Characterization of Some Physical and Photocatalytic Properties of CuO Nanofilms Synthesized by a Gentle Chemical Technique

Pure and Li-doped CuO nanofilms were synthesized on heated glass substrates using the spray-pyrolysis technique. The deposited pure CuO nanofilms were achieved at a precursor molarity of 0.2 M using a solution prepared from copper nitrate trihydrate (Cu(NO3)2·3H2O). Doped Li–CuO nanofilms were obtained using several doping concentrations (3, 6, 9, 12 and 15%) by adding a solution prepared from lithium nitrate (LiNO3). The pure and Li–CuO samples were investigated by different techniques. XRD revealed three dominant peaks (-111), (111) and (211), which are the properties of monoclinic CuO. The increase in Li-doping concentration showed the appearance of other peaks of low intensities detected at 2θ ranging from 49 to 68◦. AFM images showed a textured and inhomogeneous surface composed of spherical grains whose size decreased with increasing Li doping. UV–visible spectroscopy showed that the CuO samples were of low transparency; the transmittance was less than 50%. The band-gap energy determined from Tauc’s equation plot increased from 2.157 to 3.728 eV with the increase in Li doping. These values correspond well to the band gap of semiconducting CuO. The photocatalytic properties were accelerated by Li doping, as revealed by the discoloration of aqueous methylene-blue (MB) solution under ultraviolet irradiation.

Copper oxide is an inorganic compound with the formula CuO. It presents as blackish powder and crystallizes in the monoclinic structure with the lattice constants a = 4.6837 Å, b = 3.4226 Å, and c = 5.1288 Å, and β = 99.54 • (Figure 1). CuO is a p-type semiconductor with a narrow band gap ranging between 1.2 and 1.8 eV [12][13][14] and exhibits important physical properties.
Copper oxide can be synthesized by different methods, which can be physical methods such as atomic layer deposition [15], co-precipitation [16], sputtering [17], and Molecular Beam Epitaxy [18], or chemical methods such as chemical vapor deposition [19], spray pyrolysis, spin coating, and sol-gel [20][21][22]. It has been shown earlier [23] that, amongst these techniques, spray pyrolysis presents many advantages: it is cheaper and easy to implement, while allowing the obtention of homogeneous coatings on large areas. With this method, we can easily obtain nanoscale thin films of good quality. Copper oxide can be synthesized by different methods, which can be physical methods such as atomic layer deposition [15], co-precipitation [16], sputtering [17], and Molecular Beam Epitaxy [18], or chemical methods such as chemical vapor deposition [19], spray pyrolysis, spin coating, and sol-gel [20][21][22]. It has been shown earlier [23] that, amongst these techniques, spray pyrolysis presents many advantages: it is cheaper and easy to implement, while allowing the obtention of homogeneous coatings on large areas. With this method, we can easily obtain nanoscale thin films of good quality.
Nowadays, with the proliferation of industries, the production of pollutant gas has increased, the discharge of wastewater, in particular by the textile industry, is increasing, and human health and nature are threatened. To face this danger, many researchers are studying the photocatalytic properties of metallic nanoparticles [24,25]. It is in this context that we synthesized the oxide CuO using spray pyrolysis, after which we studied the catalytic properties of undoped and Li-doped CuO by monitoring the evolution of the absorption intensity of the solution of methylene blue subjected to UV irradiation.

Experimental Procedure
In this work, pure and Li-doped copper-oxide nanometric films were deposited onto heated glass substrates using the spray-pyrolysis technique. The primary solution that yields pure copper oxide was prepared by the use of the precursor copper nitrate trihydrate (Cu(NO3)2,·3H2O) in 100 mL deionized water. A secondary solution prepared from lithium nitrate LiNO3 for the Li doping was added to the primary solution at various concentrations ((Li/Cu) ratio: 3, 9, 12 and 15%). These solutions were sprayed onto heated glass substrates maintained at 350 °C during the entire deposition time according to the schema of Figure 2. The obtained pure and Li-doped CuO samples were characterized by techniques adapted to thin-film analysis. X-ray diffraction (XRD-X'Pert PRO from PANalytical utilizing copper line of wavelength λ = 1.54 Å) was used for structural study. Microstructural and morphological characterizations were obtained by scanning electron microscopy (SEM JEOL 7500-F Instrument) and atomic force microscopy (AFM-Dimension Edge from Bruker), respectively. The CuO band-gap energy (Eg) was graphically estimated using the Tauc's plot, which combines absorption and transmittance data extracted from UV-visible analysis using a Specord 50 plus spectrophotometer. Photocatalytic properties were investigated in a dark chamber where we placed beakers containing the methylene-blue (MB) solution + CuO samples. The whole chamber was illuminated by UV lamp of wavelength 253.7 nm. The absorption of MB solution was measured as a function of Li doping and of UV-irradiation time. Nowadays, with the proliferation of industries, the production of pollutant gas has increased, the discharge of wastewater, in particular by the textile industry, is increasing, and human health and nature are threatened. To face this danger, many researchers are studying the photocatalytic properties of metallic nanoparticles [24,25]. It is in this context that we synthesized the oxide CuO using spray pyrolysis, after which we studied the catalytic properties of undoped and Li-doped CuO by monitoring the evolution of the absorption intensity of the solution of methylene blue subjected to UV irradiation.

