Advanced Charge Utilization from NaTaO3 Photocatalysts by

Subscriber access provided by MEMORIAL UNIV

Article

Advanced charge utilization from NaTaO3 photocatalysts by multilayer reduced graphene oxide Tobias Meyer, Jacqueline B Priebe, Rafael O. da Silva, Tim Peppel, Henrik Junge, Matthias Beller, Angelika Brückner, and Sebastian Wohlrab Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm500949x • Publication Date (Web): 22 Jul 2014 Downloaded from http://pubs.acs.org on August 3, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Advanced charge utilization from NaTaO3 photocatalysts by multilayer reduced graphene oxide Tobias Meyer,† Jacqueline B. Priebe,† Rafael O. da Silva, † Tim Peppel, Henrik Junge, Matthias Beller, Angelika Brückner, and Sebastian Wohlrab* Leibniz Institute for Catalysis at the University of Rostock, A.-Einstein-Str. 29a, 18059 Rostock, Germany ABSTRACT: NaTaO3 prepared by i) a solid state reaction (SSR) with crystallite sizes between 1–5 µm and ii) by an exotemplate method (EM) with crystallite sizes of about 25 nm were tested as photocatalysts in light-driven water splitting. NaTaO3(EM) showed a 18 times higher photocatalytic hydrogen evolution rate than NaTaO3(SSR). A further improvement by a factor of 28 was achieved by mixing the EM derived material with multilayer reduced graphene oxide (m-rGO). Moreover, by depositing 0.2 wt.% Au on the surface of NaTaO3(EM), the hydrogen production efficiency has been increased by a factor of 41. Surprisingly, the hydrogen production rate could not be significantly improved with NaTaO3(SSR) under the same conditions. By using in situ EPR spectroscopy, the electronic interactions between semiconductor, co-catalyst and m-rGO have been investigated in detail. Plausible electron separation and transfer mechanisms from NaTaO3(SSR) or NaTaO3(EM), respectively, to m-rGO are discussed and compared to the catalytic testing results.

1.

Introduction

As a potential energy carrier hydrogen exhibits a high heat of combustion (ΔHC,H2 = 285.8 kJ∙mol-1) while it is converted to water being a clean exhaust.1 Efficient photocatalytic water splitting is a desirable way to produce hydrogen as a sustainable fuel. Among the wide variety of inorganic photocatalysts being developed in recent years, oxides are the most stable class of compounds for photocatalytic water splitting and potentially applicable for long term purposes. They are most often based on compounds with d0 ions (TiIV, ZrIV, NbV or TaV).2-4 During the last decades, considerable efforts have been undertaken to improve the photocatalytic performance of these materials employing several methods, such as noble metal loading5, 6, metal ion doping7, 8, anion doping9, 10, heterostructuring11, composite preparation12-14, dye sensitization15, 16, and carbon nanomaterial introduction.17 However, there are still serious drawbacks that need to be overcome to enable efficient and economically feasible water splitting, which are mainly related to fast recombination rates of the photogenerated electron-hole pairs, narrow band gaps and poor adsorption capacities. Thus, for developing photocatalysts with improved performance it is on the one hand necessary to suppress the fast recombination of photogenerated charge carriers by separation of the electron-hole pairs and/or by providing highly active uptaking sites. On the other hand, suitable spectroscopic studies are needed to elucidate relations between electronic and structural properties of the catalysts as a basis for a more rational development of them. As we

