The superior photocatalytic performance and DFT insights of S-scheme CuO@TiO2 heterojunction composites for simultaneous degradation of organics

The necessity to resolve the issue of rapid charge carrier recombination for boosting photocatalytic performance is a vigorous and challenging research field. To address this, the construction of a binary system of step-scheme (S-scheme) CuO@TiO2 heterostructure composite has been demonstrated through a facile solid-state route. The remarkably enhanced photocatalytic performance of CuO@TiO2, compared with single TiO2, which can consequence in the more efficient separation of photoinduced charge carriers, reduced the band gap of TiO2, improved the electrical transport performance, and improved the lifetimes, thus donating it with the much more powerful oxidation and reduction capability. A photocatalytic mechanism was proposed to explain the boosted photocatalytic performance of CuO@TiO2 on a complete analysis of physicochemical, DFT calculations, and electrochemical properties. In addition, this work focused on the investigation of the stability and recyclability of CuO@TiO2 in terms of efficiency and its physical origin using XRD, BET, and XPS. It is found that the removal efficiency diminishes 4.5% upon five recycling runs. The current study not only promoted our knowledge of the binary system of S-scheme CuO@TiO2 heterojunction composite photocatalyst but also shed new light on the design of heterostructure photocatalysts with high-performance and high stability.


Results and discussions
Structural analysis. The structural characteristics of pure CuO and pure TiO 2 powder were characterized as the contrasts to binary CuO@TiO 2 heterostructure nanocomposites that shown in Fig. 1a. The diffraction peaks at scattering angles (2θ) of 32.0, 35.95°, 38.21° and 48.34° were indexed to (1 1 0) (− 1 − 1 1),(1 1 1), and (2 0 2) planes of monoclinic structure of CuO, respectively, which corresponding to JCPDS No. 48-1548 13 . Furthermore, the P-25 consists of anatase and rutile phases, matching very well with the confirmation of JCPDS No. 021-1272 and 021-1276, respectively 14 . Pure TiO 2 and CuO peaks exhibit the high intensity and sharp nature, indicating pure crystalline nature. Other phases, such as impurities, Cu 2 O or metallic Cu, are not present, indicating that pure CuO could be achieved by chemical and calcination methods. The findings show that the binary composite of CuO@TiO 2 heterostructures contains two-phase TiO 2 and CuO compositions after the solid state reaction. CuO@TiO 2 heterostructure nanocomposites show more XRD peaks than TiO 2 , indicating that CuO is present on the surface of the TiO 2 15 . The higher intensity of (− 1 − 1 1) and (111) diffraction peaks in CuO@ TiO 2 , could be due to the sheild effect from the CuO nanoparticles 16 . However, a small gradual shift of the characteristic peak (101) of TiO 2 is observed towards a lower 2θ value after the formation of CuO@TiO 2 , which could be attributed to partial substitution of titanium atom (ionic radius Ti 4+ , 0.074 nm) by the larger size of Cu atom (ionic radius Cu 2+ , 0.087 nm) at surface interaction 17,18 , as we suggested and studied by DFT calculations. The same phenomena were studied and confirmed due substitution of I atom by another halogen of Br and Cl atoms in lead-free perovskites 19 .
Moreover, it is vital to note that Raman spectroscopy is a surface-probing technique, whereas XRD samples analyze the bulk. As a result, Raman spectra revealed a surface modification caused by the vibrational mode. Figure 1b illustrates the Raman spectra of TiO 2 , CuO and its binary composite CuO@TiO 2 heterostructure. In the Raman spectrum of TiO 2 , the active vibrational modes are recognized as: (i) the symmetric vibration of O-Ti-O in TiO 2 due to the doubly E g modes at 145 and 606 cm −1 ; and (ii) the symmetric bending vibration of O-Ti-O due to the B 1g modes at 437 cm −1 , while the A g peak at 272 cm −1 mode of CuO crystals, endorses the anatase-TiO 2 phase's distinctive peaks as reported in the literature 20,21 . After solid state reaction, the modification by CuO affected on the position of the Raman peaks associated with the B 1g and A 1g vibration modes and confirmed the structural phases of TiO 2 and CuO for binary CuO@TiO 2 heterostructure as shown in Fig. 1b. Additionally, there is a blue shift at 404 cm −1 which established the robust interaction obtainable in CuO@TiO 2 . Cu 2+ has a lower valence than Ti 4+ , therefore oxygen vacancies and other crystalline defects generated at the surface of TiO 2 by CuO helped to explain the spectrum behavior 22,23 . The structural and vibrational features of the TiO 2 lattice were altered when CuO was added, demonstrating that CuO and TiO 2 had a strong interaction between them 24 . Surface defects like this could act as photoactive centers, improving the charge separation efficiency 23 . Microstructure and elemental distribution. The microstructure of CuO and its binary system of CuO@TiO 2 heterostructure using SEM is shown in Fig. 2. Figure 2a provides an overview of the morphology of the CuO NPs which exhibit aggregated nanoparticles associated with irregular shape. In Fig. 2b, after solid state reaction, it revealed that the aggregated and interlinked nanocrystals, that aid in the intraparticle charge transfer. CuO nanoparticles also existed on the surface of TiO 2 nanoparticles.
