Effect of copper oxide (CuO) and vanadium oxide (V2O5) addition on the structural, optical and electrical properties of corundum (α-Al2O3)

In this work, we prepared a pure α-Al2O3, α-Al2O3/CuO (AC) and α-Al2O3/V2O5 (AV) nanocomposite. The sol–gel method was used to prepare pure α-Al2O3, (AC) and (AV) samples at 1200 °C. Structural, electrical, and optical properties of the prepared samples were investigated using the X-ray diffraction (XRD), UV–Visible spectrophotometer, and conductivity meter, respectively. The XRD results confirmed the crystalline nature and the presence of the hexagonal structure of α-Al2O3, the rhombohedra structure of CuAlO2 and the tetragonal structure of V2O5. Moreover, the crystallite size of pure α-Al2O3 was 43.1 nm, while the crystallite size of α-Al2O3 in samples AC and AV nanocompsite was 24.05 nm and 34.84 nm respectively. The optical measurements showed that the band gap α-Al2O3 decreased significantly from 5.28 eV for pure to 3.7 and 3.4 eV to AC and AV respectively. The DC electrical conductivity (σd.c) values were measured for all prepared samples at room temperature. The electrical conductivity was 2.4 × 10–7 and 1.8 × 10–7 (Ω cm)−1 in AC and AV nanocompsite respectively, while ionic conductivity (σion) decreased from 3 × 10–10 in pure α-Al2O3 to 7 × 10–5 and 1 × 10–5 in AC and AV nanocompsite, respectively. The results showed an improvement in the structural, optical, and electrical properties, which may make these materials a candidate for use in many applications, such as photocatalytic, gas sensors, optoelectronics, microelectronics, semiconductor devices, ……etc.


Synthesis Synthesis pure α-Al 2 O 3
To prepare pure α-Al 2 O 3 , 15 g of aluminum nitrate (Al(NO 3 ) 3 •9H 2 O) was dissolved in 40 ml of ethanol to obtain a 1 molar solution at room temperature by using magnetic stirrer for 20 min until became solution was homogeneous.Increasing the temperature to 80 °C and moving continuously for 20 min until became the gel by using a magnetic stirrer.The gel stayed in the beaker for 24 h, after that the gel dried in an oven at 180 °C for 2 h.Then grind until it a became soft powder, and put in the oven for 2 h at 1200 °C.

Synthesis of samples
To prepare the 0.8Al:0.2Cu(AC) sample, 12 g of aluminum nitrate (Al(NO 3 ) 3 •9H 2 O) was dissolved in 40 ml of ethanol to obtain a 0.8 M solution, and 1.933 g copper nitrate trihydrate (Cu(NO 3 ) 2 •3H 2 O) was dissolved in 40 ml of ethanol to obtain a 0.1 molar solution.Each solution was stirred separately for 20 min at room temperature until each solution became homogeneous, then all solution were mixed with each other and stirred for 20 min at room temperature until it became homogeneous then stir the homogeneity solution for 20 min at 80 °C until gel formed, the gel stayed in the beaker for 24 h, after that it was dried in the oven at 180 °C for 2 h.Then grind until it a became soft powder.All the samples were put in oven for 2 h at 1200 °C, and left until 24 h for calcinations they were ready for diagnosis.The other samples were prepared in the same way.Also, all samples were made into pellets for electrical measurements.All the pellets were prepared with a pressing machine (Carver) under a pressure of 6000 kg (diameter (d) of pellet is 13 mm and the thickness (L) was 2 mm).

