nanomaterials Article Photodehydrogenation of Ethanol over Cu2O/TiO2 Heterostructures Congcong Xing 1,2 , Yu Zhang 1 , Yongpeng Liu 3 , Xiang Wang 1 , Junshan Li 1 , Paulina R. Martínez-Alanis 4 , Maria Chiara Spadaro 5 , Pablo Guardia 1 , Jordi Arbiol 5,6 , Jordi Llorca 2, * and Andreu Cabot 1,6, * 1 Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Spain; congcongxing@irec.cat (C.X.); peterzhang@irec.cat (Y.Z.); wxiang@irec.cat (X.W.); junshanli@irec.cat (J.L.); pguardia@irec.cat (P.G.) 2 Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain 3 Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO), École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland; yongpeng.liu@epfl.ch 4 ENFOCAT-IN2UB, Universitat de Barcelona (UB), C/Martí i Franquès 1, 08028 Barcelona, Spain; paulina.martinez@ub.edu 5 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; mariachiara.spadaro@icn2.cat (M.C.S.); jordi.arbiol@icn2.cat (J.A.) 6 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain * Correspondence: jordi.llorca@upc.edu (J.L.); acabot@irec.cat (A.C.) Abstract: The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process  requires the use of highly active, stable and selective photocatalytic materials based on abundant  elements and the proper adjustment of the reaction conditions, including temperature. In this Citation: Xing, C.; Zhang, Y.; Liu, Y.; work, Cu2 O-TiO2 type-II heterojunctions with different Cu2 O amounts are obtained by a one-pot Wang, X.; Li, J.; Martínez-Alanis, P.R.; hydrothermal method. The structural and chemical properties of the produced materials and their Spadaro, M.C.; Guardia, P.; Arbiol, J.; activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. Llorca, J.; et al. Photodehydrogenation of Ethanol The Cu2 O-TiO2 photocatalysts exhibit a high selectivity toward acetaldehyde production and up to over Cu2 O/TiO2 Heterostructures. tenfold higher hydrogen evolution rates compared to bare TiO2 . We further discern here the influence Nanomaterials 2021, 11, 1399. https:// of temperature and visible light absorption on the photocatalytic performance. Our results point doi.org/10.3390/nano11061399 toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy for solar energy conversion. Academic Editor: Giuseppe Marcì Keywords: titanium dioxide; copper oxide; photodehydrogenation; ethanol; thermo-photocatalysis; Received: 28 April 2021 hydrogen Accepted: 21 May 2021 Published: 25 May 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Molecular hydrogen, a clean energy carrier and a key component in the chemical published maps and institutional affil- iations. industry, is mostly produced through partial oxidation and steam reforming of natural gas and coal gasification. To move away from the exploitation of fossil fuels, cost- and energy-effective strategies for the direct production of hydrogen from renewable sources need to be defined. In this context, biomass resources are a particularly compelling alterna- tive source of hydrogen owing to their renewable character and their near net-zero CO2 Copyright: © 2021 by the authors. footprint [1–6]. Additional advantages of the hydrogen production from dehydrogenation Licensee MDPI, Basel, Switzerland. of biomass-derived organics are the potential to co-produce valuable side organic chem- This article is an open access article icals for better process economics and the possibility to implement cost-effective waste distributed under the terms and abatement processes [7,8]. conditions of the Creative Commons Attribution (CC BY) license (https:// Among the possible dehydrogenation processes, photocatalytic routes that make use of creativecommons.org/licenses/by/ ubiquitous, abundant and renewable solar energy are especially attractive. Photocatalytic 4.0/). processes also enable the dehydrogenation reaction to take place in milder conditions, Nanomaterials 2021, 11, 1399. https://doi.org/10.3390/nano11061399 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1399 2 of 15 which further decreases costs and can increase the side product selectivity compared with thermocatalytic analogs [2]. From another point of view, the photocatalytic production of fuels can be considered as a convenient strategy to store intermittent solar energy [9,10]. In this scenario, the photodehydrogenation of ethanol to produce molecular hydrogen and acetaldehyde using solar light as the only energy input is especially appealing [6,11]. As a liquid, ethanol can be easily stored and transported. Besides, ethanol can be easily produced from several biomass-derived feedstocks and organic residues such as sewage sludge [12–14]. Additionally, bioethanol aqueous solutions can be directly used, without the need for purification. Compared with water splitting, the production of hydrogen from ethanol is thermodynamically advantageous (∆G0 = +237 kJ·mol−1 for water oxidation vs. ∆G0 = +41.5 kJ·mol−1 for ethanol oxidation to acetaldehyde), which decreases the energy input required to drive hydrogen production [2,8]. Compared with water splitting, ethanol dehydrogenation also enables a much simpler product purification, preventing the H2 and O2 back reaction. Besides, compared with ethanol photoreforming, ethanol photodehydrogenation to H2 and acetaldehyde could have a threefold higher economical profitability associated with the high economic value of the side product [15]. In terms of catalysts, while photocatalytic water splitting requires semiconductors with conduction and valence band edges sufficiently above and below the potentials for H+ reduction and water oxidation, respectively, ethanol dehydrogenation can be activated in semiconductors with significantly lower band gaps. On the other hand, the catalytic dehydrogenation of ethanol competes with the