Tuning Phase Composition of TiO2 by Sn4+ Doping for Efficient

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Tuning Phase Composition of TiO2 by Sn4+ Doping for Efficient Photocatalytic Hydrogen Generation Fenglong Wang, Jie Hui Ho, Yijiao Jiang, and Rose Amal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06287 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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ACS Applied Materials & Interfaces

Tuning Phase Composition of TiO2 by Sn4+ Doping for Efficient Photocatalytic Hydrogen Generation

Fenglong Wang1, Jie Hui Ho1, Yijiao Jiang2,* and Rose Amal1,*

1

2

School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia.

Department of Engineering, Macquarie University, Sydney, NSW 2109, Australia.

*

To whom correspondence should be addressed: Tel: +612-9385-4361 Fax: +612-9385-5966

E-mail address: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT The anatase-rutile mixed-phase photocatalysts have attracted extensive research interest due to the superior activity compared to their single phase counterparts. In this study, doping of Sn4+ ions into the lattice of TiO2 facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn4+ ions entered into the lattice of TiO2 by substituting some of the Ti4+ ions and distributed evenly in the matrix of TiO2. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn4+ ions into the TiO2 exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectroscopic spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixedphase TiO2 catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase-rutile mixtures.

KEYWORDS: Mixed-phase TiO2, tin-doping, phase transformation, photocatalysis, hydrogen production

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INTRODUCTION Solar to chemical energy conversion by photocatalysis has been regarded as a promising way to solve the increasingly serious environmental problems and to reduce our dependency on limited reserves of fossil fuels.1-2 Despite being studied for more than forty years, TiO2-based catalysts still lie in the main stream of photocatalysis research due to their abundance, low cost, high stability and non-toxicity.3 However, their wide applications have often been hindered due to the low conversion efficiency from solar to chemical energy caused by the high electron-hole recombination rate and weak absorption in the visible light region. Surface modification of TiO2 by metal nanoparticles with larger work function such as Ag Au, Pt, Pd, has been proven as an efficient way to improve the photo-generated charge carrier separation.4-5 The photo-induced electrons can be easily trapped by the metal deposits through the Schottky energy barriers formed at the interfaces between TiO2 and metal nanoparticles. At the same time, the positively charged holes left in the valence band would be involved into the oxidation reactions. Doping with other elements to form intermediate energy levels between the conduction band and valence band of TiO2 is widely used to narrow its band gap, consequently the absorption edge of TiO2 can be extended into the visible light region.6 Studies on the anatase-rutile composites have attracted great interest, since it has been shown that the heterojunction between these two phases facilitates the charge separation due to the proper band alignment.7-8 Anatase has an indirect band gap of ~3.2 eV and rutile has a direct band gap of ~3.0 eV.9 Recent studies indicate that the rutile TiO2 possesses a more negative conduction band compared to the anatase based on the simulation techniques and X-ray photoemission experiments.7 Batista et al. observed the electron transfer from the conduction band of rutile to that of anatase using the optical pump THz probe technique.10 Li’s group found that the phase junction between anatase and rutile could greatly enhance the hydrogen production due to the improved spatial charge separation.11 They further found that the phase junction between anatase and rutile can not only facilitate the spatial charge separation efficiency but also adjust the surface acid/base property, which is attributed to the promoted H2 generation and the reduced CO formation.8, 12 The above 3