The detailed morphology and microstructure of binary CuO@TiO 2 heterostructure nanocomposite were further investigated by high resolution transmission electron microscope (HR-TEM) and energy-dispersive X-ray spectroscopy (EDS). The corresponding HR-TEM image of CuO@TiO 2 heterostructure with different magnifications is presented at Fig. 2c-g. It is found that the extremely aggregated with irregularly shaped nanoparticles. The light gray regions are the TiO 2 microstructure, and the dark regions are CuO nanoparticles. This proved that the binary system of CuO@TiO 2 nanocomposite was crystalline in a uniform shape (not a physical mixture), but had the decoration of CuO on TiO 2 which formed the interface that leads to the CuO@TiO 2 heterojunction.  (Fig. 2h).
The elemental analysis in the binary system of CuO@TiO 2 nanocomposite were studied by energy-dispersive X-ray spectroscopy (EDS) mapping Fig. 3a endorses the existence of copper (Cu), titanium (Ti), and oxygen (O) elements with the inset displaying an atomic percentage proportion. The EDS mappings of each element are shown in Fig. 3b-e, and they clearly show that this Cu, Ti, and O species are homogeneously distributed throughout the entire selected area, demonstrating the continuous existence of O in the CuO particle and TiO 2 and further through their interface (Fig. 2e). This clearly illustrates the construction of the heterojunction between CuO and TiO 2 , which assists in the transfer of charges between the bands of two semiconductors, CuO 23,29 . Also, no additional peaks may be observed on the spectra of CuO@TiO 2 in the Ti 2p region, which indicates that the addition of CuO is not disturbing the TiO 2 lattice and both oxides exist as separate phases in presented materials 30 . It also confirms CuO to TiO 2 electron transport at CuO@TiO 2 heterojunction 31 . These results are supported by HRTEM. The only observed change in the Ti 2p XPS image is the decrease of intensity of the peaks, which is the natural consequence of the decreasing concentration of the TiO 2 in the materials with the addition of CuO.  33 . According to the calculation of peak areas, the concentration of O V is 35.87% over CuO@TiO 2 is higher than CuO (22.89%) and TiO 2 (13.32%), which is crucial for photocatalytic activity. The chemical composition of various types of oxygen and their percentages are summarized in Table 1 16 . The higher binding energy comes from the upshift of Cu 2p peak that shows the substantial interaction between CuO nanoparticles and TiO 2 nanoparticles, which is consistent with the literature 34,35 . Furthermore, there is no peak that relates to Cu + at around 932.7 eV, signifying that Cu species mainly exist as CuO. The existence of an unfilled Cu 3d shell corresponds to the Cu 2+ species at the CuO surface, as seen by the shakeup satellite peaks with binding energy at 942.66 and 962.2 eV 36 . This is another proof to endorse the dominant surface copper is CuO in CuO@TiO 2 heterostructure.
In summary, the surface of CuO@TiO 2 photocatalyst is oxidized and functional groups such as OH are incorporated during the calcination. The oxygen vacancies in CuO@TiO 2 form free electrons that lead to creating a reduced form of Ti species (e.g. Ti 4+ ). This is endorsed by the blue shift of the B 1g vibrational mode of TiO 2 (as identified by Raman) and the negative shift of Ti 2p to lower binding energy (as identified by XPS). Based on the XRD, HR-TEM, XPS, and Raman spectroscopy observations, we endorse the existence of TiO 2 and CuO and the successful fabrication of CuO@TiO 2 heterostructure photocatalysts with strong interface interaction.
