本研究以溶膠凝膠法製備SiO2/TiO2奈米複合粉體，實驗中以四乙氧基矽烷(TEOS)及異丙醇鈦(TTIP)做為原料，並在反應中添加檸檬酸增加合成粉體之親水性，藉由調控起始原料Si與Ti之比例以及添加不同含量之氯化鉀，探討合成粉體之化學結構、表面形貌、親疏水性、濕度感測等性質。實驗結果顯示以溶膠凝膠法可成功合成出SiO2/TiO2奈米複合粉體，且製程快速僅需數小時，不同Si/Ti比之粉體其主要成分皆為非晶相之SiO2且仍含有檸檬酸，經紅外線光譜分析後，粉體內含有Si-O-Ti之鍵結，顯示本研究之製程有成功使Ti進入到SiO2中，可增加材料之缺陷以提升濕度感測之性質。又，檸檬酸在吸附水分子的過程可以提供質子在水層中進行傳遞，增加材料之導電性以降低其阻抗值。各試樣之粒徑皆約為50~100 nm，且經水接觸角之測試後，顯示適量Ti的添加可增進粉體的親水性。
在濕度感測性質方面，結果顯示Si/Ti = 1.0 + 5 mole% KCl之試樣在相對濕度11%~95% RH的環境下具有最佳的靈敏性及線性關係，其阻抗值變化可達4個數量級，反應及回復時間分別為34秒及23秒。此外，此試樣之遲滯現象小，最大的遲滯誤差僅約3.8 %，且在高低濕度來回反覆測試下以及進行長時間之測試下，其阻抗值並無出現很大的差異，皆呈現良好的重複性及穩定性。

In this study, SiO2/TiO2 nanocomposites were prepared by a faster process and without calcination. The powders were synthesized by sol-gel method, and citric acid was added in the reaction to increase the hydrophilicity of the powders. The particle size was controlled by adjusting the stirring time and pH value, and the particle size was about 50-100 nm. Different amounts of KCl were added to the reaction, and the effects of different KCl contents on humidity sensing properties were investigated. The results showed that the sample synthesized with 5 mole% KCl at the ratio of Si/Ti = 1.0 had the best sensitivity and linearity in the relative humidity range of 11%-95% RH. The impedance value could be changed up to 4 orders of magnitude. The response and recovery time of this sample were 34 s and 23 s, respectively. It also exhibited small hysteresis and good stability. Thus, the SiO2/TiO2 nanocomposite could be successfully synthesized by sol-gel method and it was a promising material for the application of humidity sensor.

[1] H. Farahani, R. Wagiran, and M. N. Hamidon, Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review, Sensors, 14, 7881-7939 (2014).
[2] C.-D. Feng, S.-L. Sun, H. Wang, C. U. Segre, and J. R. Stetter, Humidity sensing properties of Nafion and sol-gel derived SiO2/Nafion composite thin films, Sensors and Actuators B: Chemical, 40, 217-222 (1997).
[3] V. K. Tomer, S. Duhan, A. K. Sharma, R. Malik, S. Jangra, S. P. Nehra, and S. Devi, Humidity-Sensing Properties of Ag0 Nanoparticles Supported on WO3-SiO2 with Super Rapid Response and Excellent Stability, European Journal of Inorganic Chemistry, 2015, 5232-5240 (2015).
[4] V. K. Tomer, S. Duhan, P. V. Adhyapak, and I. S. Mulla, Mn-Loaded Mesoporous Silica Nanocomposite: A Highly Efficient Humidity Sensor, Journal of the American Ceramic Society, 98, 741-747 (2015).
[5] V. K. Tomer, S. Duhan, A. K. Sharma, R. Malik, S. P. Nehra, and S. Devi, One pot synthesis of mesoporous ZnO–SiO2 nanocomposite as high performance humidity sensor, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 483, 121-128 (2015).
[6] H. Zhao, T. Zhang, R. Qi, J. Dai, S. Liu, T. Fei, and G. Lu, Development of solution processible organic-inorganic hybrid materials with core-shell framework for humidity monitoring, Sensors and Actuators B: Chemical, 255, 2878-2885 (2018).
