Optimization and modeling of preparation conditions of TiO2 nanoparticles coated on hollow glass microspheres using response surface methodology

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Highlights

  • Hollow glass microspheres (HGMs) were employed as support.

  • Response surface methodology was used to optimize synthesis conditions of TiO2–HGMs.

  • TiO2 film was coated on the HGMs by sol–gel method.

  • The structure of TiO2–HGMs prepared was core–shell.

  • Methane orange was decomposed by photocatalytic oxidation assisted with TiO2–HGMs.

Abstract

Response surface methodology was used to find and optimize the effect of the variables on the preparation, morphology and catalytic activity of nano-crystalline TiO2 thin films coated on hollow glass microspheres (HGMs) by sol–gel method. Three experimental parameters were chosen as independent variables: the calcination temperature, the amount of titanium butoxide and template F127. Morphology analysis showed that structure of TiO2–HGMs prepared was core shell, and the surface of HGMs was coated by TiO2 thin films with an average layer thickness of 728 ± 12 nm. A quadratic model was established as a functional relationship between three independent variables and the degradation efficiency of methyl orange (MO). The results of model fitting and statistical analysis demonstrated that the amount of titanium butoxide and F127 played a key role in discoloration for MO. The optimum conditions for maximum MO degradation efficiency (98.6%) were titanium butoxide of 18.64 g, F127 of 3.12 g and calcination temperature of 501.89 °C.

Introduction

Advanced oxidation processes (AOPs) can complete decompose the less reactive organic pollutants in aqueous solution quickly and non-selectively by generation of reactive species including hydroxyl radicals [1], [2]. Among AOPs, the heterogeneous photocatalytic process is an effective approach that it has been successfully employed for the decomposition of various organic pollutants such as antibiotics [3], pesticide [4], [5], dye [6], [7] and chlorophenols [8] at ambient temperature and pressure into CO2 and water under UV light irradiation with the aid of photocatalysts. Compared with various semiconductors photocatalysts such as ZnO, WO3, CdS and NiO during the photocatalysis process, TiO2 in the anatase form seems to have the most interesting attributes such as high stability, good performance and low cost [1], [9]. At present, studies of photocatalytic processes are widely carried out in slurry systems operating with TiO2 suspensions [10], [11]. However, to date, little progress has been made in the development of a photocatalytic technology for water treatment in large applications. The major problem in these systems is the separation and recycling of TiO2 nanoparticles from the slurry after wastewater treatment, which can be a time consuming and expensive process [12]. Thus, the TiO2 nanoparticles coated on supporting substrate, such as indium–tin oxide conducting glass [13], glass [14] and stainless steel [15] may be preferred to solve this problem for its commercialization.

TiO2 can be immobilized on various substrates to form thin film by many techniques, such as sol–gel method [16], [17], radio frequency magnetron sputtering [18], [19], chemical vapor deposition [20], [21], cathodic arc deposition [22], [23] and electron beam evaporation [24]. Among these techniques, the sol–gel method is one of the simplest and cost-effective synthetic techniques with several advantages, such as high purity materials, low processing temperature, homogeneity, and easy formation of large-area coating on different substrates [25], [26]. Previous investigations have typically used the traditional one-factor-at-a-time (OFAT) approach to optimize preparation conditions and photocatalytic processes, however, this univariate approach does not show the interactions between the operational factors of the process. Moreover, the OFAT approach is time-consuming and expensive due to reagent costs. There is a current trend toward replacing this inefficient practice with effective chemometric methods, such as response surface methodology (RSM), based on statistical designs of experiments [27]. This experimental strategy for seeking the optimum preparation conditions is an efficient technique for use with a multivariable system. RSM has been successfully applied to various processes to achieve optimization using experimental designs, such as photocatalytic [28], [29] and Fenton’s peroxidation [30]. In addition, hollow glass microspheres (HGMs) is a kind of cheap material, and its characteristics is low density, low thermal conductivity, high strength and excellent chemical stability. However, no report has been published on the optimization of influencing factors and their interactions using experimental design methodology during the process of immobilized TiO2 on HGMs by sol–gel method.

The present study focused on optimization the synthetic parameters for the sol–gel preparation of TiO2 thin film coated on HGMs (TiO2–HGMs) with central composite design (CCD), which is a widely used form of RSM. The synthesis parameters effect on the photocatalytic activity of prepared TiO2–HGMs is evaluated according to the degradation efficiency of methyl orange (MO). Furthermore, the predicted response values of degradation efficiency and amount of TiO2 coated using RSM were compared with experimental values. Finally, the synthesis parameters were optimized using RSM.

Section snippets

Materials

MO (C14H14N3NaO3S, Mw = 327.33 g mol−1) was selected as a model dye and which was purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Surfactant pluronic® F127 (Product number: P2443) used as template was purchased from Sigma–Aldrich. Titanium butoxide (Ti(OC4H9)4, TBT) with greater than 97% purity was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Additionally, all other chemicals used were analytical grade without further purification. The HGMs (N40, Ф300–1000 μm, floating

Characterization of TiO2–HGMs

The XRD spectra of the TiO2–HGMs were shown in Fig. 1. The XRD pattern showed that no peak existed for HGMs without coating TiO2. However, some minor peaks appeared for TiO2–HGMs due to the immobilization of titania nanoparticles on the surface of HGMs, which showed a high degree of crystallinity with characteristic peaks at 25.4°, 48.2°, 55.2° and 62.8°. These values match the characteristic peaks of TiO2 according to the database of the Joint Committee on Powder Diffraction Standards

Conclusions

In this study, TiO2 nanoparticles were successfully coated on the surface of HGMs in the form of core–shell structure by sol–gel method. The RSM was used for optimization of synthesis conditions of TiO2–HGMs, and a second-order polynomial equation was developed for describing the influence of key variables on degradation efficiency of MO. The results of model fitting and statistical analysis demonstrated that variable A and B played a key role on the photocatalytic activity of prepared TiO2

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51368004, 51208492 and 21367002) and the China Postdoctoral Science Foundation (2013M530747). In addition, authors gratefully acknowledge Manager Yongbin Huo of Qinghuangdao Qinhuang Glass Microsphere Co., Ltd. China for kindly providing of HGMs.

References (36)

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