have shown recently, in situ EPR spectroscopy is an ideal tool to directly visualize electron transfer from deposited gold nanoparticles (Au-NP) to a TiO2 semiconductor support and vice versa, depending on whether visible or UV light is used for optical excitation.18 Among the available carbon materials, graphene or graphene oxide, ultrathin 2D networks composed of sp2hybridized conjugated carbon atoms, are emerging candidates for enhancing the activity of photocatalysts because of their good electrical conductivities and high surface areas.19-21 These properties make them ideal electron shuttles and 2D supports for more active and stable photocatalysts. Examples in which such materials were used to enhance the photocatalytic activity of common semiconductors have been already described in the literature.22, 23 However, the large scale production of such carbon materials and the decoration with oxide semiconductors is still a challenging problem. Unfortunately, metal oxide syntheses often require higher temperatures which do not allow the parallel addition of preformed carbon structures due to thermal stability reasons. Therefore, solution mixing of the photocatalyst together with the carbon additive is in general the method of choice.21 In this work, we have developed a dedicated semiconductor synthesis using an easy available multilayer reduced graphene oxide (m-rGO). NaTaO3 being one of the most active tantalum-based photocatalysts24 was synthesized using a facile exotemplate method (EM) which yields nanoparticles much more active than those prepared by solid state

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reaction (SSR). Furthermore, reasons for the activity enhancement in NaTaO3(EM) catalysts by loading with Au and addition with m-rGO were studied. So far, only in a few publications the formation of photo-induced charge carriers in tantalate systems25-27 has been described and in none of them their behavior under the conditions of photocatalytic water reduction was analyzed. Therefore, in situ EPR spectroscopy was used to investigate the formation of light-induced paramagnetic species within the NaTaO3 semiconducting systems prepared by EM and SSR. This information helps to understand the different photocatalytic behavior of derived solution mixtures together with m-rGO.

2.

Experimental

2.1 Synthesis of materials NaTaO3(EM) was prepared via an adapted exotemplate method as described earlier.28 To a solution of 0.75 g NaNO3 (8.80 mmol, p.a., Merck, Germany) in 50 ml deionized water 11.4 ml of a Ta2(C2O4)5-solution (8.80 mmol, cTa = 0.77 mmol∙ml-1 H.C. Starck, Germany), 3.0 g of polyvinyl alcohol (98%, Mn = 72000 g∙mol-1, Roth, Germany) and 30.0 g of D-sucrose (99.7%, Roth, Germany) were added under stirring at 368 K. After complete dissolution the honey-like highly viscous mixture was heated at 493 K for 1 h to form carbonaceous precursor foam, from which the nanoparticles were produced by calcination at 773 K for 20 h. NaTaO3(SSR) was prepared using a solid state reaction route described by Kato and Kudo.29 Briefly, a stoichiometric mixture of 1.06 g Na2CO3 (9.96 mmol, p.a., Merck, Germany) and 4.40 g Ta2O5 (9.96 mmol, 99.99%, ChemPur, Germany) was well ground in an agate ball mill with 200 rpm for 24 h. For better distribution 20 ml of ethanol were added before milling and removed afterwards in a drying oven at 323 K. Finally, the mixture was thermally treated in a Pt-crucible at 1420 K for 10 h with a heating rate of 10 K∙min-1. Graphite powder (Sigma-Aldrich, particle size 1 µm with roundish shape and smooth surface, grown together and building aggregates. NaTaO3(EM) consists of aggregates of particles with different shapes in the nanometer range (in agreement with the calculated crystallite sizes from the respective XRD pattern). The aggregation of nanoparticles is evidenced by COMPO SEM images in Fig. SI-1. Figure 3a and b show HRTEM images of the primary NaTaO3(EM) nanoparticles building such aggregates. The sizes of the crystallites (20–25 nm) agree well with the values calculated from the powder XRD pattern. From these images lattice fringes of 3.9 Å were measured which correspond to the (101) lattice spacing. In the inserts the selected area electron diffraction (SAED) pattern are depicted which correspond to the diffraction pattern from the XRD measurements. The nanoparticles have an uneven surface seen at the rough edges of the crystals. Several of the nanoparticles build up bigger agglomerates as shown in Figure 3c and d, respectively.

Figure 3. HRTEM images of NaTaO3(EM) nanoparticles (a, b) with inserts of SAED patterns and TEM images of NaTaO3(EM) aggregates (c, d).