Textural properties. The textural nature of the catalyst CuO@TiO 2 investigated using BET analysis. The results showed that it has a surface area of 19 m 2 /g, a total pore volume of 3.12 × 10 -2 cm 3 /g, and a pore diameter of 6.08 nm. Based on the IUPAC classification, the nitrogen sorption isotherms for the catalyst display the type IV in the shape with H3 hysteresis loop and large adsorption of N 2 at P/P 0 > 0.8 (Fig. 5a). H3 loop indicates the www.nature.com/scientificreports/ existence of large mesopores (2-50 nm) with few of micropores 37 . Figure 5b depicts the plot of the BJH pore-size distribution of the catalyst CuO@TiO 2 , which shows that it is essentially mesoporous particles 38 . The low surface area of CuO@TiO 2 confirms the well-formed crystalline structure, which is extremely important for photocatalytic applications. The high pore volume of the CuO@TiO 2 increases the internal mass transfer and therefore improves the catalytic activity and enhances the photodegradation of AR8.
Optical properties. The band gap of TiO 2 , CuO, and its binary CuO@TiO 2 heterostructure catalysts by using the tauc plot are shown in Fig. 6a. It represents the direct transition of band gap energy by plotting (αhυ) 2 versus hυ. As seen, the band gap for P-25 TiO 2 was correspondingly to 3.43 eV, which is caused by the intrinsic interband absorption of TiO 2 39 . After sintering at high temperature, the Cu 2 O color was changed to black, matching the color of the CuO semiconductor. After addition of CuO nanoparticles distinctively displays the lower optical band gap energy of 2.35 eV compared to CuO and TiO 2 of about 2.5 and 3.43 eV respectively. The smaller bandgap of CuO@TiO 2 heterostructure as compared to TiO 2 can be elucidated on the basis of (a) the quantum confinement phenomena, and (b) the existence of CuO favors the formation of oxygen vacancies and/or the partial reduction of Ti sites (as identified by XPS). Taking into account the reduction of Ti sites and/or oxygen vacancies acts as a new state localized in the decreasing of the bandgap. The incorporation of CuO in TiO 2 matrix results in the formation of two closely spaced conduction bands formed by the sharing of interfacial electrons and holes between Cu 2+ and Ti 4+ . Therefore, the enhanced light absorption and interfacial charge transfer will be advantageous for improving the photocatalytic performance of the hybrid photocatalysts 21,40 As a result, the latter optical adsorption is extra proof for the presence of CuO classes in the CuO@TiO 2 heterostructure.
To determine the role of photogenerated electron-hole pairs in TiO 2 , CuO, and its binary CuO@TiO 2 heterojunction, as well as to demonstrate the mechanism liable for the remarkable boosted photocatalytic degradation of pollutants. Photoluminescence (PL) studies were investigated. The fluorescence emission spectra of samples exhibited a broadband peak at 423 nm under the excitation of 350 nm (Fig. 6b). As we know, the fluorescence intensity of the catalysts tended to be inversely proportional to the rate of recombination rate of e − /h +41 . As a result, the fluorescence intensity of the pure TiO 2 and CuO is high, indicating a higher photogenerated electron-hole binding rate 41 . The modification of TiO 2 by CuO was found to lower fluorescence intensity and more efficient separation of electron and hole by CuO@TiO 2 heterojunction that formed at the interface and therefore accounts for boosting the photocatalytic activity 42 . It also accelerates the creation of e − , and extends the lifetime of e − /h + lifetime, which was ascribed to the potential well made by the Cu 2+ hetero-junction to trap electrons 43 . The separation of electrons and holes results from the electron transfer from TiO 2 to CuO nanoparticles at the interface of CuO (p-type) and the electron-rich of TiO 2 (n-type), which is one possible clarification for the suppressed charge recombination 44,45 . Electrochemical properties, EIS. To acquire profoundly thoughtful into the influence of CuO nanoparticles on behaviors of charge transport and reaction rate of the surface of binary system of CuO@TiO 2 composite, the estimation of electrochemical impedance spectroscopy (EIS) was achieved. It is well acknowledged that the smaller arc radius of the first semi circuit matches to a reduced electron-transfer resistance (the low-frequency semicircle), implying the high efficiency of separation and charge transfer 46 , however second semi circuit is attributed to recombination resistance (the high-frequency semicircle) in p-n junction of photovoltaic devices in the fitting EIS spectra (Fig. 6c). The