One-step, hydrothermal synthesis of nitrogen, carbon co-doped titanium dioxide (N,CTiO₂) photocatalysts. Effect of alcohol degree and chain length as carbon dopant precursors on photocatalytic activity and catalyst deactivation

Abstract

A one-step, hydrothermal method for the synthesis of nitrogen, carbon co-doped titanium dioxide (N,CTiO₂) photocatalysts is demonstrated. The incorporation of nitrogen from ammonia and carbon from alcohols with different chain length (methanol, ethanol, isopropanol, 1-butanol, 2-butanol, tert-butanol) used as carbon precursors was confirmed by FTIR/DRS and XPS analyses. The UV–vis/DR absorption spectra of the modified photocatalysts extended into the visible. XRD, BET and Zeta SizerNano techniques were used for the characterization of the modified photocatalysts. The crystallite size of N,CTiO₂ was not affected by the nature of the alcohol or the pressure acquired during the modification process but surface area, particle size (crystal agglomerate) and anatase content increased with synthesis pressure. In contrast with other studies in literature, the photoactivity of the different synthesized materials was evaluated at a constant volumetric rate of photon absorption (VRPA) in an annular photoreactor. This innovative method allows the evaluation of the intrinsic photoactivity of each material. As a result, the effect of N,C-co-modification on the TiO₂ photoactivity was evaluated without interference from the amount of radiation absorbed by each suspended powder since the total radiant energy absorbed by each slurry suspension was kept constant. Phenol decomposition confirmed that activity increased with the chain length of the alcohol precursor. The highest intrinsic photoactivity was for N,CTiO₂ prepared from 2-butanol, and 1-butanol as carbon precursor which also exhibited much stronger resistance to deactivation during multiple catalyst reuse compared to pristine TiO₂ and commercial Degussa P25 photocatalysts.

Introduction

Titanium dioxide (TiO₂) is a widely used photocatalyst for water treatment, air purification, antibacterial, deodorization, self-cleaning coating or anti-stain applications [1], [2], [3], [4], [5]. The irradiation of TiO₂ with band gap photons produce electron–hole couples which after migration to the surface of the solid may be trapped by surface reducible species (e.g., molecular oxygen) and adsorbed water to produce superoxide anion radicals and hydroxyl radicals. Subsequent reactions of these radicals between themselves and with adsorbed molecules lead to oxidation/reduction reactions which in turn may degrade aqueous/air pollutants. The band-gap of TiO₂ (anatase) is 3.2 eV, in consequence photons of wavelength of 384 nm or less (UV-A) can promote the generation of electron–hole couples. However, the redox process is limited by electron–hole recombination which in practice reduces the effectiveness of TiO₂ as a photocatalyst. The narrowing of the band gap in TiO₂ and its modification allows the utilization of a wider fraction of visible light for the production of charge carriers. Metal ions (Pt, Au, Ag, Cu, Fe), metal oxides (SnO₂, WO₃, MnO₂, V₂O₅) and nonmetals (C, N, F, S, P) have been proposed as modifiers that improve the photoactivity of TiO₂ [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Nitrogen and carbon doped into substitutional sites of TiO₂ [15] results in narrow band gaps (red-shift) which may lead to higher photo-response and higher photocatalytic activity. In other studies it is revealed that doped TiO₂ undergoes electronic transitions from localized states near the valence band to their corresponding excited states for Ti³⁺ centers after visible light irradiation [[16] and reference therein]. Co-doping of titania with carbon and nitrogen simultaneously [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28] may also show synergistic effect.

In the study by Yin et al. [17] N and C co-doped titania prepared by a mechano-chemical method revealed very high photocatalytic activity for nitrogen monoxide degradation under visible light irradiation, possessing two absorption edges around 400 nm and 540 nm. Chen et al. [18] synthesized TiO₂ photocatalysts with different amounts of carbon and nitrogen by the sol–gel method. The increase of photocatalytic degradation of methylene blue under visible light irradiation was ascribed to the synergistic effect of C and N atoms in agreement with other researchers [19], [20], [21], [22], [23]. Zhang et al. [23] proposed that the degradation of methylene blue under visible light was the results of partial replacement of oxygen atoms on the TiO₂ surface with nitrogen, whereas the carbon atoms formed a mixed layer of deposited carbon and CO band species on the surface of the TiO₂ particles. N and C co-doping of titania may also lead to an increase of the BET surface area of the catalyst [20] and improved photoactivity in the visible and UV regions. Carbon and nitrogen may act as photosensitizers. The substitution of oxygen atoms by nitrogen in the TiO₂ structure improves the absorption of visible light by mixing the N2p and O2p states and/or introducing a mid-gap N2p level which is located above the top of the O2p valence band. Serpone [16] proposed that the red-shift of the absorption edge is due to the formation of color centers, including Ti³⁺ centers and defects associated with oxygen vacancies that give rise to color centers displaying these absorption bands. He argues against the narrowing of the original band-gap of TiO₂ resulting from the mixing of dopant and oxygen states as that would require heaving of an anion or cation doping. Tseng et al. [25] showed the role of the chamber atmosphere on the surface lattice structure and nitrogen and carbon content of titania prepared by a metal-organic chemical vapor deposition process and the effect on NO photooxidation by visible-light. Wang et al. [26] described the degradation of bisphenol A by visible LED light in the presence of N,C co-doped TiO₂ prepared by solvothermal synthesis.

In this study specimens nitrogen–carbon co-doped anatase titania with different optical and structural characteristics were prepared by a simple, one-step hydrothermal method with different alcohols as carbon precursors and in the presence of ammonia water (NH₄OH) vapors as the nitrogen precursor. The effect of the alcohol degree and chain length employed for C-doping was investigated. In contrast with other studies in literature, the photoactivity of the different synthesized materials was evaluated at a constant volumetric rate of photon absorption (VRPA) in the photoreactor. This different method allows the evaluation of the intrinsic photoactivity of each material. As a result, the effect of N,C-co-modification on the TiO₂ photoactivity was evaluated more accurately without interference from the amount of radiation absorbed by each suspended powder.