Journal of Applied Chemistry
Volume 2017 (2017), Article ID 4518654, 10 pages
https://doi.org/10.1155/2017/4518654

Research Article

Synthesis and Characterization of CuO, TiO₂, and CuO-TiO₂ Mixed Oxide by a Modified Oxalate Route

Ekane Peter Etape,¹ Lambi John Ngolui,² Josepha Foba-Tendo,¹ Divine Mbom Yufanyi,³ and Beckley Victorine Namondo¹

¹Department of Chemistry, Faculty of Science, University of Buea, P.O. Box 63, Buea, Cameroon
²Department of Chemistry, ENS Yaoundé, BP 47, Yaoundé, Cameroon
³Department of Chemistry, Faculty of Science, The University of Bamenda, P.O. Box 39, Bambili, Bamenda, Cameroon

Correspondence should be addressed to Josepha Foba-Tendo; jnfoba@gmail.com

Received 2 January 2017; Revised 4 April 2017; Accepted 23 April 2017; Published 4 June 2017

Academic Editor: Muthupandian Ashokkumar

Copyright © 2017 Ekane Peter Etape et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Copper oxide (CuO), titanium oxide (TiO₂), and Cu-doped TiO₂ nanoparticles have been synthesized by pyrolysis of their corresponding precursors initially prepared by precipitation in aqueous solution using A. carambola fruit juice as a natural source of the precipitating agent (oxalate). The precursors were synthesized and characterized by FTIR, TGA, and PXRD. The results revealed that the precursors obtained were CuC₂O₄, TiO₂(OH⁻)₂C₂O₄, copper-doped titanium hydroxyl oxalate, and copper titanium hydroxyl oxalate. Complete decomposition for the as-prepared precursors containing titanium ions occurs at 600°C while impurity free copper oxalate decomposed at 450°C. The as-prepared precursors were decomposed and calcined at 600°C for 4 hours and the calcination products were characterized by XRD, SEM, and EDX. The results revealed the decomposition products to correspond to CuO, TiO₂, Cu0.131Ti0.869O₂, and CuO/TiO₂.

1. Introduction

Nanosized transition metal oxides, including copper oxide and titanium oxide (TiO₂), are the subject of sustained scientific interest, because of their potential applications in sensing, catalysis, cosmetics, optoelectronic devices, biomedical field, and magnetism [1, 2]. CuO is a narrow band-gap (1.2 eV) p-type semiconductor with photoconductive and photochemical properties and has found applications in gas sensing [3, 4], in catalysis [5–7], as antimicrobial agent [8–11], and in batteries [12], magnetic devices [13–15], super capacitors [16], and field emission [17]. TiO₂ is an n-type semiconductor with wide band gap ranging from 3.2 eV to 3.6 eV. It has found applications in cosmetics, paper and medical devices coating, and gas sensors [18–20]. A significant advantage of TiO₂ is the formation of a heterojunction on reaction with another material [21, 22]. In these applications, an important parameter is the specific surface area, which is strongly related to the nanostructured morphology. Decreasing the size of materials to the nanometre scale may result in unique properties that are significantly different from those of the bulk material. The chemical and physical properties of CuO and TiO₂ depend on their microstructure, such as the morphology and the size and orientation of the constituent grains. To improve and extend the functions of these inorganic nanomaterials, one or more components are often combined to form nanocomposites for various applications in photocatalysis, electronics, and gas sensors [23, 24]. The unique properties of composite nanomaterials originate from their ability to combine the most desirable physicochemical properties of their constituents. Among these composites, CuO/TiO₂ nanocomposites have attracted interest due to their narrow band gap, stability, nontoxicity, and easy availability [21]. Preparation of high-quality nanocrystals of desired size is a prerequisite for investigating and utilising their size-dependent properties. Various approaches such as chemical precipitation [9], sol-gel method [10, 25, 26], hydrothermal method [6, 27], microwave-assisted method [28], sonochemical synthesis [29], and solvothermal synthesis [30] have been reported for the synthesis of uniform-sized (or monodisperse) metal oxide and metal oxide nanocomposites with diverse morphologies.

