Synthesis of CuAl 2 O 4 Nanoparticle and Its Conversion to CuO Nanorods

The molten salt approach was used to convert CuAl2O4 nanoparticles to CuO nanorods in this study. Molten hydroxide (NaOH) synthesis was chosen over molten salts (NaCl/KCl) for removing aluminium oxide from copper aluminate at low temperatures. The molten salt process is environmentally beneficial. Polymeric precursors were used to make nanosized copper aluminates. Alginic acid polymer is used to gel aqueous solutions of copper acetate and aluminium nitrate, yielding precursor after further heating. The precursor provides 14 nm nanosized copper aluminates after being heated at 900°C for 5 hours. XRD, FTIR, SEM, and TEM were used to characterize the nanosized copper aluminate powder. Solid state mixing and solution technique were used to investigate molten hydroxide treatment of spinel CuAl2O4. The products of the reaction were identified using XRD. FTIR and SEM are also used to analyze the sample. Using UV-DRS absorbance spectrum analysis, the optical characteristics of CuAl2O4 and CuO nanorods were examined. Using the Tauc plot method, the band gaps of CuAl2O4 and CuO were calculated to be 4.3 and 3.93 eV.

Copper aluminate is an inverse spinel with a low band gap which makes it a potential candidate as photocatalyst [19]. Various methods such as coprecipitation, sol-gel, hydrothermal, and combustion have been reported for the synthesis of nanosized CuAl 2 O 4 [20][21][22]. Weizhong et al. synthesized nanosized copper aluminate spinel by precursor approach with the help of ultrasound irradiation in the presence of argon atmosphere [11]. In the present study, we synthesized nanosized copper aluminate by alginate precursor approach without using argon atmosphere and ultrasonic irradiation.
CuO nanostructures have excellent applications such as electrode materials for lithium batteries, catalyst, and adsorbent [22]. In particular, CuO nanostructures are prepared by hydrothermal, microwave, and solvothermal, at high temperatures and complex methods. So there is a requirement for simple and low-temperature method of preparation of CuO nanostructure.
Few reports are available on the conversion of spinel oxides to metal and metal oxide in which the spinel oxide can be considered as a precursor. Yang et al. reported on the preparation of nanocrystalline "Cu" from rod-like CuFe 2 O 4 [23]. Ribeiro et al. reported the formation of surface "Ni" nanoparticles from NiAl 2 O 4 during CO 2 reforming of methane [24]. CuMn 2 O 4 catalyst showed high reactivity in water gas shift reaction due to its reduction to Cu/MnO in the presence of H 2 atmosphere at 350-400°C [25]. Formation of "Cu" on the surface of CuCr 2 O 4 makes the system an active catalyst for the selective hydrogenation of furfural to furfuryl alcohol [26]. Similarly, Ni metal dispersed on the surface of nickel spinel oxide catalyst results in stable performance in ethanol steam reforming [27]. Spinel CuFe 2 O 4 has been studied as a precursor for copper catalyst by Kameoka et al. [28]. Nillohit et al. synthesized mesoporous CuO nanoparticles from Cu(OOCPh) 2 Lut 2 precursor complex by annealing at 450°C [29]. Precursors already reported for synthesizing CuO nanoparticles are Cu 2 (OH) 3 Br, CuS, etc. [30,31]. So far, no reports are available for the synthesis of CuO from CuAl 2 O 4 precursor. However, the regeneration process of Cu or CuO from copper containing spinel oxides has not been investigated thoroughly.
Molten salt syntheses are effective low-temperature synthesis without using expensive precursors and toxic elements, and they do not require any specialized equipment for synthesizing metal oxides. Molten hydroxide synthesis is a type of molten salt synthesis in which hydroxides are used instead of salts which produces single-phase and controlled morphology [32,33]. Due to its advantages, molten hydroxide synthesis is considered to be the most promising one, and hence, it is applied to prepare simple metal oxide from complex metal oxide.
The development of new methods for preparation of cupric oxide with various morphologies has always been important because cupric oxide is a significant metal oxide. From the literature reports, it was prominent that the synthesis will have an effect on size and morphology of the particles [34]. To the best of our knowledge, no work has been done to synthesize CuAl 2 O 4 particles by polymeric precursor method using alginic acid as a complexing agent and its conversion to CuO by molten hydroxide method. There-fore, the aim of this study is to prepare nanostructured CuAl 2 O 4 powders and CuO nanopowder at low temperature and to calculate its band gap.

Experimental
2.1. Materials. The regents used were of analytical grade. Aluminium nitrate (98%) and alginic acid (19-25%) were procured from SD Fine-Chemical Limited, India. Cuprous acetate (99%) was from Sisco Lab and NaOH from Qualigens.    Journal of Nanomaterials by polymeric precursor approach. The elaborate experimental procedure for the synthesis of CuAl 2 O 4 is shown in Figure 1. The synthesis of polymeric precursor was carried out as follows. The two solutions cuprous acetate and aluminium nitrate were stirred thoroughly to get a homogeneous solution. Secondly, 60 ml of 20% alginic acid is added and kept to stirring at 70°C for 24 h till a dark blue gel formed. The gel on continuous heating gets converted to black powder. The black powder or precursor was then calcined at different (100°C, 300°C, 500°C, and 700°C) temperatures for 12 h and 900°C/5 h to produce nanosized CuAl 2 O 4 particles.   Journal of Nanomaterials crucible and heated at 100°C for 24 hours. After 24 h, a dark blue-coloured layer formed on the substance. The product was then washed multiple times with water to eliminate any sodium aluminate that had generated in the process. The powder (CAI) is then dried and tested.

