Band offset in calcium hydroxide mediated CaO-ZnO heterointerfaces
Graphical abstract
Introduction
The United Nations estimated that the current global population, as of January 2020, has reached 7.8 billion [1]. This population growth is expected to increase in the coming decades, which will further put tremendous pressure on low-cost energy generation and efficient utilization through optoelectronic devices. Non-renewable energy is limited in nature and will one day be completely depleted. Moreover, hazardous gases are generated upon burning fossil fuels that subsequently contributes to undesired changes in the earth’s climate [2]. Over the past few decades, researchers around the globe have been striving to improve the quality of life by accelerating the development of carbon-neutral technologies [3], [4].
In this context, various types of p-n and n-n semiconductor heterostructures have been employed due to their high reliability, convenient chemical synthesis, and excellent optoelectronic properties [5]. Moreover, it is widely accepted that the dynamics of charge carriers in such SHs are conditioned on potential barrier height realized by e−/h+ pairs at the heterojunction [6]. Besides, much of the research efforts are focused on identifying widely available chemical precursors that can be easily transformed into SHs using well-established routes [7], [8]. Zinc oxide (ZnO) is a highly versatile semiconductor that has applications owing to its unique optical and electronic properties resulting from its wide bandgap of 3.37 eV and chemical robustness. Significantly, it also has the largest exciton and biexciton binding energies of 60 meV assuring more efficient excitonic emission compared to most of the semiconductor oxides [9]. On the other hand, calcium oxide (CaO) having metastable nature with a wide bandgap of 7.7 eV is also widely documented in the literature [10], [11]. In recent years, CaO-ZnO heterostructures have gained immense popularity as an efficient material [12], [13], [14]. Albeit, several such studies outlining improved characteristics as well as in-depth discussion on the plausible mechanism, the role of band alignment in the CaO-ZnO heterostructures was rarely reported and thus, leaves room for further investigation. The chemical and crystallographic differences between the dissimilar materials give rise to the formation of structural and heterointerfacial complexities, hence a thorough understanding of band alignment is of great significance from practical perspectives for any application [15]. Although, band offsets in heterostructures has been a subject of wide-ranging studies [15], [16], [17], to date there is no precise experimental evidence available on the valence band offset (VBO), conduction band offset (CBO) as well as band bending in the case of CaO-ZnO nanoarchitectures using X-ray photoelectron spectroscopy (XPS) and Ultraviolet diffuse reflectance spectroscopy (UV-DRS). In this report, for the first time, a band offset estimation in CaO-ZnO heterostructures is outlined by correlating XPS and UV-DRS measurements. Additionally, the study will try to provide insights by accurately establishing the interfacial properties of the heterojunction that are essential to achieve a good quality solid-state optoelectronic device for environmental applications. The main focus of the current study is only on the energy band alignment which is anticipated to pave the way for further understanding of the electron-hole transfer mechanism originating in different band types (staggered and straddled) in the case of CaO-ZnO heterostructures.
Section snippets
Chemicals
Calcium hydroxide (Ca(OH)2), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), urea (NH2CONH2), and tri-sodium citrate dihydrate (C6H5Na3O7·2H2O) were purchased from Sigma-Aldrich Chemicals Co. The chemical precursors were of analytical reagent (AR) quality and used as received from Sigma-Aldrich. Sartorius Stedim Biotech S.A with Model-Arium 61316 was used for drawing deionized water (18.2 MΩ.cm) at 30 °C throughout the experimentation process.
Calcium hydroxide mediated Zn5(CO3)2(OH)6 transformation to CaO-ZnO
The schematic representation of chemical routes followed
Results and discussion
The evolution of n-n heterointerfaces and its stability as a function of the increase in temperature were evaluated using differential thermal analysis. The thermogravimetric curve of Zn5(CO3)2(OH)6 reaction with 10 wt%. Ca(OH)2 is presented in Fig. 2. At 30–35 °C, dehydration of organic groups is initiated and continues until 110 °C [18]. The distinctive characteristic of this stage includes an endothermic peak at 85 °C attributed to the evaporation of adsorbed hydroxyl groups. The second step
Conclusion
In summary, we reported on the formation of straddled (type II) band alignment in the CaO-ZnO hierarchical heterostructures. The valence and conduction band offsets in Ca-ZMS heterojunction synthesized at 50 °C for 12 h were measured to be ΔEV = −1.37 eV and ΔEC = 2.72 eV, respectively. Moreover, the valence and conduction band offsets in CaO-ZnO heterojunction synthesized by calcining Ca-Zn hydroxyl carbonate mixture at 600 °C for 2 h were measured to be ΔEV = −0.67 eV and ΔEC = 2.02 eV,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
SJ is thankful to the Department of Science and Technology (DST), Ministry of Science & Technology, Government of India for generous funds sanctioned under Mission Innovation IC#3 Carbon Capture Challenge with grant number: DST/TM/EWO/MI/CCUS/27(G)/27(C)/2019. All authors greatly acknowledge the facilities, scientific insights, and technical expertise provided by the Australian Microscopy & Microanalysis Research Facility at the RMIT University, Australia. Analytical support received from Y.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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