Research ArticleSol-gel derived Li and Mg incorporated nickel oxide particles: An investigation on structural and optical properties
Introduction
Nickel oxide (NiOx), a wide bandgap p-type semiconductor (3.6–4.0 eV) [1], is one of the commonly utilized metal oxides in various application areas such as anti-ferromagnetic material [2], lithium-ion batteries [3], chemical gas sensors [2], transparent p-type layers [4] in smart windows, perovskite solar cells, etc. thanks to their exciting electrical, optical, magnetic [5], catalytic [6], electrochromic [7,8] properties [9,10]. NiOx particles have been produced in numerous ways such as sputtering [4,[11], [12], [13]], pulsed laser deposition [14,15], electrodeposition [7], spin coating [[16], [17], [18], [19]], sol-gel [20], thermal decomposition [21] and spray-pyrolysis [22,23]. Among those techniques, the sol-gel method is controllable in terms of the influence of various synthesis parameters such as processing temperature, gelation time, and dwell time on chemical composition, amount of dopants, and crystallite size during the production of the powders.
In literature, incorporation of Mn [24], Sn [25], Ce [26], Fe [27], Nd [28], Li [29], and Mg [30] etc. have been studied to enhance structural and optical properties of NiOx structures. The presence of Li+ in NiOx structure individually and/or co-doping Al3+, Zr4+, or Mg2+ in addition to Li + have been recently studied. Although stoichiometric NiOx is an electrical insulator, its resistance can be reduced by the increase of Ni3+ ions resulting from the addition of monovalent atoms such as lithium, or by the appearance of nickel vacancies and/or the appearance of oxygen in NiOx. Li + ions (0.76 Å) can be effectually doped into the NiOx lattice owing to their closer ionic radius of Ni+2 (0.69 Å) [31]. Guo and co-workers examined the effects of the lithium doping concentration on the structural, electrical, and optical properties of Li:NiOx films. They reported that 0.11 mol% Li+ concentration showed the lowest resistivity of 1.33 k Ω cm and the optical transmittance of Li:NiOx decreased with increasing Li + doping concentration [32]. Garduno et al. studied about the improvement of optical and electrical properties of lithium doped nickel oxide films and showed that the electrical resistivity changed from 106 Ω cm for the NiOx films up to 102 Ω cm for the films prepared with the highest doping concentration [33]. Park et al. developed a perovskite solar cell with 5 at. % Li + doped NiOx hole transport layer showed 21% increment on average power conversion efficiency comparing with the pristine NiOx layer [34]. Besides Li+ doping, NiOx can be doped also with alkaline-earth metals such as magnesium (Mg2+) for enhancing optical and electrical properties thanks to its close ionic radii (0.71 Å) to Ni2+ (0.69 Å) [35]. Mg2+ encourages a down-shift of the valence band maximum (VBM) of the film [36,37].
Valance band maximum (VBM) and conduction band minimum (CBM) positions are very important for electronic devices, photocatalytic applications etc. Chen et al. studied inorganic hole transport layer of perovskite solar cell with Li, Mg codoped NiOx thin film. The addition of Li+ was narrowed bandgap and risen the conductivity of layer 12 times greater than pristine NiOx. Incorporation of Mg to the Li doped NiOx arranged VBM and CBM positions and caused a widening in the bandgap. Besides, carrier concentration increased to 6.46 x 1018 cm−3 (Li,Mg:NiOx) from 2.66 x 1017 cm−3 in pristine NiOx [35]. On the other hand, the optical transparency of NiO must be improved to be a transparent conductive oxide in optoelectronic devices. Mg:NiO ratio in MgNiO could be changed bandgap from 3.6 to 7.8 eV [38] Although the wide bandgap has a positive effect on transparency, it increases resistivity, so causes difficulties in applications [39]. Cao et al. studied to overcome this problem with Li doping to NiMgO and obtained %85 transparency with bandgap of 3.87 eV, resistivity of 19.26 Ω cm, hall mobility of 0.89 cm2 V−1 s−1, and a hole concentration of 3.63 1017 cm−3 [40].
Herein, we produced NiOx powders co-doped with different amounts of Li+ and Mg2+ for the first time by one-step sol-gel method. The changes in the structural and optical properties were investigated, especially with the variation in the amount of magnesium. The powders were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), photoluminescence (PL), and decay time measurements.
Section snippets
Experimental studies
In this study, NiOx, Li:NiOx, and Li,Mg:NiOx (Li0.05MgaNi(0.95-a)Ox, a: 0.00; 0.05; 0.1; 0.15; 0.30) were produced through sol-gel method. Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, Sigma-Aldrich), lithium nitrate (LiNO3, Sigma-Aldrich) and magnesium acetate tetrahydrate (Mg(CH3.CO2)2.4H2O, Sigma-Aldrich), were used in the stoichiometric amount as the starting materials. Methanol (CH3OH, Sigma-Aldrich), and glacial acetic acid (CH3CO2H, Sigma-Aldrich) were used to dissolve the starting
Results and discussion
The XRD patterns denoting the structural changes of N samples depending on annealing temperatures are given in Fig. 1. The characteristic peaks, as addressed in JCPDS card # 47-1049, at 37.48°, 43.47°, 63.12°, 75.58°, and 79.53° are associated with the crystal planes of cubic NiOx (111), (200), (220), (311), and (222), respectively [41]. According to the patterns, it can be expressed that the crystallinity of the powders are improved with increasing temperature. Consequently, NL, NLM5, NLM10,
Conclusion
In this study, NiOx, Li:NiOx, and Li,Mg:NiOx powders (Li0.05MgaNi(0.95-a)Ox, a: 0.00; 0.05; 0.1; 0.15; 0.30) were successfully produced by the sol-gel method to investigate the effect of doping on functional groups, compositions, phase structures, elemental analysis, and optical properties.
The N sample was annealed in air at various temperatures (350 °C, 500 °C, 650 °C) which caused to increase in the crystallite size and be a more crystalline structure. Besides, the increase of the Li+ and Mg2+
CRediT authorship contribution statement
Salih Alper Akalin: Akalin wrote the whole manuscript from initial draft to final revision, Methodology, Writing – original draft, preparation. Mustafa Erol: Erol, the supervisor of the project, revised and edited the whole manuscript, Supervision, Conceptualization, Writing – review & editing. Begum Uzunbayir: Uzunbayir conducted the experiments, prepared the plots and graphs for the manuscript, Methodology, Writing – original draft, preparation. Sibel Oguzlar: Oguzlar handled the PL and FTIR
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 financially supported by Dokuz Eylul University, Scientific Research Projects Coordinatorship with the project code 2019.KB.MLT.004. The authors are indebted to the infrastructural support from Dokuz Eylül University, the Center for Production and Applications of Electronic Materials (EMUM) and Deparment of Metallurgical and Materials Engineering where the research was carried out.
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