Letter pubs.acs.org/NanoLett
Size-Selected Subnanometer Cluster Catalysts on Semiconductor Nanocrystal Films for Atomic Scale Insight into Photocatalysis Maximilian J. Berr,† Florian F. Schweinberger,‡ Markus Döblinger,§ Kai E. Sanwald,‡ Christian Wolff,† Johannes Breimeier,† Andrew S. Crampton,‡ Claron J. Ridge,‡ Martin Tschurl,‡ Ulrich Heiz,‡,* Frank Jac̈ kel,†,* and Jochen Feldmann† †
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 München, Germany ‡ Chair of Physical Chemistry, Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, 85748 Garching, Germany § Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (E), 81377 München, Germany S Supporting Information *
ABSTRACT: We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size. KEYWORDS: photocatalysis, size-selected clusters, colloidal semiconductor nanocrystals, solar hydrogen, cadmium sulfide, renewable energy
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Here we introduce size-selected subnanometer cluster catalysts deposited on colloidal semiconductor nanocrystal thin films as a novel platform to obtain atomic-scale insight into photocatalytic nanosystems. Combining atomically precise sizeselection of cluster catalysts under ultrahigh vacuum (UHV) conditions and independently controlled numbers of clusters per area with colloidal semiconductor nanocrystal films allows for creating a new platform for obtaining atomic scale insight into photocatalytic nanosystems. Full control over the entire preparation procedure resulting in reproducible and stable samples is achieved. Using Pt-decorated CdS nanorods for photocatalytic hydrogen generation as a first example we demonstrate the power of our approach; first, we determine the minimum amount of Pt necessary to achieve saturation of hydrogen generation quantum efficiency, and second, we show the potential of tuning the photocatalytic activity by controlling the cluster size with atomic precision. Figure 1 illustrates our experimental approach. Colloidal CdS nanorods were prepared as reported earlier7 and spin-coated onto ITO-coated glass substrates or TEM grids followed by cluster deposition under UHV conditions.15 Either size-selected Pt46-clusters, that is, exactly 46 atoms per cluster, or unselected
hotocatalysis is an attractive approach for directly storing solar energy in renewable chemical fuels such as hydrogen, methane, or methanol.1−3 Hybrid, catalyst-decorated colloidal semiconductor nanocrystals are currently receiving renewed interest for photocatalytic hydrogen generation since advanced colloidal chemistry allows for nanoscale size-, morphology-, composition-control and thus tunable optical and electronic properties.4−9 Hybrid, catalyst-decorated semiconductor nanocrystals generate hydrogen via light absorption followed by photoelectron transfer to the catalyst in the presence of a sacrificial hole scavenger.6−8 While the semiconductor nanocrystal can be well-controlled via advanced colloidal chemistry the size-, shape-, and coverage-control over the cluster catalysts is still much more limited.9,10 However, subnanometer sized clusters have been shown to be photocatalytically active for hydrogen generation.7 In this size regime the electronic and optical cluster properties become nonscalable, that is, the extrapolation from the properties of larger clusters fails, and drastic variations of the physical and chemical properties with the precise number of atoms constituting the cluster occur.11−14 Therefore, full control over photocatalytic activities requires atomic control over the catalyst particles. However, such subnanometer control, enabling atomic scale understanding of the photocatalytic processes, in hybrid nanosystems of metal decorated colloidal semiconductor nanocrystals is still missing. © 2012 American Chemical Society
Received: September 5, 2012 Published: October 8, 2012 5903
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Figure 1. Schematic representation of the new experimental platform for atomic scale insight into photocatalytic nanosystems: Colloidal semiconductor nanocrystals (CdS nanorods) are spin-coated onto ITO substrates (a) followed by deposition of size-selected cluster catalysts by means of a laser ablation source under UHV conditions (b). The resulting samples are investigated for their photocatalytic activity in terms of hydrogen generation efficiency in aqueous solution (c).
