TiO2 photocatalysts for hydrogen production

Journal of Catalysis 330 (2015) 238–254

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Novel Au/TiO2 photocatalysts for hydrogen production in alcohol–water mixtures based on hydrogen titanate nanotube precursors Aubrey G. Dosado a,b, Wan-Ting Chen a, Andrew Chan a, Dongxiao Sun-Waterhouse a, Geoffrey I.N. Waterhouse a,b,c,⇑ a b c

School of Chemical Sciences, The University of Auckland, Auckland, New Zealand The Dodd-Walls Centre for Photonic and Quantum Technologies, New Zealand The MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand

a r t i c l e

i n f o

Article history: Received 23 May 2015 Revised 7 July 2015 Accepted 8 July 2015

Keywords: Hydrogen production TiO2 Hydrogen titanate Gold Alcohols

a b s t r a c t In this study, a series of titania nanorods with different phase compositions and surface areas were prepared by calcination of hydrogen titanate (H2Ti3O7) nanotubes at temperatures up to 1000 °C. Gold nanoparticles were subsequently deposited on the obtained TiNTx (x = calcination temperature) nanorod supports at loadings between 0.5 and 2.0 wt.%, and also onto a commercially available mixed-phase titania support (Degussa P25). TEM, XRF, UV–Vis, XPS and photoluminescence measurements confirmed the presence of gold nanoparticles of mean size 4–7 nm on the surface of the Au/TiNTx (x = 350–800) and Au/P25 photocatalysts, which suppressed electron–hole pair recombination in TiO2 under UV and created cathodic sites for hydrogen evolution. Hydrogen production tests were conducted on the Au/TiNTx and Au/P25 photocatalysts in various alcohol–water mixtures under UV excitation (6.5 mW cm2) with no external bias applied. The impact of the gold loading and the calcination temperatures on the structural, physico-chemical and photocatalytic properties of the TiNTx nanorods was investigated, and a comparison to P25 was made. A 0.5 wt.% Au/TiNT600 photocatalyst demonstrated excellent H2 production activity in all the alcohol–water systems, performing similarly to a 1.5 wt.% Au/P25 reference photocatalyst. For both the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts, H2 production rates decreased in the order triol (glycerol) > diol (1,2-ethanediol 1,2-propanediol) > ethanol > 1-propanol. Good correlations were found between the H2 production rates and alcohol properties such as the number of hydroxyl groups, polarity or standard oxidation potential. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Over the next 50 years it is envisaged that a hydrogen economy will develop, in which H2 will replace fossil fuels as the principal energy carrier [1–3]. A seamless transition from current fossil fuel-based infrastructures for electricity generation and transportation to a hydrogen-based infrastructure requires major technological advances in H2 production, distribution and storage. Producing H2 on the scale required to meet the needs of current and future societies is a considerable challenge, with the human population on Earth predicted to increase to 9 billion by 2048. Current hydrogen production is based on steam methane reforming coupled with the water–gas shift reaction [4–6], energy intensive processes that consume natural gas and liberate CO2 to the atmosphere. Hydrogen production capacity available today is ⇑ Corresponding author at: School of Chemical Sciences, The University of Auckland, Auckland, New Zealand. Fax: +64 9 373 7422. E-mail address: [email protected] (G.I.N. Waterhouse). http://dx.doi.org/10.1016/j.jcat.2015.07.014 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

already fully committed to the synthesis of ammonia and methanol (around 50% by volume), hydrotreating refineries (44%) and hydrogenation (4%), leaving very little for the energy sector. This motivates the development of alternative and sustainable technologies for H2 manufacture, amongst which water splitting and alcohol photoreforming using sunlight and semiconductor photocatalysts are considered two of the most promising routes [7–19]. In spite of enormous research effort over the past three decades, the activity or stability of all semiconductor photocatalysts reported to date for direct water splitting to H2 and O2 is too low to justify industry uptake. H2 production rates can be increased by several orders of magnitude using renewable sacrificial agents such as ethanol or glycerol, though further work is nec1 essary to realise target H2 production rates of 2–3 mmol g1 cat min under direct sunlight that are needed to attract industry interest. A wide range of semiconductor photocatalysts have been evaluated in relation to H2 production via water splitting in the presence of sacrificial agents, including oxides (e.g. TiO2, NaTaO3,

A.G. Dosado et al. / Journal of Catalysis 330 (2015) 238–254

SrTiO3), oxynitrides (e.g. TiOxNy, TaON, MTaO2N (M = Ca, Sr, Ba), GaN–ZnO solid solutions), oxysulfides (e.g. Sm2Ti2S2O5), nitrides (Ge3N4, Ta3N5), sulphides (e.g. CdS) and others. Refs. 7–19 provide a selection of comprehensive review articles covering work to date in this area. Bulk doping of wide band gap oxide semiconductors with foreign ions has also been widely practiced as a means of improving the visible light response, though this approach almost inevitably yields photocatalysts with very low activities since bulk charge imbalances or defects act as electron–hole pair recombination sites. Of all the semiconductor photocatalysts reported to date, only TiO2 satisfies the following criteria demanded of a good photocatalyst for solar H2 production: (1) low cost; (2) non-toxic; (3) stable during photoreactions; (4) the band gap Eg is less than 3.4 eV to allow solar excitation; (5) the top of the valence band is more positive than the O2/H2O redox couple (+1.23 V vs. NHE); and (6) the bottom of the conduction band is more negative than the H2O/H2 redox couple (0.0 V vs. NHE). The band gap energy of TiO2 depends on the polymorph, with values of Eg = 3.0, 3.2 and 3.3 eV reported for rutile, anatase and brookite, respectively [20–23]. Pinpointing the conduction band edges of the three polymorphs has been a focus of recent research, with the values of 0.7, 0.5 and 0.6 eV versus NHE, respectively, being commonly reported [23,24]. Whilst TiO2 satisfies the key criteria above for a direct water splitting photocatalyst, it possesses low activity for H2 production under solar or UV excitation owing to rapid electron–hole pair recombination following photo-excitation and the high overpotential for H2 production on TiO2 surfaces. These limitations can be overcome to some extent by adding sacrificial agents to water and depositing particular high work function metals (such as Ni, Pd, Pt or Au [25–49]) or suitable semiconductor co-catalysts on TiO2 [38]. Both practices facilitate charge separation in TiO2 and increase the number of charge carriers available for photoreactions that yield H2. The sacrificial agents act as electron donors, consuming photogenerated holes in the TiO2 valence band, whilst the metal co-catalysts serve as hydrogen evolution sites on the TiO2 surface, accepting photo-excited electrons from the TiO2 conduction band. Recent studies of M/TiO2 photocatalysts, where M = Pd, Pt, Au or combinations thereof, have reported H2 production rates as high as 30–40 mmol g1 h1 in alcohol–water systems under realistic solar UV fluxes [33–39], highlighting the potential of TiO2-based photocatalysts for future H2 production. Optimal metal co-catalyst loadings appear to be 0.5 wt.% for Ni, Pd or Pt, and 1.5–4.0 wt.% for Au. Deeper understanding of the roles of the TiO2 support (crystallite size, surface area, TiO2 phase composition), the metal-support interaction and alcohol hole scavengers on photocatalytic H2 production rates may allow strategic and step-change improvements in M/TiO2 photocatalyst design and performance. The effect of the TiO2 support on H2 production rates in alcohol– water systems has been subject of a number of investigations [23,33,43,44,49,50]. A general consensus of the current literature is that M/TiO2 photocatalysts based on mixed phase TiO2 supports, such as Degussa P25 TiO2 (a 6:1 mixture of anatase and rutile by weight), demonstrate superior activity to pure anatase, brookite or rutile supports of comparable surface area. In the case of P25 TiO2, the transfer of photo-excited electrons from the conduction band of rutile (a direct band gap semiconductor) to that of anatase across interfacial heterojunctions (the conduction band of anatase is 0.2 eV more positive versus NHE than that of rutile), and hole migration from the valence band of anatase to that of rutile, increases the number of charge carriers available for photoreactions [24,51]. Photoreaction rates generally scale in proportion to charge carrier concentrations. Similar positive synergies have recently been seen in brookite-anatase heterojunction photocatalysts [23,52]. TiO2 crystallite size is also important for achieving high charge carrier concentrations. Nanocrystalline TiO2 supports