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mentioned pioneering researches indicate the necessity for the preparation of highly active mixedphase catalysts. However, most approaches to prepare mixed-phase TiO2 photocatalysts rely mainly on the conventional annealing procedure at elevated temperatures which causes serious aggregation and significantly increases the crystal sizes. These effects are usually detrimental to photocatalytic reactions due to the reduction of reactive sites. Therefore, an efficient method to prepare mixedphase catalysts at lower temperature but maintaining the same crystal sizes is desired. Doping with cations possessing similar electronegativity and ionic radius to Ti4+ has been reported as an efficient way to expedite the transformation from anatase to rutile at a lower temperature. Among them, Nb, Sc, Fe, and Sn have been studied for their promoting effects on the phase transformation from anatase to rutile.13 Cao et al. found that the doped Sn4+ ions existed as substitutional species at lower calcination temperature. With increasing temperature, SnO2 seeds were formed and acted as an evolution centre of rutile TiO2 which facilitated the phase transformation from anatase to rutile.14 Besides, Sn-dopant also changed the physicochemical properties of the catalyst surface as well as the band structure. Jiang et al. reported the Sn-doped anatase TiO2 with exposed [105] facets for enhanced water splitting application compared to the bare TiO2.15 Cao et al. also found that the Sn-doping narrowed the band gap and introduced the surface defects which led to a better photocatalytic activity than the bare TiO2.16 Egdell and co-workers found that Sn-dopant could also inhibit the particle sintering during the anatase-to-rutile transformation process. A small shift in the valence band edge position and a slight decrease of the band gap were observed in the Sn-doped rutile TiO2. In addition, Sn aggregation on the surface induced the surface states which acted as the hole trapping sites, leading to the superior photocatalytic activity compared to the nitrogen-doped anatase TiO2.17 They also prepared the Sndoped anatase thin film grown epitaxially on SrTiO3 (001) by the facile dip-coating method and found that Sn-doping in anatase actually led to the increase of band gap, which is opposite to that of rutile. The narrowed band gap of Sn-doped rutile TiO2 was attributed to the internal strain caused

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by the unit cell expansion after doping and the increased band gap of Sn-doped anatase TiO2 arose from the difference in the band edge positions between TiO2 and SnO2.18 These recent studies provide us more insights into the preparation and structural properties of the Sn-doped TiO2 materials. However, how the Sn-doped anatase-rutile mixture works for photocatalytic hydrogen generation, how these two phases in the mixture interact with each other and why the Sn-doping method is superior to the conventional annealing approach is still not clear and the mechanism behind needs to be further understood. Herein, we report a facile approach for the synthesis of Sn-doped anatase-rutile mixed-phase catalysts through evaporation induced self-assembly (EISA) method. By controlling the content of Sn-dopant, we can easily manipulate the compositions between the two phases under a lower calcination temperature. The Au nanoparticles were then deposited on the surface of Sn-doped TiO2 to act as a co-catalyst for visible-light-driven (λ > 400 nm) hydrogen production. The Au/Sn-TiO2 photocatalyst with molar ratio of Sn4+ to Ti4+ of 5:100 exhibited the best activity, ca. 15 times higher than that of the undoped TiO2. For comparison, the anatase-rutile composites were also prepared through the conventional annealing process at higher temperatures. The catalysts annealed at higher temperatures showed poor activity for the hydrogen production under visible light illumination. The superior performance of the Sn-doped anatase-rutile mixture catalyst is attributed to the shift of its onset absorption to visible light and the existence of the heterojunctions while maintaining the same crystal size, relatively to the mixed-phase catalysts prepared by the conventional annealling approach at higher temperatures. The mechanism for the spatial charge separation between the two phases has been established to understand the enhanced activity on the mixed-phase catalysts, which is further confirmed by the activity test under full spectrum illuminations.

