**5.2 Functionalization**

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 ZnO [32].

**47**

**Figure 13.**

*ZnO Nanorods for Gas Sensors*

**5.3 Decoration**

**5.4 Sensitization**

**Figure 12.**

*temperature.*

*DOI: http://dx.doi.org/10.5772/intechopen.85612*

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].

The sensitivity and the selectivity of the sensor can be improved by different strategies such as incorporating dopants, applying ultraviolet (UV) irradiation, and sensitizing the sensing layer with noble metals (Au, Pt, Pd) and metal oxides (CuO, SnO2). It is known that sensitizing the sensing layer by noble metals is most

*Comparison of H2S sensing between ZnO and ZnO:Pd nanorod structures at (a) 300°C and (b) room* 

*Response of sensors based on pure and Au-modified ZnO samples versus different concentrations of H2S gas.*
