## 3 Results and Discussion
### Surface Analysis by XPS  
Figure 5 shows the survey spectra for the bare gold electrode, Au/diazo-COOH, Au-diazo-ZnO nanoparticles, Au-diazo-ZnO nanorods, ZnO-IIP and ZnO-NIP. Figure 4: SWV voltammograms in H\({}_{2}\)O/KCl 0.1 M solution after incubation of Au/Hg-IIP and Au-ZnO/Hg-IIP in \(10^{-3}\) M Hg\({}^{2+}\). Conditions: E-pulse: 0.01 V, E-step: 0.01 V, frequency: 25 Hz, equilibration time: 2 s.  
Figure 3: (**A**): Electropolymerization of pyrrole by chronoamperometry at a constant potential of 1.05 V/SSC. (black) Barrier layer, (red) Polymerization of pyrrole in the presence of L-cysteine and Hg\({}^{2+}\) (**B**): square wave voltammetry (SWV) of IIP and NIP in H\({}_{2}\)O/KCl 0.1 M solution. Firstly, comparison of the XPS spectrum of bare gold electrode with Au/diazo-COOH confirms the effective reductive electrografrating reaction of the diazonium salt on the gold electrode. Indeed, it is noticed the presence of characteristic signals of the diazo-COOH, namely C1s and N1s peaks centered at 285 eV and 400 eV, respectively (Figure 5). It should be noted that the surface-grafted layer seems to be relatively thin (less than 10 nm); this was also confirmed by the atomic percentages shown in Table 1. The atomic percentage of gold decreased from 41.6% for the bare gold electrode to 35.8% after electrografting of the diazonium salt, with a small increase in the percentage of carbon (grafting of the diazonium salt while keeping the conductive activity of the electrode). After the deposition of ZnO nanoparticles by spin-coating, we have the appearance of a peak at 1022 eV which corresponds to the Zn2p32 of ZnO (Figure 5), with an atomic percentage of 10.7%. After the ZnO growth step, the atomic percentages of zinc and oxygen increase (27.4% and 38.5%, respectively). We can also note a decrease in the peaks corresponding to the Au4f7/2 and C1s signals (Figure 5), which confirms the success of the ZnO nanorods growth step. Comparison of the XPS spectra of ZnO-IIP and ZnO-NIP (Figure 5) indicates the grafting of a polypyrrole film on the surface of the electrode. However, mercury is present only in the case of the IIP \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Samples** & **Au** & **C** & **O** & **N** & **Zn** & **Hg** \\ \hline \hline Bare gold electrode & 41.6 & 42.2 & 16.2 & - & - & - \\ \hline Au/diazo-COOH & 35.8 & 45.9 & 14.3 & 3.97 & - & - \\ \hline Au-diazo-ZnO NPs & - & 60.6 & 28.3 & 0.43 & 10.7 & - \\ \hline Au-diazo-ZnO NRs & 0.87 & 32.3 & 38.5 & 0.97 & 27.4 & - \\ \hline ZnO-IIP & 0.07 & 43.0 & 33.2 & 2.38 & 20.7 & 0.57 \\ \hline ZnO-NIP & 1.61 & 43.5 & 33.3 & 2.95 & 18.7 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface chemical composition of Au electrode, Au/diazo-COOH, Au-diazo-ZnO nanoparticles, Au-diazo-ZnO nanorods, ZnO-IIP and ZnO-NIP nanocomposites as determined by XPS. Figure 5: XPS survey scans of Au electrode, Au/diazo-COOH, Au-diazo-ZnO nanoparticles, Au-diazo-ZnO nanorods, ZnO-IIP and ZnO-NIP. material, which indicates that the template has been well trapped in the polymeric matrix. The atomic percentage of Zn for NIP and IIP decreased due to screening ZnO with polypyrrole (Table 1). As per the thickness of the aryl layer, it was determined using the Beer-Lambert exponential decay of the intensity of bare gold:  
\[I=I^{\circ}exp(-d/\lambda cos\theta)\]  
where \(I\) is the peak intensity of Au4f\({}_{7/2}\) core level signal after coating, \(I^{\circ}\) is the intensity for the bare gold, \(d\) is the aryl thickness, \(\lambda\) the mean free path of Au4f\({}_{7/2}\) in the aryl layer and \(\theta\) the analysis angle (here, \(\theta\) = 0, and \(cos\theta\) = 1). The thickness is estimated to be 0.43 nm by considering \(\lambda\) to be around 4.1 nm, which, for a monolayer of aryl groups given the fact that the size of length of benzoic acid, is around 0.49 nm. As will be shown below by AFM, ZnO NPs are large compared to the sampling depth (\(\sim\)2-10 nm) in XPS. For this reason, no gold is detected for Au-diazo-ZnO NPs.