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are generally active due to the short diffusion paths of photo-excited electrons and holes from the semiconductor bulk to the surface, thus reducing the possibility of electron–hole pair recombination. However, this only holds to a point, and often M/TiO2 photocatalysts with very small TiO2 support particles (e.g. 20, consisting of negatively charged layers of edge sharing TiO6 octahedra, with Na+ ions occupying sites between the layers. Subsequent Na+/H+ exchange at room temperature yields H2Ti3O7 nanotubes, which thermally transform to anatase nanorods when annealed at temperatures higher than 400 °C [63,65–68]. Kuo et al. [67] found that heating H2Ti3O7 nanotubes at 400 °C for 3 h formed a biphasic material composed of meta-stable TiO2 (B) phase and anatase nanoparticles, which when functionalised with Pt nanoparticles afforded a remarkably high H2 evolution rate in neat ethanol compared to P25 TiO2-based photocatalysts. Romero Ocaña et al. observed that TiO2 nanocomposites with different brookite/anatase ratios could be prepared by hydrothermal treatment of Na2Ti3O7 at 200 °C, with the brookite/anatase ratio depending on the Na2Ti3O7/water ratio in the hydrothermal reaction [52]. A series of 0.2 wt.% Pt/TiO2 photocatalysts were subsequently prepared, and evaluated for H2 production in ethanol under UV–Vis excitation. Biphasic supports containing 25–30 wt.% brookite afforded the best activity (1–1.2 mmol g1 h1). Tay et al. recently prepared Na2Ti3O7 nanotubes from TiS2 [23], then subsequently hydrothermally treated the Na2Ti3O7 nanotubes in 0.5–1.2 M NaOH at 200 °C for various times up to 24 h. The brookite/anatase ratio in the reaction products increased with NaOH concentration, with pure brookite being obtained at a concentration of 1.2 M. A 0.3 wt.% Pt/TiO2 photocatalyst, prepared using a support consisting of 88% anatase and 12% brookite, afforded a H2 production rate of 3.6 mmol g1 h1 in a 20 vol.% MeOH solution under irradiation from a 800 W Xe–Hg lamp. These studies, together with related works [68], suggest that Na2Ti3O7 ? H2Ti3O7 ? TiO2 approach can yield TiO2 supports with good characteristics for solar H2 production. A gap in the current literature is the lack of comprehensive studies comparing the H2 production performance of M/TiO2 derived from hydrogen titanate, with M/TiO2 photocatalysts synthesised using reliable reference photocatalysts such as Degussa P25, motivating a detailed investigation. Although use of alcohol–water mixtures for photocatalytic H2 production tests is standard practice in the scientific literature, the role of alcohols in promoting photocatalytic H2 production and mechanisms of alcohol photo-oxidation on semiconductor surfaces (especially in aqueous solution) remain poorly understood. Most literature studies compare the performance of a series of M/TiO2 photocatalysts in only a single alcohol or alcohol–water

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system, and in the latter case usually only at a single alcohol concentration. It is not clear at all from current literature what the optimum alcohol concentration is for H2 production in the different M/TiO2 systems, and whether the optimum concentration varies from alcohol to alcohol (alcohols typically used as sacrificial agents are methanol and ethanol, and to a lesser extent ethylene glycol and glycerol). Bowker and co-workers have made the most important contributions in this area to date [10,26–32], reporting hydrogen production rates for the photoreforming of a variety of bio-derivable feedstocks over Pd/P25 and Au/P25 under UV excitation. In the Pd/P25 system, H2 production rates at an alcohol concentration of 1 vol.% were observed to follow the order triols > diols > 2° alcohols > 1° alcohols > 3° alcohols [26,28–30]. From this pattern of reactivity, it was concluded that the sacrificial hole scavenger must have an a-H adjacent to the OH group(s), with the other main by-products of alcohol photoreforming being largely predictable based on the alcohol structure. In the Au/P25 system, H2 production rates tracked in the order methanol > 1-propanol > ethanol > 1-butanol > 2-propanol tertiary butanol [31]. Yang et al. examined UV irradiated suspensions of Pt/anatase in different alcohol–water mixtures [40], establishing a correlation between the H2 production rate and alcohol polarity. Wang et al. examined photocatalytic H2 production over Pt/TiO2 nanotubes in aqueous solutions of different aliphatic alcohols [69], with H2 production rates following the trend: methanol > ethanol > i-propanol > n-propanol > sec-butanol > tert-butanol. Chen et al. recently studied photocatalytic H2 production on 2 wt.% Au/TiO2 and 0.5 wt.% Ni/TiO2 photocatalysts over a wide range of ethanol–water concentrations under UV excitation. Highest H2 production rates were observed at EtOH:H2O volume ratios of 80:20 and 95:5, respectively [39]. These studies illustrate the general reactivity pattern of M/TiO2 photocatalysts in different alcohol–water mixtures under UV excitation. However, little work has been done attempting to correlate H2 production rates with other important parameters such as the number of OH groups on the alcohol (important for alcohol adsorption on TiO2 surfaces) or the standard oxidation potential of the alcohol (important for hole scavenging and ensuring good charge separation in TiO2 under UV), prompting further research in this area. This work aimed to address some of the existing gaps in the scientific literature identified above, by directly comparing the performance of two series of Au/TiO2 photocatalysts, one based on H2Ti3O7 calcination products and the other P25 TiO2, for hydrogen production in various alcohol–water systems under UV excitation. The objectives of this study were 3-fold; (i) to benchmark the performance of Au/TiO2 photocatalysts derived from H2Ti3O7 (i.e. Au/TiNTx) against Au/P25 photocatalysts; (ii) to examine the effect of Au loading on H2 production rates for both Au/TiNTx and Au/P25; and (iii) to explore relationships between photocatalytic H2 production rates and specific properties of the alcohols (e.g. number of hydroxyl groups, alcohol polarity or the standard oxidation potential, Eoox ).

2. Experimental section

2.2. Hydrothermal synthesis of sodium titanate and hydrogen titanate nanotubes Sodium titanate (Na2Ti3O7) nanotubes were synthesised by alkaline hydrothermal treatment of the anatase TiO2 powder according to the method described by Liu et al. [61]. Briefly, anatase TiO2 powder (6 g) was dispersed in aqueous NaOH (80 mL, 10 M). After stirring for 1 h at room temperature, the white suspension was transferred into a 100 mL Teflon-lined autoclave. This was then put in a convection oven and heated at 150 °C for 48 h. The white Na2Ti3O7 precipitate formed in the hydrothermal synthesis was collected by centrifugation, washed repeatedly with milli-Q water until the waste washing liquid was near neutral, and finally dried at 80 °C for 10 h. Hydrogen titanate (H2Ti3O7) nanotubes were synthesised from the Na2Ti3O7 nanotubes by ion exchange. Briefly, the sodium titanate nanotubes were dispersed in 1 M HCl (500 mL) for 2 h. The H2Ti3O7 nanotubes were collected by centrifugation, washed with milli-Q until the waste washing liquid was neutral and finally dried at 80 °C for 5 h. 2.3. Synthesis of TiNTx (x = 150–1000) photocatalysts The TiNTx (x = 150–1000) photocatalysts were obtained by calcination of H2Ti3O7 nanotubes at temperatures between 150 and 1000 °C for 2 h (x denotes the calcination temperature). Hydrogen titanate nanotubes (3 g) were placed in a porcelain evaporating basin, and then heated from room temperature to the desired calcination temperature at 10 °C min1 in a muffle furnace. After calcination for 2 h, the samples were removed from the furnace and allowed to quickly cool to room temperature. 2.4. Au/TiO2 photocatalyst synthesis Au nanoparticles were deposited on the H2Ti3O7 nanotubes, TiNTx (x = 350–1000) and P25 supports at nominal Au loading of 0.5, 1.0, 1.5 or 2.0 wt.% using the deposition–precipitation with urea method described by Zanella et al. [70]. Briefly, a 4.2 mM Au3+ stock solution was prepared by dissolving HAuCl43H2O (1.65 g) in 1 L of milli Q water. H2Ti3O7, TiNTx or P25 powder (2 g), Au3+ stock solution (12.5, 25.0, 37.5 or 50 mL for target nominal Au loadings of 0.5, 1.0, 1.5 or 2.0 wt.%, respectively), urea (5.04 g) and milli-Q water (200 mL less the volume of Au3+ stock solution) were added to a 500 mL glass Schott bottle, then heated with constant stirring at 80 °C for 8 h. The obtained pale yellow powders were collected by vacuum filtration, washed repeatedly with milli-Q and then air-dried at 60 °C overnight. The powders were then calcined at 300 °C for 2 h to reduce surface Au3+ to Au0. In the case of the Au/H2Ti3O7 nanotube sample, the final calcination step involved keeping the Au3+-impregnated sample in an oven at 80 °C for two weeks which eventually resulted in the reduction of surface Au3+ to Au0 without converting the H2Ti3O7 nanotube support to anatase. 2.5. Na2Ti3O7, H2Ti3O7, TiNTx and Au/TiO2 photocatalyst characterisation