Experimental Procedure
In this work, pure and Li-doped copper-oxide nanometric films were deposited onto heated glass substrates using the spray-pyrolysis technique. The primary solution that yields pure copper oxide was prepared by the use of the precursor copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) in 100 mL deionized water. A secondary solution prepared from lithium nitrate LiNO 3 for the Li doping was added to the primary solution at various concentrations ((Li/Cu) ratio: 3, 9, 12 and 15%). These solutions were sprayed onto heated glass substrates maintained at 350 • C during the entire deposition time according to the schema of Figure 2. The obtained pure and Li-doped CuO samples were characterized by techniques adapted to thin-film analysis. X-ray diffraction (XRD-X'Pert PRO from PANalytical utilizing copper line of wavelength λ = 1.54 Å) was used for structural study. Microstructural and morphological characterizations were obtained by scanning electron microscopy (SEM JEOL 7500-F Instrument) and atomic force microscopy (AFM-Dimension Edge from Bruker), respectively. The CuO band-gap energy (Eg) was graphically estimated using the Tauc's plot, which combines absorption and transmittance data extracted from UV-visible analysis using a Specord 50 plus spectrophotometer. Photocatalytic properties were investigated in a dark chamber where we placed beakers containing the methylene-blue (MB) solution + CuO samples. The whole chamber was illuminated by UV lamp of wavelength 253.7 nm. The absorption of MB solution was measured as a function of Li doping and of UV-irradiation time.  Figure 3 shows the XRD patterns of the deposited pure and Li-doped CuO thin films. More than ten peaks of different intensities were observed with the principal (−111)   Figure 3 shows the XRD patterns of the deposited pure and Li-doped CuO thin films. More than ten peaks of different intensities were observed with the principal (-111) and (111), (211) indexed planes, corresponding to 2θ diffraction angles around 36 and 38 • . The growth of CuO crystals is along these planes, as has been reported by different authors. The observation of more than one peak shows that the copper-oxide films prepared with different Li-doping concentrations were composed of different polycrystalline phases. These peaks are the most frequently shown and discussed in the literature [26,27] and are matched with the JCPDS cards 001-1117 [28], confirming the presence of the monoclinic CuO phase. No impurity peaks were detected, which means that the CuO films were of good quality. When Li doping increased, less intense peaks were observed in the diffractionangle range of 49-68 • , corresponding to (-202), (020), (202), (-113), (022), (220), respectively. This suggests that beyond 9% Li doping, Li atoms had effectively substituted the Cu sites within the CuO lattice. This is favored by the size of Li + which is equivalent to the size of Cu 2+ (r Li+ = 0.76 Å, r Cu2+ = 0.73 Å). This explains why the crystal structure is not disturbed much.  The exploitation of the major diffraction peaks (-111) and (111) allowed for the determination of the lattice parameters of the monoclinic crystal lattice and the unit-cell volume according to the following relations:

XRD Analysis
where a, b, c and  are the lattice parameters for the monoclinic structure, (h, k, l) are the Miller indices, and d is the interplanar distance. The lattice parameters a, b, c calculated from Equation (1) are summarized in Table  1: Table 1. Determination of lattice constants from XRD-spectra processing.  The exploitation of the major diffraction peaks (-111) and (111) allowed for the determination of the lattice parameters of the monoclinic crystal lattice and the unit-cell volume according to the following relations: where a, b, c and β are the lattice parameters for the monoclinic structure, (h, k, l) are the Miller indices, and d is the interplanar distance. The lattice parameters a, b, c calculated from Equation (1) are summarized in Table 1: Table 1. Determination of lattice constants from XRD-spectra processing. From this table, the lattice parameters and the unit-cell volume increase with the increase in Li doping. As r Li + is greater than r Cu 2+ ( rLi + rCu 2+ > 1.041), high Li doping probably distorted the structure by creating defects (essentially point defects as oxygen vacancies), and by increasing the lattice parameters, which explain the increase in unit-cell volume.
We can also estimate, from XRD peak widths, the variation in the average grain sizes of CuO and doped Li-CuO by using the Debye-Scherrer equation [29]: where D is the mean crystallite size, λ is the wavelength of X-ray radiation, ∆(2θ) is the angular full width half maximum (FWHM) of the diffraction peaks according to 2θ, θ is Bragg's angle of the peaks. In order to investigate the influence of Li doping on the structural lattice properties, the dislocation density (δ) [30] was estimated by the use of the following relation: We applied these relations to the most intense diffraction peak (111). The values of D and δ are shown in Table 2: From this table, the crystallite size of pure CuO is found be 18.6708 nm and decreases to 11.1749 nm with the increasing in Li doping. This behavior is due to the Li-doping effect, which contributes to the increasing CuO lattice disorder by increasing the Li atoms in CuO films. The surface roughness is clearly affected by the presence of Li atoms as shown in the following paragraph on SEM and AFM images, and similar results have been reported by other authors [31][32][33].