semicircular in EIS plots for all samples indicate the same behavior. Furthermore, the equivalent electrical circuit was shown as inset and the collected values of ohmic series resistance (R s ), charge transfer resistance (R ch ), recombination resistance (R rec ), and interface resistance were detailed in Table 2. Among all results the CuO@TiO 2 show a significantly smaller charge transfer resistance R ct (1372 Ω) than both of TiO 2 (1476 Ω) or CuO (3074 Ω), which suggests that CuO@TiO 2 has the fastest electron-transfer rate 47,48 . Comparing with bare CuO and bare TiO 2 , the binary system of CuO@TiO 2 suggests the higher separation efficiency of photoinduced e − /h + pairs at the interface, faster interfacial charge transfer, and more efficient separation of electron-hole pairs 46,49 . On the other hand, the CuO@TiO 2 shows higher recombination resistance than TiO 2 which confirms that the additional CuO interface of the binary system of CuO@TiO 2 prevents the excited electrons from recombination in TiO 2 . It is worth to mention, that the equivalent electrical circuit present also very high interface resistance for both TiO 2 (315 Ω) and CuO (1383 Ω) compared to the binary system CuO@TiO 2 (4 Ω). Therefore, the lower charge transfers resistance and higher recombination resistance of CuO@ TiO 2 favored a higher photocatalytic activity.
Computational calculations. The DFT calculations are commonly regarded as crucial to comprehending the behavior of chemical structures. We presented four possible configurations based on the XRD results (TiO 2 , Cu@TiO 2 , Ti@CuO, and CuO) that might exist as illustrated in Fig. 7. The computational calculations were performed to relax all possible structures and estimate the charge distribution and electronic properties as well [50][51][52] . Along best-fit plane's charge density distributions for the four possible configurations were carried out. The resulting data display a small displacement of Ti/Cu positions along with the surface interaction of both materials and combined with an increase in a bond length of Ti-O and Cu-O for both Cu@TiO 2 , Ti@CuO, respectively.
The experimental energy gap results of diffuse reflectance spectra are inconsistent with the preformed band structure calculations (Fig. 8). The band diagram shows an indirect bandgap for TiO 2 and proposed Cu@TiO 2 (surface interaction) from X to G symmetry point, along with the crystal symmetry directions in the first Brillouin zone. Similarly, the findings reveal a direct bandgap for both CuO   www.nature.com/scientificreports/     www.nature.com/scientificreports/ to the same orbitals with a significant contribution at maximum VB. Thus, these results at the surface interaction of CuO-TiO 2 configuration predict that a significant enhancement of CB to decrease the band gap. However, because of the contribution of copper atoms to enhancement of maximum VB in the existence of the p-orbital of titanium atoms, could achieve the charge transfer due to S-scheme mechanism and resulting in an increase in the attract of excited electrons of CuO and the holes of TiO 2 as well, during photocatalytic degradation process.
Photocatalytic activity. Kinetic studies. The photocatalytic degradation of Acid Red 8 (AR8) aqueous solution was assessed using batch mode. Figure 11a shows the catalytic activities of without catalyst (UV only), TiO 2 , CuO, and its binary CuO@TiO 2 heterojunction composites. The photocatalytic activity of the CuO@TiO-2 was found to be substantially higher than those of TiO 2 and CuO nanoparticles. The photocatalytic degradation of AR8 has also followed the pseudo-first-order kinetic model (Fig. 11b). Kinetic curves also highlight CuO@ TiO 2 exhibits 3.08 and 4.11 times the photodegradation ability of CuO and TiO 2 , respectively (Table 3). Also, the t ½ decreases with increasing the apparent rate constant (k app ) as shown in Table 3. The low decomposition rates of CuO and TiO 2 nanoparticles could be attributed to the fast recombination of e − /h + pairs and inefficient quantum yield during the photocatalytic processes. The superiority of CuO@TiO 2 binary system is due to the efficient charge transfer, enlarged light response range, and suppressed the photocarrier recombination that originated from the synergistic effects of two photocatalysts, TiO 2 and CuO that resulted from p-n heterojunction between p-type CuO and n-type TiO 2 53 . Photogenerated holes diffuse from CuO (p-type) to TiO 2 (n-type) and electrons diffuse from TiO 2 to CuO due to the existence of carrier concentration gradients. An electric field is formed at the junction when it is in the equilibrium 54 .
