氧化物气凝胶的制备、结构和性质的研究
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摘要
本论文主要探讨利用溶胶-凝胶方法制备具有纳米结构的金属氧化物气凝胶，并对其微观结构和性质进行了研究。内容包括气凝胶的制备过程、微观结构的表征和制备条件的选择，检测了金属氧化物气凝胶的物理化学性质，如热性质、光学性质和气敏性质。探讨了金属氧化物气凝胶的形成机理，丰富和发展了的氧化物气凝胶制备的理论和方法。
1．高比表面积ZrO_2气凝胶的制备
通过电解结合溶胶-凝胶法制备了氧化锆基凝胶，进一步通过超临界CO_2干燥和冷冻干燥技术对凝胶进行干燥，成功合成了高比表面的ZrO_2气凝胶。本研究提供了一个简单的以无机盐为原料制备ZrO_2气凝胶的方法，并可能用于制备其它的金属氧化物气凝胶。首先，分别配制30 mL浓度为0．3 mol／L(C_(2Y+Zr))，Y_2O_3：ZrO_2的摩尔比为0：100，3：97，6：94，8：92的水溶液，分别在25℃下直流电解池中电解。当电解进行一段时间后，溶液转变成溶胶，停止电解，向溶胶中加入异丙醇，获得ZrO_2基湿凝胶。然后将湿凝胶分成两份：一份用无水乙醇交换湿凝胶网络中的水，然后用超临界CO_2干燥，即可得到ZrO_2气凝胶块体；另一份直接进行冷冻干燥，即可得到粉末状的ZrO_2气凝胶。通过超临界干燥技术制备的气凝胶是透明块体，不含Y~(3+)的ZrO_2气凝胶平均孔径为9．7nm，比表面积约为640 m~2／g；而通过冷冻干燥技术制备的气凝胶是白色粉末，平均孔径为0．59 nm，比表面积约为400 m~2／g。500℃煅烧后，超临界干燥的产物表现为四方相和单斜相ZrO_2的混和物相，而冷冻干燥产物煅烧后为单一的四方相ZrO_2。Y~(3+)的掺杂对气凝胶的粒子尺寸、微结构、孔尺寸以及煅烧后的物相结构没有明显影响，这主要是由于Y~(3+)在500℃煅烧后未能完全均匀地进入ZrO_2晶格中所致。
2．高表面积结晶态TiO_2气凝胶的制备和光致发光性质研究
以钛酸四丁酯为原料，丙酮为溶剂，乙酰丙酮为络合剂，通过钛酸四丁酯和水发生反应，使溶液胶凝得到TiO_2湿凝胶。然后把凝胶放入15 mL高压釜中，加入丙酮，填充度为80％，分别在120、140、160℃进行溶剂热2 h处理，使TiO_2结晶，得结晶态TiO_2湿凝胶(锐钛矿相)。再将结晶TiO_2湿凝胶分别进行CO_2超临界干燥、真空干燥、常压干燥制得气凝胶。本文将溶胶-凝胶法与溶剂热法相结合，形成了一个新的制备气凝胶的工艺，利用溶胶一凝胶法，结合溶剂热方法首次制备了高表面积的结晶的TiO_2气凝胶，并研究了气凝胶在不同干燥方式下的微结构和性质。120、140、160℃溶剂热处理的样品经过干燥后表面积为220-800 m~2／g，其中140℃处理的样品的比表面积最大，经超临界、真空、常压三种干燥方式形成的产物比表面积分别为794．2 m~2／g(超临界干燥)、662．7 m2/g(真空干燥)、528．9 m~2／g(常压干燥)。本实验中所获得的TiO_2气凝胶表面积在常压干燥条件下还可达到500 m~2/g以上，其较大的比表面积应归功于溶剂热处理过程中凝胶网络强度的增大。光学性能研究表明，所有的气凝胶都具有明显的光致发光性质，这与构成气凝胶的粒子小的尺寸和表面缺陷等有关。可以看出这是一个很好的制备结晶态气凝胶材料的方法，为研究制备高表面积的结晶气凝胶又提供了一个新的思路。
3．SnO_2气凝胶薄膜的制备及气敏性质研究
本文以环氧丙烷和SnCl_4·5H_2O为原料，乙醇为溶剂，通过环氧丙烷和SnCl_4·5H_2O反应，制备了SnO_2基醇溶胶，然后采用提拉法在载玻片上镀膜，待溶胶胶凝后，经CO_2超临界萃取，制得SnO_2气凝胶薄膜。以同样的方法将SnO_2气凝胶薄膜镀到气敏元件上，制备了SnO_2气凝胶气敏传感器，并用气敏元件测试系统测试了所制备的SnO_2薄膜在不同工作温度下对乙醇、丙酮、汽油蒸汽的气敏特性。SnO_2气凝胶薄膜是由直径4-5 nm的结晶态球形粒子相互连接而成的网状结构，厚度为0．25μm，膜与基质结合紧密，厚度均匀。N_2吸附-脱附实验表明，SnO_2气凝胶薄膜的比表面积为388 m~2／g，平均孔径8．2 nm。吸附等温线为Ⅳ型，有一个H1型滞后环，说明薄膜为介孔结构。直接超临界得到的SnO_2气凝胶显示了锡石结构四方相SnO_2的特征峰，当薄膜在250℃老化后，衍射峰强度增加，说明其结晶度增强，但薄膜未出现裂纹，且SnO_2粒子未出现明显长大，其平均粒径4．7 nm。气敏性能测试结果表明，SnO_2气凝胶薄膜构成的气敏元件经250℃老化后，在160-440℃之间对浓度为2-100 ppm的三种气体都有响应，元件对三种气体均有良好的灵敏性，而且对三种气体的响应/恢复速率都非常迅速。
The nanostructured metal oxide aerogels were synthesized by a sol-gel technique. The preparation process and the microstructures of aerogels were studied, and the effects of preparation parameters, as well as the thermal properties, the optical properties, and gas-sensing properties of the prepared aerogels were investigated. Based on the experiments, the formation mechanisms of the metal oxide aerogels were proposed. This study enriched the basic theory and applications of aerogels preparated on the basis of the sol-gel process.
