59th Annual Report on Research 2014

One-pot hydrothermal synthesis of Ce_xZr_1-xO₂ (0£x£1)

Figure 1 shows the XRD patterns for a series of Ce_xZr_1-xO₂ (0£x£1) from pure CeO₂ and ZrO₂ to mixed oxides (or solid solutions) nanopowders, prepared via hydrothermal reactions at 150 ^oC with a dwell time 48 hrs. The diffraction peaks of pure CeO₂ and ZrO₂ were indexed as face-centered cubic fluorite structured CeO₂ (JCPDS file 34-0394) and monoclinic ZrO₂ (JCPDS 37-1484) with lattice parameters of a=5.156 Å, b=5.191 Å, c=5.304 Å, respectively. The XRD patterns of the mixed Ce_xZr_1-xO₂ (0<x<1) did not show any extra diffraction peaks due to the doping ZrO₂ component from x=0.9 up to x=0.2 but the presence of Zr in individual nanocrytals was evidenced by EDX analysis with a focused electron probe in STEM mode and XRF compositional analysis.

Figure 1. X-ray diffraction patterns of Ce_xZr_1-xO₂ (0£x£1) nanocrystals via a one-step hydrothermal reaction at 150 ^oC for 48 hrs.

It should be pointed out that all Ce_xZr_1-xO₂ (0£x£1) samples, reported in Figure 1, were stirred on a magnetic hot plate for 40 mins at room temperature before the hydrothermal reactions. It was found that the stirring step was playing a critical role in the control of compositional homogeneity, particle size, and morphology of final products. For example, for pure CeO₂, nanocubes with a size of 20 nm was obtained without initial stirring, but irregular shape nanocrystals with much smaller particle size (4~5 nm) were obtained if stirring the mixture of Ce(NO₃)₃ and NaOH for 40 mins before loading into the autoclave reactor, as shown in TEM images of Figure 2 (a-1 and b-1). HRTEM images and the corresponding fast Fourier transform (FFT) diffractograms are shown in Figure 2 a-2 and b-2 in order to study the detailed crystalline feature of such polyhedral nanocrystals. The inserted diffractorgrams confirm the cubic (or psudocubic) structure and single crystal characteristics of nanocrystals, and the HRTEM images show the dominant {111} surface facets for irregular particles for the stirred samples and {200} surface facets for the nanocubes in the unstirred samples.

Figure 2. TEM images of CeO₂ nanocrystals prepared by hydrothermal reaction at 150 ^oC with (a) and without (b) stirring treatment.

H₂-Temperature programmed reduction (H₂-TPR)

Figure 3 shows H₂-TPR profiles of Ce_xZr_1-xO₂ (0£x£1) nanocrystals. In pure CeO₂, the H₂-TPR profile shows two major reduction peaks corresponding to the surface and bulk reductions, respectively, which are characteristics of reduction temperature and the distribution between the surface and bulk reduction percentage. The coordinately unsaturated surface capping oxygen ions can be easily removed in the low temperature region. However, bulk oxygen requires to be transported to the surface before their reduction. Consequently, the bulk reduction takes place at higher temperature compared to the surface reduction. For pure ZrO₂ nanocrystal, the profile is relative flat, which indicates only a small quantity of H₂ was consumed or absorbed on pure ZrO₂.

Similar H₂-TPR results were obtained over Ce_xZr_1-xO₂ (0<x<1) mixed oxide nanocrystals. Unlike the two surface/bulk reduction peaks in pure CeO₂, only single H₂ consumption peak was observed for Ce_xZr_1-xO₂ (0<x<1) mixed oxides. This result clearly shows an improved low-temperature activity in Ce_xZr_1-xO₂ (0<x<1) compared to pure CeO₂. This is consistent with the literature that the reduction peak of CeO₂ shifted to lower temperature by zirconium addition.  One possible explanation for this is that oxygen defects are made in the CeO₂-ZrO₂ solid solution by the existence of zirconium and it is inferred that mobility of oxygen becomes higher, and then the rate of redox reaction becomes faster.

Figure 3. H₂-TPR profiles of Ce_xZr_1-xO₂ (0£x£1) nanocrystals.

Impact on PI and the students

Since beginning at YSU in 2010, PI has initiated a research program in materials chemistry. This ACS PRF grant strongly helped PI recruit several student researchers in the group and support PI’s research. So far, this funding also leaded to publications with graduate/undergraduate co-authors in RSC advances, Journal of Materials Chemistry and Advanced Materials Research. In PI’s laboratory, students plan and set up hydrothermal reaction, use characterization techniques including PXRD, SEM, TGA, and IR/Raman, read relevant literature, and write about and present their work. Three undergraduates and two high school students have worked in PI’s lab for a semester or more. One graduate student started to work on the current project from Fall semester 2013. In addition to three students have been coauthors on seven presentations PI has made at regional and national meetings.

Future activities

We are currently finishing up the atomic level structure and chemistry of the low-temperature active cerium based oxide nanocrystals, and preparing to publish several manuscripts.