Nanostructured ZnO Thin Films for Optical, Electrical, and

Nov 26, 2012 - Department of chemistry, Faculty of Science, University of Malaya, Kuala Lumpur-50600, Malaysia. ⊥. School of Chemistry and Materials...
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Nanostructured ZnO Thin Films for Optical, Electrical, and Photoelectrochemical Applications from a New Zn Complex Muhammad Shahid,*,† Mazhar Hamid,† Asif A. Tahir,‡ Muhammad Mazhar,§ Mohammad A. Malik,⊥ and Madeleine Helliwell⊥ †

Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan Department of Chemistry, Liverpool University, Liverpool, L69 7ZD, U.K., LE11 3TU, U.K. § Department of chemistry, Faculty of Science, University of Malaya, Kuala Lumpur-50600, Malaysia ⊥ School of Chemistry and Materials Science Centre, The University of Manchester, Oxford Road, Manchester, M13 9PL U.K. ‡

S Supporting Information *

ABSTRACT: New hexanuclear zinc complex, Zn6(OAc)8(μ-O)2(dmae)4 (1) (OAc = acetato, dmae = N,N-dimethyl aminoethanolato) has been synthesized and characterized by its melting point, elemental analysis, Fourier transform infrared spectroscopy, atmospheric-pressure chemical-ionization mass spectrometry, thermal gravimetric analysis, and single crystal X-ray analysis. The complex (1) crystallizes in the monoclinic space group C2/c. The high solubility of complex (1) in organic solvents such as alcohol, THF, and toluene and low decomposition temperature as compared to Zn(OAc)2 make it a promising single source candidate for the deposition of nanostructured ZnO thin films by aerosol-assisted chemical vapor deposition. Films with various nanostructures, morphology, and crystallographic orientation have been deposited by controlling the deposition temperature. The deposited films have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray analysis. The optical characterization of ZnO films deposited on the FTO substrate show a direct band gap of 3.31 eV, and the photoelectrochemical study revealed that the photocurrent onset is at about −0.32 V, whereas no photocurrent saturation was observed. The I−V measurements designated the deposited films as ohmic semiconductors.

1. INTRODUCTION

Commercially, dimethyl zinc and diethyl zinc are widely employed organometallic precursors in CVD for large-scale coatings. Both compounds are pyrophoric and react violently with air, water, or alcohol even at low temperature and are costly.15,16 Moreover, these compounds are made from nonoxygenated ligands; therefore, an additional source of oxygen is required to convert them into zinc oxide, while other complexes of oxygen containing ligands such as metal carboxylates and β-diketonates have low solubilities in nonpolar solvents.10 Thus, prerequisites for the developing technologies are the designing of better precursors with high solubilities, low decomposition temperature and containing all the elements of interest in the target material. Therefore, we have synthesized the hexanuclear complex Zn6(OAc)8(μ-O)2(dmae)4, by bridging the individual units with aminoalcohols in the form of an organic sheath making it more soluble in nonpolar solvents. We also report here the deposition of good-quality thin films of ZnO at relatively low temperatures from this new precursor. Thin film electrodes are also studied for their optical, electrical, and photoelectrochemical properties.

The fabrication of semiconducting metal oxide thin films in general and zinc oxide in particular has been a subject of special interest due to its widespread applications in broad areas of industry and engineering.1 Zinc oxide is chemically stable and crystallizes in the wurtzite structure with zinc atoms occupying half of the tetrahedral sites. Properties such as high carrier mobility and lower photoresistivity of zinc oxide make it a potential candidate for optoelectronic materials used in detectors,2 solar cells,3 gas sensors4, and short-wavelength UV lasers. Transparent thin films of ZnO have their applications as a substitute for expensive tin-doped In2O3 in displays and transducers.5,6 Various thin film deposition techniques such as metal−organic chemical vapor deposition (MOCVD),7−9 aerosol assisted chemical vapor deposition (AACVD),10 radio frequency magnetron sputtering,11 reactive sputtering12 and spray pyrolysis 13 have been adopted. The progressing technologies are always looking for a single source deposition technique that can deliver the target material to the substrate with greater control over the stoichiometry of the desired phase. Therefore, keeping in view all the requirements for the technological applications, we have chosen a single source precursor, with all the built-in elements, soluble in common organic solvents to deposit ZnO thin films by AACVD. The AACVD technique for fabricating thin films has the advantages of low temperature of deposition, better compositional uniformity at the molecular level, and conformal coverage in the case of film deposition.14 © 2012 American Chemical Society

