This is the submitted version of the article: Xing C., Liu Y., Zhang Y., Liu J., Zhang T., Tang P., Arbiol J., Soler L., Sivula K., Guijarro N., Wang X., Li J., Du R., Zuo Y., Cabot A., Llorca J.. Porous NiTiO3/TiO2 nanostructures for photocatatalytic hydrogen evolution. Journal of Materials Chemistry A, (2019). 7. : 17053 - . 10.1039/c9ta04763h. Available at: https://dx.doi.org/10.1039/c9ta04763h Please do not adjust margins Journal Name ARTICLE Porous NiTiO3/TiO2 Nanostructures for Photocatatalytic Hydrogen Evolution Received 00th January 20xx, Congcong Xing,ab Yongpeng Liu,c Yu Zhang,a Junfeng Liu,a Ting Zhang,d Pengyi Tang,d Jordi Arbiol,de Accepted 00th January 20xx Lluís Soler,b Kevin Sivula,c Néstor Guijarro,c Junshan Li,a Ruifeng Du,a Yong Zuo,a Andreu Cabot,*ae Jordi Llorca *b DOI: 10.1039/x0xx00000x www.rsc.org/ We present a strategy to produce porous NiTiO3/TiO2 nanostructures with excellent photocatalytic activity toward hydrogen generation. In a first step, nickel-doped TiO2 needle bundles were synthesized by a hydrothermal procedure. Through the sintering in air of these nanostructures, porous NiTiO3/TiO2 heterostructured rods were obtained. Alternatively, the annealing in argon of the nickel-doped TiO2 needle bundles resulted in NiOx/TiO2 elongated nanostructures. Porous NiTiO3/TiO2 structures were tested for hydrogen evolution in the presence of ethanol. Such porous heterostructures exhibited superior photocatalytic activity toward hydrogen generation, with hydrogen production rates up to 11.5 mmol·h- 1·g-1 at room temperature. This excellent performance is related here with the optoelectronic properties and geometric parameters of the material. photocatalytic performance over TiO2.20-22 However, the Introduction synthesis of such mixed oxides in the form of high surface area materials is extremely challenging, what has limited advances in Since first reports on photocatalytic water splitting, TiO 2 has this direction.23-27 been considered one of the most attractive photocatalysts. 1-3 To overcome this severe challenge, we present here a Among others, its interest resides on its excellent simple, high throughput and scalable hydrothermal-based optoelectronic properties, high resistance to photocorrosion, strategy to produce porous NiTiO3/TiO2 heterostructures. This natural abundance, low-cost and safety.4-6 However, TiO2 strategy is based on the synthesis of needle bundles of Ni-doped optimization for solar photocatalysis requires maximizing its TiO2 (TiO2:Ni) and their posterior sintering in air to produce surface area.7 It also requires the introduction of dopants, highly crystalline NiTiO3/TiO2 heterostructures with a significant additional phases or structural defects that allow this wide band level of porosity reminiscent of the large density of interfaces gap semiconductor to take advantage of a larger spectral range within the needle bundles. We further demonstrate here that of the solar radiation.8-13 these preliminarily optimized materials are excellent To maximize surface area, the production of hollow and photocatalysts for hydrogen generation. porous TiO2 nanostructures has recently drawn growing attention.14-16 Besides, Ti-based mixed oxides and particularly perovskite titanates MTiO3 (M=Ba, Sr, Ca and Ni) have been Experimental pointed out as a new exciting class of photoactive materials. 17- 19 Additionally heterostructured oxides, such as SrTiO /TiO and 3 2 Chemicals NiTiO3/TiO2 have been demonstrated to promote charge- Titanium(IV) isopropoxide (97%, Sigma-Aldrich), nickel(II) separation, hole-transportation and visible-light-driven nitrate hexahydrate (98%, Fluka), ethanol (96%, PanReac AppliChem), hexadecylamine (HDA, 90%, Sigma-Aldrich), potassium chloride (Sigma-Aldrich), and ammonium hydroxide a. Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 solution (28-30%, Sigma-Aldrich) were used without further Barcelona, Spain purification. b. 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 Synthesis of NiTiO3/TiO2 Heterostructures c. Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO), École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, A solution of KCl (1.9 mg) in miliQ water (0.99 g) was added Switzerland dropwise to a HDA (0.45 g) solution in ethanol (20 mL) under d. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain stirring at room temperature. To this solution, a proper amount e. ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain of Ni(NO3)2·6H2O was added (0 mg, 22.5 mg, 45 mg, 90 mg, and † Electronic Supplementary Information (ESI) available: additional TEM, HRTEM, EELS analysis and other characterizations, photocatalytic hydrogen production 225 mg to reach 0, 0.5, 1, 2, and 5 mol% concentrations, measurement. See DOI: 10.1039/x0xx00000x respectively). Then, 4.7 mL of titanium(IV) isopropoxide was This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name added. After stirring for 1 h at room temperature, we kept the light was off, and the reaction system was purged by entering solution 16 h without stirring, and afterward the solution was 20 mL∙min-1 of saturated Ar gas with the water-ethanol vapour washed with ethanol three times using centrifugation to mixture, to remove oxygen in the line. After 30 min, the UV light recover the product. This product was then dissolved in a was turned on and we monitored all photoreaction products solution containing 20 mL of MiliQ water, 40 mL of ethanol and during ca. 20-40 min by GC. Control experiments were carried 2.2 mL of ammonium hydroxide. This solution was stirred for 1 out with only the cellulose paper support and no photoactivity h and it was subsequently transferred to a 100mL Teflon-lined was measured. hydrothermal reactor. The reactor was heated at 170 °C and maintained at this temperature for 17 h. Afterward, it was Electrochemical Impedance Spectroscopy (EIS) cooled down to room temperature and the product was washed EIS analyses were performed using a Bio-Logic SP-300 three times with ethanol. Dried materials were finally annealed potentiostat in a 3-electrode configuration employing a in a tube furnace under synthetic air flow at 650 °C for 8 h. Cappuccino-type PEC cell, wherein the photocatalyst was deposited as a thin film in a conductive substrate (FTO) to be Characterization used as working electrode, whereas a Ag/AgCl electrode and a X-ray diffraction analyses (XRD, 2θ: 20 o-80o; scanning rate: Pt wired were used as a reference and a counter electrode, 5o/min) were carried out on a Bruker AXS D8 Advance X-ray respectively. The applied potential on the working electrode diffractometer with Ni-filtered Cu-Kα radiation (λ= 0.15406 Å), was sinusoidally modulated with an amplitude of 25 mV using operating at 40 mA and 40 kV. The morphology and size of the frequencies ranging from 100 kHz to 50 mHz. The analysis of the particles were characterized by transmission electron impedance response was undertaken using ZView (Scribner microscopy (TEM, ZEISS LIBRA 120), working at 120 kV and field- Associates). emission scanning electron microscopy (SEM, Zeiss Auriga) operating at 5.0 kV. High resolution transmission electron microscopy (HRTEM) images and scanning transmission Results and discussion electron microscopy (STEM) studies were conducted on a FEI Characterization of the Photocatalysts Tecnai F20 field emission gun microscope operated at 200 kV Ni-doped TiO2 (TiO2:Ni) nanostructures were produced from the with a point-to-point resolution of 0.19 nm, which was hydrothermal decomposition of titanium(IV) isopropoxide in equipped with high angle annular dark field (HAADF) and a the presence of proper amounts of nickel(II) nitrate Gatan Quantum electron energy loss spectroscopy (EELS) hexahydrate, HDA as a shape-directing agent and KCl to control detectors. Elemental analysis was carried out using an Oxford the ionic strength of the solution (see experimental section for energy dispersive X-ray spectrometer (EDX) combined with the details and Figure S2 for SEM images).32 As shown by TEM Zeiss Auriga SEM working at 20.0 kV. X-ray photoelectron characterization, the size and