Photoreduction of CO2 on TiO2/SrTiO3 Heterojunction Network Film
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Nanotube titanic acid (NTA) network film has a porous structure and large BET surface area, which lead them to possessing high utilization of the incident light and strong adsorption ability. We used NTA as the precursor to fabricate a TiO2/ SrTiO3 heterojunction film by the hydrothermal method. In the process of the reaction, part of NTA reacted with SrCl2 to form SrTiO3 nanocubes, and the remainder dehydrated to transform to the rutile TiO2. The ratio of TiO2 and SrTiO3 varied with the hydrothermal reaction time. SEM and TEM images indicated that SrTiO3 nanocubes dispersed uniformly on TiO2 film, and the particle size and crystallinity of SrTiO3 nanocubes increased with the reaction time prolonging. The TiO2/SrTiO3 heterojunction obtained by 1 h showed the best activity for CO2 photoreduction, where the mole ratio of TiO2 and SrTiO3 was 4:1. And the photo-conversion efficiency of CO2 to CH4 improved remarkably after the foreign electron traps of Pt and Pd nanoparticles were loaded. The highest photocatalytic production rate of CH4 reached 20.83 ppm/h cm2. In addition, the selectivity of photoreduction product of CO2 was also increased apparently when Pd acted as the cocatalyst on TiO2/SrTiO3 heterojunction film.
KeywordsNanotube titanic acid Porous network film TiO2/SrTiO3 heterojunction CO2 photoreduction Product selectivity
Nowadays, the fossil fuels are still the main energy resource for our society. However, the shortage of fossil fuels and the growing environmental concerns due to the emission of large amounts of CO2 during the combustion of fossil fuels have become the global problems. Conversion of CO2 into useful hydrocarbon fuels is a possible avenue to develop alternative fuels, and prevent the green house effect on the global temperature. For example, the chemical conversion of CO2 into industrially beneficial compounds is advantageous in terms of green and sustainable chemistry because CO2 is an inexpensive, nontoxic and abundant C1 feedstock . Particularly, catalytic conversion of CO2 to hydrocarbon fuels and chemicals have attracted much attention in recent years [2, 3, 4, 5, 6].
At the same time, TiO2-based materials are the most common photocatalysts because of their many advantages. Especially, one-dimensional TiO2 nanostructures have become of increasing importance in applications of photocatalysis, photoelectron-chemical process, and dye-sensitized solar cells due to their superior properties in comparison with other TiO2 nanostructured counterparts [7, 8, 9, 10, 11, 12]. Besides, TiO2-based nanomaterials, especially the layered titanate nanotubes, obtained by the hydrothermal method possess the large BET surface area, strong ion-exchange capacity, and strong adsorption ability . The high recombination of the photo-generated charge carriers leads to the low photocatalytic activity of TiO2-based nanomaterials. In order to overcome this drawback, forming a heterojunction structure by combing TiO2 with another semiconductor is considered to be one of the efficient ways to suppress the recombination of the photo-excited electron-hole pairs and to enhance the photocatalytic efficiency [14, 15]. SrTiO3 with the perovskite structure is one of semiconductors with a flat band potential lower than that of TiO2, and it is easily to be formed a heterojunction structure with TiO2 in the preparation process [16, 17, 18, 19]. In this regard, the photo-generated electrons would centralize on the conduction band of TiO2, and the holes would concentrate on the valence band of SrTiO3 under UV light irradiation, and as a result, the recombination efficiency of the photo-generated charge carriers is inhibited, and thereby the photocatalytic activity would be improved .
On the basis of above consideration, we intent to fabricate the TiO2/SrTiO3 heterojunction structure film by the hydrothermal method. Herein, the cubic SrTiO3 was achieved by hydrothermal treatment of the orthorhombic titanic acid in SrCl2 aqueous solution by adjusting pH = 13. Notably, by simply tuning reaction time, the crystallinity, morphology and the amount of SrTiO3 nanostructures can be controlled easily. The TiO2/SrTiO3 heterostructure film exhibited the good photocatalytic performance for CO2 photoreduction. In order to further improve the transformation yield, the foreign electron traps of Pt and Pd nanoparticles were loaded on the film by the photoreduction approach. The relationship between the photocatalytic properties of TiO2/SrTiO3 heterostructure film with their morphology and structure was investigated systematically.
