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Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions
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Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions Sheng Chu, Ahmet E. Becerikli, Bianca Kortewille, Freddy E. Oropeza, Jennifer Strunk* € tsstraße 150, 44801 Bochum, Germany Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universita
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abstract
Article history:
A series of Sn-modified TiO2 samples was prepared by an anhydrous grafting route and
Received 3 July 2014
applied in the photocatalytic H2 evolution from aqueous methanol solutions. The synthe-
Received in revised form
sized samples were characterized by N2 physisorption, X-ray diffraction, Raman, UVeVis
17 September 2014
reflectance and photoluminescence spectroscopy. The results revealed that the tin species
Accepted 19 September 2014
were highly dispersed on the TiO2 surface and did not alter its crystalline structure. Pho-
Available online xxx
tocatalytic results showed that Sn-grafting led to a significant improvement in the activity
Keywords:
formance of samples was found to depend highly on the Sn content. Additionally, the in-
TiO2
fluence of Rh co-modification on the photocatalytic activity of Sn-grafted TiO2 was
SnOx
investigated, and a synergetic effect between Sn and Rh was identified, which is attributed
Grafting
to the assumed electron relay among TiO2, tin species and photodeposited Rh nanoparticles.
Co-catalyst
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
compared to bare TiO2 owing to the improved charge separation. The photocatalytic per-
reserved.
Hydrogen Photocatalysis Synergetic effect
Introduction Hydrogen has been considered as an excellent energy carrier due to its high energy capacity and lack of emission of greenhouse gas when it is used as a fuel, because the reaction with O2 produces only clean water. Among various H2 production methods, semiconductor-based photocatalytic H2 generation is one of the most promising strategies, because the external potential is supplied from renewable solar energy [1]. Since the pioneering work of Fujishima and Honda [2], TiO2 photocatalysts have been the subject of extensive
investigation owing to their low cost, nontoxicity and chemical stability [3e6]. Despite its outstanding features for photocatalysis, there are still two main limitations of TiO2. Apart from the well-known limited optical response only to UV light, the other drawback of TiO2 is the fast recombination of photogenerated electronehole pairs [7]. The recombination of excited charge carriers occurs on a time scale of femtoseconds to microseconds, which is always significantly shorter than surface electrocatalytic reactions leading to the quenching of most charge carriers before they reach the surface to participate in the catalytic reactions. To address this issue, one effective strategy is to construct a heterojunction between
* Corresponding author. Tel.: þ49 2343223566; fax: þ49 234 32 14115. E-mail address: [email protected] (J. Strunk). http://dx.doi.org/10.1016/j.ijhydene.2014.09.103 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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TiO2 and partner semiconductors for the reduction of charge carrier recombination [8e11]. For example, coupling TiO2 with appropriate amounts of SnO2 can greatly enhance the charge separation efficiency, because the excited electrons can transfer from TiO2 to SnO2: the conduction band (CB) of SnO2 (0 V versus NHE at pH 7) is lower than that of TiO2 (0.5 V versus NHE at pH 7), while holes remain in the valence band (VB) of TiO2 [12e14]. Generally, the photocatalytic activity increases when loading a small amount of SnO2, whereas it decreases with a high Sn content, which is usually explained by the so-called formation of recombination centers with excess loading [13e15]. Recently, surface modification of TiO2 with small, molecular-sized metal oxides has been demonstrated to be very effective for the photodegradation of model organic pollutants owing to the enhanced separation and transfer of photogenerated charge carriers [16e22]. In this system upon light excitation, electrons can be promoted to the CB of TiO2 and then transferred to surface oxides. Besides from the excited states, it is assumed that photogenerated electrons can be directly transferred from the VB of TiO2 to surface species via interfacial charge transfer (IFCT), resulting in an efficient separation of electrons and holes. To date, several methods have been developed to fabricate the heterojunction of TiO2 with molecular metal oxides, e.g. impregnation [16e18], atomic layer deposition (ALD) [19] and chemisorptionecalcination cycle (CCC) [20e22]. Another related method is the anhydrous grafting of TiO2 with small amounts of metal-organic precursors followed by calcination, which has been widely used as an effective strategy for the synthesis of submonolayer metal oxide species on SiO2 [23e25]. Similar to CCC, the organometallic precursor is chemisorbed on the oxide support during the synthesis, and similar to ALD the hydroxyl groups of the oxide support are generally used as anchoring sites. Besides, grafting is an effective method to achieve rather high loadings of dispersed metal oxide species on the substrate without using expensive equipment [26]. During the grafting synthesis, the intrinsic properties of the bulk support are not altered. Thus, grafting is a powerful method to investigate the influence of surface properties and surface modifications on catalytic behavior. Depending on the surface concentration, two different oxide species exist: isolated species at a low content and agglomerated species with excess loading [27]. Alkoxide precursors are usually preferred over acetylacetonates when the aim is the synthesis of isolated sites by means of grafting, because previous work has indicated that acetylacetonates might lead to the formation of agglomerates already at lower loadings [28,29]. Owing to different chemical states, the catalytic behaviors of the two species for specific reactions may differ greatly. In particular, the shift of the electronic levels when going from an extended semiconducting oxide to the corresponding isolated metal oxide species needs to be taken into account. Fewer electronic states overlap when the domain size gets smaller, until only a HOMO-LUMO transition remains in case of the isolated oxide species. The photon energy needed to photoexcite the isolated oxide species is thus much greater than for the extended oxide of the same metal, and the oxidation and reduction potentials of holes and electrons, respectively, are much greater [30]. It was reported that isolated single-site cobalt species dispersed
in mesoporous silicas (SBA-15) displayed a much greater photocatalytic activity for water oxidation than that of clustered cobalt species [31]. Similarly, Wang et al. [32] reported that isolated tin species dispersed in mesoporous silicas (MCM-41) showed a much higher activity for phenol hydroxylation than that of aggregated tin species. Recently, our group reported the synthesis of highly dispersed Sn4þ and Sn2þ species on TiO2 by an organic grafting route [33]. Different from the electron transfer role of Sn4þ species, Sn2þ species acted as hole trapping sites, which was applied in our previous work to enhance the photocatalytic activity of TiO2 for methylene blue degradation [33]. However, the potential application of the aforementioned molecular junctions occurring in TiO2 modified with molecular-sized metal oxides has been less investigated with respect to photocatalytic H2 evolution and thus remains an interesting route of exploration. In this work, a series of tin(IV)-modified TiO2 samples with different Sn contents was synthesized by a grafting method and the photocatalytic H2 evolution activity from aqueous methanol solutions was studied. Additionally, we comodified TiO2 with Rh and investigated the synergetic effect of tin species and Rh on the photocatalytic performance.
Experimental section Sample preparation Sn-grafted TiO2 samples were prepared by organic grafting of a commercial anatase TiO2 (Sachtleben, BET specific surface area ¼ 100 m2/g) with tin(IV) tert-butoxide precursor (Sn [OCH(CH3)3]4, Aldrich, 99.99%) in toluene as reported in our previous paper [33]. The preparation procedures were performed under inert atmosphere with the aid of a glove box and a vacuum line. In brief, 2 g TiO2 were dried at 120 C overnight under dynamic vacuum. Then, a given amount of tin precursor in toluene was brought in contact with the dry TiO2 and stirred for 4 h. The solid was separated by means of sedimentation and washed thoroughly with toluene three times to remove any physisorbed precursor before drying under vacuum. Finally, the dried material was calcined at 300 C for 1 h in N2 and then further heated to 450 C in synthetic air for 4 h. A series of Sn-grafted TiO2 samples with different Sn coverages was prepared by introducing the desired amount of tin precursor. The resulting samples were designated as Sn(x)/ TiO2, where x was the nominal Sn loading in atoms of tin per unit of TiO2 surface area (x atoms/nm2). The actual loading was determined by chemical analysis with inductively coupled plasma optical emission spectroscopy (ICP-OES) technique, and it was very similar to the nominal loading for all samples. For comparison, a mechanical mixture sample was prepared by physical mixing of TiO2 and SnO2 (Aldrich, 99.9%) in an agate mortar followed by the same annealing and denoted as Sn(x)/TiO2-PM.
Characterization X-ray diffraction (XRD) measurements were performed on a Panalytical X'Pert Pro instrument using monochromatic CuKa radiation. Raman spectra were recorded on a Thermo Fisher
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Scientific FT-Raman spectrometer with excitation by a Nd:YAG laser (l ¼ 1064 nm). BET surface areas were measured at 77 K by N2 physisorption using a NOVA 2000 analyzer (Quantachrome, Ltd.). UVeVis diffuse reflectance spectra were collected by PerkineElmer Lambda 650 UVeVis spectrophotometer using BaSO4 as a reference. Photoluminescence (PL) spectra were acquired at room temperature on a Horiba Fluorolog-3 spectrofluorometer with an excitation wavelength of 300 nm. Transmission electron microscopy (TEM) images were obtained with an H-7100 electron microscope from Hitachi. X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultrahigh vacuum (UHV) setup equipped with a Gammadata-Scienta SES 2000 analyzer. The base pressure in the measurement chamber was 4.5 1010 mbar. Monochromatic Al Ka (1486.6 eV, 14.5 kV, 30.5 mA) was used as incident radiation. A pass energy of 200 eV was applied, resulting in an energy resolution higher than 0.6 eV. Charging effects were compensated by applying a flood gun, and binding energies were calibrated on the basis of positioning the adventitious carbon C 1s signal to 284.8 eV. Peak deconvolution was done using the Casa XPS software with Shirley-type backgrounds and a combination of GaussianeLorentzian functions and asymmetric Doniach Sunjictype functions for Rh3þ and Rh0 signals, respectively.