shape of the nanostructures spectroscopy (XPS) was examined on a SPECS system equipped obtained from this hydrothermal reaction strongly depended with a Phoibos 150 MCD-9 detector, working at 150 mW with on the amount of Ni introduced (Figure S3). In the absence of an Al anode XR50 source. Fourier transform infrared Ni, nanoparticles with an average size around 50 nm and mostly spectroscopy (FTIR, Alpha Bruker) was carried with a platinum irregular shapes, although some elongated, were obtained. attenuated total reflectance single reflection module. When introducing increasingly higher amounts of Ni, more and more elongated nanostructures were obtained. In the presence Photocatalytic Hydrogen Evolution Tests of a nominal 5% of Ni, most structures resembled nanorods with In a typical experiment, a cellulose paper impregnated with 2.0 a length in the range from 100 nm to 200 nm and thickness of mg of the photocatalyst was placed inside a photocatalytic 30-60 nm (Figures 1b, 2b and S3). A closer look to these reactor that was equipped with a UV LED (Figure S1). An Ar gas nanorods allowed discerning that they consisted of needle- stream was saturated with a water:ethanol vapour mixture shaped nanostructures assembled in bundles (Figure 2b). (90:10 ratio on a molar basis) by bubbling dry Ar gas at a flow XRD analyses showed the crystal structure of these needle rate of 20 mL∙min-1 through a saturator (Dreschel bottle) bundles to match that of anatase TiO2. Diffraction peaks shifted containing a liquid mixture of 87.5 g of H 2O and 9.92 g of to lower angles with the introduction of increasing amounts of ethanol.28-31 This Ar stream was introduced into the Ni (Figure 1a), which pointed at the presence of Ni ions within photoreactor and passed through the cellulose paper loaded the TiO2 anatase lattice. Besides anatase TiO 2, no additional with the photocatalyst. The photoreactor effluent was crystallographic phase could be discerned from XRD patterns. monitored on-line every 4 min using gas chromatography (GC) EDX analysis showed the Ni atomic concentration to be (Agilent 3000A MicroGC) using three columns: MS 5 Å, Plot U systematically higher than the nominal amount introduced, and Stabilwax. The LED UV light source (from SACOPA, S.A.U.) pointing at a higher yield of reaction of the nickel(II) nitrate than consisted of four LEDs at 365 ± 5 nm and a synthetic quartz glass the titanium(IV) isopropoxide under the hydrothermal cylindrical lens that transmitted the light to the photocatalyst. conditions used for the synthesis (Table S1). Light irradiation was measured directly with a UV-A radiation TiO2:Ni nanopowders were annealed under synthetic air monitor from Solar Light Co. and was 79.1± 0.5 mW·cm −2 at the flow at 650 °C during 8 h. Upon this thermal treatment, the sample position. At the beginning of each experiment, the UV color of the material changed from the original blueish, 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE characterizing TiO2:Ni samples, to yellow (Figure S4). TEM argon did not show full crystallographic continuity and micrographs of the annealed powder showed it to still consist contained a lower density of holes compared with the material on elongated nanostructures. However, on the contrary to the obtained from the annealing in air (Figure S10). XRD patterns of starting nanoneedle bundles, the annealed nanostructures materials annealed under argon showed anatase TiO 2 as the displayed a crystallographic continuity (Figure 2c). Additionally, main phase, with no evidence of the presence of NiTiO 3, but the annealed nanostructures presented numerous holes all with a small XRD peak at 2θ=28.6° that we associated to a NiOx over them (Figure 2d). In contrast, the annealing of undoped phase (Figure S11). The black nanopowder annealed under TiO2 nanopowders in the same conditions did not result in such argon showed a magnetic moment, which further pointed porous structure (Figure S5). toward the formation of a NiOx phase (Figure S4).38 XRD analysis showed that the porous nanostructures