Sharma Kusum, Alam Noor, Islam S S
Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia (A Central University), New Delhi 110025, India.
ACS Appl Mater Interfaces. 2020 Apr 1;12(13):15855-15866. doi: 10.1021/acsami.9b22563. Epub 2020 Mar 17.
A break in the traditional pore morphology approach in anodic alumina is presented here to see its niche merit over the conventional sensors for water vapor detection. The cylindrical pore structure was replaced with a normal cone for trace-level and inverse cone for RH-level detection. The normal conical pore was fabricated by sheer manipulation of the reaction rates of electrolytes, anodic polarization, rate and time; the procedure was reversed in the case of the inverse cone structure. A sensor with a normal cone geometry exhibits excellent response at the ppm level and slightly extended to low RH level with a detection range of 120 ppm-30% RH, having response and recovery times of 6 and 255 s, measured at 120 ppm. Lowering of the minimum detection limit further requires alteration of the conical geometric parameters, in tandem with the molecular dynamics of water vapor molecules within the pore. In contrast, a sensor developed from an inverse conical structure shows response only at the RH level, from 20% RH to 90% RH with response and recovery times of less than 60 s over the entire range. Limitations such as nonlinear response, large response-recovery time, and high hysteresis as observed in conventional anodic alumina-based humidity sensors have been removed. The sensor response in conical and inverse conical pore morphologies is compared with that of standard sensors having a cylindrical pore morphology, with a top pore diameter identical with that of the reported sensors. The standard sensors were found to detect in the RH range only, with response and recovery times below 20s. The sensing mechanisms in both structures have been suitably demonstrated and ratified with experimental data. Trace level detection is interpreted with the statistical probabilistic approach in the light of the kinetic theory of gases and Brownian energy. A correlation between top surface pore diameter (through which water molecules enter) and the optimized mean free path of vapor molecule is established, and its effectiveness has been demonstrated for humidity detection at a trace level. The results are encouraging, and the same concept may be tried for the detection of other gaseous stimuli, including organic vapors.
本文提出了一种阳极氧化铝传统孔隙形态方法的突破，以探究其相对于传统水蒸气检测传感器的独特优势。将圆柱形孔隙结构替换为用于痕量水平检测的正圆锥和用于相对湿度（RH）水平检测的倒圆锥。通过单纯控制电解质的反应速率、阳极极化速率和时间来制造正圆锥形孔隙；在制造倒圆锥结构时，步骤则相反。具有正圆锥几何形状的传感器在ppm水平下表现出优异的响应，并略微扩展到低RH水平，检测范围为120 ppm - 30% RH，在120 ppm下测量的响应和恢复时间分别为6秒和255秒。进一步降低最低检测限需要改变圆锥几何参数，并结合孔隙内水蒸气分子的分子动力学。相比之下，由倒圆锥结构开发的传感器仅在RH水平下有响应，范围为20% RH至90% RH，在整个范围内响应和恢复时间均小于60秒。传统基于阳极氧化铝的湿度传感器中观察到的诸如非线性响应、大响应恢复时间和高滞后等局限性已被消除。将圆锥形和倒圆锥形孔隙形态的传感器响应与具有圆柱形孔隙形态且顶部孔径与所报道传感器相同的标准传感器的响应进行了比较。发现标准传感器仅在RH范围内进行检测，响应和恢复时间低于20秒。两种结构中的传感机制均已通过实验数据得到适当证明和验证。痕量水平检测根据气体动力学理论和布朗能量，采用统计概率方法进行解释。建立了水分子进入的顶部表面孔径与蒸汽分子优化平均自由程之间的相关性，并已证明其在痕量水平湿度检测中的有效性。结果令人鼓舞，相同的概念可尝试用于检测其他气态刺激物，包括有机蒸汽。
