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Selective Growth of Ni Tips on Nanorod Photocatalysts Yifat Nakibli, and Lilac Amirav Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01482 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016
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Selective Growth of Ni Tips on Nanorod Photocatalysts Yifat Nakibli and Lilac Amirav* Schulich Faculty of Chemistry, The Russell Berrie Nanotechnology Institute, and The Nancy and Stephen Grand Technion Energy Program; Technion − Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: We present a synthesis for selective growth of economical Ni tips on CdSe@CdS nanorod photocatalysts. This procedure enables control over morphology, in particularly over catalyst location, size, and number of domains, which is vital for optimization of the materials potential for hydrogen reduction. The activity of the Ni decorated photocatalyst towards the water reduction half reaction was evaluated, and was found comparable to that of the Pt decorated nanorods when operated under identical conditions. We obtained an apparent quantum yield of 23%, and a turnover frequency of about 80,000 moles of hydrogen per 1 mole of catalysts per hour. The synthetic procedure presented here is expected to benefit future design of more economical photocatalysts for solar-to-fuel energy conversion.
Photocatalysis presents an appealing and promising solution for addressing the looming energy crisis and combating environmental challenges such as global worming, and water and air pollution.1-3 Photocatalytic systems, which harvest sunlight and split water into molecular hydrogen and oxygen, can provide a source of clean and renewable fuel.4-5 These systems often incorporate metallic cocatalysts, which greatly enhance the photocatalytic activity, as they offer lower activation potentials for hydrogen evolution, as well as promote charge separation from the semiconductor photocatalysts on which they are loaded. However, as they are typically made of noble metals, they also significantly increase the material cost.
location of the catalyst.18,19 In addition, the catalyst size can also play a role in affecting the efficiency.20 Hence, we present here a new and improved synthetic protocol for a well-controlled deposition of Ni nanoparticle catalysts on a photocatalytic semiconductor system.
Motivated by cost reduction considerations, recent interest was focused on utilization of earth abundant and economical substitutes for the costly noble metals. Integration of active and affordable cocatalysts is a necessity for real widespread applications. Nickel in particular has recently drawn attention as a possible candidate to substitute rare earth metals at the cocatalyst site for the water reduction half reaction.6-15 Zhukovskyi et al. reported of CdS nanosheet decorated with photodeposited Ni nanoparticles.13 Chen et al. reported of nickel loaded on CdS via hydrothermal reduction.14 Simon et al. reported on CdS nanorods decorated with Ni nanoparticles that were loaded from Ni salts.15 These prior works show the great promise of Ni as a viable alternative to Pt. However, the reported synthetic protocols for decoration of the photocatalyst semiconductor with Ni nanoparticle catalysts do not offer sufficient control on the heterostructure morphology. It was demonstrated recently that obtaining control over morphology, in particularly over metal location, number of catalyst domains, and catalyst size, is vital for optimization of the materials potential for hydrogen reduction. Hydrogen reduction is a multi electron reaction, with intermediate species. Formation of intermediates at close proximity enhances the activity. Such close proximity of intermediates can be promoted by a photocatalyst design that includes only a single cocatalytic site per each segment of the semiconductor capable of light excitation.16,17 Utilization of complex multi component heterostructure systems that are designed for improved charge separation requires prices control over spatial
Figure 1. (A-F) TEM micrograph of CdSe@CdS rods with Ni tips of different size: with 2.3 nm (D), 3 nm (A), 5 nm (B,E), 12 nm (C,F); (D-Inset) High resolution TEM demonstrating the poly crystallinity of the metal tip. (G) HR-HAADF (H) X-ray energy-dispersive spectroscopy analysis confirming presence of Ni.
For the light absorption and excitation unit we employ a wellcontrolled nanoparticle-based artificial system, which consists of a cadmium selenide (CdSe) quantum dot embedded asymmetrically within a cadmium sulfide (CdS) quantum rod.21-24 This structure
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has been widely studied optically and photo-catalytically and is well characterized. 25-28 With a single metal tip catalyst placed at the far side of the rod, the system enables efficient long lasting charge carriers' separation,18,29,30 and the formation of distinct and spatially segregated reaction sites for the different redox half reactions. The decoration of the semiconductor rods with Ni cocatalysts was achieved by the reduction of the nickel precursor Nickel(II) acetylacetonate [Ni(acac)2] by oleylamine (OA) and trioctyl phosphine (TOP) at 180°C. Our protocol is based on Carenco et al.31 procedure for Ni nanoparticle synthesis, with few modifications and adjustments. For detailed description of the procedure the reader is referred to the supporting information. A typical synthesis was initiated with injection of CdSe@CdS nanorods dissolved in 0.5 ml TOP, into a TOP solution maintained at 180°C under Ar flow. This was followed by the injection of 50-260mg of Ni(acac)2 dissolved in 2 ml of oleylanime at 130°C. The synthesis was left to run for 20 minutes, followed by quenching of the solution in a water bath. Cleaning was done by repeated addition of access of ethanol and methanol, and a final size selection wash (detailed in SI).
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Operation in continuous flow mode allowed for direct determination of the evolved gas production rate. The apparent quantum efficiency of the sample, which is defined as QE = 𝑁!! 2𝑁!! , was determined by quantifying the amount of evolved hydrogen at a given photon flux. The results are presented in Figure 2, along with the efficiency of Pt decorated rods.
