Preparation of Copper Nitride Films with Superior Photocatalytic Activity through Magnetron Sputtering

TiO2 possesses a wide forbidden band of about 3.2 eV, which severely limits its visible light absorption efficiency. In this work, copper nitride (Cu3N) films were prepared by magnetron sputtering at different gas flow ratios. The structure of the films was tested by scanning electron microscope, X-ray diffractometer, and X-ray photoelectron spectroscope. Optical properties were investigated by UV-vis spectrophotometer and fluorescence spectrometer. Results show that the Cu3N crystal possesses a typical anti-ReO3 crystal structure, and the ratio of nitrogen and Cu atoms of the Cu3N films was adjusted by changing the gas flow ratio. The Cu3N films possess an optical band gap of about 2.0 eV and energy gap of about 2.5 eV and exhibit excellent photocatalytic activity for degrading methyl orange (degradation ratio of 99.5% in 30 min). The photocatalytic activity of Cu3N mainly originates from vacancies in the crystal and Cu self-doping. This work provides a route to broaden the forbidden band width of photocatalytic materials and increase their photoresponse range.


Introduction
The accelerating industrialization has led to rapid increase in pollutant emissions and worsened environmental pollution. Effective methods must be developed to eliminate pollution and protect the environment. To resolve environmental problems, scholars have increasingly focused on semiconductor photocatalysis, which is a safe, simple, and non-secondary green technology. TiO 2 exhibits excellent photocatalytic properties and has been widely used in various fields in industry and agriculture [1][2][3][4][5]. However, the photocatalytic material of TiO 2 has a narrow photoresponse range (band gap of approximately 3.2 eV, which can only absorb ultraviolet light) and low photocatalytic quantum efficiency [2,[6][7][8][9]. CdS has a small forbidden band width and good matching with the near-ultraviolet light segment in the solar spectrum, but it is prone to photo-corrosion and has a limited service life [9][10][11][12][13]. Therefore, in research of film materials, exploratory work on photocatalytic performance is of considerable importance.
Copper nitride (Cu 3 N) possesses a special anti-ReO 3 lattice structure, and the central vacancies of the lattice are easily filled by other atoms, leading to changes in the electrical properties and the feasibility of the crystal to be used from the insulator to the conductor [14][15][16][17][18][19][20][21]. At present, research on Cu 3 N films mainly focuses on the influence of preparation parameters on the crystal structure. Changes in the Cu 3 N structure or doping of elements could lead to alterations in electrical and optical properties [16,[21][22][23]. The band gap of Cu 3 N films can be adjusted to a relatively large range. Numerous studies have shown that the optical band gap of Cu 3 N films ranges from 1.1 eV to 1.9 eV [15,[24][25][26] and varies according to experimental conditions and process parameters. In general, many reports have been published on the regulation of the band gap of Cu 3 N films by doping Figure 1a-d shows the surface morphology and cross-section of the SEM image of Cu 3 N films prepared at different values of r. The films prepared have flat and compact surface. However, as r increases, the number of large particles on the films surface increases significantly. The reason may be that as r increases, the content of Cu 3 N in the films decreases, and the elemental Cu increases accordingly. When r is low, part of the sputtered Cu atoms have no time to react with the nitrogen atoms and are deposited on the substrate. Therefore, Cu 3 N films are mainly formed by forming Cu-N bond by absorbing the nitrogen atoms and inserting a lattice of the Cu atoms. At this time, a considerable part of the Cu atoms on the surface of the films does not bond to nitrogen. Therefore, the central vacancy of the Cu 3 N lattice still contains more Cu atoms. The lattice constant of the Cu 3 N crystal is smaller than that of Cu. Many studies have reported the occurrence of preferential growth in the preparation of Cu 3 N films by magnetron sputtering [15,18,27,28,32]. This preferential growth phenomenon indicates the strong preferential growth mechanism of crystal grains during the growth of the Cu 3 N films. In addition, the diffusion process causes lattice mismatch stress, resulting in a rough surface morphology. Only when the preferential and diffusion mechanisms during growth reach the equilibrium, the films' surface tends to be flat. The cross-section image ( Figure 1e) shows that the Cu 3 N films are not dense and have some pore defects.