In Figure 4 photochemically deposited Au-NP (0.2 wt.%) on NaTaO3 from SSR and EM are shown in TEM images. On both materials the size of the evenly distributed Au-NP is in the range of 2–10 nm. Because of the lower surface area of NaTaO3(SSR) the spatial distribution of Au-NP and the variation of the particle size are higher than on the material from the EM synthesis comparable to results reported by Chan et al. on Ag/TiO2 and Au/TiO2.34 The shape of the Au-NPs is hemispherical while they show polycrystallinity.

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 9

Figure 5. a) Powder XRD pattern of graphite, multilayer graphene oxide (m-GO) and multilayer reduced graphene oxide (m-rGO); b) ATR-FT-IR-spectra of m-GO and m-rGO.

Figure 4. TEM image of photo deposited 0.2 wt.% Au-NP on NaTaO3 prepared by SSR (a, b) and EM (c, d).

XRD patterns of the graphite source, multilayer graphene oxide (m-GO) and m-rGO are shown in Figure 5a. The main reflection of graphite at 2θ = 26.5° belongs to the (002) plane with an interlayer spacing of 3.4 Å. The patterns of m-GO and m-rGO show a significant reflection at 2θ ~ 11.2° which corresponds also to the (002) plane but with a interlayer spacing of 7.9 Å which can be attributed to water incorporation between carbon sheets.35-39

In Figure 5b the ATR-FT-IR-spectra of m-GO and m-rGO are depicted. The vibration band of m-GO at νOH = 3415 cm-1 corresponds to the O-H stretching mode and disappears completely after reduction. The alkoxy (νC-O = 1055 cm-1) and epoxy (νC-O = 1229 cm-1) groups are also gone during the reduction of m-GO. The stretching band of the carboxyl bond (νC=O = 1732 cm-1) is still visible in the sample of m-rGO which shows that there are still some oxygen bearing functional groups. The vibration band of the conjugated double bond νC=C = 1625 cm-1 shifts in the spectrum of m-rGO to 1561 cm-1 as shown elsewhere by means of the reduction of graphene oxide.40 This shows that the main C=C bond of the graphene sheets remains intact. The elemental analysis of m-GO shows a C-content of 54.77%; an H-content of 1.81%, an N-content of 0.12% and an O-content of 43.3%. After chemical reduction of m-GO the O- and H-content decreases to 30.1% and 1.4%, respectively. In contrast, the C-content increases to 68.4% after reducing the m-GO, which relates to less functionalization of the carbon sheets. XPS investigations of m-rGO (Fig. SI-2) corroborates the main functional groups observed by FTIR. For instance, the C 1s peaks can be assigned to the (C-C) nonoxygenated C ring, C-O bonds, carbonyl bonds (C=O) and carboxylate carbon (C-O=O).41 It further showed significant lower oxygen content of 14.8% being present at the m-rGO surface probably due to a preferred surface reduction of the former m-rGO (Fig. SI-2 and Table SI-1). TEM investigations on dried m-rGO revealed that the single sheets are in the size range of 50-200 nm (Figure 6a, b; aggregation due to drying) with thicknesses of 5–10 nm consisting of multiple graphene layers (Figure 6c, d). Due to the curved architecture of the sheets only nanoparticles are able to access the surface efficiently. The BET surface area for m-rGO was 330 m2/g. Diffuse reflectance ultraviolet-visible spectroscopic analysis was conducted for both NaTaO3 samples. Figure SI-3 shows the absorbance and the converted reflection spectra using the Kubelka-Munk method. The direct band


Page 5 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gap of NaTaO3(EM) (3.92 eV) is slightly higher than for NaTaO3(SSR) (3.84 eV) owing to a less number of electronic states of the nanoparticles. The addition of mrGO and Au does not significantly affect the UV-vis absorption of NaTaO3(EM).

Figure 7. Photocatalytic activity of prepared materials.

Figure 8 depicts a comparison of the EPR spectra of the pure NaTaO3 samples prepared by EM (blue) and SSR (black) irradiated with UV-vis light under air, flowing helium as well as in a H2O/MeOH-saturated helium flow.

