Monolithic structured TiO2/aerogel composites were prepared from resorcinol-formaldehyde polymer aerogel (RFA) and its carbon aerogel (RFCA) derivative. A resorcinol-formaldehyde hydrogel was synthesized in a sol-gel reaction and transformed into polymer aerogel by supercritical drying. The RFA was converted to carbon aerogel by pyrolysis at 900 °C in dry N2.Amorphous and crystalline TiO2 layers were grown from TiCl4 and H2O precursors by atomic layer deposition (ALD) at 80 °C and 250 °C, respectively, on both RFA and RFCA. The substrates and the composites were studied by N2 adsorption, TG/DTA-MS, Raman, SEM-EDX and TEM techniques. Their photocatalytic activity was compared in the UV catalyzed decomposition reaction of methyl orange dye. Keywordsresorcinol-formaldehyde polymer aerogel, carbon aerogel, photocatalysis, ALD, TiO2 IntroductionBeside crystalline carbonaceous nanomaterials such as fullerene, graphene, graphene oxide, nanodiamond, carbon nanospheres, carbon nanotubes; mesoporous carbon aerogels have attracted a great deal of attention. Carbon aerogels, available also in monolithic form, have several favorable properties, for example they can be used as adsorbents or as substrates for catalysts, because of their robustness and high specific surface area [1,2]. They are also excellent thermal and phonic insulators, while conduct electricity. These properties can be tuned through their synthesis conditions. [3][4][5][6][7] Resorcinol-formaldehyde (RF) organic aerogels, that Pekala and co-workers synthetized for the first time [8], undergo two main stages during preparation. In the first stage a hydrogel is prepared by a sol-gel process and in the second stage after drying the aerogel is obtained. The resorcinol-formaldehyde polymer aerogel becomes a carbon aerogel in a consecutive third stage, which is carbonization occasionally followed by activation. Depending on the conditions carbonization or activation influence the structural and performance characteristics, like the specific surface area, significantly [9][10][11][12].Photocatalytic carbon nanocomposites have great potential in the field of environmental remediation, water splitting and self-cleaning surfaces [13,14]. Among the various semiconductor oxide photocatalysts TiO2 is researched widely, due to being stable and nontoxic. TiO2 has ideal band gap width for the half reactions of water splitting and its composites with other nanomaterials, such as carbon nanostructures, may enhance the photocatalytic activity. However, its use still faces some difficulties, such as a narrow light response range limited to UV. The carbon nanostructure inhibits the recombination by promoting the charge separation as an electron acceptor. This effect and the widening of the wavelength response range through Ti-O-C bonds as well as modifying the photocatalytic selectivity are three advantages of TiO2 composites [3,[15][16][17][18]. Moreover, a number of studies indicates that nanoporous or nanostructured carbon materials, such as activated carbon or graphene-oxide...
The improved Hummers' synthesis of graphene oxide (GO) from graphite is investigated to monitor how the functional groups form during the synthesis steps. To achieve these, samples are taken after every preparation step, and analyzed with TG-DTA/MS, FTIR, XRD and SEM-EDX techniques. It was found that the main characteristic mass loss step of GO was around 200 °C, where at first the carboxyl and lactone groups were released, and the evolution of sulphonyl groups followed them right away in a partially overlapping step. It became clear that in the as-prepared acidic GO sample the presence of H 2 SO 4 originating from the reaction solution was still dominant. The functional groups were formed only after washing the as-prepared GO with HCl. The consecutive washing step with distilled water did not alter the functional groups or the thermal properties significantly; however, it made the GO structure more ordered. The reduction of the GO structure back to reduced GO (rGO) resulted in the loss of the functional groups and a graphitic material was obtained back.
Carbon nanospheres (CNSs) were prepared by hydrothermal synthesis, and coated with TiO2 and ZnO nanofilms by atomic layer deposition. Subsequently, through burning out the carbon core templates hollow metal oxide nanospheres were obtained. The substrates, the carbon-metal oxide composites and the hollow nanospheres were characterized with TG/DTA-MS, FTIR, Raman, XRD, SEM-EDX, TEM-SAED and their photocatalytic activity was also investigated. The results indicate that CNSs are not beneficial for photocatalysis, but the crystalline hollow metal oxide nanospheres have considerable photocatalytic activity.
TiO 2 , Al 2 O 3 and ZnO are grown by atomic layer deposition (ALD) at 80 °C on graphene oxide (GO), synthesized by the improved Hummers method. The preparation steps and the products are followed by FTIR, Raman, TG/DTA-MS, SEM-EDX, XRD and TEM-ED. Both Al 2 O 3 and TiO 2 grown with ALD are amorphous, while ZnO is crystalline. Through decomposing methylene orange by UV irradiation it is revealed that the GO itself is an active photocatalyst. The photocatalytic activity of the amorphous TiO 2 , deposited by low temperature ALD, is comparable to the crystalline ZnO layer, which is the best photocatalyst among the studied 3 oxides. Al 2 O 3 used as reference suppresses the photocatalytic performance of the GO by blocking its active surface sites.
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