Photocatalytic H2 evolution Photocatalytic tests were carried out in a continuously purged stirred tank reactor as described in a previous study [34]. A nitrogen flow rate of 50 mL min1 was passed through the reactor during the experiment. 0.3 g catalyst were dispersed in an aqueous solution (600 mL) containing methanol (50 mL) as a sacrificial reagent. The reactant system was deaerated by bubbling N2 for 1 h before inner irradiation with a Hg immersion lamp (Peschl, 500 W). The light intensity was estimated to be 54 mW/cm2 by using a NOVA II PD300-UV radiation meter (OPHIR, Ltd.). The irradiation area was about 400 cm2. The H2 evolution rate was analyzed by an on-line thermal conductivity detector. In the case of photocatalytic tests with Rh co-catalyst, Rh was in situ photodeposited on the catalyst by adding an appropriate amount of Na3RhCl6$3H2O (Aldrich, 99.999%) to the reactant solution [34].
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Fig. 1 e XRD patterns of TiO2, Sn(x)/TiO2 and Sn(1.5)/TiO2PM samples.
particles, demonstrating the highly dispersed state of tin species even at a high loading using the grafting method. The Sn(x)/TiO2 samples were also investigated by Raman spectroscopy, as shown in Fig. 2. All samples give six bands (144, 197, 399, 513, 519 and 639 cm1) consistent with the I41/ amd space group of anatase TiO2 [35]. In addition, no shift or broadening of Raman peaks is observed after the grafting of tin species, demonstrating that the crystallite size and structure of the parent TiO2 remained intact. Moreover, the main Raman bands of crystalline SnO2 (476, 635 and 776 cm1) [36] were undetectable, indicating that there was no phase separation of tin oxide. On the basis of XRD and Raman results, it is reasonable to assume that tin species are highly dispersed on the surface of TiO2. Additionally, based on N2 adsorption measurements, the grafting of tin species does not affect the BET surface area of the parent TiO2 to any appreciable extent.
Results and discussion Fig. 1 shows the XRD patterns of TiO2, Sn(x)/TiO2 and Sn(1.5)/ TiO2-PM prepared by physical mixing of TiO2 and SnO2. It is clearly seen that grafting tin species onto the TiO2 surface does not alter the anatase crystal structure of the parent TiO2. The XRD peaks of Sn(x)/TiO2 do not shift compared with the Sn-free sample, indicating that tin is not incorporated into the lattice. The average crystallite size of the TiO2 in all samples based on the (101) diffraction peak is estimated to be 15 ± 1 nm using the Scherrer equation. No additional phases related to SnO2 are observed for Sn/TiO2 as a result of the high dispersion of tin species or the low tin content. However, the physically mixed Sn(1.5)/TiO2-PM sample clearly shows three peaks of rutile SnO2 phase due to the presence of large
Fig. 2 e Raman spectra of TiO2 and Sn(x)/TiO2 samples.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 3 e UVeVis absorption spectra of TiO2 and Sn(x)/TiO2 samples.
Fig. 3 shows the UVeVis absorption spectra of Sn(x)/TiO2. No obvious change in the light absorbance of TiO2 is observed after the surface modification with tin species. This is in accordance with theoretical calculation results of molecularsized SnO2 clusters adsorbed on anatase surfaces [37], indicating tin oxide derived states do not appear inside the TiO2 energy gap and thus lead to no shift of light absorption. However, it cannot be excluded that the tin-related electronic states may be close to the CB of TiO2 and thus do not exert a large influence on the optical absorption of TiO2 [21], which is in line with the low tin content. The photocatalytic H2 evolution activity was evaluated in aqueous methanol solutions. Fig. 4(A) shows the timedependent photocatalytic H2 evolution rate of TiO2 and Sn(x)/TiO2. It is found that the H2 evolution rate increases rapidly after switching on the light and then reaches a relatively stable state after about 30 min of irradiation. To better compare the photocatalytic activity, the average H2 evolution rate during the irradiation was plotted as a function of the grafted tin content, as shown in Fig. 4(B). The average H2 evolution rate was determined by dividing the total H2 evolution amount (calculated by the integral area) by the irradiation time. It can be clearly seen that there is an
approximately linear increase of H2 evolution rate with increasing tin content at low loading (<0.5 nm2), which is considered as a consequence of the increasing amount of isolated tin species. Taking into account the rather similar properties of crystallinity, surface area and light absorption for all samples, we infer that the increased photocatalytic activity is ascribed to the improved charge separation caused by the grafted tin species. Upon light excitation, electrons in the VB of TiO2 can be promoted to surface tin(IV) species, either directly via IFCT or indirectly from the CB of TiO2, both resulting in the separation of electrons and holes. Subsequently, the electrons in the tin states reduce protons to produce H2, while the holes in TiO2 oxidize methanol to CO2 via the formation of intermediates such as formaldehyde and formic acid [38]. In the content range of 0.5e1.5 nm2, the H2 evolution rate slightly decreases. It is inferred that excess Sn loading would not lead to the further increase of isolated tin species, but the formation of aggregated tin species such as SnO2 nanoclusters. The aggregation of tin species would cause a downshift of the lowest unoccupied molecular orbital (LUMO) energy level, decreasing the reducing power of photogenerated electrons for H2 evolution [22]. Similarly, Gu et al. [39] recently found that single-site tin species enhance the separation of photogenerated charge carriers and thus photocatalytic efficiency, while aggregated tin species do not. Investigation of the fate of photoinduced electronehole pairs in semiconductor particles can provide some guidelines to explain the photocatalytic reactivity of photocatalysts. PL spectroscopy is a powerful technique for these studies, since the location and intensity of PL peaks can reflect the type and relative amount of recombination centers, respectively [40]. The samples were excited at 300 nm and PL emission bands were monitored in the range of 340e560 nm as shown in Fig. 5. The emission peak around 456 nm is associated with shallow traps of TiO2 related to oxygen vacancies, which are reported 0e1 eV below the CB of TiO2 [41]. The decrease in the intensity of this peak indicates the decreased electronehole recombination, leading to higher photocatalytic efficiency. It is noted that the intensity markedly decreases after grafting tin species with a low tin content (<0.5 nm2) and slightly increases in the content range of 0.5e1.5 nm2, which is in good agreement with the photocatalytic activity results reported above. Thus, the separation efficiency of photogenerated charge carriers seems to play a dominant role in the photocatalytic
Fig. 4 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with TiO2 and Sn(x)/TiO2 samples. (B) Average H2 evolution rate as a function of tin content. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 5 e PL spectra of TiO2 and Sn(x)/TiO2 samples.
performance. It is speculated that the unoccupied Sn 5s states of tin species act as preferential electron acceptors compared to surface oxygen vacancies to promote charge separation. Without tin modification, the excited electrons in TiO2 are rapidly trapped at the surface oxygen vacancies to undergo recombination with the holes. The emission band at about 406 nm is attributed to self-trapped excitons located on the TiO6 octahedron [42]. This is an intrinsic property of bulk TiO2, which is not changed during the synthesis because grafting is a surface modification method. It is widely accepted that noble metals such as Rh and Pt can facilitate the charge separation and act as efficient H2 evolution sites with a low overpotential [43,44]. Thus, we further investigated the photocatalytic H2 evolution activity with 0.3 wt% Rh as co-catalyst obtained by in situ photodeposition. The deposition of Rh on Sn(x)/TiO2 is confirmed by the combination of TEM and XPS measurements on a typical Sn(0.5)/TiO2 sample, as shown in Fig. 6. TEM image shows that the Rh particles are well-dispersed after the photodeposition. The particle size of primary Rh particles is smaller than 5 nm. According to XPS, Rh is present in two valence states: Rh0 and
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Rh3þ. The binding energy (BE) of Rh 3d5/2 at 306.7 eV is attributed to metallic Rh, and the BE of Rh 3d5/2 at 308.3 eV corresponds to the Rh3þ valence state [45,46]. Rhodium is mostly present as Rh0 and the Rh0/Rh3þ ratio is about 2.4. It is proposed that the Rh3þ valence state is present as Rh2O3, because the BE value is approximately equal to the peak energy reported for Rh2O3 (308.5 eV) [45]. In addition, no signal of Cl is detected in the XPS survey spectrum. The photocatalytic H2 evolution rate of Sn(x)/TiO2 with 0.3 wt% Rh as co-catalyst is shown in Fig. 7(A). It can be seen that the addition of Rh co-catalyst promoted H2 evolution activity by about two orders of magnitude with a maximum value of about 11 mmol h1 for Sn(0.5)/TiO2. The H2 evolution rate displays a slight decrease after reaching a maximum, presumably due to the accumulation of reaction intermediates on the catalyst surface considering the high-rate H2 evolution [34]. Simultaneously, the signal of the reaction product CO2 is observed by an IR detector, but in much lower amounts of only 3% relative to the amount of the product H2 (not shown). This indicates that some of the methanol is fully oxidized to CO2, while the major part may form intermediates such as formaldehyde and formic acid [38]. For a better comparison, the average H2 evolution rate during the irradiation was plotted as a function of the tin content as shown in Fig. 7(B). The plot of H2 evolution rate against tin content exhibits a volcano-type curve with a maximum at 0.5 nm2. The enhanced activity at a low tin content (<0.5 nm2) is ascribed to the increase in the amount of isolated tin species, which are supposed to act as a promoter for the electron transfer from TiO2 to Rh. With a tin content of 1.5 nm2, the H2 evolution activity is the lowest among the samples, which is different from that without Rh co-catalyst. It is proposed that Rh deposited on aggregated tin sites is less active for H2 evolution than that on bare TiO2. Thus, it is assumed that the order of H2 evolution activity for different Rh deposition sites is as follows: isolated tin species > TiO2 > aggregated tin species, which will be discussed in detail below. To better compare the H2 evolution activity of different Rh deposition sites, we intentionally designed an additional experiment to deposit a very low amount of Rh on Sn(x)/TiO2 and investigated the H2 evolution activity. For a high Rh loading, Rh is likely to be deposited on all possible reduction
Fig. 6 e (A) TEM image and (B) Rh 3d XPS of 0.3 wt% Rh-loaded Sn(0.5)/TiO2 sample. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 7 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with 0.3 wt% Rh-loaded TiO2 and Sn(x)/TiO2 samples. (B) Average H2 evolution rate as a function of tin content.
