Valence State and Catalytic Role of Cobalt Ions in Cobalt TiO₂ Nanoparticle Photocatalysts for Acetaldehyde Degradation under Visible Light

Abstract

CobaltÀcarbonÀsulfur-codoped anatase photocatalysts (size <9 nm) were synthesized by hydrolyzing titanium(IV) isopropoxide in the presence of cobalt salts and ammonium thiocyanate at room temperature. The annealed photocatalysts were characterized with nitrogen adsorp-tionÀdesorption isotherms, energy-dispersive X-ray (EDX), X-ray diffraction (XRD), UVÀvisible diffuse reflectance, transmission electron microscopy (TEM), X-ray photon spectroscopy (XPS), and photoluminescence (PL) measurements. Characterization results showed that the doped cobalt, carbon, and sulfur of the TiO 2 photocatalysts have some beneficial effects on specific surface area, visible light absorption, hydroxyl radical generation, and electronÀhole pair separation. For the acetaldehyde degradation reaction, the activity of the photocatalysts follows the order 2% Co(III)/(C, S)ÀTiO 2 ≈ 2% Co(II)/(C, S)ÀTiO 2 > 1% Co(II)/(C, S)ÀTiO 2 > 2% Co(II)ÀTiO 2 > 2% Co(III)ÀTiO 2 > (C, S)ÀTiO 2 under visible light and 2% Co(II)/(C, S)ÀTiO 2 > 1% Co(II)/(C, S)ÀTiO 2 > 2% Co(II)ÀTiO 2 ≈ (C, S)ÀTiO 2 > TiO 2 ÀP25 under UV light. It was concluded that the doped cobalt ions play a vital role and act as active sites in the photodegradation reaction. A reaction mechanism is proposed that may explain the synergistic effect of the codopants in visiblelight-induced photodegradation of acetaldehyde. This new photocatalyst system, Co/(C, S)ÀTiO 2 , can have other potential applications that only need visible light as energy input.

Solar simulated hydrogen evolution using cobalt oxide nanoclusters deposited on titanium dioxide mesoporous materials prepared by evaporation induced self-assembly process

“…The CoTiO 2 samples show varied bandgap depending on the cobalt loading (Table 1). Previous research on Co-TiO 2 has shown that the bandgap varies as function of cobalt loading [14]. The bandgaps were calculated by extrapolation of the high slope region in the DRS plot to the X-axis and was found to be 3.01 eV for the non-modified titania sample and between 2.31 and 2.62 eV for the Co-modified sample.…”

“…Efforts to extend the response of titania based semiconductors into the visible region include coupling it with other low band gap semiconductors such as CdS, doping with metals, and/or non-metals and modification of its surface [11e15]. Among these, doping or surface modification of titania with transition metal ions has been studied for a variety of photocatalytic reactions such as CCl 4 dechlorination [11], reduction of nitrite and nitrate ions [16], acetaldehyde degradation [14,17], degradation of aromatic organics [18e21], and degradation of dyes [12,22e26]. However, contradictory reports exist in literature regarding the effect of doping due to variations in the location, nature, content, d electronic configuration of the dopant ions, diffusion length of the minority carriers, and the effectiveness of the dopant ions in trapping electrons and/or holes [11,27].…”

“…These researchers noted that phase of TiO 2 also has a dramatic effect on photocatalytic water-splitting with anatase producing more hydrogen compared to rutile. Doping and/or surface modification of titania with cobalt ions have been studied for photocatalytic degradation of organics and dyes [14,17,18,20,21,25,44,45]. The results in general indicate that doping with cobalt ions is either detrimental to photocatalytic activity or that there is an optimal loading at which the activity of the Co-doped titania is enhanced compared to nonmodified titania.…”

A brief review of co-doping

“…Figure 3 schematically shows the co-doped structure and the coupling mechanism. There are ongoing attempts to codope cation dopant with the mentioned anions for increasing the efficiencies of photochemical reactions, such as Co-C-S [161] and Fe-N [162], yet, theoretical studies of these complex co-doping schemes are limited. Furthermore, owing to the complex nature of photochemical reactions, the dopant can affect the surface texture and the reaction pathway [150,157,161], all of which complicate theoretical studies.…”

Nanomaterial‐Based Photocatalysts

“…A variety of metals on TiO 2 have been explored, including Fe, Cr, V, Ni, Nb, Mn, Cu, Al, Bi, and Co, and the references provided are for the best representative examples. [29][30][31][32][33][34][35][36][37][38][39] In most of these cases, publications have shown that cation doping of TiO 2 results in products that easily show visible light absorptivity; however, in general this does translate into photoactivity. However, having stated this, there have been some successful reports such as the one by Iino and coworkers where they have shown that cation doping (using Cr 3+ ) of TiO 2 results in both visible light absorptivity and photoactivity toward NO.…”