1. Preparation of zirconia aerogels with high surface area
Zirconia aerogels with high surface areas were successfully prepared by a combined electrolysis/sol-gel method, followed by supercritical extraction or freeze-drying. This provides a facile route to production of ZrO_2 aerogels using inorganic salts as precursor and might be extended to the preparation of other metal oxide aerogels. First, 30.0 mL of 0.3 mol/L ZrOCl_2·8H_2O + YCl_3·6H_2O solution, in which the molar ratio of Y_2O_3 to ZrO_2 was adjusted to 0:100, 3:97, 6:94, and 8:92, was electrolyzed in an electrolytic cell under the same conditions at 25.0℃. After several days, the solution gradually transformed to a transparent sol. Stop the electrolysing and 30.0 mL of isopropyl alcohol was added into the sol under stirring and a wet gel formed. The wet gel obtained was divided into two parts and treated through two processes. One was immersed in absolute ethanol to exchange the solvent (mainly water) in the gel network. Then, the ethanol was extracted with liquid CO_2 in a supercritical extractor to remove solvent from the gel. When the autoclave was depressurized slowly, a lump of aerogel (S-aerogels) was obtained. The other part without exchanging with ethanol was directly put into a flask and quickly frozen with liquid N_2 and then freeze-dried to give the freeze-dried aerogel (F-aerogels). The pure ZrO_2 S-aerogel was a transparent monolith with mesoporous structure (average pore size, 9.7 nm) and surface area of ca. 640 m~2/g. However, the pure ZrO_2 F-aerogel had microporous structure with surface area and mean pore size of ca. 400 m~2/g and ca. 0.6 nm. After calcination at 500℃, the pure ZrO_2 S-aerogel exhibited as a mixture of m-ZrO_2 and t-ZrO_2, while the pure ZrO_2 F-aerogel showed a single t-ZrO_2 phase. Yittria-stabilized zirconia aerogels showed similar properties including particle size, microstructure, pore size, and surface area, as well as the phase structure of the calcined samples, as the pure zirconia aerogels. Detailed investigation indicated that the Y~(3+) did not enter the zirconia crystal lattice completely, so there had not been obvious effects of the Y_2O_3 contents on the crystalline structure of the calcined zirconia aerogels.