sites, whereas for a very low Rh loading, Rh is expected to be mainly deposited on preferential sites. Fig. 8(A) shows the photocatalytic H2 evolution rate of Sn(x)/TiO2 with 0.01 wt% Rh as co-catalyst. In this circumstance, even with the lowest tin loading (0.1 nm2), the Sn/Rh molar ratio is about six, which ensures all Rh could be deposited exclusively on tin sites. Compared with the other samples, Sn(0.3)/TiO2 and Sn(0.5)/TiO2 display a different shape of the curves, with H2 evolution rates decreasing significantly after reaching a maximum. It is assumed that the high H2 evolution rates of samples with a low tin content are likely to cause the large accumulation of reaction intermediates. However, for Sn(0.1)/ TiO2, due to the lowest tin coverage, there is significant free TiO2 surface area available for photooxidation of reaction intermediates resulting in a much more stable rate of H2 evolution. For a better comparison, the average H2 evolution rate during the irradiation as a function of tin content was also plotted as shown in Fig. 8(B). With 0.01 wt% Rh as co-catalyst, it is found that Sn(0.1)/TiO2 exhibits the highest average H2 evolution rate (1.72 mmol h1), which is about 1.64 times higher than that of tin-free TiO2 (1.05 mmol h1). With increasing tin content, the H2 evolution rate remains relatively stable in the region of low tin content (<0.5 nm2), but then sharply drops when exceeding 0.5 nm2. With a tin content of 1.5 nm2, the H2 evolution activity is 0.52 mmol h1, which is
only half that of tin-free TiO2. As a comparison, the H2 evolution activity of Sn(1.5)/TiO2-PM sample is also tested. In comparison to the Sn(1.5)/TiO2 sample, Sn(1.5)/TiO2-PM shows a more similar behavior of photocatalytic performance to that of bare TiO2, indicating that a larger proportion of Rh is deposited on TiO2. This is not unexpected considering that TiO2 absorbs more light than the small amount of SnO2 to produce photoexcited electrons, and there is no apparent electron transfer between TiO2 and SnO2 in the physically mixed sample. On the other hand, the lower H2 evolution activity of Sn(1.5)/TiO2 compared to Sn(1.5)/TiO2-PM indicates that the electrons captured at aggregated tin species are inactive with respect to the H2 evolution reaction. This is ascribed to the location of the empty electronic states of aggregated tin species. Similar to the conduction band of bulk SnO2, they are located at a too positive potential to allow for H2 evolution to occur. Besides, we also calculate the ratio of H2 evolution rate of Sn(x)/TiO2 with bare TiO2 for the high Rh loading (0.3 wt%) for comparation. In such case, the H2 evolution rate of Sn(0.1)/TiO2 is only 1.02 times greater than that of tin-free TiO2 and the activity of Sn(1.5)/TiO2 is 77% that of tin-free TiO2. Undoubtedly, the activity differences among samples with low Rh loading are more significant than those with high Rh loading, which is attributed to the different Rh deposition sites and distributions as depicted in Fig. 9(A). For a
Fig. 8 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with 0.01 wt% Rh-loaded TiO2, Sn(x)/TiO2 and Sn(1.5)/TiO2-PM samples. (B) Average H2 evolution rate as a function of tin content. The black circle represents the Sn(1.5)/TiO2-PM sample. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 9 e (A) Schematic picture of the location of Rh deposition sites on TiO2 and Sn(x)/TiO2 samples with different Rh loading amount. (B) Photocatalytic H2 evolution rate from aqueous methanol solutions with different samples. Inset: Schematic picture of photocatalytic H2 evolution process over 0.01 wt% Rh-loaded Sn(0.1)/TiO2 sample.
high Rh loading, a larger proportion of Rh is deposited on TiO2 compared to the low Rh loading, resulting in a much more similar behavior of photocatalytic performance with and without tin. In contrast, for a low Rh loading, a higher proportion of Rh is loaded on tin species, either of isolated or aggregated type. This is not unexpected considering tin species function as electron acceptors upon light illumination, which facilitate the reduction of Rh3þ to metallic Rh. Note that the redox potential of Rh3þ/Rh (0.758 V versus NHE at pH 0) [47] is much more positive than that of H2 evolution (0 V versus NHE at pH 0), so the photoreduction of Rh3þ is still thermodynamically favorable even with aggregated tin species. Interestingly, as long as the tin species are fully isolated, there is a remarkable synergetic enhancement of isolated tin species and Rh for H2 evolution activity over TiO2 as shown in Fig. 9(B). The H2 evolution enhancement of TiO2 co-modified with 0.1 nm2 Sn and 0.01 wt% Rh (increase in H2 evolution rate by 1.67 mmol h1 compared to bare TiO2) far exceeds the net subtotal of single species modification (increase by 0.01 and 1.00 mmol h1 for Sn and Rh, respectively). It is proposed that the synergy originates from the electron transfer cascades along TiO2, tin species and Rh as depicted in the inset of Fig. 9(B). The tin species seem to act as interfacial mediators for the promotion of electron transfer from TiO2 to Rh, consequently decreasing the charge recombination and enhancing the photocatalytic efficiency. It has to be pointed out that our model of the assumed action mode of tin is in good agreement with that observed by Gu et al. [48]. Considering that TiO2 absorbs more light than isolated tin species to produce photoexcited electrons, and that Rh is still preferably deposited on tin species rather than TiO2, it is deduced that the photoexcited electrons of TiO2 can transfer to isolated tin species and then reduce Rh3þ. Therefore, it is proposed that the LUMO position of isolated tin species is located between the minimum of the CB of TiO2 and the reduction potential of protons, which is, however, not yet experimentally confirmed. Because the CB edge of TiO2 is only about 0.2 eV higher than that of H2 evolution potential, it is most likely that the LUMO position of isolated tin species locates very close to the CB of TiO2, which is consolidated by the unchanged bandgap after tin grafting as shown in the UVeVis absorption
spectra. Moreover, Nolan et al. [21,22] also theoretically calculated the electronic structure of tin species adsorbed on the TiO2 surface, indicating that the tin-derived electronic states were very close to the CB edge of TiO2. In this way, the photocatalytic H2 evolution reaction can be utilized as probe reaction to investigate the preferential sites for Rh deposition and relative energy levels of tin species. From the H2 evolution results, we can further deduce that Rh deposited on isolated tin species is the most effective pathway for the reduction of protons to produce H2, presumably due to the appropriate LUMO energy level of isolated tin species for H2 evolution and efficient electron relay among TiO2, isolated tin species and photodeposited Rh nanoparticles. It is supposed that tin species act as electron mediators to open a new channel for the electron transfer from TiO2 to Rh in the form of isolated TieOeSn linkages interacting with the Rh nanoparticles.
Conclusions In summary, tin(IV) grafting can enhance the photocatalytic H2 evolution activity of TiO2 from aqueous methanol solutions due to improved charge separation. Isolated tin species promote the photocatalytic efficiency in this reaction, while aggregated tin species do not. After adding Rh as co-catalyst, the H2 evolution activity is greatly enhanced and there is a synergetic effect between isolated tin species and Rh on the photocatalytic activity of TiO2. Moreover, by using H2 evolution as probe reaction, it is found that tin species rather than TiO2 are preferential sites for Rh deposition and Rh deposited on isolated tin species is the most efficient pathway for H2 evolution. This work may provide guidance for the design of other ternary photocatalysts with efficient interfacial electron transfer using the synergy of dual co-catalysts.
