High-pressure synthesis of rGO/TiO2 and rGO/TiO2/Cu catalysts for efficient CO2 reduction under solar light
TL;DR: In this paper, a one-pot method was proposed to directly synthesize the composites in supercritical CO2-ethanol medium, which resulted in an improved electron charge transfer and a better photocatalytic activity of the composite catalysts with higher rGO content.
Abstract: This work reports the synthesis of rGO/TiO2/Cu and rGO/TiO2 nanocomposites to obtain methane and CO from CO2 photoreduction with water vapour and simulated solar light. A novel, facile and efficient one-pot method is proposed to directly synthesize the composites in supercritical CO2-ethanol medium. The influence of wt% rGO on both their physicochemical properties and photocatalytic activity have been studied. The most striking changes observed have been their large increase in the visible range absorbance and absorption thresholds as compared to commercial TiO2, both effects increasing with the amount of rGO. These findings later resulted in an improved electron charge transfer and a better photocatalytic activity of the composite catalysts with higher rGO content. Nevertheless, the best catalytic performance (7.4 µmol CO2/g TiO2·h conversion rate) was achieved when the binary rGO/TiO2 nanocomposite was doped with metallic Cu at 1 wt%, due to its better electron/hole separation as compared to rGO-TiO2.
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TL;DR: In this article, a non-conventional approach to prepare titanium dioxide-reduced graphene oxide (TiO2-rGO) nanocomposites based on solar photoreduction is presented.
Abstract: A non-conventional approach to prepare titanium dioxide-reduced graphene oxide (TiO2-rGO) nanocomposites based on solar photoreduction is here presented. The standard hydro-solvothermal synthesis of the TiO2-rGO composites requires high temperatures and several steps, whereas the proposed one-pot preparation allows one to obtain the photocatalysts with a simple and green procedure, by exploiting the photocatalytic properties of titania activated by the solar irradiation. The TiO2-rGO catalysts were tested in the solar photodegradation of a widely adopted toxic herbicide (2,4-Dichlorophenoxyacetic acid, 2,4-D), obtaining the 97% of degradation after 3 h of irradiation. The as-prepared TiO2-rGO composites were more active compared to the same photocatalysts prepared through the conventional thermal route. The structural, optical, and textural properties of the composites, determined by Raman, Photoluminescence, Fourier Transform InfraRed (FTIR), UV-vis diffuse reflectance (DRS) spectroscopies, and N2 absorption-desorption measurements, showed as the solar irradiation favors the reduction of graphene oxide with higher efficiency compared to the thermal-driven synthesis. Furthermore, the possible toxicity of the as-synthesized composites was measured exposing nauplii of microcrustacean Artemia sp. to solutions containing TiO2-rGO. The good results in the 2,4-D degradation process and the easiness of the TiO2-rGO synthesis allow to consider the proposed approach a promising strategy to obtain performing photocatalysts.
15 citations
TL;DR: In this paper , the effect of carbon support on the activity and selectivity of N-doped TiO2 nanoparticles was analyzed in photoreduction of CO2 with water vapor.
Abstract: The objective of this work was to analyze the effect of carbon support on the activity and selectivity of N-doped TiO2 nanoparticles. Thus, N-doped TiO2 and two types of composites, N-doped TiO2/CNT and N-doped TiO2/rGO, were prepared by a new environmentally friendly one-pot method. CNT and rGO were used as supports, triethylamine and urea as N doping agents, and titanium (IV) tetraisopropoxide and ethanol as Ti precursor and hydrolysis agent, respectively. The as-prepared photocatalysts exhibited enhanced photocatalytic performance compared to TiO2 P25 commercial catalyst during the photoreduction of CO2 with water vapor. It was imputed to the synergistic effect of N doping (reduction of semiconductor band gap energy) and carbon support (enlarging e−-h+ recombination time). The activity and selectivity of catalysts varied depending on the investigated material. Thus, whereas N-doped TiO2 nanoparticles led to a gaseous mixture, where CH4 formed the majority compared to CO, N-doped TiO2/CNT and N-doped TiO2/rGO composites almost exclusively generated CO. Regarding the activity of the catalysts, the highest production rates of CO (8 µmol/gTiO2/h) and CH4 (4 µmol/gTiO2/h) were achieved with composite N1/TiO2/rGO and N1/TiO2 nanoparticles, respectively, where superscript represents the ratio mg N/g TiO2. These rates are four times and almost forty times higher than the CO and CH4 production rates observed with commercial TiO2 P25.