EXPERIMENTAL SECTION

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Synthesis of Sn-doped TiO2 nanoparticles. The Sn4+ doped TiO2 nanoparticles were synthesised using the EISA method.19 In a typical process, 4 ml of titanium tetraisopropoxide (TTIP) was added dropwise to 100 ml of tin chloride solution with different concentrations under vigorous stirring at room temperature. The molar ratios of Sn4+ to TiO2 were set to be 1:100, 2:100, 5:100, and 10:100, respectively. The colloid solutions were then sonicated for 30 min without cooling, and were aged in a closed beaker at room temperature for two days. The suspensions were dried at 100 ℃ until complete evaporation of residual water. Subsequently, the dried powder was calcined at 500 ℃ for 1 h in air for further use. The catalysts were denoted as Sn1-TiO2, Sn2-TiO2, Sn5-TiO2, and Sn10-TiO2, respectively. The undoped TiO2 nanoparticles were also synthesised through the same method without addition of tin chloride and denoted as TiO2. For comparison, the undoped TiO2 was calcined at 600 ℃ or 700 ℃ for 3 h in air to obtain the mixtures of anatase and rutile. The samples were denoted as TiO2-600 and TiO2-700 accordingly. Preparation of Au-decorated TiO2 nanocomposites. The Au-decorated TiO2 and Sn-TiO2 composites were prepared with the Au content of 1.0 wt. % via a chemical reduction method. Typically, 0.5 g of the Sn-doped TiO2 nanomaterials were dispersed in 100 ml of Milli-Q water, followed by an addition of the required amount of gold (III) chloride trihydrate (HAuCl4·3H2O) as Au precursor. The suspension was then sonicated and stirred for ca. 1.5 h in an ice bath. Subsequently, surplus amount of ice cold NaBH4 solution (0.1 M) was injected quickly into the suspension under vigorous stirring. After stirring for another 3 h, the products were collected by centrifugation and washing with ethanol and water for several times. The products were then dried overnight at 85 °C without further thermal treatment. The samples were denoted as Au/TiO2, Au/Sn1-TiO2, Au/Sn2-TiO2, Au/Sn5-TiO2, and Au/Sn10-TiO2. The gold contents of the catalysts were determined by the inductively coupled plasma (ICP) analysis as given in Table S1 (Supporting Information). It is found that the doping of Sn4+ ions into TiO2 did not lead to any obvious effect on the Au loading efficiency. Au was also deposited onto TiO2-600 and TiO2-700 by using the same

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method as described above. The samples were denoted as Au/TiO2-600 and Au/TiO2-700, respectively. Photocatalyst characterization. The phase compositions of the undoped and Sn-doped TiO2 samples were determined using X-ray diffraction (XRD) on a PANalytical Xpert Multipurpose Xray Diffraction System. The relative mass ratios of anatase and rutile in the samples, the weight percentage of the anatase phase, WA, was determined according to the literature (see the Supporting Information).20 The crystal sizes (D) of the photocatalysts were derived using the Scherrer Equation. Raman spectroscopy was performed to confirm the phase composition of synthesized catalysts using a Renishaw In-Via Raman microscope with an excitation line of 514 nm Argon laser. The diffuse reflectance spectra (DRS) were recorded in absorption mode on a UV-Vis spectrophotometer (Shimadzu) to determine the light absorption of the undoped and Sn-doped TiO2 nanoparticles using barium sulphate (BaSO4) as a standard. Conventional high resolution transmission electron microscopy (HRTEM) was performed to investigate the morphology and dispersions of the nanoparticles on a Philips CM 200 facility operating at 200 kV. The high-angle annular dark field scanning TEM (HAADF-STEM) images and energy dispersive X-ray spectra (EDX) were recorded on a JEOL JEM-ARM200F TEM with an EDX detector. The TEM was operated at an accelerating voltage of 200 kV. The Ti-K, O-K, Sn-L and Au-L edges were used to obtain the concentration information of each element in the EDX spectra. The surface properties and chemical states were investigated using X-ray photoelectron spectroscopy (XPS) on an EscaLab 250 Xi (Thermo Scientific, UK) spectrometer. The

119

Sn magic angle spinning nuclear magnetic

resonance (MAS NMR) experiments were performed on a Bruker Avance III 300 spectrometer at a resonance frequency of 111.8 MHz for

119

Sn. A 4 mm rotor with a spinning frequency of 12 kHz

was used. The 119Sn chemical shifts were referred to that of SnO2. Photocatalytic activity evaluation. The experiments of photocatalytic hydrogen production were performed under visible light (λ > 400 nm) irradiation in a 300 ml top-irradiation photo-

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reactor with a 300 W Xenon lamp. In a typical procedure, 25 mg of the prepared catalysts were dispersed in a 50 ml of water/methanol mixture (volume ratio is 9/1) under sonication. Argon was purged into the system for more than 30 min to completely vent out the air. Then the mixture was irradiated under magnetic stirring and a water jacket was placed on the top of the reactor to absorb the heat. The evolved gas was analyzed every 30 min using a gas chromatography (GC, Shimadzu, 8A) equipped with a thermal conductivity detector (TCD).

