2.1.

Surface-Modification of ZnO Nanowires

The surface modification of ZnO nanowires was conducted following a reported method.²⁵ The ZnO nanowire template was placed into an O₂ plasma cleaner for 15 min to promote hydroxylation with the ZnO side facing up. A silane solution was freshly prepared by mixing triethoxy-2-thienylsilane molecules with toluene and n-butylamine catalyst. The solution was preheated at 45°C for 30 min, and then the pretreated ITO/ZnO nanowire substrate was immersed into the heated solution. After that, the ITO/ZnO nanowire substrate was removed from the solution and rinsed with toluene and acetone, and dried with N₂. Postcuring was done at 110°C.

2.2.

Electrochemical Polymerization

An enclosing homemade glass cell was installed using a standard three-electrode configuration with an Ag/AgCl as the reference electrode and a coiled platinum auxiliary electrode as the counter electrode. Different substrates, including ITO and ITO/ZnO nanowires with and without surface modification, served as the working electrode, respectively. CV experiments were conducted at room temperature. The electrolyte consisting of Bu₄NFP₆ (0.1 M) and 3-hexylthiophene (3HT) monomer (0.005 M) in an anhydrous acetonitrile solution was freshly prepared. The maximum applied potentials for electrochemical polymerization were varied from 1.5 to 2.2 V with a scan rate of 0.1 V/s for each. The final films grown on the ZnO surface were washed with ethanol, acetone and then dried by N₂.

3.1.

Surface Modification of ZnO Nanowires

Figure 1 shows the SEM images of the ZnO nanowires. It can be seen that the prepared ZnO nanowires have an average diameter of ∼50 nm and a length of ∼1 μm. They are uniformly distributed on the ITO substrate.

Fig. 1

(a) Top view and (b) tilted cross-sectional SEM image of the ZnO nanowires grown on an ITO substrate.

The electrochemical polymerization method is an easy and cheap method to obtain polymers. In addition, it does not require polymers to have a side chain for solubility. In order to grow polymers by the electrochemical method, the conductive substrates are required; however this can be a further advantage for solar cell application because solar cells are fabricated using conducting substrate as electrode, such as ITO, FTO-coated glass, or plastic substrates. There has been extensive work in the electrochemical polymerization field; polythiophene,^{26, 27, 28} polypyrrole,²⁹ and poly(3,4-ethylenedioxythiophene)³⁰ have been electrochemically polymerized and well studied. One disadvantage for electrochemical polymerization method on conducting substrate is that it initiates the polymerization reaction at the electrode surface rather than that of the semiconducting oxide (e.g., ZnO),²⁰ and therefore, it may lead to a large amount of polymer formed on the substrate (electrode) rather than directly onto the semiconducting oxide. To solve this issue, the ZnO nanowires were surface modified by a layer of thienylsilane molecules capped with a thiophene ring that may promote in situ polymerization of P3HT on the ZnO nanowire surface. It would possibly help to form a homogeneous layer of P3HT directly attached onto the ZnO nanowire surface. Figure 2 (a → b) shows the process of surface functionalization of ZnO nanowires using a thienylsilane molecule, followed by the formation of P3HT synthesized by in situ electrochemical polymerization (b → c).

Fig. 2

Procedure to functionalize the ZnO nanowires using triethoxy-2-thienylsilane and then electrochemically polymerize P3HT directly onto the ZnO nanowire surface.

After the surface functionalization, the ZnO nanowire templates were characterized by XPS measurement as shown in Fig. 3. The chemical nature and surface energy of ZnO nanowires are expected to change after the modification. Figure 3 is the wide-range survey spectrum. The C (1s) signals containing three components at 285.000, 286.666, and 288.921 eV were fitted by Gaussian–Lorentzian functions. From the XPS spectra, the Si (2p) emission around 103.0 eV was found to be weak, but it could still be identified in Fig. 3. This confirms that the silane molecular layer formed on ZnO surface.

Fig. 3

(a) XPS survey spectrum of thienylsilane modified ZnO nanowires and (b) zoomed XPS in the binding energy of 100–200 eV region that shows the Si (2p) signal.

To further identify the surface modification on ZnO nanowires, water contact angle (WCA) measurements were conducted to monitor the surface properties of ZnO samples before and after surface modification. Because the surface properties of nanowires are difficult to be studied directly by this method, the ZnO NP films were prepared and measured instead. This method can be a good indicator to monitor the properties of surface-modified ZnO films. At least two samples were tested, and no less than five spots on each sample were examined to ensure accurate measurements. Figure 4 shows the WCA results of the surface-modification processes. The WCA of the pristine ZnO NP film was around 42 ± 2 deg [Fig. 4] and it became <20 deg [Fig. 4] after the O₂ plasma treatment, which caused a hydrophilic ZnO surface. This result is consistent with the O₂ plasma-treated ZnO film reported by others.^{25, 31, 32} Such hydroxylated treatment can promote the reactivity of ZnO sample with the silane reagent. After the silanization reaction, a WCA of 75 ± 2 deg [Fig. 4] was observed from multiple spots on different slides. This data are quite similar to the phenyltriethoxysilane C₆H₅Si(OCH₂CH₃)₃ (PTES)-modified ZnO nanoparticle surface, which has a WCA of 72.5 ± 4.3 deg.²⁵ This confirmed that the silane molecules attached onto the surface of ZnO films successfully and changed the ZnO surface properties. Because of the hydrophobic nature of the thiophene ring, the contact angle of ZnO sample after silanization increased greatly. In addition, the contact angle of silanized samples kept consistent after several days, which showed this surface modification is quite stable.

