Topics in Catalysis https://doi.org/10.1007/s11244-020-01360-6 ORIGINAL PAPER Layered Nanocomposite 2D‑TiO2 with ­Cu2O Nanoparticles as an Efficient Photocatalyst for 4‑Chlorophenol Degradation and Hydrogen Evolution Matías Alegría1 · Juan Aliaga1 · Luis Ballesteros2 · Clivia Sotomayor‑Torres3,4 · Guillermo González5 · Eglantina Benavente1,6 © Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract New composites formed by layered hybrid ­TiO2(stearic acid) ­(LHTiO2) and, ­Cu2O nanoparticles were studied as photocata- lysts that extend the response range to light visible for the evolution of hydrogen and the degradation of 4-chlorophenol. The results revealed that L ­ HTiO2/Cu2O exhibited a clearly improved photocatalytic degradation, about 5.6 times faster than pristine ­TiO2, and hydrogen evolution of about 2.7 times higher than the ­TiO2 anatase. The enhanced photocatalytic activity can be assigned to the properties of the two-dimensional morphology, in sheets-like arrangement of ­LHTiO2, benefitting from the high exposure of surface, with more active sites available to improve matching with the surfaces of the C ­ u 2O nanocrystals and significant reduction of migration distances of photogenerated carriers. In the photocatalytic degradation, a mechanism Z-scheme is supported, and in the photocatalytic evolution of hydrogen a mechanism type II band alignment is indicated. Photocatalytic reuse tests showed that stability and catalytic activity of ­LHTiO2/Cu2O were maintained for three cycles. Photoelectrochemical evaluation were performed through measurements of the photocurrent response and electrochemical impedance. Keywords Layered 2D-TiO2 · Cu2O · Photocatalysis · Degradation of 4-chlorophenol · Hydrogen evolution 1 Introduction and requires semiconductors that can provide charge transfer to the surface-active sites, stability, and the harvesting of a Photocatalytic reaction systems have been extensively stud- wide range of the solar spectrum [4–6]. In this context, one ied for environmental remediation and solar energy conver- of the most studied semiconductors is ­TiO2, mainly for its sion [1–3]. Photocatalysis is a semiconductor-mediated pro- low cost, low toxicity, photo-stability, reactivity and abun- cess that can absorb and utilize light for chemical reactions, dance [7, 8]. Nevertheless, its large bandgap (3.2 eV) and fast charge recombination limit its application in photocata- Electronic supplementary material The online version of this lytic processes [9, 10]. article (https​://doi.org/10.1007/s1124​4-020-01360​-6) contains supplementary material, which is available to authorized users. 3 * Guillermo González Catalan Institute of Nanoscience and Nanotechnology (ICN2) ggonzale@uchile.cl CSIC and BIST, Campus UAB Bellaterra, 08193 Barcelona, Spain * Eglantina Benavente 4 ebenaven@utem.cl ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain 5 1 Departamento de Química, Facultad de Ciencias, Departamento de Química, Facultad de Ciencias Naturales, Universidad de Chile, Las Palmeras, 3425 Santiago, Chile Matemáticas y del Medio Ambiente, Universidad 6 Tecnológica Metropolitana, Santiago, Chile Programa Institucional de Fomento a la Investigación, 2 Desarrollo e Innovación (PIDi), Universidad Tecnológica Instituto de Ciencias Químicas Aplicadas, Inorganic Metropolitana, Santiago, Chile Chemistry and Molecular Material Center, Facultad de Ingeniería, Universidad Autónoma de Chile, El Llano Subercaseaux, 2801 San Miguel, Santiago, Chile 13 Vol.:(0123456789) Topics in Catalysis An effective approach to design efficient photocatalysts 2 Experimental Section is the strategy of coupling a T ­ iO2 wide-band-gap semi- conductor with narrow-band-gap semiconductors, such as 2.1 Materials and Reagents ­Cu2O [11–14], ­RuO2 [15] and CdS [16, 17], amongst others, broadens the absorption range and can also show superior All chemical reagents used were obtained from commercial photocatalytic performance because of the charge transfer sources as guaranteed grade reagents. Ti[OCH(CH3)2]4 (tita- between the two different types of semiconductors. This nium tetraisopropoxide), C­ 18H36O2 (stearic acid), C ­ 6H 8O 6 decreases the electron/hole recombination, while maintain- (ascorbic acid) and N ­ 2H4 (hydrazine) were purchased from ing good compatibility between the two semiconductors, Sigma-Aldrich. Methanol, ethanol and 4-Chlorophenol increase the matching of the semiconductors bandgap, with (4-CP) were purchased from Merck. the essential feature of using the hole oxidation power for the degradation of water pollutants and the electron reduction 2.2 Preparation power for the evolution of hydrogen [8, 18]. Among these materials, ­Cu2O has emerged as the right 2.2.1 Layered Hybrid Titanium Nanocomposite ­(LHTiO2) candidate. It is a p-type semiconductor, abundant and low- cost, with a direct bandgap of 2.0 eV, which can be excited This was synthesized following a procedure described in the by visible light [19–21]. Some reports indicate that a draw- literature [16]. Elemental analysis for T ­ iO2(C18H35O2)1.3 × 0.7 back in C ­ u2O photocatalysts lies in their poor stability under ­H2O × 0.1(C18H35O2) was: Calc. (%): C, 61.96; H, 10.04. irradiation [22]. However, the reduction in situ of C ­ u+ to Found (%): C, 61.98; H, 10.00; T ­ iO2: 15.80 (calculated based Cu of C­ u2O after photoexcitation provides mobility of the on thermogravimetric analysis Fig S1). electronic charge and does not slow down the process of photocatalysis [23, 24]. 2.2.2 Cu2O Nanoparticles From the studies reported, n-type ­TiO2 coupling with p-type ­Cu2O exhibits good prospects for photocatalytic The nanoparticles were synthesized following a procedure applications under visible light. For example, it was used described in the literature [37]. The solid obtained was in different applications such as the removal of organic and washed and oven-dried at 45 °C for 24 h. inorganic pollutants [20, 25–28], ­CO2 reduction [29, 30], ­H2 evolution [14, 31–33], polyester textiles [34], as well as for 2.2.3 LHTiO2/Cu2O Composites antibacterial activity [35, 36]. Herein, in this work, we report a photoactive material The samples were mixed mechanically in an agate mor- formed by layered hybrid ­TiO2(stearic acid), ­(LHTiO2), tar (relation w/w). Before irradiation, the suspension was obtained by "bottom-up" methods using stearic acid as a ultrasonicated for 2 min to produce a homogeneous sample, template, following sol–gel procedures designed for the for- facilitating close contact and minimizing aggregation. mation of T ­ iO2. The material possesses a sheet-like arrange- ment intercalated between self-assembled carboxylic acid 2.3 Characterization Methods monolayers, which gives properties as a two-dimensional (2D) material. The morphology of a semiconductor per- X-ray diffraction (XRD) analyses of the products were per- forms a crucial role in defining its photocatalytic activity. formed using a Bruker D8 Advance (Cu Kα λ = 1.5418 Å). Thus, 2D material with a sizable interfacial contact area, to The images of scanning electron microscopy (SEM) were increase sunlight collection and the absorbent capacity of its obtained using a Quanta FEG250 and EVO MA 10 