Elsevier

Ceramics International

Volume 46, Issue 17, 1 December 2020, Pages 27877-27883
Ceramics International

Influence of SnO2 concentration on electrical response of α-Fe2O3 sintered with different thermal history conditions

https://doi.org/10.1016/j.ceramint.2020.07.286Get rights and content

Abstract

Hematite (α-Fe2O3) has been investigated as a promising material for the development of renewable energy technologies. However, intrinsic poor electrical conductivity and fast charge–carrier recombination rate of α-Fe2O3 are the limiting factors for such applications. A suitable approach to overcome these limitations is the insertion of electron-donor species into the α-Fe2O3 structure. This study describes the effects of incorporating SnO2 with different thermal histories on the electrical properties of α-Fe2O3. The sintering temperature at 1300 °C was responsible for a significant increase in the average grain sizes compared to the samples sintered at 1100 °C. Lower resistivity values were observed in samples obtained at 1100 °C with 2.0 wt% SnO2. As such, SnO2 was found to be an excellent modifying agent to favor the overall α-Fe2O3 electrical conductivity. Solid-state impedance spectroscopy was adopted as a versatile tool to analyze the occurrence of preferential grain boundaries for electrical current flow.

Introduction

In the past few decades, future projections on energy transition from non-renewable to renewable energy sources have enabled substantial progress in perception and development of multifunctional materials with tunable physicochemical properties. Typically, hematite (α-Fe2O3) has received considerable attention from the scientific community as a potential material in the development of renewable energy technologies, such as photoanodes for water splitting [1], anodes for rechargeable lithium-ion batteries [2], gas sensors for selective gas detection [3], and photocatalysts for the degradation of organic pollutants [4]. These technological applications have been possible because of the favorable features offered by this oxide, including n-type semiconductor with suitable indirect band gap (2.1 eV) for visible light absorption, excellent thermal and chemical stabilities, nontoxicity, natural abundance, and affordable cost [5,6]. Despite these significant advantages, the primary limitations of α-Fe2O3 are its poor electrical conductivity and fast charge–carrier recombination rate [7].

Fundamental concepts on the electrical properties of polycrystalline semiconductors consider grain boundaries (GBs) as charge-blocking barriers [8,9]. Thus, studies focusing on the specific role of GBs on electronic transport in metal oxides have been regarded as being dependent on the atomic defect concentration (vacancies, interstitial ions, substitutional atoms, etc.) [10]. However, the type and amount of bulk defects are influenced by a set of interrelated parameters, such as precursor impurities, intentional addition of doping atoms, sintering temperature and atmosphere, and cooling rate [11]. Especially, incorporation of electron-donor species into the α-Fe2O3 is an alternative approach to improve the electrical properties of this material. Among the different elements recognized as transition metals, Sn is an ideal choice for this purpose [12,13].

Following this line of research, Hufnagel et al. [14] synthesized Sn-doped α-Fe2O3 films on fluorine-doped tin oxide substrates, but with controlled depth distribution of Sn concentration in the films. According to these authors, an Sn-rich surface was beneficial in improving the films’ charge-transfer efficiency due to the suppression of surface defect states. When the Sn atoms were uniformly distributed throughout the entire α-Fe2O3 layer, they segregated at the GBs, resulting in poor electrical properties.

Previously, our research group investigated the correlation between the Schottky defects, Sn incorporation, and sintering atmospheres (O2 and N2) on the electrical behavior of polycrystalline α-Fe2O3 [15]. An important observation was the reduction of the GB resistance, as a consequence of both N2 atmosphere and interfacial Sn segregation. A more detailed study [16] revealed that Sn preferentially segregates into high-angle GBs, minimizing the voltage barriers at the solid–solid interfaces. This phenomenon was considered to play a central role in facilitating the electron flow through the polycrystalline α-Fe2O3. In addition, a heterogeneous distribution of the segregated Sn at the GBs was identified in the samples obtained under similar thermal histories (1300 °C for 6 h), suggesting distinct interfacial energies. On the basis of these results, the key insight proposed was the existence of preferential GBs for electron transport through the interfaces, which can be modulated by Sn segregation.

In the present study, SnO2-modified α-Fe2O3 samples (from 0.0 to 5.0 wt% SnO2) were prepared by using two sintering conditions (1100 °C and 1300 °C/2 h). In addition, the commercial precursor powders were subjected to a milling step to reduce their particle sizes. These experimental conditions were intentionally employed to increase the number of grains in the SnO2-modified α-Fe2O3 to investigate the correlation between the Sn segregation-induced preferential pathways (GBs with low resistance) and electrical conductivity.

Section snippets

Sample preparation

The experimental procedure was similar to that described in an earlier study [16]. Thus, α-Fe2O3 (99%, Aldrich) and SnO2 (99.9%, Aldrich) were chosen as the starting precursors and used without further purification. Initially, SnO2 at different concentrations (0.0, 0.5, 1.0, 2.0, 4.0 and 5.0 wt%) was added to the Fe2O3 powder. Further, the mixture was subjected to mechanical milling using zirconia spheres in isopropyl alcohol at 400 rpm for 2 h on a Szegvari attritor system (Union Process,

Results and discussion

The XRD patterns were employed to monitor the possible structural changes caused by the incorporation of different SnO2 concentrations in α-Fe2O3. According to our XRD data, the 1100_SN and 1300_SN samples (ranging from 0.0% to 4.0 wt% SnO2) were perfectly indexed to a single rhombohedral structure with space group R3-c reported in the Inorganic Crystal Structure Database with a Powder Diffraction File nº 33–0664. Regardless of the sintering temperature, traces of a secondary phase identified

Conclusion

In summary, polycrystalline SnO2-modified α-Fe2O3 pellets with different SnO2 concentrations were obtained using two thermal history conditions. The XRD patterns revealed that the samples crystallized as a single rhombohedral structure, when the SnO2 was added up to 2.0 wt%. Above this level, traces of secondary phase (cassiterite) were identified in all samples, except for 1300_SN modified with 4.0 wt% SnO2. The sintering temperatures yielded microstructures composed of elongated grains with

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.

Acknowledgements

This study was financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Brazil – Finance Code 001 (Grant number 88882.332718/2019–01). The authors are grateful to the financial support from the Brazilian agency FAPESP (Grant numbers 13/07296-2, 17/03135-5 and 12/14004-5), and CNPq.

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