2. EXPERIMENTAL SECTION All manipulations were carried out under an atmosphere of dry argon gas using Schlenk tube and glovebox techniques. All the Received: Revised: Accepted: Published: 16361

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two-neck round-bottomed flask was connected via rubber tubing to a quartz reactor loaded with 2.5 cm × 1 cm substrates inside a tube furnace, and a flow of argon gas was regulated using a Platon flow gauge on the other neck. The flask was fitted on an ultrasonic humidifier equipped with a piezoelectric modulator for atomization of the precursor solution to tiny droplets of aerosol that were ultimately transferred by the carrier gas into the reactor chamber. The glass substrates were washed with concentrated nitric acid, followed by washings with deionized water several times and then oven-dried at 100 °C prior to deposition. Deposition was carried out at four different temperatures, that is, 250, 325, 400, and 475 °C at a constant argon flow rate of 130 mL/min on substrates for 1.5 h. 2.4. Thin Films Characterization. Phase and crystallinity of the films was measured by a Bruker AXS D8 diffractometer using monochromatic Cu−Kα radiation. The morphology of films was determined by a FEG-SEM Philips XL30 electron microscope. Samples were carbon coated before observation and EDAX-DX4 was used to determine film composition. The thickness of the films was measured with a Dektak 8 Stylus profilometer. The current−voltage characteristics of thin films were measured with Jandel Voltmeter, model RM3, by the fourprobe method. 2.5. Optical and Photoelectrochemical Characterization. Diffused reflectance spectra of the films were recorded using the dual beam Perkin-Elmer Lambda 35 UV−vis spectrophotometermeter. The reflectance was measured as a function of wavelength in the 300−800 nm range. The current−voltage (I−V) characteristics of the ZnO electrodes were recorded in 3-electrode configuration. The current− voltage measurement was carried out in 1 M Na 2 SO 4 electrolyte, Ag/AgCl/3 M KCl as the reference electrode, a platinum wire as the counter electrode and ZnO deposited on FTO as a working electrode. A potentiostat (AutoLab, PGSTAT12) was used to control the potential of the working electrode. The steady state I−V curves were measured under illuminated and dark conditions. For PEC measurements illumination occurred from the electrode side. The illumination source was an AM 1.5 class A solar simulator (Solar Light 16S300 solar simulator), at 100 mW cm−2 light intensity, calibrated by a solar pyranometer (Solar Light Co., PMA2144 Class II).

