AVS 67 Session LD-Contributed On Demand: Leaders in Energy and the Environment Contributed On Demand Session

Carbon nanospikes show excellent energy and selectivity for electrochemical conversion of CO₂ to ethanol and N₂ to NH₃. Transmission electron microscopy (TEM) images reveal that these nanospikes are 50- 80 nm in height and terminated with a sharp tip of radius ~1 nm. It has been proposed that catalytic activity follows from an enhanced electric field from the sharp spike topography. We have tested this hypothesis through measurements of the absolute work function using ultraviolet photoemission spectroscopy (UPS). As samples were characterized after transfer through air, surface cleanliness was monitored with x-ray photoelectron spectroscopy (XPS). Annealing as-grown samples above 275 °C produces a clean surface which has a 4.13 eV work function, a half volt lower than that of flat graphite. This reduction supports the idea that a strong electric field is generated by the nanospike topography.

Contamination of the spiked surface raises the work function above that of the clean nanospikes. Work function values after exposure are increased to 4.19 eV for hydrocarbons, 4.30 for air, 4.42 for water, and 4.48 for oxygen. Blunting the spikes, by exposure to an oxygen plasma, argon sputtering, or annealing to 800 °C results in a work function close to that of flat graphite, 4.62 eV. An unusual double onset in the UPS secondary electron intensity is observed from as-grown nanospike samples and is reproduced by absorbing hydrocarbons on clean nanospikes. This double onset has not been reported on metal surfaces and appears to be unique to carbon substrates. The energy difference between the two onsets, about 0.4 eV is close to that of a typical C-H stretch in hydrocarbons suggesting the lower energy onset may originate in inelastic scattering of photoelectrons.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility

Porous CaFe₂O₄ (pCFO), a promising conversion-type anode material for next-generation lithium-ion batteries (LIB) and hybrid supercapacitors (HSC), is relatively easy to produce via solution combustion synthesis (SCS). However, despite evidence that the morphology, composition, and electrochemical performance of metal oxides can be enhanced by changing the fuel-to-oxidizer ratio during SCS, such treatment has yet been applied to pCFO. In this study it is demonstrated that increasing the ratio across a “moderately fuel-lean” to “moderately fuel-rich” range increases the porosity, inter-particle conglomeration, and specific capacity of pCFO, further cementing the importance of stoichiometric considerations in SCS. Future implications of extending the range of the ratio and optimizing calcination temperature and time in order to reach the theoretical capacity target are also discussed.

The photovoltaic properties of selenium were discovered about 150 years, and shortly afterward, selenium was utilized to fabricate the first 'thin-film' solar cell. This discovery led to modern research of semiconductors and development of related technologies. Selenium was implemented in many of these devices and applications. Following the rise of alternative semiconductors, and specifically silicon, the usage of selenium slowly declined. In recent years, the search for simple and scalable photovoltaic materials has revived the research of selenium as an elemental semiconductor that may be implemented as a possible absorber for extremely low-cost and highly scalable solar cells. Selenium has few desirable properties for these devices such as high absorption coefficient, intrinsic environmental stability, and low-temperature fabrication (below 200°C).

Herein we study the main parameters that define the quality and efficiency of selenium-based thin-film solar cells, fabricated by vacuum thermal evaporation, and optimize them using modern tools and approach. We found that careful annealing of the selenium film, very close to its melting temperature, is essential to obtain high-quality crystalline films. Moreover, the anisotropic structure of selenium, composed of 1D chains, suggests that alignment of the crystals in the thin film may improve the device properties, and we study the means to control the film orientation. Finally, we indicate the importance of light illumination (and excitation) of the film during the annealing process, enhancing and improving the film's crystallization.

Following these results, we study how the selenium bandgap (1.7 eV), can be tuned to the optimal value for photovoltaic absorber (1.2-1.4 eV), by alloying selenium with the isomorphic low bandgap semiconductor tellurium. Selenium-tellurium alloys exhibit a strong non-linear shift of the conduction band minimum energy, leading to a non-linear decrease of the bandgap. Consequently, the desired bandgap energy can be obtained by alloying a relatively small amount of tellurium (~10%). Thin-film solar cells based on the selenium-tellurium alloy indeed show improved current density and spectral response due to the optimized bandgap. The overall efficiency of the selenium-tellurium devices is still lower than the pure selenium devices, due to lower voltage and fill-factor. We indicate the leading causes for the lower properties and suggest how to resolve them and improve the device efficiency.