Acknowledgments This work was funded by the German Ministry of Education and Research (BMBF) within the scope of the funding program
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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“Technologies for Sustainability and Climate ProtectiondChemical Processes and Use of CO2”, project number 033RC1007A. The authors are grateful to Georgios Dodekatos and Dr. Harun Tu¨ysu¨z at Max-Planck-Institut fu¨r Kohlenforschung (MPIK) for the investigation of the sample by TEM. Philipp Weide is also gratefully acknowledged for the XPS measurements. The authors also appreciate the help of Vera Singer for the grafting experiment, and thank Dr. Bastian Mei and Anna Pougin for helpful discussions.
references
[1] Chen XB, Shen SH, Guo LJ, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503e70. [2] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [3] Liu G, Wang L, Yang HG, Cheng HM, Lu GQ. Titania-based photocatalystsdcrystal growth, doping and heterostructuring. J Mater Chem 2010;20:831e43. [4] Nakata K, Fujishima A. TiO2 photocatalysis: design and applications. J Photochem Photobiol C 2012;13:169e89. [5] Pulido Melian E, Gonzalez Diaz O, Ortega Mendez A, Lopez Cristina R, Nereida Suarez M, Dona Rodriguez JM, et al. Efficient and affordable hydrogen production by water photo-splitting using TiO2-based photocatalysts. Int J Hydrogen Energy 2013;38:2144e55. [6] Pulido Melian E, Lopez Cristina R, Ortega Mendez A, Gonzalez Diaz O, Nereida Suarez M, Dona Rodriguez JM, et al. Hydrogen production using Pt-loaded TiO2 photocatalysts. Int J Hydrogen Energy 2013;38:11737e48. [7] Xu MC, Gao YK, Moreno EM, Kunst M, Muhler M, Wang YM, et al. Photocatalytic activity of bulk TiO2 anatase and rutile single crystals using infrared absorption spectroscopy. Phys Rev Lett 2011;106:138302. [8] Bessekhouad Y, Robert D, Weber JV. Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions. Catal Today 2005;101:315e21. [9] Kim DW, Lee S, Jung HS, Kim JY, Shin H, Hong KS. Effects of heterojunction on photoelectrocatalytic properties of ZnOeTiO2 films. Int J Hydrogen Energy 2007;32:3137e40. [10] Chu S, Zheng XM, Kong F, Wu GH, Luo LL, Guo Y, et al. Architecture of Cu2O@TiO2 coreeshell heterojunction and photodegradation for 4-nitrophenol under simulated sunlight irradiation. Mater Chem Phys 2011;129:1184e8. [11] Wang Q, An N, Bai Y, Hang H, Li J, Lu X, et al. High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2. Int J Hydrogen Energy 2013;38:10739e45. [12] Tada H, Hattori A, Tokihisa Y, Imai K, Tohge N, Ito S. A patterned-TiO2/SnO2 bilayer type photocatalyst. J Phys Chem B 2000;104:4585e7. [13] Hou LR, Yuan CZ, Peng Y. Synthesis and photocatalytic property of SnO2/TiO2 nanotubes composites. J Hazard Mater 2007;139:310e5. [14] Sasikala R, Shirole A, Sudarsan V, Sakuntala T, Sudakar C, Naik R, et al. Highly dispersed phase of SnO2 on TiO2 nanoparticles synthesized by polyol-mediated route: photocatalytic activity for hydrogen generation. Int J Hydrogen Energy 2009;34:3621e30. [15] Lee SS, Bai H, Liu Z, Sun DD. Electrospun TiO2/SnO2 nanofibers with innovative structure and chemical properties for highly efficient photocatalytic H2 generation. Int J Hydrogen Energy 2012;37:10575e84.
[16] Murakami N, Chiyoya T, Tsubota T, Ohno T. Switching redox site of photocatalytic reaction on titanium (IV) oxide particles modified with transition-metal ion controlled by irradiation wavelength. Appl Catal A 2008;348:148e52. [17] Yu HG, Irie H, Shimodaira Y, Hosogi Y, Kuroda Y, Miyauchi M, et al. An efficient visible-light-sensitive Fe(III)-grafted TiO2 photocatalyst. J Phys Chem C 2010;114:16481e7. [18] Liu M, Qiu XQ, Miyauchi M, Hashimoto K. Cu(II) oxide amorphous nanoclusters grafted Ti3þ self-doped TiO2: an efficient visible light photocatalyst. Chem Mater 2011;23:5282e6. [19] Libera JA, Elam JW, Sather NF, Rajh T, Dimitrijevic NM. Iron(III)-oxo centers on TiO2 for visible-light photocatalysis. Chem Mater 2010;22:409e13. [20] Tada H, Jin QL, Nishijima H, Yamamoto H, Fujishima M, Okuoka S, et al. Titanium(IV) dioxide surface-modified with iron oxide as a visible light photocatalyst. Angew Chem Int Ed 2011;50:3501e5. [21] Nolan M. Surface modification of TiO2 with metal oxide nanoclusters: a route to composite photocatalytic materials. Chem Commun 2011;47:8617e9. [22] Fujishima M, Jin QL, Yamamoto H, Tada H, Nolan M. Tin oxide-surface modified anatase titanium(IV) dioxide with enhanced UV-light photocatalytic activity. Phys Chem Chem Phys 2012;14:705e11. [23] Gao X, Bare SR, Fierro JLG, Banares MA, Wachs IE. Preparation and in-Situ spectroscopic characterization of molecularly dispersed titanium oxide on silica. J Phys Chem B 1998;102:5653e66. [24] Strunk J, Vining WC, Bell AT. A study of oxygen vacancy formation and annihilation in submonolayer coverages of TiO2 dispersed on MCM-48. J Phys Chem C 2010;114:16937e45. [25] Mei B, Becerikli A, Pougin A, Heeskens D, Sinev I, Gru¨nert W, et al. Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide. J Phys Chem C 2012;116:14318e27. [26] An DL, Zhang QH, Wang Y. Copper grafted on SBA-15 as efficient catalyst for the selective oxidation of methane by oxygen. Catal Today 2010;157:143e8. [27] Ross-Medgaarden EI, Wachs IE, Knowles WV, Burrows A, Kiely CJ, Wong MS. Tuning the electronic and molecular structures of catalytic active sites with titania nanoligands. J Am Chem Soc 2009;131:680e7. € cker R, Wachs IE. [28] Carrero CA, Keturakis CJ, Orrego A, Schoma Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor. Dalton Trans 2013;42:12644e53. [29] Strunk J, Vining WC, Bell AT. Synthesis of different CeO2 structures on mesoporous silica and characterization of their reduction properties. J Phys Chem C 2011;115:4114e26. [30] Matsuoka M, Anpo M. Local structures, excited states, and photocatalytic reactivities of “single-site” Ti-oxide photocatalysts constructed within zeolites or mesoporous materials. In: Anpo M, Kamat PV, editors. Environmentally benign photocatalysts. New York: Springer ScienceþBusiness Media; 2010. p. 218. [31] Ahn HS, Yano J, Tilley TD. Photocatalytic water oxidation by very small cobalt domains on a silica surface. Energy Environ Sci 2013;6:3080e7. [32] Wang XX, Xu HB, Fu XZ, Liu P, Lefebvre F, Basset JM. Characterization and catalytic properties of tin-containing mesoporous silicas prepared by different methods. J Mol Catal A 2005;238:185e91. [33] Oropeza FE, Mei B, Sinev I, Becerikli AE, Muhler M, Strunk J. Effect of Sn surface states on the photocatalytic activity of anatase TiO2. Appl Catal B 2013;140e141:51e9.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[34] Busser GW, Mei B, Muhler M. Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions. ChemSusChem 2012;5:2200e6. [35] Swamy V, Kuznetsov A, Dubrovinsky LS, Caruso RA, Shchukin DG, Muddle BC. Finite-size and pressure effects on the Raman spectrum of nanocrystalline anatase TiO2. Phys Rev B 2005;71:184302. [36] Leonardy A, Hung WZ, Tsai DS, Chou CC, Huang YS. Structural features of SnO2 nanowires and Raman spectroscopy analysis. Cryst Growth Des 2009;9:3958e63. [37] Nolan M, Iwaszuk A, Tada H. Molecular metal oxide clustersurface modified titanium(IV) dioxide photocatalysts. Aust J Chem 2012;65:624e32. [38] Chiarello GL, Aguirre MH, Selli E. Hydrogen production by photocatalytic stream reforming of methanol on noble metal-modified TiO2. J Catal 2010;273:182e90. [39] Gu Q, Long JL, Zhou YG, Yuan RS, Lin HX, Wang XX. Singlesite Tin-grafted anatase TiO2 for photocatalytic hydrogen production: toward understanding the nature of interfacial molecular junctions formed in semiconducting composite photocatalysts. J Catal 2012;289:88e99. [40] Anpo M, Che M. Applications of photoluminescence techniques to the characterization of solid surfaces in relation to adsorption, catalysis, and photocatalysis. Adv Catal 1999;44:119e257. [41] Serpone N, Lawless D, Khairutdinov R. Size effects on the photophysical properties of colloidal anatase TiO2 particles:
[42]
[43]
[44]
[45]
[46]
[47]
[48]
9
size quantization versus direct transitions in this indirect semiconductor? J Phys Chem 1995;99:16646e54. Song XF, Gao L. Synthesis, characterization, and optical properties of well-defined N-doped, hollow silica/titania hybrid microspheres. Langmuir 2007;23:11850e6. Tada H, Kiyonagaa T, Naya SI. Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(IV) dioxide. Chem Soc Rev 2009;38:1849e58. Merka O, Bahnemann DW, Wark M. Improved photocatalytic hydrogen production by structure optimized nonstoichiometric Y2Ti2O7. ChemCatChem 2012;4:1819e27. Larichev YV, Netskina OV, Komova OV, Simagina VI. Comparative XPS study of Rh/Al2O3 and Rh/TiO2 as catalysts for NaBH4 hydrolysis. Int J Hydrogen Energy 2010;35:6501e7. Yamamoto A, Ohyama J, Teramura K, Shishido T, Tanaka T. Effect of a crystalline phase of TiO2 photocatalysts on the photodeposition of Rh metal nanoparticles. Catal Today 2014;232:165e70. Gomathisankar P, Hachisuka K, Katsumata H, Suzuki T, Funasaka K, Kaneco S. Enhanced photocatalytic hydrogen production from aqueous methanol solution using ZnO with simultaneous photodeposition of Cu. Int J Hydrogen Energy 2013;38:11840e6. Gu Q, Long JL, Zhuang HQ, Zhang CQ, Zhou YG, Wang XX. Ternary Pt/SnOx/TiO2 photocatalysts for hydrogen production: consequence of Pt sites for synergy of dual cocatalysts. Phys Chem Chem Phys 2014;16:12521e34.
