Templated Dewetting-Alloying of NiCu Bilayers on TiO2 Nanotubes Enables Efficient Noble Metal-Free Photocatalytic H2 Evolution

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1 Supporting Information Templated Dewetting-Alloying of NiCu Bilayers on TiO2 Nanotubes Enables Efficient Noble Metal-Free Photocatalytic H2 Evolution by Davide Spanu, a,b Sandro Recchia, b Shiva Mohajernia, a Ondřej Tomanec, c Štěpán Kment, c Radek Zboril, c Patrik Schmuki a,c,d * and Marco Altomare a * a Department of Materials Science and Engineering WW4-LKO, University of Erlangen- Nuremberg, Martensstrasse 7, D Erlangen, Germany b Department of Science and High Technology, University of Insubria, via Valleggio 11, Como, Italy c Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacky University, Olomouc, Šlechtitelů 27, Olomouc, Czech Republic d Chemistry Department, Faculty of Sciences, King Abdulaziz University, Jeddah, Saudi Arabia Kingdom * Corresponding author. marco.altomare@fau.de schmuki@ww.uni-erlangen.de S1

2 Experimental Section Fabrication of the TiO2 nanocavity array: Ti foils (Advent Research Materials, mm thickness, 99.6+% purity) were degreased by sonicating in acetone, isopropanol, and deionized water and then were dried in a N2 stream. Then, the Ti foils were anodized to fabricate the highly ordered TiO2 nanotube arrays in a hot electrolyte based on 3 M HF in o-h3po4 (Sigma- Aldrich). For the anodic growth, a twoelectrode configuration was used, where the Ti foil (15 mm 15 mm) and a Pt sheet were the working and counter electrodes, respectively. The anodization experiments were carried out by applying a potential of 15 V (for 2 h) using a DC power supply (VLP 2403 Voltcraft). After anodization, the TiO2 nanotube arrays on Ti metal substrates were rinsed with ethanol and dried under N2 stream. The compact TiO2 layers were prepared by anodization of Ti foils in aqueous 0.5 M H2SO4, at 20 V and room temperature, for 30min. Metal sputtering-coating and thermal treatment: In order to form Ni, Cu, alloyed NiCu and Pt nanoparticles on the TiO2 nanotube arrays, a plasma-sputtering machine (EM SCD 500, Leica) was used to sputter-coat Cu, Ni and Pt metal thin films using a 99,90% pure Cu target (Baltic Praeparation e.k.), 99,98% pure Ni target (Hauner Metallische Werkstoffe) and a 99,99% pure Pt target (Hauner Metallische Werkstoffe), respectively. In any case the applied sputtering current was 16 ma and the pressure of the sputtering chamber was set at 10-2 mbar of Ar. The amount of sputtered material was in-situ determined by an automated quartz crystal monitor, and is reported in this work as nominal thickness of the sputtered film. Subsequently, the samples were annealed at 450 C (1 h) in Ar atmosphere (Ar flux = 10 L h -1 ) to induce dewetting. S2

3 Characterization of the structures: X-ray diffraction (XRD) with an X pert Philips MPD (equipped with a Panalytical X celerator detector) was employed to examine the crystallographic properties of the materials. A fieldemission scanning electron microscope (FE-SEM, Hitachi S4800) and a transmission electron microscope (TITAN , FEI, USA) were used to characterize the morphology (by SEM and HAADF-TEM) and chemical composition (by EDS-TEM) of the samples. The chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5600, US) and peak positions were calibrated with respect to the Ti2p peak at 458 ev. UV-Vis diffuse reflectance spectra of the different Ni-, Cu- and NiCu-decorated TiO2 nanotubes were measured with an Avantes spectrophotometer (AvaSpec-ULS2048L-USB2/16S ), equipped with deuterium-halogen lamp (AvaLight-DH-S-BAL) and integrating sphere (AvaSphere-30-REFL). Photocatalytic measurements: Photocatalytic measurements for H2 generation were carried out by irradiating the oxide films in a 20 vol% ethanol water solution (ethanol was used as hole-scavenger) in a quartz tube sealed with a gas-tight cap. As light sources we used a LED UV light (Opsytec, λ = 365 nm, beam size = cm 2, power of 105 mw cm -2 ) and a solar simulator (simulated AM 1.5 illumination provided by a 300 W Xe with a Solarlight optical filter) with an irradiation power of 100 mw cm - 2 (the light intensity was measured prior to the experiments using a calibrated Si photodiode). The ethanol-water solution (kept under static conditions during the runs) and the cell head-space (volume = 5.12 ml) were purged with N2 gas for 15 min prior to photocatalysis. N2-purging is needed to remove O2 from the liquid phase and from the head space O2 would reduce the efficiency of H2 generation by competitive photocatalytic reduction to O2 (O2 can react with conduction band electrons). S3