[7] T. Zhang, R. Wnag, W. Geng, X. Li, Q. Qi, Y. He, and S. Wang, Study on humidity sensing properties based on composite materials of Li-doped mesoporous silica A-SBA-15, Sensors and Actuators B: Chemical, 128, 482-487 (2008).
[8] L. Liu, L. Y. Kou, Z. C. Zhong, L. Y. Wang, L. F. Liu, and W. Li, Preparation and Humidity Sensing Properties of KCl/MCM-41 Composite, Chinese Physics Letters, 27, 1-4 (2010).
[9] N. Yamazoe and Y. Shimizu, Humidity sensors: Principles and applications, Sensors and Actuators, 10, 379-398 (1986).
[10] K. Suri, S. Annapoorni, A. K. Sarkar, and R. P. Tandon, Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites, Sensors and Actuators B: Chemical, 81, 277-282 (2002).
[11] Y. Li, M. J. Yang, and Y. She, Humidity sensitive properties of crosslinked and quaternized poly(4-vinylpyridine-co-butyl methacrylate), Sensors and Actuators B-Chemical, 107, 252-257 (2005).
[12] B. C. Cheng, B. X. Tian, C. C. Xie, Y. H. Xiao, and S. J. Lei, Highly sensitive humidity sensor based on amorphous Al2O3 nanotubes, Journal of Materials Chemistry, 21, 1907-1912 (2011).
[13] Y. He, T. Zhang, W. Zheng, R. Wang, X. Liu, Y. Xia, and J. Zhao, Humidity sensing properties of BaTiO3 nanofiber prepared via electrospinning, Sensors and Actuators B: Chemical, 146, 98-102 (2010).
[14] Q. Qi, T. Zhang, S. Wang, and X. Zheng, Humidity sensing properties of KCl-doped ZnO nanofibers with super-rapid response and recovery, Sensors and Actuators B: Chemical, 137, 649-655 (2009).
[15] M. Parthibavarman, V. Hariharan, and C. Sekar, High-sensitivity humidity sensor based on SnO2 nanoparticles synthesized by microwave irradiation method, Materials Science and Engineering: C, 31, 840-844 (2011).
[16] M.-S. Gong, J.-U. Kim, and J.-G. Kim, Preparation of water-durable humidity sensor by attachment of polyelectrolyte membrane to electrode substrate by photochemical crosslinking reaction, Sensors and Actuators B: Chemical, 147, 539-547 (2010).
[17] C.-W. Lee, H.-S. Park, J.-G. Kim, B.-K. Choi, S.-W. Joo, and M.-S. Gong, Polymeric humidity sensor using organic/inorganic hybrid polyelectrolytes, Sensors and Actuators B: Chemical, 109, 315-322 (2005).
[18] P.-G. Su and C.-S. Wang, Novel flexible resistive-type humidity sensor, Sensors and Actuators B: Chemical, 123, 1071-1076 (2007).
[19] Z. M. Rittersma, Recent achievements in miniaturised humidity sensors—a review of transduction techniques, Sensors and Actuators A: Physical, 96, 196-210 (2002).
[20] Y. Kim, B. Jung, H. Lee, H. Kim, K. Lee, and H. Park, Capacitive humidity sensor design based on anodic aluminum oxide, Sensors and Actuators B: Chemical, 141, 441-446 (2009).
[21] S. W. Chen, O. K. Khor, M. W. Liao, and C. K. Chung, Sensitivity evolution and enhancement mechanism of porous anodic aluminum oxide humidity sensor using magnetic field, Sensors and Actuators B: Chemical, 199, 384-388 (2014).
[22] T. Wagner, S. Krotzky, A. Weiss, T. Sauerwald, C. D. Kohl, J. Roggenbuck, and M. Tiemann, A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica, Sensors, 11, 3135-3144 (2011).
[23] C.-H. Chen and C.-H. Lin, A novel method to fabricate ion-doped microporous polyimide structures for ultra-high sensitive humidity sensing, Sensors and Actuators B: Chemical, 135, 276-282 (2008).
[24] W. Yao, X. Chen, and J. Zhang, A capacitive humidity sensor based on gold–PVA core–shell nanocomposites, Sensors and Actuators B: Chemical, 145, 327-333 (2010).
