Sandip K. Pahari, Shunit Olszakier, Itamar Kahn, and Lilac Amirav*
Size Matters: Cocatalyst Size Effect on Charge Transfer and Photocatalytic Activity
Yifat Nakibli, Yair Mazal, Yonatan Dubi, Maria Wächtler*, and Lilac Amirav*
Hybrid semiconductor-metallic nanostructures play important role in a wide range of applications, and are key components in photocatalysis. Here we reveal that the nature of a nano-junction formed between a semiconductor nanorod and metal nanoparticle is sensitive to the size of the metal component.
Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic Metal Tips
Maria Wächtler,* Philip Kalisman, and Lilac Amirav
CdSe@CdS dot-in-rod nanostructures tipped with AuPt bimetallic nanoparticles as cocatalyst show increased photon-to-hydrogen conversion efficiency compared to their analogues with pure Au or Pt tips. The underlying charge-separation and recombination processes are investigated by time-resolved transient absorption spectroscopy, to unravel whether the observed enhancement of photocatalytic activity is due to charge-separation/recombination properties of the system, or to higher reactivity for proton reduction at the surface of the metal nanoparticle. We find that in the catalytically active Pt and AuPt functionalized structures charge separation occurs with similar time constants (Pt 3.5, 35 and 49 ps; AuPt 2.6, 31, and 66 ps) and the charge-separated state shows a lifetime of ~20 µs in both cases. Hence, these processes should not be regarded as source of the increased catalytic efficiency in the AuPt functionalized nanorods. The results indicate that the proton reduction at the metal nanoparticle surface itself determines the overall efficiency.
Selective Growth of Ni Tips on Nanorod Photocatalysts
Yifat Nakibli and Lilac Amirav*
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.
Stability of Seeded Rod Photocatalysts: Atomic Scale View
Eran Aronovitch, Philip Kalisman, Lothar Houben, Lilac Amirav, and Maya Bar-Sadan*
The displacement of single atoms may have detrimental effects on the functionality of nanoparticle photocatalysts. Understanding the atomic-scale phenomena is the key to the rational design of photocatalysts to prevent undesired events during their synthesis and operation. Here, we used high resolution TEM to study such phenomena within seeded rods of CdSe in CdS, mounted with bimetallic particles of Au and Pd. We show the mobility of Pd atoms during deposition and photocatalysis and relate that to photocatalyst degradation processes that limit overall photocatalyst efficiency. We also show the advantage of the bimetallic composition and the specific internal rearrangement of the two metals for better efficiency and stability to provide a more generalized understanding of prospective design principles.
We report a record 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water-splitting reduction half-reaction. This result was accomplished by utilization of nanoparticle-based photocatalysts, composed of Pt tipped CdSe@CdS rods, with a hydroxyl anion–radical redox couple operating as a shuttle to relay the holes. The implications of such record efficiency for the prospects of realizing practical over all water splitting and solar-to-fuel energy conversion are discussed.
Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods
Philip Kalisman, Yifat Nakibli, and Lilac Amirav*
A detailed video protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented. It will help new practitioners in the field avoid the common pitfalls associated with the method, as small differences in the conditions and reagents employed in this photo-oxidative growth protocol may create dramatic differences in the final product.
The Golden Gate to Photocatalytic Hydrogen Production
Philip Kalisman, Lothar Houben, Eran Aronovitch, Yaron Kauffmann,
Maya Bar-Sadan, and Lilac Amirav*
We demonstrate improved efficiency for the photocatalytic water splitting reduction half reaction by employing Au-Pt bimetallic cocatalyst. We employ nanoparticle-based photocatalyst consisting of CdSe@CdS rods tipped with Au, Pt, Au-Pt core shell or Au decorated with Pt islands. By tailoring the composition and morphology of the Au-Pt bimetallic catalysts, we achieved more than fourfold increase in activity for hydrogen production compared to pure Pt.
