Tungsten oxide production by slution combustion synthesis and its photocatalytic activity investigation

Transition Metal Oxide (TMO) has unusual physical, optical and electrical properties compared to other oxides. One of the very well TMOs is tungsten oxide. Tungsten oxide can be used in many critical applications such as CO2 reduction, water splitting, polluted air and wastewater filtration applications, lithium batteries, gas sensors, supercapacitors, electrochromic applications, hydrogen production, cancer therapy, phototherapy applications. Tungsten oxide is readily used for photocatalytic degredation applications. Especially, WO3 is capable to degrade polluted water by cationic dyes due to highly negative tungsten oxide surface charges. The reason WO3 is used in such a wide application areas is tungsten oxide compunds' variety and showing stoichometric (WO3, a-WO3 b-WO3 h-WO3) and sub-stoichometric (WO2,9, WO2, WO2,7 gibi) compunds. In particular, WOx£3 family recievs considerable attention because of its tunable structure and its unique physicochemical properties. Tungsten oxide is the first material which the electrochromic property is discovered, and therefore there is a great deal of literature on the electrochromic properties. In addition, it is very attractive with its photocatalytic properties. Tungsten oxide absorbs a fraction of the solar spectrum with a band gap of 2.5-2.8 eV and exhibits catalytic activity at visible wavelengths. This makes it one step further candidate for photocatalytic applications in comparision to very well known TiO2 photocatalysis. Solution Combustion Synthesis (SCS) is a fast, easy and high purity metal and metal oxide powder synthesis method. This method is a kind of Self Propagating High Temperature Synthesis (SHS) method conducted in solution medium. One of the greatest advantage of conducting the method in solution medium is to mix precursors at molecular level, and this gives high purity and desired phase. In addition, by tuning process parameters such as Fuel to Oxidizer ratio (F/O), fuel type, reaction medium etc. desired morphology, phase, and composition can be obtained. One of the crucial parameter is F/O ratio, and this is readily studied in literature. By changing this ratio, powder morphology, particle size and surface area can be changed and controlled. Once F/O ratio is increased, due to high amount of released gases, particle size decreases, and surface area increases. In the scope of the thesis, it is aimed to investigate SCS parameters influence on final product properties, and then photocatalytic activity investigation is conducted to observe methylene blue degredation by SCS produced WO3. Oxalic acid is determined as non investigated fuel in SCS for WO3 powder production in literature against it is readily used for other metal oxides. Beside that citric acid is also used to make a comparision with literature. By using these two fuels, different experiments having different F/O ratio, different reaction starting temperature, and different supplementary oxidizers were conducted. Also, calcination was applied to samples just to understand phase transformation and stability. According to experimental results obtained, it is understood that WO3 cannot be obtained in a single step by using citric acid. In spite of citric acid, WO3 phase is obtained by using oxalic acid in a single step. XRD pattern is matched with 01-085- 2459 ICSD card. The sample produced by using sitric acid can only be transformed to WO3 phase after calcination at 500°C during 8 hours. But now it is matched with different WO3 phase (ICSD : 00-033-1387) from the sample produced by using oxlic acid. This states that citric acid and oxalic acid produces different WO3 phases. Just to understand phase stability and transformation temperature, calcination at 500°C is also performed for samples produced by oxalic acid. After calcination, samples maintained the same crystalline phase. Different reaction starting temperature (250C°, 300°C, 350°C, 400°C) was applied to sample produced by oxalic acid. However, no reaction starting temperature influence was observed on WO3 phase, all phases were the same, and matched with 01-085-2459 ICSD pattern. Beside, it is thougt that starting temperature might affect powder morphology. As stated above, fuel to oxidizer ratio is a critical parameter in SCS. To elaborate its influence on particle morphology, and surface area samples with different F/O ratio (1, 2.5, 5) were produced for both fuel. The sample produced by citric acid with F/O ratio 5 had porous structre observed from SEM images. This result is very well matched with the literature. However, this is not the same for samples produced by oxalic acid. Although sample produced by oxalic acid, and with F/O ratio 1 has 17,5 m2/g surface area, the sample produced with F/O ratio 5 has 6,7 m2/g surface area. This result is conflicting with literature. This might occur due to high temperature reached during synthesis with high F/O ratio. Fuels we used can be burned with atmospheric oxygen, and this causes extreme temperature. Furthermore, particle coarsening, and melting at some area can take place. This might be the the reason why we obtained different results from literature. In SCS experiments , direct metal nitrates are used as a metal source but this is different for our case. Ammonium tungstate was used in our experiments due to absence of tungsten nitrates. Because of that, additional oxidizers (nitric acid and amonium nitrate) were also used to produce more stable complexes. First, amonium nitrate was added to sample produced by oxalic acid, and It is understood from XRD results ammonium nitrate addition produced more stable structure. But for citric acid, we could not obtain a single WO3 phase with the addition of ammonium nitrate. This addition caused a column type structure for the sample produced by citric acid as understood from SEM image. For the same purpose addition of 5% HNO3 to samples produced by oxalic acid was also tried. Now, addition to oxalic acid produced samples gave a different result from addition of ammonium nitrate. As obtained powder can not be indexed to any WO3 phase despite it is highly crystalline. Furthermore, obtained powder was washed with 1% HNO3 several times and the same WO3 phase (01-085- 2459 ICSD) was obtained. It is concluded that nitric acid addition created different crystalline phases as an impurity, and they were all successfully removed by nitric acid washing. As synthesized powders were photocatalytically tested. For this purpose, methylene blue was used as an organic dye. 40 mg methylene blue was dissolved in distelted 1 L water and hydrogen peroxide was added as an oxidizer. Powders synthesized by oxalic acid with different fuel to oxidezer ratio were added to the solution, and mixed in dark to reach adsorption desorption equilibrium. After that each solution was exposed to sunlight. With 30 minutes intervals, sample was taken for uv-vis spectrophotometer analysis. All as prepared samples were showed 0 adsorption after 60 minutes. Especially, the sample with F/O ratio 5 showed fastest degredation. It showed 0 adsorption before 30 minutes. For benchmarking, commercial powder was also tested. It could not degrade the methylene blue even after 3 hours. It exhibited very low amount of degradation. It can be concluded that SCS synthesized powders are much more photocatalytically active than commercial powder. In summary, WO3 phase can only be obtained by using oxalic acid in a single step. The sample produced by citric acid was transformed to WO3 phase after calcination at 500°C. From the point of photocatalytic activity, the sample produced by oxalic acid with F/O ratio 5 exhibited the best performance in comparision to commercial powder, and the sample with F/O ratio 1.