(1) Hadar, I.; Song, T. Bin; Ke, W.; Kanatzidis, M. G. Modern Processing and Insights on Selenium Solar Cells: The World's First Photovoltaic Device. Adv. Energy Mater. 2019.

(2) Hadar, I.; Hu, X.; Luo, Z.-Z.; Dravid, V. P.; Kanatzidis, M. G. Nonlinear Band Gap Tunability in Selenium–Tellurium Alloys and Its Utilization in Solar Cells. ACS Energy Lett. 2019, 4 (9), 2137–2143.

Aldol condensation creates C-C bonds between organic oxygenates, providing a useful pathway for upgrading of biomass-derived feedstock to fuels and higher value chemicals. To develop a better understanding of this reaction and factors that limit activity and selectivity on ceria, we have explored the reaction pathways for self-condensation of acetaldehyde (AcH) to crotonaldehyde (CrH) on ceria nano-octahedra using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and modeling based on DFT calculations.

As opposed to a lack of C-C coupling activity in UHV,¹ we find that CrH forms readily on ceria nano-octahedra at ambient temperature under a flow of AcH. All major peaks detected in DRIFTS are assigned conclusively by comparison with simulated IR spectra to either AcH*, trans-CrH*, or a stable intermediate state, as adsorbed on CeO₂(111). The reaction on stoichiometric sites proceeds through enolization of AcH and C-C coupling between the enolate and molecular AcH. The C-C coupled intermediate transforms into 3-hydroxylbutanal (3HBtL), which further enolizes and dehydroxylates to form CrH. A slightly less stable cis isomer of CrH* forms via the same reaction pathway with different molecular orientations in C-C coupling, but the trans isomer is more stable and is estimated to dominate in coverage. The reaction, when occurring on a point surface oxygen vacancy (V_O), leads to a build-up of the enolate of 3HBtL/V_O, with a 1620 cm^-1 peak in the DRIFTS spectra that is assigned to νC=C mode of this intermediate. Desorption of CrH from V_O is much more hindered than from stoichiometric sites, so V_O is not an active site at ambient temperature.²

Analysis of the reaction kinetics suggests that, the desorption-limited reaction rate should be zero-order in AcH partial pressure if the surface is mostly occupied, or second-order if the reaction is at low conversion. For this and similar reactions, therefore, enhancing facile product desorption would be key to having a workable catalyst based on ceria or similar oxides.

(1) Chen, T. -L.; Mullins, D. R., J. Phys. Chem. C.115 (2011) 3385-3392; Zhao, C. et al., J. Phys. Chem. C.123 (2019) 8273-8286.

(2) Bhasker-Ranganath, S. et al., ACS Catal. (2021).

Single-atom catalysts are often comprised of isolated metal atoms anchored on oxide supports and have recently gained considerable attention for their improvements in selectivity and reactivity for important industrial reactions such as preferential oxidation of CO. We have previously shown that single Pt atoms on an atomically thin copper oxide film grown on Cu(111) are able to perform low temperature CO oxidation without sintering, a common deactivation mechanism of single atom catalysts. Using a combination of scanning tunneling microscopy, infrared and x-ray photoelectron spectroscopy, and temperature programmed desorption experiments we elucidated the atomic-scale surface structure of these model catalysts and related that structure to catalytic function. We found that Pt atoms have a preferred binding site on the oxide surface and preferentially activate CO but not H₂. These results were then used to design a nanoparticle catalyst analog which functions under industrially relevant preferential oxidation of CO in H₂ (CO-PROX) conditions. Moving beyond this initial work which made use of a thin film oxide, we are now in the process of characterizing the 111 facet of a bulk Cu₂O single-crystal. This will provide a useful model catalyst surface as it better mimics the Cu₂O particles present in high surface area catalysts. Taken together, our thin-film and bulk copper oxide surfaces with isolated Pt atoms further the atomic-scale understanding of single-atom catalysts which should aid in the rational design of next-generation catalysts.