Synthesis and characterization of graphene oxide - TiO2 thin films by sol-gel for photocatalytic applications
Introduction
Nowadays, humanity faces severe environmental problems, mainly global warming; largely due to the high energy demand and the excessive fuel burning. Through the Intergovernmental Panel on Climate Change, the scientific community has made several anthropogenic global warming predictions; it is estimated that by 2050 the concentration of CO2 in the atmosphere will be approximately 470 ppm, which will increase the global temperature of the planet up to 0.9 °C respect to the temperature in 2019 [1]. Therefore, it is necessary to develop new technologies for the production of energy, especially fuels, which are committed to the environment. In particular, fuels have been widely studied considering different raw materials, one of which is carbon dioxide (CO2); CO2 is a thermodynamically stable molecule, so it requires much energy to be transformed into other products, and is mainly generated by the combustion of all organic compounds, which includes burning of currently used fuels.
In this sense, carbon dioxide is recognized not only for being one of the main greenhouse gases, but also for being a potential source of carbon to produce new fuels [2]. In this context, solar fuels are one of the major alternatives to solve problems related to energy supply and pollution at the same time. Solar fuels are energy compounds obtained through the photoreduction (or photoelectroreduction) of CO2 using solar energy. They are produce through different routes; however, the use of semiconductor materials is especially attractive, since these materials can use the solar energy through the heterogeneous photocatalysis processes to generate highly reactive species.
In photocatalytic applications, TiO2 is the most widely used semiconductor, but its performance is limited due to its large bandgap, approximately 3.2 eV for the anatase phase in powders [[3], [4], [5], [6]]. There are several techniques for modifying TiO2 in order to present a response with visible light [2]; some techniques include the doped with metals [7] and nonmetals [8], as well as several sensitization methods with dyes [9], quantum dots [10], and coupling of other semiconductors [11], among others. An effective modification consists in coupling TiO2 with graphene-derived compounds. Generally, the loading of graphene materials can enhance the photocatalytic CO2 reduction performance of TiO2 from various aspects, which include: accelerant the electron-hole separation, improving the specific surface area, enhancing the CO2 adsorption through π-π conjugation between CO2 molecules and graphene materials, activating the CO2 molecules for reduction reaction, enhancing the light utilization [12]. Titanium dioxide–graphene oxide (TiO2-GO) nanocomposites are some of the promising materials in the production of solar fuels by heterogeneous photocatalysis. This material has been synthesized both powders and coatings; some works reported in this area are presented below.
TiO2-GO nanocomposites powders have been used in some photocatalytic applications. ChongYeon et al. [13] prepared a nanocomposite base TiO2 modified with GO and CuS by a sol-gel route in 2013; they used this nanocomposite for degradation of methylene blue (MB) under visible light reaching 90.1% of degradation. By 2017, Lin et al. [14] synthetized a nanocomposite of nitrogen doped titanium dioxide modified with GO through a hydrothermal method. The authors used their nanocomposite for the photoreduction of CO2 with a 400 W Xenon lamp to reach only a CO production of 356.5 μmol g−1cat; moreover, they observed traces of H2 and they didn't register organic compounds. In 2018, Tan et al. [15] proposed a nanocomposite of O2-TiO2–GO by chemical methods (the graphene oxide was added by wet chemical impregnation); this composite was used in the form of powders, with 5% wt. of GO, for the CO2 photoreduction, obtaining up to 3.98 μmol g−1cat of CH4 under visible light. One year later, Olowoyo et al. [16] produced a nanocomposite of reduced GO with TiO2 nanoparticles by sonothermal and hydrothermal methods. They used their compound in the methanol production through CO2 photoreduction using an 8 W UVA lamp for 16 h; as results, it was reached a total yield of 50 mmol g−1 using the UVA lamp, and almost 38 mmol g−1 under visible light in a medium with acetonitrile. In the same year, Tayebi et al. [17] obtained a titanium dioxide based composite modified with reduced GO, which was used in photodegradation of methylene blue; one highlighted aspect of their work involves the demonstration of a reduction in the process of recombination of load carriers by adding reduced GO, which increases the photocatalytic activity of these composites compared with bare TiO2.