deoxygenation, reforming and decomposi- tion reactions, which makes the selectivity of the catalytic process fundamental to ensure cost-effectiveness [1]. Copper oxides, Cu2−x O, have raised increasing attention as photocatalytic materials owing to their abundance, low cost, minor environmental and health impact and suitable optoelectronic properties. Cu2−x O are p-type semiconductors with a very energetic con- duction band and a relatively low bandgap: 2.1 eV for Cu2 O and 1.2 eV for CuO, which enables absorption of the visible range of the solar spectra. As a drawback, Cu2−x O have poor photostability, being prone to photocorrosion in reaction conditions. Besides, Cu2−x O generally presents a large defect density that results in a relatively fast recombination of photogenerated charge carriers. To solve these limitations, Cu2−x O can be combined with TiO2 within p-n heterojunctions that protect Cu2−x O against photocorrosion and reduce the charge carrier recombination. The synergism between the two materials is enabled by the appropriate conduction band edges of Cu2−x O, −1.79 V for Cu2 O and −1.03 V for CuO, which allows the rapid injection of the photogenerated electrons from the Cu2−x O to the TiO2 conduction band [8,15–19]. Thus, the combination of Cu2−x O and TiO2 is regarded as a highly interesting photocatalyst to: (i) stabilize the Cu2−x O, (ii) boost the overall catalytic activity by extending the light absorption of TiO2 toward the visible light range and (iii) maximize external quantum yield by a rapid charge separation between the two phases enabled by their adequate band edges. While the concept of a p-n heterojunction between Cu2−x O and TiO2 that promotes catalytic activity is pleasantly simple, real systems are much more complex, and Cu2−x O have been reported to promote catalytic activity through several different mechanisms: (i) Cu2−x O can absorb the visible light and transfer photogenerated electrons to TiO2 , where H2 evolves, while using photogenerated holes to oxidize the organic species [20]. (ii) Cu2−x O can absorb visible light but use photogenerated electrons to evolve H2 and recombine in photogenerated holes at the Cu2−x O/TiO2 interphase within a Z-scheme mechanism [21]. (iii) Cu2−x O nanoparticles can be reduced to metallic copper during ethanol photodehydrogenation, and the resulting metal nanoparticles can act as a co- catalyst, stabilizing photogenerated electrons, promoting the water reduction reaction, simultaneously reducing the rate of charge recombination and, thus, also making more holes available for the oxidation reaction [15,22,23]. (iv) Cuδ+ and Cu0 on the surface of supported Cu clusters can also participate as catalysts in the ethanol oxidation to acetalde- hyde [17]. (v) Copper ions can be partially incorporated into the TiO2 lattice by substituting Nanomaterials 2021, 11, 1399 3 of 15 for Ti4+ ions and creating oxygen vacancies that decrease the TiO2 bandgap [15,19,24,25]. All these effects strongly depend on the synthesis procedure, the TiO2 surface area and its structural and chemical properties, which affect the Cu dispersion and oxidation states [22] and the TiO2 phase that also determines the interaction with Cu and the Cu role [26]. Most previous works assign the performance promotion of Cu2−x O/TiO2 with respect to TiO2 to the extension of light absorption toward the visible range of the solar spectra. However, in most previous works, mainly UV excitation is used, and the overall and local temperature changes associated with the visible light absorption are usually neglected. In the present work, we aim at gaining additional understanding of the mechanism behind the synergistic promotion of the catalytic performance in Cu2 O/TiO2 while simul- taneously contributing to the optimization of this system. In this direction, we present a one-pot hydrothermal synthesis strategy to produce Cu2 O/TiO2 nanocomposites with controlled Cu2 O amounts. The photocatalytic performance of Cu2 O/TiO2 toward ethanol dehydrogenation is tested using both UV and visible light irradiation. We then deter- mined the direct contribution of visible light, beyond the increasing temperature, toward increasing catalytic activity. We tested photocatalytic activity in the gas phase as it offers additional advantages, including lower light scattering, easier scale-up, higher stability, easier product recovery and even higher selectivity [7,15]. Besides, using time-resolved photoluminescence measurements and analyzing the band alignment between the two materials, we showed the activity promotion to proceed through a conventional p-n type II heterojunction. 2. Materials and Methods 2.1. Chemicals Titanium (IV) isopropoxide (97%, Sigma-Aldrich, St. Louis, MO, USA), copper (II) nitrate hexahydrate (98%, Fluka, Buchs, Switzerland), ethanol (96%, PanReac AppliChem GmBH, Darmstadt, Germany), polyvinylpyrrolidone (PVP, 90%, Sigma-Aldrich, St. Louis, MO, USA), and sodium sulfate (Alfa Aesar™, Ward Hill, MA, USA) were used without further purification. 