62nd Annual Report on Research 2017

Nanoscience is a new research direction for the Kohler Group, and the students who have been supported by this project have helped to rapidly grow our abilities and expertise in this area. Graduate student Natasha Pettinger published a paper that resulted in a new collaboration to observe the nucleation and growth of CeO₂ nanocrystals. Postdoc Sascha Froebel and Graduate student Jennifer Empey synthesized several new types of CeO₂ nanocrystals and studied them by the transient absorption (TA) technique. Undergraduate Sangeetha Natarajan will present a poster at an undergraduate science conference later this fall. The remainder of this report summarizes technical advances made during year 1.

Synthesis and Nanoparticle Characterization

We synthesized and characterized a range of CeO₂ nanoparticles (Table 1). CeO₂ nanocrystals formed through hydrolysis of Cerium(IV) Ammonium Nitrate (CAN) (CeO₂-CAN-AS), were characterized by High-Resolution Transmission Electron Microscopy (HR-TEM) at OSU's Center for Electron Microscopy and Analysis (CEMAS). These images reveal polydisperse nanoparticles 3 to 9 nm in diameter, with several crystallites per nanoparticle. The particles can be isolated by solvent evaporation (CeO₂-CAN-D), and then added to water or alcohol to make acidic, non-turbid suspensions.

Table 1. Summary of CeO₂ nanoparticle samples.

Absorption characteristics

When CeO₂-CAN-D nanoparticles are suspended in ethanol or tert-butyl alcohol, the solution assumes an orange-yellow color (Figure 1a). Addition of small amounts of glycerol to CeO₂-CAN-D in water and CeO₂-CAN-AS causes a slight red shift of the absorption onset (Figure 1b). These observations are tentatively ascribed to interfacial charge transfer from adsorbed alcohol molecules to CeO₂. Our finding that the absorption spectrum depends sensitively on the adsorbate calls into question the practice of using spectral fitting to estimate the bandgap of CeO₂.

Figure 1. (a) The absorption spectra of CeO₂-CAN-D suspended in water (green curve), ethanol (blue curve), and tert-butyl alcohol (purple curve) normalized to their respective absorbance maxima. (b) The absorption onset of CeO₂-CAN-AS/D shifts to longer wavelengths as glycerol is added to the aqueous suspension.

Interaction with UV radiation

A major aim is to understand how oxygen vacancies alter the dynamics of photogenerated carriers. While attempting to accumulate Ce³⁺ defects in our nanoparticles using UV radiation, we discovered that CeO₂-CAN nanocrystals undergo photoreductive dissolution with high quantum efficiency, releasing Ce³⁺ ions into solution (Figure 2). Photocorrosion has been observed in metal oxides like ZnO, but has not been reported previously for CeO₂.

Figure 2.  Absorption spectra during irradiation of CeO₂ at pH 1.6 in a 1 cm cuvette (open circles). The solid lines are linear combinations of the absorption spectra of CeO₂ and Ce(ClO₄)₃. The inset shows the concentration of each species vs. irradiation time.

Experiments are underway to determine how this process is affected by surface functionalization of the nanoparticle, pH, ionic strength, and nanoparticle morphology. We have observed that dissolution efficiency increases sharply in the presence of glycerol (Table 2). Increasing the ionic strength also leads to a modest (7%) increase in the photoreduction efficiency.

Table 2. Quantum yields at 300 nm for the photorelease of Ce³⁺ ions from CeO₂-CAN-AS nanoparticles as a function of glycerol and NaClO₄ concentration.

Charge-Carrier Dynamics

We conducted broadband ultrafast TA measurements on CeO₂ nanoparticles and on several nm TiO₂ nanoparticles for comparison. All three ceria systems studied (CeO₂-CAN-AS/D, CeO₂-HMTA, and CeO₂-SA) show two positive absorption bands: A stronger band with a maximum around 350 nm and a weaker band with absorption throughout the visible (Figure 3). While surface passivation and solvent dramatically affect the steady-state photochemistry, they have no effect on the TA signals in the region where there is no ground state absorption. In contrast, TA signals recorded at probe wavelengths of 320 to 450 nm, where ground state absorption is present, are shifted by a constant amount when glycerol is added but otherwise exhibit identical kinetics. Electron transfer from an adsorbed glycerol molecule appears to be very efficient with no observable back electron transfer. These results suggest that two classes of excitations are generated—surface excitations responsible for the efficient photoreduction and excitations within the bulk of the nanoparticle, which are responsible for the TA signals.

Figure 3. TA spectra of CeO₂-CAN-AS/D (blue curve), CeO₂-HMTA (yellow curve), and CeO₂-SA (green curve) normalized to the UV-peak intensity. The pump wavelength is 320 nm.

The visible absorption band decays according to a power law, with half-lives ranging between 1.5 and 16 ps, depending on nanoparticle morphology. Generally, the positive TA signals decay more slowly for larger than for smaller crystals (Figure 4).

Figure 4. Kinetic traces collected from the indicated ceria samples with a probe wavelength of 600 nm following excitation at 320 nm.

Ultrafast time-resolved infrared absorption (TRIR) measurements were performed on CeO₂-CAN-AS and CeO₂-CAN-D nanoparticles. Direct excitation of TiO₂ nanoparticles at 300 nm results in a broad positive absorption signal that decays with a half-life of 7.5 ps (Figure 5) due to free electrons in the conduction band. On the other hand, the CeO₂ nanoparticle TRIR signal closely resembles the solvent signal, with a weak, quickly decaying signal around time zero. The lack of a TA signal in the IR may indicate that electrons in CeO₂ trap to localized states within the laser pulse. Cerium-doped TiO₂ shows an intermediate decay (Figure 5a), suggesting that conduction band electrons produced in TiO₂ may trap on Ce(IV) sites and cease to absorb in the mid-IR. Our TA results suggest that electrons and holes in CeO₂ are highly localized and have low mobility.

Figure 5. TRIR kinetics following excitation at 300 nm for TiO₂ (red), Ce(IV)-doped TiO₂ (purple), CeO₂-CAN-D (blue), and nitric acid (gray) in D₂O solutions.