7 citations
TL;DR: In this article , the photoreduction of carbon dioxide (CO2) in gas phase was investigated over composites of Cu2O nanoparticles and anodized TiO2 nanotube arrays (TNAs).
Abstract: Photoreduction of carbon dioxide (CO2) in gas phase was investigated over composites of Cu2O nanoparticles (CNPs) and anodized TiO2 nanotube arrays (TNAs). CNPs with an average size of 90 nm were pulse-electrodeposited on vertically aligned hollow TiO2 nanotubes having average pore size of 90 nm. Radical trapping experiments with electron spin resonance spectroscopy suggest that photoinduced charge separation occurs via direct Z-scheme mechanism in Cu2O-loaded TNA (CNP/TNA) sample. Successively, CO2 photoreduction for CNP/TNA from water/CO2 mixture was monitored using a quadrupole mass analyzer in high vacuum, where the partial pressures of intermediate and final reaction species were measured in real time under ultraviolet-visible (UV–VIS) light irradiation (300–600 nm). The photoreduction of CO2 on CNP/TNA happens through preferred generation of formaldehyde (HCHO) and methanol (CH3OH), while platinum-loaded TNA (Pt/TNA) produced methane (CH4) and hydrogen (H2) instead of HCHO and CH3OH. Carbon monoxide (CO) formation was commonly observed for both CNP/TNA and Pt/TNA specimens. These results reveal that CO2 photoreduction occurs through hydrogenation in gas phase over CNP/TNA even in high vacuum, although CO2 deoxygenation to CH4 is conventionally dominant for gas phase reactions.
6 citations
TL;DR: In this paper , the copper−TiO2 nanosheets/graphene oxide (CTNSG) was synthesized using a hydrothermal technique in the presence of a hydrofluoric acid (HF) soft template.
Abstract: Carbon dioxide (CO2) photoreduction to high-value products is a technique for dealing with CO2 emissions. The method involves the molecular transformation of CO2 to hydrocarbon and alcohol-type chemicals, such as methane and methanol, relying on a photocatalyst, such as titanium dioxide (TiO2). In this research, TiO2 nanosheets (TNS) were synthesized using a hydrothermal technique in the presence of a hydrofluoric acid (HF) soft template. The nanosheets were further composited with graphene oxide and doped with copper oxide in the hydrothermal process to create the copper−TiO2 nanosheets/graphene oxide (CTNSG). The CTNSG exhibited outstanding photoactivity in converting CO2 gas to methane and acetone. The production rate for methane and acetone was 12.09 and 0.75 µmol h−1 gcat−1 at 100% relative humidity, providing a total carbon consumption of 71.70 µmol gcat−1. The photoactivity of CTNSG was attributed to the heterostructure interior of the two two−dimensional nanostructures, the copper−TiO2 nanosheets and graphene oxide. The nanosheets−graphene oxide interfaces served as the n−p heterojunctions in holding active radicals for subsequent reactions. The heterostructure also directed the charge transfer, which promoted electron−hole separation in the photocatalyst.
2 citations
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TL;DR: An exact derivation of the Scherrer equation is given for particles of spherical shape, values of the constant for half-value breadth and for integral breadth being obtained in this article, and various approximation methods which have been used are compared with the exact calculation.
Abstract: An exact derivation of the Scherrer equation is given for particles of spherical shape, values of the constant for half-value breadth and for integral breadth being obtained. Various approximation methods which have been used are compared with the exact calculation. The tangent plane approximation of v. Laue is shown to be quite satisfactory, but some doubt is cast on the use of approximation functions. It is suggested that the calculation for the ellipsoidal particle based on the tangent plane approximation will provide a satisfactory basis for future work.