ZEISS organic components, combined with a narrow-band semi- microscope. X-ray photoelectron spectra (XPS) were conductor, such as ­Cu2O cubic nanoparticles, that extend obtained in a STAIB system with an RQ-300 X-Ray source the absorption range, achieve optimum performance in both using monochromated Al Kα X-rays (1486.6 eV, 75 W). The photocatalytic processes. Photocatalytic degradation of UV–vis diffuse reflectance spectra (DRS) were recorded 4-chlorophenol (4-CP) and evolution H ­ 2 mechanisms studied in the range of 200–800 nm using a UV–Vis Shimadzu corroborates the synergy between both semiconductors. In double beam (model 2450 PC) spectrometer. Reflectance addition, ­LHTiO2/Cu2O catalytic reuse tests show the pres- measurements were converted to absorption spectra using ence of a minor proportion of Cu, however the stability and the Kubelka–Munk function. Thermogravimetric analyses catalytic activity of ­LHTiO2/Cu2O was retained for three were recorded in a TG/DSC 1100 SF Mettler Toledo. Pho- cycles. We also complement the study by electrochemical toluminescence (PL) spectra were recorded in a fluorescence impedance spectroscopy (EIS) and measurement efficiency spectrophotometer (PerkinElmer, model LS 55). Ultravio- of photocurrent conversion. let–visible spectra were recorded at room temperature in 13 Topics in Catalysis a UV–vis spectrometer (Jasco, model V-730). Nitrogen and purged with N­ 2 gas three times, for the removal of dis- adsorption/desorption isotherm studies were achieved with solved gases. The reactor was irradiated with a 300 W Xe Quantachrome Instruments (Nova 2200e, USA) using the arc lamp (Newport) under magnetic stirring, and the gas Brunauer–Emmett–Teller (BET) method to determine evolved during the reaction process was collected using the the surface area. The samples were degassed at 50 °C for water displacement technique. Gas aliquots of 0.5 c­ m3 were ­LHTiO2 and 200 °C for T ­ iO2, 12 h prior to the measure- taken and analysed with a Shimadzu GC-2014 using a 5 Å ments. The average pore size was measured according to the molecular sieve column and a thermal conductivity detec- Barret–Joyner–Halenda (BJH) method. Analysis of particle tor (TCD). size distribution by dynamic light scattering (DLS) was per- formed with a zetasizer NanoZS90 Malvern. 2.4.3 Hydroxyl Radicals 2.4 Measurement of Photocatalytic Activity During the photocatalytic reactions, active ‧OH radicals were and Hydroxyl Radicals detected by the PL method, using terephthalic acid (TA) 4 mM in an alkaline medium (pH 8). TA reacts with ‧OH 2.4.1 Photocatalytic Degradation and generates a highly fluorescent product, 2-hydroxytereph- thalic acid (TAOH) [38]. Every 10 min after UV irradiation, The photocatalytic activities of the nanocomposites were a small amount of reaction solution was centrifuged and ana- studied in visible light irradiation by testing the degrada- lysed in a spectrofluorometer. The total UV irradiation time tion of 4-CP as a pollutant. For this study, the characteristic was 90 min. The fluorescence product was detected as an absorption peaks of this compound, as an aqueous solution, emission peak at the maximum wavelength of 426 nm, with were examined at regular intervals at 280 and 225 nm using an excitation wavelength of 325 nm. a UV–Vis spectrophotometer. The effect of the initial con- tent of the 4-CP solutions was examined at different con- 2.5 Electrochemical Measurements centrations: 1 × 10–5 M, 1 × 10–4 M and 5 × 10–5 M, in the pH range 6–7. According to the experimental findings, the All photoelectrochemical measurements were performed best result in the degradation of 4-CP was the concentration with a Zahner Zennium Electrochemical Workstation PP211. of 5 × 10–5 M. The photocatalytic experiments were there- ­Cu2O, ­LHTiO2 and ­LHTiO2/Cu2O (1:0.1) electrodes were fore carried out by mixing 10 mg of L ­ HTiO2, with different used as photoelectrodes, platinum as counter electrode and proportions of ­Cu2O nanoparticles in 25 mL of a 5 × 10–5 M Ag/AgCl (3 M NaCl, 0.205 V vs. NHE) as reference elec- solution at pH 7.0. Before the measurements, the suspension trode. The electrodes were exposed to illumination in 0.5 M was maintained in the dark for 30 min to reach an adsorp- ­Na2SO4. A LED λ = 365 nm (50 W m–2) was used for irradia- tion/desorption equilibrium. Instead of sunlight, the suspen- tion. To prepare the working electrode, FTO (fluorine-doped sions were irradiated by a solar simulator (Sciencetech SS tin oxide) glass was ultrasonically cleaned, first with distilled 150 W) equipped with a 150 W ozone-free xenon bulb that water and then with acetone. We then prepared a thin film of produced a spectrum at an intensity of 1000 W/m2 (1 sun). sample on FTO glass by drop-casting a homogeneous ink of All samples were under constant magnetic stirring to guar- sample dispersed in ethylene glycol, controlling the area of antee a high level of homogeneity of the photocatalyst in the 1.0 ­cm2. Potential pulse measurements were taken at a 0 V suspension. Approximately 0.35 mL of the reaction mixture bias vs. Ag/AgCl reference electrode with a pulse of 20 s was taken out at different times and centrifuged to avoid under intermittent irradiation. EIS and photoelectrochemical light scattering due to the interference from the suspended impedance spectroscopy (PEIS) measurements were carried catalyst particles. The 4-CP concentration after equilibra- out in the frequency range of 100 MHz–10 kHz with an tion was considered as the initial concentration ­(C0) and was applied voltage signal of 0 V vs. reference electrode and ac monitored in the UV–vis spectra of the solution using nano- amplitude voltage of 10 mV. pure water as a reference. 2.4.2 Photocatalytic ­H2 Generation 3 Results and Discussion The photocatalytic activities were evaluated by ­H2 produc- The synthetic route to L ­ HTiO2/Cu2O was illustrated in tion. The photocatalyst ­H2 generation activity was tested in Scheme 1. X-ray diffraction (XRD) analyses of the prod- a Schlenk tube of 50 mL equipped with an inert gas supply ucts was used to study the structure, Fig. 1 shows the XRD with a dual-stage regulator, under direct light of the photo- patterns of the synthesized L ­ HTiO2, ­Cu2O, and L ­ HTiO2/ catalyst, dispersed in 20 mL of 20 vol% methanol solution. Cu2O nanocomposites. The pattern of ­LHTiO2 shows a In a typical process, the reaction suspension was evacuated series of equidistant Bragg reflections at low 2-theta angles, 13 Topics in Catalysis Scheme 1  Schematic illustration of synthetic routes for nanocomposite L ­ HTiO2/Cu2O in Fig. 1, the XRD patterns indicate that the product is a material consisting of layered hybrid ­LHTiO2 and ­Cu2O nanoparticles. The main facets (111), (200), (220) and (311) of the ­Cu2O cubic phase (JCPDS 65-3288) appeared at 2θ = 36.42°, 42.30°, 61.52° and 73.70°, respectively, and no impurities were apparent in the material. Scanning electron microscopy (SEM) was used to exam- ine the morphology and microstructure of the samples pro- duced. Figure 2a shows the morphology of a conventional 2D ­LHTiO2 sample. The AFM image Fig. 2b shows that the sample surface is stepped with steps of approx. 