solvents and reagents were purchased from Aldrich. Solvents were rigorously dried on sodium benzophenoate, while N,Ndimethylaminoethanol (dmaeH) was dried by refluxing over K2CO3 for 10 h, and distilled immediately before use. The elemental analysis was performed using a CHN analyzer LECO model CHNS-932. The melting point was determined in a capillary tube using an electrothermal melting point apparatus, model MP.D Mitamura Riken Kogyo (Japan). The FT-IR spectrum was recorded on a single reflectance ATR instrument (4000−400 cm−1, resolution 4 cm−1). The mass spectrum was recorded on a Kratos concept IS instrument. The TGA measurements were carried out by using a Seiko SSC/ S200 thermal analyzer at a heating rate of 10 °C/min under N2 gas flow. 2.1. Synthesis of [Zn6(OAc)8(μ-O)2(dmae)4]. A 0.21g portion (2.36 mmol) of dmaeH was added dropwise to a suspension of 0.8 g (3.65 mmol) Zn(OAc)2·2H2O in 20 mL of THF at room temperature. After being stirred for 2 h, the mixture was heated at 50 °C for 1 h with continuous stirring. Unreacted Zn(OAc)2·2H2O and other byproducts were eliminated by cannula filtration and the solution was concentrated to give a transparent crystalline product after 15 days at −10 °C, yield 80% (0.63g), mp 105 °C. Anal. Calcd for C36H80N4O23Zn6: C, 32.53; H, 6.07; N, 4.21. Found: C, 31.89; H, 5.85; N, 4.05%. FT-IR(KBr, cm−1): 2756m, 1601s, 1458m, 1406s, 1370m, 1340s, 1079w, 1022m, 950w, 898w, 784w, 676m, 619w. APCI-MS (positive scan) m/z: 1236.7 [M− (OAc)(O) 2 ] + , 1087.6 [Zn 6 (dmae) 3 (OAc) 7 (O)] + , 997.2 [Zn6(dmae)2(OAc)7(O)]+, 777.2 [Zn4(dmae)3(OAc)4(O)]+, 668.2 [Zn2(dmae)2(OAc)6(O)]+, 645.3 [Zn4(dmae) 2(OAc)3(O)2]+, 547.5 [Zn3(dmae)2(OAc) 3]+, 433.7 [Zn 3 (dmae)(OAc) 2 (O) 2 ] + , 368 [Zn 2 (dmae)(OAc)2(O)2]+, 340.8 [Zn2(OAc)3(O)2]+, 245.5 [Zn(dmae)(OAc)(O)2]+, 183.5 [Zn(OAc)2]+, 151.9 [Zn(dmae)]+. TGA: 78−137 °C (5 % wt. loss), 138−298 °C (37% residue mass). 2.2. X-ray Crystallography. Single crystal X-ray data were collected on a Bruker Smart Apex CCD diffractometer using Mo−Kα radiation. The structure was solved by Direct Methods17 and refined by full-matrix least-squares on F2.17 The asymmetric unit contains half the molecule, with the other half generated by inversion, together with half a disordered THF molecule. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Hydrogen atoms were placed in calculated positions, assigned isotropic thermal parameters, and allowed to ride on their parent carbon atoms; those bonded to the disordered THF molecule were omitted. Crystallographic and structure refinement data for complex (1): Empirical formula, C32H64N4O22Zn6.C4H8O; F.wt., 1321.26; temp = 100 (2) K, λ = 0.71073 Å, monoclinic, space group C2/ c, a = 3.8827(19), b = 11.9249(10), c = 21.0028(17) Å, β = 114.7890(10)°, V = 5430.4(8) Å3, Z = 4, ρcalcd = 1.616 mg m−3, μ = 2.685 mm−1, 20987 reflections collected, 5531 unique (Rint = 0.0366). Refinement was by full-matrix least-squares on F2; goodness-of-fit on F2 was 1.047. Final R indices where I > 2σ (I) are R1 = 0.0318 and wR2 = 0.0740. The R indices (all data)a are R1 = 0.0427 and wR2 = 0.0779. The largest difference peak and hole are 0.773 and −0.531 e Å−3, respectively. 2.3. Deposition of Thin Films. Zinc oxide thin films were deposited on a soda glass and fluorine-doped tin oxide (FTO) coated substrates using a self-designed aerosol-assisted chemical vapor deposition assembly described elsewhere.18 In a typical experiment, 0.1 g of precursor dissolved in 15 mL of THF in a

3. RESULTS AND DISCUSSION Zinc acetate dihydrate, Zn(OAc)2·2H2O, completely dissolves in stoichiometric amounts of N,N-dimethylaminoethanol, dmaeH, in THF solution to give complex (1) in quantitative yield as shown in chemical eq 1. 6Zn(OAc)2 · 2H 2O + 4dmaeH THF

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn6(OAc)8 (μ‐O)2 (dmae)4 + 4HOAc room temp

+ 10H 2O + 2H 2

(1)

The completion of the reaction can be observed from the disappearance of suspension of Zn(OAc)2·2H2O in the reaction flask. The complex isolated in 80% yield melts at 105 °C with decomposition and was further characterized by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), mass spectrometry, and single crystal X-ray analysis. In FT-IR spectrum, the characteristic absorptions of carboxylate appeared at 1601 cm−1 for νasy(CO2) and 1406 cm−1 for νsy(CO2). The difference Δ = 16362