RESULTS AND DISCUSSION The XRD analysis was carried out to determine the purity and phase composition of the undoped and Sn-doped TiO2 as shown in Fig. 1a. The absence of peaks corresponding to the other materials indicates the high purity of the samples. The Sn1-TiO2, Sn2-TiO2, and Sn5-TiO2 show diffraction peaks of both anatase and rutile indicating that the rutile and anatase phases co-exist in these materials. The undoped TiO2 exhibits anatase crystal structure with a trace amount of brookite whereas the Sn10-TiO2 is in a pure rutile phase. The relative mass ratios of anatase and rutile are summarized in Table 1. It can be seen that the content of rutile phase is gradually raised with the increasing content of Sn-dopant, and the mass ratios between anatase and rutile can be controlled by varying the Sn-dopant content. Further comparison of the anatase [101] diffraction peaks between the undoped TiO2 and Sn5TiO2 indicates a slight shift to a lower theta region after doping, revealing the substitutional mode of the Sn4+ ions in anatase which brought about the expansion of the unit cells.21 Since the ionic radius and electronegativity of Sn4+ are close to those of Ti4+, it is feasible for the Sn4+ ions to enter into the lattice of TiO2 and replace some of the Ti4+ ions.22 And this leads to the distortion of the crystal structure of anatase and facilitates the transformation from anatase phase to the more stable rutile phase at a lower temperature. In addition, by close inspection of the XRD pattern no characteristic peak of SnO2 or Sn metal can be observed, which further confirms that the Sn4+ ions entered into the lattice of TiO2. The absence of the SnO2 seeds as the evolution sites for rutile 8


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indicates that the produced rutile would have strong interaction with the anatase phase since the rutile phase was primarily formed on the surface of anatase, which would lead to the close-contact hetero-phase junctions. As the reference photocatalyts, the TiO2-600 and TiO2-700 samples were prepared by annealing the undoped TiO2 nanoparticles at higher temperatures of 600 ℃ and 700 ℃, respectively. The XRD patterns in Fig. S1 display that the TiO2-600 and TiO2-700 were also subjected to a clear phase transformation from anatase and rutile. A higher composition of rutile phase can be obtained at a higher calcination temperature. In this study, however, the doping of Sn4+ in the TiO2 facilitates the phase transformation of TiO2 from anatase to rutile at a lower temperature, i.e. 500 ℃. It has been reported that the phase transformation from undoped anatase to rutile phase generally occurs at temperatures between 600 to 915 ℃.13 According to the XRD results, the crystal sizes (D) of the photocatalysts were derived using the Scherrer Equation (see the Supporting Information).11 Table 1 shows that the crystalline sizes of TiO2 for all the samples are around 20-24 nm. Hence, doping Sn4+ in the TiO2 has no significant effect on the crystalline size and the induced transformation from anatase to rutile does not cause any increase of the crystalline sizes of the Sn-doped TiO2. This is in good agreement with the previous report that Sn-doping could suppress the growth of the crystal size during the anatase-torutile transformation process.17 However, the crystal sizes of the undoped samples increase significantly from 20 to 114 nm when the calcination temperature increases from 500 to 700 ℃. Together with the BET surface area measurement (Table S2 in the Supporting Information), one can see the effect of internal grain boundary interactions on the crystal size in the Sn-doped materials.17

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Fig. 1 (a) XRD patterns and (b) Raman spectra of the undoped and Sn-doped TiO2 calcined at 500 ℃.

Table 1. The phase compositions and crystal sizes of the undoped and Sn-doped TiO2 Catalysts

Weight percentage of

Weight percentage of

Crystal size, D

anatase phase, WA (%)

rutile phase, WR (%)

(nm)

TiO2-500

100

0

20

Sn1-TiO2

80

20

24

Sn2-TiO2

68

32

20

Sn5-TiO2

31

69

24

Sn10-TiO2

0

100

20

TiO2-600

96

4

44

TiO2-700

8

92

113

Raman spectroscopy was employed to further confirm the phase compositions of the undoped and Sn-doped TiO2 as displayed in Fig. 1b. The characteristic peaks of anatase and rutile were labelled according the literatures, respectively.23-24 The Raman spectra also show the transformation from anatase to rutile phase of TiO2 with the increase of the dopant content. It should be noted that the intensity of peaks for rutile phase is much lower compared to that of anatase, which is in good

