Nanofibras, microflores e nanopartículas de WO3 sintetizadas via método electrospinning e hidrotermal, e aplicadas na detecção dos gases NO2, H2 e CO
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Data
2021-08-18
Autores
Morais, Paulo Vitor de
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Universidade Estadual Paulista (Unesp)
Resumo
Nanomateriais baseados em óxido de tungstênio têm sido estudados em várias áreas de aplicação devido à suas propriedades ópticas e eletrônicas. Para o emprego desses materiais como sensor de gás observa-se algumas vantagens, tais como, possibilidade de obtenção de variadas morfologias que podem apresentar área superficial favorável para interação com gases podendo resultar em considerável resposta sensora, assim como alta depleção eletrônica. Neste trabalho foram realizadas sínteses de nanofibras pelo método electrospinning, microflores por hidrotermal assistido por micro-ondas e nanopartículas utilizando o método hidrotermal convencional. As nanofibras foram obtidas numa proporção de 1:1 de H2WO4 e PVP, e tratadas à 500 °C por 3 horas nas taxas de aquecimento de 1, 5 e 10 °C/min. Para a síntese das microflores foram utilizados 1 g de H2WO4, HCl 3 M e 2-3 g de K2SO4 e tratamento térmico de 400 °C por 3 horas. Por último, para as nanopartículas foram empregues 1 g de H2WO4 e HCl 3M, seguido por tratamento térmico de 500 °C por 3 horas. Os materiais obtidos nas sínteses foram caracterizados por técnicas microscópicas e espectroscópicas, e também por análises de área superficial específica. Na sequência os materiais foram aplicados na detecção dos gases NO2, H2 e CO. Todas as nanofibras exibiram elevada resposta ao NO2, sendo que o sensor com as nanofibras tratadas à 500 °C na taxa de 5 °C/min exibiu maior sinal de sensor de 104 de ordem de grandeza para concentrações acima de 25 ppm. As microflores sintetizadas com 2 e 3 g de K2SO4 apresentaram expressivo sinal do sensor para o NO2 que foi da ordem de até 103 para 100 ppm. Também para esta concentração, o sinal do sensor exibido pelas nanopartículas de WO3 foi da ordem de 102. A partir da comparação entre as diferentes morfologias estudadas, observou-se maior sinal de sensor exibido pelas nanofibras tratadas à 500 °C na taxa de 5 °C/min para concentrações de NO2 entre 25 e 100 ppm, enquanto para as menores concentrações, entre 2 e 10 ppm, as microflores com 2 g de K2SO4 apresentou os maiores sinais de sensor. Para o primeiro material associa-se esse maior sinal ao tamanho de grão ser menor que o tamanho crítico para o WO3. Por outro lado, as microflores com 2 g do sal apresentou o limite de detecção ao NO2 da ordem de 10-2 ppb que pode ser atribuído à elevada área superficial deste material. Análises realizadas por DRIFTS no sensor contendo as nanofibras (500 °C e 5 °C/min) revelou que a interação do NO2 com a superfície dos grãos resultou na adsorção de espécies nitrato e nitrito, e também ficou indicado a lenta dessorção dessas espécies. Além disso, as análises indicaram que a adsorção do NO2 deve ter ocorrido tanto em sítios não ocupados na superfície, quanto em sítios anteriormente ocupados por moléculas de oxigênio.