Solution Reaction
Method. CuAl 2 O 4 (0.235 g) is dissolved in 10 ml water and mixed thoroughly to get a homogenous dispersion. The sodium hydroxide solution (10 ml, 2.35 g) is added to the dispersion and refluxed in an oil bath at 100°C for 24 hours. The mixture is then filtered, rinsed, and analyzed (CAII).
2.6. Characterization. Powder X-ray diffraction was used to describe the phase present in the CuAl 2 O 4 precursor and alkali-treated copper aluminate (CAI and CAII) as manufactured powders at room temperature in a Bruker instrument (D8 Advance) using Cu Kα radiation (=1.54). The Debye-Scherrer formula was used to calculate the average particle size of the final product. The KBr disc approach was used to record the infrared spectra (FTIR spectrometer, JASSCO Model 4100). The FEI QUANTA FEG 200 HR scanning electron microscope was used to perform scanning electron microscopic analysis on the sample. TEM pictures were captured using a Philips CM 200 with a 20-200 kV working voltage. The optical properties of the powders are measured using DR UV-VIS instrument.

Results and Discussion
The XRD pattern of CuAl 2 O 4 precursor heated at different temperatures is shown in Figure 2. At each temperature, the sample is heated for 12 h in air to evaluate the formation of pure phase.
where β is the full width maximum, K is the shape factor (0.9), and λ is the wavelength of X-ray source. CuAl 2 O 4 is formed at higher temperature, and CuO is formed at 600°C.Niasari et al. synthesized nanocrystalline CuAl 2 O 4 at 800°C by modified sol-gel method. Our results are in good agreement with their reports [10].
The XRD peaks of "CAI" (Figure 3(a)) are in good agreement to pure monoclinic CuO (JCPDS No. 45-0937) whereas the XRD peaks of "CAII" (Figure 3(b)) contain CuAl 2 O4, Al 2 O 3 , and AlOH. This shows that CuAl 2 O 4 does not react with NaOH solution even when it was refluxed for 12 h.
FTIR analyses were used to identify the functional groups of nanosized spinel CuAl 2 O 4 and interaction of CuAl 2 O 4 with NaOH. The vibrations at 3419 cm -1 and 1641 cm -1 (Figure 4(a)) are due to the longitudinal and bending vibration of water. A broad peak centered at 640 cm -1 is due to the Cu-O, Al-O, and Al-O-Al longitudinal vibrations in CuAl 2 O 4 [36,37]. The IR absorption spectrum of NaOH-treated CuAl 2 O 4 (Figure 4(b)) shows a different pattern from the spinel. On treatment with NaOH, a broad peak centered at 640 cm -1 dis-appears and a small peak appeared at 460 cm -1 which confirms the formation of Cu-O. Also, the vibrations due to alumina are not observed which further indicates the complete conversion of CuAl 2 O 4 into CuO [38].
The size and the morphology of synthesized nanosized CuAl 2 O 4 were examined by SEM and TEM techniques. Figures 5(a) and 5(b) show the SEM and TEM images of nanosized CuAl 2 O 4 , respectively. The SEM image exhibits a variety of morphologies, including nanosized rod production and porous spherical particles. Thus, the homogeneity in shape is not achieved using alginate precursor. The surface area of the synthesized CuAl 2 O 4 is 27.6 m 2 g.
The TEM image ( Figure 5(b)) was done by ultrasonicating the CuAl 2 O 4 powder in acetone. The TEM image indicates that the particles are of diameter 50 nm. From the TEM image, we can further confirm that the obtained nanocrystals are not dispersed well and the cubes aggregated to form different shapes such as rods and triangles. Thus, the homogeneity in shape is not achieved using an alginate precursor.
The SEM image of CuO (CAI) formed from solid state reaction ( Figure 6) shows the rod morphology which is observed in CuAl 2 O 4 . The morphology of CuAl 2 O 4 persists on further treatment with alkali. Similar to the spinel, the rods are agglomerated and the porosity gets reduced. Similar to the reported literatures, the surface area of the synthesized CuO is 3.59 m 2 g.
With reference to the results obtained, the nano-CuAl 2 O 4 is transformed to pure nano-CuO by solid state reaction with alkali. It is well known that CuAl 2 O 4 on heating at higher temperature results in a mixture of CuAlO 2 and CuO [39]. The mechanism that exists in the formation of CuO from CuAl 2 O 4 by alkali treatment is mentioned below in the form of equations.   Figure 7 to investigate optical properties. The UV-VIS absorption spectrum of the synthesized CuAl 2 O 4 nanostructure is shown in Figure 7(a). There is an absorption band centered at 364 nm with two small shoulders at 269 and 224 nm. The absorption band for CuO is at 294 nm with a shoulder at 221 nm (Figure 7(b)) which was similar to the reported literature [41].
Tauc approach (Figure 8) was applied to calculate the band gap (E g ) of CuAl 2 O 4 and CuO using the below equation The optical band gap of CuAl 2 O 4 nanoparticles annealed at 900°C was evaluated as 4.3 eV. Similarly, for CuO, it was estimated as 3.93 eV, respectively.

Conclusion
Nano-CuAl 2 O 4 with a particle size of 20-50 nm was synthesized using the alginate precursor method. Various experimental techniques were used to characterize CuAl 2 O 4 that was synthesized. CuAl 2 O 4 has an optical band gap of 4.3 eV, whereas CuO has an optical band gap of 3.93 eV due to the absence of aluminium oxide. Molten hydroxide was used to transform CuAl 2 O 4 into pure CuO nanorods at a lower temperature. This procedure can also be used to achieve controlled morphology. The activity of spinel CuAl 2 O 4 as a precursor for the production of CuO is demonstrated in this study.

Data Availability
All data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.