Materials 2020, 13, x FOR PEER REVIEW 3 of 11 source). The effect of the films on the photocatalytic degradation of methyl orange was studied. Sampling and testing were conducted once every 3 min. The absorbance of the methyl orange solution degraded by the films was recorded using UV-vis spectrophotometer (UV-2700, Shimadzu, Kyoto, Japan).

Results and Discussion
Figure 1a-d shows the surface morphology and cross-section of the SEM image of Cu3N films prepared at different values of r. The films prepared have flat and compact surface. However, as r increases, the number of large particles on the films surface increases significantly. The reason may be that as r increases, the content of Cu3N in the films decreases, and the elemental Cu increases accordingly. When r is low, part of the sputtered Cu atoms have no time to react with the nitrogen atoms and are deposited on the substrate. Therefore, Cu3N films are mainly formed by forming Cu-N bond by absorbing the nitrogen atoms and inserting a lattice of the Cu atoms. At this time, a considerable part of the Cu atoms on the surface of the films does not bond to nitrogen. Therefore, the central vacancy of the Cu3N lattice still contains more Cu atoms. The lattice constant of the Cu3N crystal is smaller than that of Cu. Many studies have reported the occurrence of preferential growth in the preparation of Cu3N films by magnetron sputtering [15,18,27,28,32]. This preferential growth phenomenon indicates the strong preferential growth mechanism of crystal grains during the growth of the Cu3N films. In addition, the diffusion process causes lattice mismatch stress, resulting in a rough surface morphology. Only when the preferential and diffusion mechanisms during growth reach the equilibrium, the films' surface tends to be flat. The cross-section image ( Figure 1e) shows that the Cu3N films are not dense and have some pore defects.  Figure 2a shows the surface energy disperse spectra (EDS) of Cu3N films prepared at r = 1/3. The atomic percentages of various elements in the films can be obtained (see the insert table in figure). The surface of the films contains low amount of oxygen and carbon contaminations. We believe that it mainly originated from the adsorption of the films surface before the test. In the case of r = 1/3, the Cu and nitrogen ratio in the films is approximately 3.85:1. The copper-nitrogen atomic ratio value is larger than the copper-carbon ratio of 3:1 in the chemical formula of Cu3N, indicating the presence of Cu-rich atoms in the films. The reasons may be as follows: First, some Cu atoms combine with oxygen atoms in the vacuum chamber to form Cu-O, and oxygen originates from the background atmosphere of the vacuum chamber. Second, a very small amount of Cu atoms is deposited directly on the substrate before it reacts with nitrogen in the vacuum chamber. In addition, other phases such as Cu4N may exist in the films [36], causing it to deviate from the stoichiometric ratio of 3:1. The corresponding EDS elemental mapping of the surface of the Cu3N films is shown in Figure 2b.   figure). The surface of the films contains low amount of oxygen and carbon contaminations. We believe that it mainly originated from the adsorption of the films surface before the test. In the case of r = 1/3, the Cu and nitrogen ratio in the films is approximately 3.85:1. The copper-nitrogen atomic ratio value is larger than the copper-carbon ratio of 3:1 in the chemical formula of Cu 3 N, indicating the presence of Cu-rich atoms in the films. The reasons may be as follows: First, some Cu atoms combine with oxygen atoms in the vacuum chamber to form Cu-O, and oxygen originates from the background atmosphere of the vacuum chamber. Second, a very small amount of Cu atoms is deposited directly on the substrate before it reacts with nitrogen in the vacuum chamber. In addition, other phases such as Cu 4 N may exist in the films [36], causing it to deviate from the stoichiometric ratio of 3:1. The corresponding EDS elemental mapping of the surface of the Cu 3 N films is shown in Figure 2b.  