This is the submitted version of the journal article: Xing, Congcong; Liu, Yongpeng; Zhang, Yu; [et al.]. «A direct Z-Scheme for the photocatalytic hydrogen production from a water ethanol mixture on CoTiO3». ACS Applied Materials and Interfaces, Vol. 13, issue 1 (Jan. 2021), p. 449-457. DOI 10.1021/acsami.0c17004 This version is available at https://ddd.uab.cat/record/271946 under the terms of the license Page 1 of 21 ACS Applied Materials & Interfaces 1 2 3 4 5 A direct Z-scheme for the photocatalytic hydrogen production from 6 7 8 ethanol dehydrogenation on CoTiO3/TiO2 heterostructures 9 10 11 12 Congcong Xing,a,b† Yongpeng Liu,c† Yu Zhang,a Xiang Wang,a Pablo Guardia,a Liang Yao,c Xu 13 14 Han,d Ting Zhang,d Jordi Arbiol,d,e Lluís Soler,b Yufen Chen,b Kevin Sivula,c Néstor Guijarro,c 15 16 Andreu Cabot,*a,e Jordi Llorca *b 17 18 19 a 20 Catalonia Institute for Energy Research (IREC), Sant Adriàde Besòs, 08930 Barcelona, Spain 21 22 b 23 Institute of Energy Technologies, Department of Chemical Engineering and Barcelona 24 25 Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, 26 27 EEBE, 08019 Barcelona, Spain 28 29 30 c Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO), École 31 32 Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland 33 34 35 d Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 36 37 38 Bellaterra, 08193 Barcelona, Catalonia, Spain 39 40 e 41 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain 42 43 44 †These authors contributed equally to this work. 45 46 47 * E-mails: A. Cabot: acabot@irec.cat; J. Llorca: jordi.llorca@upc.edu. 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 2 of 21 1 2 3 4 5 ABSTRACT 6 7 8 Photocatalytic H2 evolution from ethanol dehydrogenation is a convenient strategy to store 9 solar energy in a highly valuable fuel with potential zero net CO2 balance. Herein, we report 10 11 on the synthesis of CoTiO3/TiO2 composite catalysts with controlled amounts of highly 12 13 distributed CoTiO3 nanodomains for photocatalytic ethanol dehydrogenation. We demonstrate 14 these materials to provide outstanding hydrogen evolution rates under UV and visible 15 16 illumination. The origin of this enhanced activity is extensively analyzed. In contrast to 17 18 previous assumptions, UV-vis absorption spectra and ultraviolet photoelectron spectroscopy 19 20 (UPS) prove CoTiO3/TiO2 heterostructures to have a type II band alignment, with the 21 conduction band minimum of CoTiO3 below the H2/H+ energy level. Additional steady-state 22 23 photoluminescence (PL) spectra, time-resolved PL spectra (TRPLS) and electrochemical 24 25 characterization prove such heterostructures to result in enlarged life times of the 26 27 photogenerated charge carriers. These experimental evidences point toward a direct Z-scheme 28 as the mechanism enabling the high photocatalytic activity of CoTiO3/TiO2 composites toward 29 30 ethanol dehydrogenation. Besides, we probe small changes of temperature to strongly modify 31 32 the photocatalytic activity of the materials tested, which could be used to further promote 33 performance in a solar thermo-photocatalytic reactor. 34 35 36 37 38 KEYWORDS: Bioethanol; photocatalysis; hydrogen; titanium dioxide; Z-scheme; titanate, 39 40 dehydrogenation 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Page 3 of 21 ACS Applied Materials & Interfaces 1 2 3 INTRODUCTION 4 5 6 Hydrogen is regarded as one of the most convenient energy vectors, owing to its 7 abundance, cleanness, and high energy density. However, as there is no natural source of 8 9 molecular hydrogen, H2 needs to be produced from chemical compounds such as water, 10 11 hydrocarbons or alcohols, what requires a supply of energy. Among the possible sources of 12 hydrogen, ethanol is particularly suitable both thermodynamically and in terms of net CO 2 13 14 emissions. The production of hydrogen from ethanol is less energetically demanding than from 15 16 water splitting, and the product can be more easily separate. At the same time, ethanol is the 17 18 most common liquid fuel derived from biomass, being worldwide produced in huge quantities 19 (1011 liters annually) by the fermentation of sugars from corn starch and sugar cane and from 20 21 large-volume low-value biomass like lignocellulose and organic waste.1-4 Ethanol biosourcing 22 23 compensates the CO2 released during combustion through its capture during photosynthesis, 24 25 leading to a near zero net CO2 footprint. Besides, being liquid at ambient temperature, ethanol 26 can be easily stored and transported, to