2.1. Materials P25 TiO2 (P99.5%), HAuCl43H2O (99%), urea (P99.5%), HCl (34–37 wt.%) and glycerol (P99%) were all obtained from Sigma– Aldrich and used without further purification. Ethanol (P99.5%), 1-propanol (99%), 1,2-ethanediol (>98%) and 1,2-propanediol (>98%) were obtained from ECP Ltd., and also used without further purification. An anatase TiO2 powder (99.0%) was sourced from Ajax Finechem. All solutions were prepared using milli-Q water (18.2 MO cm).

Powder XRD measurements were obtained using a PANalytical Empyrean Theta–Theta diffractometer system operated in the Bragg–Brentano geometry. Measurements were performed at room temperature from 2h = 5–90° (step 0.01°, dwell time 50 s) using monochromated Cu Ka1 radiation (k = 1.5418 Å, current = 40 mA, voltage = 40 kV). Anatase and rutile crystallite sizes (L) were determined from the powder XRD data using the Scherrer equation and line-widths of anatase (1 0 1) reflection at 2h = 25.3° and rutile (1 1 0) reflection at 2h = 27.4°, respectively.


The rutile:anatase ratio in the samples was determined according to the method described by Ding et al. [71]:

%Rutile ¼

1 100 ½1 þ 0:8ðIA =IR Þ

where IA is the peak intensity for the anatase (1 0 1) reflection and IR is the peak intensity for the rutile (1 1 0) reflection. TEM images were collected using a TECNAI 12 transmission electron microscope. Powder samples were dispersed in absolute ethanol, briefly sonicated, and then 1 lL of the resulting dispersion was placed on carbon coated copper TEM grids for analysis. SEM images were acquired on a Philips XL-30 field emission gun scanning electron microscope (FEGSEM). All micrographs were collected at an accelerating voltage of 10 kV. Samples were mounted on a black carbon tape and platinum sputter coated for 1 min prior to analysis. TGAs were performed on a Shimadzu TGA-50 thermogravimetric analyser. H2Ti3O7 nanotubes were heated in air from room temperature to 1000 °C at a heating rate of 10 °C min1. UV–Vis absorbance spectra were collected over the wavelength range 220–1400 nm on a Shimadzu UV-2600 spectrophotometer fitted with an integrating sphere attachment. BaSO4 powder was used as a reference. Photoluminescence measurements were performed in air at room temperature using a Perkin-Elmer LS-55 Luminescence Spectrometer. A 290 nm cutoff filter was used. Samples were excited at 310 nm and photoluminescence spectra were recorded over a range of 330–600 nm using a standard photomultiplier. N2 physisorption isotherms were collected on Micromeritics Tristar 3000 at liquid N2 temperature (195 °C). Specific surface areas were calculated according to the Brunauer–Emmett–Teller (BET) method [72]. Cumulative pore volumes and average pore diameters were calculated from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method [73]. The samples were degassed at 50 °C under vacuum for 1 h prior to analysis. XRF data were taken on a Siemens SRS3000 spectrometer. Data reduction used Siemens SPECTRA 3000 software. Samples were analysed directly as powders supported on Mylar films. XPS data were taken on the soft X-ray beamline of the Australian Synchrotron, featuring an end station equipped with a hemispherical electron energy analyser and an analysis chamber of base pressure 1 1010 Torr. Spectra were excited at a photon energy of 1486.7 eV (Al K equivalent), and calibrated against the C 1s signal of adventitious hydrocarbons at 285.0 eV. Samples were gently pressed into thin pellets of 0.1 mm thickness for the analyses. Survey scans were collected at a pass energy of 40 eV over the binding energy range 1200–0 eV, whilst core level scans were collected with a pass energy of 20 eV. 2.6. Photocatalytic hydrogen production tests Photocatalytic hydrogen production tests on the Au/TiO2 photocatalysts and selected bare TiO2 supports were carried out in a tubular Pyrex reactor (105 mL volume). Photocatalyst (6.5 mg) was placed in the reactor and flushed under a N2 flow for 30 min to remove oxygen. Then, 20 mL of a 10 vol.% alcohol–90 vol.% water mixture (alcohols tested were ethanol, 1-propanol, 1,2-ethanediol, 1,2-propanediol and glycerol) was injected into the reactor through a rubber septum and the resulting photocatalyst dispersion stirred continuously for 1 h in the dark (no UV excitation). A 1 mL sample of the headspace gas was then taken to confirm that no hydrogen formed by dark reactions, after which the reactor was exposed to UV light supplied from a Spectraline model SB-100P/F lamp (100 W, 365 nm) at a distance of 10 cm from the reactor. The photon flux at the sample was approximately 6.5 mW cm2.

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Hydrogen evolution was monitored by taking gas headspace samples (1 mL) at 20 min intervals and injecting these into a Shimadzu GC 2014 equipped with a TCD detector and Carboxen-1010 plot capillary column (L I.D. 30 m 0.53 mm, average thickness 30 lm). H2 evolved was quantified against an external calibration curve of peak area versus moles of H2. Photocatalytic tests for each sample were carried out in triplicate. 3. Results and discussion 3.1. Characterisation of sodium titanate and hydrogen titanate nanotubes Fig. 1(a) and (b) shows SEM and TEM images, respectively, for the anatase powder used in the hydrothermal synthesis of Na2Ti3O7 nanotubes. The images show crystalline anatase particles of size range 120–300 nm. Phase purity was confirmed by XRD (Fig. S1). Hydrothermal treatment in 10 M NaOH at 150 °C for 48 h successfully transformed the anatase powder into Na2Ti3O7 nanotubes (Fig. 1(c) and (d)) with an outer diameter of 8–10 nm and an aspect ratio ranging from 10 to 50. XRD analysis revealed that Na2Ti3O7 was the only compound present in the samples after 48 h hydrothermal treatment (Fig. S1) [58–66]. Immersion of the Na2Ti3O7 nanotubes in 1 M HCl for several hours yielded H2Ti3O7 nanotubes (Figs. 1(e), (f) and S1), which were morphologically similar to the Na2Ti3O7 nanotubes. Fig. S2 shows higher magnification TEM images of the Na2Ti3O7 and H2Ti3O7 nanotubes, and confirms that the nanotubes are composed of rolled-up 2D sheets, in agreement with current models for the hydrothermal synthesis of Na2Ti3O7 nanotubes from TiO2 powder precursors [58–62]. XPS survey spectra for Na2Ti3O7 and H2Ti3O7 nanotubes (Fig. S3) show that immersion of Na2Ti3O7 in 1 M HCl for several hours resulted in complete sodium ion exchange by protons, as evidenced by the absence of Na 1s and Na KLL features in the survey spectrum of the H2Ti3O7 nanotubes. Both the Na2Ti3O7 and H2Ti3O7 nanotubes are composed of sheets of edge sharing TiO6 units. The Ti 2p spectra for each compound were near identical (Fig. S3), consisting of peaks at 458.6 and 464.3 eV in a 2:1 peak area ratio, which are readily assigned to Ti 2p3/2 and 2p1/2 signals of a Ti4+ species. The O 1s spectra for the Na2Ti3O7 and H2Ti3O7 nanotubes consisted of an intense signal at 530.2 eV which is assigned to lattice oxygen in the titanate sheets, and a weaker feature at 532.0 eV due to surface hydroxyls (Fig. S3). In agreement with expectation, the hydroxyl feature was more intense in the case of H2Ti3O7. BET specific surface areas for the Na2Ti3O7 and H2Ti3O7 nanotubes, determined from the N2 physisorption isotherms shown in Fig. S4, were 185.4 and 216.4 m2 g1, respectively (Table 1). BJH pore diameters were 16.8 and 14.2 nm, respectively, although a significant fraction of the pores in the samples were below 5 nm (Fig. S4) corresponding to the inner diameter of the nanotubes. 3.2. Characterisation of TiNTx (x = 150–1000) powders formed by calcination of H2Ti3O7 nanotubes Fig. S5 shows TGA data for H2Ti3O7 nanotubes in air. The TGA plot shows a mass loss of 5% with heating up to 100 °C (attributable to desorption of physisorbed water), and a further 7% mass loss with heating from 100 to 600 °C. The latter agrees very well with the theoretical mass loss expected on transforming H2Ti3O7 ? 3TiO2 + H2O. The TGA data were collected at a dynamic heating rate of 10 °C min1 in air, suggesting that calcination of H2Ti3O7 nanotubes in air at 350 °C for several hours should be sufficient to affect the complete transformation of H2Ti3O7 to TiO2. This was confirmed in Fig. 2, which shows XRD patterns for the products obtained by calcining H2Ti3O7 nanotubes for 2 h at

