Preparation of Cu3N/MoS2 Heterojunction through Magnetron Sputtering and Investigation of Its Structure and Optical Performance.

Cu3N/MoS2 heterojunction was prepared through magnetron sputtering, and its optical band gap was investigated. Results showed that the prepared Cu3N/MoS2 heterojunction had a clear surface heterojunction structure, uniform surface grains, and no evident cracks. The optical band gap (1.98 eV) of Cu3N/MoS2 heterojunction was obtained by analyzing the ultraviolet-visible transmission spectrum. The valence and conduction band offsets of Cu3N/MoS2 heterojunction were 1.42 and 0.82 eV, respectively. The Cu3N film and multilayer MoS2 formed a type-II heterojunction. After the two materials adhered to form the heterojunction, the interface electrons flowed from MoS2 to Cu3N because the latter had higher Fermi level than the former. This behavior caused the formation of additional electrons in the Cu3N and MoS2 layers and the change in optical band gap, which was conducive to the charge separation of electrons in MoS2 or MoS2 holes. The prepared Cu3N/MoS2 heterojunction has potential application in various high-performance photoelectric devices, such as photocatalysts and photodetectors.


Introduction
Environmental problems have worsened due to the rapid consumption of fossil fuels. Thus, efficient, energy-saving, and environmentally friendly methods must be developed to solve various pollution problems [1]. Among them, photocatalysis is a widely used technology in solar energy conversion and shows good potential [2,3]. Other available methods are sewage treatment [4,5] and dye degradation [6,7]. Heterojunctions formed with 2D materials and other semiconductors have recently attracted attention from researchers due to their various potential applications. Many heterojunction materials, including semiconductors/semiconductors, metals/semiconductors, molecules/semiconductors [8,9], and multiple heterojunctions [10], have been used in photocatalysis and achieved considerable success. Heterojunction photocatalysts typically have the following advantages: strong light absorption [11], efficient charge separation and transport [12], cocatalyst effect, and strong light absorption stability [13].
Among these heterojunctions, Cu 3 N/MoS 2 shows promise as a photocatalyst because Cu 3 N is an outstanding semiconductor material that can substantially enhance the photoelectric performance of MoS 2 or introduce new functions into such heterojunctions. As a typical transition metal sulfide, MoS 2 has a S-Mo-S sandwich structure combined by the van der Waals force [14,15]. Given its unique structure and photoelectric properties, MoS 2 has attracted considerable attention from scholars [16,17] and has been widely investigated for thin film transistors [18,19], photodetectors [20,21], and solid lubrication [22,23]. Owing to its adjustable band gap, MoS 2 can be combined with

Experimental
Cu 3 N/MoS 2 heterojunctions were prepared on single-crystal silicon (100) and quartz substrates by using RF magnetron sputtering (JGP-450a, SKY Technology Development Co., Ltd, Shenyang, China). First, the silicon and quartz wafer substrates were separately sonicated in an acetone and ethanol solution for 15 min, rinsed with deionized water, and dried for further use. Second, the processed monocrystalline silicon and quartz wafers were placed in the substrate support, the target was installed, and the sputtering chamber was closed. Third, the sputtering chamber was evacuated to 2.5 Pa with a mechanical pump and the vacuum was driven to 1 × 10 −4 Pa by using a turbomolecular pump. Before the experiment, the target was presputtered in Ar atmosphere for 10 min and the surface of the target was cleaned to remove the oxide. Fourth, the MoS 2 layer was deposited on the substrate using high-purity molybdenum disulfide target (99.99%, Beijing Jingmai Zhongke Material Technology Co., Ltd., Beijing, China) in Ar atmosphere at room temperature. Total gas flow rate, vacuum chamber pressure, power, and sputtering time were set to 40 sccm, 1.0 Pa, 150 w, and 2 and 3.5 min, respectively. Fifth, a Cu 3 N layer was deposited on the MoS 2 layer with a sputtering time of 2 min using a copper target (99.99%, Beijing Jingmai Zhongke Material Technology Co., Ltd., Beijing, China). Total gas flow rate, flow ratio of N 2 and Ar, vacuum chamber pressure, power, and sputtering time were set to 40 sccm, 3:1, 1 Pa, 150 W, and 1.5 min, respectively. Under the same experimental conditions, a pure Cu 3 N layer was deposited on the blank substrate for 3.5 min. The deposition rates of MoS 2 and Cu 3 N layers are 12 and 13.3 nm/min, respectively. The diameter of molybdenum disulfide target and copper target are both 60 mm. The sputtering time for the heterojunction in the scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) was enlarged 10 times, and the rest of the conditions remained unchanged to easily observe the heterojunction structure.