the point that the already stablished infrastructure for 27 28 ethanol production and distribution makes it a very convenient precursor of other fuels, 29 30 including H2 and higher alcohols.4, 5 31 32 In terms of source of energy, the use of solar energy to generate fuels and particularly 33 34 hydrogen is especially attractive due to its abundance and ubiquitous availability at zero cost. 35 36 From another point of view, the production of solar fuels can be regarded as a suitable strategy 37 to store this intermittent form of energy.6, 7 38 39 Photocatalysts allow the production of molecular hydrogen from ethanol dehydrogenation 40 41 using solar energy as unique energy input.1 In the simplest scenario, ethanol can be 42 43 photocatalytically dehydrogenated at ambient pressure and temperature, yielding molecular 44 45 hydrogen and acetaldehyde, and releasing no CO2: 46 47 𝐶𝐻3 𝐶𝐻2 𝑂𝐻 + 2ℎ+ → 𝐶𝐻3 𝐶𝐻𝑂 + 2𝐻 + (1) 48 49 2𝐻 + + 2𝑒 − → 𝐻2 (2) 50 51 Acetaldehyde is a particularly versatile chemical precursor extensively used by the chemical 52 53 industry because the aldehyde moiety provides a functional handle that, through different 54 55 reaction pathways, can be used to yield a range of products, including upgraded fuels as well 56 57 as large volume commodity chemicals such as acetic acid or butadiene, among others.8- 58 10 Besides, this dehydrogenation reaction is also the limiting step in the synthesis of higher 59 60 alcohols through the Guerbet reaction. Thus, separating it from the rest of the process could ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 4 of 21 1 2 3 allow not only saving energy but also improving product selectivity by reducing the process 4 5 maximum temperature.4, 11, 12 6 7 To engineer an efficient photocatalyst for solar energy storage into fuels it is necessary to 8 9 ensure an effective absorbance of the solar radiation, long lasting photogenerated charge 10 11 carriers, which generally requires a means of charge separation, and a proper surface interaction 12 with all the involved chemical species. To satisfy these demanding requirements, composite 13 14 materials are generally used to extend the range of spectrum absorption, drive charge separation 15 16 and offer additional adsorption/reaction sites. In many systems it is found that the 17 18 photocatalytic reaction is most likely occurring at the interface between two different materials, 19 what makes the engineering of this interface particularly important.7 To minimize 20 21 recombination sites and defects, the interphase can be optimized by the direct growth of one 22 23 phase from the surface of the other, or even more conveniently, by the segregation of 24 25 nanodomains of the two phases from a common precursor material. To maximize the interface 26 and minimize diffusion and side reactions of the involved species, a nanoscale dispersion of 27 28 the different phases is required. 29 30 Titanium dioxide (TiO2) is the prototype photocatalyst due to its outstanding 31 32 optoelectronic properties, natural abundance, high photocorrosion resilience, lack of toxicity 33 34 and low cost.13, 14 However, TiO2 practical application is hampered by its large bandgap, that 35 limits the range of solar spectra used.15, 16 The doping of titania with different elements and its 36 37 combination with light absorbers have been demonstrated effective in extending the range of 38 39 solar spectrum absorbed. But this improvement generally comes at the cost of decreasing 40 41 internal quantum efficiencies due to increased charge carrier recombination, resulting in a 42 limited overall improvement of the photon-to-fuel energy conversion efficiency. 43 44 45 Cobalt titanate (CoTiO3) is a narrow band gap oxide semiconductor with a ABO3 46 perovskite crystal structure, large carrier mobility, excellent stability, and high absorption 47 48 coefficient.17-19 In combination with graphitic carbon nitride (g-C3N4) and Pt, it has been 49 50 recently explored for photocatalytic hydrogen generation through a direct Z-scheme, where 51 holes photogenerated in CoTiO3 participate in the oxidation reaction, electrons photogenerated 52 53 in the g-C3N4 drive the reduction reaction, and at the interphase of the two materials electrons 54 55 photogenerated in CoTiO3 recombine with holes generated in g-C3N4.17 CoTiO3/TiO2 56 57 composites have also been synthesized and used in multiple applications, including 58 photoelectrochemical water splitting;20 photocatalytic dye degradation,21 oxidative 59 60 dehydrogenation of cyclohexane;22 dry reforming of methane;23 peroxymonosulfate activation ACS Paragon Plus Environment Page 5 of 21 ACS Applied Materials & Interfaces 1 2 3 and metronidazole degradation;24 resistive humidity sensing;25 electrochemical sensing of 4 5 dopamine;26 and anode in lithium ion batteries,27 but surprisingly CoTiO3/TiO2 composites 6 7 have not been thoroughly tested as photocatalysts for H2 production from bioethanol 8 9 dehydrogenation. 