Highly Transparent ZnO/Polyvinyl Alcohol Hybrid Films with Controlled Crystallographic Orientation Growth Th. Pauporté* Laboratoire d′ Électrochimie et Chimie Analytique, UMR 7575 Ecole Nationale Supérieure de Chimie de Paris-UniVersité Paris-6, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2310–2315
ReceiVed December 21, 2006; ReVised Manuscript ReceiVed July 9, 2007
ABSTRACT: Electrodeposition has been used as a low temperature, soft method to prepare ZnO/polymer hybrid films. Polyvinyl alcohol (PVA) has been chosen as the water-soluble polymer additive. Films prepared at PVA concentrations ranging between 2 and 10 g L-1 are electrically conducting and contain well-crystallized ZnO. They are luminescent and highly transparent in the visible wavelength range. PVA film content increases continuously with PVA concentration in the deposition bath. It is shown that PVA is not only included in the film but also has a dramatic effect on ZnO properties, from which one can define polymer-assisted ZnO growth strategies. For instance, at high PVA concentration, ZnO luminescence is dominated by the near-edge emission in the UV, revealing a better stoichiometry of ZnO. We also show that it is possible to tune ZnO crystallographic orientation in the film by monitoring the PVA content of the deposition bath. PVA can be eliminated by a subsequent postannealing treatment of the layer. 1. Introduction The development of low-temperature deposition routes such as chemical and electrochemical solution methods for oxide preparation has been an important issue for the last decade with the achievement of high crystallographic and/ or optical-quality thin films.1–6 Solution deposition methods are cost-effective and can be regarded as promising synthetic means for the production of active materials included in optical and electronic devices, as well as in photovoltaic solar cells. Importantly, they also present two other advantages. First, they can proceed at mild physicochemical conditions (pH, temperature, solvent) that are compatible with plastic substrates, which are important from the technological viewpoint for the preparation of flexible and lightweight optoelectronic devices and solar cells.7–10 Second, organic additives can be easily added to the preparation bath and incorporated in the deposited films if the two components present a sufficient affinity to give rise to hybrid films.11,12 The present paper focus on ZnO, which is one of the most studied oxides because of its potential use in many applications such as gas sensors, piezoelectric transducers,13 varistors,14,15 light emitting devices,16 lasers,6,17,18 phosphors,19 thin film20 and nanostructured solar cells,21,22 transparent conducting oxide,7 etc. ZnO is a semiconductor with a room temperature bandgap at 3.37 eV and a large exciton binding energy (60 meV). It can be doped with aluminum, for instance, which yields high conductivity.7,20 ZnO films can be easily prepared by electrodeposition using the reduction of a hydroxide precursor (oxygen, hydrogen peroxide, nitrate ions) as the key reaction.3,23 Moreover, the electrochemical preparation of hybrid ZnO/organic films has been demonstrated in the presence of different molecules such as dyes (xanthene dyes,22,24 phthalocyanines,25,26 etc.), ligands,27 or surfactants.28,29 It has been shown that the interaction of the growing oxide and the molecules results in a change in the structure of ZnO with a grain size decrease and sometimes with crystallographic preferential orientation change.24 Moreover, because of the interaction at the * E-mail:
[email protected].
Figure 1. Variation of current density with time for various PVA concentrations: (a) 0, (b) 2, (c) 10, (d) 12, and (e) 14 g L-1.
molecular level of the two components (organic and inorganic), the films can present improved or novel functionality such as a charge transfer to ZnO after visible light absorption by surface dye molecules.22 These films have been found to act as efficient photoanodes in nanostructured photovoltaic solar cells. In the present paper, we show that the approach can be extended to larger molecules with a high-molecular-weight polymer as an example. The polymer chosen is a polyvinyl alcohol (PVA). PVA is known to interact with Zn2+ in solution30–32 and with ZnO crystals.33,34 ZnO growth is strongly influenced by the presence of this polymer in solution.33 Moreover, optical properties of ZnO can also be deeply governed by interaction with PVA molecules, and it has been reported recently that ZnO nanoparticle/ PVA hybrid nanofibers prepared by electrospinning yield an intense photoluminescent white-light emission under UV excitation.34 It is shown here that PVA interacts with ZnO upon the electrodeposition process to form a mixed film made of ZnO crystals embedded in a polymer matrix. The films are highly transparent and luminescent and remain conducting even at
10.1021/cg0609318 CCC: $37.00 2007 American Chemical Society Published on Web 10/18/2007
Transparent ZnO/PVA Hybrid Films with Controlled Growth
Figure 2. SEM views of ZnO films deposited at various PVA concentrations. (a,b) pure ZnO film, (c) 2, (d,e) 4, (f,g) 10, and (h) 12 g L-1 PVA.