Characterizations
The structural properties of the samples were investigated by the X-ray diffraction (XRD) technique using XD-2 X-ray diffractometer with CuKα radiation of λ = 0.154056 nm.The optical properties of the samples were investigated using a UV-Vis spectrophotometer (Hitachi U3900 with software of Varian Cary 50).The electrical conductivity measurements of the prepared samples were carried out using (conductivity meter and 3540 PH).
where λ (0.154 nm) represents the wave length of X-ray, θ indicates Bragg's angle, and (β): the from full width at half maximum (FWHM).The result were shown as in Table 3.The crystallite size of pure α-Al 2 O 3 was 43.1 nm.In the (AC) nanocomposite the crystallite size was decrease to 24.05 nm as shown in Table 3.This is decreasing in crystallite size due to that the molar concenteration of Cu +1 implying in evolution of secondary phase controls the particle size of the parent phase (α-Al 2 O 3 ) 21 .The results which obtained of the crystallite size are in a good agree with 22 .In the (AV) sample, the crystllite size decrease to 34.84 nm, this decrease may be occur due to the ionic radius of the aluminum oxide (0.54 Å) less than ionic radius of vanadium oxide (0.59 Å).Also, the molar concenteration of V +5 implying that the evolution of secondary phase controls the particle size of the parent phase (α-Al 2 O 3 ), to some extent during crystallization 21 .The results were in a good agree with 23,24 .On the other hand, due to the importance of the dislocation density (δ) in the mechanical and structural properties, it was calculated using Eq. ( 2) 25 .
All the results show in Table 2.In addition, the lattice constants (a, b and c) were calculated 2 using the Eqs.( 3) and (4).www.nature.com/scientificreports/ The calculated lattice constants were in good agreement with the last studies 22 .The unit cell volume (V) and The strain (ε) calculated using the Eqs.( 5)-( 7) 26 , respectively.The results are shown in Tables 1 and 2.

Transmission
The transmittance spectra of pure α-Al 2 O 3 , AC and AV nanocomposite was measured in the range 200-800 nm.The transmittance of pure α-Al 2 O 3 nanoparticles decreased as the wavelength increase, with the highest increased transmittance occurring at a wavelength of 320 nm, as shown in Fig. 2. The transmittance of AC and AV nanocomposite also decreased, as the wavelength increased, with transmittance values of 92% and 94%, respectively, as shown in Fig. 3.The increase in transmittance in the sample with added copper and vanadium can be attributed to the formation of new energy levels within the band gap of the α-Al 2 O 3 crystal lattice.When copper and vanadium ions are added to the α-Al 2 O 3 lattice, they introduce new energy levels that allow for the absorbed of light that allow for the absorbed by the crystal lattic, leading to an increase in transmittance.The exact mechanism behind this phenomenon is complex and depends on the specific properties of the added ions and their interaction with the α-Al 2 O 3 lattice.Ho wever, it is clear that the addition of copper and vanadium ions to the α-Al 2 O 3 crystal lattice can significantly alter its optical properties, leading to increased transmittance 16 .

Absorption
The optical absorpation of the samples were determined at room temperature using the UV-visible spectrophotometer within wavelenghth rang of 200-800 nm. Figure 4 shows the relationship between the absorpation on the Y-axis and the wavelength on the X-axis of pure α-Al 2 O 3 , the highest point of the absorpation was at 204 nm, while the lowest value was at 310 nm, then the absorpation was increase slightly with increasing the wavelength.The wavelength absorption of pure α-Al 2 O 3 are observed at 280 nm 16 .The absorption of the AC and AV nanocomposites shown as in Fig. 5. Inspectra the absorption edges are observed in the UV-Vis region as 314.8-344 nm for AC and AV nanocomposite, respectively.In these samples absorption bands are attributed to the photoexcitation of electrons from the valence band to the conduction band.Further, the absorption bands are ascribed to the electronic transitions from occupied 2p bands ofoxygen to unoccupied 3d bands of copper and vanadium 27 .
Optical band gap energy (E g ).The energy gap (E g ) was calculated by using the following equation 20,[28][29][30] : (3)   www.nature.com/scientificreports/where C 1 is a constant, h is the Planck constant and α is the optical absorption coefficient.The energy gap of pure α-Al 2 O 3 was 5.28 eV as shown in Fig. 6.This result agree with 16 .While the energy gap of AC and AV were 3.7 and 3.47 eV, respectively, as shown in Fig. 7a and b.The addition of 20% Cu +1 and 20% V +1 , reduced the band gap in the nanocomposite.The decrease in the band gap value can be attributed to the appearance of the empty levels induced by defects located in the band gap 5 .It is a well-known that the band gap of any material is influenced by the concentration of defects.In α-Al 2 O 3 , both donor (oxygen vacancies) and acceptor defects (Al interstitials) create energy levels below the conduction band and above the valence band, respectively.The creation of energy levels can be explained by the Frenkel reaction for Al interstitial defects and the Schottky reaction for the oxygen vacancy defects 31 .