10 11 Herein, CoTiO3/TiO2 photocatalysts with different CoTiO3 amounts are produced by 12 phase segregation during annealing of a Co-doped TiO2 precursor prepared by hydrothermal 13 14 reaction. CoTiO3/TiO2 photocatalysts were tested for hydrogen generation from ethanol 15 16 dehydration in gas phase. A 10% alcohol solution in water was used owing to its optimized 17 18 activity and its proximity to commercial bioethanol, produced in the form of diluted aqueous 19 solution from biomass fermentation. Notice that using aqueous solutions, thus avoiding the 20 21 costly ethanol separation and purification, is fundamental for the cost-effectiveness of the 22 23 process.2, 3, 5 24 25 RESULTS AND DISCUSSION 26 27 Photocatalysts Characterization 28 29 30 Cobalt-doped titania (TiO2:Co) nanopowders with different Co amounts were obtained by 31 the simultaneous hydrothermal reaction of titanium (IV) isopropoxide and cobalt (II) nitrate at 32 33 170 C for 14 h. XRD patterns of these materials only displayed the anatase TiO2 34 35 crystallographic phase (Figure S2). Co introduction resulted in a shift of the anatase diffraction 36 37 peaks to lower angles, which indicated the incorporation of Co inside TiO2 lattice structure. 38 EDX, XPS, ICP analysis showed the TiO2:Co nanopowders to contain Co/Ti atomic ratios 39 40 above the nominal values, which evidenced a better reaction yield of cobalt (II) nitrate than 41 42 titanium (IV) isopropoxide (Table S1). TEM micrographs (Figures 1b, S6) displayed most 43 TiO2:Co particles to have an elongated geometry, with lengths in the range 100-200 nm and 44 45 thicknesses around 30-60 nm. 46 47 Co-doped TiO2 nanopowders were used as precursors to produce Ti/Co oxide composites. 48 49 Precursor TiO2:Co nanopowders were annealed at 650 °C for 8 h with synthetic air flow 50 51 (FigureS5). XRD patterns of the annealed materials (Figure 1a) displayed the characteristic 52 53 peaks of both anatase TiO2 (JCPDS no.01-073-1764)28 and CoTiO3 (JCPDS 01-077-1373)29. 54 As expected, with Co content increasing in the precursor material, the relative intensity of the 55 56 CoTiO3 diffraction peaks became stronger (Figure 1a). Besides, the shift of the anatase peaks 57 58 detected in the precursor TiO2:Co materials disappeared, indicating the segregation of Co ions 59 to form the CoTiO3 phase. TEM analysis showed the particles within the annealed powder to 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 6 of 21 1 2 3 just partially preserve the elongated geometry of the precursor (Figure 1c, S7). Besides, 4 5 elemental maps showed the CoTiO3 phase to be evenly distributed throughout the CoTiO3/TiO2 6 7 nanocomposite, within crystal domains of a similar size as those of TiO2 (Figure 1d). 8 9 10 a CoTiO3 CoTiO3/TiO2 CoTiO3 11 12 5% * * 13 2% Intensity (a.u) Intensity (a.u) Intensity (a.u) 14 15 1% 16 17 0.5% 18 TiO2 19 CoTiO3 JCPDS:01-077-1373 20 TiO2 JCPDS:01-073-1764 21 20 30 40 50 60 70 80 25.0 25.5 32 34 36 22 23 2 (Degree) 2 (Degree) 2 (Degree) 24 25 b Co2+ d 26 Ti 27 O 28 29 200 nm 30 50 nm 31 32 33 c CoTiO3 Ti 34 35 O 36 37 38 39 100 nm 40 41 42 Figure 1. a) XRD patterns of CoTiO3/TiO2 (0, 1, 2, 5%). b,c) TEM micrograph and atomic 43 44 scheme of the precursor TiO2:Co (1%) nanopowder (b) and the CoTiO3/TiO2 (1%) 45 46 nanocomposite (c). d) STEM image of a CoTiO3/TiO2 composite and corresponding EELS 47 elemental maps for Ti, O, Co and its combinations as noted in each map. 48 49 50 As reference materials, pure TiO2 and CoTiO3 powders were synthesized by 51 decomposition of titanium (IV) isopropoxide and proper amounts of titanium (IV) 52 53 isopropoxide and cobalt (II) nitrate, respectively (see details in the experimental section). XRD 54 55 patterns of the annealed samples (Figure 2a) displayed the pure anatase TiO2 and CoTiO3 56 phases, respectively. The bandgaps of TiO2 and CoTiO3, estimated via the Tauc plot from the 57 58 UV-visible absorption spectra,30 were 3.0 eV and 2.3 eV, respectively (Figure 2b, S10). 