Large enhancement in UV emission and photocatalytic performance of Al-doped Co3O4Nanostructures under visible light
Introduction
In the daily life of human being, the impact of solid-state emitters has been no less than magnificent. The necessities of capable lighting range from traffic signals, display and telecommunications to consumer products. Hence, in the case of solid-state emitters, it is essential to have excellent quantum efficiency without energy loss to some native defects which certainly formed in solid-state materials. However, several transition metal oxides (TMs) have been used in addition to intense and broad defect radiation as well as bandgap emissions [[1], [2], [3], [4]]. Among the several TMs, owing to the significant physical and chemical properties of Co3O4, as they have versatile applications in gas sensors, fuel cell, electrochromic device and supercapacitors, etc. [5]. P-type Co3O4 having the cubic spinel structure along with mixed oxidation states (Co2+ and Co3+). Such kind of nanomaterials can be synthesized by different methods such as sol-gel method, solvothermal and hydrothermal method, etc. Among these methods, the sol-gel process is a high yield and high purity method. In view of prominent optical properties, Co3O4 NPs exhibit direct band gap [6] and two emission bands one in UV emission due to emission from excitonic band edge and other in the visible region due to intrinsic native defects (cobalt interstitial, cobalt vacancy, oxygen interstitial and oxygen vacancy). These defects are responsible for many deep level emissions (DLEs) such as violet, green and orange emissions [7] which can lower the UV emission efficiency. Therefore, the enhanced trap carriers with low UV emission efficiency make the materials incompatible for several applications. However, many researchers reported the increased near band emission (NBE) and the reduced DLEs with the decreasing surface defect density in different materials [8,9]. Moreover, +3 valency metals such as indium (In) and aluminium (Al) have been doped which improving the optical properties of ZnO [10,11].
Metal oxide semiconductors (ZnO, TiO2..) are abundant in nature and extensively used as photocatalysts because of their photosensitivity, high oxidizing power, nontoxicity, easy availability and capability to generate charge carriers when stimulate with light energy [[12], [13], [14]]. Due to a large bandgap (3.0–3.2 eV), several metal oxides such as TiO2 [15] and ZnO [16] materials show photocatalytic performance under UV radiation but exhibit poor performance under visible or solar light. In recent, a lot of works have been carried out to grow efficient photocatalytic under a visible light source. As an efficient photocatalyst [17], spinel based Co3O4 catalyst can be used as a substitute because of their optical band gap lies within the visible range (1.77–3.1 eV). Hence, Co3O4 NPs have been considered as the finest and visible light driven photocatalyst because of several factors such as lower cost, improved activity, environment-friendly and excellent chemical/physical stability [18,19]. Doping of transition metals (Ni, Mn and Co)(TMs) is one of the most effective routes to modify the photocatalytic activity of Co3O4 NPs [[20], [21], [22]]. Among the several TMs, aluminium (Al) is reflected as an appropriate choice for dopant materials in terms of non-toxicity and its availability [23]. Moreover, the doping of Al can modify the optical band gap energy of Co3O4 and enhance the photocatalytic efficiency by suppressing the photogenerated charge carrier ( and ) recombination [24]. To the best of our knowledge, photocatalytic performance of Al doped Co3O4 has not been reported yet in literature. Moreover, most of the researchers have been used methylene blue (MB) dye with the cobalt oxide. Here, reactive blue-171 (RB-171) dye has been chosen for our study because they are used in colorant in food stuffs and textiles industries, etc. RB-171 dye causes a respiratory tract of human beings as well as animals and irritation to the skin, eyes [25]. In this report, by combining the results of XRD, TEM and DRS measurements, we discussed the UV luminescence with reducing DLEs and photocatalytic performance in the decomposition of RB-171 dye under the visible light source for catalysts Co3O4 and Al-doped Co3O4 NPs.
Section snippets
Experimental
In the synthesis method, we have used high purity cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O) as a precursor material, Aluminium (III) nitrate (Al(NO3)3·9H2O) as a doping element, sodium hydroxide (NaOH) (Pellet form) and distilled water to synthesize pure and Al-doped Co3O4 nanoparticles (NPs) by sol-gel method. In this technique, the desired amount of precursor material and aluminium nitrate were dissolved in 100 mL of distilled water. The mixed solution was continuous stirred (for 2 h).
Results and discussion
The crystallinity and phase structure of the samples were examined by powder X-ray diffraction (PXRD) analysis. Fig. 1(a) shows the XRD diffraction pattern for pure and Al-doped Co3O4 NPs calcined at 700 °C. The XRD diffraction peaks demonstrate the single-phase Co3O4 formed as cubic spinel structure (space group of Fd3m) with lattice parameter a = b = c = 8.072 Å which are in good agreement with the reported literature [26] (JCPDS card No 01-076-1802). Thus, all the diffraction peaks assuredly
Conclusion
In this report, we discussed the luminescence and photocatalytic activity along with the structural properties for pure and Al-doped Co3O4 NPs prepared by sol-gel method. In XRD pattern, all the samples exhibit a spinel cubic structure without the impurity or extra phases. It has also seen that the average crystallite size (7.47–7.08 nm) decreases with increasing Al concentration. Using the DRS, the calculated value of optical band gap increases due to reduced lattice disorder as confirmed by
CRediT authorship contribution statement
M. Naseem Siddique: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Writing - review & editing. Nafees Ahmad: Software, Resources. P. Tripathi: Supervision, Validation, Visualization.
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 work was supported by the Department of Applied Physics and Department of Chemistry, AMU, Aligarh, India for providing experimental facilities. One of the authors (M. Naseem Siddique) would like to thank University Grants Commission (UGC), New Delhi, Government of India for providing financial support through Senior Research Fellowship (SRF-510591).
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