Figure 6. TEM images of multilayer reduced graphene oxide (m-rGO).

Hydrogen evolution rates over the pure semiconductor materials and over those containing the Au co-catalyst and m-rGO in solution mixture are illustrated in Figure 7. It is known that most tantalum oxides can perform photocatalytic water splitting without co-catalysts.42 Without additional co-catalyst, NaTaO3(SSR) reaches a H2 evolution rate of 0.34 mmol∙h-1∙g-1, while NaTaO3(EM) shows a significant higher activity of 5.96 mmol∙h-1∙g-1. The activity increase by a factor of 17.5 correlates to the 16 fold higher surface area of the EM derived sodium tantalate. When 0.2 wt.-% Au is deposited onto the semiconductors, the activity rises to 10.06 mmol∙h-1∙g-1 and 0.89 mmol∙h-1∙g-1 for NaTaO3(EM) and NaTaO3(SSR), respectively. After adding m-rGO to NaTaO3(EM), the H2 evolution rate reached 9.50 mmol∙h-1∙g-1 without cocatalyst and was further improved with photodeposited Au-NP to a value of 13.78 mmol∙h-1∙g-1. In the case of NaTaO3(SSR) the introduction of m-rGO has no effect on the catalytic performance and the H2 evolution rate remains low at 0.29 mmol∙h-1∙g-1. After depositing Au onto this material, the H2 evolution rate increased slightly (0.38 mmol∙h-1∙g-1) but is still lower than that of the Au/NaTaO3(SSR) catalyst without m-rGO.

Figure 8. Experimental (solid lines) and simulated (dashed line) in situ EPR spectra during irradiation with UV-vis light of NaTaO3(EM) (blue) and NaTaO3(SSR) (black) (a) in ambient air (b) under helium flow and (c) under a H2O/MeOH saturated helium flow.

In the presence of ambient air which contains paramagnetic O2, EPR signals of surface-bound paramagnetic species are usually broadened beyond detection due to their dipolar interaction with gas-phase oxygen. However, two EPR signals (Table 1, centers A and B, for spectra simulation see Fig. SI-4) are visible in NaTaO3(EM) under irradiation in ambient atmosphere at room temperature (Figure 8a), while no light-induced species could be detected for NaTaO3(SSR) under these conditions. Thus, centers A and B might be located in the subsurface and/or in the catalyst bulk because they are not affected by gas-phase oxygen. Since irradiation has no influence on the intensity of signal A (Fig. SI-5), we assign



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

it to F centers, i.e. to electrons trapped in oxygen vacancies similar to those observed in other metal oxides.43 Table 1. EPR parameters derived by spectra simulation and corresponding signal assignment for the in situ EPR spectra of the NaTaO3 samples during irradiation in helium (Figure 8a, dashed line, Fig. 9cd). EPR parameters Center

Assignment

g1

g2

g3

Ref.