59 60 TEM/HRTEM micrographs of TiO2 and CoTiO3 nanopowders showed them to be composed ACS Paragon Plus Environment Page 7 of 21 ACS Applied Materials & Interfaces 1 2 3 of irregular shaped particle with an average size of ca. 30 and 60 nm, respectively (Figures S3, 4 5 S4). 6 7 8 a b 9 10 CoTiO3 2 (h) (eV/m) Intensity (a.u) 11 12 13 2 14 TiO2 TiO2 15 CoTiO3 TiO2 (01-073-1764) 16 CoTiO3 (01-077-1373) 17 18 1.5 2.0 2.5 3.0 3.5 4.0 19 20 30 40 50 60 70 80 20 2 (Degree) Energy (eV) 21 22 23 Figure 2. a) XRD patterns of TiO2 and CoTiO3, b) Kubelka-Munk function for TiO2 and 24 CoTiO3 with a linear fit to determine the band gap energy. 25 26 27 Photocatalytic hydrogen production 28 29 Figure 3a,b displays the UV photocatalytic activity of CoTiO3/TiO2 nanopowders with 30 31 different amount of CoTiO3 in the production of hydrogen from ethanol dehydration using a 32 10% ethanol solution in water. The activities of the reference TiO2 and CoTiO3 catalysts and 33 34 that of the Co-doped TiO2 were also measured and displayed. The hydrogen evolution rate 35 36 (HER) measured under UV light (365 ±5 nm) for the reference TiO2 and CoTiO3 catalysts was 37 38 1.8 and 0.3 mmol h−1·g−1, respectively. On the other hand, among the series of CoTiO3/TiO2 39 samples tested, the CoTiO3/TiO2 (1%) photocatalyst exhibited the highest HER, 3.7 mmol 40 41 h−1·g−1, a twofold higher than TiO2 and a factor 12 above CoTiO3. Under the same conditions, 42 43 the HER of the TiO2:Co (1%) catalyst was just 0.5 mmol h−1·g−1. Actually, as shown in figure 44 S9, all the nanocomposites CoTiO3/TiO2 (0.5, 2 and 5%) displayed significantly larger HER 45 46 than those obtained from Co-doped titania, TiO2:Co (0.5, 2 and 5%). 47 48 Figure 3c shows the HER photocatalytic activities under UV light (372 ± 5 nm) when 49 50 adding visible light or heat (see experimental section for details). Experiments involved four 51 52 consecutive steps: 1) Upon turning on the UV light, HER began to increase until it stabilized. 53 54 At this point the sample temperature was ca. 24 C; 2) With the UV light on, the visible light 55 was turned on, which increased the HER of most of the samples. The addition of visible light 56 57 also increased the sample temperature, up to ca. 34-36 C; 3) Maintaining the UV light on, the 58 59 visible light was turned off, what resulted in a relatively slow decrease of the HER and a 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 8 of 21 1 2 3 temperature decrease down to 24 C. The slow HER decrease already denoted a significant 4 5 effect of temperature on the observed increase of HER with the visible light; 4) Still with the 6 7 UV light on, the reactor was heated to 34-36 C, which also resulted in an increase of the HER 8 9 for most catalysts. Comparing HER obtained in stages 2 and 4 (Figure 3c, d), it was observed 10 that the addition of visible light or heat resulted in a very significant increase of HER for most 11 12 samples. In the case of the reference TiO2 catalyst and the Co-doped TiO2, the very significant 13 14 activity enhancement obtained with the visible light was fully associated to the slight increase 15 16 of temperature. Notice that almost a twofold increase of HER was obtained with a temperature 17 change of just 10 ºC, which opens the door for an efficient solar energy conversion though 18 19 thermo-photocatalytic process. CoTiO3/TiO2 catalysts also showed an important increase of 20 21 activity with the minor increase of temperature tested. However, CoTiO3/TiO2 samples 22 23 displayed a much more significant increase of HER when incorporating the visible light 24 irradiation than when just heating the sample to the reached temperature. Among the different 25 26 CoTiO3/TiO2 composites, CoTiO3/TiO2 (1%) displayed the highest performance when 27 28 combining UV and visible light. Finally, the incorporation of visible light or temperature had 29 a minor influence on the HER of CoTiO3. 30 31 32 a 2.0 b 3.7 H2 production (mmol g ) 33 4 -1 UV 34 1.5 H2 (mmol h-1g-1) 35 CoTiO3/TiO2 (1%) CoTiO3/TiO2 (0.5%) 3 36 1.0 TiO2 1.8 2.0 1.8 37 2 CoTiO3/TiO2 (2%) 38 0.5 UV light TiO2:Co(1%) 0.3 39 on 1 0.5 0.3 40 0.0 CoTiO3/TiO2 (5%) 41 CoTiO3 0 4 TiO2 0.5% 1% 2% 5% TiO2:Co CoTiO3 42 0 5 10 15 20 25 30 35 Time (h) CoTiO 3 3/TiO2 43 44 c 1 2 3 4 d 0.6 2 45 UV UV+Vis UV UV+Heat 46 H2 rate (mmol g h ) 0.5 -1 CoTiO3/TiO2 (1%) 0.5 1 47 H2 (mmol h-1g-1) -1 0.4 48 0.4 UV 49 0 0.3 CoTiO3/TiO2 (0.5%) 0.3 UV+Vis 50 TiO2 UV+Heat 51 0.2 TiO2:Co (1%) 0.2 52 0.1 0.1 53 CoTiO3/TiO2 (5%) CoTiO3 54 0.0 0 TiO2 0.5% 1% 5% TiO2:Co CoTiO3 55 0.0 0.5 1.0 1.5 2.0 Time (h) CoTiO3/TiO2 56 57 58 Figure 3. a) Photocatalytic H2 evolution on TiO2, CoTiO3/TiO2 (1, 2, 5%), TiO2:Co (1%) and 59 60 CoTiO3 under UV light irradiation (365 ± 5 nm, 79.1 ± 0.5 mW·cm−2). b) HER from data ACS Paragon Plus Environment Page 9 of 21 ACS Applied Materials & Interfaces 1 2 3 displayed in (a). c) HER measured from CoTiO3/TiO2 (0.5, 1, 5%), TiO2, CoTiO3, TiO2:Co 4 5 (1%) under different conditions: 1) UV light irradiation (372 ±5 nm, 11.2 ±0.5 mW·cm−2); 2) 6 7 UV (372 ±5 nm, 11.2 ±0.5 mW·cm−2) plus visible light irradiation (0.017 ±0.005 mW·cm−2); 8 9 3) UV light irradiation, 4) UV light irradiation and heating to compensate temperature (34-36 10 C); d) HER obtained from data displayed in (c). 