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Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions Sheng Chu, Ahmet E. Becerikli, Bianca Kortewille, Freddy E. Oropeza, Jennifer Strunk* € tsstraße 150, 44801 Bochum, Germany Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universita
article info
abstract
Article history:
A series of Sn-modified TiO2 samples was prepared by an anhydrous grafting route and
Received 3 July 2014
applied in the photocatalytic H2 evolution from aqueous methanol solutions. The synthe-
Received in revised form
sized samples were characterized by N2 physisorption, X-ray diffraction, Raman, UVeVis
17 September 2014
reflectance and photoluminescence spectroscopy. The results revealed that the tin species
Accepted 19 September 2014
were highly dispersed on the TiO2 surface and did not alter its crystalline structure. Pho-
Available online xxx
tocatalytic results showed that Sn-grafting led to a significant improvement in the activity
Keywords:
formance of samples was found to depend highly on the Sn content. Additionally, the in-
TiO2
fluence of Rh co-modification on the photocatalytic activity of Sn-grafted TiO2 was
SnOx
investigated, and a synergetic effect between Sn and Rh was identified, which is attributed
Grafting
to the assumed electron relay among TiO2, tin species and photodeposited Rh nanoparticles.
Co-catalyst
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
compared to bare TiO2 owing to the improved charge separation. The photocatalytic per-
reserved.
Hydrogen Photocatalysis Synergetic effect
Introduction Hydrogen has been considered as an excellent energy carrier due to its high energy capacity and lack of emission of greenhouse gas when it is used as a fuel, because the reaction with O2 produces only clean water. Among various H2 production methods, semiconductor-based photocatalytic H2 generation is one of the most promising strategies, because the external potential is supplied from renewable solar energy [1]. Since the pioneering work of Fujishima and Honda [2], TiO2 photocatalysts have been the subject of extensive
investigation owing to their low cost, nontoxicity and chemical stability [3e6]. Despite its outstanding features for photocatalysis, there are still two main limitations of TiO2. Apart from the well-known limited optical response only to UV light, the other drawback of TiO2 is the fast recombination of photogenerated electronehole pairs [7]. The recombination of excited charge carriers occurs on a time scale of femtoseconds to microseconds, which is always significantly shorter than surface electrocatalytic reactions leading to the quenching of most charge carriers before they reach the surface to participate in the catalytic reactions. To address this issue, one effective strategy is to construct a heterojunction between
* Corresponding author. Tel.: þ49 2343223566; fax: þ49 234 32 14115. E-mail address: [email protected] (J. Strunk). http://dx.doi.org/10.1016/j.ijhydene.2014.09.103 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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TiO2 and partner semiconductors for the reduction of charge carrier recombination [8e11]. For example, coupling TiO2 with appropriate amounts of SnO2 can greatly enhance the charge separation efficiency, because the excited electrons can transfer from TiO2 to SnO2: the conduction band (CB) of SnO2 (0 V versus NHE at pH 7) is lower than that of TiO2 (0.5 V versus NHE at pH 7), while holes remain in the valence band (VB) of TiO2 [12e14]. Generally, the photocatalytic activity increases when loading a small amount of SnO2, whereas it decreases with a high Sn content, which is usually explained by the so-called formation of recombination centers with excess loading [13e15]. Recently, surface modification of TiO2 with small, molecular-sized metal oxides has been demonstrated to be very effective for the photodegradation of model organic pollutants owing to the enhanced separation and transfer of photogenerated charge carriers [16e22]. In this system upon light excitation, electrons can be promoted to the CB of TiO2 and then transferred to surface oxides. Besides from the excited states, it is assumed that photogenerated electrons can be directly transferred from the VB of TiO2 to surface species via interfacial charge transfer (IFCT), resulting in an efficient separation of electrons and holes. To date, several methods have been developed to fabricate the heterojunction of TiO2 with molecular metal oxides, e.g. impregnation [16e18], atomic layer deposition (ALD) [19] and chemisorptionecalcination cycle (CCC) [20e22]. Another related method is the anhydrous grafting of TiO2 with small amounts of metal-organic precursors followed by calcination, which has been widely used as an effective strategy for the synthesis of submonolayer metal oxide species on SiO2 [23e25]. Similar to CCC, the organometallic precursor is chemisorbed on the oxide support during the synthesis, and similar to ALD the hydroxyl groups of the oxide support are generally used as anchoring sites. Besides, grafting is an effective method to achieve rather high loadings of dispersed metal oxide species on the substrate without using expensive equipment [26]. During the grafting synthesis, the intrinsic properties of the bulk support are not altered. Thus, grafting is a powerful method to investigate the influence of surface properties and surface modifications on catalytic behavior. Depending on the surface concentration, two different oxide species exist: isolated species at a low content and agglomerated species with excess loading [27]. Alkoxide precursors are usually preferred over acetylacetonates when the aim is the synthesis of isolated sites by means of grafting, because previous work has indicated that acetylacetonates might lead to the formation of agglomerates already at lower loadings [28,29]. Owing to different chemical states, the catalytic behaviors of the two species for specific reactions may differ greatly. In particular, the shift of the electronic levels when going from an extended semiconducting oxide to the corresponding isolated metal oxide species needs to be taken into account. Fewer electronic states overlap when the domain size gets smaller, until only a HOMO-LUMO transition remains in case of the isolated oxide species. The photon energy needed to photoexcite the isolated oxide species is thus much greater than for the extended oxide of the same metal, and the oxidation and reduction potentials of holes and electrons, respectively, are much greater [30]. It was reported that isolated single-site cobalt species dispersed
in mesoporous silicas (SBA-15) displayed a much greater photocatalytic activity for water oxidation than that of clustered cobalt species [31]. Similarly, Wang et al. [32] reported that isolated tin species dispersed in mesoporous silicas (MCM-41) showed a much higher activity for phenol hydroxylation than that of aggregated tin species. Recently, our group reported the synthesis of highly dispersed Sn4þ and Sn2þ species on TiO2 by an organic grafting route [33]. Different from the electron transfer role of Sn4þ species, Sn2þ species acted as hole trapping sites, which was applied in our previous work to enhance the photocatalytic activity of TiO2 for methylene blue degradation [33]. However, the potential application of the aforementioned molecular junctions occurring in TiO2 modified with molecular-sized metal oxides has been less investigated with respect to photocatalytic H2 evolution and thus remains an interesting route of exploration. In this work, a series of tin(IV)-modified TiO2 samples with different Sn contents was synthesized by a grafting method and the photocatalytic H2 evolution activity from aqueous methanol solutions was studied. Additionally, we comodified TiO2 with Rh and investigated the synergetic effect of tin species and Rh on the photocatalytic performance.
Experimental section Sample preparation Sn-grafted TiO2 samples were prepared by organic grafting of a commercial anatase TiO2 (Sachtleben, BET specific surface area ¼ 100 m2/g) with tin(IV) tert-butoxide precursor (Sn [OCH(CH3)3]4, Aldrich, 99.99%) in toluene as reported in our previous paper [33]. The preparation procedures were performed under inert atmosphere with the aid of a glove box and a vacuum line. In brief, 2 g TiO2 were dried at 120 C overnight under dynamic vacuum. Then, a given amount of tin precursor in toluene was brought in contact with the dry TiO2 and stirred for 4 h. The solid was separated by means of sedimentation and washed thoroughly with toluene three times to remove any physisorbed precursor before drying under vacuum. Finally, the dried material was calcined at 300 C for 1 h in N2 and then further heated to 450 C in synthetic air for 4 h. A series of Sn-grafted TiO2 samples with different Sn coverages was prepared by introducing the desired amount of tin precursor. The resulting samples were designated as Sn(x)/ TiO2, where x was the nominal Sn loading in atoms of tin per unit of TiO2 surface area (x atoms/nm2). The actual loading was determined by chemical analysis with inductively coupled plasma optical emission spectroscopy (ICP-OES) technique, and it was very similar to the nominal loading for all samples. For comparison, a mechanical mixture sample was prepared by physical mixing of TiO2 and SnO2 (Aldrich, 99.9%) in an agate mortar followed by the same annealing and denoted as Sn(x)/TiO2-PM.
Characterization X-ray diffraction (XRD) measurements were performed on a Panalytical X'Pert Pro instrument using monochromatic CuKa radiation. Raman spectra were recorded on a Thermo Fisher
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Scientific FT-Raman spectrometer with excitation by a Nd:YAG laser (l ¼ 1064 nm). BET surface areas were measured at 77 K by N2 physisorption using a NOVA 2000 analyzer (Quantachrome, Ltd.). UVeVis diffuse reflectance spectra were collected by PerkineElmer Lambda 650 UVeVis spectrophotometer using BaSO4 as a reference. Photoluminescence (PL) spectra were acquired at room temperature on a Horiba Fluorolog-3 spectrofluorometer with an excitation wavelength of 300 nm. Transmission electron microscopy (TEM) images were obtained with an H-7100 electron microscope from Hitachi. X-ray photoelectron spectroscopy (XPS) measurements were performed in an ultrahigh vacuum (UHV) setup equipped with a Gammadata-Scienta SES 2000 analyzer. The base pressure in the measurement chamber was 4.5 1010 mbar. Monochromatic Al Ka (1486.6 eV, 14.5 kV, 30.5 mA) was used as incident radiation. A pass energy of 200 eV was applied, resulting in an energy resolution higher than 0.6 eV. Charging effects were compensated by applying a flood gun, and binding energies were calibrated on the basis of positioning the adventitious carbon C 1s signal to 284.8 eV. Peak deconvolution was done using the Casa XPS software with Shirley-type backgrounds and a combination of GaussianeLorentzian functions and asymmetric Doniach Sunjictype functions for Rh3þ and Rh0 signals, respectively.