A

F centers

2.003

2.003

2.003

43

B

Sub-surface oxygen radical O•−

2.013

2.013

1.999

a

C

surface superoxide radical O2•−

2.038

2.015

1.999

a

D

surface species

oxygen

2.026

2.015

1.990

a

E

surface species

oxygen

2.033

2.024

2.014

a

F

carbon-based localized conduction electrons

2.003

2.003

2.003

44

a

This work

In contrast, signal B increases strongly under UV-vis light excitation (Fig. SI-5 d). In general, irradiation with light of energy equal to or larger than the band gap of the semiconductor should lead to charge carrier separation and two paramagnetic species may be expected: electrons excited into the conduction band from which they can be trapped at metal centers such as Ta5+ (ecb– + Ta5+ → Ta4+) as well as positive holes in the valence band, which can react with lattice oxide species (h+ + O2– → O•–). As known from other works27, 45, Ta4+ is not detectable at room temperature (290 K). Thus, we assign signal B to trapped holes (O•– species), which are not located on the outermost NaTaO3 surface but in subsurface layers reached by light radiation. Obviously, the concentration of light-induced charge carriers is higher in the sample prepared by EM than in the sample obtained by SSR. This may be due, on the one hand, to the much higher irradiated surface area of the EM sample. On the other hand, fast recombination of charge carriers may be suppressed in NaTaO3(EM), due to the lower particle (Figure 4) and crystallite size (Figure 1). This might also be a reason for the higher photoactivity in H2 evolution.3 When air is replaced by helium flow, complex EPR spectra for both, the catalyst prepared by EM as well as by SSR are detected (Figure 8b). Spectrum simulation for NaTaO3(EM) revealed three additional species (In the presence of ambient air which contains paramagnetic O2, EPR signals of surface-bound paramagnetic species are usually broadened beyond detection due to their dipolar interaction with gas-phase oxygen. However, two EPR

Page 6 of 9

signals (Table 1, centers A and B, for spectra simulation see Fig. SI-4) are visible in NaTaO3(EM) under irradiation in ambient atmosphere at room temperature (Figure 8a), while no light-induced species could be detected for NaTaO3(SSR) under these conditions. Thus, centers A and B might be located in the subsurface and/or in the catalyst bulk because they are not affected by gas-phase oxygen. Since irradiation has no influence on the intensity of signal A (Fig. SI-5), we assign it to F centers, i.e. to electrons trapped in oxygen vacancies similar to those observed in other metal oxides.43 Table 1, Fig. SI-4, centers C, D and E) which seem to be also present in the NaTaO3(SSR) sample, the EPR spectrum of which shows the same signals but with lower intensities. Since these three signals appear only in the absence of oxygen, those species are most probably localized on the outermost catalyst surface. In order to further explore the nature of the surface-bound species (C, D, E), the pure Au-free NaTaO3(EM) catalyst was treated with aqueous hydrogen peroxide prior to spectra recording at 200 K (Fig. SI-6) as performed by other groups using TiO2.46 Apart from slight differences in line width and g1 component, it is obvious that the same species C appeared after H2O2-treatment on NaTaO3(EM) under irradiation. This indicates that signal C of the irradiated pure NaTaO3 arises from surface-bound superoxide radicals O2•– showing the characteristic line shape for these species bound at metal oxide surfaces.47 Such superoxide species O2•– are usually formed upon trapping of photo-excited electrons at adsorbed dioxygen (ecb– + O2 → O2•–).48, 49 However, when small amounts of gas-phase oxygen (1.5 %) were added to the helium flow, the EPR signals broadened immediately and the signal amplitude decreased as a consequence of dipolar interaction of paramagnetic gas-phase O2 with those surface O2•– species (Fig SI-7). This was confirmed, too by Howe et al. who could detect O2•– species on the surface of TiO2 only after evacuation.50 For proper reproduction of the experimental spectrum it was necessary to include two more signals (D and E) in the spectrum simulation. Most probably, these arise also from adsorbed paramagnetic oxygen radicals, yet with unknown nature. By saturating the helium flow with the reactants, H2O and MeOH, the signals attributed to surface-bound oxygen species (signals C, D and E) as well as to subsurface O•– radicals (signal B) strongly lost intensity. Remarkably, signals C, D and E already lost intensity upon addition of water alone without the hole scavenger MeOH, indicating that the corresponding surface oxygen radicals are either consumed by water molecules or their formation is prevented by addition of H2O (Fig. SI-8). Subsequent introduction of MeOH barely reduced the signals C, D and E further, but an intensity loss of signal B attributed to trapped holes at oxide anions (O•–) is noticeable. Since the H2 evolution rate of NaTaO3(EM) was enhanced by deposition of gold and/or m-rGO, we studied the impact of these additives on the EPR properties in order to explain this effect. For these