Figure 2. HAADF-STEM images of CdS nanorords decorated with (a,b) Ptn≥36 clusters at a coverage of 0.04 e/nm2 corresponding to ∼23 clusters per nanorod. Pt46 cluster at coverages of 0.04 e/nm2 (c,d) and 0.07 e/nm2 (e,f) (∼40 clusters per nanorod). In contrast to Pt46 clusters, Ptn≥36 clusters show a higher polydispersity. Recording the cluster current during deposition allows for controlling the final cluster coverage. The highresolution images (b,d,f) show the attachment of the clusters to the nanorods and also reveal the lattice planes of the CdS nanorods with Wurtzite structure, demonstrating their single-crystalline nature and a growth direction along [001].
clusters (Ptn≥36), that is, narrow size distribution with n ≥ 36, were deposited using a laser ablation cluster source in combination with size-selection by a quadrupole mass spectrometer (QMS).16 Structural characterization was performed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Photocatalytic hydrogen generation under UV−vis illumination in the presence of the sacrificial hole scavenger triethanolamine (TEA) was evaluated using gas chromatography (GC).7,17 For full experimental details see the Supporting Information. Figure 2 displays HAADF-STEM images of representative samples. Panels a and b show Ptn≥36 clusters on CdS nanorods at a coverage of 0.04 e/nm2 (∼23 clusters per nanorod; see Supporting Information for calibration). Panels c,d and e,f show Pt46 clusters on CdS nanorods at coverages of 0.04 and 0.07 e/ nm2 (∼23 and ∼40 clusters per nanorod), respectively. The Ptn≥36 clusters appear more polydisperse than the Pt46 clusters.
This is confirmed by a detailed analysis of the size distribution (see Supporting Information). Furthermore, the images demonstrate control over the cluster coverage, which is achieved by monitoring the integrated cluster current during deposition (see Supporting Information for details). Between spin-coating of nanorods, cluster deposition in UHV, and STEM characterization the samples were exposed to ambient conditions for several weeks which demonstrates the stability of the samples similar to other systems.18 In particular, cluster diffusion and Ostwald ripening appears negligible on this time scale. To understand the influence of the Pt cluster coverage and polydispersity on the photocatalytic activity, we varied systematically the coverage and type of Pt clusters deposited. Figure 3 displays the hydrogen evolution under UV−vis illumination from individual samples with Pt46 and Ptn≥36 clusters at different coverages (0.04, 0.07, and 0.11 e/nm2 or 5904
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Figure 4. Hydrogen generation after four hours of illumination and monolayer quantum efficiencies (ML-QE) for Ptn≥36 and Pt46 clusters deposited onto CdS nanorods as a function of cluster coverage. At coverages below 15 clusters per nanorod, only small amounts of hydrogen are generated. A clear increase of both hydrogen generation and QE can be observed for coverages between 20 and 30 clusters per nanorod, followed by a saturation at higher coverages. The data points represent the averages over multiple measurements (see Supporting Information). At saturation, the Pt46 clusters show a better performance than the Ptn≥36 clusters. The red line indicates the maximum QE observed for colloidal CdS nanorods from the same batch when decorated with Pt cluster via photodeposition at a coverage of ∼300 clusters per nanorod.
Figure 3. Hydrogen generation as function of time for Ptn≥36 and Pt46 clusters deposited on CdS nanorods or on a blank substrate. Clearly, only clusters deposited on CdS nanorods are photocatalytically active. In the range displayed here, hydrogen generation increases with coverage for Pt46 clusters and at a coverage of 0.04 e/nm2 Ptn≥36 exhibits a larger hydrogen generation efficiency than Pt46.
∼23, ∼40 and ∼63 clusters per nanorod, respectively) over four hours. While Ptn≥36 clusters on a blank substrate do not exhibit significant hydrogen production, all samples containing both CdS nanorods and Pt clusters show significant hydrogen generation. Since it has been shown that Pt clusters are necessary for photocatalytic hydrogen evolution from CdS nanorods,7 this proves that only the Pt clusters on the CdS nanorods are catalytically active. Furthermore, clear differences in hydrogen generation between different cluster size dispersions and coverages can be observed throughout the whole duration of the experiment. The constant hydrogen generation rates suggest that the samples are stable over the course of the experiment. Figure 4 displays hydrogen generation and the monolayer (ML) quantum efficiency (QE) of CdS nanorods decorated with either Ptn≥36 or Pt46 clusters as function of cluster coverage. ML-QE, that is, QE corrected for the absorption of a ML, is reported since only CdS nanorods in the uppermost layer are decorated with Pt clusters (see Supporting Information). Both the amount of generated hydrogen and ML-QE are small for coverages below 15 clusters per nanorod. Between 15 and 30 clusters per nanorod, a strong increase followed by saturation of both quantities at higher coverages is observed. The QEs are larger than those observed for colloidal suspensions of nanocrystals from the same batch when decorated with Pt clusters via photodeposition. In the latter case, the cluster coverage is higher (∼300 cluster per nanorod) and the cluster size is not controlled.7 Furthermore, sizeselected Pt46 clusters exhibit larger QEs in the saturation regime, that is, more than 30 clusters per nanorod, than Ptn≥36 clusters.7 The observed saturation of the hydrogen generation efficiency allows, for the first time, for determining the minimum amount of catalyst necessary while maintaining maximum efficiency. This is of practical importance for cost efficient design of photocatalytic nanosystems. In the present case, a coverage of ∼30 Pt clusters per nanorod is sufficient to achieve saturation of the QE.