obtained after the annealing of TiO2:Ni in air consisted mainly a NiTiO3 * NiTiO3 NiTiO3/TiO2 on anatase TiO2 (Figure 2a, S6). With the annealing process, the shift of the XRD peaks observed in the precursor TiO 2:Ni ** 5% * * Intensity (a.u) material disappeared, pointing at the outward diffusion of the 2% Ni ions from the TiO2 lattice. Besides the anatase phase, XRD patterns displayed the presence of a second crystal structure, 1% which was identified as NiTiO3.33 The relative intensity of the TiO2 peaks associated to this NiTiO3 phase clearly increased with the nominal amount of Ni (Figure 2a). Rietveld analysis showed the TiO2 JCPDS:01-073-1764 relative amount of the NiTiO3 phase to correspond to a 4.6 20 30 40 50 60 70 80 24.8 25.6 32 34 36 mol%, 9.0 mol% and 15 mol%, for samples obtained from 2 (Degree) 2 (Degree) 2 (Degree) nominal Ni concentration of 1 mol%, 2 mol% and 5 mol%, respectively (Figure S7).34 Within its experimental error, EDX b c analysis showed the Ni molar concentrations to be maintained after the annealing process (Table S1). a TiO2:Ni b 650 °C 5% Intensity (a.u.) 2% air 100 nm 1% TiO2 100 nm e TiO2 JCPDS:01-073-1764 20 30 40 50 60 70 80 24.8 25.6 200 nm d 2 (Degree) 2(Degree) Figure 1. a) XRD patterns of TiO2 and TiO2:Ni (1%, 2%, 5%) nanopowders. b) TEM image of TiO2:Ni (5%) nanopowder. HRTEM analysis confirmed the annealed material to contain 5 0 n m 2 0 0 n Tim mostly anatase TiO2 (Figures S5 and S6).35 EELS elemental maps of the annealed materials showed Ni, Ti and O to be evenly O 5 0 n m distributed throughout the nanostructures (Figure 2e, S5). The lack of evidences of the presence of large NiTiO 3 crystal domains Ni and the homogeneous distribution of Ni throughout the 5 0 n m material observed by EELS and EDX analyses pointed at the 200 nm growth of small NiTiO3 domains as a discontinuous shell on the 5 0 n m Ti-O-Ni surface of the porous TiO2 nanostructures.36 XPS analyses of the Figure 2. a) XRD patterns of NiTiO3/TiO2 (0, 1%, 2%, 5%) annealed samples displayed only one chemical state for Ti, nanopowders obtained after annealing in air. b) TEM assigned to Ti4+, and one for Ni, assigned to Ni2+, consistently micrograph of TiO2:Ni (5%) needle bundles. c, d) TEM with XRD, HRTEM, EELS and EDX characterization (Figures S8 micrographs of NiTiO3/TiO2 (5%) porous structures obtained and S9).37 XPS analysis showed the amount of Ni to be almost a after annealing in air. e) HAADF micrograph of a NiTiO3/TiO2 twofold of that measured by EDX, pointing at its preferential (5%) rod and corresponding EELS elemental maps for Ti, O, Ni surface location (Table S1). and Ti-O-Ni. When the Ni-doped material was annealed under Ar instead We hypothesize that the porous geometry of the of air, the sample color changed from blue to black, instead of NiTiO3/TiO2 structures obtained after annealing TiO 2:Ni in air yellow (Figure S4), but the elongated geometry of the annealed was created by the sintering of the nanoneedle bundles. The structures was conserved. In contrast to the annealing in air, the large density of interfaces of the initial bundles translated in the nanostructuration of the original nanoneedle bundles partially formation of a large density of holes on the sintered material, persisted after annealing in argon. The particles annealed in which could be aided by the simultaneous outward diffusion of This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name Ni ions to form NiTiO3 domains. On the contrary, annealing in SEM characterization showed the morphology of the argon did not allow a similar level of sintering of the NiTiO3/TiO2 hetero-nanostructures to remain unmodified after nanostructures, which was promoted in air by the ubiquitous the catalytic test (Figure 3c). presence of oxygen. Thus, the material obtained from the We believe the enhancement of the photocatalytic annealing process in argon maintained smaller crystal domains properties of TiO2 with the NiTiO3 introduction to be related and presented a less porous structure. With the annealing with an improvement