4 The amount of produced H2 (which was accumulated over the irradiation time in the head space of the tube) was measured by using a gas chromatograph (GCMSQO2010SE, Shimadzu) equipped with a thermal conductivity detector and a Restek micropacked Shin Carbon ST column (2 m 0.53 mm). GC measurements were carried out at a temperature of the oven of 45 C (isothermal conditions), with the temperature of the injector set at 280 C and that of the TCD fixed at 260 C. The flow rate of the carrier gas, i.e., argon, was 14.3 ml min 1. The experiments lasted 4-30 hours. Apparent quantum efficiency (AQE): The apparent quantum efficiency (AQE) was determined for the most active photocatalyst under monochromatic UV light illumination taking into account the following experimental details: Illumination source = LED UV light, λ = 365 nm, power density = 105 mw cm -2 Sample 5Ni5Cu-TiO2 leads to the highest H2 evolution rate = 6.0 µl h -1 cm -2 (data in Figure 2b) The apparent quantum efficiency can be calculated as in equation 1: AQE % = 2 generated H 2 molecules φ (photon flux) 100% (equation 1) Taking into account the following equations: r H2 = V t A (equation 2) p V = n R T (equation 3) φ (photon flux) = H λ h c (equation 4) Where: S4

5 rh2 = H2 generation rate [ L s m 2] t = illumination time [h] A = illuminated sample area [m 2 ] p = pressure [atm] V = volume of generated H2 [L] n = moles of generated H2 [mol] R = gas constant [ T = temperature [K] L atm K mol ] # of photons φ = photon flux [ ] s m 2 H = power density [ W m 2] λ = wavelength [m] h = Planck s constant [6.63 x J s] c = speed of light [3.0 x 10 8 m s ] Equation 1 can be expressed as follows, using equations 2-4: AQE % = 2 r H2 p R T N A H λ h c Where: NA = Avogadro s number [6.022 x % (equation 5) 23 # of molecules # of moles 2 = correction factor assuming that 2 photons are required to generate 1 H2 molecule ] The resulting AQE is ~ 0.043%. The value is relatively low compared to data in the literature (e.g. 1 ). This can be explained considering that the short depth of the TiO2 nanotubes used in the S5

6 present work (~ 200 nm) may not lead to full light absorption and also by taking into account that that a non-noble metal cocatalyst was used in contrast, in ref. 1 longer nanotubes (15 µm) are used that are loaded with noble metal (Pt) cocatalyst nanoparticles. Optimization of the annealing, TiO2 crystallization and metal dewetting parameters: The effect of dewetting time (as well as of other parameters e.g. temperature, metal film thickness) on kinetics and morphological evolution of metal thin films has been discussed in the literature by various authors (e.g. 2 4 ). In general, for polycrystalline metal layers with a thickness of few tens of nm exposed to T in the C (i.e. under experimental conditions that are comparable to those adopted in our work), the morphological evolution of metal films occurs to a large extent within the first minutes. Typically thinner layers dewet faster. Dewetting may be considered completed when parameters such as substrate coverage, metal particle size and crystallographic domain size remain constant with increasing annealing time. In the present work key is that NiCu dewetting-alloying takes place at the same time of TiO2 NT crystallization. While dewetting may be completed within < 30 minutes, previous work on anodic TiO2 nanotube layers 5 7 for photocatalysis and photo-electrochemical application reveals that a set of most suitable annealing conditions to reach an optimized crystallization of TiO2 is a temperature of ~ 450 C and an annealing time of ~ 1 h. Shorter times may lead to poor TiO2 crystallization and also to an uncompleted metal dewetting. On the other hand, annealing time > 1 h lead to an extensive formation of TiO2 rutile phase in the NTs that was found detrimental for photocatalytic H2 evolution 8. The heating rate may have an effect on the morphology of the dewetted metal films (and therefore on the photocatalytic activity of the metal/tio2 NT structures) if the dewetting process has not reached the equilibrium for a given dewetting time and/or dewetting temperature. S6

7 Figure S1 SEM images of different TiO2 nanotube arrays: (a) pristine nanotube; (b) nanotube coated with a 10 nm thick Ni layer; (c) nanotube coated with a 10 nm thick Cu layer. Figure S2 XRD pattern of sample 5Ni5Cu crystallized (and dewetted) in argon at 450 C. TiO2 NTs annealed at 450 C in argon gas are converted into an anatase-rutile mixed phase. The XRD peaks at 2θ = 25.3 and 27.5 can be assigned to TiO2 (101) anatase and (110) rutile reflections, respectively. S7

8 Figure S3 (a-d) Metal particle size distribution for samples (a) 10Ni, (b) 10Cu, (c) 5Ni5Cu and (d) 5Ni5Cu on compact TiO2 (the corresponding structure is shown in (d)). The cocatalyst nanoparticle size was estimated from the SEM images of the various samples by using the software Image J. For each sample, the average size was determined from ~ 70 particles. (e) SEM image of a 5 nm Ni 5 nm Cu bilayer sputter-dewetted on a compact TiO2 anodic layer under identical experimental condition (450 C, 1 h, in argon atmosphere). The compact TiO2 layers were prepared by anodization of Ti foils in aqueous 0.5 M H2SO4, at 20 V and room temperature, for 30min. S8

