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
Graphical abstract
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Highlights
► Optical and structural properties of N,CTiO2 depend on the alcohol precursor used. ► N,CTiO2 deactivates much less than TiO2 P25 and has much higher activity after reuse. ► A new approach for the evaluation of the intrinsic photocatalytic activity is proposed. ► Photocatalytic activity of slurries is measured at equal rate of photon absorption.
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.
Section snippets
Materials
Commercial P25 TiO2 (Degussa, Germany) with BET surface area of 55.5 m2 g−1 was used as a reference photocatalyst. A commercial titanium dioxide (TiO2/A) supplied by Chemical Factory “Police” S.A. (Poland) with BET surface area of 238 m2 g−1 was used as a pristine material for the synthesis of N,C modified TiO2. Alcohols (methanol, ethanol, isopropanol, 1-butanol, 2-butanol, tert-butanol, POCH, Gliwice, Poland) and ammonia water (30%, reagent grade, ACS, ISO, Scharlau Chemie) used as carbon and
UV–vis/DR
Fig. 1, shows the UV–vis/DR spectra of pristine TiO2 together with the spectra obtained for N,CTiO2/2-butanol. Pristine TiO2 absorbed radiation in the ultraviolet, however, the absorption spectra of the modified photocatalysts extended into the visible. The UV–vis/DR spectra for all the samples was plotted using the Kubelka–Munk model [32] that allows the calculation of the reflectance from materials that both scatter and absorb light. This so-called “two-flux” model, where only diffuse light
Conclusions
In summary, this study has demonstrated a simple one-step method for the synthesis of N,C-modified TiO2 photocatalysts. The incorporation of N from ammonia and C from alcohols into the photocatalysts was confirmed by FTIR/DRS and XPS studies. The nature of the alcohol used as C precursor was found to have a significant role on the optical (absorption of radiation) and physico-chemical (surface area, particle and crystallite size) properties of the modified materials.
A new approach for the
Acknowledgments
This work has been supported by the Polish Ministry of Science and Higher Education as a scientific project. Prof. Morawski thanks Prof. Barbara Grzmil from West Pomaranian Univeristy of Technology for the XRD analysis and Dr Dariusz Moszyński also from West Pomaranian Univeristy of Technology for the XPS analysis.G. Li Puma is grateful to NATO (grant CBP.EAP.SFPP 982835) for funding the studies on catalyst activity and kinetic analysis. The authors are grateful to the EU Commission for
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