high PVA content. Another important observation is that the ZnO crystallographic orientation can be tuned by monitoring the PVA content of the deposition bath. At least, because of the intimate mixture of both components, we reasonably suggest that the hybrid films must present a better mechanical compatibility with flexible substrates than rigid electrodeposited pure zinc oxide. 2. Experimental Section The deposition solutions were prepared with MilliQ quality water and contained 5 mM ZnClO4 (Alfa Aesar, 99%) and 0.1 M LiClO4 (Fluka 98%). Perchlorate ions were preferred to chloride ions as the supporting anion to avoid film contamination by chloride.20 The solution was heated before to add 2–15 g L-1 PVA (PVA, Alfa Aesar, 87%, high molecular weight, MW 88 000–97 000) under stirring to ensure a good solubilizing of the polymer. The electrochemical cell was placed in a thermoregulated bath and the deposition temperature was 85 °C. The solution was bubbled with O2 gas for 30 min before the deposition experiment was started. The
Crystal Growth & Design, Vol. 7, No. 11, 2007 2311
Figure 3. (A) FTIR spectra of a ZnO/PVA hybrid film deposited from a bath containing 2 g L-1 PVA: (a) as-deposited film, (b) same film postannealed at 400 °C. (B) XRD spectra of films prepared from different PVA bath content (same counting time). The SnO2 substrate reflections are identified by asterisks. Table 1. Analysis of PVA Concentration Effects on Film Composition and Faradaic Efficiency (deposition time 4000 s) [PVA] (g L-1) ttotal (µm) tZnO (µm) ttotal/tZnO Ff (%)
0
2
4
5
6
8
10
2 2 1 0.95
1.5 1.40 1.07 0.76
1.9 1.47 1.30 0.78
1.8 1.25 1.44 0.74
1.7 1.14 1.49 0.69
1.9 1.13 1.68 0.72
1.9 0.95 2 0.67
F-doped SnO2 conducting glass substrates were cleaned under ultrasonics (5 min in acetone, 5 min in alcohol), rinsed abundantly with water, and finally treated for 2 min in a 45% nitric acid solution. The substrates were fixed to a rotating electrode and the deposition was performed at 300 rotations per minutes (rpm). A constant potential of -1 V was applied between the working electrode and a saturated calomel electrode
2312 Crystal Growth & Design, Vol. 7, No. 11, 2007
Pauporté
Figure 4. Variation of the thickness ratio, ttotal/tZnO, and F with PVA concentration in the deposition bath. reference (SCE) with an Autolab PGSTAT30 potentiostat. A zinc wire was used as a counter electrode. The deposition time was 4000 s. The film specular optical transmission spectra were measured with a two beam Varian Cary 100 spectrophotometer. The reference was a bare sheet of conducting glass. The as-deposited film photoluminescence was measured at room temperature with an excitation source at 266 nm provided by a YAG:Nd quadrupled frequency laser source. The emission was analyzed using a HR250 monochromator (Jobin-Yvon) coupled with an UV-enhanced intensified charge coupled device (ICCD) (Princeton Instrument). The FTIR spectra of ZnO/PVA films deposited on silicon wafers were recorded with a Nicolet 5700 FTIR instrument in a transmission mode. The postannealed film was heated several hours at 400 °C in air. The film morphology was observed by scanning electron microscopy (SEM) (Stereoscan 440 from Leica). The zinc content was determined by ICP-AES after complete dissolution of a film of known surface area in 1 mL of 10% HCl. The X-ray diffraction (XRD) experiments were carried out with a Siemens D5000 type diffractometer, using a Co KR1 radiation (λ ) 1.7889Å) and a back graphite monochromator. The diffraction pattern was scanned by steps of 0.02° (2θ) between 20 and 50°.
3. Results and Discussion 3.1. Deposition Current Transients. The films were deposited at a constant applied potential, and the variation with time of current density, j, was recorded at different PVA concentrations and compared to a pure ZnO curve (Figure 1). Steady state j of pure ZnO is about 0.7 mA cm-2. It remains high, with a value of 0.54–0.55 mA cm-2, after PVA addition ranging between 2 and 10 g L-1. In the presence of 12 g L-1 PVA, the current density is slightly lowered at 0.45 mA cm-2. A dramatic fall is observed at 14 g L-1 PVA and above. These curves demonstrate that the deposited films are good electrical conductors when the PVA concentration does not exceed 12 g L-1, because the current density collected at the electrode is kept at a significant value. Another interesting feature of the curves in Figure 1 is the presence of a cathodic wave at low deposition time (