Figure 3a shows the XRD results of the Cu3N films. The diffraction peaks of the crystal planes of Cu3N crystals are sharp, which indicates that the Cu3N films prepared by magnetron sputtering has a good crystallinity under different r. When r is small, the Cu3N films show preferential growth in the crystal planes of (100) and (200). By contrast, the films show preferential growth the crystal plane of (110). This result is similar to most research reports, which indicate that Cu3N prepared by magnetron sputtering has an anti-ReO3 structure [19,31,37]. The reason for the preferential growth orientation of the Cu3N films is as follows: At low r, films are mainly formed by Cu-N bonds due to the adsorption of nitrogen atoms into the crystal lattice of Cu atoms; and then Cu3N (100) and Cu3N (200) planes are grown. At high r, sufficient nitrogen atoms combine with Cu on the target surface or substrate surface to form Cu-N. The Cu3N (110) plane is grown on the films according to the principle that the free energy of the crystal plane has the lowest priority. Another explanation is that as the flow of nitrogen increases, the number of Cu atoms reaching the surface of the substrate decreases. As such, insufficient Cu atoms combine with high-energy nitrogen, resulting in the weakening of the Cu3N (110) peak intensity [28,31]. When the experimental parameters change, the preferential growth of the Cu3N crystal plane shifts from (100) to (111) [38]. This phenomenon may be caused by different preparation methods employed. At the same time, it is not difficult to find that there are two very weak diffraction peaks around 31.5° and 48.1°, which belong to the diffraction peaks of CuO and Cu, respectively [39]. The XRD diffraction pattern of the TiO2 films is shown in Figure 3b. The peaks at 25.3°, 38.0°, 48.1°, and 53.3° correspond to the diffraction peaks of the (101), (112), (200), and (211) crystal planes of TiO2 [8,40], respectively, which belong to anatase TiO2 as referred to JCPDS card (NO. . The peaks at 54.8° correspond to the diffraction peak of the (211) crystal plane of TiO2, belonging to the tetragonal rutile phase [9]. The main body of the TiO2 films prepared by magnetron sputtering is anatase structure. Compared with rutile, anatase has more oxygen vacancies and can easily trap electrons, resulting in better electron-hole separation effect [41]. Figure 4a shows the survey scan of the XPS spectrum of Cu3N films prepared at different r values. A strong oxygen peak appears in the spectrum, indicating that the surface of the films contains more oxygen. Oxygen mainly originates from the moisture and atmospheric absorption of the films and the residual oxygen in the vacuum chamber. As the r increases, the CuLMM peak and Cu2P peak in the films increase significantly, thereby indicating that increasing flow rate of N2 is more favorable for the formation of a Cu3N structure by nitrogen and Cu atoms in the vacuum chamber. To analyze the bonding mode and valence state of Cu in the films, we performed Lorentz fitting on the Cu2P peak of the XPS spectrum. Figure 4b shows the XPS fitting of Cu2P for the Cu3N films. In the range of 930.0-940.0 eV, the spectral line can be fitted by two Cu characteristic peaks, namely, Cu 2+ Cu2P3/2 (334.5 eV) and Cu 1+ Cu2P3/2 (333.0 eV), indicating that the valence of Cu on the surface of the films is +2 or +1 [42,43]. At the same time, according to the fitting results, we obtained by simple estimation that the ratio of Cu 1+ to Cu 2+ in the films deposited under the conditions of r = 1/1 and r = 1/3 is 3.66  Figure 3a shows the XRD results of the Cu 3 N films. The diffraction peaks of the crystal planes of Cu 3 N crystals are sharp, which indicates that the Cu 3 N films prepared by magnetron sputtering has a good crystallinity under different r. When r is small, the