Photocatalytic H2 evolution Photocatalytic tests were carried out in a continuously purged stirred tank reactor as described in a previous study [34]. A nitrogen flow rate of 50 mL min1 was passed through the reactor during the experiment. 0.3 g catalyst were dispersed in an aqueous solution (600 mL) containing methanol (50 mL) as a sacrificial reagent. The reactant system was deaerated by bubbling N2 for 1 h before inner irradiation with a Hg immersion lamp (Peschl, 500 W). The light intensity was estimated to be 54 mW/cm2 by using a NOVA II PD300-UV radiation meter (OPHIR, Ltd.). The irradiation area was about 400 cm2. The H2 evolution rate was analyzed by an on-line thermal conductivity detector. In the case of photocatalytic tests with Rh co-catalyst, Rh was in situ photodeposited on the catalyst by adding an appropriate amount of Na3RhCl6$3H2O (Aldrich, 99.999%) to the reactant solution [34].
3
Fig. 1 e XRD patterns of TiO2, Sn(x)/TiO2 and Sn(1.5)/TiO2PM samples.
particles, demonstrating the highly dispersed state of tin species even at a high loading using the grafting method. The Sn(x)/TiO2 samples were also investigated by Raman spectroscopy, as shown in Fig. 2. All samples give six bands (144, 197, 399, 513, 519 and 639 cm1) consistent with the I41/ amd space group of anatase TiO2 [35]. In addition, no shift or broadening of Raman peaks is observed after the grafting of tin species, demonstrating that the crystallite size and structure of the parent TiO2 remained intact. Moreover, the main Raman bands of crystalline SnO2 (476, 635 and 776 cm1) [36] were undetectable, indicating that there was no phase separation of tin oxide. On the basis of XRD and Raman results, it is reasonable to assume that tin species are highly dispersed on the surface of TiO2. Additionally, based on N2 adsorption measurements, the grafting of tin species does not affect the BET surface area of the parent TiO2 to any appreciable extent.
Results and discussion Fig. 1 shows the XRD patterns of TiO2, Sn(x)/TiO2 and Sn(1.5)/ TiO2-PM prepared by physical mixing of TiO2 and SnO2. It is clearly seen that grafting tin species onto the TiO2 surface does not alter the anatase crystal structure of the parent TiO2. The XRD peaks of Sn(x)/TiO2 do not shift compared with the Sn-free sample, indicating that tin is not incorporated into the lattice. The average crystallite size of the TiO2 in all samples based on the (101) diffraction peak is estimated to be 15 ± 1 nm using the Scherrer equation. No additional phases related to SnO2 are observed for Sn/TiO2 as a result of the high dispersion of tin species or the low tin content. However, the physically mixed Sn(1.5)/TiO2-PM sample clearly shows three peaks of rutile SnO2 phase due to the presence of large
Fig. 2 e Raman spectra of TiO2 and Sn(x)/TiO2 samples.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 3 e UVeVis absorption spectra of TiO2 and Sn(x)/TiO2 samples.
Fig. 3 shows the UVeVis absorption spectra of Sn(x)/TiO2. No obvious change in the light absorbance of TiO2 is observed after the surface modification with tin species. This is in accordance with theoretical calculation results of molecularsized SnO2 clusters adsorbed on anatase surfaces [37], indicating tin oxide derived states do not appear inside the TiO2 energy gap and thus lead to no shift of light absorption. However, it cannot be excluded that the tin-related electronic states may be close to the CB of TiO2 and thus do not exert a large influence on the optical absorption of TiO2 [21], which is in line with the low tin content. The photocatalytic H2 evolution activity was evaluated in aqueous methanol solutions. Fig. 4(A) shows the timedependent photocatalytic H2 evolution rate of TiO2 and Sn(x)/TiO2. It is found that the H2 evolution rate increases rapidly after switching on the light and then reaches a relatively stable state after about 30 min of irradiation. To better compare the photocatalytic activity, the average H2 evolution rate during the irradiation was plotted as a function of the grafted tin content, as shown in Fig. 4(B). The average H2 evolution rate was determined by dividing the total H2 evolution amount (calculated by the integral area) by the irradiation time. It can be clearly seen that there is an
approximately linear increase of H2 evolution rate with increasing tin content at low loading (<0.5 nm2), which is considered as a consequence of the increasing amount of isolated tin species. Taking into account the rather similar properties of crystallinity, surface area and light absorption for all samples, we infer that the increased photocatalytic activity is ascribed to the improved charge separation caused by the grafted tin species. Upon light excitation, electrons in the VB of TiO2 can be promoted to surface tin(IV) species, either directly via IFCT or indirectly from the CB of TiO2, both resulting in the separation of electrons and holes. Subsequently, the electrons in the tin states reduce protons to produce H2, while the holes in TiO2 oxidize methanol to CO2 via the formation of intermediates such as formaldehyde and formic acid [38]. In the content range of 0.5e1.5 nm2, the H2 evolution rate slightly decreases. It is inferred that excess Sn loading would not lead to the further increase of isolated tin species, but the formation of aggregated tin species such as SnO2 nanoclusters. The aggregation of tin species would cause a downshift of the lowest unoccupied molecular orbital (LUMO) energy level, decreasing the reducing power of photogenerated electrons for H2 evolution [22]. Similarly, Gu et al. [39] recently found that single-site tin species enhance the separation of photogenerated charge carriers and thus photocatalytic efficiency, while aggregated tin species do not. Investigation of the fate of photoinduced electronehole pairs in semiconductor particles can provide some guidelines to explain the photocatalytic reactivity of photocatalysts. PL spectroscopy is a powerful technique for these studies, since the location and intensity of PL peaks can reflect the type and relative amount of recombination centers, respectively [40]. The samples were excited at 300 nm and PL emission bands were monitored in the range of 340e560 nm as shown in Fig. 5. The emission peak around 456 nm is associated with shallow traps of TiO2 related to oxygen vacancies, which are reported 0e1 eV below the CB of TiO2 [41]. The decrease in the intensity of this peak indicates the decreased electronehole recombination, leading to higher photocatalytic efficiency. It is noted that the intensity markedly decreases after grafting tin species with a low tin content (<0.5 nm2) and slightly increases in the content range of 0.5e1.5 nm2, which is in good agreement with the photocatalytic activity results reported above. Thus, the separation efficiency of photogenerated charge carriers seems to play a dominant role in the photocatalytic
Fig. 4 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with TiO2 and Sn(x)/TiO2 samples. (B) Average H2 evolution rate as a function of tin content. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 5 e PL spectra of TiO2 and Sn(x)/TiO2 samples.
performance. It is speculated that the unoccupied Sn 5s states of tin species act as preferential electron acceptors compared to surface oxygen vacancies to promote charge separation. Without tin modification, the excited electrons in TiO2 are rapidly trapped at the surface oxygen vacancies to undergo recombination with the holes. The emission band at about 406 nm is attributed to self-trapped excitons located on the TiO6 octahedron [42]. This is an intrinsic property of bulk TiO2, which is not changed during the synthesis because grafting is a surface modification method. It is widely accepted that noble metals such as Rh and Pt can facilitate the charge separation and act as efficient H2 evolution sites with a low overpotential [43,44]. Thus, we further investigated the photocatalytic H2 evolution activity with 0.3 wt% Rh as co-catalyst obtained by in situ photodeposition. The deposition of Rh on Sn(x)/TiO2 is confirmed by the combination of TEM and XPS measurements on a typical Sn(0.5)/TiO2 sample, as shown in Fig. 6. TEM image shows that the Rh particles are well-dispersed after the photodeposition. The particle size of primary Rh particles is smaller than 5 nm. According to XPS, Rh is present in two valence states: Rh0 and
5
Rh3þ. The binding energy (BE) of Rh 3d5/2 at 306.7 eV is attributed to metallic Rh, and the BE of Rh 3d5/2 at 308.3 eV corresponds to the Rh3þ valence state [45,46]. Rhodium is mostly present as Rh0 and the Rh0/Rh3þ ratio is about 2.4. It is proposed that the Rh3þ valence state is present as Rh2O3, because the BE value is approximately equal to the peak energy reported for Rh2O3 (308.5 eV) [45]. In addition, no signal of Cl is detected in the XPS survey spectrum. The photocatalytic H2 evolution rate of Sn(x)/TiO2 with 0.3 wt% Rh as co-catalyst is shown in Fig. 7(A). It can be seen that the addition of Rh co-catalyst promoted H2 evolution activity by about two orders of magnitude with a maximum value of about 11 mmol h1 for Sn(0.5)/TiO2. The H2 evolution rate displays a slight decrease after reaching a maximum, presumably due to the accumulation of reaction intermediates on the catalyst surface considering the high-rate H2 evolution [34]. Simultaneously, the signal of the reaction product CO2 is observed by an IR detector, but in much lower amounts of only 3% relative to the amount of the product H2 (not shown). This indicates that some of the methanol is fully oxidized to CO2, while the major part may form intermediates such as formaldehyde and formic acid [38]. For a better comparison, the average H2 evolution rate during the irradiation was plotted as a function of the tin content as shown in Fig. 7(B). The plot of H2 evolution rate against tin content exhibits a volcano-type curve with a maximum at 0.5 nm2. The enhanced activity at a low tin content (<0.5 nm2) is ascribed to the increase in the amount of isolated tin species, which are supposed to act as a promoter for the electron transfer from TiO2 to Rh. With a tin content of 1.5 nm2, the H2 evolution activity is the lowest among the samples, which is different from that without Rh co-catalyst. It is proposed that Rh deposited on aggregated tin sites is less active for H2 evolution than that on bare TiO2. Thus, it is assumed that the order of H2 evolution activity for different Rh deposition sites is as follows: isolated tin species > TiO2 > aggregated tin species, which will be discussed in detail below. To better compare the H2 evolution activity of different Rh deposition sites, we intentionally designed an additional experiment to deposit a very low amount of Rh on Sn(x)/TiO2 and investigated the H2 evolution activity. For a high Rh loading, Rh is likely to be deposited on all possible reduction
Fig. 6 e (A) TEM image and (B) Rh 3d XPS of 0.3 wt% Rh-loaded Sn(0.5)/TiO2 sample. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 7 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with 0.3 wt% Rh-loaded TiO2 and Sn(x)/TiO2 samples. (B) Average H2 evolution rate as a function of tin content.