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νasy(CO2) − νsy(CO2) suggests a dominant chelating or chelating-bridging behavior for the acetate ligands.19,20 The APCI-MS spectrum of the complex (1) does not show a molecular ion peak, while the complex breaks down under experimental conditions to give multiple fragments of different m/z ratios, with the heaviest fragment observed being [M− (OAc)(O)2]+, thus indicating the instability of the complex under the MS conditions. The molecular structure for compound (1) depicted in Figure 1, shows that the complex (1) consists of two inversion

consists of oxygen atoms [O(3), (5)] of two bridging acetate ligands, a triply bridged μ3-oxygen [O(1)], oxygen [O(2)], and nitrogen atoms [N(1)] of a chelating-bridging dmae group. The geometry of Zn(1) is not perfectly square pyramidal due to one of equatorial substituent groups, the doubly bridged O(2) of dmae ligand, being displaced toward O(3) and away from O(5), giving rise to an angle O(3)−Zn(1)−O(2) being reduced from the ideal 180° to 146.10(8)°, and O(5)− Zn(1)−O(2) being enlarged from 90° to 103.82(9)°. The coordination environment of Zn(2) is distorted octahedral, with a ZnO5N set of ligating atoms, composed of a triply bridged μ3-oxygen [O(1)], connecting Zn(1), Zn(2), Zn(3), three oxygen atoms [O(4), O(6), O(8)] of bridging acetate groups and two donor atoms [O(7), N(2)] of chelating dmae ligand. The overall geometry around Zn(3) is almost tetrahedral, with a ZnO4 core, generated by a triply bridged μ 3 -oxygen [O(1)], an oxygen atom [O(10)] of the monodentate acetate ligand, one oxygen atom [O(9)] of a bridging acetate and another μ2-oxygen [O(2)] of a dmae ligand that links Zn(3) and Zn(1) of two asymmetric units. The Zn−O bond distances (Table S1, see Supporting Information) significantly increase with the increase in coordination number at zinc, that is, Zn(3) 1.931(2)− 1.971(2); Zn(1) 1.982(2)−2.0507(19); Zn(2) 2.047(2)− 2.217(2) Å, while all Zn−N bond distances in the complex are similar irrespective of the coordination number at the metal center which is in good agreement to other similar complexes.21−23 3.1. Pyrolysis Studies. The thermal behavior of the complex (1) has been studied by thermogravimetric analysis and differential thermogravimetric measurements in the temperature range of 50−500 °C under dynamic atmosphere of nitrogen gas at 130 mL/min with a scanning rate of 10 °C/ min. The TG/DTG curve (Figure 3) of the complex shows

Figure 1. A view of labeled ORTEP drawing of the molecular structure of [Zn6(OAc)8(μ-O)2(dmae)4] (1). Thermal ellipsoids are drawn on 50% probability level. The disordered THF molecule is omitted for clarity.

related asymmetric units, each having three zinc atoms interconnected by two μ2-oxygen atoms of the dmae ligands. The six metal atoms are linked together by four oxygen atoms, two of which are μ3-O atoms bridging the three metal centers in each asymmetric unit and the other two oxygen atoms belong to the chelating-bridging dmae groups. Both ligands span a range of coordination modes. The acetate groups are either monodentate [O(10)], or bridging two metals through [O(5), O(6); O(8), O(9)]. As far as dmae ligands are concerned, they are chelating [O(7), N(2] and chelating-bridging two metals [O(2), N(1)]. In each asymmetric unit, all the three zinc atoms have different geometric environments with Zn(1) square pyramidal, Zn(2) octahedral, and Zn(3) tetrahedral coordination spheres (Figure 2) which are discussed in detail below. The environment around Zn(1) atom can be described as a distorted square pyramidal with the ligation set ZnO4N which

Figure 3. TGA and DTG curves for complex (1) recorded at a heating rate of 10 °C/min under flowing atmosphere of nitrogen.

two-step weight loss, the first step beginning at 78 °C and completed at 137 °C with a weight loss of approximately 5% which may be due to the loss of solvate molecules. The second and major step of weight loss starts at 138 °C and is completed at 298 °C with a maximum weight loss of 58%, resulting in a residue of 37% of the initial weight of the complex (1). The residual mass (37%) is close to the expected value 36.7%, calculated for complete conversion of complex (1) to ZnO