Cu 3 N films show preferential growth in the crystal planes of (100) and (200). By contrast, the films show preferential growth the crystal plane of (110). This result is similar to most research reports, which indicate that Cu 3 N prepared by magnetron sputtering has an anti-ReO 3 structure [19,31,37]. The reason for the preferential growth orientation of the Cu 3 N films is as follows: At low r, films are mainly formed by Cu-N bonds due to the adsorption of nitrogen atoms into the crystal lattice of Cu atoms; and then Cu 3 N (100) and Cu 3 N (200) planes are grown. At high r, sufficient nitrogen atoms combine with Cu on the target surface or substrate surface to form Cu-N. The Cu 3 N (110) plane is grown on the films according to the principle that the free energy of the crystal plane has the lowest priority. Another explanation is that as the flow of nitrogen increases, the number of Cu atoms reaching the surface of the substrate decreases. As such, insufficient Cu atoms combine with high-energy nitrogen, resulting in the weakening of the Cu 3 N (110) peak intensity [28,31]. When the experimental parameters change, the preferential growth of the Cu 3 N crystal plane shifts from (100) to (111) [38]. This phenomenon may be caused by different preparation methods employed. At the same time, it is not difficult to find that there are two very weak diffraction peaks around 31.5 • and 48.1 • , which belong to the diffraction peaks of CuO and Cu, respectively [39]. The XRD diffraction pattern of the TiO 2 films is shown in Figure 3b.  [8,40], respectively, which belong to anatase TiO 2 as referred to JCPDS card (NO. . The peaks at 54.8 • correspond to the diffraction peak of the (211) crystal plane of TiO 2 , belonging to the tetragonal rutile phase [9]. The main body of the TiO 2 films prepared by magnetron sputtering is anatase structure. Compared with rutile, anatase has more oxygen vacancies and can easily trap electrons, resulting in better electron-hole separation effect [41]. Figure 4a shows the survey scan of the XPS spectrum of Cu 3 N films prepared at different r values. A strong oxygen peak appears in the spectrum, indicating that the surface of the films contains more oxygen. Oxygen mainly originates from the moisture and atmospheric absorption of the films and the residual oxygen in the vacuum chamber. As the r increases, the CuLMM peak and Cu2P peak in the films increase significantly, thereby indicating that increasing flow rate of N 2 is more favorable for the formation of a Cu 3 N structure by nitrogen and Cu atoms in the vacuum chamber. To analyze the bonding mode and valence state of Cu in the films, we performed Lorentz fitting on the Cu2P peak of the XPS spectrum. Figure 4b shows the XPS fitting of Cu2P for the Cu 3 N films. In the range of 930.0-940.0 eV, the spectral line can be fitted by two Cu characteristic peaks, namely, Cu 2+ Cu2P 3/2 (334.5 eV) and Cu 1+ Cu2P 3/2 (333.0 eV), indicating that the valence of Cu on the surface of the films is +2 or +1 [42,43]. At the same time, according to the fitting results, we obtained by simple estimation that the ratio of Cu 1+ to Cu 2+ in the films deposited under the conditions of r = 1/1 and r = 1/3 is 3.66 and 0.69, respectively. As the nitrogen pressure in the vacuum chamber decreases, the Cu + component in the films decreases. Figure 4c shows the N1s in the Cu 3 N films. The peak of binding energy of N is mainly at 400.0 eV, and the peak at 397.8 eV is relatively small. The binding energy at 397.8 eV indicates that the nitrogen atom in the form of beta-N (β-N) exists in the crystal, and that at 400.0 eV indicates that the nitrogen atom in the form of gamma-N (γ-N) exists in the crystal or in the form of N-N and N-O bond [44]. Similarly, according to the Lorentz fitting results of the N1s peak, the ratio of β-N to γ-N in the films deposited under the conditions of r = 1/1 and r = 1/3 is estimated to be 2.68 and 2.03, respectively.