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agreement with the previous report.25 The absence of Raman bands of SnO2 indicates that Sn does not exist as a separate crystalline oxide phase, corroborating well with the XRD results. The optical absorption properties of the catalysts were characterized by a UV-Vis spectrometer. Fig. 2a shows the UV-Vis absorption spectra of the undoped and Sn-doped TiO2. It can be seen that there is a gradual shift towards the higher wavelength region when the content of doped Sn4+ increased from 1 to 5 at. %, and further increase of the Sn4+ content did not lead to more red shift. The DRS UV-Vis spectra showed that the onset of the absorption approaches that of rutile TiO2 as the anatase content decreases. The band gaps determined from the UV-Vis spectra of the undoped TiO2 and Sn10-TiO2 are 3.20 and 2.98 eV, respectively. The calculated band gap of anatase agrees well with the previously reported values and the measured band gap of rutile is slightly narrow.7 The narrowing of band gap in the Sn-doped rutile was also found by Egdell et al. by using valence band XPS and DRS measurement.17 As can be seen from the DRS spectra of the Au deposited catalysts in Fig. 2b, the surface modification with Au nanoparticles on the TiO2 materials did not induce any change of the light absorption edge. This is attributed to the fact that the Au nanoparticles were deposited on the surface but not doped into the TiO2 lattice. In addition, the strong absorbance signals at ~ 550 nm can be attributed to the local surface plasmonic resonance (SPR) effect of the Au nanoparticles.26

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Fig. 2 The UV-Vis spectra of the undoped and Sn-doped TiO2 before (a) and after (b) Au deposition.

The morphological structure and elemental distribution of the catalysts were studied by TEM and EDX mapping technique, respectively. Fig. 3a shows the HRTEM of the Au/Sn5-TiO2. As indicated by the characteristic lattice spacing, it can be confirmed that the anatase crystal sits close to the rutile crystal forming a close-contact phase junction.27 It was reported that because of these kinds of phase junctions which facilitate the charge separation between the two phases, an enhanced activity was observed on the catalysts.7, 11 The photo-induced electrons in the conduction band of rutile phase by the visible light (λ > 400) will migrate to the conduction band of anatase leaving the holes still in the valence band of rutile, thus a better visible light activity is expected on the mixed phase catalysts. Fig. 3b shows the TEM image of the Au/Sn5-TiO2 at a low magnification. It can be clearly seen that the spherical Au nanoparticles with a diameter smaller than 4 nm are well dispersed on the TiO2 supports. A TEM image of the Au/Sn5-TiO2 with lower magnification is provided Fig. S2, to further show that sizes of the Au nanoparticles fall in 2-4 nm region. (Supporting Information) It has been reported that the Au nanoparticles with such a fine particle size often exhibited high activity due to the negative shift in the Fermi level as compared to the bigger Au nanoparticles.28 When the fine Au nanoparticles are deposited onto the Sn5-TiO2, they can facilitate charge separation by allowing the photo-excited electrons to transfer from the semiconductor nanoparticles to the Au nanoparticles, thus preventing the recombination of electron-hole pair. A recent study conducted by Qian et al. revealed that the small Au nanoparticles (< 10 nm) showed weak SPR effect to sensitize the TiO2 nanoparticles.28 It is therefore assumed that in this study, the fine Au nanoparticles deposited on the surface of TiO2 mainly act as the electron sinks to trap the photo-

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induced electrons from the conduction band of TiO2 and the co-catalysts to provide the reactive sites for proton reduction.29

Fig. 3 (a) HRTEM of the phase junction between anatase and rutile on the Au-Sn5/TiO2, (b) TEM image showing the Au nanoparticles distribution on the Au-Sn5/TiO2, (c) HAADF-STEM image of the Au-Sn5/TiO2, (d) Ti EDX mapping, (e) Sn EDX mapping, and (f) Au EDX mapping on the Au/Sn5-TiO2.