2.2. Synthesis of Photocatalysts PVP (0.45 g) was dissolved in Milli-Q water:ethanol (1:2) (40 mL) under stirring at room temperature. To this solution, a proper amount of Cu (NO3 )2 ·6H2 O was added (0, 11.3, 22.5, 45 and 112.5 mg to reach 0%, 0.5%, 1%, 2% and 5%, respectively) by stirring for 5 min. Then, 2.3 mL of titanium (IV) isopropoxide was added dropwise, followed by stirring for 10 h at room temperature. Finally, the suspension was transferred to a 50-mL Teflon-lined autoclave and maintained at 170 ◦ C for 14 h. 2.3. Structural and Chemical Characterization The morphology and size of the particles were obtained by transmission electron microscopy (TEM) using a ZEISS LIBRA 120 (Carl Zeiss, Jena, Germany) instrument. Elemental analysis was carried out using an Oxford energy dispersive X-ray spectrometer (EDX) combined with the Zeiss Auriga SEM (Carl Zeiss, Jena, Germany) working at 20.0 kV. The crystal structure of the samples was determined by X-ray diffraction (XRD) using a D8 Advance (Bruker, Billerica, MA, USA) equipment with Ni-filtered Cu-Kα radiation (λ = 0.15406 Å) operating at 40 mA and 40 kV. UV-Vis absorption spectra were recorded on a UV-Vis spectrophotometer (Shimadzu, UV-3600i Plus, Tokyo, Japan), and BaSO4 was used as a reference standard. The spectra were recorded at room temperature in the air within the range of 300–800 nm. High-resolution transmission electron microscopy (HRTEM) images and scanning transmission electron microscopy (STEM) studies were conducted on an FEI Tecnai F20 field emission gun microscope operated at 200 kV with a point-to-point resolution of 0.19 nm, which was equipped with high angle annular dark- field (HAADF) and a Gatan Quantum electron energy loss spectroscopy (EELS) detectors. X-ray photoelectron spectroscopy (XPS) was done on a SPECS system (SPECS GmbH, Nanomaterials 2021, 11, 1399 4 of 15 Berlin, Germany) equipped with an Al anode XR50 source operating at 150 mW and a Phoibos 150 MCD-9 detector (SPECS GmbH, Berlin, Germany). Data processing was performed with the CasaXPS program (Casa Software Ltd., Teignmouth, UK). Steady-state photoluminescence (PL) spectra were conducted by a high-resolution photoluminescence spectrofluorometer (Horiba Jobin Yvon Fluorolog-3, Palaiseau, France). For the time- resolved photoluminescence spectroscopy (TRPL) measurements, a nanosecond LED with a 350-nm peak wavelength (Horiba NanoLED N390, Palaiseau, France, pulse width < 1.3 ns) was applied to excite the samples. The TRPL decay was resolved at 400 nm. Average lifetimes were obtained by fitting the TPPL spectra with DAS6 software (Horiba, Palaiseau, France). 2.4. Photoelectrochemical Measurements Photoelectrochemical (PEC) properties were measured using CHI760e (CHI 760E, CH Instrument, Austin TX, USA) in a three-electrode cell with a platinum mesh as the counter electrode, and an Ag/AgCl reference electrode. Na2 SO4 (0.5 M) was used as the electrolyte solution. The working electrode was prepared by depositing Cu2 O/TiO2 on an indium tin oxide (ITO) glass electrode (1 cm × 1 cm) and heating at 200 ◦ C for 1 h. Potentials vs. Ag/AgCl were converted into potentials vs. reversible hydrogen electrodes (RHE), according to the Nernst equation (ERHE = EAg/AgCl + 0.059 pH + 0.196). Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal ac perturbation of 5 mV applied over the frequency range of 0.01–100,000 Hz. The transient photocurrent (TPC) of the as-prepared photocatalysts was measured with an AM1.5G solar power system used as the light irradiation source (100 mW·cm−2 ) at an ambient temperature and without any light irradiation source. Mott–Schottky (M–S) measurements were carried out in the dark with a scanning speed of bias potential ranging from −1.4 to 0.2 V at a scan rate of 0.01 V·s−1 . The linear sweep voltammetry was carried out with a scanning speed of bias potential ranging from −1.2 to 0.6 V at a scan rate of 0.01 V·s−1 . 2.5. Photocatalytic Test In a typical experiment, a cellulose paper impregnated with 2.0 mg of the photocatalyst was placed inside a photocatalytic reactor that was equipped with UV LEDs (365 ± 5 nm, from SACOPA S.A.U, Gerona, Spain) (Figure S1). A light irradiation of 79.1 ± 0.5 mW·cm−2 was measured for UV light at the sample position. A saturated Ar gas stream was prepared by bubbling dry Ar gas through a Dreschel bottle with a water:ethanol vapor mixture (9:1, molar ratio, 20 mL·min−1 ). The photoreactor effluent was monitored online every 4 min using gas chromatography (GC) (Agilent 3000A MicroGC, Santa Clara, CA, USA) with three columns: MS 5 Å, Plot U and Stabilwax. The system was purged with the saturated Ar stream (20 mL·min−1 , 30 min) to remove oxygen before performing the experiments. The UV-visible light source contained two LEDs emitting at 372 ± 5 nm and two LEDs emitting visible light (correlated color temperature (CCT) 6099 K and color rendering index (CRI) 74) in Figure S1. In this system, UV light irradiation was 11.2 ± 0.5 mW·cm−2 at the sample position. 2.6. Apparent Quantum Yield (AQY) Calculation The AQY was estimated using the following equation: 2nH2 2nNA AQY = ·100 = ·100 (1) np ET /Ep where nH2 is the number of evolved hydrogen molecules, and np is the number of incident photons reaching the catalyst. The number of incident photons can be calculated by np = ET /Ep , where ET is the total energy reaching the catalyst, and Ep is the energy of a photon. ET can be calculated by ET = PSt, where P (W·m−2 ) is the power density of the incident monochromatic light, S (m2 ) is the irradiation area and t (s) is the duration of Nanomaterials 2021, 11, 1399 5 of 15 the incident light exposure. Ep can be calculated by Ep = hc/λ, where h is the Planck’s constant, c the speed of light and λ (m) is the wavelength of the incident monochromatic light. The number of hydrogen molecules can be calculated as nH2 = nNA , where n is the H2 moles evolved during the time of light exposure (t), and NA is the Avogadro constant. In our experimental conditions with UV light, the wavelength of the incident light was λ = 365 nm, the power density of the incident light at the paper surface was P = 79.1 mW·cm−2 and the irradiation area was S = πR2 = 3.14 × 0.752 = 1.77 cm2 . 