In situ formation of large-scale Ag/AgCl nanoparticles on layered titanate honeycomb by gas phase reaction for visible light degradation of phenol solution

https://doi.org/10.1016/j.apcatb.2011.06.018Get rights and content

Abstract

Highly uniform AgCl nanoparticles (NPs) were grown in situ on a titanate honeycomb (THC) structure. The honeycomb structure is composed of vertically grown, intertwined one dimensional (1D) titanate nanowires from its side walls. This unique morphology of the THC surface structure was prepared by a modified hydrothermal approach within a short autoclave treatment time. The growth of AgCl crystals on the THC firstly makes use of a facile ion-exchange process by soaking the as-prepared THC in HNO3 solution and AgNO3 in sequence, during which Na+ ion in the interlayer of titanate is consequently replaced by H+ and Ag+ ions without changing its morphology. The obtained Ag-THC then readily reacts with HCl vapor to form the AgCl particles on THC. Finally, the visible-light-driven plasmonic photocatalyst Ag/AgCl/THC is obtained by partially reducing Ag+ ions from AgCl particles with the aid of Xe lamp illumination. The as-prepared photocatalyst exhibited high activity in the visible region of the solar spectrum for the degradation of phenol solution. The degradation performance and mechanism were discussed based on the high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC–MS).

Highlights

• In situ formation of large-scale and uniformly distributed Ag/AgCl nanoparticles on titanate honeycomb (THC) surface nanostructures. • The formation of uniform and dense AgCl nanoparticles on THC is by a facile technique combining ion-exchange and vapor phase reaction. • The Ag/AgCl/THC plasmonic photocatalysts are grown on a substrate so there is no concern about the catalyst separation as in the particle system. • The degradation mechanism of phenol is discussed based on the high performance liquid chromatography and gas chromatography–mass spectrometry.

Introduction

Devising suitable semiconductor photocatalysts for the decomposition of organic compounds using solar energy is one of the important issues in the photocatalysis research. An optimal material would combine an ability to dissociate organic molecules, having a band gap that absorbs light in the visible range and to remain stable in aqueous media. Besides, it should be non-toxic, abundant and easily processable into a desired shape [1], [2]. Up to now, various inorganic semiconductors have been reported for photocatalytic decomposition of organic compounds under visible-light irradiation by different approaches [2], [3], [4], [5]. For example, substitutional doping with various ions, photosensitizing with suitable dye and metal complexes, and developing new materials have been reported to be effective methods to extend the light response of photocatalysts. Despite of the focused attention on visible-light photocatalysts, there are still some drawbacks hindering their practical application, such as short photogenerated electron–hole pair lifetimes, low stability or limited spectrum of visible-light photo-response. New and more efficient visible-light photocatalysts are needed to meet the requirements of future environmental and energy technologies driven by solar energy.

Recently, a series of highly efficient plasmonic photocatalysts were developed for photocatalytic decomposition under ultraviolet [6], [7] (UV)/visible-light illumination [8], [9], [10], [11]. Among them, silver halide particles showed the faster degradation rate than nitrogen-doped TiO2 in the visible light region under the same condition [8]. Silver halides are extensively used as photosensitive materials in chemical photography. When they absorb the photons, the Ag+ ion is photoreduced to Ag0 atom due to the photogenerated electron. Ultimately, a cluster of silver atoms is formed within a silver halide particle upon repeated absorption of photons. Therefore, silver halides are seldom used as photocatalysts due to their instability under sunlight [8], [9]. However, the recent intensive studies have indicated that the silver halides or silver/silver halides possesses excellent photocatalytic activity and high stability under solar irradiation [8], [9], [10], [11], [12], [13], [14], e.g., H2 generation was observed and hydrogen was continuously evolved for 200 h without destruction of AgBr although Ag0 was detected under UV illumination [12]. So far, reported excellent photoactivity and stability of silver halides has attracted attention of many researcher groups worldwide. Hu et al. prepared the AgBr/TiO2 by deposition–precipitation method and found that the inhibiting decomposition of AgBr may be due to Ag0 species scavenged hVB+ (Ag0 + hVB+  Ag+) and then trapped eCB in the process of photocatalytic reaction [9], [15]. From electron spin resonance (ESR) analysis and H2O2 measurement, they found that radical dotOH or O2radical dot radicals were produced in the visible light illuminated aqueous Ag/AgBr/TiO2 suspension, and the AgBr was the main photoactive species for the destruction of organic dye under visible light. Their work of Ag/AgBr/Al2O3 shows that the degradation of pollutants came from both photoexcited AgBr and plasmon-excited Ag NPs [16]. Huang’ group proposed that the stable Ag@AgX (X = Cl or Br) under the visible light was due to their surface plasmon resonance (SPR) [8], [10]. Synergy between the enhanced absorption of visible light by strong SPR of the Ag nanograins and polarization field provided by the AgX core facilitates electron–hole separation and interfacial charge transfer. This means that a photon is absorbed by the silver NPs under visible light, and the electron separated from an absorbed photon remains in the Ag NPs rather than being transferred to the Ag+ ions of the AgX lattice. The hole transfers to AgX and oxidizes X to X0, which is the reactive radical species. They are able to oxidize organic dye and be reduced to X ions again. Therefore, the O2radical dot and X0 (Cl0 or Br0) radicals contribute to the degradation of dye. Very recently, experiment by Hu's group [11] confirmed that the two electron transfer processes were present from the excited Ag NPs to AgI and from 2-CP to the Ag NPs, and the main active species, O2radical dot and excited h+ on Ag NPs were involved in the photoreaction system of Ag-AgI/Al2O3. Therefore, Ag NPs were not corrupted by light and resulted in significantly stable AgI. We suspect that the different mechanisms reported might be due to the difference in band gaps and band edge positions, so the charge-separation mechanisms of the silver halides may differ. As a result, the photocatalytic activity differs too.