Tungsten oxide-based nanomaterials have been studied in various application areas due to their optical and electronic properties. For the use of these materials as a gas sensor, there are some advantages. One of them is the possibility of obtaining different morphologies that can present a friendly surface area for interaction with gases. It can result in a considerable sensor response, as well as high electronic depletion. In this work, syntheses of nanofibers by the electrospinning method, microflowers by microwave assisted hydrothermal, and nanoparticles using the conventional hydrothermal method were performed. The nanofibers were obtained in a 1:1 ratio of H2WO4 and PVP, calcined at 500°C for 3 hours at heating rates of 1, 5, and 10°C/min. For the microflowers, 1 g of H2WO4, 3 M HCl, and 2-3 g of K2SO4 were used and thermal treatment at 400°C for 3 hours. Finally, 1 g of H2WO4 and 3M HCl was used for the nanoparticles, followed by heat treatment at 500°C for 3 hours. The characteristics of the materials were investigated by microscopic and spectroscopic techniques and also by specific surface area analyses. Afterward, NO2, H2, and CO detection were performed. WO3 nanofibers exhibited a high response to NO2, and the sensor heat treated at 500°C in the rate of 5°C/min exhibited the highest 104 order of magnitude sensor signal for concentrations above 25 ppm. The microflowers synthesized with 2 g of K2SO4 showed an expressive sensor signal for NO2, which was in the order of up to 103 to 100 ppm. Also, for this concentration, the sensor signal displayed by the WO3 nanoparticles was of the order of 102 . A comparison of the morphologies studied reveals a higher sensor signal displayed by nanofibers heat treated at 500 °C (5°C/min) for NO2 concentrations between 25 and 100 ppm. While for the lowest concentrations, between 2 and 10 ppm, microflowers obtained using 2 g of K2SO4 showed the highest sensor signals. For the first material, this higher signal is related to the smaller grain size than the critical size for WO3. Still, the microflowers with 2 g of the salt presented X the detection limit for NO2 in the order of 10-2 ppb, which can be attributed to the high surface area of this material. Analyzes carried out by DRIFTS on the sensor containing the nanofibers heat treated at 500°C (5°C/min) revealed that the interaction of NO2 with the grain surface resulted in the adsorption of nitrate and nitrite species, and slow desorption of nitrate species was also indicated. Furthermore, the analyses indicated that the adsorption of NO2 must have occurred both in sites not occupied on the surface and in surface sites previously occupied by oxygen molecules.
Tungsten oxide-based nanomaterials have been studied in various application areas due to their optical and electronic properties. For the use of these materials as a gas sensor, there are some advantages. One of them is the possibility of obtaining different morphologies that can present a friendly surface area for interaction with gases. It can result in a considerable sensor response, as well as high electronic depletion. In this work, syntheses of nanofibers by the electrospinning method, microflowers by microwave assisted hydrothermal, and nanoparticles using the conventional hydrothermal method were performed. The nanofibers were obtained in a 1:1 ratio of H2WO4 and PVP, calcined at 500°C for 3 hours at heating rates of 1, 5, and 10°C/min. For the microflowers, 1 g of H2WO4, 3 M HCl, and 2-3 g of K2SO4 were used and thermal treatment at 400°C for 3 hours. Finally, 1 g of H2WO4 and 3M HCl was used for the nanoparticles, followed by heat treatment at 500°C for 3 hours. The characteristics of the materials were investigated by microscopic and spectroscopic techniques and also by specific surface area analyses. Afterward, NO2, H2, and CO detection were performed. WO3 nanofibers exhibited a high response to NO2, and the sensor heat treated at 500°C in the rate of 5°C/min exhibited the highest 104 order of magnitude sensor signal for concentrations above 25 ppm. The microflowers synthesized with 2 g of K2SO4 showed an expressive sensor signal for NO2, which was in the order of up to 103 to 100 ppm. Also, for this concentration, the sensor signal displayed by the WO3 nanoparticles was of the order of 102 . A comparison of the morphologies studied reveals a higher sensor signal displayed by nanofibers heat treated at 500 °C (5°C/min) for NO2 concentrations between 25 and 100 ppm. While for the lowest concentrations, between 2 and 10 ppm, microflowers obtained using 2 g of K2SO4 showed the highest sensor signals. For the first material, this higher signal is related to the smaller grain size than the critical size for WO3. Still, the microflowers with 2 g of the salt presented X the detection limit for NO2 in the order of 10-2 ppb, which can be attributed to the high surface area of this material. Analyzes carried out by DRIFTS on the sensor containing the nanofibers heat treated at 500°C (5°C/min) revealed that the interaction of NO2 with the grain surface resulted in the adsorption of nitrate and nitrite species, and slow desorption of nitrate species was also indicated. Furthermore, the analyses indicated that the adsorption of NO2 must have occurred both in sites not occupied on the surface and in surface sites previously occupied by oxygen molecules.
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Palavras-chave
Sensores de gás, Óxido de tungstênio, Nanomateriais de WO3, Detecção de gases, Gas sensors, Tungsten oxide, WO3 nanomaterials, Gas detection