Materials 2020, 13, x FOR PEER REVIEW 5 of 11 and 0.69, respectively. As the nitrogen pressure in the vacuum chamber decreases, the Cu + component in the films decreases. Figure 4c shows the N1s in the Cu3N films. The peak of binding energy of N is mainly at 400.0 eV, and the peak at 397.8 eV is relatively small. The binding energy at 397.8 eV indicates that the nitrogen atom in the form of beta-N (β-N) exists in the crystal, and that at 400.0 eV indicates that the nitrogen atom in the form of gamma-N (γ-N) exists in the crystal or in the form of N-N and N-O bond [44]. Similarly, according to the Lorentz fitting results of the N1s peak, the ratio of β-N toγ-N in the films deposited under the conditions of r = 1/1 and r = 1/3 is estimated to be 2.68 and 2.03, respectively.   Materials 2020, 13, x FOR PEER REVIEW 5 of 11 and 0.69, respectively. As the nitrogen pressure in the vacuum chamber decreases, the Cu + component in the films decreases. Figure 4c shows the N1s in the Cu3N films. The peak of binding energy of N is mainly at 400.0 eV, and the peak at 397.8 eV is relatively small. The binding energy at 397.8 eV indicates that the nitrogen atom in the form of beta-N (β-N) exists in the crystal, and that at 400.0 eV indicates that the nitrogen atom in the form of gamma-N (γ-N) exists in the crystal or in the form of N-N and N-O bond [44]. Similarly, according to the Lorentz fitting results of the N1s peak, the ratio of β-N toγ-N in the films deposited under the conditions of r = 1/1 and r = 1/3 is estimated to be 2.68 and 2.03, respectively.    Figure 5a shows the UV-vis absorption spectrum of the Cu 3 N films. The films show good absorption in the visible light band, indicating its feasibility for catalytic application to visible light. The absorption coefficient α of the Cu 3 N films can be obtained directly by the absorption spectrum of the films as follows: where T and d are the transmittance and thickness of the Cu 3 N films. The optical band gap (E g ) of the Cu 3 N films can be obtained by the Tauc equation as follows: where h, υ, and A are the Planck constant, optical frequency, and constant, respectively. The E g of the films prepared by magnetron sputtering is within 1.96-2.09 eV (Figure 5b), and the result is slightly larger than that in literature [28,45,46]. The reason may be caused by different preparation techniques and preparation parameters.
Materials 2020, 13, x FOR PEER REVIEW 6 of 11 Figure 5a shows the UV-vis absorption spectrum of the Cu3N films. The films show good absorption in the visible light band, indicating its feasibility for catalytic application to visible light. The absorption coefficient α of the Cu3N films can be obtained directly by the absorption spectrum of the films as follows: where T and d are the transmittance and thickness of the Cu3N films. The optical band gap (Eg) of the Cu3N films can be obtained by the Tauc equation as follows: where h, υ, and A are the Planck constant, optical frequency, and constant, respectively. The Eg of the films prepared by magnetron sputtering is within 1.96-2.09 eV (Figure 5b), and the result is slightly larger than that in literature [28,45,46]. The reason may be caused by different preparation techniques and preparation parameters. The photoluminescence (PL) spectra of Cu3N films prepared under different r at 370 nm excitation wavelength are shown in Figure 6. The room-temperature PL bands of the Cu3N films prepared under different r are mainly concentrated in the blue-violet light region. The PL spectrum of the sample shows three peaks that are located at 418, 488, and 507 nm. The peak position at 418 nm is very strong, and the peak positions at 488 and 507 nm are relatively weak. We think that the peak position at 418 nm is a band-edge emission of the Cu3N films, and the two other peaks are caused by defect-state fluorescence [28]. The change in r has little effect on the intensity of each luminous peak. Based on this spectrum, the intrinsic luminescence energy gap of the Cu3N film is within 2.34-2.97 eV, which is higher than the optical band gap Eg obtained in Figure 5b (1.96-2.09 eV) [47,48]. The energy level difference is about 0.6 eV. The light emitting region of Cu3N in the blueviolet light range is mainly due to the following: Unbonded Cu atoms (which can be regarded as selfdoping) that form the electronic transition from the defect level to the valence band [49]; the energy levels of the center vacancy defects in the Cu nitride crystal and the electronic transitions between the composite defects; and the electronic transitions between the interface defects near the Cu3N grain boundary and the valence band [28]. The photoluminescence (PL) spectra of Cu 3 N films prepared under different r at 370 nm excitation wavelength are shown in Figure 6. The room-temperature PL bands of the Cu 3 N films prepared under different r are mainly concentrated in the blue-violet light region. The PL spectrum of the sample shows three peaks that are located at 418, 488, and 507 nm. The peak position at 418 nm is very strong, and the peak positions at 488 and 507 nm are relatively weak. We think that the peak position at 418 nm is a band-edge emission of the Cu 3 N films, and the two other peaks are caused by defect-state fluorescence [28]. The change in r has little effect on the intensity of each luminous peak. Based on this spectrum, the intrinsic luminescence energy gap of the Cu 3 N film is within 2.34-2.97 eV, which is higher than the optical band gap E g obtained in Figure 5b (1.96-2.09 eV) [47,48]. The energy level difference is about 0.6 eV. The light emitting region of Cu 3 N in the blue-violet light range is mainly due to the following: Unbonded Cu atoms (which can be regarded as self-doping) that form the electronic transition from the defect level to the valence band [49]; the energy levels of the center vacancy defects in the Cu nitride crystal and the electronic transitions between the composite defects; and the electronic transitions between the interface defects near the Cu 3 N grain boundary and the valence band [28].