3. Results and Discussion 3.1. Structural, Chemical and Optical Properties Cu2 O/TiO2 nanocomposites with different Cu2 O loading, between 0.5% and 5%, were synthesized by the hydrothermal reaction of copper (II) nitrate hexahydrate and titanium (IV) isopropoxide at 170 ◦ C for 12 h. Figure 1a shows the XRD patterns of the TiO2 and Cu2 O/TiO2 nanopowders. The main XRD peaks of all patterns could be indexed with the tetragonal anatase TiO2 phase (JCPDS No. 01-071-1167). Additional XRD peaks at 2θ = 36.4◦ and 42.3◦ were identified in the Cu2 O/TiO2 samples containing 1% and higher Cu2 O amounts and were associated with the (111) and (200) family planes of the cubic Cu2 O cuprite phase. From the XRD patterns, using the Scherrer equation, the size of the TiO2 and Cu2 O crystal domains was calculated to be ca. 7 nm and 50 nm, respectively, which pointed at the presence of some large Cu2 O crystals. TEM micrographs showed TiO2 and Cu2 O/TiO2 nanopowders that consisted of small nanoparticles with irregular shapes and an average size of ca. 10 nm (Figure 1b and Supplementary Figure S2). HRTEM characterization of the 1% composite further confirmed the presence of both the tetragonal anatase TiO2 and cubic Cu2 O phases (Figure 1c). STEM- EELS compositional map displayed the elemental distribution (Figure 1d). By performing the quantitative relative compositional analysis, we could extrapolate that the Ti and O compositions oscillate between 30–35% and 65–70%, respectively. Only traces of Cu could be detected from the 1% composite (TS1). This limitation and the small size of the Cu2 O domains observed by HRTEM resulted in a STEM-EELS compositional map showing homogeneous-like copper distributions (Figure 1d). SEM-EDX analysis showed the Cu concentration to match the nominal amount in low Cu-loaded samples but to be lower than expected in 2% and 5% Cu2 O/TiO2 nanocomposites (Table S1). XPS spectra showed the incorporation of Cu not to influence the Ti chemical state (Figure 1e and Supplementary Figure S3), which displayed the Ti 2p3/2 and Ti 2p1/2 - binding energies at 458.5 eV and 464.2 eV, respectively, consistent with Ti4+ within a TiO2 chemical environment [27–29]. Besides, the Cu 2p3/2 and Cu 2p1/2 -binding energies were 931.9 eV and 951.9 eV, pointing at a Cu+ chemical state [30,31]. The surface composition of Cu matched the nominal amount of Cu in the 1% Cu2 O/TiO2 nanocomposite, but it was lower for the 2% Cu2 O/TiO2 nanocomposite, which is, in part, consistent with SEM-EDX analysis and, in part, associated to the formation of relatively large Cu2 O particles when increasing the Cu loading, as observed by XRD. Figure 2 shows the UV-vis spectra of TiO2 and Cu2 O/TiO2 nanopowders and the corresponding Tauc plot calculated as (αhν)1/2 vs. hν to determine the direct bandgap of TiO2 (Figure 2b) and, as (αhν)2 vs. hν, to determine the indirect bandgap of Cu2 O (Figure 2c). UV-vis absorption data showed a clear absorption edge at around 3.2 eV consistent with the TiO2 bandgap. No clear shift of the absorption edge was observed with the introduction of Cu, which ruled out a possible bandgap change related to the incorporation of Cu ions within the TiO2 lattice. Besides, when incorporating Cu2 O, additional light absorption in the visible region and with an absorption edge of ca. 2.0 eV was clearly observed, consistent with the presence of the Cu2 O phase [32]. Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 15 Nanomaterials 2021, 11, 1399 6 of 15 Figure 1. (a) Powder XRD pattern of TiO2 and 0.5%, 1%, 2% and 5% Cu2O/TiO2 nanocomposites. Figure 1. (a) Powder XRD pattern of TiO2 and 0.5%, 1%, 2% and 5% Cu2O/TiO2 nanocomposites. (b) (b) TEM micrograph of 1% Cu2O/TiO2, with a scale bar of 200nm. (c) HRTEM analysis of the 1% TEM micrograph of 1% Cu2O/TiO2, with a scale bar of 200nm. (c) HRTEM analysis of the 1% CuCu2O/TiO2 sample. The upper image shows a crystal with a tetragonal anatase phase of TiO2 2O/TiO2 sample. The upper image shows a crystal with a tetragonal anatase phase of TiO2 visu- visualized along the zone alized along the [010] [010] axis. zoneTheaxis.lower The lower image image shows shows a cubicaCucubic Cu2O crystallite visualized 2O crystallite visualized along the [111] zone axis. (d) STEM-ADF and STEM-EELS analysis of the 1% Cu21% along the [111] zone axis. (d) STEM-ADF and STEM-EELS analysis of the Cu2O/TiO2 O/TiO 2 sample. sample. Cu L- Cu L-edges at 931 eV (red), O K-edge at 532 eV (green) and Ti L-edge at 456 edges at 931 eV (red), O K-edge at 532 eV (green) and Ti L-edge at 456 eV (blue). (e) High eV (blue). (e) High resolution XPS spectra XPS resolution for the Ti 2pfor spectra core thelevel ofcore Ti 2p and of TiO2level 1%, 2% and TiO2 Cu2O/TiO 1%, 2%2 nanocomposites. Cu2O/TiO2 nanocomposites. Figure 2 shows the UV-vis spectra of TiO2 and Cu2O/TiO2 nanopowders and the cor- responding Tauc plot calculated as (αhν)1/2 vs. hν to determine the direct bandgap of TiO2 (Figure 2b) and, as (αhν)2 vs. hν, to determine the indirect bandgap of Cu2O (Figure 2c). UV-vis absorption data showed a clear absorption edge at around 3.2 eV consistent with the TiO2 bandgap. No clear shift of the absorption edge was observed with the introduc- tion of Cu, which ruled out a possible bandgap change related to the incorporation of Cu ions within the TiO2 lattice. Besides, when incorporating Cu2O, additional light absorption Nanomaterials 2021, 11, 1399 in the visible region and with an absorption edge of ca. 2.0 eV was clearly observed,7 con- of 15 sistent with the presence of the Cu2O phase [32]. a b 2.0 c 0.3 TiO2 0.5% Cu2O/TiO2 1/2 Absorbance (a.u.) 2 1% Cu2O/TiO2 1.5 (αhν) / (eV/m) / (eV/m) 2% Cu2O/TiO2 5% Cu2O/TiO2 0.2 1.0 2 1/2 0.1 (αhν) 0.5 TiO2 Cu2O 3.2 eV 2.0 eV 0.0 0.0 300 400 500 600 700 800 2.0 2.5 3.0 3.5 4.0 4.5 2.0 2.5 3.0 Wavelength (nm) Energy (eV) Energy (eV) Figure 2. UV-vis absorption spectra (a) and Tauc plot calculated as (αhν)1/21/2 vs. hν (b) and as (αhν)2 vs. hν t (c) for TiO 2 Figure 2. UV-vis absorption spectra (a) and Tauc plot calculated as (αhν) vs. hν (b) and as (αhν)2 vs. hν t (c) for TiO2 and and 0.5%, 1%, 2% and 5% Cu2 O/TiO2 nanocomposites. 