Photocatalytic conversion of CO2 to hydrocarbon fuel using carbon and nitrogen co-doped sodium titanate nanotubes
Graphical abstract
Introduction
The excessive release of CO2 into atmosphere is considered as a major cause of the challenging issues of global warming, climate changes and environmental pollution. To overcome the issue of increased atmospheric CO2 concentration, one effective approach is to convert atmospheric CO2 to hydrocarbon fuels. CO2 is a thermodynamically stable molecule [1]; hence significant energy is required for its conversion. In this regard, solar-spectrum photocatalysis is considered a favorable strategy for conversion of CO2 into useful energy compounds, such as CO, methane, formic acid, methanol, etc. [2], [3], [4]. Enormous research have been conducted in developing a various photocatalyst materials for photocatalytic CO2 conversion like ZnO [5], ZnGe2O4 [6], Ga2O3 [7], Co3O4 [8], TaO3 [9], BiVO4 [10], g-C3N4 [11], Si nanocrystals [12], GO [13] and TiO2 [14]. Amongst them, Titania (TiO2) or Ti-based photocatalysts have been intensively studied due to their abundant availability, nontoxicity, corrosion resistance, and chemical stability [15], [16], [17]. However the relatively wide bandgap of TiO2, approximately 3.2 eV, which is responsible for its excellent corrosion stability, limits its absorption of electromagnetic energy to UV wavelengths, comprising only some 4% of the terrestrial solar-spectrum energy [18]. Numerous approaches have been used for reducing the TiO2 bandgap as a means of absorbing, and ideally utilizing, a greater portion of the solar spectrum energy, including metal ion implantation [19], [20], non-metal doping [21], [22], [23], synthesis of low bandgap hybrid nanomaterials [24], dye-sensitization [25], and doping with an up-conversion luminescence agent [26].
TiO2 modifications with metals co-catalysts e.g. Pt, Ag, Cu, etc. which acts as electron traps, suppressing the recombination of photogenerated electron–hole pairs and improving the photocatalytic activity [19], [20]. On contrary to loading of expensive metals, doping or co-doping of non-metals e.g. N, S, C, F, etc. into TiO2 lattice are inexpensive and less likely to create recombination centers with significant narrowing of bandgap resulting in improved light absorption [27]. Xue et al. [28] reported a markedly increased CO2 photoreduction to formic acid (439 μmol/g h) by C doped TiO2 having low band gap as compared to undoped TiO2 with significant extension of light absorption to visible range. Zhang et al. synthesized Iodine doped TiO2 (I-TiO2) and detected high CO2 photoreduction to CO (2.4 μmol/g h) under visible light as compared to undoped TiO2 [29]. Upon co-doping I-TiO2 with Cu, Zhang et al. [30] found further increase in amount of CO produced (6.7 μmol/g h) with traces of CH4 and CH3Cl. They observed iodine doping mainly responsible for visible light activity while the Cu species improved charge transfer and enhanced CO2 photoreduction. Moreover, they found that activity of Cu-I-TiO2 is higher under UV light as compared to single I-TiO2, but under visible light no significant difference among them.
TiO2 can also form various nanostructures such as nanofibers [31], nanosheets [32], nanotubes [33] in which above mentioned modifications are possible with stable morphology. Among the variety of available nanostructures layered-titanate nanotubes (TNTs), innovated by Kasuga et al. [34], synthesized by alkaline, e.g. NaOH, hydrothermal treatment of crystalline TiO2 nanoparticles, have been widely studied for an extensive range of applications [35], [36]. Advantages of the material architecture include large surface areas, improved adsorption capacity, moderate photocatalytic activity, and improved thermal and chemical stability [37], [38], [39]. It is recognized that during hydrothermal synthesis of the layered-TNTs that Na+ ions intercalate between the titanate layers. These Na+ ions are believed to act as recombination centers that decrease photocatalytic activity [40], [41]. However, it has recently been reported that Na ions (Na+) bound to TNT-interlayers improves CO2 adsorption capacity that, in turn, influence rates of CO2 photoconversion into hydrocarbons [42], [43]. The layered structure of the hydrothermally synthesized TNTs makes them ideally suited for investigating the effects of single or multi-anion doping as a means for improving their photocatalytic properties. Carbon and nitrogen are well known dopants with admirable ability to improve the photocatalytic activity and enormous amount of investigations have been done for C and N doping in TiO2 nanoparticle systems [44], [45].
Motivated by the unique qualities of the layered-TNTs and the potential for simultaneous co-doping of the materials as a means to obtain improved photocatalytic properties we have investigated and herein reported on the application of carbon and nitrogen co-doped sodium titanate nanotubes (C,N-TNTs) for photoconversion, under simulated solar-light, of CO2 and water-vapor into hydrocarbon fuels. C,N-TNT are prepared by a simple two step synthesis route. The first step of synthesis involves alkaline hydrothermal treatment of available TiO2 (anatase), producing sodium titanate nanotubes which are then mixed with desired amount of urea and calcined in the second step to obtain C,N-TNT. A series of the C,N-TNT is prepared by varying the composition of urea and its influence on physicochemical properties and photocatalytic activity of the materials is demonstrated. The materials are characterized by various instrumental techniques. It is reported, the most stable nanotube structure is obtained via alkaline hydrothermal treatment of TiO2 (anatase) with NaOH as compared to nanotubes formed when other alkali metal oxides are used [46], [47]. The stable nanotube structure is also an important parameter for photocatalytic activity [48], thus herein NaOH was used during alkaline hydrothermal treatment step. To the best of our knowledge, this is the first report demonstrating and investigating key parameters for photocatalytic performance of C,N-TNT by photocatalytic conversion of CO2 into hydrocarbons.
Section snippets
Materials and reagents
Sodium hydroxide (Reagents Duksan, 94%), TiO2 anatase (Daejung, >98%), urea (Sigma–Aldrich, 98%), methylene blue (Sigma–Aldrich) and deionized water (resistivity > 18 μm) was used throughout the experiments.
Preparation of sodium titanate nanotubes (TNT)
The synthesis route followed for preparation of sodium titanate nanotubes is reported earlier [49]. Typically 2.0 g of commercially available TiO2 powder (anatase) is mixed with 20 mL of 10 M NaOH solution in a beaker. The suspension was stirred for 1 h to get a uniform mixture. The mixture was
Crystalline structure
The XRD pattern of bare TNT and C,N-TNT samples are shown in Fig. 1(A). The samples exhibit diffraction peaks at 2θ value of 10.2°, 24.2°, 28.6°, 33.8°, 38.6° and 61.6° corresponding to d2 0 0, d1 1 0, d3 1 0, d31-2, d0 0 4 and d0 2 0 respectively, indicating the presence of titanate phase [51]. The peaks appearing near 2θ values of 10.2° and 28.6° correspond to C,N-TNT interlayer spacing [52], [53]. Compared to un-doped TNT sample, the peaks of higher urea content samples are shifted to higher angles,
Conclusions
In summary, bare TNT and C, N co-doped TNT samples were prepared by hydrothermal method followed by calcination process. The photocatalytic performance of the prepared samples was mainly investigated for photocatalytic conversion of CO2 into CH4. The prepared materials were characterized by various analytical instruments such as XRD, TEM, SEM, Raman, XPS, and BET. While the Na+ ions serve to promote CO2 adsorption and act as electron–hole recombination centers, the amount of C,N-doping affects
Acknowledgments
This work was supported by the DGIST R&D Program of the Ministry of Science, ICT & Future Planning (15-BD-0404). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1008678).
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These authors contributed equally to this work.