Titanium oxide is found in three different crystalline forms: rutile, anatase, and brookite. The anatase phase has been reported to display higher photocatalytic activity than the others, owing to its higher density of localized states [31]. Different synthetic methods have been reported for the synthesis of anatase [31, 32], rutile [33], and brookite [34] phases. The precipitation of titanium by oxalate routes has been reported [35] with the precipitate formulated as TiO(C₂O₄)·H₂O which has as complex isomer Ti(OH)₂(C₂O₄)·()H₂O, with (in solid state the product might be polymeric or amorphous).

In spite of all the progress made, the synthesis of CuO, TiO₂, Cu-doped TiO₂, and CuO/TiO₂ nanomaterials of controlled size and shape remains a challenge. Size and shape need to be tailored by an appropriate choice of the synthesis methods and conditions. Fine-tuning of the morphology is of key importance, since the surface energy, the electronic structure, bonding, and the chemical reactivity of nanomaterials are all directly related to surface morphology [36]. The purity and stoichiometry (quality of the nanoparticles generated) depend on the synthesis route. Synthesis of nanomaterials by thermal decomposition using cheap, readily available, nontoxic precursors emerges as the most suitable and cost-effective approach. We have been interested in investigating how selected thermal decomposition conditions can influence the nanoscale formation of first row transition metal oxide nanoparticles and their resulting physicochemical properties [22, 37]. In this paper we report on the synthesis of copper oxide (CuO), titanium oxide (TiO₂), and Cu-doped TiO₂ nanoparticles by a simple modified oxalate process (using A. carambola fruit juice as source of ligand to precipitate the metal oxalates). The influence of amount of dopant on the composition and morphology of precursor and calcined samples is investigated. This method has the advantage of mixing the component salt at microscopic level without the use of an additional solvent.

2. Experimental

2.1. Materials and Methods

Titanium trichloride (20% in 3% hydrochloric acid) (Alfa Aesar, Germany), calcium chloride (MERCK), copper chloride (Fluka), commercial oxalic acid, and potassium permanganate were of analytical grade and were used without further purification. HPLC A. carambola fruit juice (source of the precipitating agent) was also used.

2.2. Processing and Characterization of the Fruit Juice

The fruit juice was processed and characterized as previously reported by Nguimezong et al. [37].

2.3. Synthesis of Precursors

The oxide nanocomposites and the doped materials were both obtained in the same manner via two experimental steps. Solutions (0.04 M) of the various metal chlorides (Cu²⁺ and Ti³⁺) were prepared in 200 mL of distilled water. The precursor was synthesized by coprecipitation in aqueous solution containing the metal chlorides using A. carambola fruit juice as the source of the precipitating agent. Using copper titanyl oxalate as an example, 200 ml portion of the juice extract was poured into a 400 mL round bottom flask at ambient temperature. The metal ion solutions (in the proportions specified in Table 1) were slowly and/simultaneously added to the juice while stirring. The mixture in each case was further stirred for 2 hours. The slurry obtained was aged for 8 hours. The precipitate obtained was filtered, washed successively with distilled water and ethanol, and dried in an oven at 80°C. The synthesis of the precursors and their thermal decomposition into the oxides are shown in Scheme 1.

Table 1: Combination volumes of the metal chloride and A. carambola fruit juice solutions.

Sample code
(mL)
(mL)
(mL)
(mL)

E₁
44
56
200
300

E₂
100
—
200
300

E₃
—
100
200
300

E₄
13.1
86.9
200
300

Scheme 1: Synthesis of the precursor complexes and metal oxides.

2.4. Determination of Oxalate Ion Concentration

The concentration of oxalate ions in the fruit juice was determined by titration with permanganate solution. The permanganate solution was standardized using a standard solution of commercial oxalic acid.

2.5. Thermal Decomposition of the Precursors

The as-prepared precursor powders were calcined in a ceramic combustion boat holder at 600°C in a muffle oven (with temperature ramp of 10°C/min) for 4 hours under air flow.