From a fundamental perspective, the strong initial increase can be explained considering the charge carrier dynamics in such photocatalytic nanosystems.19,20 At low Pt coverage, photoelectron transfer to the Pt can not compete with electron hole recombination and charge carrier trapping since the number of Pt clusters is too small. With increasing coverage, the photoelectron transfer to Pt becomes more probable and increasing hydrogen generation is observed. The threshold for saturation is expected for distances between the Pt clusters comparable to the spatial extent of the electronic wave functions. In the present case, this allows for estimating this distance to 5 to 8 nm. At larger coverages, the saturation can be attributed to the two-electron reduction process involved in hydrogen generation. Increased Pt coverage accelerates photoelectron transfer, but more catalytic sites compete for the available photoelectrons that allows for recombination of electrons at Pt clusters with newly generated photoholes. This latter effect becomes more severe for larger coverages, which explains why the colloidal QE at ∼300 clusters per nanorod is lower than efficiencies reported for the present coverage range. Indeed, for larger coverages we expect to observe a decrease in efficiencies. An additional fundamental insight enabled by our new platform can be inferred from the better performance of Pt46 clusters compared to Ptn≥36 clusters in the saturation regime. Since the size distribution of the unselected Ptn≥36 clusters is peaked at the size of Pt46 clusters, the catalytic activity of Pt46 must be larger than the size-averaged catalytic activity of the Ptn≥36 clusters. This is direct evidence that differently sized clusters have different catalytic activity and that the precise number of atoms in the cluster is the determining factor. This opens the possibility to tweak the electronic states (lowest 5905
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and by the Bundesministerium für Bildung und Forschung through IC4 is gratefully acknowledged.
unoccupied molecular orbitals) of clusters with respect to the lower edge of the conduction band of the semiconductor as well as to the chemical potential of the H+/H2 partial reaction by varying cluster size. Furthermore, for cluster sizes and materials with occupied electronic states (highest occupied molecular orbitals) between the upper edge of the valence band and the chemical potential of the O2/H2O partial reaction the water oxidation can be tuned. Eventually the complete photocatalytic water splitting reaction may become accessible with our approach. The herein introduced experimental platform is not limited to Pt-decorated CdS nanorods but is applicable to any combination of colloidal nanocrystals and cluster catalyst materials in the nonscalable size regime, including sequential deposition of multiple materials.21 The approach is not restricted to the photocatalytic water splitting reaction but also includes reactions, such as the CO2-reduction, relevant for renewable energies. These new materials can further be investigated employing advanced optical spectroscopies for elucidating the fundamental mechanistic details on the ensemble and single-particle level. Such studies may include cluster size- and coverage dependence of photocatalytic activities, intermediate and transition states of reactions, and charge carrier dynamics giving atomic scale insight into these important reactions. This will be of crucial importance for the understanding and design of future photocatalytic nanosystems. In conclusion, we introduced a novel experimental platform that combines atomically precise size-selected supported cluster catalysts and colloidal semiconductor nanocrystal films for quantitative studies of these promising photocatalytic nanosystems. The platform allows for atomic scale insight into photocatalytic processes of precisely defined nanosystems and is applicable to a range of important materials and reactions. In this study, Pt-cluster-decorated CdS nanorods were used to quantitatively correlate the dependence of the photocatalytic activity for hydrogen generation on Pt cluster size and coverage. The minimum amount of Pt necessary to achieve saturated efficiencies was determined, and evidence for the cluster size dependence of the catalytic activity was presented. The ability to determine the saturation point for hydrogen generation and to tune the QE by optimizing the cluster size is of practical importance for the cost efficient design of photocatalytic nanosystems.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Full experimental details and additional experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (U.H.)
[email protected], (F.J.) frank.jaeckel@ physik.uni-muenchen.de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the Nanosystems Initiative Munich (NIM) and LMUexcellent, by the Bavarian State Ministry of Sciences, Research and the Arts through Solar Technologies Go Hybrid 5906
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