2. Synthesis and photoluminescence properties of crystalline TiO_2 aerogels with high-surface- area
TiO_2 wet gels were prepared in acetone by a sol-gel method using tetrabutyl titanate as a precursor and acetylacetone as a stabilizer. Then, the gel was poured into a 15 mL autoclave, and the acetone was added in up to the 80% volume of the autoclave. The autoclave was heated at a designed temperature (120, 140, 160℃) for 2 h to give the crystalline TiO_2 wet gel (anatase). The anatase wet-gels were respectively dried by CO_2 supercritical drying, vacuum drying and atmosphere drying to give the TiO_2 aerogels. A new route came into being for preparation of aerogels by a combined solvothermal/sol-gel method. The properties and microstructures of aerogels obtained under different drying conditions were studied. The surface areas of all aerogels obtained were between 220 and 800 m~2/g. The samples solvothermally treated at 140℃had maximum surface area in all the samples obtained with different drying methods. The surface areas were 794.2 m~2/g (supercritical drying)、662.7 m~2/g(vacuum drying)、528.9 m~2/g(atmosphere drying), respectively. The higher surface area of TiO_2 aerogels dried at atmosphere were over 500 m~2/g, which might due to the strengthed network of the gel during the solvothermal process. All TiO_2 aerogels showed obvious PL peaks in spectra, which is due to the small particles and surface vacancy. This was a efficient method for for preparing crystalline TiO_2 aerogels and may be extended to the preparation of othe ??rcrystalline aerogels.
3. The preparation and gas-sensing properties of SnO_2 aerogel films
SnO_2 sol was prepared by a reaction of SnCl_4·5H_2O and 1,2-epoxypropane in ethanol system. Then, the SiO_2 substrates were dipped into the solution and withdrawn from the bath at a constant rate to coat film. After sol geled, the coated substrates were dried by CO_2 supercritical drying to give the SnO_2 aerogel films. SnO_2 aerogel films also were coated on gas-sensing components by the above method to test its gas-sensing properties for ethanol, acetone and gasoline. SnO_2 aerogel film was composed of 4-5 nm crystalline particles. The thickness of film was 250 nm. The surface area was 388 m~2/g and average pore size was 8.2 nm(mesoporous structure). The XRD patterns showed its cassiterite structure of the prepared SnO_2 film. After aging at 250℃, the film was stale and no cracks on the film surface can be observed and the particles did not grew up, but the crystalline degree was increased. The gas-sensing components structured by SnO_2 aerogel films had responses for 2-100 ppm ethanol, acetoneand and gasoline in the temperature range of 160℃-440℃with a rapid response/recovery speed.