Figure 2. Geometric spheres of (1), (a) distorted square pyramidal Zn(1), (b) octahedral Zn(2) and (c) tetrahedral Zn(3). 16363

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material. The differential curve of TG shows the changes in weight at 118 and 227 °C, respectively. Further heating of the residue to 500 °C did not bring any change in weight indicating complete conversion of complex (1) to a stable phase of zinc oxide. The zinc atom in complex (1) is coordinatively saturated by oxygen atoms of acetate and dmae ligands, thus eliminating the need of extraneous oxygen to decompose complex (1) in to zinc oxide. A detailed thermal decomposition mechanism of related zinc complexes is well depicted, and it has been proved experimentally that the course of thermal decomposition is unaffected by the reaction atmosphere, either oxygen or helium.24−26 3.2. Deposition and Characterization of Thin Films. Considering the decomposition behavior from the TGA curve the deposition was carried out at four different temperatures of 250, 325, 400, and 475 °C as described in Experimental Section. At all four temperatures, thin films having good adherence to the substrate as verified by the “scotch-tape test” were produced. Moreover, the films reflected light in multishaded colors depending upon the crystallite size and thickness of the deposited films. Phase and crystallinity of ZnO films deposited from complex (1) was investigated by X-ray diffraction (XRD) measurements. The XRD patterns (Figure 4) of films deposited on soda glass substrates at temperatures

Figure 5. Powder X-ray diffractograms of ZnO films deposited from complex (1) on FTO glass substrate at 400 °C; the asterisk (∗) corresponds to SnO2 from FTO.

in preferred orientation clearly shows a link between the structure of crystallites in the films and the nature of the substrate. The crystallite size of the electrode was estimated by using the Scherrer equation (eq 2).28 τ=

kλ β cos θ

(2)

where k = 0.9 (assuming the crystallites are spherical), λ is the wavelength of the X-rays, β is the full width at half-maximum of the reflection and θ is the Bragg diffraction angle of the reflection. The crystallite size estimated for the reflection at 2θ ≈ 36.4° was found 20.4 nm. Analysis of the XRD diffractograms found the lattice constants of the ZnO electrode to be a = 3.160 Å and c = 5.063 Å with an axial ratio (c/a) of 1.602. These values are slightly smaller than the lattice parameters of an ideal wurtzite ZnO crystal (c/a =1.633).29 The SEM topographic images in Figure 6 show that the films deposited at different temperatures have different morphology and particle size. Films deposited at 250 °C gave relatively small, poorly defined individual particles that are in the range 0.1−0.3 μm in size, while relatively large individual grains, 0.2− 0.7 μm, were obtained from the films deposited at 475 °C. At growth temperatures of 325 and 400 °C, films are more compact and smooth with homogeneously dispersed grains falling in the size range given in Table 1. The particles with spherical appearance have good orientation and well-defined grain boundaries. In contrast, films deposited from THF solution onto the crystalline FTO surface display a more regular appearance as densely packed rectangular shaped particles (Figure 7) which are evenly distributed. We believe that the formation of well developed isolated crystalline particles is due to the homogeneous decomposition pathway in AACVD.30 In this case the ZnO particles are formed in the gaseous state by the decomposition of precursor and subsequently deposit on a substrate surface. The formation of a ZnO well-defined nanostructure instead of agglomeration of nanoparticles is also well supported by the deposition of ZnO thin films using single source precursors.30−33 These observations reveal that morphology of films also depends on deposition temperature as well as other variables such as nature of substrate and solvent, concentration of solution, and flow rate of the carrier gas.

Figure 4. Powder X-ray diffractograms of ZnO films obtained from complex (1) at (a) 250, (b) 325, (c) 400, and (d) 475 °C.

