Defect-promoted photo-electrochemical performance enhancement of orange-luminescent ZnO nanorod-arrays

Abstract

Intentionally defect-rich zinc oxide (ZnO) nanorod-arrays were grown from solution by carefully adjusting the concentration ratio of the growth-precursors used followed by various post-deposition thermal treatments. Post-deposition rapid thermal annealing (RTA) at moderate temperatures (350 °C-550 °C) and in various atmospheres was applied to vary the defect composition of the grown nanorod-arrays. It is demonstrated that, intense, defect-related orange emission occurs solely upon RTA around 450 °C and is essentially independent of the atmosphere used. Extensive materials characterization was carried out in order to evaluate the origin of the orange-luminescent defects and what influence they have on the ZnO material properties. It is concluded that the oxygen vacancy-zinc interstitial defect complex (V-Zn) is responsible for the orange luminescence in the prepared materials. A kinetic formation mechanism of the V-Zn complex dependent on the RTA temperature is proposed and shown to be in accordance with the experimental findings. Furthermore it is shown that this bulk deep-level defect could act as a trap state for photo-generated electrons prolonging the charge carrier lifetime of photo-generated holes and therefore improving the charge carrier separation in the material. As a result the photo-current density under simulated sunlight is found to increase by almost 150% over as-grown samples. The potential use of this defective material in applications such as solar water splitting is outlined.

ZnO Nanorod-Arrays as Photo-(Electro)Chemical Materials: Strategies Designed to Overcome the Material's Natural Limitations

“…38 In that paper evidences have been presented that the oxygen vacancy-zinc interstitial defect complex V O -Zn i might be responsible for the observed orange emission. Due to advances in computational studies of ZnO over the past decades, it is now widely accepted that the deep-level oxygen vacancy is thermodynamically stable in its neutral V O and doubly ionized state V O ++ with V O being more abundant due to the lower formation energy.…”

“…38 It is noteworthy that samples prepared on ALD deposited seed layers showed generally less orange emission when compared to nanorodarrays grown on spin-coated seed layers. When co-annealing nanorodarrays prepared on different seed layers and different substrates (glass, gallium nitride, FTO, sapphire) it was found that the substrate does not influence the orange emission.…”

“…The broad feature in the visible is frequently observed for samples grown from solution and the origin of this emission has mainly been attributed to surface adsorbents from the growth solution. 33,34,38 As the bandband recombination of ZnO would be expected to be closer to 380 nm the peak in the near UV is likely caused by contributions of multiple recombination processes (i.e. band-band and recombination over shallow defects).…”

“…38. In short, 0.025 M ZNH, 0.15 M HMTA and 4 drops of 5%wt HCl were dissolved in H 2 O to yield 100 ml of growth solution which was then constantly stirred for 1 h and transferred into a sealable plastic bottle.…”

“…The origin of this emission is not yet fully understood and several different explanations Journal of The Electrochemical Society, 165 (4) H3034-H3044 (2018) H3035 have been debated in the literature. [33][34][35][36][37][38] Since the defect-chemistry will strongly influence the later device performance, careful defectengineering is necessary to accelerate the use of ZnO as photo-active material.…”