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Fig. 1. (Left) SEM images for (a) anatase precursor; (c) Na2Ti3O7 and (e) H2Ti3O7. All images were taken at a magnification of 150,000, scale bar = 200 nm. (Right) TEM images for (b) anatase precursor; (d) Na2Ti3O7 and (f) H2Ti3O7, scale bar = 100 nm.

Table 1 Summarised optical, structural and physico-chemical data for sodium titanate, hydrogen titanate and products obtained by the calcination of hydrogen titanate at temperatures between 150 and 1000 °C for 2 h. The calcined samples are labelled TiNTx, where x = 150–1000, respectively.

a

Sample

Band gap energy (eV)

TiO2 phase composition (%) A

R

A

R

Anatasea Na2Ti3O7 H2Ti3O7

3.20 3.52 3.38

100 – –

– – –

180 – –

TiNT150 TiNT200 TiNT250 TiNT300 TiNT350 TiNT425 TiNT500 TiNT600 TiNT700 TiNT800 TiNT900 TiNT1000

3.25 3.22 3.20 3.18 3.27 3.27 3.25 3.19 3.19 3.19 3.04 2.90

– – – – – 100 100 100 100 100 82.5 –

– – – – – – – – – – 17.5 100

– – – – – 11 22 24 36 57 249 –

Anatase powder used in the synthesis of Na2Ti3O7 nanotubes.

Mean crystallite size (nm)

BET surface area (m2 g1)

BJH cumulative pore volume (cm3 g1)

BJH average pore diameter (nm)

– – –

8.5 185.4 216.4

0.019 0.703 0.686

13.6 16.8 14.2

– – – – – – – – – – 272 1093

192.8 190.2 184.8 168.4 201.2 165.3 93.5 68.2 41.0 31.0 9.6 0.5

0.613 0.661 0.680 0.670 0.709 0.706 0.555 0.453 0.255 0.150 0.026 0.001

12.8 12.6 13.6 16.0 14.8 17.4 25.8 28.7 29.4 23.9 14.9 23.9

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(a)

(b)

TiNT500

TiNT250

TiNT300

TiNT350 (x 0.5)

TiNT600

TiNT700

Intensity (arbitrary units)

Intensity (arbitrary units)

H2Ti3O7

TiNT800

TiNT900

TiNT425 (x 0.5)

10

20

30

40

50

2 (degrees)

60

70

80

TiNT1000

10

20

30

40

50

60

70

80

2 (degrees)

Fig. 2. Powder XRD patterns for (a) H2Ti3O7 and TiNTx (x = 250–425), where TiNTx are the products obtained by calcining H2Ti3O7 for 2 h at temperatures between 250 and 425 °C; and (b) TiNTx (x = 500–1000).

Fig. 3. SEM images of the TiNTx products obtained by calcining H2Ti3O7 for 2 h at selected temperatures in the range 300–1000 °C. (a) TiNT300; (b) TiNT425; (c) TiNT600; (d) TiNT700; (e) TiNT800; and (f) TiNT1000. All images were taken at 150,000 magnification (scale bar = 200 nm).

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selected temperatures in the range 250–1000 °C. At calcination temperatures below 350 °C, the TiNTx samples contain a mixture of H2Ti3O7 and nanocrystalline anatase, with the anatase fraction increasing with increasing temperature. For calcination temperatures between 350 and 800 °C (TiNT350–TiNT800), anatase was the only phase identified by XRD, with the characteristic anatase reflections sharpening and intensifying with increasing temperature up to 800 °C. Mean anatase crystallite sizes determined from the XRD data are summarised in Table 1. The product obtained by H2Ti3O7 calcination at 900 °C was a mixture of anatase and rutile (approximate weight fractions 4.7:1), whilst complete transformation of H2Ti3O7 to rutile was achieved when the calcination temperature was increased to 1000 °C. The onset temperature of the anatase–rutile phase transition is known to depend on the method of anatase synthesis, and the initial anatase crystallite size [74–78]. Several studies have suggested that the anatase crystallite size needs to reach 30 nm before the phase transition to rutile commences [74–78]. Onset temperatures for the thermal transformation of anatase to rutile can be as low as 500 °C for anatase powders synthesised by sol–gel or hydrothermal routes. The data presented in Fig. 2 and Table 1 suggest that anatase nanorod powders synthesised here from H2Ti3O7 by calcination have a high thermal stability and high anatase–rutile phase transition onset temperature compared with anatase powders synthesised by more conventional sol–gel routes. Remarkably, the TiNT800 sample retained phase purity and a high BET surface area of 31 m2 g1

(Table 1). The TiNTx products may therefore be useful in high temperature applications requiring anatase, including ceramics and catalytic applications [77]. The morphological evolution of the TiNTx products with calcination temperature was followed by SEM (Fig. 3) and TEM (Fig. S6). The TiNT300 sample (Fig. 3(a)) possessed the nanotubular habit characteristic of H2Ti3O7. After calcination at 425 °C, the product consisted of anatase nanotubes and also anatase nanorods (Figs. 3(b) and S6(b)), the latter having a mean aspect ratio of 9:1 and diameter around 10–12 nm. With increasing temperature between 425 and 800 °C, the anatase nanorods became more granular and crystalline (Fig. 3(b)–(e)), with well-defined lattice fringes for anatase appearing in TEM images at temperatures above 500 °C. Fig. S6(d) shows a fringe spacing of 0.352 nm corresponding to the anatase (1 0 1) plane. Pan et al. recently examined the photoreactivity of different anatase TiO2 facets [53], and contrary to popular understanding, observed that clean {1 0 1} facets demonstrated superior reactivity to clean {0 0 1} facets in photoreductions. The fact that the TiNT500–TiNT800 photocatalysts produced in the current study possess well-defined anatase {1 0 1} facets, amongst others, may in part contribute to their excellent performance below in photocatalytic H2 production tests. Dramatic sintering of TiO2 particles occurred at 1000 °C (Figs. 3(f) and S6(f)), which coincided with the complete transformation of anatase to rutile. BET surface areas and BJH cumulative pore volumes decreased progressively with increasing temperature in the

2.0

2.0

(b)

(a) Na2Ti3O7 H2Ti3O7 TiNT150 TiNT200 TiNT250 TiNT300 TiNT350

1.0

1.0

0.5

0.5

0.0

TiNT425 TiNT500 TiNT600 TiNT700 TiNT800 TiNT900 TiNT1000

1.5

Absorbance

Absorbance

1.5

0.0

300

400

500

600

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300

Wavelength (nm)

400

500

600

5

5

(d)

(c) 4

)

)

4

(eV cm

-1/2

-1/2

1/2

3 Na 2Ti 3O7 H2Ti3O7

1

0 2.5

3.0

3.5

4.0

Energy (eV)

4.5

1/2

TiNT150 TiNT200 TiNT250 TiNT300 TiNT350

2

( h )

1/2

(eV cm

1/2

( h )

700

Wavelength (nm)

3

2

TiNT425 TiNT500 TiNT600 TiNT700 TiNT800 TiNT900 TiNT1000

1

5.0

0 2.5

3.0

3.5

4.0

4.5

5.0

Energy (eV)