Page 7 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


studies, sample NaTaO3(EM) has been selected, since it showed the most pronounced enhancement of H2 production after adding Au and m-rGO. Interestingly, none of the signals B to E detected in pure NaTaO3(EM) was observed under the same conditions in the catalysts loaded with Au and/or m-rGO (Figure 9). The reason might be that deposition of co-catalysts such as Au or mrGO onto the NaTaO3 semiconductor surface enables an electron transfer of the photo-excited conduction band electrons of the NaTaO3 to these components with high electron affinity. This transfer is regarded as one reason for the enhanced photocatalytic activity, since it might further suppress the recombination of photo-separated charge carriers.51 Thus, instead of trapping the photoexcited electrons at adsorbed oxygen to form e.g. superoxide radicals such as center C, these electrons are transferred to the additives. When injected into the Au conduction band, such electrons are not detectable under ambient conditions.52 In this case, only defect centers reflected by signal A are seen which do not change upon irradiation in He.

electrons trapped in oxygen vacancies similar to those observed in other metal oxides.43 Table 1) at the same position as signal A (g = 2.003), yet with markedly higher intensity. It is assigned to localized conduction electrons of carbon-inherited defects (dangling bonds) in graphene planes of the m-rGO phase.44 A much larger intensity of signal F was observed in case of loading Au and m-rGO together onto NaTaO3(EM) (Figure 9d) resulting also in higher photocatalytic activity. This indicates that an electron transfer might be facilitated due to synergistic effects of co-loading with Au and m-rGO, which leads to such an intense signal attributed to carbon-based conduction electrons. Its intensity slightly decreased (~5 % of the original value) in H2O/MeOH/He flow under irradiation (Fig. SI-9) indicating a consumption of these electrons by the reactants. Here, the main intensity loss was as well provoked by the introduction of water rather than by methanol similar to the behavior of signal C (Fig. SI-8).

4.

Figure 9. Experimental in situ EPR spectra in helium flow during irradiation with UV-vis light of NaTaO3(EM) (a) pure, (b) loaded with 0.2 wt.-% Au, (c) loaded with m-rGO and (d) loaded with 0.2 wt.-% Au and m-rGO.

The spectrum of the m-rGO-containing catalysts exhibits a sharp isotropic signal F (Figure 9c, In the presence of ambient air which contains paramagnetic O2, EPR signals of surface-bound paramagnetic species are usually broadened beyond detection due to their dipolar interaction with gas-phase oxygen. However, two EPR signals (Table 1, centers A and B, for spectra simulation see Fig. SI-4) are visible in NaTaO3(EM) under irradiation in ambient atmosphere at room temperature (Figure 8a), while no light-induced species could be detected for NaTaO3(SSR) under these conditions. Thus, centers A and B might be located in the subsurface and/or in the catalyst bulk because they are not affected by gas-phase oxygen. Since irradiation has no influence on the intensity of signal A (Fig. SI-5), we assign it to F centers, i.e. to

Conclusion

NaTaO3 being one of the most active tantalum-based photocatalysts was synthesized using a facile exotemplate method (EM) yielding nanoparticles much more active than those prepared by solid state reaction (SSR). Decorating their surface with gold nanoparticles (Au-NP) by photodeposition improved the activity in photocatalytic hydrogen generation. In contrast to NaTaO3(SSR), the addition of multilayer reduced graphene oxide (m-rGO) to a dispersion of NaTaO3(EM) led to an improved photocatalytic performance. The activity of the Au loaded NaTaO3(EM) is further improved by m-rGO leading to a H2 evolution rate 41 times higher than the NaTaO3(SSR) reference. In situ EPR spectroscopy revealed the underlying differences in the formation and distribution of the light-induced paramagnetic species. Fast recombination of charge carriers is suppressed in NaTaO3(EM), due to the lower particle size. Electrons being transferred from NaTaO3(EM) to the additives lead to enhanced activities.

