Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO₂

Abstract

New nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO₂ having only the homogeneous anatase crystalline phase and high surface area were successfully synthesized by a modified sol–gel route. The catalysts were characterized by EDX, XRD, BET, UV–vis, IR, and Raman spectroscopy. The effects of the experimental parameters on the visible light reactivity of the catalysts were evaluated for the photodegradation of gaseous acetaldehyde as a model indoor pollutant. The activity results show that the silver(I) ion, Ag⁺, doping significantly promotes the visible light reactivities of carbon and sulfur-doped TiO₂ catalysts without any phase transformation from anatase to rutile. Moreover, Ag/(C, S)–TiO₂ photocatalysts degrade acetaldehyde 10 times faster in visible light and 3 times faster in UV light illuminations than the accredited photocatalyst P25–TiO₂. The commendable visible photoactivities of Ag/(C, S)–TiO₂ new nanoparticle photocatalysts are predominantly attributable to an improvement in anatase crystallinity, high surface area, low band gap and nature of precursor materials used.

Introduction

The phenomenon of photocatalysis is defined as the combination of photochemistry and catalysis. More precisely, the term “photocatalysis” herein implies that the light and a catalyst are essential to enhance the rates of thermodynamically favored but kinetically slow photophysical and photochemical transformations. Indeed, photocatalysis emerged as a new scientific area when Fujishima and Honda carried out photolysis of water into environmentally clean fuels (hydrogen and oxygen) using a titanium dioxide electrode in an electrochemical cell [1]. Ever since, heterogeneous photocatalysis by means of TiO₂ has been widely accepted and exploited as an efficient technology for killing bacteria and degrading organic and inorganic pollutants [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Moreover, titanium dioxide (TiO₂) has been regarded as an excellent semiconductor photocatalyst because of its performance, low cost, nontoxicity, stability, and availability. Unfortunately, because of its wide band gap (anatase: 3.2 eV; rutile: 3.0 eV), the extensive exploitation of TiO₂ created an expectation to use merely 3–4% UV light of the whole radiant solar energy.

Fortunately, there are, however, a number of empirical ways to design and develop a second generation of visible-light-sensitive photocatalysts of titanium dioxide by means of physical and chemical processes [14], [15]. Of the various processes cited in the literature, ion implantation methods require more expensive and more sophisticated equipment, whereas chemical methods are more economical and can be conducted at or near ambient temperatures. In context, a great deal of effort has shown that doping with transition metals, such as Cr, Co, V, and Fe, extends the spectral response of TiO₂ well into the visible region and enhances the photoreactivity [16], [17], [18], [19]. However, transition metal ion-doped TiO₂ suffers from some serious drawbacks, such as thermal instability and low quantum efficiency of the photoinduced charge carriers (electron–hole pairs) [20]. Besides metal ion-doped TiO₂ systems, there are numerous recent reports on nonmetal-doped TiO₂, for example, carbon, nitrogen, phosphorus, sulfur and fluorine doped photocatalysts [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Basically, doping with carbon, nitrogen and sulfur effectively narrows the band gap of TiO₂ (<3.0 eV) [34], [35], [36]. Moreover, band gap narrowing emanates from the electronic perturbations caused by change of the lattice parameters and/or the presence of the trap states within conduction and valence bands of TiO₂. Consequently, the photons of lower energy ($\lambda >420\text{nm}$) can induce electron–hole pairs in TiO₂. These photoinduced electrons and holes, which are in fact very powerful reducing and oxidizing agents, migrate to the surface of TiO₂ and eventually become available for direct or indirect consecutive reduction and oxidation reactions. Furthermore, because of the presence some trap states within the band gap of titanium dioxide, the lifetime of the so-called photoinduced charge-carriers increases in such a way that it predominates over the fast charge-recombination process, thereby resulting in an enhanced visible light reactivity.