Fig. 4. (a, b) UV–Vis absorbance spectra and (c, d) corresponding Tauc plots for Na2Ti3O7, H2Ti3O7 and the TiNTx powders (x = 150–1000).


range 350–1000 °C (Table 1), reflecting the change in the sample morphology from nanotubes to anatase nanorods and finally rutile nanorods, together with the stepwise increase in the mean TiO2 particle size. UV–Vis absorption spectra for Na2Ti3O7, H2Ti3O7 and TiNTx (x = 150–1000) powders are shown in Fig. 4, along with the corresponding Tauc plots. Band gap energies for each sample determined from the Tauc plots by extrapolation to (ahm)1/2 = 0 are summarised in Table 1. Band gap energies for Na2Ti3O7 and H2Ti3O7 nanotubes were 3.52 and 3.38 eV, respectively, in good agreement with previous literature reports [58–66]. The TiNT350–TINT800 samples all had band gap energies around 3.2 eV, characteristic for anatase TiO2 [20–23]. The band gap for the TiNT1000 sample was 2.9 eV, consistent with the literature reports of 2.9–3.0 eV for rutile [20–23]. Photoluminescence spectra collected in air for Na2Ti3O7, H2Ti3O7 and TiNTx powders (x = 150–1000) are shown in Fig. S7. The spectra originate from radiative electron–hole pair recombination following photo-excitation [36,37,79–80], and provide useful

245

information about excitation/de-excitation processes in different materials. The Na2Ti3O7 nanotubes gave a very intense PL signal centred at 370 nm. Transforming the Na2Ti3O7 nanotubes to H2Ti3O7 nanotubes redshifted and partially attenuated the PL signal. The redshift seen in the PL spectra between Na2Ti3O7 and H2Ti3O7 matched the redshift seen in the absorbance spectra of the compounds (Fig. 4(a)). The PL signal for the TiNTx samples was progressively attenuated with temperature between 150 and 300 °C (Fig. S7(a) and (b)), coinciding with the onset of the transformation of H2Ti3O7 to nanocrystalline anatase (Fig. 2). The PL signal intensified again with calcination temperature from 400 to 800 °C, which presumably reflects crystallisation of the anatase TiO2 network and an associated increase in the concentration of charge carriers (electron–hole pairs) generated in the TiNT400– TiNT800 powders under UV excitation. PL intensity is a function of both charge carrier concentrations and charge carrier lifetimes. For the TiNT1000 sample, the PL signal redshifted and was further attenuated, due to the formation of rutile nanorods with low specific surface area.

Fig. 5. TEM images for (a) 0.5 wt.% Au/H2Ti3O7; (b) 0.5 wt.% Au/TiNT350; (c) 0.5 wt.% Au/TiNT500; (d) 0.5 wt.% Au/TiNT600; (e) 0.5 wt.% Au/TiNT800; and (f) 0.5 wt.% Au/ TiNT1000.

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Table 2 H2 production rates for Au/H2Ti3O7, Au/TiNTx (x = 300–1000) and Au/P25 TiO2 photocatalysts in different ethanol–water mixtures under UV irradiation (6.5 mW cm2). Selected data sets have been highlighted in grey to aid the discussion.

a

Based on nominal Au loading.

3.3. Characterisation of Au/TiNTx and Au/P25 photocatalysts To activate the TiNTx and P25 photocatalysts for H2 production, Au nanoparticles were deposited on the supports by the deposi tion–precipitation with urea method [36,37,70], at nominal Au loadings ranging from 0.5 to 2.0 wt.%. Previous studies of Au/TiO2 photocatalysts, including Au/P25, have established that the optimum Au loading for H2 production in ethanol or ethanol–water systems is between 1.5 and 4.0 wt.% [33,37]. It was of particular interest to establish whether the optimum Au loading in the Au/TiNTx and Au/P25 systems was similar. Figs. 5 and S8 show representative TEM micrographs at different magnifications for 0.5 wt.% Au/H2Ti3O7 and selected 0.5 wt.% Au/TiNTx photocatalysts. Mean Au particle sizes determined from multiple TEM images collected for each sample, are shown in Fig. S9 and Table 2. The data show that for the 0.5 wt.% Au/TiNTx (x = 300– 800) photocatalysts, the mean Au particle size was similar, generally falling in the range 4–6 nm. A comparable mean Au particle size was seen for the 0.5–2.0 wt.% Au/TiNT600 and 0.5–2.0 wt.% Au/P25 series of photocatalysts (Table 2). The data show that dep osition–precipitation with urea method thus affords a very narrow Au particle size distribution irrespective of Au loading or TiO2 support, a point noted previously by other research groups [36,70]. Au loadings determined by XRF for the Au/TiNTx and Au/P25 samples were in excellent agreement with nominal loadings (Table 2), and the Au-loaded photocatalysts had slightly lower specific surface areas than the bare TiO2 supports (c.f. Tables 1 and 2). Following functionalisation with the Au nanoparticles, the Au/TiNTx and Au/P25 photocatalysts were a distinctive purple colour, characteristic of nanocrystalline Au0 dispersed on TiO2. Fig. 6 shows digital photographs for 0.5 wt.% Au/H2Ti3O7 and selected 0.5 wt.% Au/TiNTx photocatalysts, showing that the shade of purple varied from sample to sample. Au 4f XPS data for the selected Au/TiNTx samples (Fig. 6) showed two peaks at 83.6 and 87.4 eV, in a 4:3 area ratio, which are readily assigned to Au 4f7/2 and Au 4f5/2 signals, respectively, confirming the presence of only metallic Au on the TiNTx supports [33,36,37]. Differences in the colour of the 0.5 wt.% Au/TiNTx samples seen in Fig. 6 cannot therefore be

due to differences in Au speciation, rather must depend on the properties of the support and the strength of the gold-support interaction. The purple colour of the Au/TiNTx and Au/P25 photocatalysts arises from stimulation of the Au localised surface plasmon resonance (LSPR) by light of appropriate frequency. The Au LSPR position depends on Au particles size (above a critical size 20 nm), Au particle shape and the dielectric constant of surrounding medium [43,44]. In the case of supported Au nanoparticles, the strength of the metal support interaction has a profound influence on Au dispersion and particle shape [37,78,81–84]. Fig. 7 shows UV–Vis absorbance spectra for 0.5 wt.% Au/H2Ti3O7 and selected 0.5 wt.% Au/TiNTx photocatalysts. All samples show strong absorption below 420 nm due to band gap excitation in the supports (Table 1), and a Au LSPR feature at visible wavelengths. The position and intensity of the Au LSPR feature were somewhat dependent on the calcination temperature of the TiNTx support (Fig. 7(b)). For the 0.5 wt.% Au/TiNT500–Au/TiNT1000 samples, the Au LSPR position was largely independent of the support calcination temperature, although the absorbance maximum and FWHM of the LSPR feature varied considerably between samples. Differences in Au particle shape (but not size) induced by the nature of the Au nanoparticle interaction with the different TiNTx supports is the most plausible explanation for the latter. Powder XRD confirmed that the phase composition of the TiNTx supports was unaffected by Au deposition (Fig. S10), though Au related peaks were too weak to identify with confidence in the XRD patterns at an Au loading of 0.5 wt.%. Fig. S11 shows that Au deposition suppressed electron–hole pair recombination for both the H2Ti3O7 and TiNTx supports. Noble metal co-catalysts are proposed to create a rectifying Schottky contact on TiO2 surfaces, acting as ‘‘sinks’’ for electrons photo-excited into the conduction band of TiO2 and thereby reducing electron– hole pair recombination [26–49]. The attenuation of the PL signals for the different TiO2 supports following Au deposition is entirely consistent with this hypothesis. Figs. 8 and 9 examine the effect of Au loading on the optical properties of the Au/TiNTx and Au/P25 photocatalysts. Fig. 8 shows that the purple colouration of the Au/TiNTx photocatalysts