Current-voltag (I-V) measurments
The ohmic resistance (R) of pure α-Al 2 O 3, AC and AV nanocomposites were calculated from the I-V curve according to ohm law Eq.( 9) (ohm law) 32 .
where L (Thiknecess), d (Diameter), r (Radius; r = d/2) and A (Area; A = л r 2 ).The conductivity of material is determined by the presence of free charge carriers, such as electrons or ions, that can move freely within the material 34 .In the case of pure α-Al 2 O 3 , there are no free charge carriers available, resulting in zero conductivity.When copper nitrate (Cu(NO3)2) is added to α-Al 2 O 3 , it introduces copper ions (Cu 2 + ) into the material.These copper ions can act as charge carriers and contribute to the conductivity of the material 35 .However, the conductivity is still weak because the conductivity is still weak because the concentration of copper ions is relatively low.Similarly, when copper nitrate NH 4 VO 3 is added to α-Al 2 O 3 , it introduces vanadium ions (V 5 + ) into the material.These copper ions can act as charge carriers and contribute to the conductivity.However, like with copper nitrate, the conductivity of vanaduim ions is relatively low, resulting in weak conductivity.It is important to note that both copperr and vanaduim are transition metals with partially filled d-orbitals in their electronic configurations.This allows them to easily donate or accept electrons and participate in charge transport within a mateial.The value obtained in this work for electrical conductivity(σ d.c ) is in agree with conventional value of σ d.c of semiconductors (10 4 -10 −9 Ω −1 cm −1 ) 36 , also is consistent with the average value of α-Al 2 O 3 (6.87 × 10 -12 ± 1 .22× 10 -14 Ω −1 cm −1 ) 37 , CuO (1.1 × 10 −4 and 2.77 × 10 -4 Ω −1 cm −1 ) 38 and V 2 O 5 (2.53 × 10 -4 Ω −1 cm −1 ) 39 , (2.48 × 10 −6 and 6.16 × 10 −8 Ω −1 cm −1 ) 40 .

Ionic conductivity (σ ion )
The ionic conductivity σ ion of the elctrolyte was measured at room temperture.The ionic conductivity was found to be greater than the electrical conductivity, and this increase may be attributed to the contribution of charged carriers in the liquid, as shown in Table 3.

Conclusions
In the summary, pure α-Al 2 O 3 , (AC) and (AV) nanocomposite were prepared using Sol-Gel method at 1200 °C.X-ray diffraction showed, the high crystallinity of all samples.The crystallite size dimension was calculated from diffraction data using the formula Debye-Scherrer.The results showed that the crystallite size (D) of pure α-Al 2 O 3 was 43.1 nm with hexagonal structural, the crystal size of α-Al 2 O 3 in AC nanocomposite was 24.05 nm, the crystal size of V 2 O 5 was 21.47 nm with tetragonal structure and the crystal size of CuAlO 2 was 25.2 nm with rhombohedra structure.The band gab of pure α-Al 2 O 3 was 5.28 eV, while the band gap of AC and AV nanocomposite were 3.7 and 3.47 eV respectively.The resistance was decreasing with addition concentration of Cu +1 and V +5 .

Figure 5 .
Figure 5.The absorption of AC and AV nanocomposite.

Figure 7 .
Figure 7.The energy gap of (a) AC nanocomposite and (b) AV nanocomposite.

Table 1 .
Structure properties of the pure α-Al 2 O 3 , AC and AV nanocomposite.

Table 2 .
Structure properties of the pure α-Al 2 O 3 , AC and AV nanocomposite.