11 12 13 A comparison of the XRD patterns and TEM images of the CoTiO3/TiO2 (5%) sample 14 before and after the photocatalytic tests (Figures S11, S12) revealed no apparent changes, 15 16 suggesting that the CoTiO3/TiO2 heterojunction possessed a good structural photostability. 17 18 The photocatalytic performance of semiconductor materials is closely related to the charge 19 20 separation, transport and transfer processes that strongly depend on their relative electronic 21 22 energy level positions. To gain insights from the photocatalytic process and to understand the 23 24 enhanced performance of CoTiO3/TiO2 composites, a set of spectroscopic analysis was carried 25 out. The steady-state PL spectra recorded on TiO2 and CoTiO3/TiO2 (1%) showed a 26 27 characteristic peak around 400 nm (Figure 4a), which was in line with recent reports on PL 28 29 spectra of TiO2.31, 32 The lower PL intensity of the CoTiO3/TiO2 composite originated from the 30 reduced radiative charge recombination in TiO2 due to CoTiO3 incorporation. Similar PL 31 32 attenuation effects were reported by Chen et al. on noble-metal-decorated TiO2 nanocrystals32 33 34 and by Zuo et al. on SnS2–Pt nanoheterostructures.33 To analyze the role of CoTiO3, TRPL 35 36 spectra of TiO2 and CoTiO3/TiO2 heterostructures were measured and compared. The TRPL 37 spectra tracked at 400 nm is shown in Figure 4b. The PL intensity of CoTiO3/TiO2 (1%) was 38 39 observed to decay significantly slower than for pristine TiO2. The fitting revealed an average 40 41 lifetime of 35.8 ns for CoTiO3/TiO2 (1%) and 5.9 ns for TiO2. The longer lifetime recorded 42 43 over CoTiO3/TiO2 proved that the heterojunction contributed to the charge separation, 44 increasing in this way their recombination time. 45 46 47 a 5 b 103 48 TiO2 PL intensity (a.u.) 49 4 PL intensity (a.u.) 50 102 51 3 CoTiO3/TiO2(1%) 52  = 35.8 ns 53 2 101 54 55 1 TiO2 56 CoTiO3/TiO2(1%)  = 5.9 ns 57 0 100 58 350 400 450 500 550 600 0 30 60 90 120 150 59 Wavelength (nm) Time (ns) 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 10 of 21 1 2 3 Figure 4. a) Steady-state photoluminescence spectra. b) Time-resolved photoluminescence 4 5 spectra with corresponding fitting curves. 6 7 TPC and EIS measurements were conducted using photocatalysts films deposited on FTO 8 9 substrates (see experimental section for details). As shown in Figure 5a, the loading of CoTiO3 10 11 on TiO2 resulted in enhanced photocurrent responses, with a higher steady-state photocurrent 12 at 0.6 V vs RHE that may be related to an improved charge transfer process. Notably, TPC 13 14 spikes, whose amplitude represents the degree of surface recombination,34 were larger in 15 16 pristine TiO2 than in CoTiO3/TiO2 (1%). This result indicated that the CoTiO3/TiO2 17 18 heterojunction efficiently suppresses the surface charge recombination process. Similar TPC 19 response have been reported on other photocatalyst systems, for example, carbon nanotube 20 21 loaded TiO2,35 TiO2/Fe2TiO5/Fe2O3 triple-heterojunction,36 and Bi2O3@TiO2 photocatalyst.37 22 23 EIS measurements showed that qualitatively speaking, and as deduced from the Nyquist 24 25 plots (Figure 5b), CoTiO3/TiO2 (1%) was less resistive than TiO2,38 indicating that the 26 27 formation of the heterojunction facilitated the charge transport process. As shown in the inset 28 29 of Figure 5b, a Randles equivalent circuit was employed to fit the data, which consisted of a 30 series resistor 𝑅𝑆 , a bulk resistor 𝑅𝑐𝑡,𝑏𝑢𝑙𝑘 for charge transport resistance, and a bulk capacitor 31 32 𝐶𝑏𝑢𝑙𝑘 for space charge region capacitance.39 The data fitting showed that CoTiO3/TiO2 (1%) 33 34 had two times lower 𝑅𝑐𝑡,𝑏𝑢𝑙𝑘 than TiO2. With only 1% incorporation of CoTiO3 on TiO2 the 35 36 value of 𝑅𝑐𝑡,𝑏𝑢𝑙𝑘 was reduced a twofold, from 5.77 k to 2.68 k. Likewise, the electron 37 38 lifetime, 𝜏𝑛 , could be estimated by the RC time constant of the Randles circuit.30 Generally 39 speaking, 𝜏𝑛 characterizes the electron recombination; a shorter 𝜏𝑛 means a faster 40 41 recombination process. The formation of the CoTiO3/TiO2 heterojunction boosted 𝜏𝑛 from 1.3 42 43 ms to 2.9 ms, which implies that the heterojunction reduced charge recombination probably 44 45 due to a promoted charge separation, consistent with TRPL results. 