Apart from doping TiO₂ with a single metal or nonmetal, it is highly anticipated that doping TiO₂ with an appropriate combination of metals and/or nonmetals would, of course, result in more visible light sensitive photocatalysts for a desired application. In this context, Li et al. synthesized N–F-codoped TiO₂ photocatalysts by spray pyrolysis (SP) using TiCl₃ and NH₄F precursors and observed an enhanced photoreactivity of the materials in visible light [37]. Luo et al. prepared a Br and Cl-codoped TiO₂ system and demonstrated the efficiency of the material for photocatalytic splitting of water into H₂ and O₂ in the presence of Pt co-catalyst and UV light irradiation [38]. These recent efforts and strategies have revealed that codoping TiO₂ with a metal and a nonmetal can result in the development of additional visible active photocatalysts [39], [40], [41].

It is well known that noble metals such as Ag, Au, and Pt possess unique electronic and catalytic properties. For example, Li reported that Au³⁺-doped TiO₂ exhibited visible light reactivity for the photodegradation of methylene blue [42]. Likewise, Kim et al. prepared Pt ion-doped TiO₂, and examined its visible light activity for the photodegradation of chlorinated organic compounds [43]. From an economic viewpoint, gold and platinum are very expensive and unaffordable metals for extensive use in photocatalysis. Compared to gold and platinum, silver is a more affordable metal and deserves further investigation. Munevver et al. reported a Ag–TiO₂ system for killing E. coli under UV light illumination [44]. There have been some reports on the preparation of Ag⁺-doped TiO₂ films and nanoparticles that degrade some textile dyes (methyl orange, crystal violet, and methyl red) in aqueous medium [45], [46], [47]. Furthermore, silver halides, such as AgBr/SiO₂ and AgCl catalysts, have also been used in photocatalysis [48], [49]. It appears that doping with Ag⁺ ions makes a dramatic improvement in the performance of TiO₂ photocatalysts; however, the aforementioned silver based photocatalysts function only under UV light. Earlier, researchers in our group have reported the synthesis and characterization of some nanocrystalline metal oxides and mixed metal oxides. It was discovered that materials in the nano-size domain exhibit a unique surface chemical reactivity for the destructive absorption of acid gas and chemical warfare agents [50], [51]. Moreover, this unique surface reactivity of nanomaterials was attributed to the high surface areas and the presence of defects, edges, and corners.

Therefore, it is rationally more desirable to synthesize new nanoparticle visible-light-driven photocatalysts based on silver, carbon and/or sulfur-doped TiO₂. To accomplish the desired work, a quest for a suitable precursor material seems indispensable for carbon and sulfur dopants. On this line of research, Park et al. prepared carbon-doped TiO₂ nanotube arrays at an elevated temperature range 500–800 °C by using carbon monoxide precursor, and tangibly demonstrated the catalytic efficiency of the C-doped TiO₂ nanotubes for water splitting under visible light irradiation [52]. Further, Choi et al. fabricated C-doped TiO₂ photocatalysts by oxidative annealing of TiC and ended up with the conclusion that C-doped TiO₂ powder exhibited superior photoactivity for the photodecomposition of methylene blue and water under UV light [53]. Thus, carbon monoxide can be used, but it is rather dangerous and is not a suitable precursor for carbon dopant on a large-scale synthesis. Also, oxidative annealing of TiC requires hundreds of hours or progressively very high temperatures (600–750 °C) for optimal activity of the materials and such a high temperature synthesis results in materials inclusive of anatase–rutile phases and lower surface areas, thereby making them less worthwhile for photocatalysis. We thus realize, in essence, the need for a more suitable precursor for carbon and sulfur dopants other than TiC, CO, and NH₂CSNH₂. Herein, we express a strong preference for carbon and sulfur as nonmetal dopants, because these elements can stabilize Ag⁺ ions in doped TiO₂ systems. Moreover, the well-dispersed Ag⁺ ions trap photoinduced electrons, leading to a substantial increase in electron–hole separation and a concomitant decrease in charge-carrier recombination.

To the best of our knowledge, there has been no publication on the synthesis of a silver, carbon and sulfur-doped TiO₂ system and its photocatalytic performance in visible light. In this article, we report Ag/(C, S)-doped TiO₂ photocatalysts and the structural and photocatalytic properties of these new nanoparticle photocatalysts. The photoreactivity of the synthesized materials was evaluated for the degradation of gaseous acetaldehyde (a major indoor pollutant) under UV and visible light.