sites, whereas for a very low Rh loading, Rh is expected to be mainly deposited on preferential sites. Fig. 8(A) shows the photocatalytic H2 evolution rate of Sn(x)/TiO2 with 0.01 wt% Rh as co-catalyst. In this circumstance, even with the lowest tin loading (0.1 nm2), the Sn/Rh molar ratio is about six, which ensures all Rh could be deposited exclusively on tin sites. Compared with the other samples, Sn(0.3)/TiO2 and Sn(0.5)/TiO2 display a different shape of the curves, with H2 evolution rates decreasing significantly after reaching a maximum. It is assumed that the high H2 evolution rates of samples with a low tin content are likely to cause the large accumulation of reaction intermediates. However, for Sn(0.1)/ TiO2, due to the lowest tin coverage, there is significant free TiO2 surface area available for photooxidation of reaction intermediates resulting in a much more stable rate of H2 evolution. For a better comparison, the average H2 evolution rate during the irradiation as a function of tin content was also plotted as shown in Fig. 8(B). With 0.01 wt% Rh as co-catalyst, it is found that Sn(0.1)/TiO2 exhibits the highest average H2 evolution rate (1.72 mmol h1), which is about 1.64 times higher than that of tin-free TiO2 (1.05 mmol h1). With increasing tin content, the H2 evolution rate remains relatively stable in the region of low tin content (<0.5 nm2), but then sharply drops when exceeding 0.5 nm2. With a tin content of 1.5 nm2, the H2 evolution activity is 0.52 mmol h1, which is
only half that of tin-free TiO2. As a comparison, the H2 evolution activity of Sn(1.5)/TiO2-PM sample is also tested. In comparison to the Sn(1.5)/TiO2 sample, Sn(1.5)/TiO2-PM shows a more similar behavior of photocatalytic performance to that of bare TiO2, indicating that a larger proportion of Rh is deposited on TiO2. This is not unexpected considering that TiO2 absorbs more light than the small amount of SnO2 to produce photoexcited electrons, and there is no apparent electron transfer between TiO2 and SnO2 in the physically mixed sample. On the other hand, the lower H2 evolution activity of Sn(1.5)/TiO2 compared to Sn(1.5)/TiO2-PM indicates that the electrons captured at aggregated tin species are inactive with respect to the H2 evolution reaction. This is ascribed to the location of the empty electronic states of aggregated tin species. Similar to the conduction band of bulk SnO2, they are located at a too positive potential to allow for H2 evolution to occur. Besides, we also calculate the ratio of H2 evolution rate of Sn(x)/TiO2 with bare TiO2 for the high Rh loading (0.3 wt%) for comparation. In such case, the H2 evolution rate of Sn(0.1)/TiO2 is only 1.02 times greater than that of tin-free TiO2 and the activity of Sn(1.5)/TiO2 is 77% that of tin-free TiO2. Undoubtedly, the activity differences among samples with low Rh loading are more significant than those with high Rh loading, which is attributed to the different Rh deposition sites and distributions as depicted in Fig. 9(A). For a
Fig. 8 e (A) Time course of photocatalytic H2 evolution rate from aqueous methanol solutions with 0.01 wt% Rh-loaded TiO2, Sn(x)/TiO2 and Sn(1.5)/TiO2-PM samples. (B) Average H2 evolution rate as a function of tin content. The black circle represents the Sn(1.5)/TiO2-PM sample. Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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Fig. 9 e (A) Schematic picture of the location of Rh deposition sites on TiO2 and Sn(x)/TiO2 samples with different Rh loading amount. (B) Photocatalytic H2 evolution rate from aqueous methanol solutions with different samples. Inset: Schematic picture of photocatalytic H2 evolution process over 0.01 wt% Rh-loaded Sn(0.1)/TiO2 sample.
high Rh loading, a larger proportion of Rh is deposited on TiO2 compared to the low Rh loading, resulting in a much more similar behavior of photocatalytic performance with and without tin. In contrast, for a low Rh loading, a higher proportion of Rh is loaded on tin species, either of isolated or aggregated type. This is not unexpected considering tin species function as electron acceptors upon light illumination, which facilitate the reduction of Rh3þ to metallic Rh. Note that the redox potential of Rh3þ/Rh (0.758 V versus NHE at pH 0) [47] is much more positive than that of H2 evolution (0 V versus NHE at pH 0), so the photoreduction of Rh3þ is still thermodynamically favorable even with aggregated tin species. Interestingly, as long as the tin species are fully isolated, there is a remarkable synergetic enhancement of isolated tin species and Rh for H2 evolution activity over TiO2 as shown in Fig. 9(B). The H2 evolution enhancement of TiO2 co-modified with 0.1 nm2 Sn and 0.01 wt% Rh (increase in H2 evolution rate by 1.67 mmol h1 compared to bare TiO2) far exceeds the net subtotal of single species modification (increase by 0.01 and 1.00 mmol h1 for Sn and Rh, respectively). It is proposed that the synergy originates from the electron transfer cascades along TiO2, tin species and Rh as depicted in the inset of Fig. 9(B). The tin species seem to act as interfacial mediators for the promotion of electron transfer from TiO2 to Rh, consequently decreasing the charge recombination and enhancing the photocatalytic efficiency. It has to be pointed out that our model of the assumed action mode of tin is in good agreement with that observed by Gu et al. [48]. Considering that TiO2 absorbs more light than isolated tin species to produce photoexcited electrons, and that Rh is still preferably deposited on tin species rather than TiO2, it is deduced that the photoexcited electrons of TiO2 can transfer to isolated tin species and then reduce Rh3þ. Therefore, it is proposed that the LUMO position of isolated tin species is located between the minimum of the CB of TiO2 and the reduction potential of protons, which is, however, not yet experimentally confirmed. Because the CB edge of TiO2 is only about 0.2 eV higher than that of H2 evolution potential, it is most likely that the LUMO position of isolated tin species locates very close to the CB of TiO2, which is consolidated by the unchanged bandgap after tin grafting as shown in the UVeVis absorption
spectra. Moreover, Nolan et al. [21,22] also theoretically calculated the electronic structure of tin species adsorbed on the TiO2 surface, indicating that the tin-derived electronic states were very close to the CB edge of TiO2. In this way, the photocatalytic H2 evolution reaction can be utilized as probe reaction to investigate the preferential sites for Rh deposition and relative energy levels of tin species. From the H2 evolution results, we can further deduce that Rh deposited on isolated tin species is the most effective pathway for the reduction of protons to produce H2, presumably due to the appropriate LUMO energy level of isolated tin species for H2 evolution and efficient electron relay among TiO2, isolated tin species and photodeposited Rh nanoparticles. It is supposed that tin species act as electron mediators to open a new channel for the electron transfer from TiO2 to Rh in the form of isolated TieOeSn linkages interacting with the Rh nanoparticles.
Conclusions In summary, tin(IV) grafting can enhance the photocatalytic H2 evolution activity of TiO2 from aqueous methanol solutions due to improved charge separation. Isolated tin species promote the photocatalytic efficiency in this reaction, while aggregated tin species do not. After adding Rh as co-catalyst, the H2 evolution activity is greatly enhanced and there is a synergetic effect between isolated tin species and Rh on the photocatalytic activity of TiO2. Moreover, by using H2 evolution as probe reaction, it is found that tin species rather than TiO2 are preferential sites for Rh deposition and Rh deposited on isolated tin species is the most efficient pathway for H2 evolution. This work may provide guidance for the design of other ternary photocatalysts with efficient interfacial electron transfer using the synergy of dual co-catalysts.
Acknowledgments This work was funded by the German Ministry of Education and Research (BMBF) within the scope of the funding program
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
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“Technologies for Sustainability and Climate ProtectiondChemical Processes and Use of CO2”, project number 033RC1007A. The authors are grateful to Georgios Dodekatos and Dr. Harun Tu¨ysu¨z at Max-Planck-Institut fu¨r Kohlenforschung (MPIK) for the investigation of the sample by TEM. Philipp Weide is also gratefully acknowledged for the XPS measurements. The authors also appreciate the help of Vera Singer for the grafting experiment, and thank Dr. Bastian Mei and Anna Pougin for helpful discussions.
references
[1] Chen XB, Shen SH, Guo LJ, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503e70. [2] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [3] Liu G, Wang L, Yang HG, Cheng HM, Lu GQ. Titania-based photocatalystsdcrystal growth, doping and heterostructuring. J Mater Chem 2010;20:831e43. [4] Nakata K, Fujishima A. TiO2 photocatalysis: design and applications. J Photochem Photobiol C 2012;13:169e89. [5] Pulido Melian E, Gonzalez Diaz O, Ortega Mendez A, Lopez Cristina R, Nereida Suarez M, Dona Rodriguez JM, et al. Efficient and affordable hydrogen production by water photo-splitting using TiO2-based photocatalysts. Int J Hydrogen Energy 2013;38:2144e55. [6] Pulido Melian E, Lopez Cristina R, Ortega Mendez A, Gonzalez Diaz O, Nereida Suarez M, Dona Rodriguez JM, et al. Hydrogen production using Pt-loaded TiO2 photocatalysts. Int J Hydrogen Energy 2013;38:11737e48. [7] Xu MC, Gao YK, Moreno EM, Kunst M, Muhler M, Wang YM, et al. Photocatalytic activity of bulk TiO2 anatase and rutile single crystals using infrared absorption spectroscopy. Phys Rev Lett 2011;106:138302. [8] Bessekhouad Y, Robert D, Weber JV. Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions. Catal Today 2005;101:315e21. [9] Kim DW, Lee S, Jung HS, Kim JY, Shin H, Hong KS. Effects of heterojunction on photoelectrocatalytic properties of ZnOeTiO2 films. Int J Hydrogen Energy 2007;32:3137e40. [10] Chu S, Zheng XM, Kong F, Wu GH, Luo LL, Guo Y, et al. Architecture of Cu2O@TiO2 coreeshell heterojunction and photodegradation for 4-nitrophenol under simulated sunlight irradiation. Mater Chem Phys 2011;129:1184e8. [11] Wang Q, An N, Bai Y, Hang H, Li J, Lu X, et al. High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2. Int J Hydrogen Energy 2013;38:10739e45. [12] Tada H, Hattori A, Tokihisa Y, Imai K, Tohge N, Ito S. A patterned-TiO2/SnO2 bilayer type photocatalyst. J Phys Chem B 2000;104:4585e7. [13] Hou LR, Yuan CZ, Peng Y. Synthesis and photocatalytic property of SnO2/TiO2 nanotubes composites. J Hazard Mater 2007;139:310e5. [14] Sasikala R, Shirole A, Sudarsan V, Sakuntala T, Sudakar C, Naik R, et al. Highly dispersed phase of SnO2 on TiO2 nanoparticles synthesized by polyol-mediated route: photocatalytic activity for hydrogen generation. Int J Hydrogen Energy 2009;34:3621e30. [15] Lee SS, Bai H, Liu Z, Sun DD. Electrospun TiO2/SnO2 nanofibers with innovative structure and chemical properties for highly efficient photocatalytic H2 generation. Int J Hydrogen Energy 2012;37:10575e84.
[16] Murakami N, Chiyoya T, Tsubota T, Ohno T. Switching redox site of photocatalytic reaction on titanium (IV) oxide particles modified with transition-metal ion controlled by irradiation wavelength. Appl Catal A 2008;348:148e52. [17] Yu HG, Irie H, Shimodaira Y, Hosogi Y, Kuroda Y, Miyauchi M, et al. An efficient visible-light-sensitive Fe(III)-grafted TiO2 photocatalyst. J Phys Chem C 2010;114:16481e7. [18] Liu M, Qiu XQ, Miyauchi M, Hashimoto K. Cu(II) oxide amorphous nanoclusters grafted Ti3þ self-doped TiO2: an efficient visible light photocatalyst. Chem Mater 2011;23:5282e6. [19] Libera JA, Elam JW, Sather NF, Rajh T, Dimitrijevic NM. Iron(III)-oxo centers on TiO2 for visible-light photocatalysis. Chem Mater 2010;22:409e13. [20] Tada H, Jin QL, Nishijima H, Yamamoto H, Fujishima M, Okuoka S, et al. Titanium(IV) dioxide surface-modified with iron oxide as a visible light photocatalyst. Angew Chem Int Ed 2011;50:3501e5. [21] Nolan M. Surface modification of TiO2 with metal oxide nanoclusters: a route to composite photocatalytic materials. Chem Commun 2011;47:8617e9. [22] Fujishima M, Jin QL, Yamamoto H, Tada H, Nolan M. Tin oxide-surface modified anatase titanium(IV) dioxide with enhanced UV-light photocatalytic activity. Phys Chem Chem Phys 2012;14:705e11. [23] Gao X, Bare SR, Fierro JLG, Banares MA, Wachs IE. Preparation and in-Situ spectroscopic characterization of molecularly dispersed titanium oxide on silica. J Phys Chem B 1998;102:5653e66. [24] Strunk J, Vining WC, Bell AT. A study of oxygen vacancy formation and annihilation in submonolayer coverages of TiO2 dispersed on MCM-48. J Phys Chem C 2010;114:16937e45. [25] Mei B, Becerikli A, Pougin A, Heeskens D, Sinev I, Gru¨nert W, et al. Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide. J Phys Chem C 2012;116:14318e27. [26] An DL, Zhang QH, Wang Y. Copper grafted on SBA-15 as efficient catalyst for the selective oxidation of methane by oxygen. Catal Today 2010;157:143e8. [27] Ross-Medgaarden EI, Wachs IE, Knowles WV, Burrows A, Kiely CJ, Wong MS. Tuning the electronic and molecular structures of catalytic active sites with titania nanoligands. J Am Chem Soc 2009;131:680e7. € cker R, Wachs IE. [28] Carrero CA, Keturakis CJ, Orrego A, Schoma Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor. Dalton Trans 2013;42:12644e53. [29] Strunk J, Vining WC, Bell AT. Synthesis of different CeO2 structures on mesoporous silica and characterization of their reduction properties. J Phys Chem C 2011;115:4114e26. [30] Matsuoka M, Anpo M. Local structures, excited states, and photocatalytic reactivities of “single-site” Ti-oxide photocatalysts constructed within zeolites or mesoporous materials. In: Anpo M, Kamat PV, editors. Environmentally benign photocatalysts. New York: Springer ScienceþBusiness Media; 2010. p. 218. [31] Ahn HS, Yano J, Tilley TD. Photocatalytic water oxidation by very small cobalt domains on a silica surface. Energy Environ Sci 2013;6:3080e7. [32] Wang XX, Xu HB, Fu XZ, Liu P, Lefebvre F, Basset JM. Characterization and catalytic properties of tin-containing mesoporous silicas prepared by different methods. J Mol Catal A 2005;238:185e91. [33] Oropeza FE, Mei B, Sinev I, Becerikli AE, Muhler M, Strunk J. Effect of Sn surface states on the photocatalytic activity of anatase TiO2. Appl Catal B 2013;140e141:51e9.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[34] Busser GW, Mei B, Muhler M. Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions. ChemSusChem 2012;5:2200e6. [35] Swamy V, Kuznetsov A, Dubrovinsky LS, Caruso RA, Shchukin DG, Muddle BC. Finite-size and pressure effects on the Raman spectrum of nanocrystalline anatase TiO2. Phys Rev B 2005;71:184302. [36] Leonardy A, Hung WZ, Tsai DS, Chou CC, Huang YS. Structural features of SnO2 nanowires and Raman spectroscopy analysis. Cryst Growth Des 2009;9:3958e63. [37] Nolan M, Iwaszuk A, Tada H. Molecular metal oxide clustersurface modified titanium(IV) dioxide photocatalysts. Aust J Chem 2012;65:624e32. [38] Chiarello GL, Aguirre MH, Selli E. Hydrogen production by photocatalytic stream reforming of methanol on noble metal-modified TiO2. J Catal 2010;273:182e90. [39] Gu Q, Long JL, Zhou YG, Yuan RS, Lin HX, Wang XX. Singlesite Tin-grafted anatase TiO2 for photocatalytic hydrogen production: toward understanding the nature of interfacial molecular junctions formed in semiconducting composite photocatalysts. J Catal 2012;289:88e99. [40] Anpo M, Che M. Applications of photoluminescence techniques to the characterization of solid surfaces in relation to adsorption, catalysis, and photocatalysis. Adv Catal 1999;44:119e257. [41] Serpone N, Lawless D, Khairutdinov R. Size effects on the photophysical properties of colloidal anatase TiO2 particles:
[42]
[43]
[44]
[45]
[46]
[47]
[48]
9
size quantization versus direct transitions in this indirect semiconductor? J Phys Chem 1995;99:16646e54. Song XF, Gao L. Synthesis, characterization, and optical properties of well-defined N-doped, hollow silica/titania hybrid microspheres. Langmuir 2007;23:11850e6. Tada H, Kiyonagaa T, Naya SI. Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(IV) dioxide. Chem Soc Rev 2009;38:1849e58. Merka O, Bahnemann DW, Wark M. Improved photocatalytic hydrogen production by structure optimized nonstoichiometric Y2Ti2O7. ChemCatChem 2012;4:1819e27. Larichev YV, Netskina OV, Komova OV, Simagina VI. Comparative XPS study of Rh/Al2O3 and Rh/TiO2 as catalysts for NaBH4 hydrolysis. Int J Hydrogen Energy 2010;35:6501e7. Yamamoto A, Ohyama J, Teramura K, Shishido T, Tanaka T. Effect of a crystalline phase of TiO2 photocatalysts on the photodeposition of Rh metal nanoparticles. Catal Today 2014;232:165e70. Gomathisankar P, Hachisuka K, Katsumata H, Suzuki T, Funasaka K, Kaneco S. Enhanced photocatalytic hydrogen production from aqueous methanol solution using ZnO with simultaneous photodeposition of Cu. Int J Hydrogen Energy 2013;38:11840e6. Gu Q, Long JL, Zhuang HQ, Zhang CQ, Zhou YG, Wang XX. Ternary Pt/SnOx/TiO2 photocatalysts for hydrogen production: consequence of Pt sites for synergy of dual cocatalysts. Phys Chem Chem Phys 2014;16:12521e34.
Please cite this article in press as: Chu S, et al., Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.09.103