pubs.acs.org/JPCL
Photocatalytic Hydrogen Production with Tunable Nanorod Heterostructures Lilac Amirav and A. Paul Alivisatos* Department of Chemistry, University of California, Berkeley, California 94720, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
ABSTRACT We report the design of a multicomponent nanoheterostructure aimed at photocatalytic production of hydrogen. The system is composed of a platinum-tipped cadmium sulfide rod with an embedded cadmium selenide seed. In such structures, holes are three-dimensionally confined to the cadmium selenide, whereas the delocalized electrons are transferred to the metal tip. Consequently, the electrons are now separated from the holes over three different components and by a tunable physical length. The seeded rod metal tip samples studied here facilitate efficient long-lasting charge carrier separation and minimize back reaction of intermediates. By tuning the nanorod heterostructure length and the seed size, we were able to significantly increase the activity for hydrogen production compared to that of unseeded rods. This structure was found to be highly active for hydrogen production, with an apparent quantum yield of 20% at 450 nm, and was active under orange light illumination and demonstrated improved stability compared to CdS rods without a CdSe seed. SECTION Nanoparticles and Nanostructures
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hotocatalytic production of hydrogen from water using solar energy is a potentially clean and renewable source for hydrogen fuel, but there are still many materials-related obstacles to its widespread use.1 It is particularly difficult to find a stable semiconductor system with suitable band gap and electron affinity for visible light absorption and for driving the subsequent redox chemistry. Additional challenges facing the photocatalytic process include the quick recombination of photoinduced charge carriers, back reaction of intermediates on the catalyst surface, and the back reaction of the products. Semiconductor photocatalysts are often loaded with metallic cocatalyst, which presumably promotes charge separation of photogenerated electrons and holes and also acts as the site for hydrogen generation.2,3 However, the charge separation might not be long-lived as the electron may diffuse back into the semiconductor particle to recombine with a hole within a time frame faster than what is required for a chemical reaction. The most efficient photocatalytic activity reported for overall water splitting, under UV light, was attributed in part to the physical separation of the reaction sites for hydrogen and oxygen evolution.4 Thus, a systematic way to vary the spatial arrangement of the various semiconductor and metal components on the nanoscale can provide a useful system to better understand the underlying photophysical and photochemical phenomena. With advances in size,5 shape,6 and composition control,7 colloidal synthesis of inorganic nanostructures is now developing toward more sophisticated construction, where multicomponent nanostructures can be tailored in a predictable
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manner for a particular demand. We report herein the design of a multicomponent nanoheterostructure aimed at photocatalytic production of hydrogen. Hydrogen production with the CdS/Pt photochemical system has been studied extensively over the years. This system has an appropriate band gap and provides an adequate driving force for hydrogen evolution but suffers from limited photochemical stability. Prior studies have featured stochastic arrangements of the semiconductor and metal components. Here, we investigate a spatially controlled nanoheterostructure composed of a platinum-tipped cadmium sulfide (CdS) rod with an embedded cadmium selenide (CdSe) seed. In such structures, holes are three-dimensionally confined to the CdSe, whereas the delocalized electrons8,9 are transferred to the metal tip, as illustrated in Figure 1. Consequently, the electrons are now separated from the holes over three different components and by a tunable physical length. The ability to control parameters such as the distance between the reaction sites (via control over the nanorod length) and the degree of charge separation (via control over the relative band alignment by tuning the seed size10) makes this heterostructure an interesting model photocatlaytic system. It enables a systematic study of the influence of the distance between the metal particle and the region where the hole is confined on the photocatalytic yield. We can determine if the formation of distinct reaction sites, which are further Received Date: January 20, 2010 Accepted Date: March 2, 2010 Published on Web Date: March 08, 2010
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DOI: 10.1021/jz100075c |J. Phys. Chem. Lett. 2010, 1, 1051–1054
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containing 70 nm long seeded rods was four times more active for hydrogen production compared to the sample of 27 nm long rods with the same seed (3.1 nm). A similar trend was obtained for the 20, 40, and 60 nm long samples with a seed diameter of 2.3 nm. These results demonstrate that longer rods, with distinct reaction sites that are further apart, provide higher activity. Rods with comparable length (6070 nm) but a smaller seed (2.3 nm vs 3.1 nm) showed higher activity, presumably as a result of improved charge separation. With the use of 2-propanol as the hole scavenger, the highest hydrogen production rate was 162 μmol/h (40 mmol/ h-g Cat), with an apparent quantum yield of 20% at 450 nm. By tuning the nanorod heterostructure length and seed size, we obtained a sample that was 28 times more active for hydrogen production compared to the unseeded rods. Bao et al.14 reported an apparent quantum yield of 60% at 420 nm from Pt-loaded CdS nanoporous structures. However, these nanoporous photocatalysts lost 20% of their original activity within 12 h of illumination. Rever et al.15 reported an apparent quantum yield of 37% at 450 nm from platinized suspensions of cadmium zinc sulfide modified by silver sulfide, but the materials were not stable. The hydrogen production rate was monitored with time for evaluation of photocatalyst stability. The results for both Pt-SR (red) and Pt-tipped CdS rods (black) are shown in Figure 2B. The hydrogen production rate from the Pt-tipped CdS