Design and development of visible-light-responsive two-dimensional carbon-based nanomaterials for photocatalytic reduction of carbon dioxide into methane

2017-02-24T00:51:58Z (GMT) by Ong, Wee Jun
The continuously increasing concentration of carbon dioxide (CO2) in the atmosphere is universally accepted as the most prominent cause of global warming. The ability to harness the power of CO2 on a large scale and integrate it back into the utilization cycle as a sustainable form of energy production is highly desirable. Among various renewable projects, semiconductor photocatalysis has gained considerable interdisciplinary research fascination for their diverse potentials in energy and environmental applications. In this regard, photocatalytic reduction of CO2 with H2O vapor into useful hydrocarbons under the visible light irradiation is a compelling solution to circumvent both energy and environmental crisis to mimic the natural photosynthesis in plants. Among the developed photocatalysts, the focus in this study has been on the recently researched TiO2 with exposed {001} facets (TiO2-001) and graphitic carbon nitride (g-C3N4) in the solar energy conversion due to their high chemical stability, corrosion resistant and environmentally benign nature. However, the rapid recombination of photogenerated charge carriers during the reaction process and low electrical conductivity greatly impair the efficiency of TiO2-001 and g-C3N4. Consequently, the state-of-the-art technology is far from being optimal before it can be considered as a viable process. Thus, various modifications have been employed to enhance the performance of the photocatalysts. This project focused on the use of a continuous, gas-phase heterogeneous photocatalytic system, which was performed under the irradiation of visible light. In the initial phase, the incorporation of graphene with high energy {001} facets of nitrogen-doped TiO2 (N-TiO2-001/GR) composite was designed for the conversion of CO2 into CH4. The N-TiO2-001/GR sample exhibited a total CH4 evolution of 3.70 µmol/gcatalyst, which was 11-fold higher activity compared to the TiO2-001 sample. The presence of {001} facets of TiO2 possessed a higher surface energy than the thermodynamically stable and low surface energy of {101} facets, rendering it to be more reactive for the photoactivity. Meanwhile, the nitrogen doping and graphene increased the light absorbance of the samples and reduced band gap energies. Importantly, graphene acted as an electron storage channel for efficient charge separation to inhibit the charge recombination. In the subsequent phases of this study, as an alternative to the conventional UV-active TiO2 photocatalyst, visible-light-responsive pristine g-C3N4 was modified to enhance the photocatalytic activity by coupling with other semiconductors such as silver halides (AgX, X = Br or Cl) forming Type I or Type II heterojunction. The types of heterojunction were influenced by the electronic band potentials of g-C3N4, AgBr and AgCl. Upon the light irradiation, Ag metal was formed, which promoted the photocatalytic activity due to its surface plasmon resonance (SPR) effect. The well-matched energy bands of Ag, AgBr and g-C3N4, which exhibited Type II heterojunction, resulted in enhanced photoactivity for the CO2 reduction to CH4 by 34.1 and 4.2 times higher compared to those of bare AgBr and g-C3N4, respectively due to the efficient migration of charge carriers in space apart from each semiconductor. In a separate study, noble-metal Pt was incorporated with g-C3N4 to form a binary nanocomposite photocatalyst. As such, 2 wt% of Pt metal clusters demonstrated the highest CH4 evolution of 13.02 µmol/gcatalyst after 10 h of reaction. Pt served as reduction co-catalysts for electron trapping sites to suppress the recombination of charge carriers and also displayed broad absorption in the visible light region. It is highly in demand in the search for metal-free photocatalytic systems for the large scale applications. In another study, a 2D graphene was employed as a reduction co-catalyst with g-C3N4 to form 2D/2D heterointerfaces. Two synthesis approaches to form graphene-modified g-C3N4 were carried out, namely (1) a one-pot impregnation-thermal reduction strategy and (2) a novel surface charge promoted self-assembly technique. For the former approach, the sublimation amount of urea (precursor of g-C3N4) was difficult to be controlled during the annealing process and the main challenge still revolved in the development of an intimate interfacial contact between graphene and g-C3N4. Therefore, to overcome this bottleneck, further improvements were subsequently conducted by altering the surface charge of g-C3N4 via acid pretreatment of HCl to fabricate protonated g-C3N4 (pCN) for the ease of coupling with rGO to form rGO/pCN sample via electrostatic attraction. Interestingly, the rGO/pCN photocatalyst exemplified the highest CH4 production of 13.93 µmol/gcatalyst compared to pCN (2.58 µmol/gcatalyst) after 10 h of reaction due to its exceptional 2D/2D heterojunction interface. Overall, this research accentuates the scientific aspects of photocatalytic process and fundamental understanding behind the enhancement of each carbon-based photocatalytic system. As a whole, the as-developed hybrid nanocomposite is anticipated to be a robust means to address various energy and environmental-related issues via photocatalytic process for a sustainable energy future.<br><br>Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Engineering, 2016.