2.6. Characterization Techniques

FTIR spectra were recorded from 4000 to 400 cm⁻¹ on a PerkinElmer Spectrum Two Universal Attenuated Total Reflectance Fourier Transform Infrared (UATR-FTIR) spectrometer. Thermogravimetric analysis (TGA) was obtained using a Pyris 6 PerkinElmer TGA 4000 thermal analyzer. The TGA analysis was conducted between 30 and 900°C under nitrogen atmosphere at a flow rate of 20 mL/min and a temperature ramp of 10°C/min. The XRD diffractograms of the precursors and the decomposition products were recorded on a Bruker D8 Advance X-ray diffractometer, using a Cu Kα radiation source ( nm, 40 kV, and 40 mA). Scans were taken over the 2θ range from 10° to 100° in steps of 0.01° at room temperature in open quartz sample holders. The phases were identified with the help of the Bruker DIFFRACplus evaluation software in combination with the ICDD powder diffraction database (International Centre for Diffraction Data). The surface morphology of the samples was examined by a Scanning Electron Microscope (SEM) (JEOL JSM-5600 or TESCAN VEGA III XMU, Q150 TЄ). Prior to examination, samples were prepared by mounting about 0.5 mg of powder onto a 5 mm × 5 mm double sided carbon tape, on an aluminum stub. The powder was then sputter-coated for 40 seconds with carbon or gold.

3. Results and Discussion

Reaction of the metal salts with the A. carambola juice at ambient temperature generally led to colored complexes as shown in Table 2.

Table 2: Composition and colors of the precursor and decomposition products.

Sample code
Color of precursor
Color of decomposition product
Formula of decomposed product

E1
Greenish blue
Black
CuO/TiO₂

E2
Greenish blue
Pale brown
TiO₂

E3
Greenish blue
Black
CuO

E4
Greenish blue
Black
Cu-TiO₂

3.1. Oxalate Ion Concentration

The oxalate concentration in A. carambola fruit juice, determined by titration with standardized potassium permanganate, compared to that of 0.125 mole/L solution of commercial oxalic acid revealed that the concentration of oxalates in the juice was 78 mg/100 cm³. This concentration conforms to literature values which fall in the range 76 mg/dL to 80 mg/dL [38]. The volume of permanganate required in each case is shown in Table 3.

Table 3: Volume of required in the determination of oxalate in A. carambola fruit juice.

Commercial oxalic acid
A. carambola fruit juice

Approx.
Accurate
Accurate
Approx.
Accurate
Accurate

2nd burette reading
47.2
46.6
46.6
6.6
12.4
18.2

1st burette reading
0.00
0.00
0.00
0.00
6.6
12.4

Titre/cm³
47.2
46.6
46.6
6.6
5.8
5.8

46.6
5.8

3.2. FTIR Spectral Characterization

The formation of pure oxalate phases is indicated by FTIR analysis. Figure 1 shows the FTIR spectra of all four complexes (E1, E2, E3, and E4) synthesized using carambola juice as precipitating agent. They all exhibit similar characteristics and comparable patterns except for the broad band centered at 3250 cm⁻¹ present in E2 and E4 but less pronounced in E1 and E3. The broad band at 3250 cm⁻¹ is attributed to the stretching vibration of the O-H of (metal hydroxyl and/or the hydroxyl of water molecules of crystallization). The reduction in the intensity of the bands in E2 and E3 (copper dominated complexes) indicates the reduced amount of water of crystallization in these complexes. The strong band at 1580 cm⁻¹ can be attributed to the antisymmetric stretching vibration modes of the carboxylate groups C=O of oxalates. The three bands at 1487–1400 cm⁻¹, 1365–1337 cm⁻¹, and 1324–1292 cm⁻¹ are attributed to the asymmetric and symmetric stretching modes of OC=O. These results are in conformity with literature [32]. The bands between 827 and 821 cm⁻¹ are attributed to the bending vibrations of O-M-O. The bands between 750 and 490 cm⁻¹ are characteristic M-O (Ti-O and Cu-O) vibrations. The absence of the free carbonyl, , vibration, expected at 1735–1705 cm⁻¹, suggests that the entire carboxylate group is probably engaged in the formation of the coordination complex. These results suggest that the complexes represented by E2 and E4 contain the hydroxyl and oxalates ( ()) whereas E1 and E3 contain only the oxalate [Cu] as ligand (which is expected since the main titanium(IV) species in the titanyl oxalate solution is Ti(OH⁻)₂) as earlier reported in literature [33].