of three fundamental parameters: i) process in air, pure TiO2 nanopowders did not result in porous surface area; ii) light absorption; and iii) charge separation. We materials as those obtained from TiO 2:Ni because they did not estimate that the introduction of Ni had associated a factor 3 present a proper initial nanostructuration. increase of the surface area with respect to pure TiO 2 through the formation of the porous NiTiO3/TiO2 hetero-nanostructures Photogeneration of Hydrogen during the annealing process. Additionally, the presence of To test their photocatalytic properties toward hydrogen NiTiO3 provided a higher light absorption coefficient at the used evolution, the annealed materials were supported on a excitation wavelength (Figure S15). In case of using solar conventional filter paper and placed on a reactor containing a radiation, the presence of NiTiO3 would provide an even larger UV LED light source. An Ar flow saturated with a water-ethanol improvement with respect to pure TiO2, as it would allow vapor mixture was flowed through this reactor. Exhaust gases absorbing a larger portion of the solar spectra due to its smaller were analyzed using GC, being acetaldehyde and hydrogen the band gap. Besides, the proper band alignment of TiO 2 and main products of the reaction (see details in the experimental NiTiO3 allowed an efficient spatial charge separation that section and Figure S1). significantly reduced recombination of the photogenerated charge carriers before reaction. a 5 b To gain additional insight into the band structure alignment 0.5%, 1%, 2%, 5% 1% 12 H2 (mmol gcat-1 h-1) 4 NiTiO3/TiO2 and the interfacial charge carrier dynamics at the NiTiO 3/TiO2 H2 (mmol gcat-1) 10 8 heterostructure, EIS analyses were carried out on TiO 2, 3 NiOx/TiO2 0.5% 6 NiTiO3/TiO2 (1%) and NiTiO3 thin films in dark conditions. 2 TiO2 TiO2:Ni 4 Representative impedance responses in the form of Nyquist 5% 1 2% 2 plot for TiO2 and NiTiO3/TiO2 (1%) are displayed in Figure 4a. NiTiO3 0 Responses typically feature a single semicircle. Interestingly, the 0.5% NiOx/TiO2 0 1% 2% TiO2 NiTiO3 5% TiO2:Ni 0 10 20 30 40 size of the semicircle obtained for the TiO2 sample was reduced Reaction Time (min) NiTiO3/TiO2 with the incorporation of Ni. A more quantitative description of c the EIS data can be provided by modelling the electrical response with an equivalent circuit, such as a Randles circuit UV light (Figure 4a inset), which has been previously reported to successfully describe nanostructured TiO2 and other photocatalysts.22,39 This circuit consists of a resistor 𝑅𝑆 in series with a bulk capacitance 𝐶𝑏𝑢𝑙𝑘 , attributed to the space charge 400 nm 400 nm region, and a second resistor 𝑅𝑐𝑡,𝑏𝑢𝑙𝑘 that represents the Figure 3. a) H2 production of TiO2, NiTiO3/TiO2 (1%, 2%, 5%), charge transport resistance, both in parallel. At a first glance, NiOx/TiO2 (1%) and NiTiO3. b) H2 production rate of TiO2, the decrease in the size of the semicircle with the incorporation NiTiO3/TiO2 (1%, 2%, 5%), NiOx/TiO2 (1%) and NiTiO3. c) Optical of Ni in TiO2 suggests the reduction in the charge transport and SEM images of NiTiO3/TiO2 (5%) before and after 30 min resistance,39,40 which is further confirmed with the decrease of photocatalytic test. 𝑅𝑐𝑡,𝑏𝑢𝑙𝑘 from 15.4 k to 9.8 k pointing out the amelioration Figure 3a shows the accumulated hydrogen production of the charge transport process. In addition, the electron during a 32 min reaction for NiTiO3/TiO2 hetero-nanostructures lifetime 𝜏𝑛 can be calculated as the inverse of the angular having different nominal Ni concentrations. Data for bare TiO 2 frequency at the maximum of the Nyquist plot ( 2𝜋𝑓𝑚𝑎𝑥 ),22 and pure NiTiO3 prepared following the same procedure (see highlighted in Figure 4a. Results reveal that 𝜏𝑛 increases from experimental section for details) were also plotted in Figure 3a 4.5 ms to 6.6 ms when the heterostructure is formed. In as reference. NiTiO3/TiO2 hetero-nanostructures with a nominal general, 𝜏𝑛 is considered a metric of electron recombination, 1% of Ni systematically