0.5%, 1%, 2% and 5% Cu2O/TiO2 nanocomposites. 3.2. 3.2. Photocatalytic Photocatalytic Activity Activity Figure 3a,b displays Figure 3a,b displays the theUV UV(365 (365±± 55 nm)nm) photocatalytic photocatalyticactivityactivityofofTiO TiO 2, Cu2O/TiO2 2 , Cu2 O/TiO2 and Cu O nanopowders toward hydrogen production Cu2 O nanopowders toward hydrogen production from a gas phase 10% 2 from a gas phase 10% ethanol ethanolso- lution solution ininwater. water. TheThe composition compositionofofthe theeffluent effluentgas gaswaswasmonitored monitoredusing usinggas gas chromatog- chromatog- raphy, which showed showedacetaldehyde acetaldehyde(2) (2)andandhydrogen hydrogen (3)(3) in in a 1:1 molar a 1:1 molarratio to be ratio tothe be two the unique two uniqueproductsproductsof theofreaction. the reaction.TheseThese resultsresults proved both that proved boththe hydrogen that the hydrogen was gener- was ated from the generated from dehydrogenation the dehydrogenation of ethanol and not of ethanol andfrom waterwater not from splitting and that splitting andthethatreac- the tion proceeded with very high selectivity toward acetaldehyde reaction proceeded with very high selectivity toward acetaldehyde production, following production, following the scheme the scheme [33]:[33]: CH3 CH2 OH + 2h+ → CH3 CHO + 2H+ (2) CH CH OH + 2h → CH CHO + 2H (2) 2H+ + 2e− → H2 (3) 2H + 2e → H (3) The hydrogen evolution rate (HER) measured under UV light for the reference TiO2 was 2.4Themmol h−1 · g− hydrogen 1 (Figure 3a,b). HER strongly increased with the introduction of Cu O evolution rate (HER) measured under UV light for the reference TiO 2 2 (Table was 2.4S2).mmolAmongh ·gthe(Figure −1 −1 series of Cu2 O/TiO 3a,b). 2 samples HER strongly tested, the increased withhighest HRE were obtained the introduction of Cu2O for theS2). (Table 0.5% and 1% Among theCu 2 O/TiO series of Cu2 samples 2O/TiO2 thatsamplesdisplayed tested,athe HER of 20.5 highest HREmolwere·g−1 ·obtained h−1 and 24.5the mmol − ·hand1 − 1 ·g1%, Cu a factor of2 10 abovethat baredisplayed TiO2 . Higher Cuof for 0.5% 2O/TiO samples a HER 2 O20.5 loadings mol·g−1that ·h−1 resulted and 24.5 in lower−1HER, mmol·h ·g−1, a 13.6 factor andof 10.7 10 abovemmolbare ·g−1TiO ·h−12. for the 2% Higher Cuand 5% samples, 2O loadings respectively. that resulted We in lower hypothesize the lower HER obtained when increasing HER, 13.6 and 10.7 mmol·g ·h for the 2% and 5% samples, respectively. We hypothesize −1 −1 the Cu loading above 1% to be related with an increase of the recombination rate associated the lower HER obtained when increasing the Cu loading above 1% to be related with an with a faster recombination of the charge increase of the carriers photogenerated recombination in the Cuwith rate associated 2 O phase a fasterthan in the TiO2 phase. recombination of theBesides, charge the formation carriers of larger Cu photogenerated in2 O thedomains Cu2O phase whenthanincreasing in the TiO the2Cu loading phase. couldthe Besides, also play an formation important of larger Cu role. The AQY of the 1% Cu2 O/TiO 2O domains when increasing the Cu2 was 6.4%,could loading whereasalsothe playAQY for TiO2 , 0.5%, an important role. 2% and 5% Cu O/TiO were 0.6%, 5.3%, 3.5% and 2.8%, The AQY of the 1% Cu2O/TiO2 was 6.4%, whereas the AQY for TiO2, 0.5%, 2% and 5% 2 2 respectively (Figure S4). Table S2 displays a comparison of the AQY obtained here with those Cu2O/TiO2 were 0.6%, 5.3%, 3.5% and 2.8%, respectively (Figure S4). Table S2 displays a obtained in previous works. On the otherofhand, comparison the AQY the HER of bare obtained hereCuwith 2 O wasthose very moderate, obtained just 0.8 mmol in previous works. ·h− 1 −1 On·gthe, demonstrating other hand, the bothHERthe important of bare Cu2O was rolevery played by TiO2just moderate, in the0.8 separation mmol·h−1·gof −1, charge carriers demonstrating and the synergism between the two materials to optimize photocatalytic both the important role played by TiO2 in the separation of charge carriers and the syner- activity. Figure S5 displays the HER of the 1% gism between the two materials2 to optimizeCu O/TiO 2 sample, measured three consecutive photocatalytic activity. Figure S5 displays the times during 1 h, showing the notable HER stability of the system. Besides, in contrast to some previous HER of the 1% Cu2O/TiO2 sample, measured three consecutive times during 1 h, showing works, we observed no color change of our samples during the photocatalytic reaction the notable HER stability of the system. Besides, in contrast to some previous works, we in the presence of ethanol [34,35]. It should be noted, that beyond the convenient use of aqueous ethanol solutions, as produced from biomass processing, the presence of water is beneficial to increase of the catalyst activity and stability by preventing active sites to be blocked by acetaldehyde, which exhibits a strong affinity towards inorganic oxide surfaces [36]. observed no color change of our samples during the photocatalytic reaction in the pres- ence of ethanol [34,35]. It should be noted, that beyond the convenient use of aqueous ethanol solutions, as produced from biomass processing, the presence of water is benefi- Nanomaterials 2021, 11, 1399 8 of 15 cial to increase of the catalyst activity and stability by preventing active sites to be blocked by acetaldehyde, which exhibits a strong affinity towards inorganic oxide surfaces [36]. Figure 3. (a) Photocatalytic H2 evolution on TiO2 , Cu2 O, 0.5%, 1%, 2% and 5% Cu2 O/TiO2 nanocomposites under UV light Figure 3. (a) (365 irradiation Photocatalytic ± 5 nm and H279.1 evolution ± 0.5 mW ·cm2−, 2Cu on TiO 2O, HER ). (b) 0.5%,from 1%, 2% and data 5% Cu2O/TiO displayed 2 nanocomposites