Mechanothermal synthesis of Ag/TiO₂ for photocatalytic methyl orange degradation and hydrogen production

Highlights

• Synthesis of a highly activity Ag/TiO₂ photocatalyst through a two-step method.

• Synergy between silver and titania enhanced the photophysical properties.

• Ag/TiO₂ exhibited excellent stability over 3 cycles for photocatalytic degradation of Methyl orange.

Abstract

Photocatalysis offers a promising route to address the challenges of future energy production and anthropogenic environmental pollution. Here we demonstrated the synthesis of a high activity Ag/TiO₂ photocatalyst through a two-step, sol-gel and mechanothermal decomposition method employing a silver acetate precursor. Bulk and surface characterization revealed the formation of dispersed metallic silver nanoparticles (∼9 nm diameter) decorating anatase crystallites (∼14 nm) which stabilized a significant concentration of Ti³⁺ surface species. Synergy between silver and titania enhanced the photophysical properties, narrowing the band gap and suppressing charge-carrier recombination. Ag/TiO₂ exhibited good visible light activity and excellent stability over 3 cycles for the aqueous phase photocatalytic degradation of methyl orange dye (38 μmol/h/g_cat), and excellent hydrogen production from water splitting (910 μmol/h/g_cat).

Introduction

Global energy demand is predicted to rise more than 50 percent between 2013–2040, largely due to population growth and associated expansion of transportation, industrial, residential and commercial sectors (US Energy Information Administration, 2016). A parallel increase in the quantity and diversity of pollutants released into the environment is also predicted, due to an over-reliance on chemicals for agriculture and manufacturing and the concomitant release of contaminated waste. Photocatalysis offers a valuable approach to address both solar fuels production and environmental depollution (Khan et al., 2014a; Yan et al., 2013; Sivula and Krol, 2016; Wang et al., 2016; Tachibana et al., 2012; Ge et al., 2016a), with hydrogen generation via photocatalytic water splitting (Khan et al., 2014a; Yan et al., 2013; Sivula and Krol, 2016; Wang et al., 2016; Tachibana et al., 2012; Ge et al., 2016a). In regard of the environmental remediation, water pollution accounts for >840,000 fatalities annually worldwide, with 80% of associated contaminants arising from the discharge of toxic, organic compounds by industrial and agricultural processes (World Health Organization WHO, 2017). Organic azo dye such as methyl orange is difficult to treat by conventional bio- and/or physicochemical processing (Chan et al., 2009; Awual et al., 2015a,b; Awual and Hasan, 2015; Awual et al., 2016) and their concentrations can reach 500 ppm in textile effluents (Chequer et al., 2013). Advanced oxidation processes such as Fenton and photo-Fenton oxidation are promising solutions to their oxidative removal from wastewater but require significant quantities of H₂O₂ and are prone to metal leaching. Photocatalytic solutions to wastewater depollution are therefore desirable.

In recent years, solid state nanomaterials such as semiconductors, nanoparticles, nanowires, nanotubes, nanoporous, and hollow materials have found which have applications to energy and environment science, especially in photocatalysis (Khan et al., 2014a; Ge et al., 2016a). Such nanomaterials offer high surface areas, rapid charge transport and selective chemical transformations. For over three decades, titania has been the most widely used photocatalyst due to its high thermochemical stability, low toxicity, abundance, and conduction band energy which renders it suitable for oxidation processes (Khan et al., 2014a; Yan et al., 2013; Ge et al., 2016a; Luttrell et al., 2014), notably photodegradation of organic pollutants under UV irradiation. While compared with brookite and rutile phase of TiO₂, anatase phase show high catalytic performance because of oxidation and reduction potential favor for removal of contamination under UV illumination (Luttrell et al., 2014). However, the wide band gap and poor quantum efficiency of pure TiO₂ is a barrier to solar photocatalysis (Khan et al., 2014a; Yan et al., 2013; Ge et al., 2016a; Luttrell et al., 2014; Pelaez et al., 2012; Kumar et al., 2017), although diverse methods exist to engineer the physical/electronic structure and chemical composition of TiO₂ including through nanocomposite formation (Pelaez et al., 2012, 2012; Kumar et al., 2017) in order to utilize visible light. Ag/TiO₂ has shown potential for photocatalytic energy and environmental applications (Zhou et al., 2011, 2014; Ubonchonlakate et al., 2012; Hu et al., 2016; Gomes et al., 2017; Wang et al., 2013; Fei and Li, 2014; Lim et al., 2014; Wu et al., 2013; Ravishankar et al., 2015), including through plasmonic enhancement of dye sensitized solar cells (Lim et al., 2014); silver is particularly attractive due to its low toxicity to humans and visible surface plasmon resonance (Gomes et al., 2017; Zhou et al., 2014; Ubonchonlakate et al., 2012; Hu et al., 2016; Gomes et al., 2017; Wang et al., 2013; Fei and Li, 2014; Lim et al., 2014; Wu et al., 2013; Ravishankar et al., 2015). Ag/TiO₂ photocatalysts are typically synthesized through (thermos) chemical reduction, which hinders control over the resulting dimensions, composition and phase of the resulting material, hence new synthetic approaches desirable to elucidate the nature of active sites and synergy between components.

Herein, we report a new route for Ag/TiO₂ through a combined sol-gel and subsequent mechanothermal synthesis which obviates the need for a reduction step, and affords a simple and cost-effective route to high activity photocatalysts for the photocatalytic degradation of azo dyes, and production of hydrogen from water, under UV or/and solar irradiation.