Scientific report 2016 January September. Designing composite nanoarchitectures for hydrogen production and environmental depollution

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1 Scientific report 2016 January September Designing composite nanoarchitectures for hydrogen production and environmental depollution Synthesis of the spherical Pt nanoparticles We have chosen to synthesize spherical nanoparticles due to their superior efficiency in photocatalysis. Hexachloroplatinic acid was used as a platinum precursor and sodium borohydride as a reducing agent, the reaction took place in a solution of trisodium citrate. This synthesis has already been described in the literature by our research group 1. The obtained Pt nanoparticles were studied by transmission electron microscopy (Figure 1). Figure 1: TEM micrographs of the spherical nanoparticles. The bar graph shows the size distribution of the individual platinum nanoparticles. As it was expected from the synthesis pathway, the obtained dominant shape was the spherical one. The size distribution was relatively uniform, with 85% of the nanoparticles having size of 5±1 nm. The deposition of the as synthetized Pt nanoparticles on the surface of the semiconductors was also successful. Besides, XRD measurements were also performed in order to gain more information about the crystalline structure of the materials. It can be concluded, that the deposition of the noble metal nanoparticles is not affecting the crystal phases of the semiconductors, the diffraction patterns remaining unchanged compared to the basesemiconductors. 1 G. Kovács, Sz. Fodor, A. Vulpoi, K. Schrantz, A. Dombi, K. Hernádi, V. Danciu, Zs. Pap, L. Baia, Journal of Catalysis, 2015, 325,

2 The optical properties of the Pt-based composites were characterized using diffuse reflectance spectroscopy (Figure 2). It can be clearly visible, that the deposition of Pt nanoparticles on the surface of titania influences the optical properties of the semiconductor, changing its color to darker (grey), even when a small concentration (1%) was used Reflectance (%) (nm) AA-Pt(s) AR-Pt(s) P25-Pt(s) AA AR P25 Figure 2: Diffuse reflectance spectra of Pt-based TiO2 composites. Using the Kubelka-Munk theory, we have calculated the band-gap energy of the as obtained materials. It was observed, that the deposition of the nanoparticles have changed the band-gap values of the semiconductor, lowering its band-gap value by ev. Synthesis of tungsten trioxide from ammonium metatungstanate (AMT) In 12.5 ml of distilled water were added 0.77 g of TMA and 0.53 ml HCl under continuous stirring. This mixture was subjected to hydrothermal treatment (180 C, 4 hours) thereby achieving a yellow colloidal suspension. The oxide was collected, washed by centrifugation (1600 rpm, 15 minutes) with distilled water, dried (6 hours at 70 C) and finally heat-treated (500 C for 30 min). The WO3 part of the composite was also analyzed using various morphological and structural characterization methods. From the scanning electron microscopy we have observed that the morphology of the microcrystals (WO3-AMT) obtained from ammonium metatungstate (AMT) was star-like (Figure 3). These stars mean diameter was 3 4 μm and were composed from microfibers of 3 4 μm length. These were built from several smaller nanowires with a 2

3 diameter =10 15 nm (Figure X), this result being confirmed by XRD measurements, where it was also found that the dominant crystal phase is the monoclinic one. Figure 3: SEM micrographs of WO3 nanostructures. From DRS measurements it was found that the WO3-AMT has a relatively low bandgap energy (2.25 ev). Interestingly, there was a break in the light absorption threshold of this material, which may require additional experimental work to be explained. Preparation of the composites The prepared TiO2 and WO3 composites contain 76 wt.% titanium dioxide and 24 wt.% tungsten trioxide. To prepare the composites, the isoelectric point of composites with platinum TiO2 (Pt) (IEP = 6.74), WO3 (1% Pt) (IEP = 1.22) and semiconductors (without noble metal) TiO2 (P25) (IEP = 5.50, lit 6.40), WO3 (1% Pt) (IEP = 1.50) was determined. The method of determining the isoelectric point was detailed described in the 2015 scientific report. The ph of the distilled water was adjusted to the average of the isoelectric points of TiO2(Pt) and WO3 (average IEP 4.12) and TiO2 and WO3(Pt) (average IEP 3.36). After the ph was adjusted to 4.12, 190 mg of TiO2(Pt) were added to distilled water and ultrasonicated for 10 minutes. The suspension was stirred for 15 minutes then WO3 has been added (60 mg) and stirred for another 15 minutes. The final product was dried for 12 hours at 70 C. The procedure was repeated for preparation of the composite containing TiO2(P25) (190 mg) and WO3(Pt) (60 mg). TiO2 (P25 catalyst) used in the experiments was produced by Evonik, the commercially available P25 Aeroxide containis 90% anatase and 10% rutile and a diameter of nanoparticles of 20 nm (for anatase) and 40 nm (for rutile). 3

4 Evaluation of the photocatalytic properties of the obtained composites In order to asses the effeciency of these photocatalysts, photodegradation tests were conducted on model organic pollutants (methyl-orange). The composite TiO2WO3(Pt) has degraded 40% of the model pollutant in two hours, while the composite TiO2WO3(Pt) has degraded 34% of pollutant until the end of measurements. Hydrogen production rates of the composites The hydrogen production experiments were executed in a Pyrex glass photoreactor with 170 ml reactor volume, filled with 45 mg - TiO2(Pt)76%WO3AMT24%, 70 mg - WO3(Pt)24%TiO276% and 150 mg Au-P25-WO3 suspension, respectively) thermostated at 25 C and surrounded by W fluorescent, low pressure mercury lamps (λmax 365 nm). The suspension s concentration was 1.0 g/l and the applied sacrificial agent was oxalic acid (50 mm). During the photocatalytic runs the suspension was continuously purged with N2 (50 ml/min) to avoid the presence of O2. The H2 gas evolved was determined with a Hewlett- Packard 5890 type gas chromatograph equipped with a thermal conductivity detector, using a 2 ml sampling loop (sampling times: in the first hour 10 min, in the second hour 20 min, the duration of the experiment was 2h) The best results were obtained when the platinum was selectively deposited on the tungsten oxide (Figure 4). Figura 4: Evolution of hydrogen in the presence of TiO2(Pt) 76%WO3AMT24%, WO3(Pt)24% TiO276% si Au-P25-WO3 (sacrificial agent - oxalic acid) 4

5 Conclusions During these nine months we have obtained nanocomposites from titanium dioxide and tungsten trioxide nanoparticles with controlled deposition of platinum using the isoelectric point method. Selective deposition of the nobel metal was verified using SEM-EDX and their size distribution was evaluated with DRS and TEM. The individual composite morfological and structural analysis was also performed (DRS, TEM, SEM-EDX, XRD). These materials were subjected to photocatalytic degradation of pollutants and photocatalytic hydrogen production tests. The best results were obtained when the platinum was selectively deposited on the tungsten oxide. 5