The HAADF-STEM image in Fig. 3c further confirms that the Au nanoparticles with a size of around 2~4 nm are well dispersed on the surface of TiO2. Besides, the absence of a bright spot around the TiO2 nanoparticles indicates that there are no separated tin species formed, corroborating well with the XRD and Raman results. The EDX mapping of Ti and Sn in Fig. 3d and 3e shows the uniform distribution of Sn element in the matrix of TiO2 indicating that the Sn4+ ions entered into 13



the lattice of TiO2, thus forming a well-mixed solid solution. The Au mapping shown as Fig. 3f confirms that the bright tiny spots are Au nanoparticles. Surface electronic properties of the catalysts were studied by XPS spectroscopy. As seen from the Ti 2p XPS spectra of the Au/TiO2 and Au/Sn5-TiO2 shown in Fig. 4a, two peaks for the Ti 2p located at 464.6 eV and 458.9 eV are assigned to Ti 2p1/2 and Ti 2p3/2, respectively.30 The splitting between both Ti 2p1/2 and Ti 2p3/2 peaks is about 5.7 eV indicating the presence of Ti4+ in the TiO2 powder.31 No obvious shift can be observed for the two peaks of the Ti 2p spectra upon doping of Sn4+ ions. The chemical state of Au is shown in Fig. 4b. The Au 4f7/2 peaks located at 83.75 and 83.52 eV for the Au/TiO2 and the Au/Sn5-TiO2, respectively. It indicates that the Au species exist in the metallic state.32 The metallic Au nanoparticles can not only serve as electron sinks to trap the photo-generated electrons from the conduction band of TiO2 thus prolonging the lifetime of photoexcited charge carriers, but also act as co-catalysts to provide proton reduction sites.33 Fig. 4c shows the Sn 3d core-level XPS peak obtained on the Au/Sn5-TiO2 catalyst. The doublet peak at 486.5 and 494.6 eV can be attributed to Sn 3d5/2 and Sn 3d3/2 of the substitutional Sn4+ dopants in the lattice with the chemical environment of -Sn-O- since the peak of Sn 3d5/2 is located between SnO2 (486.6 eV) and metallic Sn (484.4 eV).34 The XPS spectrum further confirms that the Sn atoms entered into the lattice of TiO2 forming -O-Sn-O-Ti- species.

(a) Au 4f

Au/Sn5-TiO2

(b)

Au/Sn5-TiO2

Au/TiO2

(c)

Au/Sn5-TiO2 Sn 3d

83.52 eV

Intensity (a.u)

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464.9 eV

Ti 2p

494.9 eV

459.2 eV

486.5 eV

Au/TiO2 83.75 eV

464.6 eV

470

468

466 464 462 460 Binding Energy / eV

458.9 eV

458

456

89

88

87

86

85

84

83

82 500

Binding Energy / eV

498

496

494 492 490 488 Binding Energy / eV

486

484

Fig. 4 (a) XPS Ti 2p and (b) Au 4f spectra of the Au/TiO2 and Au/Sn5-TiO2 samples. (c) XPS Sn 3d spectrum of the Au/Sn5-TiO2 sample. 14


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The

119

Sn MAS NMR spectrum of the Sn5-TiO2 is displayed in Fig. 5. The absence of the

characteristic peak of SnO2 at δ=-604 ppm further confirms that there are no isolated SnO2 species formed and all the Sn4+ ions entered into the lattice of TiO2 and substituted the Ti4+ ions, corroborating well with the TEM, Raman, XRD, and XPS results.35 The appearance of two peaks at δ=-657 ppm and δ=-710 ppm indicates that there are two Sn species with different chemical environments. The intensity ratio of these two signals is approximately 1:2, which is quite close to the mass ratio of anatase and rutile in the Sn5-TiO2 sample (see Table 1). The Sn NMR peaks at δ=657 ppm and δ=-708 ppm can be assigned to the Sn-O-Ti species in anatase and rutile, respectively.36 It is evidenced that the Sn-O-Ti species were formed after the substitution of Ti4+ by Sn4+. The shielding effect of the electron cloud on the Sn-O-Ti is reduced because of the lower electronegativity of Ti leading to a down-field shift of the Sn MAS NMR peaks compared to the SnO2. Upon Sn4+ entering into the lattice of TiO2, the distortion of the crystal structure of the bare TiO2 occurred due to the different atomic diameters and the metastable arrangement among the atoms.13 After thermal treatment, this metastable structure tends to transform into a more stable form and this might be a reason for the phase transformation from anatase to rutile at a lower calcination temperature. -710

-657

Fig. 5 119Sn MAS NMR spectrum of the Sn5-TiO2 sample.