rods decreased sharply, to about 50% of its original value over the course of 12 h, while the production rate from the Pt-SR sample remained unaffected. This result implies that the addition of the CdSe seed contributes to improved stability of the nanorod heterostructure. Examination of photocatalyst samples at relatively early stages of degradation revealed that many rods have lost the Pt tip. This observation suggests that the interface between the semiconductor and the metal is the first to degrade. In light of this finding, the improved stability of the seeded rods might be attributed to the ability of the seed to isolate the oxidizing holes from the sensitive Pt/CdS interface. The activity of hydrogen production started to decrease after a few days under illumination but was revived upon the addition of the consumed methanol. Figure 2C shows the quantum efficiency of hydrogen production as a function of the wavelength of the incident light. The quantum efficiency decreases with increased wavelength, and the longest wavelength suitable for hydrogen production coincides with the absorption edge of the sample, which indicates that the reaction proceeds through light absorption by the photocatalyst. The incorporation of the CdSe seed into the rod introduces absorption at more redshifted wavelengths, enabling the utilization of more abundant parts of the incoming solar energy. The seeded rods maintain some activity for hydrogen production even under orange light illumination at 600 nm. The hydrogen production rate was found to be linear with the light intensity (Figure 2D). Although two electrons are needed for the production of a single hydrogen molecule, the reaction seems to be of first order with respect to photon flux. This finding implies that the Pt-bonded hydrogen atom, involved in the first step of the hydrogen evolution reaction,16
Figure 1. (Left top corner) An illustration of the multicomponent nanoheterostructure, composed of a Pt-tipped CdS rod with an embedded CdSe seed. (Top middle) An illustration of the energy band diagram indicating that holes are confined to the CdSe while electrons are transferred to the Pt and are thus separated from the holes over three different components and by a tunable physical length. (Bottom) TEM images of Pt tipped seeded rods with two different lengths, 27 and 70 nm on average (bar is 20 nm). (Top right corner) The Pt tip is clearly seen in the HRTEM (with a 3 nm tip) and HAADF STEM (of a 40 nm long rod) micrographs.
apart, does in fact minimize the back reaction of intermediates. The synthesis of the multicomponent nanoheterostructure photocatalyst consists of four separate steps. Preformed CdSe nanocrystal seeds11 are injected into a reaction mixture containing the precursors for CdS rod growth.12 Control over the rod length is attained by varying the amount of seeds or sulfur precursor and the reaction time. Next, the growth of the Pt cocatalyst tips, forming the water reduction sites, is done utilizing the procedure developed by Mokari and coauthors.13 Figure 1 shows TEM images of Pt-tipped seeded rods (Pt-SR) with two different lengths, 27 and 70 nm on average. The Pt tip is clearly seen in the high-resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF) scanning TEM micrographs. Finally, the organic ligands are exchanged with polar ligands to allow the particles to be well dispersed and suspended in water. Methanol was added to the water as a hole scavenger. The photocatalyst solutions were placed in a gas-tight reaction cell purged with argon and were illuminated through a quartz window with a 300 W Xe arc lamp. The hydrogen gas evolved was determined using an online gas chromatograph equipped with a thermal conductivity detector. See the Supporting Information for experimental details. The relative quantum efficiency for hydrogen production, obtained from platinum-tipped unseeded CdS rods (yellow), and five different samples of Pt-SR, with seed diameters of 3.1 (red) or 2.3 nm (green), are shown in Figure 2A. The photocatalytic activity of the samples investigated was normalized to the sample absorption (at 450 nm), with a typical sample amount of about 4 mg (in 15 mL of solution). The sample
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Figure 2. (A) Relative quantum efficiency for hydrogen production, obtained from platinum-tipped unseeded CdS rods (yellow), and five different samples of platinum-tipped seeded rods, with seed diameters of 3.1 (red) or 2.3 nm (green). Underneath each bar is the corresponding average sample length. (B) Hydrogen production rate from Pt-tipped seeded rods (red) and Pt-tipped CdS rods (black) monitored over the course of 12 h. (C) Influence of the wavelength of the incident light on the apparent quantum yield for hydrogen production. Data obtained from a sample of 40 nm long seeded rods with a seed diameter of 2.3 nm. Absorbance values do not correspond to those of the solution in the reaction cell. (D) Influence of the light intensity (white light, 0-1.3 W) on the hydrogen evolution rate.
is stable while awaiting a second photon absorption. Thus, physically separating the reaction sites for reduction and oxidation along the rod length seems highly relevant and beneficial. The seeded rod metal tip samples studied here facilitate efficient long-lasting charge carrier separation and minimize back reaction of intermediates. This structure was found to be highly active for hydrogen production, with an apparent quantum yield of 20% at 450 nm, was active under orange light illumination, and demonstrated improved stability. More comprehensive research on this interesting model system, aimed toward improved photocatalytic efficiency, is currently underway.
ACKNOWLEDGMENT This work was funded by the Helios Solar
SUPPORTING INFORMATION AVAILABLE Experimental
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Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors acknowledge support of the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory for the use of their microscope facilities. L.A. thanks the Weizmann Institute of Science Postdoctoral Award Program for Advancing Women in Science for partial financial support.
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details of the synthesis, and experimental setup. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: alivis@ berkeley.edu.
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