3 mn high. This characteristic is consistent with the width of a single layer of ­LHTiO2, 2.78 nm (Fig. 1), as well as with the aver- age thickness of micrometer particles of material of ~ 18 nm, as estimated by the Scherrer equation, corresponding to about 6 single hybrid layers per particle [16]. Figure 2c shows the very regular cubic ­Cu2O nanoparticles of 60 nm (particle diameter measured by dynamic light scattering Fig. 1  XRD patterns of a ­LHTiO2; b ­Cu2O and c ­LHTiO2/Cu2O (DLS) in Fig S2). Figure 2d illustrates the morphology of (1:0.1) the ­LHTiO2/Cu2O, where a combination of the structures, such as layers of L­ HTiO2 and cubes of C ­ u2O nanoparticles, can be seen. Figure 2e shows the energy dispersive X-ray characteristic of an ordered layered arrangement. The dif- spectroscopy (EDX) pattern, confirming the presence of fraction peaks at 2θ values of 3.19°, 6.52° and 9.47° are Ti and Cu in the composite. EDX elemental mapping of in good agreement with the first three (00l) reflections in ­LHTiO2/Cu2O provides the dominant distribution of Ti and a lamellar solid with a basal-distance along the c-axis of less dense regions of Cu in the nanostructure (Fig. S3). 27.8 Å. The low intensity peaks at 19.7° may be ascribed Diffuse reflectance spectroscopy (DRS) was employed to a small excess of free stearic acid (JCPDS 03-0252). The to characterize the samples. The absorbed light wavelength XRD results confirm the lamellar nature of L ­ HTiO2, con- distribution is directly influenced by their electronic bandgap stituted by a layer of semiconductor scattered among self- structure, which determines the photocatalytic activity of assembled carboxylic acid molecules [16]. This structure is the materials. Figure 3a shows the UV–vis DRS of ­LHTiO2, also retained in the composite L ­ iHTiO2/Cu2O. Moreover, 13 Topics in Catalysis Fig. 2  a SEM images of ­LHTiO2; b AFM images of ­LHTiO2; c SEM images of ­Cu2O cubic nanoparticles; d SEM images of ­LHTiO2/Cu2O composite and e EDX pattern of L ­ HTiO2/Cu2O composite ­Cu2O, and the L­ HTiO2/Cu2O composite. A sharp fundamen- XPS was utilized to analyse the composition and chemical tal absorbance edge for ­LHTiO2 was observed at 378 nm, state of Cu, Ti and O in ­Cu2O, and L ­ HTiO2. In Fig. 4a, the exhibiting a wide bandgap of 3.28 eV. C ­ u2O can absorb a Cu 2p3/2 and Cu 2p1/2 binding energy appeared at 932.5 eV certain wavelength range of visible light of 530 nm, with a and 952.2 eV, respectively, confirming the presence of the forbidden bandwidth of 2.34 eV. The bandgap of the sam- ­Cu+ characteristic peak of C ­ u2O [44]. Meanwhile, the sat- ples was analysed using the Tauc method. The energy (Eg), ellite peaks of the Cu 2p1/2 and Cu 2p3/2 binding modes estimated from a plot of (αhʋ)2 versus the photon energy of the Cu and ­Cu+2 are not detected [23]. In Fig. 4b, the and the intercept of a tangent to the x-axis, was recorded. characteristic peak of anatase ­TiO2 was observed at the As shown in the insert, ­Eg were determined from the spectra binding energy of Ti 2p3/2 and Ti 2p1/2, respectively, at [39, 40]. The results reveal that the combination of catalysts 458.7 eV and 464.2 eV. The binding energies of the Cu and presents a broad absorbance in the UV and visible range, Ti modes are in good accord with those of prior reports [32, demonstrating that the composite L ­ HTiO2/Cu2O may allow 43, 44]. XPS of O1s spectra are presented in Fig. S4. The more visible light absorption, thus improving its photocata- XPS results indicate that the C ­ u+, ­Ti+4 and O ­ −2 states exist lytic behavior [25, 41]. in the initial sample at the beginning of the photocatalysis Photoluminescence (PL) emission spectra were used to studies. The XPS spectra of the sample with the composite appreciate the separation of the photoinduced electrons and of ­LHTiO2/Cu2O have been difficult to interpret due to the holes in the samples and understand the transitions involved changes in copper, and the charge correction in the adven- of the PL bands. Figure 3b shows the typical PL spectra titious carbon being influenced by the organic surfactant. of ­LHTiO2 and ­LHTiO2/Cu2O in the wavelength range of According to literature we assigned the peak of 930 eV as an 360–550 nm. For the assignment of the peaks, a Gaussian artefact for the adjustment of C1s [45, 46]. (Fig S4). adjustment was used in the PL spectra. The emission peaks The results of the porosity analysis and the surface prop- at 380 nm is attributed to the band PL phenomenon, of the erties obtained by BET of T ­ iO2 and ­LHTiO2 are shown in energy bandgap of anatase, the emission peak at 490 nm is Fig. 5. The pore size calculated using the BJH method was ascribed to the excitation in the titanium states of ­LHTiO2, approximately 2.897 nm and 4.102 nm (inset in Fig. 5) for and the signal at 533 nm is assigned to the oxygen vacan- ­TiO2 and ­LHTiO2, respectively. The BET surface area was cies [13, 42]. The luminescence intensities for the L­ HTiO2/ 10.468 ­m2g−1 for ­TiO2 and 37.428 m2 g−1 for ­LHTiO2. The Cu2O photocatalyst is lower than ­LHTiO2, indicating a low isotherms exhibit a similar type II curve for ­TiO2, which recombination and thus better charge carrier life-time [43]. shows the non-porous nature of the T ­ iO2 nanoparticles [47], and for the L­ HTiO2 sample, the isotherm showed a type IV 13 Topics in Catalysis perceived in the control sample of 4-CP under irradiation. Both the ­Cu2O nanoparticles alone and L ­ HTiO2 composite show an inferior photocatalytic behaviour compared to the ­LHTiO2/Cu2O composites. The photocatalytic activity of the ­LHTiO2/Cu2O (1:0.2) compound does not show a sig- nificant change with the performance of the L ­ HTiO2/Cu2O (1:0.1) compound, possibly due to the opacity of the sample, excess ­Cu2O covers the surface active sites of the L ­ HTiO2, thus hindering light absorption [49]. Therefore, the optimal ratio for the ­LHTiO2/Cu2O of 1:0.1 was selected. Indeed, as observed in Fig. 6a, the photocatalytic efficiency of L­ HTiO2/ Cu2O (1:0.1) after 300 min of irradiation was about 73.3%, which is significantly higher than that of ­LHTiO2 at 43.9%, whereas only 3% degradation was observed without catalyst. Figure 6b illustrates the changes in the UV–visible absorp- tion spectra of 4-CP at different irradiation times. 