Elsevier

Journal of Electroanalytical Chemistry

Volume 763, 15 February 2016, Pages 149-154
Journal of Electroanalytical Chemistry

Studies on the structural, morphological, optical and electrical properties of Al-doped ZnO nanorods prepared by electrochemical deposition

https://doi.org/10.1016/j.jelechem.2015.12.037Get rights and content

Highlights

  • High quality undoped and Al-doped ZnO thin films were prepared using a simple electrochemical rout.

  • Al doping increases the band gap energy values.

  • Al-doped ZnO thin films exhibits aligned nanorods arrays for sample obtained at Al  1 at.%.

  • The better photocurrent properties are found for the film with 2% Al doping.

  • The addition of the Al changes the nucleation and growth mechanism of ZnO from progressive to instantaneous.

Abstract

A study about the growth mechanism of zinc oxide (ZnO) nanorods and Al-doped zinc oxide (AZO) electrodeposited from the reduction of hydrogen peroxide in zinc chloride solutions was reported. The variations of the electrochemical, morphological, structural, optical and photoelectrochemical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. X-ray diffraction spectra demonstrate that films crystalline with the Würtzite structure with preferential (002) crystallographic orientation having c-axis perpendicular to the substrate. The AZO films obtained forms aligned hexagonal nanorods and depending on the increasing aluminium concentration, the surface morphologies of the films are changed. As Al concentration increased the optical band gap was also found to be increase from 3.31 to 3.45 eV and in the carrier densities from 1.06 × 1018 to 2.91 × 1018 cm 3 are observed. The blue shift in the band gap energy was attributed to the Burstein-Moss effect. Changes in the photocurrent response are also discussed in the light of Al doping. The amplitude of the photocurrent generated increases steadily from undoped ZnO to the AZO film (2 at.%) going from 21 to 58 μA at 1.0 V.

Introduction

Zinc oxide (ZnO) is a wide band gap (3.37 eV) semiconductor with large exciton binding energy (60 meV) at room temperature, and has been attracting increasing attentions in many fields [1], [2], [3]. Researchers found that the presence of intrinsic defects such as oxygen vacancies and interstitial atom leads to ZnO with high resistivity. The electrical and optical properties of ZnO are changed by Al doping. The optical band gap is widened in proportion to the Al-doping concentration [4]. The electrical conductivity, charge carrier density, and mobility are substantially improved in Al-doped ZnO (AZO) [5]. Usually, Al3 + enters into ZnO lattice and replaces the Zn2 + producing an extra carrier, thus improves the electrical conductivity [6].

AZO coatings exhibit high transparency and low resistivity and these materials are suitable for fabricating transparent electrodes in solar cells, gas sensors and ultrasonic oscillators [7], [8], [9]. They are also found in applications such as surface acoustic devices, optical waveguides and micro-machined actuators. The AZO is one of the best alternative materials to tin oxide and indium tin oxide not only in terms of cost but also in terms of non-toxicity, and possess comparable electrical and optical properties [10]. It was shown that the optical and electrical properties of AZO thin films could be obviously improved by optimized deposition conditions and doping [11] [12]. To fabricate high quality AZO thin films, various growth methods including sputtering [13], pulsed-laser deposition [14], chemical vapour deposition [15] were extensively studied. However, the equipment or process of these techniques is relatively complex and they are also not suitable for preparing large area films. Electrodeposition is a versatile method for the growth of one-dimensional ZnO nanorods arrays with features of simplicity, simple process, cost-efficiency, large-area deposition, good adhesion and high quality at relative low synthesis temperatures. For the electrodeposition of ZnO many efforts have been reported [16], [17], [18], [19], [20], [21]; however, by this approach there are few researches to focus on elaboration of doped ZnO nanorods [22].

Consequently, in this paper, Al-doped ZnO nanorods arrays were prepared on ITO substrates by electrodeposition method from aqueous solution of chloride: zinc chloride and aluminium chloride. Previous work has focused on the use of molecular oxygen (O2) or nitrate (NO3) as precursors of hydroxide (OH) for electrodeposition of AZO. Different from their investigation, the influences of dopant concentration using the hydrogen peroxide (H2O2) as precursor on the structural, morphological, electrical, optical and photoelectrochemical properties were investigated.

Section snippets

Experimental

The electrodeposition procedure was performed in a classical three-electrode electrochemical cell using a solution containing 5 mM ZnCl2, 0.1 M KCl as supporting electrolyte and 5 mM H2O2 as OH precursor [23]. Different concentrations of the doping compound AlCl3 in the electrolyte are explored: 0, 0.5, 1, 2 and 4 at.%, respectively. For clarity, the doped proportion of Al3 + is labelled as AZO (x%), where x = Al/(Al + Zn) (at.%). The substrate used was glass sheets coated with indium tin oxide (ITO).

Nucleation and growth mechanism studies

The cathodic electrodeposition of ZnO thin films from a chloride solution occurred according to an electrochemical-chemical process beginning via the electrochemical reduction of H2O2 according to the Eq. (1). The generated hydroxide ions then react chemically with Zn2 + ions in the solution to form Zn(OH)2 at the cathode which is kinetically favored at first, and subsequently Zn(OH)2 is spontaneously dehydrated and forms the seeds permitting the growth of the zinc oxide film according to the

Conclusion

Al doped ZnO thin films were prepared on glass substrates by the electrochemical method. Thin films have been investigated using scanning electron microscopy, X-ray diffraction, UV–vis transmittance, electrochemical impedance spectroscopy and photocurrent spectroscopy. The addition of the Al changes the nucleation and growth mechanism of ZnO from progressive to instantaneous. X-ray diffraction patterns confirmed that all films of undoped and Al doped ZnO have polycrystalline nature correspond

Acknowledgments

We would like to thank Pr. Federico Rosei and Dr. Riad Nechache of Energy Materials Telecommunications centre of INRS Research University in Quebec for their assistance in acquiring the SEM images.

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