SEM and AFM Analysis
The SEM images in Figure 4 are unfortunately blurry; for this reason we have only presented images of the pure and 6% Li-doped CuO. These images show a homogeneous CuO surface composed of granular structures that are clearer in the Li-doped sample. The grains are spherical in shape and uniformly distributed along the surface, resulting in a rough surface. have been reported by other authors [31][32][33].

SEM and AFM Analysis
The SEM images in Figure 4 are unfortunately blurry; for this reason we have only presented images of the pure and 6% Li-doped CuO. These images show a homogeneous CuO surface composed of granular structures that are clearer in the Li-doped sample. The grains are spherical in shape and uniformly distributed along the surface, resulting in a rough surface. For the surface morphology, we characterized the pure CuO sample and the samples doped with lithium at 3, 6 and 12% by AFM microscopy. AFM images are embedded in Figure 5. The AFM image corresponding to the pure CuO films shows that the surface For the surface morphology, we characterized the pure CuO sample and the samples doped with lithium at 3, 6 and 12% by AFM microscopy. AFM images are embedded in Figure 5. The AFM image corresponding to the pure CuO films shows that the surface is composed of islands containing dense spherical CuO grains that are agglomerated and organized in flocks with high porosity, as indicated by voids (dark areas) of irregular shapes. The average flock size was estimated from the profile (Figure 5b,e,h,k) and varied from 20 to 90 nm. The obtained AFM images of CuO films doped with lithium are different from the AFM images of pure CuO. The size of the CuO particles, organized in flocks, increased with the increase in Li doping and become increasingly separated by a great void. The surface roughness, expressed by rms (root mean square), decreased from 48 to 30 nm when Li doping increased from 3 to 12% (Figure 5c,f,i,l).

Optical Properties
The transmittance T and absorbance α were used for defining the optical properties of CuO. T and α were determined for our samples by using UV-visible spectrometry with a double-beam spectrophotometer operated in the wavelength range 200-900 nm. As our CuO-deposit films were pale in color tending towards dark, the spectra of the transmittance signals that we recorded were very low, not exceeding 50% for all samples, which is in agreement with other works [34,35]. In contrast, the absorbance spectra were intense. This allows us to conclude that the CuO films were not very transparent. This is attributed to the inhomogeneity of the surface as observed in the AFM images.
By combining the transmittance and absorption data and applying Tauc's relation [36]: where α is the absorption yield, hν is the photon energy, A is a constant, E g is the band-gap energy, we determined the values of the band gap E g of CuO from the plot of Equation (5) by extrapolation the linear portion of the curve to (αhν) 2 = 0, which intersects the energy axis ( Figure 6). The intersection shown by dashed lines gives the approximate value of the band-gap energy. These values are summarized in Table 3. It was observed that Eg was sensitive to Li doping. The band-gap values of the deposited CuO films, shown in Table 3, varied from 2.1722 eV for pure CuO to 3.5608 eV for 15% Li-CuO. The increase in the gap may be due to the formation of Cu-Li-O nanocrystallites at higher doping concentrations by introducing some defects and disorder due to the substrate temperature, which reduce the size of CuO nanocrystallites. It is admitted that the gap of nanostructured systems is greater than the gap of the bulk materials [37][38][39]. According to the AFM images, which showed discontinued CuO deposits composed of different areas and according to the obtained Eg values, we can conclude that the areas where CuO was deposited were nanostructured. organized in flocks with high porosity, as indicated by voids (dark areas) of irregular shapes. The average flock size was estimated from the profile (Figures 5b,e,h,k) and varied from 20 to 90 nm. The obtained AFM images of CuO films doped with lithium are different from the AFM images of pure CuO. The size of the CuO particles, organized in flocks, increased with the increase in Li doping and become increasingly separated by a great void. The surface roughness, expressed by rms (root mean square), decreased from 48 to 30 nm when Li doping increased from 3 to 12% (Figures 5c,f,i,l).