4-CP has strong absorption bands at wavelengths of 223 and 278 nm. As the reaction develops, the characteristic peaks gradually decrease as the reaction mixture is exposed to the irradia- tion [50]. The photocatalytic properties of the samples were evalu- ated under comparable conditions. All the curves were nor- malized after allowing the suspension to reach adsorption/ desorption equilibrium in the dark. For a better understand- ing of the photocatalytic efficiency of the catalysts, the kinetic analysis of the photocatalytic degradations, shown in Fig. 6c can be described by a pseudo-first-order kinetics, with ln(C0/C) = kt, where k is the corresponding kinetic con- stant, and t is the irradiation time. The apparent rate constant ­(kapp, ­min–1) of the ­LHTiO2/Cu2O (1:0.1) nanocomposite is about 5.6 times faster than that of the ­TiO2 anatase. The photocatalytic performance observed in the composites Fig. 3  a UV–vis diffuse reflectance spectra of C­ u2O, ­LHTiO2 and in visible light validates the synergistic effects mentioned ­LHTiO2/Cu2O (1:0.1). The insert is the Tauc plots of ­LHTiO2/Cu2O above. The better photocatalytic performance of ­LHTiO2/ (1:0.1); b Photoluminescence (PL) emission spectra of L ­ HTiO2 and ­LHTiO2/Cu2O (1:0.1) Cu2O (1:0.1) could be ascribed to the lower recombination of the photo-generated electrons and holes and a greater close interface between the 2D sheets with the flat surfaces curve with a type H3 hysteresis loop, which correlated with of the C­ u2O. Besides, the presence of a hydrophobic organic aggregates of platelike particles, characteristic of layered moiety in ­LHTiO2 contributes significantly to the photocata- nanocomposites [48, 49]. In addition, the increase in the lytic increase, previously observed in photocatalytic studies specific surface area in ­LHTiO2 is 3.7 times greater than the of organo-inorganic hybrids of ­TiO2 and ZnO [16, 37]. ­TiO2, which significantly improves photocatalytic proper- ties, providing more surface-active sites as a 2D material. 3.2 Proposed Degradation Mechanism 3.1 Photocatalytic Activity To further investigate the probable mechanistic effects asso- ciated with the role of the composites, we performed experi- The photocatalytic activity of the L ­ HTiO2/Cu2O nanocom- ments aiming to detect the radical species involved in the posites was studied by testing the degradation of 4-CP as 4-CP photodegradation using particular radical scavengers. a pollutant with different irradiation times under sunlight. For these experiments, we used oxalate of ammonium (OA) Figure 6a shows the dye degradation in the presence of as a hole ­(h+) scavenger, isopropanol (IPA) as an ‧OH scav- ­LHTiO2/Cu2O composites with L ­ HTiO2 molar ratios of enger, and benzoquinone (BQ) as an ‧O2− scavenger, at con- 1:0.05, 1:0.1 and 1:0.2, comparing them to those of the centrations of 1 mmol [51]. Specifically, we tested the 4-CP ­Cu2O and ­LHTiO2 separately. No photocatalytic activity was degradation rate in the presence of each of the scavengers, 13 Topics in Catalysis Fig. 4  High resolution spectra of a Cu 2p XPS spectra of ­Cu2O; b Ti 2p XPS spectra of ­LHTiO2 for two hours, comparing them with those studied in the absence of any radical scavenger (Fig. 6d). The addition of OA had a slight diminution on the 4-CP degradation efficiency of L ­ HTiO2/Cu2O. Conversely, the degradation efficiency was significantly reduced in the presence of BQ, indicating that ‧O2− plays a dominant role in the degradation of 4-CP. It was noted that there is a decline in the degrada- tion rate of 4-CP with the introduction of IPA, suggesting that ‧OH is also an active species in the degradation process, checked by the PL studies with terephthalic acid (TA). To analyse the charge transfer mechanism, we measured the rates of ‧OH formation in the samples in aqueous solu- tion under irradiation by PL, using TA as the probe mol- ecule. TA could react with ‧OH to form highly fluorescent 2-hydroxyterephthalic acid (TAOH). From Fig. 7a, the production rate of radical ‧OH during irradiation can be easily observed, indicating that the surface of the sample aids in the production of ‧OH radicals and is favourable to improve photocatalytic activity. The fluorescence intensity at 426 nm significantly improves with increasing irradiation time, demonstrating that ‧OH radicals form as intermediates of photocatalytic reactions in the degradation of 4-CP [38, 52]. In terms of possible mechanisms, when the sample was illuminated under visible light, in the case of C ­ u2O, this perfectly correlates with its bandgap (2.34 eV). For ­LHTiO2 (3.28 eV), the UV irradiation (~ 5% of visible light) is suf- ficient to initiate the semiconductor, producing mobile elec- trons in the conduction band (CB) and holes in the valence band (VB) [53]. Since the CB of C ­ u2O is positioned above the CB of ­LHTiO2, the electrons photogenerated by ­Cu2O should be inserted into the CB of L ­ HTiO2 from the CB of ­Cu2O, owing to the potential difference of the band, and the collected electrons on the L ­ HTiO2 CB can convert O ­ 2 to Fig. 5  The nitrogen adsorption/desorption isotherms. The inserts are ‧ O2−, the redox potential (­ Eo ­(O2/‧O2−) = -0.33 V vs NHE) the pore size distributions for a ­TiO2 and b ­LHTiO2 composite 13 Topics in Catalysis Fig. 6  a Photocatalytic performance of samples, b UV absorption dation of 4-CP solution, d Photodegradation of ­LHTiO2/Cu2O (1:0.1) spectra of the degradation of 4-CP using L ­ HTiO2/Cu2O (1:0.1) sam- in the presence of specific radical scavengers ple, c Photodegradation kinetics of the selected samples for the degra- Fig. 7  a ‧OH trapping photoluminescence spectra of L ­ HTiO2/Cu2O (1:0.1) in a solution of terephthalic acid at room temperature. b DRX pat- terns of samples before and after reuse in photocatalytic degradation and hydrogen evolution 13 Topics in Catalysis presented in the scheme of the photocatalytic mechanism 3.3 Photocatalytic Hydrogen Production in Fig. 8. On the other hand, the potential of L­ HTiO2 photo holes is high, sufficient for water oxidation of hydroxyl In the present study, the efficiency of H ­ 2 production of groups to hydroxyl radicals ‧OH. Meanwhile, the photo the photocatalysts was evaluated using various sacrificial holes generated by C ­ u2O excitation do not have the energy agents. Since one of the crucial factors for the photocatalytic required for the oxidation of water to hydroxyl radicals ­(Eo evolution of H­ 2 is the rapid recombination of pairs of pho- ­(H2O/‧OH) = 2.73 V vs NHE) [54]. Given the experimental togenerated electrons, the rapid removal of photogenerated results, we proposed that there should be negative charge holes through sacrificial hole scavengers in the reaction transfer from energy levels of the CB of ­LHTiO2 to the BV was observed [59, 60]. The effect of the sacrificial agents of ­Cu2O, through a type Z-scheme mechanism, to describe was evaluated with formic acid, glucose, sodium sulphite, the electron transfer processes through the interface in p-n methanol and glycerol in samples of T ­ iO2, ­LHTiO2, ­LHTiO2 heterojunctions then of the simultaneous excitation of semi- with ­Cu2O of 200 nm, and ­LHTiO2 with ­Cu2O of 60 nm, to conductors. The photogenerated hole in the L ­ HTiO2 unites validate that the role of C­ u2O in photocatalysis is strongly with the ­H2O to produce ‧OH, avoiding the recombination influenced by its particle size [57]. Also, control experi- of the photogenerated electron–hole formed. Moreover, it ments were carried out to investigate the production of ­H2 should be observed that the charge-carrier movement for the with only sacrificial reagents under solar irradiation without direct Z-scheme photocatalyst is physically feasible, since photocatalysts. The quantities of ­H2 produced by the samples the migration of photogenerated electrons is valuable due to in the presence of the selected hole collectors are compared the electrostatic attraction of the electron and the hole [20, in Fig. 9a. 37, 55]. In addition, we have observed by DRX that after The results for the ­LHTiO2/Cu2O(60 nm) composite irradiation of the