in panel under UV (a). (c) HER measured on light TiO2 , irradiation (365 ± 5 nm and 79.1 ± 0.5 mW·cm −2). (b) HER from data displayed in panel (a). (c) HER measured on TiO2, 0.5%, 1%, 2% and 5% Cu2 O/TiO2 nanocomposites under different conditions: (1) UV light irradiation (372 ± 5 nm and 11.2 0.5%, ± 0.51%, mW 2% ·cm and −2 ),5%(2)Cu UV2O/TiO2 nanocomposites under different (372 ± 5 nm and 11.2 ± 0.5 mW·cm−2 )conditions: plus visible(1)light UV irradiation light irradiation (0.017(372 ± 5 nm ± 0.005 mWand·cm11.2 −2 ), ± 0.5 mW·cm−2), (2) UV (372 ± 5 nm and 11.2 ± 0.5 mW·cm−2) plus visible light irradiation (0.017 ± 0.005 mW·cm ◦ −2), (3) UV (3) UV light irradiation and (4) UV light irradiation and heating to compensate for the temperature (~36−37 C). (d) HER light irradiation and (4) UV light irradiation and heating to compensate for the temperature (~36−37 °C). (d) HER obtained obtained from the data displayed in panel (c). from the data displayed in panel (c). Figure 3c shows the photocatalytic HER activities of Cu2 O/TiO2 under UV light (372Figure ± 5 nm)3c and showswhen thecombining photocatalytic HERwith UV light activities visibleoflight Cu2O/TiO or heat2 under (see theUV light (372 experimental ±section 5 nm) foranddetails). when combining UV light with visible light or heat (see the It should be noted that, under visible light, there is an increase in the experimental section for details). temperature of the Itphotocatalyst; should be noted thus,that, it isunder visible necessary light, there to separate theiseffect an increase on HERinofthe the temperature of the photocatalyst; thus, it is necessary to separate the temperature increase and the photogenerated charge carriers obtained with the visible light effect on HER of the temperature increase absorption. Thus, theand the photogenerated photocatalytic chargeinto test was divided carriers four obtained consecutive with the (i) steps: visible After light absorption. Thus, the photocatalytic test was divided into four turning on the UV light, HER began to rise until it stabilized. At this stage, the sample consecutive steps: (i) After turning on the UV light, ◦ HER began to rise until it stabilized. temperature was ca. 25 C. (ii) Keeping the UV light on, the visible light was turned on, At this stage, the sam- ple whichtemperature increasedwasthe ca. HER 25 of °C.all(ii) KeepingThe samples. theintroduction UV light on,of the visible visible light light was also turned increased on, thewhich sampleincreased the HER temperature, upofto allca.samples. 36–37 The ◦ introduction C (Table of visible S3). (iii) With thelightUValso increased light on, the visible light was turned off, which resulted in a relatively slow decline of the HER and a temperature decrease down to 25 ◦ C. The slow HER decrease already denoted a significant effect of temperature on the increase of HER observed with the visible light. (iv) Finally, still maintaining the UV light on, the reactor was heated to 36–37 ◦ C (Table S3), which also resulted in an increase of the HER for all catalysts. By comparing stages 2 and 4, the effect of temperature and photogenerated electron–hole pairs can be differentiated. Nanomaterials 2021, 11, 1399 9 of 15 Notice that the addition of visible light increased the HER of TiO2 by a factor of two, which was associated with a 10 ◦ C increase in temperature (Figure 3c,d). This twofold HER increase points toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy of solar energy conversion. Such a strong influence of temperature on HER is likely related to the high adsorption energy of acetaldehyde on the oxide surface, blocking the catalyst active sites and, thus, slowing down the reaction. A moderate increase in temperature can significantly reduce the acetaldehyde adsorption strength, thus unblocking active sites and increasing the activity [37]. Cu2 O/TiO2 catalysts displayed a much higher increase of activity with the addition of visible light (Figure S6) by close to a factor of three in 1% Cu2 O/TiO2 . Only a small fraction of this increase in activity can be associated with the increase of temperature, as observed in Figure 3c,d. The much larger increase of HER obtained with visible light irradiation compared to the sample heating to the same temperature suggests a significant contribution of photogenerated charge carriers in Cu2 O. 3.3. Photoluminescence and Photoconductivity The photocatalytic performance of the semiconductor photocatalyst is tightly related to their charge transport, separation and transfer processes, which closely rely on their relative electronic energy level positions. To understand the photocatalytic process and to gain insights from the enhanced performances of the Cu2 O/TiO2 nanocomposites, a series of spectroscopic analyses was performed. Figure S7 displays the steady-state PL spectra of TiO2 and 1% Cu2 O/TiO2 . In both spectra, a peak at around 400 nm, associated with the band-to-band radiative recombination in TiO2 , was observed [38–42]. The presence of Cu2 O resulted in a decrease of the peak intensity, which denoted an influence of Cu2 O on the recombination of charge carriers photogenerated in TiO2 . Figure 4a displays the TRPL spectra of TiO2 and 1% Cu2 O/TiO2 at 400 nm. The PL intensity of both samples was observed to decay at a similar rate, with an average photocarrier lifetime of 32.3 ns for 1% Cu2 O/TiO2 and 34.0 ns for TiO2 . This result demonstrated a minor influence of Cu2 O on the band-to-band recombination within TiO2 , thus pointing again toward a minor or null influence of Cu within the TiO2 lattice. The photoelectrochemical behavior of TiO2 and 1% Cu2 O/TiO2 samples supported on an ITO-covered glass substrate were measured under dark and 100-mW·cm−2 AM 1.5G irradiation. As shown in Figure 4c, the