Photogenerated electrons can migrate the conduction band of TiO 2 (n-type) and holes to the valence band of CuO (p-type) during photocatalysis. The p-n CuO@TiO 2 heterojunction increases the photocatalytic activity by facilitating the separation of photogenerated electrons and holes. Additionally, the sensitization by CuO increased the optical absorption properties of TiO 2 and improved charge carrier lifetime in the system that leads to enhancing the photocatalytic activity of CuO@TiO 2 . As indicated by XRD, HR-TEM, XPS, UV-Vis., PL, and www.nature.com/scientificreports/ EIS characterizations as well as DFT investigations, heterojunction formation considerably influenced the photocatalytic activity of CuO@TiO 2 due to its influences on the photocatalyst microstructure and band structure.
Quantum efficiency. The quantum yield can be detected by estimating the rate of disappearing of the reactant molecule or the formation of the product molecule divided by the photons absorbed per unit time, which can be used to quantify the heterogeneous catalysis. A considerable portion of the incident light is reflected or scattered by dispersed photocatalysts and is not absorbed by the dye solution. In most cases, there is no way to measure the amount of light absorbed by the photocatalyst experimentally. Another parameter frequently stated is the apparent quantum yield (Q app ), which avoids the challenges of estimating the quantum yields in the photocatalytic reaction 55 . Table 3 clarifies that Q app for UV/CuO@TiO 2 system is higher than that of the UV/TiO 2 and UV/CuO systems. As a result of the poor quantum yield, CuO and TiO 2 nanoparticles had the lowest photocatalytic activity.
Investigations of E EO . The economic analysis of photocatalysis is critical factor that accounts for a large portion of operating costs. As a result, it is vital to evaluate the photocatalytic process' electrical energy consumption under experimental settings. Because it follows the pseudo first order kinetic model, the electrical energy per order (E EO ) is a useful indicator of the photocatalysis process 56 . The E EO enables a quick calculation of the cost of electrical energy and indicates the overall power needed. The treatment efficiency for the samples is evaluated using E EO values for comparative study.
The E EO values were calculated using the inverse of the slope of a plot of log (C o /C) versus UV dose (Fig. 11c). It was determined that the figure-of-merit method is appropriate for calculating the electrical energy efficiency. It is not only demonstrating the decline in the amount of electricity required by the photocatalytic system, but also the significant impact of the UV dose on the E EO in the process. From Table 3, the E EO values were established to be depending on the photocatalyst's nature. Furthermore, the E EO values of the UV/CuO@TiO 2 system are lower than those of the blank UV/TiO 2 or UV/CuO system, indicating that the lower energy consumption is attributable to the higher applied potential and formation of highly reactive radical species. In summary, these insights can be used to design photocatalytic systems that use less electrical energy, have a higher rate constant, and cost less to operate.

Role of reactive oxygen species.
To get insights into the photodegradation mechanism of AR8 dye over the CuO@TiO 2 binary system and understand the role of photo-generated holes and radicals in the photodegradation process, the charge trapping studies were conducted. To explore the role of reactive oxygen species (ROS), the photocatalytic activity of CuO@TiO 2 was greatly suppresed by the addition of EDTA, K 2 Cr 2 O 7 , benzoquinone (BQ), and tert-Butyl alcohol (TBA) as model scavengers to capture holes (h + ), electron (e − ), superoxide radical ( · O 2 -), and hydroxyl radical ( · OH), respectively 57 . The AR8 degradation efficiency by CuO@TiO 2 in the existence of various scavengers is shown in Fig. 11d. Both BQ and TBA had stronger suppressing effects on the photocatalytic degradation of AR8 than that for EDTA and K 2 Cr 2 O 7 , suggesting that · O 2 − and · OH are the principal active species in the photocatalytic degradation of AR8 in the presence of a binary system of CuO@TiO 2 .