sample (Fig. 7b), the electrons photoex- showed a significantly higher volume of H ­ 2 evolved in the ­ u2O to metal Cu. The Cu cited cause a partial reduction of C presence of methanol, followed by glycerol and 1 M sodium can additional improve the separation of the charge, acting sulphite. The best results were always obtained for methanol as a conductive assistance for the mobility of the electronic as a sacrificing reagent in the photocatalytic evolution of H­ 2 charge [56] and, impeding recombination in cuprous oxide from aqueous solutions. This can be mainly attributed to its its main defect [31, 57, 58]. We also tested the recycling ability to produce an electron donor in the conduction band, of the ­LHTiO2/Cu2O compound (1:0.1) by sampling three preventing their recombination and increasing the potential times during 120 min of irradiation. The photocatalytic effi- for the evolution of ­H2 [43, 61]. ciency of ­LHTiO2/Cu2O(1:0.1) remained constant until the The hydrogen evolution reactions were performed using third cycle, as shown in Fig S5. The recycling indicates the different photocatalyst ratios to estimate which ratio has the effective reuse of the ­LHTiO2/Cu2O without significant loss best efficiency for hydrogen evolution. Figure 9b shows the in the degradation efficiency. hydrogen evolution over time in the presence of ­LHTiO2/ Cu2O composites with L ­ HTiO2 molar ratios of 1:0.05, 1:0.1, and 1:0.2. For all samples, the amount of produced H ­ 2 increased with irradiation time. The optimal photocatalytic hydrogen generation rate, achieved with the ­LHTiO2/Cu2O (1:0.1) composite, was 20.34 mmol g−1 h−1, which was about 2 orders of magnitude higher than that of the composite ­LHTiO2 catalyst (10.4 mmol g−1 h−1) and 2.7 times higher than the ­TiO2 anatase (7.58 mmol g−1 h−1). Recently, similar rates of ­H2 evolution for heterostructured systems of ­TiO2 with ­Cu2O (16.00 mmol g−1 h−1 and 24.83 mmol g−1 h−1) [13, 33] have been reported. 3.4 Mechanism of Hydrogen Production The possible mechanism proposed for the evolution of hydrogen from an aqueous methanol solution using ­LHTiO2/Cu2O is shown in the Fig. 8. Under photoirradia- tion, ­LHTiO2 and ­Cu2O separately generate electron–hole Fig. 8  Scheme of photocatalytic mechanism of the Cu (formed in- pairs at CB and VB, respectively. Despite the conduction situ)/2D ­LHTiO2/Cu2O photocatalysts under sunlight for degradation band potentials of ­LHTiO2 and ­Cu2O being more nega- of 4-CP and the evolution H ­ 2 tive than the reduction potential of ­2H+/H2, the rate of ­H2 13 Topics in Catalysis have observed, by DRX, the presence of Cu after irradia- tion (Fig. 7b), which would provide additional sites for the reduction reaction with protons, favouring the evolution of hydrogen, and additionally, the Cu could limit the prob- lem of self-oxidation to ­Cu2O [58, 62]. Through the results obtained, we proposed a type II mechanism band alignment, formed between L ­ HTiO2 and C ­ u2O. Therefore, the forma- tion of the p-n heterojunction of L ­ HTiO2/Cu2O suppresses load recombination, improving the photocatalytic activity (Fig. 9) [63, 64]. The recycling study was carried out to evaluate the sta- bility of the photocatalyst ­LHTiO2/Cu2O (1:0.1) in a 20% aqueous solution of methanol. Each cycle was maintained for 2.5 h, and at the beginning of the second and third cycles, 3 mL of methanol was added. After each cycle, the system was left for 24 h, tightly sealed with vacuum and N­ 2. Fig. S6 shows the results of the recycled L ­ HTiO2/Cu2O composite, up to 7 h. To verify the structure of L ­ HTiO2/Cu2O (1:0.1), the recycled sample was measured by XRD. The results of XRD patterns indicated that the characteristic diffraction peaks of L ­ HTiO2/Cu2O and the diffraction peaks of metal- lic Cu are maintained. According to the results obtained, the prolonged activity in the photocatalytic process retains the coexistence of Cu and ­Cu2O in the reaction. 3.5 Electrochemical Measurement To confirm the behaviour of the photocurrent response con- cerning the dynamics of charge transfer and percentage of recombination at the L­ HTiO2/Cu2O/electrolyte interface, for individual and combined systems, potential pulse measure- ments were made under constant illumination, following Fig. 9  a Effect of the sacrificial agents evaluated with formic acid, the methodology described by L.M. Peter [65]. Figure 10 glucose, sodium sulphite, methanol, and glycerol in samples and b represents the photocurrent response of C ­ u2O, ­LHTiO2, and Hydrogen evolution of samples in methanol ­LHTiO2/Cu2O, respectively, at a 0 V bias versus Ag/AgCl reference electrode with a pulse of 20 s under intermittent evolution was fairly low due to the rapid recombination of irradiation LED λ = 365 nm (50 W m–2), in a 0.5 M solution photo-excited electron–hole pairs. Alternatively, for the of ­Na2SO4. ­LHTiO2/Cu2O composite under visible light, both L ­ HTiO2 For all systems, an increase is seen in the steady-state and ­Cu2O are photoexcited to produce electrons and holes, photocurrent response, which is the sum of the currents in the acoupling of semiconductors p-n. Thus, the enhanced recorded during the lighting of the electrodes, added to activity can be asignated to the fact an internal electrical the effect of the curvature of the bands down and the bulk field is produced in the interface which improves the elec- of the semiconductor, promoting the migration of pho- tron–hole separation efficiency as compared to L ­ HTiO2.. In togenerated carriers ­( e −/h +). In addition, the response the ­LHTiO2/Cu2O, the CB photogenerated electrons of ­Cu2O in photocurrent increases for all systems, following the should be inserted into the CB of L­ HTiO2, due to the poten- same efficiency pattern that was registered for the degra- tial band difference between semiconductors, and react fast dation and hydrogen photogeneration process: L ­ HTiO2/ with protons producing hydrogen. Furthermore, sacrificial Cu2O > LHTiO2 > Cu2O, which also corroborates with the agents (water/methanol) are consumed by photogenerated order of catalytic efficiency of all the samples. In general, holes in the VB of ­Cu2O and ­LHTiO2, facilitating the charge the relationship between initial photocurrent ­(j0) and the separation and transfer during the photoexcitation process, stationary photocurrent (­ j1) is very close in all cases to the which aids the improvement of H ­ 2 production [31, 43]. We unit. Therefore, the recombination rates for ­Cu2O, ­LHTiO2 and the ­LHTiO2/Cu2O systems are low [66]. 