247


intensified with increasing nominal Au loading in the range 0.5– 2.0 wt.%. XRF (Table 2) and XPS (Fig. 8) confirmed that the experimental and nominal Au loadings were near identical. Similar data were obtained for the 0.5–2.0 wt.% Au/P25 photocatalysts and reported elsewhere [36,37]. Fig. 9(a) and (b) shows TEM images for the 1.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts, respectively. Mean Au particle sizes for each were 4.6 ± 1.3 nm and 5.5 ± 1.7 nm, respectively. Note that for the Au/P25 sample, the Au nanoparticles are preferentially located on anatase–rutile heterojunctions, which are proposed photocatalytic ‘‘hotspots’’ for H2 production [37]. Further TEM data for the 1.5 wt.% Au/P25 photocatalyst, collected at different magnifications and shown in Fig. S12 provide a better indication about the Au dispersion in this sample. For both the Au/TiNT600 and Au/P25 samples, Au LSPR feature intensified non-linearly and red-shifted slightly with increasing nominal Au loading up to 2.0 wt.% (Fig. 9(c) and (e), respectively). Fig. 9(d) and (f) shows photoluminescence spectra

Au/H2Ti3O7

Au/TiNT350

for the different Au/TiNT600 and Au/P25 photocatalysts, respectively, and demonstrates that electron–hole pair recombination in the TiO2 supports was progressively suppressed as the Au loading increased. In Fig. S13, we have plotted the PL intensity versus the absorbance maximum of the Au LSPR feature for the Au/TiNT600 and Au/P25 photocatalysts. To our knowledge, this is the first such study to attempt such a correlation. The analysis reveals excellent linearity for both sets of Au/TiO2 photocatalysts. Such a relationship is not surprising, given the respective dependencies of the Au LSPR absorbance and PL intensity on the Au loading. Results suggest a very close relationship between the optical properties and charge separation characteristics of Au/TiO2 photocatalysts. 3.4. Photocatalytic H2 production tests on the Au/TiNTx and Au/P25 photocatalysts Fig. 10 shows plots of H2 production versus UV irradiation time for the 0.5 wt.% Au/TiNTx series of photocatalysts in ethanol–water mixtures. The experiments were conducted at EtOH:H2O volume

Au/TiNT425

2.0

Au/TiNT800

Au/TiNT600

Au/TiNT900

Au/H2Ti3O7 Au/TiNT300 Au/TiNT350 Au/TiNT425 Au/TiNT500 Au/TiNT600 Au/TiNT700 Au/TiNT800 Au/TiNT900 Au/TiNT1000

1.5

Au/TiNT700

Absorbance

Au/TiNT500

(a)

Au/TiNT1000

1.0

0.5

0.0

83.6

300

400

500

600

700

0.8

Au/TiNT425

Au/TiNT600

Au/TiNT800

Au/TiNT1000

(b) 600

0.7

590 0.6 580 0.5 570 0.4 560 0.3

550

540 88

86

84

82

80

Binding energy (eV) Fig. 6. (Top) Digital photographs of 0.5 wt.% Au/H2Ti3O7 and the 0.5 wt.% Au/TiNTx powders; (Bottom) Au 4f XPS spectra for 0.5 wt.% Au/H2Ti3O7 and selected 0.5 wt.% Au/TiNTx powders.

200

400

600

800

Maximum absorbance of Au LSPR

Au/H2Ti3O7

Wavelength of Au LSPR maximum (nm)

Normalised intensity (arbitrary units)

610

90

800

Wavelength (nm)

0.2 1000

o

Calcination temperature ( C) Fig. 7. (a) UV–Vis absorbance spectra for 0.5 wt.% Au/H2Ti3O7 and the 0.5 wt.% Au/ TiNTx photocatalysts; and (b) plot of Au LSPR wavelength and absorbance as a function of H2Ti3O7 calcination temperature.


0.5 wt.%

1.0 wt.%

1.5 wt.%

2.0 wt.%

83.6

ratios of 10:90 (Fig. 10(a)) and 80:20 (Fig. 10(b)) to examine the effect of ethanol concentration. Previous studies have shown that an EtOH:H2O ratio of 80:20 is optimal for Au/TiO2 photocatalysts [37,39], whilst the 10:90 represents a more technically feasible ratio for large scale H2 production. Under the photoreaction conditions used here, H2 production occurs from both ethanol photoreforming (CH3CH2OH + 3H2O ? 2CO2 + 6H2 or CH3CH2OH + H2O ? CH4 + CO2 + 2H2) and photocatalytic water splitting (H2O ? H2 + 1/2 O2) [26–32,35]. At both ethanol concentrations, the 0.5 wt.% Au/TiNTx photocatalysts displayed excellent stability during the tests, as evidenced by the linearity of the H2 production plots with UV irradiation time. Further, the H2 production rates were strongly dependent on the TiNTx support, with the 0.5 wt.% Au/TiNT600 sample displaying the highest rates (14.4 and 31.8 mmol g1 h1 at EtOH:H2O ratios of 10:90 and 80:20, respectively). Table 2 and Fig. 11 summarise H2 production rates for all the Au/TiNTx and Au/P25 photocatalysts in the ethanol–water mixtures. H2 production rates normalised against catalyst mass and catalyst surface area for the 0.5 wt.% Au/TiNTx samples are plotted in Fig. 11 to aid the discussion of the data. In both plots, the Au/TiNT600 photocatalyst was most active. Table 2 shows that the Au/TiNT600 samples had the smallest mean Au particle size (3.6 ± 1.4 nm), suggesting that Au particle size might be an important factor in the high H2 production rates realised by this sample. However, previous studies of photocatalytic H2 production over M/TiO2 photocatalysts have shown that reaction rates are largely independent of metal nanoparticle size, at least over the size range 3–12 nm [33]. This is clearly contrary to dark reactions over Au/TiO2 photocatalysts, such as low temperature CO oxidation, where metal nanoparticle periphery sites are mechanistically important and for which a distinct Au particle size dependence is observed. Hence, the very high activity of the 0.5 wt.% Au/TINT600 photocatalyst is instead rationalised in terms of a near ideal anatase crystallite size (24 nm) and a relatively high specific surface area (61.1 m2 g1). A further possibility was that the TiNT600 support possessed a high proportion of reactive anatase surface facets, such as anatase {1 0 1}, though further HRTEM studies are needed to establish the nature of all the anatase facets exposed by the TiNT600 support. Intriguingly, the 0.5 wt.% Au/TiNT600 photocatalyst was more active than the 0.5 wt.% Au/P25 photocatalyst at both ethanol concentrations, even when rates were normalised against photocatalyst surface area (Table 2). Au/P25 photocatalysts generally demonstrate superior H2 production activity compared to Au/anatase photocatalysts under UV [37], since in the former case higher charge carrier concentrations are generated in the P25 support by electron transfer from the conduction band of rutile to that of anatase across interfacial heterojunctions (with valence band holes migrating in the opposite direction). In the current study, the intrinsic characteristics of the TiNT600 support (anatase particle size, high surface area, exposed facets) appear to match and better the synergies realised when using mixed phase P25 as photocatalytic support. Interestingly, the Au/TiNT1000 sample afforded the highest area normalised H2 production rates (Fig. 11(b)). Generally, the photocatalytic activity of anatase is considered to be superior to that of rutile, since charge carrier lifetimes in rutile are several orders of magnitude lower than in anatase. However, the data of Fig. 11(a) and (b) and Table 2 suggest that the activities of Au/anatase and Au/rutile photocatalysts are actually quite similar when corrected for surface area, which can be rationalised in terms of the Au nanoparticles creating cathodic sites on anatase and rutile for H2 evolution, therein suppressing the electron–hole pair recombination that is normally highly detrimental to the photocatalytic activity of rutile. Fig. 12 shows plots of H2 production versus UV irradiation time for the 0.5–2.0 wt.% Au/TiNT600 and 0.5–2.0 wt.% Au/P25 samples in EtOH:H2O = 80:20 mixtures. For the Au/TiNT600 photocatalyst

Au 4f

0.5 wt.% Au Intensity (arbitrary units)

248

1.0 wt.% Au

1.5 wt.% Au

2.0 wt.% Au

90

88

86

84

82

80

Binding energy (eV) Fig. 8. (Top) Digital photographs of the 0.5–2.0 wt.% Au/TiNT600 photocatalysts; (Bottom) Au 4f XPS spectra for the 0.5–2.0 wt.% Au/TiNT600 photocatalysts showing a stepwise increase in the intensity of the Au 4f signals with increasing Au nominal loading.