The surface morphology of heterojunction was characterized with a field emission scanning electron microscope (S-4800, Hitachi, Tokyo, Japan). The crystal structure of heterojunction was characterized using X-ray diffractometry (XRD, X'Pert PRO, PANalytical, Holland). Elemental characterization was conducted via X-ray photoelectron spectroscopy (XPS, Escalab, Thermo Fisher Scientific, MA, USA). The optical band gap of heterojunction was investigated with an ultraviolet-visible (UV-vis) spectrometer (UV-2600/2700, Shimadzu, Kyoto, Japan).

Results and Discussion
The XRD test patterns of MoS 2 layer, Cu 3 N layer, and Cu 3 N/MoS 2 heterojunction are shown in Figure 1. The 2θ in the MoS 2 (002), (100), (101), and (110) crystal planes was found at 14.55 • , 32.3 • , 36.5 • , and 49.4 • , respectively [22,35]. The absence of other impurity peaks in the spectrum indicated the high purity of the obtained MoS 2 . Moreover, the sharp diffraction peak of MoS 2 revealed the good crystallinity of MoS 2 . The average grain size of the heterojunction was 14.9 nm, which was smaller than the grain size observed in the SEM image. This may have been because the size of the grain aggregate observed by the SEM was not the size of the single grain; therefore, the particle size value observed by SEM was often larger than the calculated value. When the Cu 3 N layer was deposited on the MoS 2 layer to form the Cu 3 N/MoS 2 heterojunction, the sample still showed reflection on the (100), (101), and (110) crystal planes of MoS 2 . However, the intensity of the (100), (101), and (110) crystal planes of MoS 2 in the Cu 3 N/MoS 2 heterojunction was weaker than that in the pure MoS 2 layer, indicating the reduced crystal quality of the MoS 2 layer after Cu 3 N deposition. Compared with that of the pure Cu 3 N layer, the strength of the (100) and (200) crystal planes of Cu 3 N in the Cu 3 N/MoS 2 heterojunction decreased, possibly due to the interdiffusion of atoms that occurred at the interface between MoS 2 and Cu 3 N and led to the reduced quality of heterogeneous crystals.

Results and Discussion
The XRD test patterns of MoS2 layer, Cu3N layer, and Cu3N/MoS2 heterojunction are shown in Figure 1. The 2θ in the MoS2 (002), (100), (101), and (110) crystal planes was found at 14.55°, 32.3°, 36.5°, and 49.4°, respectively [22,35]. The absence of other impurity peaks in the spectrum indicated the high purity of the obtained MoS2. Moreover, the sharp diffraction peak of MoS2 revealed the good crystallinity of MoS2. The average grain size of the heterojunction was 14.9 nm, which was smaller than the grain size observed in the SEM image. This may have been because the size of the grain aggregate observed by the SEM was not the size of the single grain; therefore, the particle size value observed by SEM was often larger than the calculated value. When the Cu3N layer was deposited on the MoS2 layer to form the Cu3N/MoS2 heterojunction, the sample still showed reflection on the (100) Figure 2. This surface of the MoS2 layer was generally smooth and flat with block particles of uniform size and no evident cracks. These characteristics were beneficial to the growth of the Cu3N layer on the surface of the MoS2 layer. Figure 2b shows the thickness of the heterojunction. The MoS2 and Cu3N layers had a thicknesses of approximately 300 and 200 nm, respectively. The boundary between the layers clearly showed the heterojunction structure. The relationship between the film deposition thickness and deposition time suggested that the Cu3N/MoS2 heterojunction was approximately 50 nm-thick.  Figure 2. This surface of the MoS 2 layer was generally smooth and flat with block particles of uniform size and no evident cracks. These characteristics were beneficial to the growth of the Cu 3 N layer on the surface of the MoS 2 layer. Figure 2b shows the thickness of the heterojunction. The MoS 2 and Cu 3 N layers had a thicknesses of approximately 300 and 200 nm, respectively. The boundary between the layers clearly showed the heterojunction structure. The relationship between the film deposition thickness and deposition time suggested that the Cu 3 N/MoS 2 heterojunction was approximately 50 nm-thick. The distribution of elements in the Cu3N/MoS2 heterojunction and the atomic percentage of each element are shown in Figure 3. EDS data revealed that the ratio of Cu to N atoms in the heterojunction was around 1. Moreover, its