13 Topics in Catalysis in the semiconductor space charge zone and the electrified interfaces [71]. The decrease of the semicircle arc diam- eter in the Nyquist plots under illumination (­LHTiO 2/ Cu2O > LHTiO2 > Cu2O) correlates with the most effective separation of the photogenerated charge carriers ­(e–, ­h+) and, therefore, accelerates the charge transfer across the photo- electrode/solution interface [72]. The inset in Fig. 11b, is the equivalent circuit diagram used in the simulation, considering the charge transfer across the semiconductor/electrolyte interface. Rs corresponds to the resistance of the solution. The resistance to load transfer in the double layer is represented by Rdl, while CPEdl cor- responds to the load of the double layer. The Rct // CPEsc configuration describes the resistance to load transfer from the semiconductor and the capacitance of the semiconduc- tor space charge zone, both components associated with the processes of transmission of charge carriers, as well as the separation and recombination of pairs e­ –/h+. The substitution Fig. 10  Photocurrent transients for electrodes (black line) ­Cu2O, (red line) ­LHTiO2, (blue line) L­ HTiO2/Cu2O. Photocurrent pulses were of the capacitive components of the circuit by constant phase realized in 0.5 M ­Na2SO4 using an LED λ = 365 nm (50 W m−2) elements (CPE) was carried out considering that the systems analysed have a heterogeneous structure and high micro- scopic roughness, which leads to a non-ideal capacitance, The interfacial charge transfer processes occurring at associated with a non-uniform distribution of the current the semiconductor-electrolyte interfaces have been inves- [73]. Rct decreases significantly upon illumination for all tigated using EIS and PEIS [67]. photoelectrodes (EIS-Rct: C ­ u2O 8.663 kΩ c­ m–2, ­LHTiO2 Figure 11a and b represent the EIS and PEIS Nyquist 18.051 kΩ c­ m , ­LHTiO2/Cu2O 10.727 kΩ c­ m–2, and PEIS- –2 plots of the samples ­Cu2O, ­LHTiO2, and ­LHTiO2/Cu2O, Rct: ­Cu2O 5.681 kΩ ­cm–2, ­LHTiO2 4.215 kΩ ­cm–2, ­LHTiO2/ respectively. In the Nyquist plots for EIS, it is observed Cu2O 2.201 kΩ c­ m–2), indicating that illumination greatly that the arc of the semicircles decreases at low frequen- accelerates the charge transfer reaction at the semiconductor/ cies ­(Cu2O < LHTiO2/Cu2O < LHTiO2) according to the solution interface due to the photoinduced increase of car- relationship between the decrease in electrical resistivity rier density. The variation trend of Rct with L ­ HTiO2/Cu2O ­(Cu2O 1.6 × 104 Ω cm [68], ­TiO2 2.34 × 104 Ω cm [69]) composite agrees well with that of the photocurrent den- and the acceleration of the interfacial transfer processes, sity and the smallest Rct value, both in the dark and under recorded by EIS measurements [70]. illumination, suggesting that the ­LHTiO2/Cu2O composite Figure 11b shows only one semicircle in the Nyquist accelerates the charge transfer across the semiconductor/ plot under illumination (PEIS) for the three electrodes, solution interface, which explains the good photocatalytic which is associated with the processes of charge transfer performance of the compound. Fig. 11  a EIS and b PEIS spec- tra for electrodes (open square) ­Cu2O, (open circle) ­LHTiO2, (open triangle) L­ HTiO2/Cu2O. The insert corresponds to the equivalent circuit used in the simulation, considering the electrode and the dissolution interface 13 Topics in Catalysis 4 Conclusion 5. Wu H, Tan HL, Toe CY et al (2020) Photocatalytic and photo- electrochemical systems: similarities and differences. Adv Mater 32:1904717. https​://doi.org/10.1002/adma.20190​4717 The formation of a novel photocatalyst of 2D-TiO2 in lay- 6. Li D, Yu JCC, Nguyen VH et al (2018) A dual-function photo- ers of a sheets-like arrangement with C ­ u2O nanoparticles catalytic system for simultaneous separating hydrogen from water was prepared and characterized using various techniques. splitting and photocatalytic degradation of phenol in a twin-reac- tor. Appl Catal B Environ 239:268–279. https:​ //doi.org/10.1016/j. The results indicated that the synergy between the semi- apcat​b.2018.08.010 conductors generated the separation of electrons and pho- 7. Nakata K, Fujishima A (2012) ­TiO2 photocatalysis: design and togenerated holes and exhibited an improved visible light applications. J Photochem Photobiol C Photochem Rev 13:169– absorption, demonstrating the clear contribution of C ­ u 2O 189. https​://doi.org/10.1016/j.jphot​ochem​rev.2012.06.001 8. Kim J, Monllor-Satoca D, Choi W (2012) Simultaneous pro- and the 2D structure of ­LHTiO2 in the photocatalytic effi- duction of hydrogen with the degradation of organic pollutants ciency of the compound for degradation of 4-chlorophenol using ­TiO2 photocatalyst modified with dual surface components. and hydrogen evolution. Photoinduced charge separation Energy Environ Sci 5:7647–7656. https​://doi.org/10.1039/c2ee2​ mechanisms, a Z-scheme for photocatalytic degradation, and 1310a​ 9. Riente P, Noël T (2019) Application of metal oxide semiconduc- a type II heterojunction to evolution photocatalytic hydrogen tors in light-driven organic transformations. Catal Sci Technol are proposed. The recyclability experiments showed that the 9:5186–5232. https​://doi.org/10.1039/c9cy0​1170f​ photocatalysts could be reused for at least three cycles with- 10. Nasr M, Eid C, Habchi R et al (2018) Recent progress on titanium out a significant change in their activity. dioxide nanomaterials for photocatalytic applications. Chemsu- schem 11:3023–3047. https​://doi.org/10.1002/cssc.20180​0874 The photoelectrochemical evaluation corroborates the 11. Hu Z, Wang X, Dong H et al (2017) Efficient photocatalytic deg- order of catalytic efficiency of all the samples, showing that radation of tetrabromodiphenyl ethers and simultaneous hydro- the compound ­LHTiO2/Cu2O exhibits an improved visible gen production by T ­ iO2-Cu2O composite films in N ­ 2 atmosphere: light absorption with low recombination rates and an effec- influencing factors, kinetics and mechanism. J Hazard Mater 340:1–15. https​://doi.org/10.1016/j.jhazm​at.2017.07.009 tive separation and charge transfer efficient carriers pho- 12. Wei T, Zhu Y-N, An X et al (2019) Defect modulation of togenerated in photocatalytic reactions. Z-scheme ­TiO2/Cu2O photocatalysts for durable water splitting. ACS Catal 9:8346–8354. https​://doi.org/10.1021/acsca​tal.9b017​ Acknowledgements This work was supported by Project funded by the 86 Research Continuity Project Fund, year 2019, code LPC19-01, Univer- 13. Trang TNQ, Tu LTN, Man TV et al (2019) A high-efficiency sidad Tecnologica Metropolitana, Universidad de Chile, FONDECYT photoelectrochemistry of ­Cu2O/TiO2 nanotubes based compos- 1171803, CMST Spanish MINECO projects PHENTOM (FIS2015- ite for hydrogen evolution under sunlight. Compos Part B Eng 70862-P) and Severo Ochoa (SEV-2013-0295) and the CERCA Pro- 174:106969. https​://doi.org/10.1016/j.compo​sites​b.2019.10696​9 gramme/Generalitat de Catalunya, Programa Fondequip XPS EQM 14. Cheng WY, Yu TH, Chao KJ, Lu SY (2014) ­Cu2O-decorated 140044 and SEM EQM 150101. We want to thank Dra. Marjorie mesoporous ­TiO2 beads as a highly efficient photocatalyst for Segovia for the help in the samples preparation of nanoparticles ­Cu2O. hydrogen production. ChemCatChem 6:293–300. https​://doi. org/10.1002/cctc.20130​0681 15. Uddin MT, Babot O, Thomas L et al (2015) New insights into the Compliance with Ethical Standards photocatalytic properties of ­RuO2/TiO2 mesoporous heterostruc- tures for hydrogen production and organic pollutant photodecom- Conflicts of interest The authors have no conflicts of interest. position. J Phys Chem C 119:7006–7015. https:​ //doi.org/10.1021/ jp512​769u Research Involving Human and/or Animal Rights There were no human 16. Benavente E, Maldonado C, Devis S et al (2016) A