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Photocatalytic hydrogen generation under visible light irradiation (λ > 400 nm) on the Sndoped TiO2 with the presence of Au nanoparticles are shown in Fig. 6. As expected, the bare TiO2 did not show any activity under visible light irradiation. After deposition of Au nanoparticles, the Au/TiO2 shows a hydrogen production of 25 µmol/g after 3 h irradiation due to the weak visible light absorption of TiO2 and the efficient charge separation promoted by the Au nanoparticles as electron sinks and co-catalysts for proton reduction. With the increase of Sn4+ content from 0 to 5 at. %, the Au deposited Sn-doped catalysts show an increased hydrogen production which could be attributed to the increase content of rutile phase. The Au/Sn5-TiO2 shows the best activity with a hydrogen production of 360 µmol/g, which is nearly 15 times higher compared to that on the Au/TiO2. However, further increase of the Sn4+ content led to a decrease of the hydrogen evolution over Au/Sn10-TiO2. This might be attributed to the exceeding amount of rutile which often shows less activity compared to the anatase TiO2 due to the low mobility of the electrons in the conduction band and the short lifetime of the photo-excited charge carriers.9-10 Kinetics plot of the visible-lightdriven hydrogen generation activity of the Au supported TiO2 materials are shown in Fig. S3. (Supporting information)

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Fig. 6 Hydrogen evolution on the photocatalysts under visible light irradiation (λ > 400 nm). Reaction conditions: 25 mg catalyst, irradiation of 3 h, 50 ml of water/methanol mixture (volume ratio of 9:1).

According to the UV-Vis absorption spectra in Fig. 2, doping of Sn4+ can cause a red-shift of the absorption region thus leading to a higher absorption in the visible light region due to the presence of rutile phase possessing a narrower band gap (2.98 eV). The enhanced visible light absorption indicates the increase of the photo-excited electron-hole pairs under irradiation. This could be the reason why the Sn-doped catalysts show high photocatalytic activity. The activity trend shows that doping of an appropriate content of Sn4+ ions into the lattice of TiO2 leads to a high activity in the visible-light-driven hydrogen production, which is mainly attributed to the improved visible light absorption of the rutile phase and the efficient charge separation at the interface between the two phases. Further investigation of the photocatalytic activity under irradiation with λ > 420 nm shows no hydrogen produced on all the samples, which may be attributed to the rather low light absorption of the TiO2 nanoparticles in this region. The lack of hydrogen evolution under irradiation with λ > 420 nm also confirms that the production of hydrogen is caused by the excitation of TiO2 support but not by the localized SPR effects induced by the Au nanoparticles. This indicates that the Au nanoparticles on the TiO2 surface mainly act as the electron traps and co-catalysts for proton reduction. To understand the synergistic effect of the mixed-phase catalysts, a diagram illustrating the spatial charge separation process between anatase and rutile should be established. For this purpose, valence band XPS spectra were measured on pure TiO2 and Sn10-TiO2, as shown in Fig. S4.(Supporting Information) It clearly indicates that there is a shift of the valence band edge measured on Sn10-TiO2 (pure rutile) to the lower binding energy level compared to that of pure TiO2 (anatase). Closer observation of the spectra reveals that the valence band edges of anatase and 17