Based on the outcomes of the trapping tests, the following Eqs. (1-6) can be offered as a possible photodegradation mechanism of AR8 dye in the presence of CuO@TiO 2 heterojunction. (1) 2 e − + HO · 2 + H + → · OH + OH − ,  www.nature.com/scientificreports/ Stability and recyclability of CuO@TiO 2 photocatalyst. From the economic point of view, the stability and good recyclability are two essential features for large-scale application of photocatalysis 58 . Using the same protocol, the degradation of AR8 were done on subsequent repeated cycles by reusing the CuO@TiO 2 collected after each cycle. Then, the degradation percentage was calculated in each run. The stability and recyclability of CuO@TiO 2 towards AR8 dye photodegradation were evaluated after five consecutive runs and the obtained data are presented in Fig. 12a. It is clear that the reduction in the removal efficiency was negligible (< 4.5%), which denotes the nature of high stability after 5 cycles. Wherefore, CuO@TiO 2 is an economically suitable photocatalyst for industrial application from a practical point of view.
To evaluate the stability of CuO@TiO 2 photocatalyst, the XRD patterns were compared before and after the first cycle of the photocatalytic degradation of AR8 dye as shown in Fig. 12b. Obviously, the maintained structure after photocatalysis was evidenced by the unchanged XRD reflections. All peaks corresponding to the interpretation based on Fig. 1a, represent the stability of CuO@TiO 2 .
As it is known, the porous structure is one of the most important requirements for the ideal photocatalysis process. Table 4 shows the textural nature of sample CuO@TiO 2 and its recycled sample after photocatalysis that was investigated by BET analysis (Fig. 5). There is a slight decrease in the surface area after recycling indicating low structural changes occurrence by recycling ( Table 4). The pore volume and pore diameter were also slightly reduced. Even after the recycling, the catalyst still had a feasible pore diameter that can accommodate the flow of reactant intermediate molecules) through the photocatalyst during the reaction.S BET surface area based on BET equation, S mico micropores surface area, L 0 micropore width, V T total pore volume, V meso mesopores volume.
To discuss the mechanism of the photocatalytic reaction for CuO@TiO 2 , an XPS analysis was investigated the elemental valence states on the surface of CuO@TiO 2 before and after AR8 removal. The changes of element www.nature.com/scientificreports/ state before and after the photocatalytic reaction are revealed in Fig. 13 and Table 5. Figure 13a shows that the O 1s peaks of CuO@TiO 2 were divided into three peaks. After photocatalysis, the peak at 529.39 eV belonged to the lattice oxygen (O L ), including Ti-O and Cu-O, was shifted to 529.07 e.V 29 . Also, the peak at 530.94 e.V attributed to ions O 2 − staying in the oxygen vacancy (OV) that is shifted to 530.84 e.V. The peak at 530.53 eV was attributed to hydroxyl oxygen (O H ) that shifted to 530.76 e.V after photocatalysis. The relative ratios of elements for catalysts have been calculated and are shown in Table 5. The oxygen content in TiO 2 and CuO mainly composed of O L , and the proportion of O H and O V is very small (Table 1), which reduced to 24.36% in the composite. After photocatalysis, the concentration of O L is reduced to 10.02%, which confirmed the contribution of O H and O V in the photocatalytic process (Table 5) and supported the results of the role of reactive oxygen species (Fig. 11d) and photodegradation mechanism (Eqs. 1-6). Furthermore, CuO@TiO 2 displayed lower O H than before photocatalysis, which was ascribed to the high oxidation ability of the CuO@TiO 2 structure (Table 5). Also, CuO@TiO 2 displayed much higher oxygen vacancy (Ov/O) than before photocatalysis, which was attributed to the greater loss of lattice oxygen by TiO 2 nanoparticles to introduce more oxygen vacancies to the composite (Table 1).
After UV illumination, Fig. 13b show the peaks of Cu 2p 3/2 and Cu 2p 1/2 moved to 932.76 and 952.71 eV, respectively, being assigned to Cu +59 . These revealed that most of surface Cu 2+ was reduced to Cu + upon the UV irradiation 60 . The photocatalytic process introduced more oxygen vacancies on the surface of CuO@TiO 2 , which is reflected in Table 5. Under UV light irradiation, the electron can transfer from the valence band to the surface CuO through interface charge transfer, leading to the reduction of Cu 2+ to Cu +3 .