hybrid or animal subjects involved in this research. organic-inorganic layered ­TiO2 based nanocomposite for sunlight photocatalysis. RSC Adv 6:18538–18541. https:​ //doi.org/10.1039/ c5ra2​6981d​ 17. Liu S, Zhang N, Tang ZR, Xu YJ (2012) Synthesis of one-dimen- sional CdS@TiO2 core-shell nanocomposites photocatalyst for References selective redox: the dual role of T ­ iO2 shell. ACS Appl Mater Inter- faces 4:6378–6385. https​://doi.org/10.1021/am302​074p 1. Xu C, Ravi Anusuyadevi P, Aymonier C et al (2019) Nanostruc- 18. Peng W, Li Y, Zhang F et al (2017) Roles of two-dimensional tured materials for photocatalysis. Chem Soc Rev 48:3868–3902. transition metal dichalcogenides as cocatalysts in photocatalytic https​://doi.org/10.1039/c9cs0​0102f​ hydrogen evolution and environmental remediation. Ind Eng 2. Luo J, Zhang S, Sun M et al (2019) A critical review on energy Chem Res 56:4611–4626. https​://doi.org/10.1021/acs.iecr.7b003​ conversion and environmental remediation of photocatalysts 71 with remodeling crystal lattice, surface, and interface. ACS Nano 19. Tsai CE, Yeh SM, Chen CH, Lin HN (2019) Flexible photo- 13:9811–9840. https​://doi.org/10.1021/acsna​no.9b036​49 catalytic paper with ­Cu2O and Ag nanoparticle-decorated ZnO 3. Wang Q, Domen K (2020) Particulate photocatalysts for light- nanorods for visible light photodegradation of organic dye. driven water splitting: mechanisms, challenges, and design strate- Nanoscale Res Lett. https​://doi.org/10.1186/s1167​1-019-3034-7 gies. Chem Rev 120:919–985. https​://doi.org/10.1021/acs.chemr​ 20. Ye F, Su Y, Quan X et al (2019) Constructing desired interfacial ev.9b002​01 energy band alignment of Z-scheme ­TiO2-Pd-Cu2O hybrid by con- 4. Zhang L, Ran J, Qiao S-Z, Jaroniec M (2019) Characterization trolling the contact facet for improved photocatalytic performance. of semiconductor photocatalysts. Chem Soc Rev 48:5184–5206. Appl Catal B Environ 244:347–355. https​://doi.org/10.1016/j. https​://doi.org/10.1039/c9cs0​0172g​ apcat​b.2018.10.066 13 Topics in Catalysis 21. Liao Y, Deng P, Wang X et al (2018) A facile method for prepa- nanoparticles for sunlight photocatalysis. J Mater Sci 54:13523– ration of ­Cu2O-TiO2 NTA heterojunction with visible-photocat- 13536. https​://doi.org/10.1007/s1085​3-019-03878​-x alytic activity. Nanoscale Res Lett. https​://doi.org/10.1186/s1167​ 38. Bubacz K, Kusiak-Nejman E, Tryba B, Morawski AW (2013) 1-018-2637-8 Investigation of OH radicals formation on the surface of ­TiO2/N 22. Zheng Z, Huang B, Wang Z et al (2009) Crystal faces of C ­ u 2O photocatalyst at the presence of terephthalic acid solution. Esti- and their stabilities in photocatalytic reactions. J Phys Chem C mation of optimal conditions. J Photochem Photobiol A Chem. 113:14448–14453. https​://doi.org/10.1021/jp904​198d https​://doi.org/10.1016/j.jphot​ochem​.2013.04.003 23. Sinatra L, Lagrow AP, Peng W et al (2015) A Au/Cu2O-TiO2 39. Tauc J, Menth A (1972) States in the gap. J Non Cryst Solids system for photo-catalytic hydrogen production. A pn-junction 8–10:569–585. https​://doi.org/10.1016/0022-3093(72)90194​-9 effect or a simple case of in situ reduction? J Catal 322:109–117. 40. Murphy AB (2007) Band-gap determination from diffuse https​://doi.org/10.1016/j.jcat.2014.11.012 reflectance measurements of semiconductor films, and appli- 24. Tamiolakis I, Papadas IT, Spyridopoulos KC, Armatas GS (2016) cation to photoelectrochemical water-splitting. Sol Energy Mesoporous assembled structures of ­Cu2O and ­TiO2 nanoparticles Mater Sol Cells 91:1326–1337. https​://doi.org/10.1016/j.solma​ for highly efficient photocatalytic hydrogen generation from water. t.2007.05.005 RSC Adv 6:54848–54855. https​://doi.org/10.1039/c6ra0​8546f​ 41. Cheng Y, Gao X, Zhang X et al (2018) Synthesis of a ­TiO2-Cu2O 25. Chu S, Zheng X, Kong F et al (2011) Architecture of C ­ u2O@TiO2 composite catalyst with enhanced visible light photocatalytic core-shell heterojunction and photodegradation for 4-nitrophenol activity for gas-phase toluene. New J Chem 42:9252–9259. https​ under simulated sunlight irradiation. Mater Chem Phys 129:1184– ://doi.org/10.1039/c8nj0​0409a​ 1188. https​://doi.org/10.1016/j.match​emphy​s.2011.06.004 42. Mathew S, Kumar Prasad A, Benoy T et al (2012) UV-visible pho- 26. Surikanti GR, Bandarapu AK, Sunkara MV (2019) A facile toluminescence of T ­ iO2 nanoparticles prepared by hydrothermal one pot synthesis of ­Cu2O@TiO2: a nanocomposite catalyst for method. J Fluoresc 22:1563–1569. https​://doi.org/10.1007/s1089​ enhanced visible light driven photocatalysis #. ChemistrySelect 5-012-1096-3 4:2249–2257. https​://doi.org/10.1002/slct.20190​0003 43. Praveen Kumar D, Lakshmana Reddy N, Srinivas B et al (2016) 27. Ding RC, Fan YZ, Wang GS (2018) High efficient ­Cu2O/TiO2 Stable and active C­ uxO/TiO2 nanostructured catalyst for proficient nanocomposite photocatalyst to degrade organic pollutant under hydrogen production under solar light irradiation. Sol Energy visible light irradiation. Chem Select 3:1682–1687. https​://doi. Mater Sol Cells 146:63–71. https​: //doi.org/10.1016/j.solma​ org/10.1002/slct.20170​2650 t.2015.11.030 28. Liu J, Wen M, Chen H et al (2014) Assembly of ­TiO2-on-Cu2O 44. Lalitha K, Sadanandam G, Kumari VD et al (2010) Highly stabi- nanocubes with narrow-band C ­ u 2 O-induced visible-light- lized and finely dispersed C ­ u2O/TiO2: a promising visible sensi- enhanced photocatalytic activity. ChemPlusChem 79:298–303. tive photocatalyst for continuous production of hydrogen from https​://doi.org/10.1002/cplu.20130​0278 glycerol:water mixtures. J Phys Chem C 114:22181–22189. https​ 29. Aguirre ME, Zhou R, Eugene AJ et al (2017) ­Cu2O/TiO2 hetero- ://doi.org/10.1021/jp107​405u structures for C ­ O2 reduction through a direct Z-scheme: protecting 45. Greczynski G, Hultman L (2020) X-ray photoelectron spectros- ­Cu2O from photocorrosion. Appl Catal B Environ 217:485–493. copy: towards reliable binding energy referencing. Prog Mater Sci https​://doi.org/10.1016/j.apcat​b.2017.05.058 107:100591. https​://doi.org/10.1016/j.pmats​ci.2019.10059​1 30. Xu H, Ouyang S, Liu L et al (2014) Porous-structured ­Cu2O/ 46. Greczynski G, Hultman L (2017) C 1s peak of adventitious carbon TiO2 nanojunction material toward efficient C ­ O2 photoreduction. aligns to the vacuum level: dire consequences for material’s bond- Nanotechnology. https:​ //doi.org/10.1088/0957-4484/25/16/16540​ ing assignment by photoelectron spectroscopy. ChemPhysChem. 2 https​://doi.org/10.1002/cphc.20170​0126 31. Xi Z, Li C, Zhang L et al (2014) Synergistic effect of ­Cu2O/TiO2 47. Nava-Núñez MY, Jimenez-Relinque E, Grande M et al (2020) heterostructure nanoparticle and its high H ­ 2 evolution activity. Int Photocatalytic ­BiOX mortars under visible light irradiation: com- J Hydrogen Energy 39:6345–6353. https:​ //doi.org/10.1016/j.ijhyd​ patibility, Nox efficiency and nitrate selectivity. Catalysts 10:226. ene.2014.01.209 https​://doi.org/10.3390/catal​10020​226 32. Zhang YH, Li YL, Jiu BB et al (2019) Highly enhanced photo- 48. Thommes M, Kaneko K, Neimark AV et al (2015) Physisorption catalytic ­H2 evolution of ­Cu2O microcube by coupling with ­TiO2 of gases, with special reference to the evaluation of surface area nanoparticles. Nanotechnology. https​://doi.org/10.1088/1361- and pore size distribution (IUPAC Technical Report). Pure Appl 6528/aafcc​b Chem 87:1051–1069. https​://doi.org/10.1515/pac-2014-1117 33. Li G, Huang J, Chen J et al (2019) Highly active photocatalyst 49. Zhang J, Yu J, Zhang Y et al (2011) Visible light photocatalytic of ­Cu2O/TiO2 octahedron for hydrogen generation. ACS Omega H2-production activity of CuS/ZnS porous nanosheets based on 4:3392–3397. https​://doi.org/10.1021/acsom​ega.8b034​04 photoinduced interfacial charge transfer. Nano Lett 11:4774–4779. 