1．高比表面积ZrO_2气凝胶的制备
通过电解结合溶胶-凝胶法制备了氧化锆基凝胶，进一步通过超临界CO_2干燥和冷冻干燥技术对凝胶进行干燥，成功合成了高比表面的ZrO_2气凝胶。本研究提供了一个简单的以无机盐为原料制备ZrO_2气凝胶的方法，并可能用于制备其它的金属氧化物气凝胶。首先，分别配制30 mL浓度为0．3 mol／L(C_(2Y+Zr))，Y_2O_3：ZrO_2的摩尔比为0：100，3：97，6：94，8：92的水溶液，分别在25℃下直流电解池中电解。当电解进行一段时间后，溶液转变成溶胶，停止电解，向溶胶中加入异丙醇，获得ZrO_2基湿凝胶。然后将湿凝胶分成两份：一份用无水乙醇交换湿凝胶网络中的水，然后用超临界CO_2干燥，即可得到ZrO_2气凝胶块体；另一份直接进行冷冻干燥，即可得到粉末状的ZrO_2气凝胶。通过超临界干燥技术制备的气凝胶是透明块体，不含Y~(3+)的ZrO_2气凝胶平均孔径为9．7nm，比表面积约为640 m~2／g；而通过冷冻干燥技术制备的气凝胶是白色粉末，平均孔径为0．59 nm，比表面积约为400 m~2／g。500℃煅烧后，超临界干燥的产物表现为四方相和单斜相ZrO_2的混和物相，而冷冻干燥产物煅烧后为单一的四方相ZrO_2。Y~(3+)的掺杂对气凝胶的粒子尺寸、微结构、孔尺寸以及煅烧后的物相结构没有明显影响，这主要是由于Y~(3+)在500℃煅烧后未能完全均匀地进入ZrO_2晶格中所致。
2．高表面积结晶态TiO_2气凝胶的制备和光致发光性质研究
以钛酸四丁酯为原料，丙酮为溶剂，乙酰丙酮为络合剂，通过钛酸四丁酯和水发生反应，使溶液胶凝得到TiO_2湿凝胶。然后把凝胶放入15 mL高压釜中，加入丙酮，填充度为80％，分别在120、140、160℃进行溶剂热2 h处理，使TiO_2结晶，得结晶态TiO_2湿凝胶(锐钛矿相)。再将结晶TiO_2湿凝胶分别进行CO_2超临界干燥、真空干燥、常压干燥制得气凝胶。本文将溶胶-凝胶法与溶剂热法相结合，形成了一个新的制备气凝胶的工艺，利用溶胶一凝胶法，结合溶剂热方法首次制备了高表面积的结晶的TiO_2气凝胶，并研究了气凝胶在不同干燥方式下的微结构和性质。120、140、160℃溶剂热处理的样品经过干燥后表面积为220-800 m~2／g，其中140℃处理的样品的比表面积最大，经超临界、真空、常压三种干燥方式形成的产物比表面积分别为794．2 m~2／g(超临界干燥)、662．7 m2/g(真空干燥)、528．9 m~2／g(常压干燥)。本实验中所获得的TiO_2气凝胶表面积在常压干燥条件下还可达到500 m~2/g以上，其较大的比表面积应归功于溶剂热处理过程中凝胶网络强度的增大。光学性能研究表明，所有的气凝胶都具有明显的光致发光性质，这与构成气凝胶的粒子小的尺寸和表面缺陷等有关。可以看出这是一个很好的制备结晶态气凝胶材料的方法，为研究制备高表面积的结晶气凝胶又提供了一个新的思路。
3．SnO_2气凝胶薄膜的制备及气敏性质研究
本文以环氧丙烷和SnCl_4·5H_2O为原料，乙醇为溶剂，通过环氧丙烷和SnCl_4·5H_2O反应，制备了SnO_2基醇溶胶，然后采用提拉法在载玻片上镀膜，待溶胶胶凝后，经CO_2超临界萃取，制得SnO_2气凝胶薄膜。以同样的方法将SnO_2气凝胶薄膜镀到气敏元件上，制备了SnO_2气凝胶气敏传感器，并用气敏元件测试系统测试了所制备的SnO_2薄膜在不同工作温度下对乙醇、丙酮、汽油蒸汽的气敏特性。SnO_2气凝胶薄膜是由直径4-5 nm的结晶态球形粒子相互连接而成的网状结构，厚度为0．25μm，膜与基质结合紧密，厚度均匀。N_2吸附-脱附实验表明，SnO_2气凝胶薄膜的比表面积为388 m~2／g，平均孔径8．2 nm。吸附等温线为Ⅳ型，有一个H1型滞后环，说明薄膜为介孔结构。直接超临界得到的SnO_2气凝胶显示了锡石结构四方相SnO_2的特征峰，当薄膜在250℃老化后，衍射峰强度增加，说明其结晶度增强，但薄膜未出现裂纹，且SnO_2粒子未出现明显长大，其平均粒径4．7 nm。气敏性能测试结果表明，SnO_2气凝胶薄膜构成的气敏元件经250℃老化后，在160-440℃之间对浓度为2-100 ppm的三种气体都有响应，元件对三种气体均有良好的灵敏性，而且对三种气体的响应/恢复速率都非常迅速。
The nanostructured metal oxide aerogels were synthesized by a sol-gel technique. The preparation process and the microstructures of aerogels were studied, and the effects of preparation parameters, as well as the thermal properties, the optical properties, and gas-sensing properties of the prepared aerogels were investigated. Based on the experiments, the formation mechanisms of the metal oxide aerogels were proposed. This study enriched the basic theory and applications of aerogels preparated on the basis of the sol-gel process.