6,907 citations
TL;DR: A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this article, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters.
Abstract: Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.
2,570 citations
TL;DR: A series of titanium dioxide and graphene sheets composites were synthesized with a sol-gel method using tetrabutyl titanate and graphite oxide (GO) as the starting materials.
Abstract: A series of titanium dioxide and graphene sheets (GSs) composites were synthesized with a sol–gel method using tetrabutyl titanate and graphite oxide (GO) as the starting materials. The obtained TiO2/GSs photocatalysts are characterized by X-ray diffraction, N2 adsorption analysis, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy. The photocatalytic activity of the as-prepared samples was evaluated by hydrogen evolution from water photo-splitting under UV-vis illumination. The influence of GSs content and calcinations atmosphere on the photocatalytic activity was also investigated. The results show that both GSs content and the calcinations atmosphere can affect the photocatalytic activity of the obtained composites.
985 citations
TL;DR: The proposed mechanism for the observed photocatalytic performance of TiO(2) nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response and for the first time shows a significant enhancement in the H(2)-production activity by using metal-free carbon material as an effective co-catalyst.
Abstract: Graphene-modified TiO2 nanosheets with exposed (001) facets (graphene/TiO2) were prepared by microwave-hydrothermal treatment of graphene oxide (GO) and hydrothermally synthesized TiO2 nanosheets with exposed (001) facets in an ethanol–water solvent. These nanocomposite samples showed high photocatalytic H2-production activity in aqueous solutions containing methanol, as sacrificial reagent, even without Pt co-catalyst. The optimal graphene content was found to be ∼1.0 wt%, giving a H2-production rate of 736 μmol h−1 g−1 with a quantum efficiency (QE) of 3.1%, which exceeded the rate observed on pure TiO2 nanosheets by more than 41 times. This high photocatalytic H2-production activity is due to the deposition of TiO2 nanosheets on graphene sheets, which act as an electron acceptor to efficiently separate the photogenerated charge carriers. The observed enhancement in the photocatalytic activity is due to the lower absolute potential of graphene/graphene˙− (−0.08 V vs. SHE, pH = 0) in comparison to the conduction band (−0.24 V) of anatase TiO2, meanwhile the aforementioned absolute value is higher than the reduction potential of H+ (0 V), which favors the electron transfer from the conduction band (CB) of TiO2 to graphene sheets and the reduction of H+, thus enhancing photocatalytic H2-production activity. The proposed mechanism for the observed photocatalytic performance of TiO2 nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response. This work not only shows a possibility for the utilization of low cost graphene sheets as a substitute for noble metals (such as Pt) in the photocatalytic H2-production but also for the first time shows a significant enhancement in the H2-production activity by using metal-free carbon material as an effective co-catalyst.
721 citations
TL;DR: In this article, the state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations, is presented.
Abstract: The continuous combustion of non-renewable fossil fuels and depletion of existing resources is intensifying the research and development of alternative future energy options that can directly abate and process ever-increasing carbon dioxide (CO2) emissions. Since CO2 is a thermodynamically stable compound, its reduction must not consume additional energy or increase net CO2 emissions. Renewable sources like solar energy provide readily available and continuous light supply required for driving this conversion process. Therefore, the use of solar energy to drive CO2 photocatalytic reactions simultaneously addresses the aforementioned challenges, while producing sustainable fuels or chemicals suitable for use in existing energy infrastructure. Recent progress in this area has focused on the development and testing of promising TiO2 based photocatalysts in different reactor configurations due to their unique physicochemical properties for CO2 photoreduction. TiO2 nanostructured materials with different morphological and textural properties modified by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals have been tested. This review presents contemporary views on state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations. In this review, we discuss existing and recent TiO2 based supports, encompassing comparative analysis of existing systems, novel designs being employed to improve selectivity and photoconversion rates as well as emerging opportunities for future development, crucial to the field of CO2 photocatalytic reduction. The influence of different operating and morphological variables on the selectivity and efficiency of CO2 photoreduction is reviewed. Finally, perspectives on the progress of TiO2 induced photocatalysis for CO2 photoreduction will be presented.
714 citations