Modular assembly of photochemical water splitting catalysts from KCa2Nb3O10 nanosheets, IrO2, and Pt nanoparticlesOwen C. Compton, Elizabeth C. Carroll, Cory Mullett, Shirley Chiang, Delmar S. Larsen, Frank E. Osterloh Department of Chemistry, University of California, Davis, CA 95616 *fosterloh@ucdavis.edu

Introduction

References

Acknowledgment This work was funded by the Energy Innovations Small Grant (EISG) program of the California Energy Commission.

Conclusion

Photochemical H2 evolution Excited state dynamics

Photoreduction of IrO2 (cit)

Predicted increases in global energy consumption and climate change driven by fossil fuel emissions have sparked a renewed interest in carbon-neutral energy sources. A recent analysis has shown that the conversion of solar energy into hydrogen is one of the most promising renewable energy technologies.1 Exfoliation of the layered perovskite KCa2Nb3O10 produces 1.6 nm thin nanosheets (bandgap 3.53 eV) composed of triple stacks of NbO6 octahedra. Nanosheet edge lengths range from 0.2 nm to 1.8 nm with a mean value of 1.2 nm. We have previously observed that dispersions of exfoliated TBA[Ca2Nb3O10] nanosheets are photocatalytically active for the conversion of water into H2 and unidentified water oxidation products.2 Viewing these nanosheets as templates, we have developed a facile method for the synthesis of multi-component photocatalysts. Here we demonstrate that the activity of the nanosheets can be altered by addition of IrO2(cit) and Pt(cit) nanoparticles serving as co-catalysts. The nanoparticles are either linked through γ-aminoalkylsilyl groups, or, in the case of Pt, photochemically grown on the nanosheet surface. The resulting nanostructures were characterized by TEM, HRTEM, UV/vis, fluorescence, XPS, and IR spectroscopy. The performance of the catalysts was tested under UV irradiation with a low pressure Hg lamp. Transient absorption spectroscopy was used to determine the excited state lifetimes of the nanosheets.

In conclusion we have demonstrated a modular assembly approach for the generation of two-component nanostructures for photocatalytic hydrogen evolution from water. The structures are supported by direct chemical bonds between the nanomaterials or held together by γ-aminoalkylsilyl linkers. Under UV irradiation, all structures are catalytically active for photochemical H2 evolution from water with decreasing activity in the series Pt-[HCa2Nb3O10] (3) > Pt(cit)-APS-[Ca2Nb3O10] (5) > Pt(cit)-AEAUS-[Ca2Nb3O10] (4) > IrO2(cit)-APS-[Ca2Nb3O10] (7) > TBA[Ca2Nb3O10] (2). The trend can be rationalized by assuming that citrate molecules reduce the catalytically active area on Pt and organic linkers impede electron transport from the nanosheets to the Pt cocatalyst. The same trend is observed when the electrochemical potentials for water reduction are compared. In the absence of Pt, little H2 is evolved due to the lack of water reduction sites on the catalysts. Hydrogen but no O2 evolution from water is achieved with IrO2(cit)-APS-[Ca2Nb3O10] (7), which undergoes photoreduction to Ir-APS-[Ca2Nb3O10]. Ongoing work5 in this laboratory is devoted to elucidating the excited state dynamics in these systems, determining the fate of O2, and to extending the synthetic concept to optimized three component nanostructures with separate sites for water oxidation and reduction.

TEM images of photocatalysts

Optical properties

Transient absorption measurements of APS-[Ca2Nb3O10] (2) sheets in water. Transient spectra at 1 ps, 10 ps, 100 ps, 1 ns, and 6.8 ns show that transient decay was faster for APS functionalized nanosheets than non-fuctionalized. Normalized kinetics at 400 nm, 500 nm, 600 nm, and 695 nm reveal recombination of shallowly trapped carriers occurs relatively quickly compared to deeply trapped carriers at longer wavelengths.

Disperse reflectance spectra of photocatalysts show a consistent band gap absorption at 350 nm (3.53 eV). IrO2(cit) and Pt(cit) nanoparticles have absorption bands in the vislble.

TEM images reveal variation in the attachment of co-catalysts. In absence of APS, photoreduced Pt nanoparticles grow in clusters. When APS is present isolated Pt nanoparticles are formed instead. IrO2 (cit) nanoparticles are found as globular clusters ~30 nm in diameter that are supported by interactions among the citrate surfactants.4

1. Lewis, N. S.; Nocera, D. G., PNAS, 2006, 103, 15729-15735. 2. Compton, O. C.; et al., J. Phys. Chem. C, 2007, 111, 14586-14592.3. Hara, M.; et al., Electrochim. Acta., 1983, 28, 1073-1081. 4. Hoertz, P. G.; et al., J. Phys. Chem. B, 2007, 111, 6845-6856.5. Carroll, E. C.; et al., J. Phys. Chem. C, 2008, 112, 2394-2403.

Assembly of photocatalysts

[Ca2Nb3O10]- IrO2 (cit)

Linker (L)

Si

H3CO

H3CO

H3CO

R

Pt (cit)

[HCa2Nb3O10] (1) Pt-[HCa2Nb3O10] (3)H2PtCl6/hν

MeOH

APSor

AEAUSDMSO

L-[Ca2Nb3O10] (2)

Pt-APS-[Ca2Nb3O10] (4)

Pt (cit)-APS-[Ca2Nb3O10] (5)

IrO2 (cit)-APS-[Ca2Nb3O10] (7)

H2PtCl6/hνMeOH

Pt (cit)H2O

IrO2 (cit)

H2O

Platinization of the nanosheets significantly increases photocatalytic activity. Activity of platinized catalysts decreases due to the presence of chemical linkers or citrate surfactant. No O2 was evolved. Results are for 100 mg of catalyst suspended in 50 mL of water irradiated by 750 W Hg lamp.

200 nm

1

200 nm

2

200 nm

3

200 nm

4

100 nm

5 6

100 nm

D

13

2

4

57

Infrared spectra

Strong absorption bands for ν(Si-O) in APS and AEAUS functionalized species confirm attachment of the γ-aminoalkylsilyl linkers.

Irradiation of IrO2(cit)-APS-[Ca2Nb3O10) reduces IrO2 nanoparticles from an initial mixture of Ir4+/Ir3+ to a final mixture of Ir4+/Ir(0), with 10-20% of Ir(0). The latter is observed in the XPS spectrum as a high saddle between the peaks and the shoulder on the 4f7/2 peak.3 TEM data reveals that globular IrO2 (cit) clusters transform to islands of individual particles after irradiation.

200 nm

IrO2 (cit)

Pt (cit)

50 nm

64.7

61.8

64.5

61.9

(7) after irradiation

1

32456

R = (APS)

(AEAUS)

NHNH2

NH2

Pt (cit)-AEAUS-[Ca2Nb3O10] (6)

7

pre-irr.

post-irr.

1

3

2

4

5

6

7

Cyclic voltammetry measurements at pH=14 show that the overpotential for water reduction is reduced when Pt is present as a cocatalyst. Increasing the linker length from C3 to C12 in AEAUS increases the water reduction overpotential. Attachment of IrO2(cit) reduces the water oxidation overpotential by 0.6V. However, the catalytic effect of IrO2 is reduced when the potential is cycled repeatedly to -1.47V. All potentials are given vs. NHE.

Electrochemical measurements