(250−475 °C) point toward their polycrystalline nature, and the peak positions correspond to those reported in literature for ZnO.27 Further this study indicates that 400 °C is a most suitable temperature for the growth of highly crystalline ZnO thin films from precursor (1) by AACVD. The same is proven later on by morphological studies of the films where the best resolved images with densely packed and relatively smaller crystallite size are obtained from the films deposited at 400 °C. Keeping all the deposition parameters in view, ZnO photoelectrodes were fabricated at 400 °C on FTO-coated glass substrates (Figure 5). The intensity of all the SnO2 reflections corresponding to the FTO substrate is marked as an asterisk (∗). Strong and weak reflections corresponding to the (100), (002), (101), (102), (110), (103) and (112) planes were assigned to hexagonal ZnO. It is evident that ZnO films coated on FTO glass substrates have preferred orientation along (101) as compared to those deposited on soda glass substrate, which is oriented along (002) direction. This change 16364

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Figure 6. SEM micrographs of films deposited from complex (1) at (a) 250 (b) 325 (c) 400 and (d) 475 °C on soda glass.

The energy dispersive X-ray analysis (Figure 8) detected zinc and oxygen as the only elements present onto the films other than constituent elements present in the substrates; however, the exact ratio of atomic zinc to oxygen could not be determined due to inclusion of extra oxygen in the results from oxides present in the substrates. The EDX analysis also ensures the clean decomposition of the complex, eliminating the possibility of incorporation of carbon impurities either from organic parts of the complex or the solvent, which is difficult to avoid in previously, reported organozinc precursors such as dimethyl zinc and diethyl zinc.34

Table 1. Numerical Data for the Particle Size, Thickness, and Electrical Properties of ZnO Films Deposited at Different Temperatures for Complex (1) deposition temp (°C)

particle size (μm)

film thickness (μm)

250 325 400 475

0.1−0.3 0.1−0.4 0.2−0.5 0.2−0.7

0.71 1.06 1.57 1.55

resistivity (Ω cm)

1.8

Figure 7. SEM image of film deposited by the AACVD of complex (1) onto FTO-coated glass substrate at 400 °C. 16365

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Figure 8. EDX spectrum of films deposited from complex (1) onto FTO coated glass substrate at 400 °C.

The variation of the ZnO film’s thickness as a function of growth temperature is shown in Figure 9. The plot shows three

(1 − R ∞)2 K = ≡ F(R ∞) S 2R ∞

(3)

where S represents the portion of light scattered per unit of vertical length, K represents the portion of light absorbed per unit of vertical length, R∞ is the reflectance of an infinitely thick sample relative to a standard and F(R∞) is the remission function of the electrode. ZnO is known to have direct optical band gap transitions, therefore, the band gap and the absorption threshold can be evaluated by plotting [F(R∞) hν]2 versus hν for direct band gap transitions (inset of Figure 10).35,36 The optical band gap estimated was 3.31 eV for the deposited ZnO electrode.

Figure 9. Variation of ZnO film thickness as a function of substrate temperature with a dynamic nitrogen flow rate of 130 mL/min.

distinct temperature regimes. At low temperature, the thickness of the film strongly depends on substrate temperature, indicating that kinetic factors are predominant. In this region, the thickness of film is controlled by the decomposition of the precursors. An increase in deposition rate is observed at a temperature range of 340−432 °C, suggesting an increase in the mass transfer via diffusion of species through the boundary layer. Finally, above 432 °C, the film thickness decreases with increasing temperature because of increased depletion of reactants by the reaction at the reactor walls. 3.3. Optical and Photoelectrochemical Characterization. The optical band gap of ZnO film deposited from complex (1) was estimated by measuring UV−vis diffused reflectance spectrum and films were found to absorb primarily in the near to far UV region and displayed the highest degree of reflection in the visible light region. The measurement of diffused reflectance spectrum was preferred over absorption spectra to avoid light scattering effect expected in samples with high aspect ratio of nanostructures.35 The optical absorption threshold of ZnO electrode was estimated by applying the Kubelka−Munk model to the diffuse reflectance data. The equation for the Kubelka−Munk model is shown below.35

Figure 10. UV−vis diffused reflectance spectrum and optical absorption threshold determination from plot of [F(R∞)hv]2 vs hv for ZnO electrode.

The photoelectrochemical properties of the ZnO electrode were investigated in 3-electrode mode under AM 1.5 illumination. The steady-state I−V plots in Figure 11 show the dependence of photocurrent density on the applied potential. The photocurrent onset of the ZnO electrode is at about −0.32 V vs Ag/AgCl. The dark current remains low up to approximately 1.2 V. The I−V characteristics show that the 16366

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curve in Ω and d is the film’s thickness (Table 1), describing the semiconducting nature of the films.