After photocatalysis, the 457.75 and 463.75 e.V positions were assigned to typical Ti 2p 3/2 and Ti 2p 1/2 peaks in CuO@TiO 2, respectively, which indicate there is a small shift of all peaks when compared to the composite before photocatalysis (Fig. 13c). This shift of binding energy values is attributed to the redistribution of electric charge in the composite materials 61 . In addition, the spectra of Ti 2p region showing the presence of Ti 3+ that formed from the reduction of Ti 4+ and denoting the influence of UV light on the nature of the Ti phase, in agreement with TiO 2 results previously reported 28 . Despite the certain reduction observed the difference in BE between Ti 2p3/2 and Ti 2p1/2 components are always maintained at 5.7 eV.
Structure activity correlation and proposed photocatalytic mechanism. The plausible mechanism of the photocatalytic degradation of AR8 dye over CuO@TiO 2 heterojunction composite is proposed in Fig. 14 based on the aforementioned experimental findings and characterization analysis. In this system, upon direct irradiation, the e − /h + pairs would be formed on both TiO 2 and CuO. In the transfer process, the lifetime of the excited electrons and holes is extended, resulting in better quantum efficiency (Table 3).
Furthermore, this considerably improves the separation of the photogenerated electron-hole pairs and limits their quick recombination, resulting in CuO@TiO 2 having a higher photocatalytic activity than TiO 2 56 . When the combination of TiO 2 with CuO, an inner electrical field forms at the interface lead to improving the electron-hole separation 62 .
Because of TiO 2 of lower EF and connected with CuO of higher EF, the CuO@TiO 2 heterojunction is formed. However, the electrons will transfer from CuO to the TiO 2 easily until the EF at the interface tends to equilibrate. Concurrently, CuO shows downward interface band bending and is positively charged at the interface owing to loss of electrons; while TiO 2 shows upward interface band bending and is negatively charged at the interface owing to an accumulation of electrons. Meanwhile, the electrons transfer creates an internal electric field at interfaces pointing from CuO to TiO 2 . When the CuO@TiO 2 heterojunction is exposed to light, under the combined effect of an internal electric field, interface band bending and Coulomb interaction, the photogenerated electrons in CB of TiO 2 easily transfer to VB of CuO and recombine with the photogenerated holes of CuO. Meanwhile, the photogenerated holes on VB of TiO 2 and the photogenerated electrons on CB of CuO are maintained, which participate in the photocatalytic redox reaction, respectively [63][64][65][66] .
Because CuO's conduction band is more negative than TiO 2 's, photo-generated electrons have an affinity to transfer from CuO's conduction band to TiO 2 's conduction band because of the difference in potential, reducing the possibility of photoinduced e − /h + recombination and thus facilitating reduction reactions for the destruction of pollutants 67 . Meanwhile, because TiO 2 's VB is more positive than CuO's, the holes transfer from TiO 2 's VB into CuO's VB, which carry out oxidation reactions with water to form a proton and intermediate products. This causes the photogenerated electrons to be accumulated at the conduction band of TiO 2 and also to be accumulated of holes at the valence band of CuO. As a result of the higher conduction and valence band of CuO than TiO 2 , the transfer process is thermodynamically advantageous 68,69 . Hence, the e − /h + pair recombination rate in the CuO@TiO 2 is substantially lower than in pure TiO 2 . Furthermore, the electrons help Ti 3+ ions in having a longer lifetime and improve ions transference at the interface between CuO and TiO 2 . Overall, the p-n heterojunction may prevent the recombination of the photogenerated electrons from recombining with holes.
According to the abovementioned results of the effect of reactive oxygen species and the results from XPS before and after photocatalysis, the ongoing electrons reduced dissolved oxygen, generating · O 2 − (Eq. 2) and the following · OH (Eq. 4) which was the governing active species for the pollutant attacking that consistent with the results of effect of scavengers (Fig. 11d). On the other hand, the holes stay in the VB of CuO that helps in the Table 4. Textural characteristics of the CuO@TiO 2 before and after photocatalysis.

Samples
S BET (m 2 g −1 ) S micro (m 2 g −1 ) L o (nm) V T (cm 3 g −1 ) Pore diameter (nm) V meso (cm 3 g −1 )  ) (Eq. 2). Continuous production of highly strong oxidants ( · O 2 − and · OH) leads to the oxidation AR8 to CO 2 and H 2 O 21 . According to the thoughts presented above, the highly boosted photocatalytic activity of CuO@TiO 2 heterostructures compared to pure TiO 2 can be attributed to a number factors, including the intensification in the photo-absorption, perfection in the charge separation efficiency and direct oxidation of AR8 dye with CuO after the construction of p-n heterojunctions. The synergistic impact (for example, heterojunction-induced effects on CuO@TiO 2 photocatalysts) can aid to strengthen the separation of photo-generated electrons and holes (Fig. 14). As a result, the CuO@TiO 2 composites outperform ordinary TiO 2 in terms of photocatalytic activity. Another benefit for photocatalytic decomposition by CuO@TiO 2 heterojunction nanocomposite is to form electrons that help Ti 3+ ions for extending the lifetime and improve the transfer of electrons at the p-n junction which suppresses the electron/hole recombination.

Materials and chemicals
TiO 2 (Degussa P 25, average particle size about 30 nm, 70% anatase form and of surface area about 50 m 2 g −1 ) was purchased from Degussa company, Germany. CuCl 2 ·2H 2 O, NaOH and glucose were purchased from Aldrich. All aqueous solutions were prepared using distilled water from the Millipore instrument. Acid red 8 dye with molecular formula C 18 H 14 N 2 Na 2 O 7 S 2 , and molecular weight of 480.42 g/mol, with λ max 508 nm was purchased from Fluka.

Synthesis of photocatalysts.
CuO nanoparticles were prepared by free-template method by dissolving 1 g of copper precursor in 50 mL solvent (water/ethanol) under a constant stirring at room temperature. A precipitate was produced when NaOH solution (3 M, 10 mL) is added dropwise to the above solution. After being stirred for 5 min, d-glucose powder (0.2 g) was added into the dark precursor and the temperature was raised up to 70 °C with stirring for 15 min. The precipitate gradually turns into brick red and then it was allowed to cool to room temperature and the obtained precipitates were centrifuged. The precipitate was allowed to centrifuge twice more in de-ionized water and anhydrous ethanol, respectively. Finally, the precipitates product was dried at 50 °C overnight.
The S-scheme CuO@TiO 2 heterojunction nanocomposite was prepared through a solid-state reaction route. 50 wt. of Cu 2 O nanoparticles were added to 50 wt.% TiO 2 which have been mixed uniformly and made fine by  www.nature.com/scientificreports/ grinding in ball mill for 2 h to get fine nanopowders. The resultant powders were calcined in air at 500 C for 3 h in an electric muffle furnace.

Conclusions
To the best of our knowledge, this is the first time we have shown that the fabricating a binary system of S-scheme CuO@TiO 2 heterojunction composite photocatalyst via a simple solid-state reaction is a promising strategy for boosting the photocatalytic degradation of toxic organics. The existence of heterojunction between TiO 2 and CuO was demonstrated by the measurements of catalytic activity, including HRTEM, Raman, XPS, UV-Vis. PL and EIS. Also, the experimental optical results showed that the CuO@TiO 2 reduced the band gap by shifting the band-gap of commercial TiO 2 (3.43 eV) into a new band gap of CuO@TiO 2 (2.35 e.V) and DFT calculation explained that by details. The S-scheme CuO@TiO 2 heterojunction composite, displaying the lowest PL intensity, shows the highest activity, and reveals the apparent quantum efficiency of 3.2%. The higher photocatalytic activity was qualified to the well-designed S-scheme CuO@TiO 2 heterojunction, which remarkably aids the separation of photogenerated electrons and holes by the synergistic impacts at the interface of the catalyst components. Further, the most active species in the photodegradation of AR8 dye was detected as · OH and · O 2 − . However, the experimental results demonstrated that the h + , and e − are also involved in the degradation process. The plausible degradation mechanism by p-n S-scheme CuO@TiO 2 heterojunction was determined and discussed based upon detection of reactive oxygen species, PL and EIS, which found that the the electron transfer from CuO to TiO 2 , followed by hole transfer from TiO 2 to CuO was attributed to be the most likely mode of charge transfer in the S-scheme CuO@TiO 2 system. The recycling experiment revealed that the AR8 dye is diminished by 4.5% after five runs of CuO@TiO 2 . The result certified the suitable stability of CuO@TiO 2 for long-term applications. Further theoretical work is currently underway to investigate the formation of S-scheme CuO@TiO 2 heterojunction. Potentially, this study offers a new way for the strategy of the highly efficient S-scheme heterojunction photocatalyst for the effective degradation of organic pollutants in large scales.

Data availability
All data generated or analyzed during this study are included in this article (and its Supplementary Information file). www.nature.com/scientificreports/