Fig. 4

WCA measurements of (a) ZnO NP film, (b) ZnO NP film treated with O₂ plasma, and (c) surface-modified ZnO NP film via silanization.

3.2.

Electropolymerization of P3HT film on ITO/ZnO nanowire substrate

In order to study the effect of thienylsilane functionalization treatment on the electrochemical deposition of P3HT, the maximum applied potentials (V_max) in the potentiodynamic CV experiments were varied quantitatively on different substrates: (i) ITO substrate, (ii) ITO/ZnO nanowires without thienylsilane group surface modification, and (iii) ITO/ZnO nanowires with thienylsilane group surface modification. In the rest of the work, they will be labeled as ITO, ITO/ZnO, ITO/ZnO/Si, respectively.

P3HT was deposited by sweeping the working electrode potential from 0 to V_max (verse Ag/Ag⁺) at a scan rate of 0.1 V/s for a specific number of cycles. The representative CV sweeps of growing P3HT on three different substrates (ITO, ITO/ZnO, ITO/ZnO/Si) at a V_max of 1.8 and 1.9 V, respectively, are shown in Fig. 5. The evidence for successful P3HT growth was the increase of the anodic and cathodic peak current in each successive cycle. During the experiments, a dark color film was observed to form on the electrodes. By increasing V_max, the area beneath each curve was increased, which indicated that more P3HT was formed on the electrode.

Fig. 5

Cyclic voltammogram of the 3-hexylthiophene monomer at different substrate ITO (a) 1.8 V, (d) 1.9 V; ITO/ZnO (b) 1.8 V, (e) 1.9 V, and ITO/ZnO/Si (c) 1.8 V, (f) 1.9 V.

As shown in Fig. 5, when the number of cycles increased, the oxidization and reduction were also subsequently changed. When comparing the polymerization under the same V_max, the area inside the CV curves was in the order of ITO/ZnO < ITO < ITO/ZnO/Si for both V_max = 1.8 V and V_max = 1.9 V. The reason for ITO/ZnO < ITO was that ITO had a conducting surface, but ZnO had a semiconducting surface; therefore, P3HT was easier to be oxidized on the ITO substrate than that of ITO/ZnO substrate. However, compared to the ZnO nanowire substrate with thienylsilane surface modification, the P3HT polymerization onset potential was greatly decreased. This indicated that the thienylsilane functionalized ZnO surface acted as a favorable nucleation site for electrochemical polymerization and led to a low-surface electrical potential for nucleation.

For a more quantitative analysis and comparison of the effects of substrates and variations in V_max on P3HT growth, the charge density for each curve was calculated. The charge density versus numbers of CV sweeps obtained from the anodic waves were plotted in Fig. 6. It was clearly found that the charge density for ITO and ITO/ZnO substrate changed insignificantly at the V_max of 1.8 V. However, it was obviously increased for the ITO/ZnO/Si substrate after the self-assembling of a molecular layer of thienylsilane linker. This further proved that the surface modification could enhance P3HT polymerization at lower potentials.

Fig. 6

Charge density versus cycle number calculated from the anodic waves of potentiodynamic CV experiments.

3.3.

Optical Absorption of Electropolymerized P3HT Film

Figure 7 shows the UV-visible absorption spectra of the ZnO nanowires and electropolymerized P3HT films with and without thienylsilane surface modification. The ZnO nanowires show absorption up to 375 nm with a characteristic shape. In the presence of P3HT, the absorption was broadened to 650 nm for both the samples of ZnO nanowires with and without surface modification. In addition, it was evident that the UV-Vis absorption was much stronger in the sample with the thienylsilane surface modification than the sample without it. The reason is that the thienylsilane-functionalized layer acted as a favorable nucleation site for electrochemical polymerization and caused more P3HT to grow onto ZnO nanowires. This is consistent with the higher charge density found in the sample with thienylsilane surface modification.

Fig. 7

UV-Vis absorption spectra of ZnO nanowires on an ITO substrate (ITO/ZnO) and electropolymerized P3HT on ZnO nanowires without (ITO/ZnO/P3HT) and with (ITO/ZnO/Si/P3HT) surface modification.