A Systematic Study of the Effect of Silver on the Chelation of Formic Acid to a Titanium Precursor and the Resulting Effect on the Anatase to Rutile Transformation of TiO₂

Abstract

Anatase to rutile transition in an unmodified synthetic titania usually occurs at a temperature range of 600-700°C. Various methods such as addition of metallic and nonmetallic dopants and modifying the precursor have previously been reported to influence the anatase to rutile transition temperature. In the current study, the effect of addition of increasing amounts of silver to the extent of chelation of a formate group to a titanium precursor and the resulting effects on the transformation of anatase to rutile has been studied. The addition of silver (0, 1, 3, and 5 mol %) on the anatase to rutile transformation temperature has been systematically followed by Fourier transform infrared (FTIR), Raman, X-ray diffraction (XRD), differential scanning calorimetry, and X-ray photoelectron spectroscopy (XPS) studies. From the FTIR and Raman spectroscopy studies it was observed that the incorporation of silver caused a reduction in the intensity of the COO -stretches indicating that the titania formate bridging complex is becoming weaker in the presence of silver. XRD studies indicated an early rutile formation for the silver-doped samples. XRD of the samples calcined at 700°Cshowed that 5 mol % Ag TiO 2 contained both anatase (46%) and rutile (54%), whereas the undoped sample primarily consists of anatase (95%). At 800°C all silver doped samples converted to 100% rutile and the undoped TiO 2 consisted of both anatase (55%) and rutile (45%). XPS analysis showed that Ag 0 and Ag 2 O has been formed on the surface of the titania formate complex without calcination (>100°C) indicating that photo-oxidation has occurred. FTIR, Raman, and XPS studies confirmed that the presence of silver in the xerogel before calcination may be responsible for the reduction of the titanium formate bridge. It was concluded that the presence of silver (Ag 2 O and Ag 0 ) hindered bridging ligands, which resulted in a weakened titanium gel network. This structurally weakened gel network could easily be collapsed during calcination, and it favors an early rutile formation.

Effect of N-doping on the photocatalytic activity of sol–gel TiO2

“…An endothermic peak was observed at ∼100 • C for both doped and undoped TiO 2 , which represents unbound water being removed from the TiO 2 surface [16]. However, for undoped TiO 2 the endothermic peak was of greater intensity indicating a greater capacity for the material to absorb water due to increased porosity.…”

“…Anatase and rutile are the most widely investigated polymorphs, with anatase being reported as the most photocatalytically active of the three [3,14,15]. Typically, anatase transforms to rutile under heat treatment (600-700 • C) [16,17]. Parameters that affect the anatase-to-rutile transformation include; particle shape/size [18], source effects [19], atmosphere [20], reaction conditions [21] and purity [22].…”

Chelated Nitrogen-Sulphur-Codoped TiO2: Synthesis, Characterization, Mechanistic, and UV/Visible Photocatalytic Studies

“…TiO 2 exists in nature in three polymorphic forms (anatase, rutile, and brookite), which differ only in the arrangement of their TiO 6 octahedra; anatase (tetragonal) consists of octahedral shearing vertices; rutile (tetragonal) is connected by edges; and in brookite (orthorhombic), both edges and vertices are connected. Thermodynamically, rutile is the most stable phase while both anatase and brookite are metastable, transferring to rutile under heat treatment, typically at temperature ranging between 600 and 700°C [2]. Among the TiO 2 crystalline forms, anatase and rutile are the most widely investigated polymorphs, with anatase generally found to be more photocatalytically active than brookite and rutile, but some reports illustrate the synergistic effect found in the composite mixture of anatase-brookite and anatase-rutile to be responsible for the enhanced photocatalytic activity [3,4].…”

“…However, Ihara et al [7] suggested that addition of nitrogen only increases the stabilization of oxygen vacancies and as a consequence helps to improve the visible light activity of N-TiO 2 . While, work by Irie et al [8] showed that nitrogen atoms can be substituted for the lattice oxygen sites to form an isolated impurity energy level (N2p) above the valence band (O2p) of TiO 2 , and under visible light, electrons are excited to the nitrogen impurity energy level. Many other research efforts have also been made with F, S, I, B, Br, C, Cl, and P to develop anionic-doped TiO 2 with improved visible light activity [2,9], and among these nonmetal dopants, sulphur doping in TiO 2 has received much attention due to its anionic (sulfide, S 2− ) and cationic (hexavalent (S 6+ ) and tetravalent (S 4+ )) characteristics.…”

Solar photocatalysis for water disinfection: materials and reactor design

“…However, introduction of artificial UV light sources is not practical in remote areas where there is a lack 70 of power supplies. Utilisation of the main part of the solar spectrum by the development of photocatalysts (Figure 2) that can yield high photocatalytic activity under visible light 7,[11][12][13][14][15][16][17][18][19][20][21] would be highly beneficial in remote regions. Hydroxyl radicals have the most positive electrochemical 10 reduction potential (+2.8 V vs. normal hydrogen electrode (NHE)) among other substances generally employed for water disinfection, e.g.…”