Band offset in calcium hydroxide mediated CaO-ZnO heterointerfaces

https://doi.org/10.1016/j.mseb.2020.115005Get rights and content

Highlights

  • •

    Zinc hydroxide carbonate conversion to ZnO using Ca(OH)2 at 50 °C is demonstrated.

  • •

    Nanostructured CaO leads to formation of type II (staggered) band alignment in CaO-ZnO.

  • •

    CaO-ZnO synthesized at 50 °C/12 h showed ΔEV = −1.37 eV and ΔEC = 2.72 eV, respectively.

  • •

    CaO-ZnO synthesized at 600 °C/2 h showed ΔEV = −0.67 eV and ΔEC = 2.02 eV, respectively.

  • •

    Bandgap restructuring due to n-n heterojunctions and high aspect ratio.

Abstract

Chemical routes for synthesizing semiconductor heterostructures (SHs) from viewpoints of improved band energetics in an optoelectronic device continue to be a lucrative area. Here we report on the experimental and theoretical band alignment correlated by XPS and UV-DRS analysis at the n-n nanointerfaces of CaO-ZnO heterostructure. A wet synthesis route was utilized to obtain SHs by zinc hydroxide carbonate (Zn5(CO3)2(OH)6) conversion to ZnO using Ca(OH)2 at near room temperature and by calcination method. The heterostructures achieved by reacting Zn5(CO3)2(OH)6 with Ca(OH)2 at near room temperature and by calcination method are referred to as Ca-ZMS and CaO-ZnO, respectively. The band gaps of individual components namely, CaO and ZnO were found to be 4.67 and 3.32 eV, respectively. In each case, band alignment was found to be of type II (staggered) in contrast to what is reported in the literature as type I (straddled). This change was attributed to the presence of nanostructured CaO. An interfacial valence band and conduction band offset of −1.37 and 2.72 eV was observed in Ca-ZMS, while CaO-ZnO demonstrated valence band and conduction band offset of −0.67 and 2.02 eV, respectively.

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.

References (31)

  • I. Roger et al.

    Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

    Nat. Rev. Chem.

    (2017)
  • T.P. Senftle et al.

    The holy grail: chemistry enabling an economically viable co2 capture, utilization, and storage strategy

    Acc. Chem. Res.

    (2017)
  • A. Agarwal et al.

    Type I and type II band alignment in ZnO/MgZnO bilayer films

    Appl. Phys. Lett.

    (2014)
  • H.J. Kim et al.

    Highly sensitive and selective gas sensors using p-type oxide semiconductors: an overview

    Sens. Actuators B Chem.

    (2009)
  • Ü. Özgür et al.

    A comprehensive review of ZnO materials and devices

    J. Appl. Phys.

    (2005)

    • Cited by (3)

    • Fabrication of orderly changed ZnO hierarchical structures by calcining different zinc precursors and morphology-depended photocatalytic property

      2022, Inorganic Chemistry Communications
      Citation Excerpt :

      It is well known that when C6H12N4 (Hexamethylenetetramine, HMT) or CO(NH2)2 as an alkali source was added to the reaction solution, because C6H12N4 can produce NH3 (then further formation of OH–) and CO32– groups, and the generated OH– and CO32– groups can simultaneously react to generate the precursors of zinc carbonate hydroxides (ZCHs), porous ZnO nanosheets, spheres or nanoparticles, nanobelts and nanowalls can be obtained due to the thermal decomposition of the ZCHs precursors. According to the previous reports, there are many ZCHs precursors, such as Zn5(CO3)2(OH)6 [11,12] or/and Zn4(CO3)2(OH)6·nH2O [13], Zn2CO3(OH)2 [14], Zn4CO3(OH)6·H2O [15,16], and Zn4CO3(OH)6 [17] by these hydrothermal method or solution environment. Meanwhile, NH4F was used as a morphology modification [18], fluorine source [19], mineralizer [20], control agent of supersaturation [21], dopant [22], etc. has been reported.

    • Surfactant-enhanced ZnO<inf>x</inf>/CaO catalytic activity for ultrasound-assisted biodiesel production from waste cooking oil

      2023, Reaction Chemistry and Engineering

    • Qualitative Analysis of the Valence and Conduction Band Offset Parameters in FeNiO/CuNiO Bilayer Film Using X-Ray Photoelectron Spectroscopy

      2022, Physica Status Solidi (B) Basic Research
    View full text
    © 2020 Elsevier B.V. All rights reserved.