On the other hand, regarding these TiO2-GO composites deposited in coatings, including thin films, research is more limited. On 2014, Fa et al. made a heterostructure of thin films of TiO2 deposited on ITO by AACVD, subsequently they deposited a graphene oxide film by dip coating; the device was used in photoelectrochemical water splitting [18]. One year later, Sim et al. created a nanocomposite based on TiO2 nanotubes by anodization of Ti foil, the coating was decorated with reduced GO and Pt nanoparticles; the authors used the composite in the photoreduction of CO2 registering a total yield of CH4 around 10.5 μmol m−2 compared with 5.75 μmol m−2 for a TiO2 nanotubes anode [19]. In the same year, Cheng et al. created a photoelectrocatalytic system for reduction of CO2 using a Pt-modified reduce GO cathode and a Pt-modified TiO2 anode; using a 300W Xe arc lamp, they produce up to 750 nmol cm−2 h−1 of H2, 275 nmol cm−2 h−1 of C2H5O, 200 nmol cm−2 h−1 of CH3COOH, among others [20]. The next year, Saleem et al. used TiO2-GO in photovoltaic applications in solar hybrid cells in 2016; the composite was obtained by sol-gel using a spin coater and recorded good optical properties [21]. Abdelmajid Timoumi et al. [22] prepared TiO2-GO nanocomposite thin films by a sol-gel route using a spin coating technique in 2018; they shown an important reduction on the band gap of the composite. Datcu et al. [23] also used the spin coating technique to prepared thin films of TiO2-GO, they proved their photocatalytic performance on the blenching of methylene blue. However, in addition of these and other works, only a small number of reports appeared on the synthesis of TiO2-GO nanocomposite by a dip coating method, like Zeynali et al., that prepared a TiO2-GO composite by dip coating in vacuum, placing a GO coating on a TiO2 and alumina support for hydrogen separation [24].
According to the author's knowledge, works have not been reported about the synthesis of TiO2 thin films modified with GO, incorporated in the starting solution, through a sol-gel route by means of dip coating. Therefore, the purpose of this investigation is to show a simple and low cost synthesis procedure, which also has reduced environmental impact, to prepare thin films of TiO2 modified with GO for photocatalytic applications.
Section snippets
Materials
Graphite powder; titanium isopropoxide (97%), hydrogen peroxide (50%) and methylene blue were purchased from Sigma Aldrich; potassium permanganate (≥99.0%) and sodium bisulfite from Meyer; ethanol (99.91%), hydrofluoric acid (48-51%), sulfuric acid (97.9%) and phosphoric acid (85%) from J. T. Baker; Chromotropic acid disodium salt dehydrate form ACS, reagent European Pharmacopoeia and deionized water. All chemicals were used without further purification.
Graphene oxide synthesis
Graphene oxide (GO) was prepared by a
Graphene oxide synthesis and characterization
The optical properties of the obtained GO was investigated by UV-VIS spectroscopy and Raman Spectroscopy. Fig. 1 shows the UV-VIS absorption spectra of the synthetized GO and the graphite used as precursor, measured in water as solvent; a maximum absorption peak appears at 236 nm, which is ascribed to π→π* transitions of aromatic C-C bonds (sp2 domains) [[26], [27], [28], [29]]. For the graphite case, the band for π→π* transitions appears at shorter wavelengths than those of the GO, at 224 nm,
Conclusions
In this work the synthesis of TiO2 thin films modified with graphene oxide (GO/TiO2) is proposed through a sol-gel route using a dip-coating deposition technique. The spectroscopic analysis confirms the successful obtaining of graphene oxide after the modification of the Hummers' method. According to the TEM, the graphene oxide shows a sheet like morphology with sizes between 0.5 and 2 μm; the DRX results confirm the exfoliation of the sheets and show the loss of crystallinity of the material
CRediT authorship contribution statement
A. Velasco-Hernández: Conceptualization, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. R.A. Esparza-Muñoz: Data curation, Writing - review & editing. F.J. de Moure-Flores: Data curation, Formal analysis, Writing - review & editing. J. Santos-Cruz: Formal analysis, Data curation, Writing - review & editing. S.A. Mayén-Hernández: Formal analysis, Data curation, Conceptualization, Writing - original draft, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Authors are grateful for technical support and facilities at Universidad Autónoma de Querétaro (UAQ) and Universidad Nacional Autónoma de México (UNAM) Campus Juriquilla, CFATA, as well as the Mexican National Research Council CONACYT (México) for the scholarship of Arturo Velasco-Hernández. We would also like to thank the technical staff and advisors of UNAM campus Juriquilla: Lourdes Palma Tirado (Instituto de Neurobiología) for TEM micrographs, Marina Vega González (Centro de Geociencias)
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