4. CONCLUSIONS New hexanuclear zinc complex (1) was synthesized by direct reaction of zinc acetate dihydrate and dmaeH in THF solution and characterized by various analytical techniques. The complex was applied as a single source precursor for fabrication of nanostructured crystalline ZnO thin films on soda and FTOcoated glass substrates using the AACVD technique. The growth rate of ZnO was found to increase with an increase in deposition temperature up to 400 °C then decreases. The morphology and orientation of crystallites strongly depend on the nature of the substrate. I−V measurements of the films indicate their ohmic semiconductor behavior. Optical and photoelectrochemical studies show that the ZnO electrodes display high reflectance in the visible light region due to scattering and photodiodic behavior. It is observered that under dark current and illumination conditions, they are suitable candidates for photoelectrochemical as well as the n-type wide band gap material for solar cell applications.

Figure 11. Steady state I−V curves of films deposited from complex (1) under light and dark conditions.

electrodes displayed the typical diode behavior in the dark. Even under illumination, the cathodic forward current of the electrode appears to show ideal photodiode properties. However, as the applied potential was increased beyond the photocurrent onset, the system diverges from this model. The I−V measurements displayed a sigmoid shaped curve, because an “over-potential” had to be applied before a significant rise in photocurrent could be observed. This effect may be due to surface recombination in the depletion layer of the electrode.37 Under illumination ZnO is known to undergo oxidative dissolution of the semiconductor in aqueous solutions when under anodic polarization. As a result, the film shows no photocurrent saturation in the reverse bias region. 3.4. Electrical Characterization. From the graph of voltage−current shown in Figure 12 for the ZnO films

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ASSOCIATED CONTENT

S Supporting Information *

Table showing selected bond lengths and angles and separate CIF file for complex 1. This material is available free of charge via the Internet at http://pubs.acs.org. CDC No. 654596 contains the supplementary crystallographic data for complex 1. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax, (+44) 1223-336-033; or E-mail, deposit@ ccdc.cam.ac.uk.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +92 322 5111064. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S., M.H. and M.M. are thankful to HEC Pakistan for financial support. M.M. is also grateful for financial support from University of Malaya through HIR/UMRG Grant No. UM.C/625/1/1/6 and RG097/10 AET.

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REFERENCES

(1) Pearton, S. J.; Norton, D. P.; Ip, K.; Heo, Y. W.; Steiner, T. Recent advances in processing of ZnO. J. Vac. Sci. Technol. B 2004, 22, 932−948. (2) Liang, S.; Sheng, H.; Liu, Y.; Hio, Z.; Lu, Y.; Shen, H. ZnO schottky ultraviolet photodetectors. J. Cryst. Growth 2001, 225, 110− 113. (3) Keis, K.; Vayssieres, L.; Lindquist, S. E.; Hagfeldt, A. Nanostructured ZnO electrodes for photovoltaic applications. Nanostruct. Mater. 1999, 12, 487−490. (4) Golego, N.; Studenikin, S. A.; Cocivera, M. Sensor photoresponse of thin-film oxides of zinc and titanium to oxygen gas. J. Electrochem. Soc. 2000, 147, 1592−1594. (5) Addonizo, M. L.; Antonaia, A.; Cantele, G.; Privato, C. Transport mechanisms of r.f. sputtered Al-doped ZnO films by H2 process gas dilution. Thin Solid Films 1999, 349, 93−99.

Figure 12. I−V curve of zinc oxide film deposited from complex (1) at 400 °C.

deposited on bare soda glass substrate at 400 °C, the I−V curve is linear, thus the films prepared are found to be ohmic. From the slope of the curve, the resistance is 2.5 × 103 Ω for the films deposited at 400 °C. An accurate reading could not be obtained for the films deposited at other temperatures because of the fluctuation in the readings. The resistivity of the films at 400 °C was found to be 1.8 Ω cm which was calculated using the relationship ρ = 4.532(V/I) × d, where ρ is the resistivity in Ω cm, 4.532 is the instrument constant, V/I is the slope of 16367

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