1. Preparation of zirconia aerogels with high surface area
Zirconia aerogels with high surface areas were successfully prepared by a combined electrolysis/sol-gel method, followed by supercritical extraction or freeze-drying. This provides a facile route to production of ZrO_2 aerogels using inorganic salts as precursor and might be extended to the preparation of other metal oxide aerogels. First, 30.0 mL of 0.3 mol/L ZrOCl_2·8H_2O + YCl_3·6H_2O solution, in which the molar ratio of Y_2O_3 to ZrO_2 was adjusted to 0:100, 3:97, 6:94, and 8:92, was electrolyzed in an electrolytic cell under the same conditions at 25.0℃. After several days, the solution gradually transformed to a transparent sol. Stop the electrolysing and 30.0 mL of isopropyl alcohol was added into the sol under stirring and a wet gel formed. The wet gel obtained was divided into two parts and treated through two processes. One was immersed in absolute ethanol to exchange the solvent (mainly water) in the gel network. Then, the ethanol was extracted with liquid CO_2 in a supercritical extractor to remove solvent from the gel. When the autoclave was depressurized slowly, a lump of aerogel (S-aerogels) was obtained. The other part without exchanging with ethanol was directly put into a flask and quickly frozen with liquid N_2 and then freeze-dried to give the freeze-dried aerogel (F-aerogels). The pure ZrO_2 S-aerogel was a transparent monolith with mesoporous structure (average pore size, 9.7 nm) and surface area of ca. 640 m~2/g. However, the pure ZrO_2 F-aerogel had microporous structure with surface area and mean pore size of ca. 400 m~2/g and ca. 0.6 nm. After calcination at 500℃, the pure ZrO_2 S-aerogel exhibited as a mixture of m-ZrO_2 and t-ZrO_2, while the pure ZrO_2 F-aerogel showed a single t-ZrO_2 phase. Yittria-stabilized zirconia aerogels showed similar properties including particle size, microstructure, pore size, and surface area, as well as the phase structure of the calcined samples, as the pure zirconia aerogels. Detailed investigation indicated that the Y~(3+) did not enter the zirconia crystal lattice completely, so there had not been obvious effects of the Y_2O_3 contents on the crystalline structure of the calcined zirconia aerogels.
2. Synthesis and photoluminescence properties of crystalline TiO_2 aerogels with high-surface- area
TiO_2 wet gels were prepared in acetone by a sol-gel method using tetrabutyl titanate as a precursor and acetylacetone as a stabilizer. Then, the gel was poured into a 15 mL autoclave, and the acetone was added in up to the 80% volume of the autoclave. The autoclave was heated at a designed temperature (120, 140, 160℃) for 2 h to give the crystalline TiO_2 wet gel (anatase). The anatase wet-gels were respectively dried by CO_2 supercritical drying, vacuum drying and atmosphere drying to give the TiO_2 aerogels. A new route came into being for preparation of aerogels by a combined solvothermal/sol-gel method. The properties and microstructures of aerogels obtained under different drying conditions were studied. The surface areas of all aerogels obtained were between 220 and 800 m~2/g. The samples solvothermally treated at 140℃had maximum surface area in all the samples obtained with different drying methods. The surface areas were 794.2 m~2/g (supercritical drying)、662.7 m~2/g(vacuum drying)、528.9 m~2/g(atmosphere drying), respectively. The higher surface area of TiO_2 aerogels dried at atmosphere were over 500 m~2/g, which might due to the strengthed network of the gel during the solvothermal process. All TiO_2 aerogels showed obvious PL peaks in spectra, which is due to the small particles and surface vacancy. This was a efficient method for for preparing crystalline TiO_2 aerogels and may be extended to the preparation of othe ??rcrystalline aerogels.
3. The preparation and gas-sensing properties of SnO_2 aerogel films
SnO_2 sol was prepared by a reaction of SnCl_4·5H_2O and 1,2-epoxypropane in ethanol system. Then, the SiO_2 substrates were dipped into the solution and withdrawn from the bath at a constant rate to coat film. After sol geled, the coated substrates were dried by CO_2 supercritical drying to give the SnO_2 aerogel films. SnO_2 aerogel films also were coated on gas-sensing components by the above method to test its gas-sensing properties for ethanol, acetone and gasoline. SnO_2 aerogel film was composed of 4-5 nm crystalline particles. The thickness of film was 250 nm. The surface area was 388 m~2/g and average pore size was 8.2 nm(mesoporous structure). The XRD patterns showed its cassiterite structure of the prepared SnO_2 film. After aging at 250℃, the film was stale and no cracks on the film surface can be observed and the particles did not grew up, but the crystalline degree was increased. The gas-sensing components structured by SnO_2 aerogel films had responses for 2-100 ppm ethanol, acetoneand and gasoline in the temperature range of 160℃-440℃with a rapid response/recovery speed.
引文
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