Fabrication of branched-TiO2 microrods on the FTO glass for photocatalytic reduction of Cr(VI) under visible-light irradiation

https://doi.org/10.1016/j.jiec.2019.01.032Get rights and content

Highlights

  • Branched-TiO2 (B-TiO2) microrods on the FTO glass was fabricated as visible-light responsive photocatalyst.

  • Cr(VI) was readily reduced to Cr(III) under visible LED light with AuNPs/B-TiO2/FTO.

  • Mechanism of Cr(VI) reduction for AuNPs/B-TiO2 was different with that of prinstine-TiO2.

  • Reduced Cr(III) was also readily removed by adsorption on the substrate from aqueous phase.

Abstract

It is important to prepare a visible-light responsive photocatalyts, which has enhanced absorbance in the range of visible-light. In addition, metal nanoparticles deposited on TiO2 induced the localized surface plasmon resonance (LSPR) effect of Au nanoparticles (AuNPs), and thus enhanced the photocatalytic activity. Herein, TiO2 microrods with nano-branch (B-TiO2) on the fluorine-doped tin oxide glass (FTO) was prepared by hydrothermal method, and AuNPs was deposited on the B-TiO2. Its photocatalytic activity was evaluated by reduction of toxic Cr(VI) to less-toxic Cr(III) under visible-light irradiation. AuNPs/B-TiO2/FTO was successfully photocatalytic reduced target ions and Cr(III) ions were also removed from aqueous phase.

Graphical abstract

Branched-TiO2 nanorods on FTO glass for photocatalytic reduction of Cr(VI) under visible light irradiation

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Introduction

Photocatalysts, which are semiconducting materials are attracting attention for various applications, including the purification of water and air and the decomposition of organic dyes. Owing to its advantages of being inexpensive, stable, and abundant, titanium dioxide (TiO2) has been widely used as a photocatalyst to eliminate various pollutants in sewage treatment systems [1], [2]. However, TiO2 photocatalyst was not easily worked under visible light. Namely, the well-known limitation of TiO2 is its wide bandgap (3.2 eV) that has reactivity under only ultraviolet (UV) irradiation [3]. In addition, similar to other photocatalysts (ZnO, CdS), TiO2 powder-type has freshwater ecotoxicity and human toxicity [4], [5], [6], [7], [8], [9], [10], [11]. To prevent the release of TiO2 powder into the environment, the fabrication of a nanostructure on the glass substrate has emerged as a solution [12].

CdS, Ag2S, and PbS/TiO2 were reported as visible-light-induced photocatalysts to overcome the limitations of TiO2 [13], [14], [15]. In addition, the localized surface plasmon resonance (LSPR) effect of metal nanoparticles can be used to generate photoexcited electrons. LSPR arises from the collective oscillation of free electrons at the metallic interface or in small metallic nanostructures [16]. Au nanoparticles (AuNPs) or Ag nanoparticles (AgNPs) were mainly deposited on TiO2 to transfer the electrons generated from the metal nanoparticles to the semiconductor [17], [18], [19]. These metallic nanoparticles were enhanced the electron transfer and prevented the recombination of electrons and holes, and thus NPs deposited on TiO2 showed generally enhanced photocatalytic activity.

In environmental applications, hexavalent Cr [Cr(VI)] is one of the major contaminants in sewage released from mining, electroplating, leather tanning, and chromate manufacturing [20]. The reduction of toxic Cr(VI) to the less toxic Cr(III) using photocatalytic reaction is an essential process with regard to environmental pollution. To increase the efficiency of photocatalytic reduction, more electrons must be deposited via various methods.

In this study, we fabricated a branched-TiO2 (B-TiO2) microrod structure fixed on fluorine-doped tin oxide (FTO) glass via a hydrothermal method using an autoclave. B-TiO2 was not detached from FTO glass, so it does not cause additional environmental pollutants. After photocatalytic reaction, B-TiO2/FTO substrate was easily recovered from aqueous phase. To increase the amount of AuNPs deposited on the surface of the B-TiO2/FTO, (3-aminopropyl)triethoxysilane (APTES) was used as the binder. The photocatalytic activity of AuNPs/B-TiO2/FTO glass was evaluated via a photoreduction of Cr(VI) to Cr(III) under visible-light irradiation.

Section snippets

Fabrication of TiO2 structure on FTO glass

In the fabrication process of B-TiO2, titanium(IV) butoxide (TBT, Sigma–Aldrich, 97%) and titanium(III) trichloride (TiCl3, Kanto Chemical, 20%) were used as precursor of TiO2 (Fig. S1). 30 mL of DI water was mixed with 30 mL of hydrochloric acid (HCl, Duksan, 36%) in a vial. The mixture was stirred at room temperature for 5 min, and 1 mL of TBT was slowly added dropwise. Then, the mixture was stirred for 10 min to disperse TBT evenly. For ultrasonically cleaning the FTO glass, acetone, ethanol, and

Morphological characterization of as-made AuNPs/B-TiO2

The morphological change of B-TiO2 and AuNPs/B-TiO2/FTO glass was investigated using HR-FE-SEM (Fig. 1) and TEM (Fig. 2). As shown in Fig. 1b and d, TiO2 microrods on FTO glass were grown vertically to a length of approximately 2 μm. While the outer surface of the TiO2 microrods was smooth (Figs. 1b and 2a), after the sideways growth of TiO2 nano-branch, as-made TiO2 microrods exhibited a very rough surface, compared with the previous step (Figs. 1d, e, and 2b–e) [21], [22], [23], [24], [25]. TiO

Conclusions

AuNPs/B-TiO2 mirorods immobilized on FTO glass were prepared via the hydrothermal method using an autoclave and the dip-coating method. AuNPs were easily deposited on the surface of branced-TiO2 microrods by using DI water and an APTES binder. The AuNPs/B-TiO2/FTO had a higher absorbance in the visible range and narrower bandgap, compared to that of parent TiO2. The photocatalytic activity of the samples under visible LED light was investigated via photocurrent measurements and the reduction of

Acknowledgements

This work was supported by the National Research Foundation of Korea(NRF-2017R1A2B4001829) and the Research Grant of Kwangwoon University(2019).

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