The highly efficient silver halides with good stability for the degradation of organic compounds have attracted great interest in both scientific and engineering fields. It is known that for a given photocatalyst, increasing the surface area and avoiding aggregation plays an important role in improving its photocatalytic activity. For example, the uniform Ag or Au NPs on AgCl nanowires via in situ oxidation of Ag nanowires in Fe3+ solution by the poly(vinyl pyrrolidone) (PVP) surfactant [17], [18]. Sun et al. reported monodispersed Ag/AgCl NPs in ethylene glycol with assistance of PVP [19]. The hybrid NPs were produced by partial reduction of AgCl nanocubes at an elevated temperature. Ding and Li used a two-step route to prepare porous Ag/AgCl involving the formation of nanoporous silver and subsequent surface chlorination process [20]. Moreover, Yu and co-workers [21] demonstrated the Ag/AgCl NPs can be immobilized on other substrates (e.g., TiO2 nanotube) via an impregnating–precipitation–photoreduction method for several times. Despite that these reports have demonstrated the feasibility of silver halides NPs preparation, a facile and inexpensive scheme is still lacking to achieve controllable morphologies, ideally of uniform distribution at large-scale. Meanwhile, numerous works have shown that the titanate materials possess excellent ability for cation-exchange. Since the cations are uniformly distributed in the interlayer of titanate crystal structure [22], [23], [24], [25], [26], [27], it makes it attractive for the synthesis of controllable nanoparticles using ion-exchanged titanate nanostructures as a starting material. Following this concept, we develop a novel process to produce highly uniform AgCl NPs on a prepared titanate honeycomb (THC) surface nanostructure. In addition to the plasmonic effect by the Ag NPs, the networked THC structures further enhance the adsorption of the reactive species and absorption efficiency of the incident light [28].

Section snippets

Preparation of THC films

THC films were grown on titanium foil (0.127 mm thick, Sigma–Aldrich) by a modified hydrothermal method. The titanium foil (5.5 cm × 4 cm) was washed by deionized water and then placed perpendicularly to the bottom of a 125 mL Teflon-lined stainless steel autoclave (Parr Instrument). The autoclave was filled with 75 mL of 1 M aqueous NaOH solution and kept inside an oven at 220 °C for 6 h. After the hydrothermal reaction, the titanium foil was taken out of the autoclave and immersed in low surface

Scheme of AgCl/THC formation

Scheme 1 shows the schematic illustration of the synthesis process for AgCl/THC heterostructure. The quantitative EDX data in Fig. S1 in the Supporting Information provides the elemental composition change with different processing stages. Firstly, the sodium THC (Na-THC, Fig. S1a) was synthesized after the hydrothermal treatment, and then the sample was immersed in HNO3 solution for the hydrogen ions exchange (Fig. S1b). Secondly, the obtained hydrogen THC (H-THC) went through silver ion

Conclusions

In this work, a strategy combining ion-exchange and gas phase reaction was employed to synthesize highly uniform and densely distributed AgCl NPs on titanate honeycomb network thin films. After illumination, the Ag/AgCl/THC photocatalyst exhibits excellent photocatalytic activity for the decomposition of phenol. The degradation mechanism was also proposed based on the GC–MS analysis. This room-temperature synthesis route could be easily extended to prepare various solar light responsive

Acknowledgements

The authors thank the Environment and Water Industry Programme Office (EWI) under the National Research Foundation of Singapore (grant MEWR651/06/160) for the financial support of the work. The authors thank Drs. Pierre Pichat, Simo Pehkonen, and Alexander Orlov for their constructive advice and time for discussion.

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