series, the 0.5 wt.% Au loading was optimal and the H2 production rate decreased with increasing Au loading (Table 2 and Fig. 13). This was unexpected, since previous studies of Au/P25, Au/anatase and Au/rutile photocatalysts have found that H2 production rates generally increase with Au loading up to a maximum centred anywhere from 1.5 to 4 wt.% [33,37], as seen in Fig. 12(b) and Fig. 13 for the Au/P25 photocatalyst series for which an Au loading of 1.5 wt.% gave the highest H2 production rate. Based on the photoluminescence data presented here for the 0–2.0 wt.% Au/TiNT600 photocatalyst series (Figs. 9(d) and S13), the decrease in the H2 production rate with increasing Au loading is somewhat counter-intuitive and demands further investigation. Currently we conducting picosecond spectroscopy measurements on the Au/TiNT600 and Au/P25 photocatalysts in air and various alcohol–water mixtures to better understand the photophysics of these materials, and in particular the role of Au in facilitating charge separation which clearly has important downstream consequences for H2 production. From these measurements, we hope to establish a relationship between H2 production rates and charge carrier lifetimes. The fact that the Au/TiNT600 series of photocatalysts displayed high activity at low Au loadings is obviously interesting in the context of future solar H2 production, where minimising the amount of precious noble metal co-catalyst used will be a research priority. Amongst all the photocatalysts tested in the ethanol–water mixtures, the 0.5 wt.% Au/TiNT600 photocatalyst had the best 1 1 rate per gram Au (6.36 mol g1 and 2.88 mol g1 at 80:20 Au h Au h and 10:90 EtOH:H2O ratios, respectively). In Table 3 and Fig. 14 we explore the effect of the alcohol sacrificial hole scavenger on photocatalytic H2 production rates under UV excitation. The experiments were performed using the

249


(b)

(a)

800

2.0

(c)

Photoluminescence intensity

1.5

Absorbance

(d)

TiNT600 0.5 wt.% Au/TiNT600 1.0 wt.% Au/TiNT600 1.5 wt.% Au/TiNT600 2.0 wt.% Au/TiNT600

1.0

0.5

0.0

TiNT600 0.5 wt.% Au/TiNT600 1.0 wt.% Au/TiNT600 1.5 wt.% Au/TiNT600 2.0 wt.% Au/TiNT600

600

400

200

0 300

400

500

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Wavelength (nm)

400

450

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(e)

(f)

P25 TiO 2

P25 TiO 2 0.5 wt.% Au/P25 TiO2

0.5 wt.% Au/P25 TiO 2

1.5

Photoluminescence intensity

1.0 wt.% Au/P25 TiO 2 1.5 wt.% Au/P25 TiO 2 2.0 wt.% Au/P25 TiO 2

Absorbance

550

Wavelength (nm)

2.0

1.0

0.5

0.0

500

1.0 wt.% Au/P25 TiO2

800

1.5 wt.% Au/P25 TiO2 2.0 wt.% Au/P25 TiO2

600

400

200

0

300

400

500

600

700

800

Wavelength (nm)

350

400

450

500

550

Wavelength (nm)

Fig. 9. (a) TEM image of the 1.5 wt.% Au/TiNT600 photocatalyst; (b) TEM image of 1.5 wt.% Au/P25 photocatalyst; (c) UV–Vis absorbance spectra for the 0–2 wt.% Au/TiNT600 photocatalysts; (d) photoluminescence spectra for the 0–2 wt.% Au/TiNT600 photocatalysts; (e) UV–Vis absorbance spectra for the 0–2.0 wt.% Au/P25 photocatalysts; and (f) photoluminescence spectra for the 0–2.0 wt.% Au/P25 photocatalysts.

0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts in aqueous solutions of ethanol, 1-propanol, 1,2-ethanediol, 1,2propanediol or glycerol (alcohol concentration 10 vol.%).

Fig. 14(a) shows that the H2 production activity (in mmol g1 h1) of the two photocatalysts was very similar in all alcohol–water mixtures tested, with rates following the general order

250


likely complex, as the number of OH groups influences alcohol polarity, alcohol adsorption strength and binding mode on TiO2 and also the oxidation potential of the alcohol. Polarities (Y) for the different alcohols are shown in Table 3, and were calculated from alcohol permittivities (es) according to the equation Y = (es 1)/(es + 2), using es = 24.6, 19.9, 37.7, 32.0 and 47.0 for ethanol, 1-propanol, 1,2-ethanediol, 1,2-propanediol and glycerol, respectively. Fig. 15(b) shows a good correlation between H2 production rates and the alcohol polarity, in agreement with the previous work of Yang et al. [40] for Pt/anatase in different 1° and 2° alcohol–water mixtures. Alcohol standard oxidation potentials (Eoox ) were calculated from standard Gibbs free energy changes (G°) for the alcohol reforming reaction CxHyOz + (2x z)H2O ? xCO2 + nH+ + ne using the Eoox (V) vs. NHE = G°/nF, and are

50

(a) EtOH:H2O = 10:90 Au/H2Ti3O7 Au/TiNT300 Au/TiNT350 Au/TiNT425 Au/TiNT500 Au/TiNT600 Au/TiNT700 Au/TiNT800 Au/TiNT900 Au/TiNT1000

-1

H2 produced (mmol g )

40

30

20

10

35

0 0.0

(a) 0.5

1.0

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10:90 EtOH:H 2O 80:20 EtOH:H 2O

3.0 30 -1

H2 production rate (mmol g h )

Time (h) -1

100

(b) EtOH:H2O = 80:20 Au/H2Ti3O7

-1


80

60

Au/TiNT300 Au/TiNT350 Au/TiNT425 Au/TiNT500 Au/TiNT600 Au/TiNT700 Au/TiNT800 Au/TiNT900 Au/TiNT1000

25

20

15

10

5

40

0 200

20

400

600

800

1000

o

Calcination temperature ( C) 0.7

(b) 1.0

1.5

2.0

2.5

3.0

10:90 EtOH:H2O 80:20 EtOH:H2O

0.6

-2

Time (h)

-1

0.5

Surface area normalised rate (mmol m h )

0 0.0

Fig. 10. H2 evolution versus UV exposure time for 0.5 wt.% Au/H2Ti3O7 and the 0.5 wt.% Au/TiNTx photocatalysts in (a) 10 vol.% ethanol and (b) 80 vol.% ethanol. The UV flux was 6.5 mW cm2.

glycerol > 1,2-ethanediol 1,2-propanediol > ethanol > 1-propanol. In simpler terms, this order can be written as triols > diols > 1° aliphatic alcohols > 2° aliphatic alcohols. The 1.5 wt.% Au/P25 photocatalyst demonstrated slightly superior activity once rates were normalised against photocatalyst surface area (Table 3, Fig. 14(b)), though the 0.5 wt.% Au/TiNT600 outperformed the 1.5 wt.% Au/P25 photocatalyst by a factor of 3–3.5 when rates were normalised against the mass of Au (Table 3). Considering the data sets collected in ethanol and 1,2-ethanediol (same number of carbons), and those collected in 1-propanol, 1,2-propanediol and glycerol (again the same number of carbons), it is evident that H2 production rates track the number of hydroxyl groups on the alcohol. For both Au/TiO2 photocatalysts, a good correlation was established between the H2 production rate and the number of OH groups on the alcohol (Fig. 15(a)). The reason why H2 production rates increase with the number of OH groups on the alcohol is

0.5

0.4

0.3

0.2

0.1

0.0 200

400

600

800

1000

Calcination temperature (oC) Fig. 11. (a) Mass normalised H2 production rates for the 0.5 wt.% Au/TiNTx photocatalysts plotted as a function of the TiNTx calcination temperature; and (b) corresponding surface area normalised H2 production rates.

251


40

100

(a)

Au/P25 TiO2 Au/TiNT600

-1

-1

-1


80


(a)

0.5 wt. % Au/TiNT600 1.0 wt. % Au/TiNT600 1.5 wt. % Au/TiNT600 2.0 wt. % Au/TiNT600

60

40

30

20

10

20

0 0.5

1.0

1.5

2.0

Gold loading (wt. %) 1.0

1.5

2.0

2.5

3.0

0.8

(b)

-1

0.5


0 0.0

Au/P25 TiO2 Au/TiNT600

-2

Time (h) 100

(b)

0.5 wt. % Au/P25 TiO2 1.0 wt. % Au/P25 TiO2 1.5 wt. % Au/P25 TiO2 2.0 wt. % Au/P25 TiO2

-1


80

60

40

0.6

0.4

0.2

0.0 0.5

1.0

1.5

2.0

Gold loading (wt. %)

20

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 13. (a) Mass normalised H2 production rates for the Au/TiNT600 and Au/P25 photocatalysts as a function of the gold loading; and (b) surface area normalised H2 production rates for the Au/TiNT600 and Au/P25 photocatalysts as a function of the gold loading. The rates were determined from the data in Fig. 12.

