high as 98.5%) in water disinfection experiment, and the photocatalytic efficiency for antibacterial still remains stable after three cycles.
However, the single metal sulfides catalysts exhibit low recovery rate and high photocorrosion, and easy to form agglomerated particles [39], [40]. It is reported that composite-based metal-sulfides catalysts could effectively solve the above issues. The CdxZn1-xS, a visible light-sensitive photocatalyst, exhibits tunable photocatalytic performance. Furthermore, coupling TiO2-based nanotubes with CdxZn1-xS, which leads to the formation of the heterojunction, is effective and promising method to improve photocatalytic activity [41]. Chen et al. developed the Cd0.5Zn0.5S/titanate nanohybrid catalyst, and the samples demonstrated higher photocatalytic hydrogen production rate (347.7 μmol/h) than pure Cd0.5Zn0.5S with an appropriate amount of sacrificial reagent (0.75 M of Na2SO3 and 1.05 M of Na2S) under irradiation by visible light. The improved photocatalysis is attributed to the higher separation efficiency of charge carriers provided by the unique enwrapped structure [42]. Besides, it was reported that, during the process of preparing TiO2-based nanotubes, titanic acid nanotubes (H-TNTs) obtained by pickling with hydrochloric acid (0.1 mol/L) showed better photocatalysis performance [43]. CdxZn1-xS, as a co-catalyst, possesses better visible light response, and H-TNTs as charge carriers, possess large specific surface area, high porosity, and strong ions exchange capacity. Thus, the CdxZn1-xS loaded H-TNTs composite should exhibit extended visible light response, enhanced charge carrier separation and better photocatalysis.
Herein, an alkaline heat method was adopted to synthesize H-TNTs. Subsequently, a novel series of Cd0.5Zn0.5S loaded H-TNTs composites (Cd0.5Zn0.5S/TNTs) with varied composition ratios were achieved by the in-situ growth method. The TEM analysis was carried out to investigate microstructure and morphology of the samples. The XRD was used to analyse the samples’ crystal structure. Various types of functional groups contained in the catalysts and the chemical environment were identified by vibrational spectroscopy (FT-IR). The X-ray photoelectron spectroscopy (XPS) studies revealed the surface chemical oxidation states qualitatively. The photoresponse of the prepared samples was analyzed using UV–vis DRS. The fluorescence spectroscopy (FL) and photocurrent tests helped to understand photo electron-hole pair separation in the samples qualitatively. The photocatalytic performance of the prepared samples was evaluated by the reduction of Cr(VI) ions and degradation of rhodamine B (RhB) solution. The electron spin-resonance (ESR) spectra were tested to identify the significant active radicals during the photocatalytic reaction process.
Section snippets
Preparation of the samples
Preparation of H-TNTs: The preparation process of the H-TNTs was shown in Scheme 1. Firstly, 0.8 g of P25-TiO2 was added into the NaOH solution (50 mL, 10 mol/L) and stirred for 30 min. The homogeneous suspension was transferred to the polytetrafluoroethylene reactor. The hydrothermal reaction was conducted at 150 °C for 48 h and the reactor was allowed to cool down to room temperature naturally. Secondly, the samples were washed using deionized (DI) water repeatedly until the attainment of
Results and discussion
Fig. 1a-1d showed the TEM images of 10 %Cd0.5Zn0.5S/TNTs at different magnifications. Fig. 1a and 1b showed that 1D tubular structure had been formed from 3D grain structure under hydrothermal reaction for 48 h. The measured diameter of the H-TNTs was ~ 5 nm (Fig. 1c), and the darker spots uniformly distributed over the surface of the H-TNTs and attributed to the adhered Cd0.5Zn0.5S nanoparticles was ~ 10 nm. Furthermore, The high-resolution transmission electron micrograph of the 10 %Cd0.5Zn0.5
Conclusions
A new series of Cd0.5Zn0.5S/TNTs with various proportions were prepared by combining hydrothermal with the in-situ growth method using commercial P25 as a raw material. The morphological and microstructural studies confirmed that the Cd0.5Zn0.5S nanoparticles were distributed homogeneously over the surface of the H-TNTs. The UV–vis DRS results revealed the intensity of visible light absorption of the Cd0.5Zn0.5S/TNTs became stronger with the amount of loaded Cd0.5Zn0.5S nanoparticles. The FL
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 National Natural Science Foundation of China (No. 51802245), Shaanxi Province Innovative Talent Promotion Plan-Young Science and Technology Star (2021KJXX-43), the Natural Science Basic Research Plan in the Shaanxi Province of China (No. 2020JQ-828, 2021JQ-654 and 2021JQ-655), the Shaanxi Provincial Association of Science and Technology Youth Talents Lifting Plan (No. 20180418), Science and Technology Guidance Project Plan of China National Textile and Apparel
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