chemical ratio differed from the standard ratio of Cu3N, possibly because of the free Cu atoms in the heterojunction. These free atoms did not combine with the N atoms to form Cu3N. However, fewer Mo and S atoms were detected on the surface of the heterojunction. The ratio of Mo and S atoms was about 1.87. This phenomenon occurred because MoS2 can be found in the lower layer, and some Mo atoms in the lower layer had moved to the surface. Meanwhile, the S atoms had difficulty in reaching the surface. The atomic ratio of Cu and N in the film Cu3N was 1:1, which does not meet the standard atomic ratio of Cu3N, in which the N element was significantly more than the Cu element. This is because during the deposition process, the ratio of N2 to Ar was high, and a part of the free N atoms were adsorbed between the crystal grains. However, when Mo: S = 3.11:1.66 = 1:0.53 ≈ 1.87:1, there were fewer S atoms. This may have beeen because when MoS2 is deposited, a part of S becomes a single substance to escape, resulting in fewer S atoms. Mo atoms that were not combined into MoS2 combined with MoO3 to oxygen (as shown in Figure 4e,f). In addition, the XPS spectrum also proved that there were a large number of Cu + , N − (as shown in Figure 4c   The distribution of elements in the Cu 3 N/MoS 2 heterojunction and the atomic percentage of each element are shown in Figure 3. EDS data revealed that the ratio of Cu to N atoms in the heterojunction was around 1. Moreover, its chemical ratio differed from the standard ratio of Cu 3 N, possibly because of the free Cu atoms in the heterojunction. These free atoms did not combine with the N atoms to form Cu 3 N. However, fewer Mo and S atoms were detected on the surface of the heterojunction. The ratio of Mo and S atoms was about 1.87. This phenomenon occurred because MoS 2 can be found in the lower layer, and some Mo atoms in the lower layer had moved to the surface. Meanwhile, the S atoms had difficulty in reaching the surface. The atomic ratio of Cu and N in the film Cu 3 N was 1:1, which does not meet the standard atomic ratio of Cu 3 N, in which the N element was significantly more than the Cu element. This is because during the deposition process, the ratio of N 2 to Ar was high, and a part of the free N atoms were adsorbed between the crystal grains. However, when Mo: S = 3.11:1.66 = 1:0.53 ≈ 1.87:1, there were fewer S atoms. This may have beeen because when MoS 2 is deposited, a part of S becomes a single substance to escape, resulting in fewer S atoms. Mo atoms that were not combined into MoS 2 combined with MoO 3 to oxygen (as shown in Figure 4e,f). In addition, the XPS spectrum also proved that there were a large number of Cu + , N − (as shown in Figure 4c The distribution of elements in the Cu3N/MoS2 heterojunction and the atomic percentage of each element are shown in Figure 3. EDS data revealed that the ratio of Cu to N atoms in the heterojunction was around 1. Moreover, its chemical ratio differed from the standard ratio of Cu3N, possibly because of the free Cu atoms in the heterojunction. These free atoms did not combine with the N atoms to form Cu3N. However, fewer Mo and S atoms were detected on the surface of the heterojunction. The ratio of Mo and S atoms was about 1.87. This phenomenon occurred because MoS2 can be found in the lower layer, and some Mo atoms in the lower layer had moved to the surface. Meanwhile, the S atoms had difficulty in reaching the surface. The atomic ratio of Cu and N in the film Cu3N was 1:1, which does not meet the standard atomic ratio of Cu3N, in which the N element was significantly more than the Cu element. This is because during the deposition process, the ratio of N2 to Ar was high, and a part of the free N atoms were adsorbed between the crystal grains. However, when Mo: S = 3.11:1.66 = 1:0.53 ≈ 1.87:1, there were fewer S atoms. This may have beeen because when MoS2 is deposited, a part of S becomes a single substance to escape, resulting in fewer S atoms. Mo atoms that were not combined into MoS2 combined with MoO3 to oxygen (as shown in Figure 4e,f). In addition, the XPS spectrum also proved that there were a large number of Cu + , N − (as shown in Figure 4c,d), and Mo-S bonds (as shown in Figure 4e,f) in the film. It showed that the heterojunction is mainly composed of Cu3N and MoS2, and contains only a small amount of MoO3 and N atoms, which does not affect the formation of Cu3N/MoS2 heterojunction. In addition, the Si atoms detected via EDS originated from the monocrystalline silicon substrate.  