Preparation of the Film Photocatalysts
Ti foil with a size of 2 cm × 4 cm was put into an autoclave containing a concentrated 10 M NaOH aqueous solution, and then reacted at 120 °C for 24 h. After cooling down, the obtained film was washed with distilled water several times, and then immersed in 0.1 M HCl aqueous solution for 12 h to obtain the titanic acid nanotubes film (TAN). After that, TAN was put into an autoclave containing 80 mL 0.05 M SrCl2 aqueous solution, and the pH value of the solution was adjusted to 13 by NaOH solution. The autoclave was kept at 120 °C for 1 h, 2 h, and 3 h respectively. The as-fabricated films were washed with deionized water several times, and then dried with the stream of N2. The samples obtained in the different reaction time were denoted as TS1, TS2, and TS3, respectively. In order to further increase the photocatalytic performance, the foreign electron traps of Pt and Pd nanoparticles were deposited on the TS1 surface by photoreduction of H2PtCl6 and PdCl2 solution under the irradiation of the high-pressure mercury lamp for 1 h. The obtained products were denoted as TS1-Pt and TS1-Pd.
X-Ray powder diffraction (XRD) patterns of the films were measured on a Philips X’Pert Pro X-ray diffractometer (Holland) (Cu Kα radiation; 2θ range 5 ~ 70°, step size 0.08°, time per step 1.0 s, accelerating voltage 40 kV, and applied current 40 mA). The morphologies of the samples were taken on SEM (JSM-7100 F, JEOL Co., Japan) and TEM (JEM-2010, JEOL Co., Japan). X-ray photoelectron spectra (XPS) were recorded with a Kratos AXIS Ultra spectrometer (excitation source: monochromatized Al Kα (hν = 1486.6 eV); voltage 15 kV, current 10 mA). And the C 1 s binding energy of hydrocarbon (284.8 eV) was used as the standard for the correction of charging shift.
Evaluation of Photocatalytic Activity
The photocatalytic reduction of CO2 was conducted in a flat closed reactor with the inner capacity of 358 mL containing 20 mL 0.1 mol/L KHCO3 solution. The prepared samples were located in the center of the reactor and then the ultra-pure gaseous CO2 and water vapor was flowed through the reactor for 2 h to achieve the adsorption-desorption equilibrium. Before illumination, the reactor was sealed. The light source was the high pressure Hg lamp with 300 W, and the intensity of the incident light was measured to be 10.4 mW/cm2. Both sides of the Ti foil have transformed to TiO2/SrTiO3 heterojunction film, but only one side under the light irradiation took part in the CO2 photo-reduction reaction. The photocatalytic reaction was typically performed at room temperature for 6 h. The concentration of CO, CO2, and CH4 were measured by a gas-chromatography (GC). Moreover, the electrochemical impedance spectroscopy (EIS) properties were measured in 0.05 M Na2SO3 aqueous solution using a three-electrode photoelectrochemical cell with TS film as the working electrode, an Ag/AgCl electrode as the reference, and a platinum meshwork as the counter electrode.
Results and Discussion
Phase Structure of the Porous Film
Morphology and Composition Analysis of TiO2/SrTiO3 Heterostructure
Photoreduction of CO2 on TiO2/SrTiO3 Heterojunction Films
To confirm the real photocatalytic reduction process of CO2 to CH4 on TiO2/SrTiO3 heterojunction films, some related reference experiments were conducted. When the reaction was preceded in dark, there was no CH4 detectable, indicating that the photo-excited process of TS film was essential in the photo-reduction of CO2. When the experiment was conducted in the absence of H2O, almost no CH4 was produced. That should be due to no reduce species (H+) took part in the photo-reduction of CO2. If we use the NTA nanotube film to replace TS film, there was also no photoactivity. The above comparison experiments illustrated that the conversion of CO2 to CH4 on TS films was indeed the photo-reduction process.
Photocatalytic Activity on Pt (or Pd) Loaded TiO2/SrTiO3 Heterojunction Films
In summary, TiO2/SrTiO3 heterojunction network films were prepared successfully by hydrothermal method using nanotube titanic acid film (NTA) as the precursor. In the basic reaction process, part of NTA reacted with SrCl2 to form SrTiO3 naocubes, and the residues transformed to rutile TiO2. As prolonging the reaction time to 2 h, NTA transformed to SrTiO3 naocubes completely. The TiO2/SrTiO3 heterojunctions obtained at 1 h exhibit the best photocatalytic performance for the photoreduction of CO2. The increased photocatalytic activity can be responded by the enhanced charge separation derived from the coupling effect the TiO2 and SrTiO3 components, large surface area (BET) and strong adsorption ability of TS1 network porous film. In addition, the photoreduction activity of CO2 to CH4 increased from 3.67 to 11.37 and 20.83 ppm/h cm2 when Pt and Pd loaded on TS1 film. Especially, Pd also played the important role to increase the selectivity of photoreduction CO2 to CH4.
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Nos. 21103042 and 21471047), and Program for Science & Technology Innovation Talents in University of Henan Province (No. 15HASTIT043).
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