9 Figure S4 SEM images of TiO2 nanotube arrays decorated with dewetted-alloyed NiCu nanoparticles of different compositions: (a) 7Ni3Cu and (b) 3Ni7Cu. Figure S5 SEM images of TiO2 nanotube arrays coated with a NiCu bilayer (5 nm Ni 5 nm Cu) and dewetted at 450 C. The images are taken (a) before and (b) after the photocatalytic H2 evolution runs. S9

10 Figure S6 UV-Vis diffuse reflectance spectra of various Ni-, Cu- and NiCu-decorated TiO2 nanotubes measured in the (a) nm and (b) nm range. S10

11 Figure S7 (a) XRD patterns of TiO2 nanotube arrays decorated with dewetted-alloyed NiCu nanoparticles of different compositions. Inset: detail of the XRD patterns in the range. (b) Predicted and experimental cell parameter (a) plotted as a function of the composition of the alloyed NiCu NPs. The composition is expressed as molar fraction of Ni. The predicted and experimental cell parameter data are obtained from the XRD results by applying the Vegard s law. Additional data and equations are reported below (Table S2). S11

12 The patterns are shown in the full range (i.e ) as well as in the range (Inset) to highlight the (111) and (200) reflections of the NiCu particles. The position of peak attributed to the (111) reflection (at ) is taken into account as reference signal to evaluate the NiCu composition. One can also notice that another series of peaks (attributed to the (200) reflection) appear in the range. While also for these peaks a shift can be observed that correlates with the NiCu particle composition, this signal becomes weak with increasing the Ni composition (this is well in line with previous work 9 ). Besides, we observed that in the presence of only Cu, the TiO2 NTs undergo crystallization forming mainly rutile phase. S12

13 Scheme S1 The improved photocatalytic activity of TiO2 NTs decorated with dewetted-alloyed NiCu NPs is attributed to the specific cocatalyst composition that can lead to a faster H atom adsorption/recombination over Ni cocatalytic centers combined with a more favorable H2 molecule desorption from adjacent Cu sites 10,11. S13

14 Energy Scheme S2 Cu φ Cu = 4.65 ev Ni φ Ni = 5.15 ev CB Ni E F Cu E CB - φ Cu φ Cu φ Ni E F φ BCu φ Cu E CB - φ Ni EF φ BNi φ Ni φ BCu < φ BCuNi < φ BNi VB TiO 2 TiO 2 Cu TiO 2 Ni An additional reason for the improved activity of TiO2 NTs decorated with dewetted-alloyed NiCu NPs is the work function of NiCu alloyed particles with respect to that of pure Cu and Ni (i.e ev and 5.15 ev 12, respectively). According to theoretical calculations 13 15, the work function of NiCu alloy increases linearly from 4.65 ev to 5.15 ev with increasing the Ni content. This leads in principle to an increase of the Schottky barrier height (φb) at the NiCu/TiO2 interface, which can enable a more efficient electron-hole separation. As the highest rate of H2 evolution is achieved with a 1:1 Ni:Cu ratio, one can conclude that such composition of the NPs enables at the same time an efficient electron-hole separation (due to a higher Schottky barrier height compared to pure Cu), and a fast kinetics of hydrogen adsorption-recombinationdesorption at the NiCu alloy surface (as outlined in Scheme S1). S14

15 Table S1 Sample Name* Nominal thickness (nm) Nominal composition (xni) Real thickness (nm) Ni Cu Ni Cu Real composition (xni) 5Cu Cu Ni7Cu Ni5Cu 400 C 5Ni5Cu 450 C 5Cu5Ni 450 C 5Ni5Cu 500 C Ni3Cu Ni Ni *The order of the metal refers to the sputtering sequence. Sample name and cocatalyst composition of different TiO2 nanotube arrays decorated with dewetted Ni, Cu and alloyed NiCu nanoparticles. S15

16 Table S2 Sample name Real composition (xni) Peak position 2 ( ) d (Å) a predicted (Å) a experimental (Å) 10Cu Ni7Cu Ni5Cu Ni3Cu Ni Summary of the XRD data of TiO2 nanotube arrays decorated with dewetted Ni, Cu and alloyed NiCu nanoparticles of different composition. The predicted and experimental cell parameter data are obtained from the XRD data by applying the equations below. According to the Bragg's law: Thus: 2 d sin = n λ d = λ / (2 sin ) Where d is the lattice spacing, λ = 1,5406 Å, and is determined as the maximum of the lorentzian fitting of the (111) reflection for each sample (data in Table S2 and Figure 2d). In the case of Ni, Cu and NiCu alloys: d = a / (h 2 + k 2 + l 2 ) ½ Where a is the lattice parameter, and the Miller indices h, k and l = 1. Thus: a = d / (3) ½ S16

17 According to the Vegard s law: aab = x aa + (1-x) ab Where x is the molar fraction of A in the AB alloy. S17

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