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rutile are 2.83 eV and 2.38 eV vs NHE, respectively (Fig. S4 inset). This result places the valence band of rutile 0.45 eV above that of anatase. In combination with the band gaps determined from DRS measurement, one can deduce that the conduction band minima of anatase and rutile in this study are -0.37 eV and -0.6 eV vs NHE, respectively. This indicates that the electron transfer from the conduction band of rutile to that of anatase is thermodynamically favourable due to the potential gradient at the interfaces. The smooth electron transfer from conduction band of rutile to that of anatase would prolong the lifetime of the photo-generated charge carriers and lead to the improved hydrogen production. Thus, a scheme for the photo-excited charge carrier separation process can be proposed as displayed in Fig. 7. Since rutile has a narrower band gap than anatase, it can achieve higher visible-light absorption. When the mixed-phase catalysts are illuminated by visible light, most of the rutile is excited instead of anatase. In such a case, the electrons can be injected from rutile to anatase due to the potential gradient between the conduction bands of anatase and rutile.7, 10 The Au nanoparticles deposited on TiO2 surface would then act as the electron sink to trap the electrons and as the co-catalysts for the hydrogen production. Meanwhile, the positive holes remain in the valence band of rutile to oxidize methanol. In the Au/Sn5-TiO2, an optimal content of both phases is achieved.

Fig. 7 Schematic of the proposed reaction mechanism for the phase-junction between anatase and rutile with the enhanced photocatalytic hydrogen production.

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To verify that doping of Sn4+ ions into TiO2 is a promising approach to prepare efficient mixed-phase photocatalysts under visible light irradiation, the undoped TiO2 samples were calcined at higher temperatures of 600 ℃ and 700 ℃ to obtain the mixture of anatase and rutile and then were deposited with Au nanoparticle under identical conditions. Unlike the Au/Sn5-TiO2, both of the Au/TiO2-600 and Au/TiO2-700 catalysts show no hydrogen production under visible light irradiation (λ > 400 nm) though both of them possess a mixture of anatase and rutile as shown in Table 1. This result demonstrates that the performance in the visible-light-driven hydrogen production depends not only on the phase composition, but also on the other properties such as the crystal size. In this study, doping of Sn4+ ions in the TiO2 facilitates the phase transformation of TiO2 from anatase to rutile at a lower temperature while maintaining the crystal size constant, which would be the reason that the Sn-doped TiO2 shows superior activity than the mixed-phase TiO2 obtained from conventional annealing process.

CONCLUSIONS In this study, Sn-doped anatase-rutile composite photocatalysts were prepared by a simple evaporation induced self-assembly method for efficient hydrogen production under visible light irradiation. The phase composition can be easily tuned by varying the content of Sn-dopant. Various characterisation results show that the Sn4+ ions entered into the lattice of TiO2 and substituted some of the Ti ions leading to the transformation from anatase to rutile phase at a lower calcination temperature, which resulted in the formation of the close-contact phase junctions. The enhanced hydrogen generation activity under visible light irradiation can be attributed to the efficient charge separation in these phase junctions between anatase and rutile. A schematic diagram derived from valence band XPS and DRS measurement shows that the electron transfer from the conduction band of rutile to that of anatase is thermodynamically downhill. Compared to the conventional annealing method at higher temperatures providing the mixed phase photocatalysts with significant increased crystal sizes, the main advantage of this method is that the prepared catalysts maintain the same

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smaller crystal size of the undoped one. It is believed that doping of Sn4+ in the lattice of TiO2 provides a new approach for the fabrication of mixed-phase TiO2 catalysts with high photocatalytic activity under visible light irradiation.

Supporting Information. Au contents in the undoped and Sn-doped TiO2 materials measured by ICP-AES; XRD patterns of TiO2-600 and TiO2-700; TEM image of Au/Sn5-TiO2 at lower magnification; BET surface areas of the catalysts; Chemical compositions of Au/Sn5-TiO2 determined by EDX and XPS; Photocatalytic hydrogen production profile of the catalysts under full spectrum irradiation and valence band XPS spectra of pure TiO2 and Sn10-TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements Financial support through the ARC Discovery Early Career Researcher Award (DE120100329) and ARC Discovery Project (DP140102432) are gratefully acknowledged. We also thank the UNSW Mark Wainwright Analytical Centre at UNSW Australia for use of facilities. The use of JEOL JEMARM200F TEM funded by ARC Linkage Infrastructure, Equipment and Facilities grant (LE120100104) located at the UOW Electron Microscopy Centre is also greatly appreciated. We are grateful to the anonymous reviewers for their valuable comments and suggestions.

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Graphical Abstract

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Tuning Phase Composition of TiO2 by Sn4+ Doping for Efficient

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