The degradation rate reaches 99.8% when r = 1/3. As shown in Figure 7, the photodegradation rate of methyl orange increases with the extension of the illumination time. The photocatalytic degradation rate of methyl orange on the films reaches 93.5% after 30 min of illumination, and r = 1/3 is the best photocatalytic activity (degradation rate reaches 99.5%). This finding is due to the large number of voids and free Cu coexisting in the Cu nitride structure prepared at r = 1/3, and the combination of the two exhibits good photocatalytic activity. The superior photocatalytic properties of the Cu 3 N films are mainly due to its special anti-ReO 3 lattice structure. In Cu 3 N crystals, Cu atoms do not occupy the tight position of the lattice (111) plane but leave many voids in their crystal structure, causing the other atoms to easily fill the central vacancy of the Cu 3 N lattice. After this void is filled with other atoms, structural defects or impurity replacement defects are formed in the Cu 3 N crystal. The presence of these defects plays an important role in photocatalysis. The degradation rate reaches 99.8% when r = 1/3. As shown in Figure 7, the photodegradation rate of methyl orange increases with the extension of the illumination time. The photocatalytic degradation rate of methyl orange on the films reaches 93.5% after 30 min of illumination, and r = 1/3 is the best photocatalytic activity (degradation rate reaches 99.5%). This finding is due to the large number of voids and free Cu coexisting in the Cu nitride structure prepared at r = 1/3, and the combination of the two exhibits good photocatalytic activity. The superior photocatalytic properties of the Cu3N films are mainly due to its special anti-ReO3 lattice structure. In Cu3N crystals, Cu atoms do not occupy the tight position of the lattice (111) plane but leave many voids in their crystal structure, causing the other atoms to easily fill the central vacancy of the Cu3N lattice. After this void is filled with other atoms, structural defects or impurity replacement defects are formed in the Cu3N crystal. The presence of these defects plays an important role in photocatalysis.
A schematic of the degradation of methyl orange by the Cu3N films is shown in Figure 8. In photocatalysis, the formation of •OH free radicals is very important [50]. Each Cu3N particle in the films can be regarded as a small short-circuit photoelectrochemical cell, which can generate wideranging electrons (e − ) and holes (h + ) under the photoelectric effect. The e − and h + migrate to different positions on the surface of the Cu3N grains under the action of the electric field. The photo-generated electrons e − on the surface of the Cu3N grains are easily captured by oxidizing substances, such as dissolved oxygen in the water. The h + can oxidize organic substances adsorbed on the surface of the Cu3N grains or first oxidize •OH and H2O molecules adsorbed on the surface of the Cu3N grains into •OH radicals. The oxidizing ability of •OH radical is the strongest among the oxidants in the water body, and it can oxidize most of organic matter and inorganic pollutants in the water and mineralize them into inorganic small molecules, carbon dioxide, water, and other harmless substances. The possible chemical formulas are as follows: ℎ + H O → • OH + H OH + H → ⋯ → CO + H O.
