**5.3 Decoration**

*Nanorods and Nanocomposites*

units and their assembly structure.

**5. Modification of ZnO nanorods**

response and selectivity [31].

**5.2 Functionalization**

**5.1 Doping**

**4.4 Hollow microhemispheres**

(AL) intrinsic defects. ZnO semiconductor with high response and selectivity to H2S as well as good stability was obtained by controlling the shape-defined building

Self-assembling ZnO nanorods into 3D ZnO hollow microhemispheres is simply done by solvothermal reaction of zinc and alkali source in the presence of ethylene glycol. The 3D ZnO hollow microhemispheres were synthesized by the primary formation of glycolate precursors and subsequent transformation into ZnO. Glycol such as EG and the solvothermal process play the critical role in the synthesis. Numerous orderly and radical nanorods in the 3D ZnO hollow microhemispheres have the diameter of about 50 nm and length of several hundred nanometers. Furthermore, high sensitivity for ethanol and ammonia as well as quick response and recovery time at room temperature were obtained for the 3D ZnO hollow microhemisphere-based gas sensor due to the high surface-to-volume ratio [29].

The ZnO-sensing performance can be effectively improved by doping with transition metal elements. Vertically aligned yttrium-doped ZnO nanorod (YZO NR) arrays were synthesized by a one-pot hydrothermal method [30]. The Y doping concentration strongly influences the surface morphology of the NRs. With the dopant concentration increasing, longer and sharper NRs with high aspect ratio formed, and the aspect ratio of the YZO NRs was increased from 11 to 25. The Y doping reduced the ZnO NR resistivity to its minimum by a factor of 115 for the Y0.10ZnO. And the corresponding sensor gave the highest sensitivity toward all the tested gas species with the lowest breakdown voltage. The sensitivity of the sensor based on Y-doped ZnO nanorod arrays was enhanced up to sixfold, and the breakdown voltage from the highly conductive YZO NRs significantly reduced. The Cr-doped ZnO sensor presents the response of 104 to 100 ppm acetone at 300°C, as is four times higher than that of ZnO sensor. The increase of electron concentration optimized by Cr doping improved the sensing

The sensitivity and responding kinetics of metal oxide semiconductors (MOS) can be improved by the surface functionalization with noble metal NPs (e.g., Pd, Au, and Pt) because it provides the preferred adsorption and activation sites for the target analyte to react with the ionosorbed oxygen. CuO with a low band gap of 1.2–2.0 eV is a p-type semiconductor and can form p-n junctions with n-type metal oxides to extend the space-charge region, as locally narrows the conducting channel for the charge carriers in the ZnO. The p-n junction is thus more sensitive to gas molecule-induced charge transfer, and the gas-sensing selectivity and sensitivity is improved. CuO-functionalized ZnO nanorods were synthesized by a chemical bath deposition method. And their sensor showed enhanced gas response compared with bare ZnO NRs. According to both a collective- and a local-site approach, the deposition of CuO altered the overall band structure and surface properties of the

**46**

ZnO [32].

Pd-decorated ZnO nanorod structures were fabricated by sputter depositing Pd on the hydrothermally grown ZnO nanorods and the subsequent oxidation for detecting H2S gas at room temperature. The dissociation of H2S gas molecules on Pd accelerated, as facilitated the room temperature-sensing capability [33]. Therefore, the Pd decoration enhanced the sensing response to H2S at the examined operation temperatures. **Figure 12(a)** shows higher response level of ZnO:Pd nanorod structure than the ZnO nanorod structure at 300°C. **Figure 12(b)** shows a more dramatic effect of Pd at room temperature because the response of the ZnO:Pd sensor to 500 ppm H2S is approximately ∼16% compared with the pure ZnO nanorod of no H2S-sensing response at RT. The Pd-decorated ZnO nanorods showed a stable performance as well as good stability and repeatability. The response time was approximately 12 min, but the recovery time was much longer, approximately 10 h. Moreover, the CaO-decorated n-ZnO nanorods showed stronger response to NO2 than the pristine ZnO nanorods [34].
