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

Overview

The PRF support enables the team to pursue rare earth based nanomaterials (RENs) for solar cell applications. Taking advantage of RENs' unique optical properties, that is, absorption in the near infrared (NIR), and emission visible photons via up conversion (UC). The RENs can be placed at the rear of, for example, petroleum-based photovoltaic cells to harvest NIR radiation. The photoluminescent photons can then be reflected back to the cell to improve the overall photoelectrical conversion efficiency. Improvement of absorption and UC efficiency is thus the key to realizing RENs' application. Two series of nanostructures are planned: (a) NaYF₄:Yb³⁺/Er³⁺ and NaYF₄:Yb³⁺/Tm³⁺ core with a NaYF₄ shell for optimal luminescent intensity, and (b) NaYF₄:Yb³⁺/Er³⁺ and NaYF₄:Yb³⁺/Tm³⁺ core with a SiO₂ shell for improved stability and facilitation of further function and assembly on surfaces. High quality, well-dispersed RENs will be produced and characterized using X-ray diffraction, electron microscopy and spectroscopy. Using a home-built apertureless near-field scanning optical microscope (NSOM), single particle spectroscopy will be acquired in conjunction with high resolution imaging to further understand the impact of macroscopic variables like size, geometry, coating and sample preparation on absorption and emission intensity and lifetime. The impact of local structure around the lanthanide dopants will be probed with nuclear magnetic resonance (NMR) spectroscopy. Collectively, the proposed investigation shall provide new insight into the structure-emission relationship, and guide the optimization of RENs for photovoltaic applications.

Progress in year 1

Improving Hematite's Solar Water Splitting Efficiency by Incorporating Rare Earth Upconversion Nanomaterials

Composite materials consisting of hematite films and rare earth nanocrystals of NaYF₄:Yb³⁺/Er³⁺ have been produced and characterized using AFM, TEM and spectroscopy. We have demonstrated that RENs enable harnessing IR radiation for water splitting by hematite. AFM, SEM and EDS characterizations verify the chemical and structural integrity of hematite films and RENs in those materials. The spectroscopy studies indicate (a) the absorption of 980 nm radiation by RENs and emission at 550 nm and 670 nm regions via upconversion process; (b) the emitted photons at 550 nm regions are absorbed by surrounding hematite. The photocurrent enhancement is clearly observed when the composite materials are illuminated by a 980 nm laser, as shown in Figure 1. These observations collectively demonstrate that incorporation of RENs into hematite films leads to higher efficiency and performance in solar water splitting, because the RENs enable harvesting more photons at IR range than conventional hematites which only absorb UV and visible radiation. This concept of using RENs could be applied to general semiconductor photoelectrodes.

Figure 1. (A) A schematic diagram of the photoelectrochemical measurements. (B) Current density (red) of the REN/hematite electrodes was measured in the duration of the experiments, in which the 980 nm laser was turned on and off. The same electrode without RENs was also measured (green) as a negative control for comparison. (C) I-V measurements were taken for REN/hematite composite films with (red) and without (green) 980 nm illuminations.

PRF support enabled us to support a postdoctoral researcher and a graduate student, as well as establish collaborations with Boston University and Peking University in China. The first manuscript of this work is in press in ACS Nano.

Solid State NMR Study of the Local Environment of Ln Ions

The atomic level environment of the RENs have been systematically studied using solid state NMR. 19F and 23Na NMR spectra were obtained for these powdered solids using a homebuilt 9.4 T spectrometer based on a Tecmag Apollo pulse programmer. Both inversion and saturation recovery pulse sequences were used to measure the 19F and 23Na spin lattice relaxation times in these samples.

The 19F spin lattice relaxation times for the RENs were measured using powdered solid as a function of Yb and Er content. The 19F spin lattice relaxation rate 1/T1 was calculated from the values, matching simple functions of

1/T1(Ybx) = (0.613 kHz/equiv) x – 0.012 KHz

1/T1(Ery) = (1.922 kHz/equiv) x – 0.012 KHz

1/T1(YbxEry) = (0.571 kHz/equiv) x + (1.76 kHz/equiv) y – (8.61 kHz/equiv2) x y – 0.011 KHz

where R2 ≥ 0.96 and RMSE ≤ 0.013 in all cases.

The PRF support enabled us to support a graduate student for this portion of the project. Manuscript of this work is in preparation.