The superior photocatalytic degradation activity of the Cu3N films can be explained by frontier molecular orbital theory [51,52]. In this photocatalytic reaction system, the 2P electron orbital of oxygen atom is the frontier molecular orbit (HOMO), and the metal Cu orbital in the films is the lowest unoccupied molecular orbital (LUMO). The electrons are transitioned from the oxygen atom under specific wavelength illumination. For the Cu atom, the atomic HOMO needs to reach stable

Conclusion
Cu3N films were prepared by magnetron sputtering on silicon and quartz substrate under different gas flow ratios. Characterization and analysis of the microstructure and optical properties A schematic of the degradation of methyl orange by the Cu 3 N films is shown in Figure 8. In photocatalysis, the formation of •OH free radicals is very important [50]. Each Cu 3 N particle in the films can be regarded as a small short-circuit photoelectrochemical cell, which can generate wide-ranging electrons (e − ) and holes (h + ) under the photoelectric effect. The e − and h + migrate to different positions on the surface of the Cu 3 N grains under the action of the electric field. The photo-generated electrons e − on the surface of the Cu 3 N grains are easily captured by oxidizing substances, such as dissolved oxygen in the water. The h + can oxidize organic substances adsorbed on the surface of the Cu 3 N grains or first oxidize ·OH and H 2 O molecules adsorbed on the surface of the Cu 3 N grains into ·OH radicals. The oxidizing ability of ·OH radical is the strongest among the oxidants in the water body, and it can oxidize most of organic matter and inorganic pollutants in the water and mineralize them into inorganic small molecules, carbon dioxide, water, and other harmless substances. The possible chemical formulas are as follows:

Conclusion
Cu3N films were prepared by magnetron sputtering on silicon and quartz substrate under different gas flow ratios. Characterization and analysis of the microstructure and optical properties of the films were performed using modern analytical testing techniques and methods. The Cu3N films prepared in this work have a good crystalline phase and present an anti-ReO3 crystal structure. The Cu3N films contains Cu-rich atoms and have part of structure vacancies. These defects play a decisive role with regard to the structure and light absorption properties of the films. Experimental results show that under the irradiation of mercury lamp, the degradation rate of methyl orange by the Cu3N films can reach more than 99.5% (when r = 1/3, the best effect is achieved), which is superior to that of TiO2 films with the same thickness prepared by magnetron sputtering. This finding is due to the presence of more defects and Cu-rich atoms in the films, resulting in effective separation of photogenerated electron-hole pairs. This phenomenon also expands the absorption band edge red shift, enhances the light absorption ability, and confers excellent photocatalytic performance. Hence, the fabricated Cu3N films can oxidize pollutants into carbon dioxide and water for effective degradation and would be a promising low cost and nontoxic material for photocatalysis.   The superior photocatalytic degradation activity of the Cu 3 N films can be explained by frontier molecular orbital theory [51,52]. In this photocatalytic reaction system, the 2P electron orbital of oxygen atom is the frontier molecular orbit (HOMO), and the metal Cu orbital in the films is the lowest unoccupied molecular orbital (LUMO). The electrons are transitioned from the oxygen atom under specific wavelength illumination. For the Cu atom, the atomic HOMO needs to reach stable electrons to form ·OH with e in water molecules, thereby effectively degrading the methyl orange molecule and achieving the photocatalytic effect.

Conclusions
Cu 3 N films were prepared by magnetron sputtering on silicon and quartz substrate under different gas flow ratios. Characterization and analysis of the microstructure and optical properties of the films were performed using modern analytical testing techniques and methods. The Cu 3 N films prepared in this work have a good crystalline phase and present an anti-ReO 3 crystal structure. The Cu 3 N films contains Cu-rich atoms and have part of structure vacancies. These defects play a decisive role with regard to the structure and light absorption properties of the films. Experimental results show that under the irradiation of mercury lamp, the degradation rate of methyl orange by the Cu 3 N films can reach more than 99.5% (when r = 1/3, the best effect is achieved), which is superior to that of TiO 2 films with the same thickness prepared by magnetron sputtering. This finding is due to the presence of more defects and Cu-rich atoms in the films, resulting in effective separation of photogenerated electron-hole pairs. This phenomenon also expands the absorption band edge red shift, enhances the light absorption ability, and confers excellent photocatalytic performance. Hence, the fabricated Cu 3 N films can oxidize pollutants into carbon dioxide and water for effective degradation and would be a promising low cost and nontoxic material for photocatalysis.