Figure 1: FTIR spectra of the metal oxalate complexes.

3.3. X-Ray Diffraction

The precursors E₁, E₂, E₃, and E₄ and their decomposition products (ED1, ED2, ED3, and ED4) obtained by pyrolysis of the precursors at 600°C for 4 hours in air were analyzed by X-ray powder diffraction (PXRD). The XRD patterns of the precursors are shown in Figures 2 and 3 while those of the decomposition products are shown in Figure 4. The PXRD patterns for the precursors are different from those of the corresponding decomposition products. The patterns for the decomposition products were indexed with ICDD files for TiO₂ anatase phase and CuO tenorite phase.

Figure 2: PXRD patterns for the as-prepared precursors (E1 to E4).

Figure 3: XRD pattern of E2 and E4 showing masked peaks.

Figure 4: PXRD patterns of the decomposition products (ED1, ED2, ED3, and ED4).

The XRD pattern obtained for the decomposition product ED3 reveals crystallinity with major peaks at 2θ values 35.52, 38.72, 48.82, 61.34, and 68.08 corresponding to the database 2θ values of JCPDS card number 48-1548 exhibiting copper(II) oxide (tenorite phase). On the other hand, the XRD pattern obtained for the decomposition product ED2 reveals crystallinity with major peaks at 2θ values 25.3, 37.88, 47.98, 55.28, and 62.78 corresponding to the database 2θ values of JCPDS card number 21-1272 exhibiting titanium oxide (anatase phase). The structural properties of Cu-doped TiO₂ (ED4) and 0.56 CuO/0.44 TiO₂ (ED1) as-prepared were characterized by PXRD in the range 20° to 80°. Diffraction peaks in the pattern obtained were indexed, respectively, to titanium oxide (anatase phase) and CuO (tenorite phase). The values for ED4 were in good agreement with the database 2θ values of TiO₂ with lattice planes (101, 004, 200, 105, 211, and 204). The values for ED1 show partial agreement with each of the titanium oxide (anatase phase) (2θ value, 25.36, and lattice plane (101)) and CuO (tenorite phase) (2θ values, 35.54 and 38.82, and lattice planes (002, 111)). The results obtained from powdered XRD can be put under two categories (Figure 4): one which indicates just a single phase in each case corresponding to the decomposition products ED2 and ED3, and the other which indicates the presence of two phases ED1 and ED4. CuO (tenorite phase) ED3 was obtained from E3, while the mixed phase, ED1, was identified as CuO/TiO₂ (obtained from E1). The absence of any copper phase (copper metal or CuO) in the PXRD pattern for decomposition product ED2 can be explained by the fact that during pyrolysis Cu was incorporated into the TiO₂ crystal structure by substituting Ti. This is possible because the ionic radii of Cu²⁺ and Ti⁴⁺ (0.72 pm and 0.68 pm) are very close. The decomposition product ED4 corresponds to Cu0,15Ti0,85O₂ while ED1 corresponds to 0.56 CuO/0.44 TiO₂. The crystallite sizes () for the as-prepared samples were determined (by measuring the broadening of the most intense peak of the phase in a diffraction pattern for the as-prepared samples) according to Debye-Scherrer equation as follows:

where is the crystallite size, is a correction factor which accounts for the particle shape, is the full width at half maximum (FWHM) of the most intense diffraction plane, is the wave length of Cu target (0.15406 nm), and θ is the Bragg’s angle. The results obtained are summarized in Table 4.

Table 4: Average crystallite sizes of the as-prepared nanosized metal oxides.