[25] D. Zhang, D. Wang, P. Li, X. Zhou, X. Zong, and G. Dong, Facile fabrication of high-performance QCM humidity sensor based on layer-by-layer self-assembled polyaniline/graphene oxide nanocomposite film, Sensors and Actuators B: Chemical, 255, 1869-1877 (2018).
[26] A. I. Buvailo, Y. J. Xing, J. Hines, N. Dollahon, and E. Borguet, TiO2/LiCl-Based Nanostructured Thin Film for Humidity Sensor Applications, ACS Applied Materials & Interfaces, 3, 528-533 (2011).
[27] S. Lei, D. J. Chen, and Y. Q. Chen, A surface acoustic wave humidity sensor with high sensitivity based on electrospun MWCNT/Nafion nanofiber films, Nanotechnology, 22, 7-14 (2011).
[28] M. Bedoya, G. Orellana, and M. C. Moreno-Bondi, Fluorescent optosensor for humidity measurements in air, Helvetica Chimica Acta, 84, 2628-2639 (2001).
[29] C. Meng, Y. Xiao, P. Wang, L. Zhang, Y. Liu, and L. Tong, Quantum-Dot-Doped Polymer Nanofibers for Optical Sensing, Advanced Materials, 23, 3770-3774 (2011).
[30] Z. Chen and C. Lu, Humidity sensors: A review of materials and mechanisms, Sensor Letters, 3, 274-295 (2005).
[31] 彭永福, 以溶膠凝膠法製備SiO2薄膜作TFT閘極絕緣層材料, 國立中山大學光電工程學系碩士論文, 12-20 (2009).
[32] L. Ye, Y. Zhang, X. Zhang, T. Hu, R. Ji, B. Ding, and B. Jiang, Sol–gel preparation of SiO2/TiO2/SiO2–TiO2 broadband antireflective coating for solar cell cover glass, Solar Energy Materials and Solar Cells, 111, 160-164 (2013).
[33] C. Kapridaki and P. Maravelaki-Kalaitzaki, TiO2–SiO2–PDMS nano-composite hydrophobic coating with self-cleaning properties for marble protection, Progress in Organic Coatings, 76, 400-410 (2013).
[34] S. Son, S. H. Hwang, C. Kim, J. Y. Yun, and J. Jang, Designed Synthesis of SiO2/TiO2 Core/Shell Structure As Light Scattering Material for Highly Efficient Dye-Sensitized Solar Cells, ACS Applied Materials & Interfaces, 5, 4815-4820 (2013).
[35] F. Yang, J. Zhu, X. Zou, X. Pang, R. Yang, S. Chen, Y. Fang, T. Shao, X. Luo, and L. Zhang, Three-dimensional TiO2/SiO2 composite aerogel films via atomic layer deposition with enhanced H2S gas sensing performance, Ceramics International, 44, 1078-1085 (2018).
[36] Y. Tang, D. Ao, W. Li, X. Zu, S. Li, and Y. Q. Fu, NH3 sensing property and mechanisms of quartz surface acoustic wave sensors deposited with SiO2, TiO2, and SiO2-TiO2 composite films, Sensors and Actuators B: Chemical, 254, 165-1173 (2018).
[37] P.-G. Su and W.-Y. Tsai, Humidity sensing and electrical properties of a composite material of nano-sized SiO2 and poly(2-acrylamido-2-methylpropane sulfonate), Sensors and Actuators B: Chemical, 100, 417-422 (2004).
[38] P.-G. Su and S.-C. Huang, Electrical and humidity sensing properties of carbon nanotubes-SiO2-poly(2-acrylamido-2-methylpropane sulfonate) composite material, Sensors and Actuators B: Chemical, 113, 142-149 (2006).
[39] Q. Qi, T. Zhang, X. Zheng, and L. Wan, Preparation and humidity sensing properties of Fe-doped mesoporous silica SBA-15, Sensors and Actuators B: Chemical, 135, 255-261 (2008).
[40] Q. Yuan, N. Li, J. Tu, X. Li, R. Wang, T. Zhang, and C. Shao, Preparation and humidity sensitive property of mesoporous ZnO–SiO2 composite, Sensors and Actuators B: Chemical, 149, 413-419 (2010).
