Reports: DNI1051804-DNI10: Hybrid Inorganic-Organic Nanoscale Arrays: Structural Control and Mechanisms of Formation
Gordana Dukovic, University of Colorado, Boulder
In
this reporting period, we published the results of work done in the previous
reporting period focusing on the role of phosphonic acids in the synthesis of
ZnO nanocrystals (Chemistry of Materials,
2013, 25, 4321-4329). Briefly, we found that Zn(OAc)2 reacts
with phosphonic acids to form insoluble layered Zn-phosphonates, which then can
react with 1-undecanol to produce isotropic, soluble ZnO nanocrystals. To
obtain non-isotropic nanorod-shaped nanocrystals, both the molecular Zn(OAc)2
and the heterogeneous Zn-phosphonate precursors are required. As nanocrystal
synthesis is usually described in terms of homogeneous nucleation of soluble
molecular precursors, the use of insoluble layered phosphonates in the
synthesis adds a new perspective on the possible synthetic pathways to
nanoparticles.
During this reporting period, we have focused on the
fundamental structural and photochemical properties of nanoparticles of a
mixed-metal oxynitride (Ga1-xZnx)(N1-xOx).
(Ga1-xZnx)(N1-xOx) is a fascinating
recently discovered semiconductor because it absorbs visible light, with the
absorption onset dependent on the value of x, even though the constituent
semiconductors GaN and ZnO absorb in the UV. My group has developed a method to
synthesize (Ga1-xZnx)(N1-xOx) nanoparticles
with a wide range of x values (0.2 < x < 0.99) and band gaps ranging from
2.7 to 2.2. eV, using ZnO nanoparticles as one of the precursors. In the
PRF-supported work, we have investigated how the composition impacts the
ability of this material to perform photochemical reactions, such as reduction
of methyl viologen, driven by visible light. Photochemical reactivity is a
complicated function of valence and conduction band energies, surface
chemistry, and other factors. Our goal has been to unravel this complexity and
elucidate the underlying mechanisms of light-driven reactivity.
Additionally, we have worked on understanding the mechanism
of formation of oxynitride nanocrystals. In the synthesis reaction,
nanoparticles of wurtzite ZnO and spinel ZnGa2O4 are
mixed in the ratio that produces the desired value of x. The mixture is then
heated under NH3. By examining the time course of the synthesis, we
have found that spinel disappears over the course of several hours and the
wurtzite peaks in the powder x-ray diffraction (XRD) pattern shift to the
oxynitride position over the same time period. The visible absorption arises
during this time as well. No intermediate phases are found. Interestingly,
wurtzite ZnO is necessary for the formation of wurtzite oxynitride. Figure 1
shows XRD patterns of nitridation of spinel ZnGa2O4 in
the absence of ZnO, and formation of wurtzite is not observed. We are currently
writing a manuscript describing the formation of oxynitride and dependence of
various properties on composition (i.e., the value of x).
This PRF grant has had an important impact in launching my
independent academic career. It was my first nationally-recognized grant and it
allowed my students and I to pursue new and interesting research directions. In
addition to financial support, this award provided external recognition for my
research program. This recognition was helpful for obtaining other funding and
awards, such as the NSF CAREER Award, Cottrell Scholar Award, Sloan Research
Fellowship, Beckman Young Investigator Award, as well as funding from the DOE
and AFOSR. In addition, two
graduate students were trained in the course of the work supported by PRF, and
others have benefited directly and indirectly from this resource in my research
group.
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