Sample
Plane (hkl)
FWHM
Bragg’s angle (θ)
Crystallite size ()/nm
Average ()/nm

E1 (CuO/TiO₂)
101
0.0058
12.65
24.3

002
0.0058
17.76
25.3
25.43

110
0.0055
19.36
26.7

E2 (TiO₂)
101
0.0088
12.65
16.1
16.1

E3 (CuO)
002
0.0024
17.76
60.4
61.25

111
0.0024
19.42
62.1

E4 Cu-doped TiO₂
101
0.0082
12.67
17.3
17.3

3.4. Thermogravimetric Analysis

The thermogravimetric and differential thermograms are presented in Figure 5. The thermograms for E2 and E4 are very similar. They each present three weight loss steps. The first occurs between 90 and 280°C, showing a weight loss of 26.3% (cf. 26.2% theoretically determined from the formula of the as-prepared precursor) which is attributed to the loss of water of crystallization. The second weight loss portion between 280 and 350°C shows a weight loss of 13.19% (cf. 13.6%) attributed to the elimination of one mole of carbon monoxide. The last weight loss portion occurs between 350 and 600°C, which shows a weight loss of 13.19% (cf. 23%) which can be attributed to partial decomposition of the intermediate. These results suggest that total decomposition of the reaction intermediate requires prolonged heating (calcinations for longer period). E1 also presents three weight loss steps but different from E2 and E4. The first portion between 90 and 270°C shows a weight loss of 12.87% (cf. 13.15%) attributed to the loss of water of crystallization. The second step between 270 and 370°C shows a weight loss of 33.66% (cf. 30%) attributed to the loss of carbon monoxide and partial elimination of carbon dioxide. The final weight loss of 9.0% (cf. 11.03%) can be attributed to the complete decomposition of the carbonate intermediate. In the case of E3, there is a relatively weak weight loss portion between 30 and 400°C corresponding to 8% by weight of the initial weight which can be attributed to water present in the matrix of the sample, though the relatively low representation indicates that the amount of water in E3 complex is greatly reduced. This is in agreement with FTIR results which indicated the absence of the O-H stretching vibration band characteristic of water of crystallization. The second weight loss in the E3 sample corresponding to 46.12% can be attributed to the decomposition of the oxalate to the corresponding oxide.

Figure 5: TGA/DTG of the metal oxalate precursors (E1, E2, E3, and E4).

The reaction pathway for E4 is the sum of reaction equation 1 and reaction equation 3 to give the product CuO/TiO₂ as shown in Scheme 2.

Scheme 2: Proposed reaction pathways for the formation of the products (E1 to E3).

3.5. SEM and EDX Analyses

The elemental composition was obtained using EDX while the morphologies and microstructures of ED1, ED2, ED3, and ED4 were studied by SEM. Figures 6(a)–6(h) show the SEM/EDX results of the samples. As evident from the micrographs, the polycrystalline nature of the ED2 and ED4 residues is obvious with well-defined polyhedral particles with grain sizes of 2 µm whereas the ED1 and ED2 residues show less-defined grain sizes. The agglomeration of particles into larger sizes as the impurity of Ti into the sample occurs is also evident. The level of agglomeration in the copper-doped anatase appears to be lower than for the pure phase. This observed agglomeration could play a vital role in the properties of the target materials such as variation in electrical conductivity due to the change in specific surface area.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 6: (a) SEM of CuO/TiO₂ (ED1), (b) EDX of CuO/TiO₂ (ED1), (c) SEM of TiO₂ (ED2), (d) EDX of TiO₂ (ED2), (e) SEM of CuO (ED3), (f) EDX of CuO (ED3), (g) SEM of Cu-doped TiO₂ (ED4), and (h) EDX of Cu-doped TiO₂ (ED4).

The results of semiquantitative analysis performed by EDX on the decomposition products ED1, ED2, ED3, and ED4 are summarized in Table 5. The Ti/Cu atomic ratios for E1 and E4 were, respectively, 0.79 (cf. 0.81 expected) and 6.66 (cf. 6.69 expected). The slight differences observed between the expected and obtained values can be explained by the limitation of EDX analysis as a quantitative method.

Table 5: EDX results for ED1, ED2, ED3, and ED4.

Sample code
Ti content (%)
Cu content (%)
Ti/Cu atomic ratio observed
Ti/Cu atomic ratio expected

ED1
44.0
56.0
0.79
0.81

ED2
100
—
0
0

ED3
—
100
0
0

ED4
86.95
13.05
6.66
6.69

Nanoregimes of the sample were analyzed by SEM and Scherrer formula. The results of the structural studies revealed that all samples had crystallites in the nanosize range; Cu²⁺ addition into the TiO₂ increases the crystallite size which may be due to the replacement of Ti⁴⁺ ions by Cu²⁺ ions which have a higher ionic radius. Increase in crystallite size leads to weaker quantum confinement and modification of structure.