[41] R. Wang, X. Liu, Y. He, Q. Yuan, X. Li, G. Lu, and T. Zhang, The humidity-sensitive property of MgO-SBA-15 composites in one-pot synthesis, Sensors and Actuators B: Chemical, 145, 386-393 (2010).
[42] J. Tu, N. Li, W. Geng, R. Wang, X. Lai, Y. Cao, T. Zhang, X. Li, and S. Qiu, Study on a type of mesoporous silica humidity sensing material, Sensors and Actuators B: Chemical, 166-167, 658-664 (2012).
[43] V. K. Tomer, P. V. Adhyapak, S. Duhan, and I. S. Mulla, Humidity sensing properties of Ag-loaded mesoporous silica SBA-15 nanocomposites prepared via hydrothermal process, Microporous and Mesoporous Materials, 197, 140-147 (2014).
[44] V. K. Tomer and S. Duhan, Nano titania loaded mesoporous silica: Preparation and application as high performance humidity sensor, Sensors and Actuators B: Chemical, 220, 192-200 (2015).
[45] V. K. Tomer, S. Devi, R. Malik, S. P. Nehra, and S. Duhan, Fast response with high performance humidity sensing of Ag–SnO2/SBA-15 nanohybrid sensors, Microporous and Mesoporous Materials, 219, 240-248 (2016).
[46] W.-P. Tai and J.-H. Oh, Fabrication and humidity sensing properties of nanostructured TiO2–SnO2 thin films, Sensors and Actuators B: Chemical, 85, 154-157 (2002).
[47] P.-G. Su and L.-N. Huang, Humidity sensors based on TiO2 nanoparticles/polypyrrole composite thin films, Sensors and Actuators B: Chemical, 123, 501-507 (2007).
[48] A. Sun, L. Huang, and Y. Li, Study on humidity sensing property based on TiO2 porous film and polystyrene sulfonic sodium, Sensors and Actuators B: Chemical, 139, 543-547 (2009).
[49] X. J. Yue, T. S. Hong, X. Xu, and Z. Li, High-Performance Humidity Sensors Based on Double-Layer ZnO-TiO2 Nanofibers via Electrospinning, Chinese Physics Letters, 28, 1-4 (2011).
[50] Z. Y. Wang, L. Y. Shi, F. Q. Wu, S. A. Yuan, Y. Zhao, and M. H. Zhang, The sol-gel template synthesis of porous TiO2 for a high performance humidity sensor, Nanotechnology, 22, 9-17 (2011).
[51] W.-D. Lin, C.-T. Liao, T.-C. Chang, S.-H. Chen, and R.-J. Wu, Humidity sensing properties of novel graphene/TiO2 composites by sol–gel process, Sensors and Actuators B: Chemical, 209, 555-561 (2015).
[52] M. Gong, Y. Li, Y. Guo, X. Lv, and X. Dou, 2D TiO2 nanosheets for ultrasensitive humidity sensing application benefited by abundant surface oxygen vacancy defects, Sensors and Actuators B: Chemical, 262, 350-358 (2018).
[53] E. Poonia, P. K. Mishra, V. Kiran, J. Sangwan, R. Kumar, P. K. Rai, and V. K. Tomer, Aero-gel assisted synthesis of anatase TiO2 nanoparticles for humidity sensing application, Dalton Transactions, 47, 6293-6298 (2018).
[54] Z. Li, A. A. Haidry, B. Dong, L. Sun, Q. Fatima, L. Xie, and Z. Yao, Facile synthesis of nitrogen doped ordered mesoporous TiO2 with improved humidity sensing properties, Journal of Alloys and Compounds, 742, 814-821 (2018).
[55] L. Sun, A. A. Haidry, Q. Fatima, Z. Li, and Z. Yao, Improving the humidity sensing below 30% RH of TiO2 with GO modification, Materials Research Bulletin, 99, 124-131 (2018).
[56] W. Geng, R. Wang, X. Li, Y. Zou, T. Zhang, J. Tu, Y. He, and N. Li, Humidity sensitive property of Li-doped mesoporous silica SBA-15, Sensors and Actuators B: Chemical, 127, 323-329 (2007).