Optical Properties
The transmittance T and absorbance α were used for defining the optical properties of CuO. T and α were determined for our samples by using UV-visible spectrometry with a double-beam spectrophotometer operated in the wavelength range 200-900 nm. As our CuO-deposit films were pale in color tending towards dark, the spectra of the transmittance signals that we recorded were very low, not exceeding 50% for all samples, (αhν) 2 = A(hν − Eg) (5) where α is the absorption yield, hν is the photon energy, A is a constant, Eg is the band-gap energy, we determined the values of the band gap Eg of CuO from the plot of Equation (5) by extrapolation the linear portion of the curve to (αhν) 2 = 0, which intersects the energy axis ( Figure 6).

Photocatalytic Activity
Nanostructured semiconductors present interesting photocatalytic properties [40][41][42][43]. ZnO and TiO 2 were the first nanostructured materials to be studied as photocatalysts against environmental pollutants [44,45]. CuO nanofilms have the capacity to remove contaminants of organic dyes as illustrated by the bleaching of aqueous methylene-blue (MB) solution under ultraviolet illumination.
The study of the catalytic properties consisted of introducing samples, both of pure CuO and doped with Li, into a solution of methylene blue (MB) that was subjected to UV irradiation. We recorded the decrease in the absorbance signal α after each 30 min of irradiation. The absorbance signal was composed of two peaks situated around 630 and 670 nm (Figure 7).
In Figure 7, we show the absorbance degradation of MB solution versus UV-irradiation time for pure CuO and for intermediate Li doping at 6 and 15%. From the curves, we observed that the MB solution was more degraded by the presence of Li-doped CuO than pure CuO. After 17 h of UV illumination, the absorbance decreased from 1.90 to 1.20 in the presence of 15% Li-doped CuO, while it has decreased to 1.30 in the case of pure CuO, which corresponded to an absorbance-signal decrease from 68 to 63% for both cases. In Figure 7d, the decrease in the absorbance signal with the increase in Li doping can be clearly observed.
We interpret these results in terms of electronic excitation processes caused by the ultraviolet irradiation and the differing morphology of the samples. When illuminated by UV light with an excitation energy (4.88 eV) greater than the band gap of Li-CuO (3.5608 eV for 15% Li-CuO), electron-hole pairs are created. Hydroxide groups created by the irradiation react with the photogenerated holes, producing O-H radicals. These radicals react with the MB solution dye and degrade it into non-toxic organic compounds.
As discussed in the previous paragraphs, the XRD and AFM results showed, for high Li doping (12-15%), a reduction in the CuO nanoparticle size. It is suggested that small CuO particles capture photons more efficiently, thereby generating electron-hole pairs in larger quantities and hence causing the more heavily doped CuO to exhibit a higher photocatalytic activity in the degradation of MB. As reported in our previous work [46], the photocatalytic phenomenon can be explained by the following steps: (i) Step 1: (ii) Step 2: The species interact with the solution and degrade it during UV irradiation.

Conclusions
The present study led us to successfully synthesize pure and Li-doped copper-oxide films via the spray-pyrolysis technique. On the basis of the characterization results, we analyzed the effect of Li doping on the structural, morphological, optical and photocatalytic properties. CuO crystallization in monoclinic structure was revealed by XRD patterns. The surface morphology and its roughness were monitored by AFM microscope. The inhomogeneous and textured CuO surface was improved by Li doping, which reduced grain sizes. The optical and photocatalytic properties were investigated in detail. The band gap of CuO films was in the range 2.1722-3.5608 eV for 15% from pure to 15% Lidoped CuO. The photo degradation of MB solution activated by Li-CuO particles allows displaying CuO as interesting material for eventual employ in environmental domain.
Author Contributions: M.G. proposed, designed the study and wrote the paper; S.A. and M.A.D. performed out the experiments and discussed the results, C.F. and D.T. contributed to the interpretations of the results and they take care of the article processing charge. In general all authors contributed to the preparation of this article. All authors have read and agreed to the published version of the manuscript.
Funding: This research is carried out jointly by the "Laboratoire des Sciences de la Matière Condensée (LSMC)" and the "Centre Interdisciplinaire de Nanosciences de Marseille" in the framework of a cooperation project. The authors would like to thank the French National Agency of Research (ANR) (project 'Nanoptix' # ANR-18-CE42-0016-02) for financial support.
Institutional Review Board Statement: Not applicable.