Figures 6(a)–6(h) show SEM images of the as-prepared TiO₂, Cu-doped TiO₂, CuO, and CuO/TiO₂ nanosized particles. It shows that pure and doped TiO₂ nanosized crystallites agglomerate into porous polyhedral particles which measure 10–20 μm in size while CuO and CuO/TiO₂ present as coarsening particles with low level of agglomeration.

4. Conclusion

We have been able to synthesize pure CuO, TiO₂, Cu0.131Ti0.869O₂, and CuO/TiO₂ nanoparticles by a modified oxalate process using A. carambola fruit juice as the source of the precipitating agent. Particle sizes of 61.25 nm for CuO, 17.3 nm for Cu-doped TiO₂, 16.1 for TiO₂, and 25.43 for CuO/TiO₂ were obtained from calculations as confirmed by PXRD analysis. Introduction of Cu²⁺ impurities into the microstructure of TiO₂ increases the crystallite size which may be due to the replacement of Ti⁴⁺ ions by Cu²⁺ ions which have a higher ionic radius. Increase in crystallite size leads to weaker quantum confinement but the doping fraction was too small to reflect modification in the structure. Decomposition of copper oxalate to copper oxide occurred at 450°C while decomposition of Ti and Ti/Cu oxalate to corresponding metal oxides (pure, doped, and composites) occurred at a later temperature 600°C. The purity of both our precursors was justified from the FTIR while EDX results indicated the elemental representation of the decomposition products. SEM results revealed that the nanoparticles agglomerated to crystallites which are about 2  in width. Our results demonstrate that the proposed modified coprecipitation technique by using a precipitating agent extract from A. carambola juice is simple and presents as an environmental friendly, low cost, and controllable method, which can also be applied to prepare a wide variety of mixed metal oxides of interest.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this article.

References

M. Diab, B. Moshofsky, I. Jen-La Plante, and T. Mokari, “A facile one-step approach for the synthesis and assembly of copper and copper-oxide nanocrystals,” Journal of Materials Chemistry, vol. 21, no. 31, pp. 11626–11630, 2011.

R. S. Devan, R. A. Patil, J.-H. Lin, and Y.-R. Ma, “One-dimensional metal-oxide nanostructures: recent developments in synthesis, characterization, and applications,” Advanced Functional Materials, vol. 22, no. 16, pp. 3326–3370, 2012.

C. Yang, X. Su, J. Wang, X. Cao, S. Wang, and L. Zhang, “Facile microwave-assisted hydrothermal synthesis of varied-shaped CuO nanoparticles and their gas sensing properties,” Sensors and Actuators, B: Chemical, vol. 185, pp. 159–165, 2013.

J. Zhang, J. Liu, Q. Peng, X. Wang, and Y. Li, “Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors,” Chemistry of Materials, vol. 18, no. 4, pp. 867–871, 2006.

H. R. Shaterian, F. Moradi, and M. Mohammadnia, “Nano copper(II) oxide catalyzed four-component synthesis of functionalized benzo[a]pyrano[2,3-c]phenazine derivatives,” Comptes Rendus Chimie, vol. 15, no. 11-12, pp. 1055–1059, 2012.

N. Scotti, D. Monticelli, and F. Zaccheria, “Dispersed copper oxide: a multifaceted tool in catalysis,” Inorganica Chimica Acta, vol. 380, no. 1, pp. 194–200, 2012.

Y. Zhao, X. Song, Z. Yin, and Q. Song, “One-step self-assembled synthesis of CuO with tunable hierarchical structures and their electrocatalytic properties for nitrite oxidation in aqueous media,” Journal of Colloid and Interface Science, vol. 396, pp. 29–38, 2013.

A. Azam, A. S. Ahmed, M. Oves, M. S. Khan, and A. Memic, “Size-dependent antimicrobial properties of CuO nanoparticles against Gram-positive and-negative bacterial strains,” International Journal of Nanomedicine, vol. 7, pp. 3527–3535, 2012.