photocurrent density measured for 1% Cu2 O/TiO2 was higher than that obtained for TiO2 . Figure 4b displays the TPC data obtained from the TiO2 and Cu2 O/TiO2 composites with different Cu2 O loadings. The 1% Cu2 O/TiO2 electrode showed the highest photocurrent densities, well above those obtained for bare TiO2 . The stable photocurrent of all Cu2 O/TiO2 samples pointed at a good stability of the composites under illumination in the solution. The 5% Cu2 O/TiO2 sample showed the largest TPC transient spikes, indicating the highest degree of surface charge recombination, which is consistent with its lower HER catalytic performance (Figure 3b) and suggests that an excessive amount of Cu2 O hampers the photocatalytic activity due to excessive charge carrier recombination [43]. Figure 4d displays the Nyquist plot with the EIS data obtained from TiO2 and 1% Cu2 O/TiO2 in the dark and under illumination. EIS analysis showed the 1% Cu2 O/TiO2 sample to be much less resistive than TiO2 [44], suggesting that the formation of the het- erojunction facilitates the charge transport and injection. Data were fitted with a Randles equivalent circuit consisting of a series resistor RS , a bulk resistor Rct,bulk for charge trans- port resistance and a bulk capacitor Cbulk for space charge region capacitance (Table S4) [45]. With the incorporation of only 1% of Cu2 O, the value of Rct,bulk was reduced from 4.18 Ω to 15.5 Ω in the dark and to even lower values under AM1.5G irradiation. transport resistance and a bulk capacitor 𝐶 for space charge region capacitance (Table Nanomaterials 2021, 11, 1399 S4) [45]. With the incorporation of only 1% of Cu2O, the value of 𝑅 , was reduced 10 of 15 from 4.18 Ω to 15.5 Ω in the dark and to even lower values under AM1.5G irradiation. Figure 4. (a) TRPL decay of the TiO2 and 1% Cu2 O/TiO2 composites. (b) Transient photocurrent response for TiO2 and Figure1%, 0.5%, 4. (a) 2% TRPL and 5%decay of the 2TiO Cu2 O/TiO 2 and 1% Cu composites. O/TiO2 composites. (c)2Current (b) Transient density vs. potential photocurrent (RHE) response and (d) Nyquist plot for withTiO 2 and the EIS 0.5%, 1%, 2% and 5% Cu 2O/TiO2 composites. (c) Current density vs. potential (RHE) and (d) Nyquist plot with the EIS data obtained from TiO2 and the 1% Cu2 O/TiO2 composite in the dark (off) and under illumination (on) at the AM1.5G data obtained solar from 100 power system TiOmW 2 and the ·cm−21% Cuirradiation. light 2O/TiO2 composite in the dark (off) and under illumination (on) at the AM1.5G solar power system 100 mW·cm−2 light irradiation. 3.4. Determination of Heterojunction Band Position 3.4. Determination of Heterojunction Band Position To determine the band alignment of the Cu O/TiO heterojunction; the M–S analysis To determine the band alignment of the Cu22O/TiO22 heterojunction; the M–S analysis was performed on pristine TiO2 , Cu2 O and 1% Cu2 O/TiO2 , considering: was performed on pristine TiO2, Cu2O and 1% Cu2O/TiO2, considering: 22 kTkT   −2 C =ND εε0 e VV−− C = Vfb − V −e (4) (4) ND εε e e where C is the space charge capacitance in the semiconductor, ND is the electron carrier density, e is the elementary charge (1.60 × 10−19 C) and ε0 is the vacuum permittivity (8.85 × 10−12 F·m−1 ). The considered relative permittivity was ε = 55 for TiO2 and ε = 6.3 for Cu2 O [46]. Nanomaterials 2021, 11, 1399 11 of 15 Figure 5a shows the M–S plots of TiO2 , Cu2 O and 1% Cu2 O/TiO2 . ND is determined as:  h i  −1 2 d 12 C  ND = × (5) eε0 ε dvs " h i # −1 d 1 h i C2 where is the best fit of their linear range of 1 vs. V (12 × 109 cm4 ·F−2 for dvs C2 TiO2 and 9 × 1010 cm4 · F−2 for Cu2 O). As expected, TiO2 shows a positive value in the linear region in accordance with its n-type character, while Cu2 O shows a negative value Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 15 consistent with its p-type behavior [30]. The M–S analysis resulted in ND = 2.14 × 1020 cm−3 for TiO2 and ND = 2.5 × 1020 cm−3 for Cu2 O. Figure 5. 5. (a) (a)M-S M-Sanalysis of of analysis TiO 2 and 1% Cu2O/TiO2. (b) M-S analysis of a Cu2O. (c) Energy band diagrams for Cu2O and TiO Figure 2 and 1% Cu2 O/TiO2 . (b) M-S analysis of a Cu2 O. (c) Energy band diagrams for TiO2 before contact. (d) Scheme of the Energy band structure of a Cu2O/TiO2 heterojunction and the ethanol dehydrogena- Cu2 O and TiO2 before contact. (d) Scheme of the Energy band structure of a Cu2 O/TiO2 heterojunction and the ethanol tion reaction. dehydrogenation reaction. 4. Conclusions The effective density of states in the conduction band (NC ) is given by: A simple one-pot method for the synthesis of p-n Cu2O/TiO2 heterostructures was 3 presented. Using UV-vis spectroscopy, and2πm M–Sdeanalyses, we showed the formation of a  kT 2 p-n heterojunction between Cu2ON C ≡TiO and 2 2, which 2 favors the (6) separation of electron–hole h pairs. They obtained nanocomposites at 0.5%, 1%, 2% and 5% Cu2O loading were tested for the photocatalytic dehydrogenation of ethanol in water:ethanol vapor mixture. We demonstrated the composites to be photostable catalysts capable of working in a light ab- sorption towards the visible range, with an outstanding selectivity to the production of acetaldehyde and hydrogen from ethanol. The optimum composition contained 1% of −1 −1 Nanomaterials 2021, 11, 1399 12 of 15 where mde is the density-of-state effective mass for electrons of nano-crystalline anatase TiO2 , h is Planck’s constant (6.62607004 × 10−34 m2 ·kg·s−1 ), k is Boltzmann’s constant (1.38064852 × 10−23 m2 ·kg·s−2 ·K−1 ) and T is the absolute temperature (298 K). For TiO2 a mde =10 m0 was used for Nc calculations, where m0 (9.109 × 10−31 ) is the mass of a free electron. For Cu2 O mde = 0.58 m0 is taken as the effective hole mass. Boltzmann statistics was applied to determine