[57] Z. Li, H. Zhang, W. Zheng, W. Wang, H. Huang, C. Wang, A. G. MacDiarmid, and Y. Wei, Highly sensitive and stable humidity nanosensors based on LiCl doped TiO2 electrospun nanofibers, Journal of the American Chemical Society, 130, 5036-5037 (2008).
[58] L. Wang, D. Li, R. Wang, Y. He, Q. Qi, Y. Wang, and T. Zhang, Study on humidity sensing property based on Li-doped mesoporous silica MCM-41, Sensors and Actuators B: Chemical, 133, 622-627 (2008).
[59] J. Tu, R. Wang, W. Geng, X. Lai, T. Zhang, N. Li, N. Yue, and X. Li, Humidity sensitive property of Li-doped 3D periodic mesoporous silica SBA-16, Sensors and Actuators B: Chemical, 136, 392-398 (2009).
[60] H. Zhao, S. Liu, R. Wang, and T. Zhang, Humidity-sensing properties of LiCl-loaded 3D cubic mesoporous silica KIT-6 composites, Materials Letters, 147, 54-57 (2015).
[61] H. Zhang, Z. Li, L. Liu, C. Wang, Y. Wei, and A. G. MacDiarmid, Mg2+/Na+-doped rutile TiO2 nanofiber mats for high-speed and anti-fogged humidity sensors, Talanta, 79, 953-958 (2009).
[62] X. W. He, W. C. Geng, B. L. Zhang, L. M. Jia, L. B. Duan, and Q. Y. Zhang, Ultrahigh humidity sensitivity of NaCl-added 3D mesoporous silica KIT-6 and its sensing mechanism, RSC Advances, 6, 38391-38398 (2016).
[63] S. B. Ge, X. W. He, L. M. Jia, L. B. Duan, S. Zhang, Q. Y. Zhang, W. C. Geng, Facile fabrication of NaCl-added mesoporous silica HMS composite and its humidity responsing performance, Journal of Sol-Gel Science and Technology, 82, 635-642 (2017).
[64] Q. Qi, Y. Feng, T. Zhang, X. Zheng, and G. Lu, Influence of crystallographic structure on the humidity sensing properties of KCl-doped TiO2 nanofibers, Sensors and Actuators B: Chemical, 139, 611-617 (2009).
[65] M. Anbia and S. E. M. Fard, Improving humidity sensing properties of nanoporous TiO2–10mol% SnO2 thin film by co-doping with La3+ and K+, Sensors and Actuators B: Chemical, 160, 215-221 (2011).
[66] W. Geng, Q. Yuan, X. Jiang, J. Tu, L. Duan, J. Gu, and Q. Zhang, Humidity sensing mechanism of mesoporous MgO/KCl–SiO2 composites analyzed by complex impedance spectra and bode diagrams, Sensors and Actuators B: Chemical, 174, 513-520 (2012).
[67] W. Zhang, R. Wang, Q. Zhang, and J. Li, Humidity sensitive properties of K-doped mesoporous silica SBA-15, Journal of Physics and Chemistry of Solids, 73, 517-522 (2012).
[68] R. Al-Oweini and H. El-Rassy, Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors, Journal of Molecular Structure, 919, 140-145 (2009).
[69] I. A. Mudunkotuwa and V. H. Grassian, Citric Acid Adsorption on TiO2 Nanoparticles in Aqueous Suspensions at Acidic and Circumneutral pH: Surface Coverage, Surface Speciation, and Its Impact on Nanoparticle-Nanoparticle Interactions, Journal of the American Chemical Society, 132, 14986-14994 (2010).
[70] S. Agarwal and G. L. Sharma, Humidity sensing properties of (Ba, Sr) TiO3 thin films grown by hydrothermal-electrochemical method, Sensors and Actuators B-Chemical, 85, 205-211 (2002).
[71] Z. G. Zhao, X. W. Liu, W. P. Chen, and T. Li, Carbon nanotubes humidity sensor based on high testing frequencies, Sensors and Actuators A: Physical, 168, 10-13 (2011).