M. S. Hassan, T. Amna, O.-B. Yang, M. H. El-Newehy, S. S. Al-Deyab, and M.-S. Khil, “Smart copper oxide nanocrystals: synthesis, characterization, electrochemical and potent antibacterial activity,” Colloids and Surfaces B: Biointerfaces, vol. 97, pp. 201–206, 2012.

T. Pandiyarajan, R. Udayabhaskar, S. Vignesh, R. A. James, and B. Karthikeyan, “Synthesis and concentration dependent antibacterial activities of CuO nanoflakes,” Materials Science and Engineering: C, vol. 33, no. 4, pp. 2020–2024, 2013.

R. Wahab, S. T. Khan, S. Dwivedi, M. Ahamed, J. Musarrat, and A. A. Al-Khedhairy, “Effective inhibition of bacterial respiration and growth by CuO microspheres composed of thin nanosheets,” Colloids and Surfaces B: Biointerfaces, vol. 111, pp. 211–217, 2013.

O. Waser, M. Hess, A. Güntner, P. Novák, and S. E. Pratsinis, “Size controlled CuO nanoparticles for Li-ion batteries,” Journal of Power Sources, vol. 241, pp. 415–422, 2013.

T. Ahmad, R. Chopra, K. V. Ramanujachary, S. E. Lofland, and A. K. Ganguli, “Canted antiferromagnetism in copper oxide nanoparticles synthesized by the reverse-micellar route,” Solid State Sciences, vol. 7, no. 7, pp. 891–895, 2005.

D. Gao, J. Zhang, J. Zhu et al., “Vacancy-mediated magnetism in pure copper oxide nanoparticles,” Nanoscale Research Letters, vol. 5, no. 4, pp. 769–772, 2010.

S. J. Stewart, M. Multigner, J. F. Marco, F. J. Berry, A. Hernando, and J. M. González, “Thermal dependence of the magnetization of antiferromagnetic copper(II) oxide nanoparticles,” Solid State Communications, vol. 130, no. 3-4, pp. 247–251, 2004.

A. Pendashteh, M. F. Mousavi, and M. S. Rahmanifar, “Fabrication of anchored copper oxide nanoparticles on graphene oxide nanosheets via an electrostatic coprecipitation and its application as supercapacitor,” Electrochimica Acta, vol. 88, pp. 347–357, 2013.

Y. W. Zhu, T. Yu, F. C. Cheong et al., “Large-scale synthesis and field emission properties of vertically oriented CuO nanowire films,” Nanotechnology, vol. 16, no. 1, pp. 88–92, 2005.

F. A. Deorsola and D. Vallauri, “Synthesis of TiO₂ nanoparticles through the Gel Combustion process,” Journal of Materials Science, vol. 43, no. 9, pp. 3274–3278, 2008.

M. Kobayashi, K. Tomita, V. Petrykin, M. Yoshimura, and M. Kakihana, “Direct synthesis of brookite-type titanium oxide by hydrothermal method using water-soluble titanium complexes,” Journal of Materials Science, vol. 43, no. 7, pp. 2158–2162, 2008.

X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications and applications,” Chemical Reviews, vol. 107, no. 7, pp. 2891–2959, 2007.

C. H. Ashok, V. K. Rao, and C. H. S. Chakra, “CuO/TiO₂ metal oxide nanocomposite synthesis via room temperature ionic liquid,” Journal of Nanomaterials & Amp; Molecular Nanotechnology, vol. 5, no. 1, 4 pages, 2016.

F. L. Roussin, P. T. Kenfack, M. Delphine, H. Sophie, D. Arnaud, and L. J. Ngolui, “Coprecipitation of nickel and zinc malonate: A facile and reproducible synthesis route for Ni1-xZnxO nanoparticles and Ni1-xZnxO/ZnO nanocomposite via pyrolysis,” Journal of Solid State Chemistry, vol. 230, pp. 381–389, 2015.

P. H. C. Camargo, K. G. Satyanarayana, and F. Wypych, “Nanocomposites: synthesis, structure, properties and new application opportunities,” Materials Research, vol. 12, no. 1, pp. 1–39, 2009.