peaks located at 232.0 and 228.5 eV could be inferred as the Mo3d3/2 orbital peak in the Mo-S hybrid bond structure in MoS2 that characterized Mo 4+ in MoS2. The two weak peaks at 223.46 and 232.95 eV could be attributed to the S2s orbital and Mo-O hybrid bond structure that indicated the formation of sulfide and partial oxidation on the film surface [23]. However, the Mo-S hybrid bond of the Mo3d orbital peak of pure MoS2 was located at the binding energies of 228.4 and 231.5 eV. The XPS spectra were consistent with the XRD and EDS results and indicated a large number of MoS2 and Cu3N in the heterojunction.
Materials 2020, 13, x FOR PEER REVIEW 6 of 9   The chemical composition of the Cu 3 N/MoS 2 heterojunction was characterized using XPS, and the test results are shown in Figure 4. Figure 4a,b illustrates the total spectrum of the Cu 3 N/MoS 2 heterojunction and MoS 2 thin film, respectively. Figure 4a depicts that the binding energies of the S2p, Mo 3d, N1s, O1s, and Cu2p peaks were 162.6, 227.8, 397.4, 530.5, and 931.3 eV, respectively [15]. Figure 4b shows that the S2p and Mo 3d peaks appeared at the binding energies of 161.5 and 229.5 eV, respectively. The O1s peak appearing in the spectrogram might be due to the inevitable atmospheric pollution in the heterojunction during transfer and testing. However, a weak S2p peak appeared at 162.6 eV possibly due to the trace S atoms sputtered out during the deposition of the remaining MoS 2 layer in the vacuum chamber, thus causing pollution in the Cu 3 N layer. The high intensity peaks with binding energies of 931.1 and 952.1 eV corresponded to the Cu2p 3/2 and Cu2p 1/2 orbital peaks, respectively, as shown in Figure 4c. Figure 4d illustrates the fitting of Cu2p peaks in the Cu 3 N layer with the Cu2p 3/2 and Cu2p 1/2 orbital peaks at the binding energies of 932.81 and 952.76 eV, respectively [27]. In Figure 4c,d, weaker peaks appeared at the binding energies of 933.9, 952.6 eV and 934.5, 954.4 eV, which belong to Cu 2+ , which is caused by the slight oxidation of Cu + . Figure 4e,f depicts the Mo3d spectra in the Cu 3 N/MoS 2 heterojunction and pure MoS 2 layers, respectively. After the XPS spectrum of Mo3d was fitted, four peaks appeared with different intensities. The two main peaks located at 232.0 and 228.5 eV could be inferred as the Mo3d 3/2 orbital peak in the Mo-S hybrid bond structure in MoS 2 that characterized Mo 4+ in MoS 2 . The two weak peaks at 223.46 and 232.95 eV could be attributed to the S2s orbital and Mo-O hybrid bond structure that indicated the formation of sulfide and partial oxidation on the film surface [23]. However, the Mo-S hybrid bond of the Mo3d orbital peak of pure MoS 2 was located at the binding energies of 228.4 and 231.5 eV. The XPS spectra were consistent with the XRD and EDS results and indicated a large number of MoS 2 and Cu 3 N in the heterojunction. Figure 5 presents the energy band diagram of the Cu 3 N/MoS 2 interface. The valence (VBO or ∆Ev) and conduction (CBO or ∆Ec) band offsets of the interface were calculated on the basis of the XPS spectra and corresponding band gap data as follows [13]: where E s m represents the energy of characteristic m in sample s, E MoS 2 VBM ; E The UV-vis transmission spectrum and diagram of band gap calculation of the Cu3N/MoS2 heterojunction are shown in Figure 6. The transmittance of the pure Cu3N layer in the wavelength range of 350-650 nm was clearly improved after a heterojunction was formed between Cu3N and MoS2. According to the transmission spectrum curve of the thin film, the Eg of Mo-Cu based compound region can be calculated using the optical constant law (3) and Tauc Equation (4) as follows [31]: where T represents the transmittance, d denotes the film thickness, indicates the absorption coefficient, ℎ refers to the photon energy, and a signifies the constant. The band gap Eg could be inferred from the relationship curve between ℎ and ℎ by linearly fitting the straight part of the curve as shown in Figure 6b. The optical band gaps of the Cu3N layer, MoS2 layer, and Cu3N/MoS2 Mo-Cu-based compound region were 1.83, 2.43, and 1.98 eV, respectively. changed the optical band gap. The formed heterojunction can be applied to various high-performance photoelectric devices, such as photocatalysts and photodetectors.