46 47 48 a 4 b 6 49 5 Photocurrent (A) 3 50 CoTiO3/TiO2 (1%) -Z'' (k cm2) 51 4 52 2 3 TiO2 53 TiO2 54 1 2 55 CoTiO3/TiO2 (1%) 1 56 0 57 0 58 0 10 20 30 40 50 60 0 1 2 3 4 5 6 59 Time (s) Z' (k cm2) 60 ACS Paragon Plus Environment Page 11 of 21 ACS Applied Materials & Interfaces 1 2 3 Figure 5. a) Transient photocurrent response. b) Nyquist plot of impedance response with 4 5 corresponding equivalent circuit for fitting. 6 7 Photocatalytic Mechanism 8 9 10 Using UPS, the valence band maximum (VBM) of the TiO2 and CoTiO3 reference 11 materials was determined to be at −6.57 eV and −7.13 eV vs vacuum, respectively (Figure 12 13 6a).15, 40,41 Taking into account the measured band gaps and VBM position, the band alignment 14 15 between TiO2 and CoTiO3 corresponded to that of a type II heterostructure (left scheme in 16 Figure 7). Thus, considering a type-II heterojunction charge-transfer mechanism, within the 17 18 CoTiO3/TiO2 heterojunction photogenerated electrons would travel from TiO2 to CoTiO3, and 19 20 holes from CoTiO3 to TiO2. However, because the conduction band maximum (CBM) of 21 22 CoTiO3 is at lower position than the H2/H+ level, the free electrons of CoTiO3 see an energy 23 barrier to reduce H+ into H2 (left scheme in Figure 7).41 This does not match well with the 24 25 enhanced photocatalytic performance observed in CoTiO3/TiO2 composites with respect to 26 27 TiO2. 28 29 Actually, taking into account the electron energy band position of the two materials and 30 31 intrinsic n-type character of TiO2 and p-type character of CoTiO3, upon placing the two 32 materials in contact an injection of electrons from the TiO2 to the CoTiO3 will occur. This 33 34 charge redistribution will create an electric field at the interphase. In view of the small size of 35 36 the crystallites and the moderate charge carrier concentration characterizing these type of 37 38 materials, this electric field will probably extend through the whole TiO2 and CoTiO3 domains. 39 The electric field will thus drive electrons photogenerated within the TiO2 and holes 40 41 photogenerated within CoTiO3 away from the interphase, and holes photogenerated within the 42 43 TiO2 and electrons photogenerated within the CoTiO3 towards the interphase, where they 44 45 probably recombine (right scheme in Figure 7). As a result, photogenerated electrons in the 46 conduction band of TiO2, thus having a high reduction potential, are driven toward the surface 47 48 with a reduced probability of recombination with valence band holes. Thus TiO2 49 50 photogenerated electrons could reduce protons into hydrogen more effectively. Likewise, 51 photogenerated holes in the valence band of CoTiO3, i.e. with a strong oxidation potential, are 52 53 driven to the surface with a lower chance to recombine with conduction band electrons. Thus, 54 55 CoTiO3 photogenerated holes can more efficiently drive ethanol dehydrogenation to 56 57 acetaldehyde and protons. Overall, a clear synergism between the two materials is stablished 58 through this direct Z-scheme charge transfer mechanism.42-46 59 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 12 of 21 1 2 3 4 a b O 1s 5 CoTiO3 Intensity (a.u.) Intensity (a.u.) 6 CoTiO3/TiO2 (1%) 7 8 9 -7.1 eV CoTiO3/TiO2 (0.5%) 10 TiO2 CoTiO3 11 12 TiO2 13 -6.6 eV 14 -10 -5 0 5 10 540 537 534 531 528 525 15 Binding energy (eV) 16 Binding energy (eV) 17 18 Figure 6. a) UPS spectrum for TiO2 and CoTiO3. b) O 1s XPS spectra of TiO2, CoTiO3, 19 20 CoTiO3/TiO2 (0.5%) and CoTiO3/TiO2 (1%). 21 22 Finally, figures 6b and S8 show the O1s, Ti 2p and Co 2p XPS spectra of the reference 23 24 samples and two CoTiO3/TiO2 composites. The most interesting aspect of these analyses was 25 26 the additional peak (532.5 eV) appearing at lower binding energy than the main O 1s peak 27 (529.6 eV) corresponding to lattice oxygen. This new peak should be associated with OH− 28 29 groups.47 The notable increase of surface adsorbed species and particularly hydroxyl groups in 30 31 the CoTiO3/TiO2 composites is indicative of the promoted charge separation and tendency of 32 33 part of the charge carriers to migrate toward the surface, away from an interphase, where they 34 can promote molecular adsorption and redox reactions. 