Time (h) Fig. 12. H2 evolution versus UV exposure time for (a) 0–2.0 wt.% Au/TiNT600 photocatalysts; and (b) 0–2.0 wt.% Au/P25 photocatalysts in 80 vol.% ethanol. The UV flux was 6.5 mW cm2.

presented in Table 3. If we consider the electron transfer reaction from the alcohol to the TiO2 valence band holes as a simple donor-to-acceptor reaction [85], the experimental rate constant (kexp) for such an electron transfer reaction could be expected to follow an Arrhenius type relation:

EoVBðTiO Þ Eoox

kexp ¼ A exp

2

.

RT

ð1Þ

where EoVBðTiO2 Þ and Eoox are the valence band potential of TiO2 and the oxidation potential of the donor (e.g. alcohol), respectively, and A is the pre-exponential factor. Further, if electron transfer from the alcohols to TiO2 was the overall rate limiting step in photocatalytic H2 production over Au/TiO2 photocatalyts, then H2 production rates o could therefore be expected to correlate with expEox or

o

o

expðEVBðTiO2Þ Eox Þ . In Fig. 15(c) and (d) we have plotted H2 production o

o

o

rates against expEox or expðEVBðTiO2Þ Eox Þ , respectively, in the latter case using EoVBðTiO2Þ ¼ 2:7 V (the potential of the anatase valence band). Both analyses confirm a linear relationship, suggesting that H2 production rates reported here for the different Au/TiO2 photocatalysts may indeed be rate limited by the kinetics of simple electron transfer reactions between the alcohol sacrificial agents and TiO2 valence band holes. A remarkable feature of the data reported in Table 3 and Fig. 15 is the similarity in H2 production rates between the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts in the different alcohol–water systems. An important point to note is that the linear relationships observed here between H2 production rates and specific properties of the alcohols may not hold at other alcohol concentrations. Thus, the current study should be extended to encompass a wider range of alcohol concentrations, and to identify the optimum concentration for photoreforming each alcohol. We are currently conducting detailed GC analyses of the other reaction products formed during Au/TiO2 photo-excitation in the different alcohol– water mixtures, with the aim of better understanding the alcohol

252


Table 3 H2 production rates for 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 TiO2 photocatalysts in different alcohol–water mixtures under UV irradiation (6.5 mW cm2). The alcohol concentration in all experiments was 10 vol.%.

a

Alcohol (10 vol.%)

Alcohol polarity

Alcohol OH groups

Alcohol a-H

Alcohol Eoox (V) vs. NHE

0.5 wt.% Au/TiNT600 (mmol g1 h1)

(mmol m2 h1)

1 a (mol g1 ) Au h

(mmol g1 h1)

(mmol m2 h1)

1 a (mol g1 ) Au h

Ethanol 1-propanol 1,2-ethanediol 1,2-propanediol Glycerol

0.887 0.864 0.924 0.912 0.939

1 1 2 2 3

2 2 4 3 5

0.084 0.105 0.009 0.047 0.004

14.4 12.7 22.7 23.0 29.2

0.236 0.207 0.371 0.377 0.478

2.88 2.54 4.54 4.60 5.84

12.6 10.3 23.1 22.7 27.4

0.277 0.218 0.488 0.479 0.579

0.84 0.69 1.54 1.56 1.83


Based on nominal Au loading.

35 1.5 wt. % Au/P25 TiO2 0.5 wt. % Au/TiNT600

-1

-1


(a) 30 25

ethanol 20 15 10

1-propanol 5

l g ly ce ro

-et ha ne d 1,2

pa -pr o 1,2

-2

1,2-ethanediol

0.7

(b)

-1


i ol

i ol ne d

an ol rop 1-p

eth an ol

0

1.5 wt. % Au/P25 TiO2 0.5 wt. % Au/TiNT600

0.6 0.5

1,2-propanediol

0.4 0.3 0.2 0.1

glycerol

g ly ce rol

1,2

-et ha ne d

iol

an ed i ol 1,2 -pr op

pa no l 1-p ro

eth an ol

0.0

Fig. 14. (a) Mass normalised H2 production rates for the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts in different alcohol–water systems; and (b) surface area normalised H2 production rates for the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts in different alcohol–water systems. The alcohol concentration was 10 vol.% in all cases, and the UV flux was 6.5 mW cm2.

photo-oxidation pathway and role in promoting H2 production. This will also allow the relative contributions of alcohol photoreforming and water-splitting to the observed H2 yields to be determined, and meaningful comparisons to be drawn with prior related studies on

the Pd/TiO2 system [26–32]. Further photocatalytic tests are also being conducted under visible and full solar excitation, to determine the extent to which the plasmonic effect [35,46–48] can enhance H2 production rates.

253


35

35

(a)


(b) H2 production rate (mmol g h )

-1

30

1.5 wt.% Au/P25 TiO2 0.5 wt.% Au/TiNT600

30

-1

-1

-1


0.5 wt.% Au/TiNT600

25

20

15

10

5 0.5

1.0

1.5

2.0

2.5

3.0

25

20

15

10

5 0.86

3.5

0.88

Number of OH groups 35 1.5 wt.% Au/P25 TiO2

(d)

0.5 wt. % Au/TiNT600 -1


30

0.94

0.96

1.5 wt. % Au/P25 TiO2 0.5 wt. % Au/TiNT600

30

-1

-1

-1

0.92

35

(c) H2 production rate (mmol g h )

0.90

Alcohol polarity

25

20

15

10

5 0.98

1.00

1.02

1.04

1.06

Exp

o (Eox )

1.08

1.10

25

20

15

10

5 0.066

1.12

0.068

0.070

0.072

0.074

0.076

o

Exp (- E )

Fig. 15. H2 production for the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au/P25 photocatalysts plotted as a function of (a) number of OH groups on the alcohol; (b) alcohol polarity; o o o (c) expEox ; and (d) expðEVBðTiO2Þ Eox Þ .

4. Conclusion Calcination of hydrothermally synthesised H2Ti3O7 nanotubes at 400–800 °C yields anatase nanorods with high specific surface areas and high anatase–rutile phase transition temperatures. A 0.5 wt.% Au/TiNT600 photocatalyst, prepared using an anatase support obtained by calcining H2Ti3O7 nanotubes at 600 °C for 2 h, demonstrated excellent photocatalytic activity for H2 production in alcohol–water mixtures under UV excitation. H2 production rates of 14.4 and 31.8 mmol g1 h1 were achieved in 10:90 and 80:20 EtOH:H2O mixtures, respectively, at solar-like UV fluxes. These rates were superior to those achieved with a 0.5 wt.% Au/P25 photocatalyst under the same reaction conditions. For Au/P25, an Au loading of 1.5 wt.% was optimal, whereas for the Au/TiNT600 series the lower loading of 0.5 wt.% was optimal. H2 production rates (in mmol g1 h1) were similar for the 0.5 wt.% Au/TiNT600 and 1.5 wt.% Au./P25 photocatalysts in different alcohol–water mixtures, decreasing in the order glycerol > 1,2-ethanediol 1,2-propanediol > ethanol > 1-propanol. Correlations were found between the H2 production rates and the number of OH groups on the alcohol, alcohol polarity, and the alcoo

ðEoVBðTiO Þ Eoox Þ

hol oxidation potential (specifically, expEox and exp

2

).

Results suggest that titanias derived by calcination from H2Ti3O7 nanotubes have great potential in photocatalytic and renewable energy applications. Acknowledgments The authors would like to acknowledge funding support from the Dodd-Walls Centre for Photonic and Quantum Technologies, the MacDiarmid Institute for Advanced Materials and Nanotechnology, the Energy Education Trust of New Zealand and the University of Auckland FRDF fund. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.07.014. References [1] [2] [3] [4]

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