Zhufeng Hou

Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect

Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone-Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale.

Electronic structure of N-doped graphene with native point defects

Nitrogen doping in graphene has important implications in graphene-based devices and catalysts. We have performed the density functional theory calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. The interference between native point defect and N dopant strongly modifies the role of N doping regarding the free carrier production in the bulk pi bands. For some of the defects and N dopant-defect complexes, localized defect pi states are partially occupied. Discussion on the possibility of spin polarization in such cases is given. We also present qualitative arguments on the electronic structures based on the local bond picture. We have analyzed the 1s-related x-ray photoemission and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales defect in connection with the experimental ones. We also discuss characteristic scanning tunneling microscope (STM) images originating from the electronic and structural modifications by the N dopant-defect complexes. STM imaging for small negative bias voltage will provide important information about possible active sites for oxygen reduction reaction.

Interplay between nitrogen dopants and native point defects in graphene

To understand the interaction between nitrogen dopants and native point defects in graphene, we have studied the energetic stability of N-doped graphene with vacancies and Stone-Wales (SW) defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants in defective graphene. In the presence of monovacancy, the most stable position for N dopant is the pyridinelike configuration, while for other point defects studied (SW defect and divacancies) N prefers a site in the pentagonal ring. The effect of native point defects on N dopants is quite strong: While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective graphene. Our results imply that the native point defect and N dopant attract each other, i.e., cooperative effect, which means that substitutional N dopants would increase the probability of point defect generationandviceversa.OurfindingsaresupportedbyrecentexperimentalstudiesontheNdopingofgraphene. Furthermore we point out possibilities of aggregation of multiple N dopants near native point defects. Finally we make brief comments on the effect of Fe adsorption on the stability of N dopant aggregation.

Defect States Induced by Oxygen Vacancies in Cubic SrTiO3: First-Principles Calculations

Using the first-principles calculations with the correction of on-site Coulomb interaction, we found that single O vacancy (V O ) in cubic SrTiO 3 takes a magnetic ground state in which V O induces a singly-occupied localized state in the band gap and a delocalized state in the conduction band. Our calculations showed that V O energetically prefers the singly-ionized state V O + rather than forming a pair of neutral V O 0 and doubly ionized V O 2+ states from 2V O + . Furthermore, we found that two O vacancies of a vacancy pair complex tend to align in a magnetically anti-parallel shoulder-to-shoulder configuration in the x y plane, rather than in a non-magnetic head-to-head configuration along the z direction as predicted in an available theoretical work. For O divacancies in the all five configurations considered here, localized states were found in the band gap. Our results not only clarify the puzzling findings reported in literature, but also can provide a better understanding of the role of O vacanc...

Theoretical characterization of X-ray absorption, emission, and photoelectron spectra of nitrogen doped along graphene edges.

K-edge X-ray absorption (XAS), emission (XES), and photoelectron (XPS) spectra of nitrogen doped along graphene edges are systematically investigated by using first-principles methods. In this study we considered pyridinium-like, pyridine-like, cyanide-like, and amine-like nitrogens at armchair and zigzag edges and pyrrole-like nitrogen at armchair edge as well as graphite-like nitrogen at graphene interior site. Our results indicate that nitrogen configuration and its location (armchair or zigzag edge) in nitrogen-doped graphene can be identified via the spectral analysis. Furthermore, some controversial spectral features observed in experiment for N-doped graphene-like materials are unambiguously assigned. The present analysis gives an explanation to the reason why the peak assignment is usually made differently between XPS and XAS.

Effects of Al addition on the native defects in hafnia

Two occupied native defect bands are experimentally detected in pure HfO2. The density of states of band one in the middle of the band gap reduces drastically with the Al addition, while that of band two slightly above the valence-band maximum remains rather unaffected. We attribute the two bands to the charged oxygen vacancy, and the oxygen-interstitial-related defect states of the HfO2, respectively. We demonstrate that the added Al passivates the VO+ induced midgap states but has little effect on other aspects of the electronic structure of the material.

Effects of Y doping on the structural stability and defect properties of cubic HfO2

First-principles calculations have been performed to study the structural and electronic properties of pure and Y-doped cubic HfO2. It is found that Y doping in HfO2 would increase the stability of the cubic phase relative to the monoclinic phase by reducing the energy difference and the phase transition pressure. This result is consistent with the observed stabilization of the cubic phase of HfO2 by the addition of Y. The calculated formation energy of the VO–YHf complex defect in different charged states indicates that the single positively charged state (VO–YHf)+ is more stable than the neutral state (VO–YHf)0 and the double positively charged state (VO–YHf)++ in Y-doped cubic HfO2. Because the number of d-electrons of Y is less than that of Hf by one and substitutional Y for Hf introduces holes in the oxygen p-band, Y doping would make the highest occupied defect level induced by (VO–YHf)+ fall into the valence band rather than the energy gap, which explains the experimental observation that gap state...

Effect of Al addition on the microstructure and electronic structure of HfO2 film

We have investigated the microstructures and electronic structures of a series of hafnium aluminate (HfAlO) films with Al concentration ranging from 0% to 100%. When the films evolve from pure HfO2 to pure Al2O3 by increasing the aluminum content, we find changes in their radial distribution functions, which disclose the short-range order of the materials, despite the amorphous nature of all films. The HfAlO films (with Al∕Hf ratio ranging from 0.25 to 5.8) appear to be a single glassy phase of Hf, Al, and O, instead of simple mixtures of HfO2 and Al2O3. The Hf (Al)–O, Hf (Al)–Al, and Hf–Hf bonds are observed to be insensitive to the amount of Al in the film, except when the Al concentration is large (Al∕Hf∼5.8), in which case the bonding is similar to that in pure Al2O3. Although the local symmetry of Hf in amorphous HfO2 is suggested by the electron energy-loss spectrum taken at an oxygen K edge, it is largely disrupted when Al is introduced. The valence electron energy-loss spectroscopy reveals three d...

Energetics and electronic structure of aluminum point defects in HfO2: A first-principles study

Using the plane-wave pseudopotential method within the generalized gradient approximation, we studied the atomic structure, energetics, and electronic structure of the interstitial and substitutional point defect of dopant aluminum in monoclinic HfO2. Our results show that the doped Al atom energetically prefers to substitute for the Hf atom under the oxygen-rich condition. Substitution of Al for Hf creates a shallow acceptor level near the valence band maximum, whereas both substitution of Al for O and interstitial Al introduce deep levels in the band gap of HfO2. We also discussed the possible effect of Al doping on the electronic properties of HfO2.

Designing Nanostructures for Phonon Transport via Bayesian Optimization

Phonon transport---the movement of vibrational wave packets in a solid---in nanostructures is a key element in controlling solid heat conduction, but it remains a complex design challenge. A new framework uses informatics and phonon transport calculations to greatly accelerate the design process and reveals nonintuitive structures that are more effective than their traditional counterparts.