the position of the bottom of the conduction band ECB for TiO2 and the maximum of the valence band EVB for Cu2 O (3):   NC E − EF = kT ln (7) ND where EF is the Fermi level position (EF = Vfb ). EF was found to be 0.033 eV below the ECB for TiO2 and 0.081 eV above the EVB for Cu2 O [47]. Based on the M–S analysis, the electronic band structure of Cu2 O and TiO2 is displayed in Figure 5c. The Vfb values of TiO2 and Cu2 O are −0.44 eV and 0.72 eV vs. RHE, respectively. The ECB for TiO2 was −0.41 eV and the EVB for Cu2 O is 0.64 eV. As the two materials are brought into contact, there is a net transfer of electrons from n-type TiO2 to p-type Cu2 O that results in a bending of the band structure at the interface. Due to the small size of the crystal domains, this bending extends through all the whole TiO2 and Cu2 O crystals that are in contact with each other. (Figure 5d). In the resulting heterostructure, photogenerated electrons in the conduction band of Cu2 O tend to move toward TiO2 , where hydrogen generation takes place, and photogenerated holes in TiO2 tend to move toward the Cu2 O, where ethanol is oxidized to acetaldehyde (Figure 5d) [48,49]. 4. Conclusions A simple one-pot method for the synthesis of p-n Cu2 O/TiO2 heterostructures was presented. Using UV-vis spectroscopy, and M–S analyses, we showed the formation of a p-n heterojunction between Cu2 O and TiO2 , which favors the separation of electron–hole pairs. They obtained nanocomposites at 0.5%, 1%, 2% and 5% Cu2 O loading were tested for the photocatalytic dehydrogenation of ethanol in water:ethanol vapor mixture. We demonstrated the composites to be photostable catalysts capable of working in a light absorption towards the visible range, with an outstanding selectivity to the production of acetaldehyde and hydrogen from ethanol. The optimum composition contained 1% of Cu2 O and showed a yield for HER of 24.5 mmol·g−1 ·h−1 and an AQY = 6.4%. The EIS analysis showed the 1% Cu2 O/TiO2 sample to be less resistive than TiO2 sample and suggested that the heterojunction facilitated the charge transport and injection. The addition of visible light increased the HER of the samples by a factor of two, which was partially associated with an increment in the reaction temperature of around 10 ◦ C. We further discerned the influence of temperature and photogenerated electron–hole pairs in the HER increase upon visible light irradiation, demonstrating the important role of photogenerated charge carriers in the presence of Cu2 O. Besides, our results open new opportunities for efficient solar energy conversion by the combination of energy sources in thermo-photocatalytic reactors. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano11061399/s1, Figure S1. (a) Photograph of the system used to test photocatalytic hydrogen generation: 1 displays the flask containing the ethanol-water solution (1:9) and 2 displays the actual photoreactor. (b) Scheme of the photoreactor under UV-light, (c) Scheme of the central part of the photoreactor. Figure S2. TEM micrograph of TiO2 nanocrystals. Figure S3. (a) Survey XPS spectra of TiO2 and 1% and 2% Cu2O/TiO2 nanocomposites. (b) High-resolution XPS spectra for Cu 2p core level of 1% and 2% Cu2O/TiO2 nanocomposites. Figure S4. AQY data of HER obtained on TiO2 and 0.5%, 1%, 2%, 5% Cu2O/TiO2 nanocomposites. Figure S5. Three consecutive cycles of photocatalytic hydrogen production under UV light using the same 1% Cu2O/TiO2 sample. Figure S6 Emission spectrum of the visible LED used for visible illumination recorded by an ocean optics spectrometer (USB2000+XR1-ES). Figure S7. Photoluminescence spectra of TiO2 (black line) and 1% Nanomaterials 2021, 11, 1399 13 of 15 Cu2O/TiO2 nanocomposite (red line). Table S1. Ti and Cu atomic concentrations of 0%, 0.5%, 1%, 2%, 5% Cu2O/TiO2 nanocomposites. Table S2. Comparison of the hydrogen evolution rates reported on Cu2O-TiO2 systems. Table S3. Temperature evolution in the photocatalytic reaction of the 0%, 0.5%, 1%, 2%, 5% Cu2O/TiO2 nanocomposites. Table S4. EIS data fitting results obtained from TiO2 and 1% Cu2O/TiO2 nanocomposite in the dark (off) and under 100 mW·cm-2 AM 1.5G irradiation (on). Author Contributions: The manuscript was prepared through the contribution of all authors. A.C. and J.L. (Jordi Llorca) guided the project and supervised the work. C.X., Y.Z., X.W., P.R.M.-A. and A.C. conceived and prepared the manuscript. J.L. (Jordi Llorca) designed the hydrogen production experi- ments. C.X. and Y.Z. produced the samples. Y.Z. and J.L. (Junshan Li) performed the electrochemical measurements and analyzed the results. Y.L. performed PL and TRPL experiments and analyzed the results. P.G., M.C.S. and J.A. performed TEM, HRTEM and STEM-EELS and discussed these results. J.L. (Jordi Llorca) performed XPS measurements and discussed these results. The manuscript was corrected and improved by all authors. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the MINECO project COMBENERGY (PID2019-105490RB- C32) and MICINN/FEDER project RTI2018-093996-B-C31. J.A. and M.C.S. thank funding from GC 2017 SGR 327 and MINECO project (ENE2017-85087-C3). ICN2 thanks supporting from the Severo Ochoa Program (MINECO, No. SEV-2013-0295) and the CERCA Program/Generalitat de Catalunya. Y.Z., C.X., and X.W. acknowledge the China Scholarship Council (CSC) for scholarship support. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. J.L. is a Serra Hunter fellow and is grateful to GC 2017 SGR 128 and ICREA Academia program. Y.L. thank the Swiss National Science Foundation (SNF) for funding under the Ambizione Energy (No. PZENP2_166871). M.C.S. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 754510 (PROBIST) and the Severo Ochoa programme. 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