The UV-vis transmission spectrum and diagram of band gap calculation of the Cu 3 N/MoS 2 heterojunction are shown in Figure 6. The transmittance of the pure Cu 3 N layer in the wavelength range of 350-650 nm was clearly improved after a heterojunction was formed between Cu 3 N and MoS 2 . According to the transmission spectrum curve of the thin film, the E g of Mo-Cu based compound region can be calculated using the optical constant law (3) and Tauc Equation (4) as follows [31]: where T represents the transmittance, d denotes the film thickness, α indicates the absorption coefficient, hv refers to the photon energy, and a signifies the constant. The band gap E g could be inferred from the relationship curve between (αhv) 2 and hv by linearly fitting the straight part of the curve as shown in Figure 6b. The optical band gaps of the Cu 3 N layer, MoS 2 layer, and Cu 3 N/MoS 2 Mo-Cu-based compound region were 1.83, 2.43, and 1.98 eV, respectively. The UV-vis transmission spectrum and diagram of band gap calculation of the Cu3N/MoS2 heterojunction are shown in Figure 6. The transmittance of the pure Cu3N layer in the wavelength range of 350-650 nm was clearly improved after a heterojunction was formed between Cu3N and MoS2. According to the transmission spectrum curve of the thin film, the Eg of Mo-Cu based compound region can be calculated using the optical constant law (3) and Tauc Equation (4) as follows [31]: where T represents the transmittance, d denotes the film thickness, indicates the absorption coefficient, ℎ refers to the photon energy, and a signifies the constant. The band gap Eg could be inferred from the relationship curve between ℎ and ℎ by linearly fitting the straight part of the curve as shown in Figure 6b. The optical band gaps of the Cu3N layer, MoS2 layer, and Cu3N/MoS2 Mo-Cu-based compound region were 1.83, 2.43, and 1.98 eV, respectively.

Conclusions
Cu3N/MoS2 heterojunction was prepared on silicon and quartz substrates through magnetron sputtering. Microstructure analysis revealed the good crystallinity of MoS2 and Cu3N in the heterojunction, the uniform distribution of spherical grains on the surface, the thickness of the heterojunction at approximately 50 nm, and the formation of a heterojunction structure with a clear interface. This work provided a simple and effective method for preparing heterojunctions. XPS and UV correlation spectra and data revealed that the prepared Cu3N/MoS2 heterojunction had an

Conclusions
Cu 3 N/MoS 2 heterojunction was prepared on silicon and quartz substrates through magnetron sputtering. Microstructure analysis revealed the good crystallinity of MoS 2 and Cu 3 N in the heterojunction, the uniform distribution of spherical grains on the surface, the thickness of the heterojunction at approximately 50 nm, and the formation of a heterojunction structure with a clear interface. This work provided a simple and effective method for preparing heterojunctions. XPS and UV correlation spectra and data revealed that the prepared Cu 3 N/MoS 2 heterojunction had an arrangement of II-type staggered band. The VBO and CBO of the Cu 3 N/MoS 2 interface were 1.42 and 0.82 eV, respectively. This condition facilitated the charge separation of electrons in MoS 2 . The prepared heterojunction can be applied to various high-performance photoelectric devices, such as photocatalysts and photodetectors.