35 36 Type II 37 heterostructure Direct Z-scheme 38 transitions 39 40 -3 41 e-  e- -1 42 E vs vacuum (eV) -4 H2  E vs SHE (eV) 43 0 e- 2H+ e- 44 -5 45 1 46 CH3CHO+2H+ CH3CHO+2H+ 47 -6 C2H5OH C2H5OH 48 2 h+ 49 -7  h+ 50 h+ h+  3 51 -8 52 TiO2 CoTiO3 TiO2 CoTiO3 53 54 55 Figure 7. Schematic illustration of the relative energy levels of TiO2 and CoTiO3 and 56 simplified charge transfer mechanism between TiO2 and CoTiO3 in a conventional type II 57 58 heterostructure and a direct Z-scheme. 59 60 ACS Paragon Plus Environment Page 13 of 21 ACS Applied Materials & Interfaces 1 2 3 4 5 6 CONCLUSION 7 8 A simple method for the synthesis of CoTiO3/TiO2 heterostructures with tuned CoTiO3 9 10 amounts was presented. The materials showed an excellent activity in the photocatalytic 11 hydrogen evolution from ethanol dehydrogenation, with HER under UV light well above those 12 13 of TiO2, CoTiO3 and Co-doped TiO2. We also observed a significant influence of visible light 14 15 on the HER of CoTiO3/TiO2 heterostructures beyond the important effect of the increased 16 temperature. Electrochemical impedance spectroscopy and static and time resolved PL 17 18 measurements showed the formed heterostructures to promote charge separation, extending the 19 20 life time of photogenerated charge carriers. UV-vis spectroscopy and UPS analysis showed the 21 22 alignment of the energy levels of the two materials to resemble a type II heterostructure, with 23 the CBM of the CoTiO3 below the H2/H+ energy level. In such type II heterostructures electron 24 25 transfer from n-type TiO2 to p-type CoTiO3 is to be expected, generating an electric field within 26 27 the material that drives photogenerated electrons within TiO2 and photogenerated holes within 28 29 CoTiO3 away from the interphase and photogenerated holes in TiO2 and electrons in CoTiO3 30 towards the interphase where they recombine. All these evidences pointed out toward a direct 31 32 Z-scheme mechanism as responsible of the enhanced photocatalytic ethanol dehydrogenation 33 34 activity of CoTiO3/TiO2 composites over that of TiO2. 35 36 EXPERIMENTAL SECTION 37 38 Chemicals: Titanium (IV) isopropoxide (97%), hexadecylamine (HDA, 90%) and ammonium 39 40 hydroxide (28-30%) were ordered from Sigma Aldrich. Ethanol (96%) was purchased from 41 42 PanReac AppliChem ITW Reagents. Cobalt (II) nitrate hexahydrate (98%) was purchased from 43 Fluka. 44 45 46 Synthesis of Photocatalysts (TiO2, CoTiO3 and CoTiO3/TiO2): First, 0.45 g HDA was mixed 47 with 40 mL ethanol at room temperature and stirred for 5 mins. Then, an accurate quantity of 48 49 Co(NO3)2·6H2O (0, 11.3, 22.5, 45, 112.5 and 1.2 g to get 0, 0.5, 1, 2, 5 and 100 mol% 50 51 concentrations, respectively) was introduced under stirring for another 5 min. At this point, 2.3 52 mL titanium isopropoxide was added dropwise and kept stirring 16 h at ambient temperature. 53 54 The mixture was washed 3 times with ethanol by centrifuging to collect the final product. The 55 56 product was subsequently mixed with a solution including 20 mL of ethanol, 10 mL of MiliQ 57 58 water and 2.2 mL of ammonium hydroxide, kept for 16 h without stirring. The mixture was 59 then placed in an autoclave reactor with a Teflon liner (50 mL) and maintained at 170 °C for 60 ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 14 of 21 1 2 3 14 h. TiO2 and CoTiO3 were obtained by annealing dried samples at 650 °C for 8 h with 4 5 synthetic air flow. 6 7 Characterization: The particle morphology and size were analyzed using transmission electron 8 9 microscopy (TEM) in a ZEISS LIBRA 120, working at 120 KeV. High angle annular dark field 10 11 (HAADF) scanning TEM and high-resolution TEM (HRTEM) analysis were performed on a 12 FEI Tecnai F20 field emission gun microscope operated at 200 keV combined with a Gatan 13 14 Quantum electron energy-loss spectroscopy (EELS) detector. Scanning electron microscopy 15 16 (SEM) was conducted using a ZEISS Auriga microscope. X-ray diffraction analyses (XRD) 17 18 were carried out on a Bruker D8-Advance X-ray diffractometer (2θ: 20o-80o, scanning rate was 19 set to 5o/min). The optical properties of photocatalysts were evaluated by UV/Visible diffuse 20 21 spectroscopy (UV-Vis-NIR Shimadzu 3600). X-ray photoelectron spectroscopy (XPS) was 22 23 conducted with an Al anode XR50 source on a SPECS equipment using a Phoibos 150 MCD- 24 25 9 detector (150 W). Steady-state photoluminescence (PL) spectra were conducted by a high- 26 resolution photoluminescence spectrofluorometer (Horiba Jobin Yvon Fluorolog-3). For the 27 28 time-resolved photoluminescence spectroscopy (TRPL) measurements, a nanosecond LED 29 30 with 350 nm peak wavelength (Horiba NanoLED N-390, pulse width < 1.3 ns) was applied to 31 excite the samples. TRPL decay was resolved at 400 nm. Average lifetimes were obtained by 32 33 fitting the TPPL spectra with DAS6 software (Horiba). Transient photocurrent (TPC) and 34 35 electrochemical impedance spectroscopy (EIS) analyses were conducted using a Bio-Logic SP- 36 37 300. Photocatalysts were coated on transparent conducting substrates (FTO) and were used as 38 working electrode. Ag/AgCl was used as reference electrode and a Pt wire as counter electrode. 39 40 For impedance characterization, a sinusoidal potential was applied to the working electrode 41 42 with an amplitude of 25 mV and a frequency in the range 100 kHz - 50 mHz. Impedance 43 44 response fitting was conducted using ZView (Scribner Associates). In TPC measurements, the 45 same three-electrode configuration was employed and a Xenon arc lamp (Newport, 450 W) 46 47 was used as light source. UV photoelectron Spectroscopy (UPS) measurements were 48 49 performed on a Thermo Fisher Scientific Escalab 250Xi. 50 51 Photocatalytic Hydrogen Evolution Measurements: A cellulose paper impregnated with the 52 53 photocatalyst (2.0 mg) was holded inside a photocatalytic reactor equipped with 365 ± 5 nm 54 UV LEDs (Figure S1). Light irradiation at the sample position was was 79.1 mW·cm−2. 55 56 Saturated Ar gas stream was prepared by bubbling Ar (20 mL∙min-1) through a Dreschel bottle 57 58 containing a water-ethanol vapour mixture with a molar ratio 9. The photoreactor exit was 59 60 analysed online by gas chromatography equipped with Plot U, MS 5 Å, and Stabilwax columns ACS Paragon Plus Environment Page 15 of 21 ACS Applied Materials & Interfaces 1 2 3 every 4 min. Saturated Ar was purged into system (20 mL∙min-1, 30 min) to remove oxygen 4 5 before performing the experiment. The UV-visible light source contained two LEDs emitting 6 7 at 372 ±5 nm and two LEDs emitting visible light (correlated color temperature 6099 K, color 8 9 rendering index 74) Figure S1. In this system light irradiation at the sample position was 11.2 10 ±0.5 mW·cm−2 for UV light and 0.017 ±0.005 mW·cm−2 for visible light.48-50 11 12 13 14 15 16 ASSOCIATED CONTENT 17 18 Supporting Information. Supporting information contains photocatalytic reactor, additional 19 20 TEM and HRTEM characterization, XPS analysis, Elemental composition analysis, 21 22 Photocatalytic H2 production rates and UV-vis analysis. 23 24 AUTHOR INFORMATION 25 26 27 Corresponding Authors 28 29 * A. Cabot: acabot@irec.cat 30 31 * J. Llorca: jordi.llorca@upc.edu 32 33 34 ORCID 35 36 Congcong Xing: 0000-0001-7674-6720 37 38 39 Yongpeng Liu: 0000-0002-4544-4217 40 41 Yu Zhang: 0000-0002-0332-0013 42 43 Pablo Guardia: 0000-0001-9076-4642 44 45 46 Jordi Arbiol: 0000-0002-0695-1726 47 48 Lluís Soler: 0000-0003-1591-3366 49 50 51 Kevin Sivula: 0000-0002-8458-0270 52 53 Néstor Guijarro: 0000-0002-3277-8816 54 55 Andreu Cabot: 0000-0002-7533-3251 56 57 58 Jordi Llorca: 0000-0002-7447-9582 59 60 AUTHOR CONTRIBUTIONS ACS Paragon Plus Environment ACS Applied Materials & Interfaces Page 16 of 21 1 2 3 # C. Xing and Y. Liu contributed equally to this work. The manuscript was written through 4 5 contributions of all authors. All authors have given approval to the final version of the 6 7 manuscript. 8 9 NOTES 10 11 12 The authors declare no competing financial interest. 13 14 ACKNOWLEDGEMENTS 15 16 This work was supported by projects MICINN/FEDER RTI2018-093996-B-C31, RTI2018- 17 18 095498-J-I00. J.A., T.Z. and X.H. thank funding from GC 2017 SGR 327 and MINECO project 19 20 (ENE2017-85087-C3). ICN2 thanks supporting from the Severo Ochoa Program (MINECO, 21 No. SEV-2013-0295) and the CERCA Program/Generalitat de Catalunya. N.G. and Y.L. thank 22 23 the Swiss National Science Foundation (SNF) for funding under the Ambizione Energy (No. 24 25 PZENP2_166871). Y.Z, C.C.X., X.W. and T.Z. acknowledge the China Scholarship Council 26 27 (CSC) for scholarship support. Part of the present work has been performed in the framework 28 of Universitat Autònoma de Barcelona Materials Science PhD program. J.L. is a Serra Hunter 29 30 fellow and is grateful to GC 2017 SGR 128 and ICREA Academia program. 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Page 17 of 21 ACS Applied Materials & Interfaces 1 2 3 REFERENCE 4 5 6 1. 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