provided the highest hydrogen i.e., the longer the value the lesser the charge recombination. production rates, with values up to 11.5 mmol·h-1g-1, well above The lengthening of 𝜏𝑛 when TiO2 is interfaced with NiTiO3 those previously reported for other Ni-Ti-O systems (Table S2). underpins the effective charge carrier separation across the n-n The hydrogen production rate of NiTiO3/TiO2 (1%) was almost a heterojunction and thus the enhanced photocatalytic activity of fourfold higher than that of bare TiO 2 and 60 times larger than the composite with respect TiO2. Similar behavior has recently that of pure NiTiO3 (Figure 3b, S12, S13, S14). The presence of been reported by Wei and co-workers on NiTiO3/TiO2 larger amounts of Ni did not improve the evolution rate with composite for inorganic sensitized solar cells.22 respect to NiTiO3/TiO2 (1%). We associate this experimental In an attempt to further interrogate the energy band evidence with the blocking of the TiO2 surface active sites when positions, Mott–Schottky (M–S) plots were constructed from too large amounts of NiTiO3 were grown on the TiO2 surface. 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE 𝐶𝑏𝑢𝑙𝑘 values extracted from EIS measurements on TiO2 and where ℎ is Planck constant. 𝑁𝐶 is estimated to be 7.7×1020 NiTiO3 and further analyzed using the M–S equation: cm–3 for TiO2, and 7.5×1019 cm–3 for NiTiO3. Since the ratio between 𝑁𝐷 and 𝑁𝐶 is in all cases less than 0.05, both TiO2 and 2 𝑘𝑇 NiTiO3 could be treated as nondegenerate semiconductors, and −2 𝐶𝑏𝑢𝑙𝑘 = (𝑉 − 𝑉𝑓𝑏 − ) therefore, the Boltzmann statistics can be applied:44,47 𝑞𝐴2 𝜀𝜀0 𝑁𝐷 𝑞 where 𝑞 is elementary charge, 𝐴 is taken as the geometric 𝑁𝐶 area (assuming a low surface roughness spin-coated thin film), 𝐸𝐶𝐵 − 𝐸𝐹 = 𝑘𝑇 ln ( ) 𝑁𝐷 𝜀 is relative permittivity (taken to be 55 and 15 for TiO 241 and where 𝐸𝐶𝐵 is the bottom of the conduction band and 𝐸𝐹 is NiTiO3,42 respectively), 𝜀0 is vacuum permittivity, 𝑁𝐷 is donor the Fermi level position. 𝐸𝐶𝐵 is found to be around 168 mV and density, 𝑉 is applied potential, 𝑉𝑓𝑏 is flat band potential, 𝑘 is 126 mV above 𝑉𝑓𝑏 for TiO2 and NiTiO3, respectively. Based on Boltzmann constant, and 𝑇 is absolute temperature. The the aforementioned calculations, a scheme of the electronic positive slope in the linear region (dashed line in Figure 4b) band structures is illustrated in Figure 4c, where the relative unambiguously confirmed the n-type character of both NiTiO3 band alignment between NiTiO3 and TiO2 allows for efficient and TiO2, whereas 𝑉𝑓𝑏 values of –0.186 V and –0.276 V vs. RHE charge transfer. In such Ni-TiO2 system, photogenerated were obtained for TiO2 and NiTiO3, respectively. Likewise, 𝑁𝐷 electrons in the conduction band of NiTiO3 could transfer to the shows values of 1.1×1018 cm–3 for TiO2 and 5.5×1017 cm–3 for conduction band of TiO2 to promote water reduction while NiTiO3. Similar values have also been reported by Thimsen et al. photogenerated holes in TiO2 could migrate to NiTiO3 to drive on nanocrystalline anatase TiO2,42 and by Trari and co-workers ethanol dehydrogenation to acetaldehyde and hydrogen. on nanostructured NiTiO3.43 We note that the depletion width Therefore, the formation of NiTiO3/TiO2 n-n junction can is estimated to be 20 nm in the linear region of M–S plot for effectively suppress the electron-hole recombination process both TiO2 and NiTiO3 so that M–S analysis can be reasonably within TiO2. As a result, NiTiO3/TiO2 composites at optimal applied in the case of not fully depleted feature. 44 Considering NiTiO3 loading condition shows a significant increased the density-of-state effective mass for electrons 𝑚𝑑𝑒 for photocatalytic activity. In fact, incorporating NiTiO3 in other nanocrystalline anatase TiO2 is 9.1×10–30 kg,41 and for semiconductor photoabsorber materials such as TiO 2,22 - nanostructured NiTiO3 is 1.9×10–30 kg,44 the effective density of Fe2O347 and Ag3VO418 has also shown to enhance the charge states in the conduction band 𝑁𝐶 is given by:44,47 separation efficiency. 