ASSOCIATED CONTENT Supporting Information. Additional SEM images, XPS C1s and O1s data of m-rGO, UV-vis of NaTaO3 (EM and SSR) and EPR spectra of pure NaTaO3 (SSR) and NaTaO3 (EM) in the dark and under irradiation as well as in different atmospheres and H2O2-pretreatment, derived simulation parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions


Chemistry of Materials †

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Authors Tobias Meyer, Jacqueline B. Priebe and Rafael O. da Silva contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We would like to thank Marga-Martina Pohl, Martin Adam, Matthias Schneider, and Ursula Bentrup for valuable insight and contributions. We would like to further thank Dr. Jörg Radnik for the XPS measurements and evaluation. Dr.-Ing. Ralph Krähnert and Paul Benjamin (TU Berlin) for the SEM measurements.

ABBREVIATIONS Au-NP, gold nanoparticles; EM, exotemplate method; EPR, electron paramagnetic resonance; m-GO, multilayer graphene oxide; m-rGO, multilayer reduced graphene oxide; PDF, powder diffraction file; SSR, solid state reaction.

5.

REFERENCES

1. Lide, D. R. CRC Handbook of Chemistry and Physics. CRC Press: 2007; Vol. 88th. 2. Osterloh, F. E. Chem. Mater. 2008, 20, 35-54. 3. Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278. 4. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503-6570. 5. St. John, M. R.; Furgala, A. J.; Sammells, A. F. J. Phys. Chem. 1983, 87, 801-805. 6. Jin, Z.; Zhang, X.; Lu, G.; Li, S. J. Mol. Catal. A: Chem. 2006, 259, 275-280. 7. Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669-13679. 8. Choi, J.; Park, H.; Hoffmann, M. R. J. Phys. Chem. C 2010, 114, 783-792. 9. Mukherji, A.; Sun, C.; Smith, S. C.; Lu, G. Q.; Wang, L. J. Phys. Chem. C 2011, 115, 15674-15678. 10. Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Appl. Catal., A 2004, 265, 115-121. 11. Lin, Y.; Zhou, S.; Liu, X.; Sheehan, S.; Wang, D. J. Am. Chem. Soc. 2009, 131, 2772-2773. 12. So, W.-W.; Kim, K.-J.; Moon, S.-J. Int. J. Hydrogen Energy 2004, 29, 229-234. 13. Zhang, N.; Zhang, Y.; Pan, X.; Yang, M.-Q.; Xu, Y.-J. J. Phys. Chem. C 2012, 116, 18023-18031. 14. Wang, P.; Chen, P.; Kostka, A.; Marschall, R.; Wark, M. Chem. Mater. 2013, 25, 4739-4745. 15. Gurunathan, K.; Maruthamuthu, P.; Sastri, M. V. C. Int. J. Hydrogen Energy 1997, 22, 57-62. 16. Sreethawong, T.; Yoshikawa, S. Mater. Res. Bull. 2012, 47, 1385-1395. 17. Jiang, T.; Tao, Z.; Ji, M.; Zhao, Q.; Fu, X.; Yin, H. Catal. Commun. 2012, 28, 47-51. 18. Priebe, J. B.; Karnahl, M.; Junge, H.; Beller, M.; Hollmann, D.; Brückner, A. Angew. Chem., Int. Ed. 2013, 52, 11420-11424. 19. Dreyer, D. R.; Jia, H.-P.; Bielawski, C. W. Angew. Chem., Int. Ed. 2010, 49, 6813-6816. 20. Zhang, N.; Zhang, Y.; Xu, Y.-J. Nanoscale 2012, 4, 57925813. 21. Han, L.; Wang, P.; Dong, S. Nanoscale 2012, 4, 58145825. 22. Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. J. Am. Chem. Soc. 2011, 133, 10878-84. 23. Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. J. Am. Chem. Soc. 2011, 133, 11054-11057.