L. I. Trakhtenberg, G. N. Gerasimov, V. F. Gromov, T. V. Belysheva, and O. J. Ilegbusi, “Gas semiconducting sensors based on metal oxide nanoparticles,” Journal of Materials Science Research, pp. 156–168, 2012.

M. M. Ba-Abbad, A. A. H. Kadhum, A. B. Mohamad, M. S. Takriff, and K. Sopian, “Synthesis and catalytic activity of TiO₂ nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation,” nt. J. Electrochem. Sci., vol. 7, pp. 4871–4888, 2012.

H. A. Monreal, J. G. Chacon-Nava, U. Arce-Colunga, C. A. Marti, P. G. Casillas, and A. Martinez-Villafane, “Sol–gel preparation of titanium dioxide nanoparticles in presence of a linear polysaccharide,” Micro & amp; Nano Letters, vol. 4, no. 4, pp. 187–191, 2009.

Q. Zhang and L. Gao, “Preparation of oxide nanocrystals with tunable morphologies by the moderate hydrothermal method: insights from rutile TiO₂,” Langmuir, vol. 19, no. 3, pp. 967–971, 2003.

M. Rashad, M. Rüsing, G. Berth, K. Lischka, and A. Pawlis, “CuO and Co₃O₄ nanoparticles: synthesis, characterizations, and raman spectroscopy,” Journal of Nanomaterials, vol. 2013, Article ID 714853, 6 pages, 2013.

S. Anandan, G.-J. Lee, and J. J. Wu, “Sonochemical synthesis of CuO nanostructures with different morphology,” Ultrasonics Sonochemistry, vol. 19, no. 3, pp. 682–686, 2012.

B.-M. Wen, C.-Y. Liu, and Y. Liu, “Solvothermal synthesis of ultralong single-crystalline TiO₂ nanowires,” New Journal of Chemistry, vol. 29, no. 7, pp. 969–971, 2005.

Y. Li, Z. Qin, H. Guo et al., “Low-temperature synthesis of anatase TiO2 nanoparticles with tunable surface charges for enhancing photocatalytic activity,” PLoS ONE, vol. 9, no. 12, Article ID e114638, 2014.

A. L. Castro, M. R. Nunes, A. P. Carvalho, F. M. Costa, and M. H. Florêncio, “Synthesis of anatase TiO₂ nanoparticles with high temperature stability and photocatalytic activity,” Solid State Sciences, vol. 10, no. 5, pp. 602–606, 2008.

M. S. Anwar, S. Kumar, F. Ahmed, N. Arshi, C. G. Lee, and B. H. Koo, “One step synthesis of rutile TiO₂ nanoparticles at low temperature,” Journal of Nanoscience and Nanotechnology, vol. 12, no. 2, pp. 1555–1558, 2012.

T. C. Monson, M. A. Rodriguez, J. L. Leger, T. E. Stevens, and D. L. Huber, “A simple low-cost synthesis of brookite TiO₂ nanoparticles,” Journal of Materials Research, vol. 28, no. 3, pp. 348–353, 2013.

D. P. Kharkar and C. C. Patel, “Peroxy titanium oxalate,” Proceedings of the Indian Academy of Sciences - Section A, vol. 44, no. 5, pp. 287–306, 1956.

J. Xia, H. Li, Z. Luo, K. Wang, S. Yin, and Y. Yan, “Ionic liquid-assisted hydrothermal synthesis of three-dimensional hierarchical CuO peachstone-like architectures,” Applied Surface Science, vol. 256, no. 6, pp. 1871–1877, 2010.

M. B. N. Nguimezong, J. Foba-Tendo, D. M. Yufanyi, E. P. Etape, J. N. Eko, and L. J. Ngolui, “Averrhoa carambola: A Renewable Source of Oxalic Acid for the Facile and,” Green Synthesis of Divalent Metal (Fe, Co, Ni, Zn, and Cu) Oxalates and Oxide Nanoparticles, Journal of Applied Chemistry, vol. 2014, no. Article ID, Article ID 767695, p. pp, 2014.

L. K. Massey, “Food Oxalate: Factors Affecting Measurement, Biological Variation, and Bioavailability,” Journal of the American Dietetic Association, vol. 107, no. 7, pp. 1191–1194, 2007.