3 2𝜋𝑚𝑑𝑒 𝑘𝑇 2 𝑁𝐶 ≡ 2 ( ) ℎ2 a 14 b 3.5 c (1010 cm4 F2) 12 3.0 TiO2 -Z'' (k cm2) 10 35.5 Hz 2.5 8 TiO2 2.0 6 24 Hz 1.5 NiTiO3 4 1.0 Cbulk 2 2 0.5 NiTiO3/TiO2 (1%) 0 0.0 0 2 4 6 8 10 12 14 -0.3 -0.2 -0.1 0.0 0.1 0.2 2 Z' (k cm ) V vs. RHE (V) Figure 4. a) Representative impedance response of TiO2 (black sphere) and NiTiO3/TiO2 (1%) (orange sphere) with the corresponding fit (solid lines) in Nyquist plot. The maximum point of the Nyquist plot is highlighted (hollow), and the equivalent circuit used is included as an inset. b) Mott–Schottky plot of TiO2 (black sphere) and NiTiO3 (red sphere) including a linear fit (dashed lines). c) Schematics of the electronic band structures of NiTiO3/TiO2 n-n heterojunction. generation from ethanol-water solutions at room temperature. NiTiO3/TiO2 hetero-structures provided unprecedented H2 Conclusions production rates up to 11.5 mmol·h-1g-1. This was associated to Porous NiTiO3/TiO2 heterostructures were prepared from the the improvement of three parameters, sur-face area, light annealing in air of TiO2:Ni nanoneedle bundles obtained from a absorption and charge separation. We estimated a factor 3 hydrothermal route. Pores were generated during the sintering increase of the surface area with the introduction of Ni through of the nanostructured bundles due to the presence of a large the formation of porous NiTiO3/TiO2 hetero-nanostructures density of interfaces on the precursor material. NiTiO3/TiO2 during the annealing in air. Additionally, the pres-ence of heterostructures were used for photocatalytic hydrogen NiTiO3, with a lower band gap energy, provided a higher light absorption coefficient at the excitation wavelength. Besides, This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name TiO2 and NiTiO3 presented a proper band alignment that allows 15 S. C. Yang, D. J. Yang, J. Kim, J. M. Hong, H. G. Kim, I. D. Kim an efficient spatial charge separation of the photogenerated and H. Lee, Adv. Mater., 2008, 20, 1059–1064. 16 J. Y. Hwang, H. M. Kim, S. K. Lee, J. H. Lee, A. Abouimrane, charge carriers before reaction, as confirmed by EIS analysis. M. A. Khaleel, I. Belharouak, A. Manthiram and Y. K. Sun, Adv. Energy Mater., 2016, 6, 1–7. 17 H. Fan, H. Li, B. Liu, Y. Lu, T. Xie and D. Wang, ACS Appl. Mater. Interfaces, 2012, 4, 4853–4857. Conflicts of interest 18 B. Inceesungvorn, T. Teeranunpong, J. Nunkaew, S. Suntalelat and D. Tantraviwat, Catal. Commun., 2014, 54, There are no conflicts of interest to declare. 35–38. 19 X. Wang, C.-N. Xu, H. Yamada, K. Nishikubo and X.-G. Zheng, Adv. Mater., 2005, 17, 1254–1258. Acknowledgements 20 F. Wu, Y. Yu, H. Yang, L. N. German, Z. Li, J. Chen, W. Yang, L. Huang, W. Shi, L. Wang and X. Wang, Adv. Mater., 2017, 29, This work was supported by the European Regional 1–7. Development Funds and by the Spanish Ministerio de Economía 21 Y. Qu, W. Zhou, Z. Ren, S. Du, X. Meng, G. Tian, K. Pan, G. y Competitividad through project SEHTOP (ENE2016-77798-C4- Wang and H. Fu, J. Mater. Chem., 2012, 22, 16471–16476. 22 Y.-Y. Li, J.-G. Wang, H.-H. Sun and B. Wei, ACS Appl. Mater. 3-R and ENE2015-63969-R). Y.L. and N.G. thank the Swiss Interfaces, 2018, 10, 11580–11586. National Science Foundation (SNF) for funding under the 23 S. Liu, S. Xue, W. Zhang, J. Zhai and G. Chen, J. Mater. Chem. Ambizione Energy grant no. PZENP2_166871. J.L. is a Serra A, 2014, 2, 18040–18046. Hunter fellow and is grateful to ICREA Academia program and 24 J. Y. Lee, J. H. Lee, S. H. Hong, Y. K. Lee and J. Y. Choi, Adv. GC 2017 SGR 128. T.Z., P.Y.T. and J.A. acknowledge funding from Mater., 2003, 15, 1655–1658. 25 J. H. Pan, C. Shen, I. Ivanova, N. Zhou, X. Wang, W. C. Tan, Q. Generalitat de Catalunya 2017 SGR 327 and the Spanish H. Xu, D. W. Bahnemann and Q. Wang, ACS Appl. Mater. MINECO project ENE2017-85087-C3. ICN2 acknowledges Interfaces, 2015, 7, 14859–14869. support from the Severo Ochoa Programme (MINECO, Grant no. 26 Y. Xin, J. Jiang, K. Huo, T. Hu and P. K. Chu, ACS Nano, 2009, SEV-2013-0295) and is funded by the CERCA Programme / 3, 3228–3234. Generalitat de Catalunya. Part of the present work has been 27 S. Murcia-López, M. Moschogiannaki, V. Binas, T. Andreu, P. Tang, J. Arbiol, J. Jacas Biendicho, G. Kiriakidis and J. R. performed in the framework of Universitat Autònoma de Morante, ACS Appl. Mater. Interfaces, 2017, 9, 40290– Barcelona Materials Science PhD program. C.C.X., Y.Z and T.Z. 40297. thank the China Scholarship Council for scholarship support. 