34. Zeghioud H, Assadi AA, Khellaf N et al (2019) Photocatalytic https​://doi.org/10.1021/nl202​587b performance of ­CuxO/TiO2 deposited by HiPIMS on polyester 50. Castañeda C, Tzompantzi F, Gómez R, Rojas H (2016) Enhanced under visible light LEDs: oxidants, ions effect, and reactive oxy- photocatalytic degradation of 4-chlorophenol and 2,4-dichloro- gen species investigation. Materials (Basel) 12:412. https​://doi. phenol on in situ phosphated sol-gel ­TiO2. J Chem Technol Bio- org/10.3390/ma120​30412​ technol 91:2170–2178. https​://doi.org/10.1002/jctb.4943 35. Ibrahim MM, Mezni A, El-Sheshtawy HS et al (2019) Direct 51. Liu W, Wang M, Xu C et al (2013) A ­ g3PO4/ZnO: An efficient Z-scheme of ­C u 2O/TiO 2 enhanced self-cleaning, antibacte- visible-light-sensitized composite with its application in photocat- rial activity, and UV protection of cotton fiber under sunlight. alytic degradation of Rhodamine B. Mater Res Bull 48:106–113. Appl Surf Sci 479:953–962. https​: //doi.org/10.1016/j.apsus​ https​://doi.org/10.1016/j.mater​resbu​ll.2012.10.015 c.2019.02.169 52. Qu X, Kirschenbaum LJ, Borish ET (2000) Hydroxyterephtha- 36. Li H, Zhong J, Zhu H et al (2019) Hybrid ­Cu 2O/TiO 2 nano- late as a fluorescent probe for hydroxyl radicals: application to composites with enhanced photocatalytic antibacterial activity hair melanin. Photochem Photobiol 71:307–313. https​://doi. toward acinetobacter Baumannii. ACS Appl Bio Mater 2:4892– org/10.1562/0031-8655(2000)07103​07haa​fpf2.0.co2 4903. https​://doi.org/10.1021/acsab​m.9b006​44 53. Youssef Z, Colombeau L, Yesmurzayeva N et al (2018) Dye- 37. Segovia M, Alegría M, Aliaga J et al (2019) Heterostructured sensitized nanoparticles for heterogeneous photocatalysis: 2D ZnO hybrid nanocomposites sensitized with cubic ­C u2O cases studies with T ­ iO2, ZnO, fullerene and graphene for water 13 Topics in Catalysis purification. Dye Pigment 159:49–71. https​://doi.org/10.1016/j. prepared with two-dimensional semiconducting nanocrystals. J dyepi​g.2018.06.002 Am Chem Soc 136:1872–1878. https:​ //doi.org/10.1021/ja4094​ 65k 54. Wardman P (1989) Reduction potentials of one electron couples 65. Peter LM (2013) Energetics and kinetics of light-driven oxygen involving free radicals in aqueous solution. J Phys Chem Ref Data evolution at semiconductor electrodes: the example of hematite. 18:1637–1755. https​://doi.org/10.1063/1.55584​3 J Solid State Electrochem 550:315–326. https​://doi.org/10.1007/ 55. Low J, Jiang C, Cheng B et al (2017) A review of direct s1000​8-012-1957-3 Z-scheme photocatalysts. Small Methods 1:1700080. https​://doi. 66. Yang L, Wang W, Zhang H et al (2017) Electrodeposited ­Cu2O org/10.1002/smtd.20170​0080 on the 101 facets of T ­ iO2 nanosheet arrays and their enhanced 56. Das D, Makal P (2020) Narrow band gap reduced ­TiO2-B: Cu photoelectrochemical performance. Sol Energy Mater Sol Cells nanowire heterostructures for efficient visible light absorption, 165:27–35. https​://doi.org/10.1016/j.solma​t.2017.02.026 charge separation and photocatalytic degradation. Appl Surf Sci 67. Yu D, Yu D, Dai J et al (2020) Growth of ­Cu2O/TiO2 heterojunc- 506:144880. https​://doi.org/10.1016/j.apsus​c.2019.14488​0 tion and its photoelectrochemical properties. Mater Lett 263:1–4. 57. Li L, Xu L, Shi W, Guan J (2013) Facile preparation and size- https​://doi.org/10.1016/j.matle​t.2019.12722​5 dependent photocatalytic activity of ­Cu2O nanocrystals modified 68. Niveditha CV, JabeenFatima MJ, Sindhu S (2016) Electrochemi- titania for hydrogen evolution. Int J Hydrogen Energy 38:816– cal synthesis of p-type copper oxides. Nanosyst Phys Chem Math 822. https​://doi.org/10.1016/j.ijhyd​ene.2012.10.064 7:747–751 58. Janczarek M, Endo M, Zhang D, Wang K, Kowalska E (2018) 69. Senain I, Nayan N, Saim H (2010) Structural and electrical prop- Enhanced photocatalytic and antimicrobial performance of erties of ­TiO2 thin film derived from sol-gel method using tita- cuprous oxide/titania: the effect of titania matrix. Materials nium (IV) butoxide. Int J Integr Eng 4:29–35 (Basel) 11:2069. https​://doi.org/10.3390/ma111​12069​ 70. Szaniawska E, Bienkowski K, Rutkowska IA et al (2018) 59. Denisov N, Yoo JE, Schmuki P (2019) Effect of different hole Enhanced photoelectrochemical C ­ O2-reduction system based on scavengers on the photoelectrochemical properties and photo- mixed ­Cu2O–nonstoichiometric ­TiO2 photocathode. Catal Today catalytic hydrogen evolution performance of pristine and Pt-dec- 300:145–151. https​://doi.org/10.1016/j.catto​d.2017.05.099 orated ­TiO2 nanotubes. Electrochim Acta 319:61–71. https​://doi. 71. Santamaria M, Conigliaro G, Di Franco F, Di Quarto F (2014) org/10.1016/j.elect​acta.2019.06.173 Photoelectrochemical evidence of ­Cu2O/TiO2nanotubes hetero- 60. Sreethawong T, Puangpetch T, Chavadej S, Yoshikawa S (2007) junctions formation and their physicochemical characterization. Quantifying influence of operational parameters on photocatalytic Electrochim Acta 144:315–323. https​://doi.org/10.1016/j.elect​ ­H2 evolution over Pt-loaded nanocrystalline mesoporous T ­ iO2 acta.2014.07.154 prepared by single-step sol-gel process with surfactant template. 72. Yin H, Wang X, Wang L et al (2015) ­Cu2O/TiO2 heterostructured J Power Sources 165:861–869. https​://doi.org/10.1016/j.jpows​ hollow sphere with enhanced visible light photocatalytic activity. our.2006.12.050 Mater Res Bull 72:176–183. https​://doi.org/10.1016/j.mater​resbu​ 61. Guzman F, Chuang SSC, Yang C (2013) Role of methanol sac- ll.2015.07.030 rificing reagent in the photocatalytic evolution of hydrogen. Ind 73. Oh JT, Chowdhury SR, Lee II T, Misra M (2019) Synergetic Eng Chem Res 52:61–65. https​://doi.org/10.1021/ie301​177s influence of Au/Cu2O core-shells nanoparticle on optical, photo- 62. Zhen W, Jiao W, Wu Y et al (2017) The role of a metallic copper electrochemical, and catalytic activities of Au/Cu2O/TiO2 nano- interlayer during visible photocatalytic hydrogen generation over a composite. Dye Pigment 160:936–943. https​://doi.org/10.1016/j. Cu/Cu2O/Cu/TiO2 catalyst. Catal Sci Technol 7:5028–5037. https​ dyepi​g.2018.09.003 ://doi.org/10.1039/c7cy0​1432e​ 63. Haque F, Daeneke T, Kalantar-zadeh K, Ou JZ (2018) Two-dimen- Publisher’s Note Springer Nature remains neutral with regard to sional transition metal oxide and chalcogenide-based photocata- jurisdictional claims in published maps and institutional affiliations. lysts. Nano-Micro Lett 10:1–27. https​://doi.org/10.1007/s4082​ 0-017-0176-y 64. Ida S, Takashiba A, Koga S et al (2014) Potential gradient and photocatalytic activity of an ultrathin p-n junction surface 13