57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The use of nanomaterials, such as metal oxides, quantum dots, and metal nanoparticles, in future dye sensitized solar cell (DSSC) configurations can greatly benefit from all the nanocomponents being constructed in aqueous media. In the first year of this project we have been able to successfully synthesize crystalline TiO₂, quantum dots, and core/alloy nanoparticles using a microwave mediated hydrothermal processing route. The long-term goal of this work is to be able to fabricate these materials in a heterostructured manner due to the ability for each nanomaterial to be crafted using similar conditions. 

For each of these materials, its important to tailor the nucleation and growth, as well as provide the energy needed for annealing of a highly crystalline material. For this, we use a synthetic microwave reactor to provide microwave irradiation (MWI) as a heating source. This allows us to rapidly tune hydrothermal temperature (T_H), kinetic ramping, and temperature quenching. Temperature is monitored in-situ during synthesis via an integrated IR-sensor, and the instrument is equipped with an active pressure monitoring system.  Using this approach, we firstly synthesized TiO₂ nanoparticles with the goal of achieving an anatase crystal structure, which has been shown by others, to be the most suitable for charge transfer in dye sensitized systems. We probed the use of a number of different Ti-precursors, ligands, solvents, and heating temperatures.  These materials were probed by powder XRD and UV-vis absorption to assay the properties.  An optimized product consisted of TiO₂ nanoparticles with TEM determined diameters of d = 7.2 ± 0.7 nm. The powder XRD resulted in a highly crystalline anatase structure.  The optimum hydrothermal (or solvothermal) temperature was found to be ~150 ^oC for 15 min.  Suitable capping ligands included oleylamine, or the ionic liquid 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, [bdim]⁺[BF₄].  The titanium precursor was titanium butoxide.

Next, these anatase TiO₂ were used as ‘seeds’ upon which to deposit CdSe quantum dots.  The CdSe were chosen as a model quantum dot, and the resulting photophysical properties were used as a probe to investigate heterstructured TiO₂|Qdot formation.  Upon growth of CdSe by adding both Cd and Se precursors (cadmium perchlorate, and selenourea, respectively), we observed the growth of CdSe rods at the TiO₂ seeds.  These materials had low quantum yields (<0.5%), and zinc-blende crystal structures.  The low QY compared to controls, suggests the quenching of photoluminescence by TiO₂, while the absorption profile matched that of quantum dot absorption.  In contrast, when the Se and Cd precursors were added in a layer-by-layer process (i.e. SILAR-like), the resulting heterostructures had little growth, which suggests the need for both a TiO₂ and CdSe nuclei to be present for heterostructured growth.  We are currently optimizing this system in order to fabricate TiO₂|Qdot heterostructures with controlled stoichiometry (i.e., TiO₂:CdSe) and morphology (i.e., aspect ratios). We also investigated the energy transfer of these materials by probing their PL-decay, and we have plans to collect transient absorption results in year 2.

In a closely related project we investigated the hydrothermal processing method to fabricate ‘core/alloy’ metallic nanoparticles.  In these systems, a metal core (such as gold), has deposited on it a thin layer (~0.5 nm) of a second metal (such as palladium), and the system is heated at hydrothermal temperatures like the TiO₂ described above.  Under these conditions, we have been able to show that alloying occurs at the core/shell interface, forming a core/alloy structure.  Using these nanoparticles with alloy interfaces, we were interested in the growth of a third, ternary, component.  In this case, we wanted to investigate whether or not the phase behavior of the alloy will influence further growth, which up to this point had been symmetrical.  Using a Au/Pd core with a low percentage of Au, a silver shell was deposited and heated at a hydrothermal temperature of 120 ^oC.  The resulting Au/Pd-Ag nanoparticles showed a clear asymmetric growth, in which the Ag-deposited (or ripened) at one side/face of the core.  Interestingly, as the number of shells (n) increased, the Ag domain grew until its size was comparable to the initial Au/Pd core (up to 12-15 nm).  A novelty of this system was that the as-synthesized solutions contained >95% of heterostructures, and the growth was not due to a shape driving ligand, such as citrate. The heterostructured interface was probed by UV-vis absorption, XPS, and electrocatalytic studies using MeOH electrooxidation, the results of which showed the linear increase of Ag in the heterostructure, and the presence of both Pd and Ag reactivity.  This proved that both metal surfaces are solvent exposed and present. Interestingly, control experiments at higher temperatures (>160 ^oC), resulted in improved alloying, and symmetric growth. We are currently investigating the phase segregated growth at the core/alloy interfaces, particularly at the time and location of initial shell deposition.  For this, we are exploring collaborations for the study of the interface using high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) for composition analysis.

Finally, a explored a small synthesis that was inspired by the TiO₂ and core/alloy work described above. This included the synthesis of non-noble metal alloys, and oxides. We combined the insights provided above and synthesized Fe/FeCr core/alloy nanoparticles for the first time, and preliminarily studied the resulting oxidation tendencies. The Fe/FeCr nanoparticle was hypothesized to have properties similar to its bulk analogue, stainless steel.  This small summer synthesis led to a very novel discovery that the resulting nanoparticles have highly tailorable oxidation, which can result in a range of morphologies, ranging from; solid Fe₃O₄ nanoparticles, to hollow Fe₃O₄, to solid Fe nanoparticles, and even particles with a novel void morphology. A key discovery here is how to synthesize the crystalline Fe core (with b.c.c. structure), and the FeCr alloy shell. 

Taken together this summary shows that we have made positive progress towards the goals set forth in our proposal.  We have deviated slightly by adding a study of core/alloy nanoparticles due to the high novelty of those systems.  In year 2 we will show further photophysical insights into the TiO₂/qdot system, and investigate the oxidation in stainless steel nanoparticles.