The Ni tipped CdSe@CdS rods are presented in Figure 1. A single Ni tip of controlled and variable size is observed at the end of the nanorod (Figure 1 A-F). High resolution TEM demonstrates the poly crystallinity of the metal tip (D insert), and along with Xray energy-dispersive spectroscopy analysis confirms presence of metallic Ni (H). Evidence of partial surface oxidation was observed for old samples, though this shell is likely reduced again to metallic Ni during the catalytic operation. Control over the Ni nanoparticle size, location and number of sites on the CdS surface was attained via adjustments and fine tuning of the nickel precursor concentration (absolute and relative to the rods), reaction temperature and time, as well as the ligands and solvent that were used. In particularly, nickel precursor concentration was found to be the key parameter for controlling tip size and the number of catalytic domains on the rod. Increasing the concentration enabled us to tune the Ni size from 2 nm to 12 nm, with a reasonable size distribution of 0.6 nm for most samples, and up to 2.5 nm for the larger tips (9, 12 nm). This tip size distribution is narrower than the typical distribution for Pt tipping. In addition, the length of the ligand was found to affect the kinetics of the growth and application of the shorter ligand tributyl phosphine, rather than trioctyl phosphine, resulted in smaller Ni tips. Note that sample handling and following cleaning steps were more challenging with this ligand. Decreasing the temperature below 200°C encouraged heterogonous growth rather than homogeneous nucleation of freestanding Ni nanoparticles. Below 140°C the temperature was found to be too low to initiate the reaction. The activity of the Ni decorated CdSe@CdS nanorods towards hydrogen production was examined. Different sets of rods were examined throughout this work, with preference to 50 nm long rods, with 2.3 nm seed size. Solutions containing about 1015 rods suspended in water with isopropyl alcohol (10% by volume) acting as a hole scavenger (electron doner), at neutral pH conditions, were placed in a custom-built gas-tight reaction cell purged with argon. The samples were then illuminated with a 455nm LED adjusted to 50 mW (equivalent to a photon flux of 1.15×1017 photons/sec). We analyzed the evolving hydrogen using an online gas chromatograph equipped with a thermal conductivity detector.
Figure 2. Photocatalytic quantum efficiency for the hydrogen reduction half reaction obtained with CdSe@CdS nanorod photocatalysts decorated with Ni or Pt tips.
Rods decorated with a single Ni catalyst were compered to identical rods decorated with Pt catalyst. It is important to note that the absolute activity of the sample is strongly related to the rod morphology. Hence, only relative evaluation of the activity is given significance for these sets. A comparable activity was found for the Ni and Pt tipped rods, with 10.6% QE for the CdSe@CdS-Pt sample (purple bar) and 11.8% QE for the CdSe@CdS-Ni sample (turquoise bar). Numerous Ni tipped samples were examined and the maximal apparent quantum yield obtained in the course of this work was 23% (Figure 2, left turquoise bar). This corresponds to a flow of 1.3×1016 hydrogen molecules per sec, with about 22 hydrogen molecules that are generated from a single rod every second. These numbers reveal that each rod produces a hydrogen molecule every 45 msec. We thus obtain a turnover frequency of close to 80,000 moles of hydrogen per 1 mole of catalysts per hour. This activity is equivalent to the maximal activity for hydrogen reduction half reaction obtained with Pt tipped seeded rods, when operated under neutral conditions (QE of 27%). With comparable activity at a fraction of the price, these results demonstrate that the Ni tip is a viable alternative to Pt. Following the changes in H2 production rate over time offers insights into the dynamic processes that take place on the surface of the photocatalyst.32 The performances over time of the Pt tipped and Ni tipped photocatalysts differed significantly in terms of the induction time until full activity was reached (figure 3). One possible explanation for the induction time is reorganization of the lig-
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ands on the photocatalysts surface. The longer induction time obtained with the Ni cocatalysts might be related to formation of an oxide layer on the surface, which is being reduced during the operation of the catalysts as a reduction reaction site.33 In addition, these results testify for the excellent long-term stability of the Ni-tipped sample.
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Figure 3. Normalized photocatalytic quantum efficiency for the hydrogen reduction half reaction, over time, as obtained with CdSe@CdS nanorod photocatalysts decorated with Ni and Pt tips.
In conclusion, we present a new synthetic procedure for a wellcontrolled Ni tipping of CdSe@CdS nanorod photocatalysts, with comparable activity to that of Pt towards the water reduction half reaction. We anticipate that this work will ultimately advance our ability to realize a stable and efficient photocatalyst for solar-to-fuel energy conversion.
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ASSOCIATED CONTENT Supporting Information Supporting Information includes detailed description of the typical preparation of Ni tipped CdSe@CdS nanorods, and techniques used in characterization of these particles. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION
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Corresponding Author * Corresponding author email:
[email protected] Author Contributions
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The manuscript was written through contributions of all authors. 17.
Funding Sources No competing financial interests have been declared.
ACKNOWLEDGMENT This research was supported by the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (Grant No. 152/11), and by a grant from GIF, the German-Israeli Foundation for Scientific Research and Development (Grant 2307-2319.5/2011). We thank the Schulich Faculty of Chemistry and the Technion − Israel Institute of Technology for the renovated laboratory and startup package.
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