Eran Aronovitch, Philip Kalisman, Shai Mangel, Lothar Houben, Lilac Amirav, Maya Bar-Sadan*
The enhanced catalytic properties of bimetallic particles has made them the focus of extensive research. We compare the photocatalytic activity for hydrogen production of core-shell structures of Au@Pd and Au@(Au/Pd alloy) on seeded rods of CdSe@CdS and show that Au@alloy was superior towards hydrogen production. Our finding reveals that the promotion effects of Au in Pd originate both from the alteration of the electronic structure by the Au core as well as by the atomic rearrangement of the surface. Long-term monitoring of the activity of the photocatalysts offered insights into the dynamic processes during the illumination showing that the tip morphology influenced the stability of the hybrid structures. The Au core served as a physical barrier, protecting the CdS rod against cation exchange reactions with the Pd. The coupling of these factors to achieve synergistic effects is therefore a prime aspect in the rational design of efficient co-catalysts.
Yifat Nakibli, Philip Kalisman, and Lilac Amirav *
We provide evidence that for a multi electron reaction such as hydrogen reduction, the photocatalyst design should only include a single co-catalytic site per each segment of the semiconductor capable of light excitation. This is to insure that intermediates are formed at close proximity. These findings are demonstrated by evaluating the efficiency for hydrogen production over a nanoparticle-based photocatalyst consisting of Pt decorated CdSe@CdS rods. Rods decorated with a single Pt catalyst were found to be the most active for hydrogen production, with QE of 27%, while rod having two reduction sites reached only QE of 18% and rods with multiple sites showed very low activity. The advantage of using a single catalytic site became negligible when the rods were employed in catalyzing a single electron reaction. We believe the implications of this finding are of significance for the proper design of photocatalysts aimed at solar-to-fuel energy conversion.
Philip Kalisman, Yaron Kauffmann, and Lilac Amirav*
The combination of photochemical oxidation with colloidal synthesis enables us to re-think the design of photocatalysts with an eye towards overall water splitting. Here, IrO2 nanoparticle oxidative catalysts that were photodeposited on colloidal CdSe@CdS nanorod photocatalysts revealed a mediated oxidative pathway, and afforded the rods remarkable photochemical stability under prolonged illumination in pure water.
Modular Synthesis of a Dual Metal - Dual Semiconductor Nano Heterostructure
Lilac Amirav, Fadekemi Oba, Shaul Aloni and A. Paul Alivisatos*
We report on the design and modular synthesis of a dual metal - dual semiconductor heterostructure, with control over the dimensions and placement of its individual components. Analogous to molecular synthesis, colloidal synthesis is now evolving into a series of sequential synthetic procedures with separately optimized steps. We detail the challenges and parameters that must be considered when assembling such a multicomponent nanoparticle, and their solutions. Our multicomponent nanosystem, Ru-CdSe@CdS-Pt, was designed to achieve charge carrier separation and directional transfer across different interfaces toward two separate redox catalysts. This heterostructure may potentially serve as a nanometric closed circuit photoelectrochemical cell.
Angew. Chem. Int. Ed. 2015, 24, 7007-7011.
Luminescence Studies of Individual Quantum Dot Photocatalysts
Lilac Amirav and A. Paul Alivisatos*
Using far-field optical microscopy we report the first measurements of photoluminescence from single nanoparticle photocatalysts. Fluence-dependent luminescence is investigated from metal-semiconductor heterojunction quantum dot catalysts exposed to a variety of environments, ranging from gaseous argon to liquid water containing a selection of hole scavengers. The catalysts each exhibit characteristic nonlinear fluence dependence. From these structurally and environmentally sensitive trends, we disentangle the separate rate-determining steps in each particle across the very wide range of time scales, which follow the initial light absorption process. This information will significantly benefit the design of effective artificial photocatalytic systems for renewable direct solar-to-fuel energy conversion.
JACS, 2013. 135, 13049-13053
Hydrogen Production with Tunable Nanorod Heterostructures
Lilac Amirav and A. Paul Alivisatos*
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.