Page 8 of 9

24. Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082-3089. 25. Tanaka, T.; Nojima, H.; Yamamoto, T.; Takenaka, S.; Funabiki, T.; Yoshida, S. Phys. Chem. Chem. Phys. 1999, 1, 52355239. 26. Laguta, V. V.; Glinchuk, M. D.; Bykov, I. P.; Cremona, A.; Galinetto, P.; Giulotto, E.; Jastrabik, L.; Rosa, J. Radiat. Eff. Defects Solids 2002, 157, 721-727. 27. Laguta, V. V.; Zaritskii, M. I.; Glinchuk, M. D.; Bykov, I. P.; Rosa, J.; Jastrabík, L. Phys. Rev. B 1998, 58, 156-163. 28. Wohlrab, S.; Weiss, M.; Du, H. C.; Kaskel, S. Chem. Mater. 2006, 18, 4227-4230. 29. Kato, H.; Kudo, A. J. Phys. Chem. B 2001, 105, 42854292. 30. Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. ACS Nano 2010, 4, 4806-4814. 31. Adamski, A.; Spałek, T.; Sojka, Z. Res. Chem. Intermed. 2003, 29, 793-804. 32. Mitchell, R. H.; Liferovich, R. P. J. Solid State Chem. 2004, 177, 4420-4427. 33. Scherrer, P. Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen 1918, 2, 98. 34. Chan, S. C.; Barteau, M. A. Langmuir 2005, 21, 55885595. 35. Dubin, S.; Gilje, S.; Wang, K.; Tung, V. C.; Cha, K.; Hall, A. S.; Farrar, J.; Varshneya, R.; Yang, Y.; Kaner, R. B. ACS Nano 2010, 4, 3845-3852. 36. Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S., Nature 2007, 448, 457-460. 37. Park, S.; Lee, K.-S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. ACS Nano 2008, 2, 572-578. 38. Buchsteiner, A.; Lerf, A.; Pieper, J. J. Phys. Chem. B 2006, 110, 22328-22338. 39. Lerf, A.; Buchsteiner, A.; Pieper, J.; Schöttl, S.; Dekany, I.; Szabo, T.; Boehm, H. P. J. Phys. Chem. Solids 2006, 67, 11061110. 40. Guardia, L.; Villar-Rodil, S.; Paredes, J. I.; Rozada, R.; Martinez-Alonso, A.; Tascon, J. M. D. Carbon 2012, 50, 1014-1024. 41. Jo, G.; Choe, M.; Cho, C. Y.; Kim, J. H.; Park, W.; Lee, S.; Hong, W. K.; Kim, T. W.; Park, S. J.; Hong, B. H.; Kahng, Y. H.; Lee, T. Nanotechnology 2010, 21, 175207 (6pp). 42. Kato, H.; Kudo, A. Catal. Today 2003, 78, 561-569. 43. Sun, Y.; Egawa, T.; Zhang, L.; Yao, X. J. Mater. Sci. Lett. 2003, 22, 799-802. 44. Panich, A. M.; Shames, A. I.; Aleksenskii, A. E.; Dideikin, A. Solid State Commun. 2012, 152, 466-468. 45. Sweeney, K. L.; Halliburton, L. E.; Kappers, L. A. Phys. Lett. A 1986, 116, 81-84. 46. M. Murphy, D.; W. Griffiths, E.; C. Rowlands, C.; M. Murphy, D.; E. Hancock, F.; Giamello, E. Chem. Commun. 1997, 2177-2178. 47. Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E.; Paganini, M. Top. Catal. 1999, 8, 189-198. 48. Coronado, J. M.; Maira, A. J.; Conesa, J. C.; Yeung, K. L.; Augugliaro, V.; Soria, J. Langmuir 2001, 17, 5368-5374. 49. Attwood, A. L.; Murphy, D. M.; Edwards, J. L.; Egerton, T. A.; Harrison, R. W. Res. Chem. Intermed. 2003, 29, 449-465. 50. Howe, R. F.; Gratzel, M. J. Phys. Chem. 1987, 91, 39063909. 51. Sá, J.; Fernández-García, M.; Anderson, J. A. Catal. Commun. 2008, 9, 1991-1995. 52. Claus, P.; Brückner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem. Soc. 2000, 122, 11430-11439.


Page 9 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


Advanced Charge Utilization from NaTaO3 Photocatalysts by

Recommend Documents