Following the growth of ZnO clusters on graphite by in situ X-ray Absorption Near-Edge Spectroscopy

Highlights

• Time-resolved in situ XANES monitored the hydrothermal growth of ZnO on graphite.

• Zinc chemical species evolution: from Zn⁺² ions in solution to solid ZnO

• Diaminopropane as a ZnO assembling and structure-growth control agent

• Designing micro-structured ZnO films: from flower-like to hollow hexagonal rods

Abstract

This work reports on time-resolved in situ X-ray Absorption Near-Edge Spectroscopy (XANES) investigation at the Zn K-edge during the hydrothermal synthesis of ZnO structures on graphite. Such challenging and scarcely explored experiment enabled to monitor the evolution of Zn chemical species during the film deposition, from Zn⁺² ions in solution to solid ZnO. In addition, other experimental techniques were applied to better comprehend the formation and growth of the ZnO. For instance, scanning electron microscopy (SEM) was used to track the morphological changes at different deposition stages, and XRD (X-ray diffraction) provided the long range structural order. XPS (X-ray photoelectron spectroscopy) analysis, excited with synchrotron radiation, allowed probing the composition and electronic structure of the resulting ZnO structures. Diaminopropane concentration turns out to be an essential parameter of the synthesis reaction, and its influence on the ZnO morphology was evaluated.

Introduction

The challenging growth of innovative semiconductor nanostructures has attracted great interest, mainly induced by the search of novel properties and applications. Zinc oxide (ZnO) is considered a potential material in several areas, including photovoltaic devices, light-emitting diodes, photocatalysts, solar cells and biosensors [[1], [2], [3], [4], [5], [6]]. Moreover, it presents attracting physical properties, such as high thermal stability, wide band gap (3.37 eV at room temperature), high electron mobility, associated with a chemical flexibility that enables to combine ZnO with polymers or other organic molecules [[7], [8], [9]]. ZnO is a biocompatible material that is being used to immobilize proteins, opening the possibility to develop biosensors to detect molecules that are important in biological processes (such as glucose, urea, and cholesterol among others) [[10], [11], [12], [13]].

Nanostructured and microstructured ZnO films can be produced with diverse morphologies that are strongly dependent on the preparation methodology and synthesis conditions [14]. Thin (or thick) films can be prepared by forced hydrolysis of Zn⁺² ions. This growth method has many advantages since it is relatively simple with low cost, convenient for scale-up; it can be performed at mild temperatures either at ambient pressure or in a closed vessel and it is compatible with in situ studies [15]. This method is named hydrothermal growth if carried out at temperatures ranging from 100 to 300 °C in an autoclave. The resulting ZnO morphology is directly affected by the reaction parameters, including the chemicals' concentration, temperature, substrate, stirring condition and growth time. One attribute of this deposition method is the production of assorted self assembled morphologies [3,8]. The control and design of nano- and micro-sized ZnO structures have increasingly developed into the main path to further improve ZnO based devices. The use of self assembling growth methods with highly controllable morphology and structure as an alternative to post deposition patterning is an important economical issue.

X-ray absorption spectroscopy (XAS) is an element specific technique that probes the atomic local structure around a selected atom in a matrix [[15], [16]]. The XANES (X-ray Absorption Near-Edge Spectroscopy) investigation of the ZnO film during its preparation, using a chemical approach, enables to investigate the chemical surroundings of the zinc atoms during the formation of the ZnO nanostructures. Nevertheless, it is a challenging experiment and to our knowledge little explored in the literature [15]. To perform in situ measurements it is required to build an apparatus, where the deposition of ZnO occurs on the surface of a low Z substrate, which is surrounded by the aqueous reaction solution that is continuously heated (up to 100 °C). It is desirable the use of a low absorbance substrate and due to the strong absorption of water, the X-ray path must be adjustable [17].

In the present paper we report on the time-resolved in situ Zn K-edge XANES investigations during the ZnO deposition on graphite by the hydrothermal method. This experiment allowed surveying the entire synthesis of ZnO nanostructures on graphite, while quantifying the involved chemical species. In this study, the ZnO film deposition is based on the homogeneous precipitation route [18]. Diaminopropane (DAP), (CH₂)₃(NH₂)₂, is a bidentate ligand that forms [Zn(DAP)₂]²⁺ complexes, which control the free Zn⁺² concentration in solution. The heating promotes the dissociation of the Zn complex leading to a controlled supersaturation of Zn⁺² in the solution. The hexamethylenetetramine (HMT) thermally decomposes yielding formaldehyde and ammonium hydroxide (NH₄OH), and the released OH⁻ promptly reacts with the Zn⁺² species forming ZnO and H₂O. The precipitation of Zn(OH)₂ occurs at slightly basic pH [19].

The in situ XANES enabled to comprehend the effect of the DAP concentration on the ZnO growth kinetics, while scanning electron microscopy (SEM) was used to evaluate its effect on the film morphology at distinct growth stages. The structural and electronic properties of the deposited ZnO were further probed by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy), respectively.