PNNL-13501 - Pacific Northwest National Laboratory

demonstrated that quantum dots with the desired
electronic and optical properties can be formed via a selfassembled
process.
Synthesis of Self-Assembled Cu2O Quantum Dots
We successfully synthesized Cu2O quantum dots on
SrTiO3 and TiO2 (anatase) substrates using the molecular
beam epitaxial (MBE) method. The structure and
chemical states of these Cu2O quantum dots were
characterized using x-ray diffraction (XRD) and x-ray
photoelectron spectroscopy (XPS). These data showed
that by carefully controlling the partial pressure of oxygen
and Cu flux during the growth we were able to selectively
grow Cu2O or CuO dots under different conditions.
Figure 1 is an atomic force microscopy (AFM) image that
shows Cu2O quantum dots grown on a SrTiO3 substrate.
By controlling the growth time and substrate temperature,
we were able to control the dot size and density. For
example Cu2O dots with diameters ranging from less than
10 nm to over 100 nm were demonstrated by varying
growth conditions.
Figure 1. An atomic force microscopy image showing selfassembled
Cu 2O quantum dots grown on a SrTiO 3 substrate
High-resolution scanning-Auger microscopy was used to
determine the elemental distribution of the resulting
surface after the growth to verify chemical identity of the
dots observed by atomic force microscopy. These data
confirmed that protrusions from the atomic force
microscopy images were Cu2O, as shown in Figure 2.
The scanning-Auger microscopy also showed that the
amount of copper in areas between the protrusions was
only about 1.5%, less than a monolayer of Cu2O. This
suggests that SrTiO3 substrates were exposed in the areas
not covered by the Cu2O dots—a prerequisite for surface
charge separation and simultaneous photo-reduction and
oxidation reactions upon above-gap-photon excitation.
These results suggest that from the geometric-and-
340 FY 2000 Laboratory Directed Research and Development Annual Report
(a) (b)
Figure 2. Scanning-Auger microscopy image showing
distribution of copper (a) and titanium (b) on a
Cu 2O/SrTiO 3 surface determined by scanning Auger
microscopy
electronic-structure point of view, these Cu2O/ SrTiO3
systems can be used as electrodes for
photoelectrochemical reactions.
Determination of Electronic Structures and Band
Offsets Between Cu2O and SrTiO3
In addition to surface morphology and elemental
distributions, x-ray photoelectron spectroscopy was used
to examine the chemical state and electronic structure of
Cu2O/SrTiO3. These data showed that the copper oxide
grown under the appropriate conditions was indeed
copper (I) in valance, consistent with the x-ray
photoelectron spectroscopy finding. In addition, by
conducting x-ray photoelectron spectroscopy
measurements at different stages of growth, we
determined that the conduction and valance band offsets
between Cu2O and SrTiO3 were 0.6 and 1.8 eV,
respectively. Combining this result with the known flat
band potential of SrTiO3, we found that energetics of
Cu2O/SrTiO3 were favorable for splitting water via a
photocatalysis reaction, as shown in Figure 3. We further
examined the stability of Cu2O quantum dots grown on
SrTiO3 substrates. We exposed a Cu2O/SrTiO3 sample to
water (pH=9) and irradiated with white light (75W) for
45 minutes. Atomic force microscopy and x-ray
photoelectron spectroscopy measurements of Cu2O
quantum dots before and after the water exposure showed
no changes in Cu2O morphology and chemical states.
Synthesis and Reaction Tests of Heterostructured
Cu2O/SrTiO3 Powders
We synthesized heterostructure Cu2O/TiO2 powders
through chemical routes to significantly increase the
active surface area. By carefully controlling reaction
routes, we were able to successfully synthesize single
phase Cu2O powders and coupled Cu2O/TiO2 powders.
X-ray diffraction of these heterostructure powders showed