28 E. Molins, M. Benito, I. Mata, L. Martínez, L. Soler and J. Llorca, MRS Adv., 2017, 2, 3499–3504. 29 L. Martínez, M. Benito, I. Mata, L. Soler, E. Molins and J. Notes and references Llorca, Sustain. Energy Fuels, 2018, 2, 2284–2295. 30 L. Martínez, L. Soler, I. Angurell and J. Llorca, Appl. Catal. B 1 X. Wu, G. Q. Lu and L. Wang, Energy Environ. Sci., 2011, 4, Environ., 2019, 248, 504–514. 3565–3572. 31 A. May-Masnou, L. Soler, M. Torras, P. Salles, J. Llorca and A. 2 D. P. Wang and H. C. Zeng, Chem. Mater., 2009, 21, 4811– Roig, Front. Chem., 2018, 6, 110. 4823. 32 R. Imani, R. Dillert, D. W. Bahnemann, M. Pazoki, T. Apih, V. 3 Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, Kononenko, N. Repar, V. Kralj-Iglič, G. Boschloo, D. Drobne, 6575–6578. T. Edvinsson and A. Iglič, Small, 2017, 13, 1700349. 4 Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han and C. Li, Chem. Rev., 33 K. Huo, Y. Li, R. Chen, B. Gao, C. Peng, W. Zhang, L. Hu, X. 2014, 114, 9987–10043. Zhang and P. K. Chu, Chempluschem, 2015, 80, 576–582. 5 D. Su, S. Dou and G. Wang, Chem. Mater., 2015, 27, 6022– 34 B. H. Toby and R. B. Von Dreele, J. Appl. Cryst., 2013, 46, 6029. 544–549. 6 J. Liu, H. Bai, Y. Wang, Z. Liu, X. Zhang and D. D. Sun, Adv. 35 P. D. Cozzoli, A. Kornowski and H. Weller, J. Am. Chem. Soc., Funct. Mater., 2010, 20, 4175–4181. 2003, 125, 14539–14548. 7 B. Liu and H. C. Zeng, Chem. Mater., 2008, 20, 2711–2718. 36 H. Bin Wu, H. H. Hng and X. W. D. Lou, Adv. Mater., 2012, 24, 8 N. O. Gopal, H. H. Lo and S. C. Ke, J. Am. Chem. Soc., 2008, 2567–2571. 130, 2760–2761. 37 J. Huang, Y. Jiang, G. Li, C. Xue and W. Guo, Renew. Energy, 9 C. Burda, Y. Lou, X. Chen, A. C. S. Samia, J. Stout and J. L. 2017, 111, 410–415. Gole, Nano Lett., 2003, 3, 1049–1051. 38 J. H. Kim, K. Zhu, Y. Yan, C. L. Perkins and A. J. Frank, Nano 10 M. Chiodi, C. P. Cheney, P. Vilmercati, E. Cavaliere, N. Lett., 2010, 10, 4099–4104. Mannella, H. H. Weitering and L. Gavioli, J. Phys. Chem. C, 39 Y. Zuo, Y. Liu, J. Li, R. Du, X. Yu, C. Xing, T. Zhang, L. Yao, J. 2012, 116, 311–318. Arbiol, J. Llorca, K. Sivula, N. Guijarro and A. Cabot, ACS Appl. 11 R. Bouchet, A. Weibel, P. Knauth, G. Mountjoy and A. V. Mater. Interfaces, 2019, 11, 6918–6926. Chadwick, Chem. Mater., 2003, 15, 4996–5002. 40 X. Zhu, N. Guijarro, Y. Liu, P. Schouwink, R. A. Wells, F. Le 12 M. Murdoch, G. I. N. Waterhouse, M. A. Nadeem, J. B. Formal, S. Sun, C. Gao and K. Sivula, Adv. Mater., 2018, 30, Metson, M. A. Keane, R. F. Howe, J. Llorca and H. Idriss, Nat. 1801612. Chem., 2011, 3, 489–492. 41 E. Thimsen, A. B. F. Martinson, J. W. Elam and M. J. Pellin, J. 13 G. I. N. Waterhouse, A. K. Wahab, M. Al-Oufi, V. Jovic, D. H. Phys. Chem. C, 2012, 116, 16830–16840. Anjum, D. Sun-Waterhouse, J. Llorca and H. Idriss, Sci. Rep., 42 S. H. Chuang, M. L. Hsieh and D. Y. Wang, J. Chinese Chem. 2013, 3, 2849. Soc., 2012, 59, 628–632. 14 J. H. Pan, X. Zhang, A. J. Du, D. D. Sun and J. O. Leckie, J. Am. 43 L. Boudjellal, A. Belhadi, R. Brahimi, S. Boumaza and M. Trari, Chem. Soc., 2008, 130, 11256–11257. Mater. Sci. Semicond. Process., 2018, 75, 247–252. 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal Name ARTICLE 44 Y. Liu, F. Le Formal, F. Boudoire, L. Yao, K. Sivula and N. Guijarro, J. Mater. Chem. A, 2019, 7, 1669–1677. 45 M. A. Ruiz Preciado, A. Kassiba, A. Morales-Acevedo and M. Makowska-Janusik, RSC Adv., 2015, 5, 17396–17404. 46 Y. Zhang, J. Gu, M. Murugananthan and Y. Zhang, J. Alloys Compd., 2015, 630, 110–116. 47 M. Rudan, Physics of Semiconductor Devices, 2015. This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name TOC graphical abstract 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins