arXiv subjects

Walter Orellana

Publications and source records attributed to Walter Orellana.

8 recordsLinked to original sources

arXiv2026

Understanding the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Metal Intercalated Biphenylene Bilayers

We conducted an {\it ab initio} study of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-encapsulated biphenylene bilayers, B/M/B, with M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, W, Os and Pt. In most systems, the intercalated metal sits at the square carbon sites (C$^{468}$) of the biphenylene lattice. Using a computational hydrogen electrode approach, we evaluated the reaction energetics at these active sites. Several B/M/B systems show competitive ORR and OER performance. Among the investigated systems, Cu, Pt, Ru, and Mn exhibit the lowest ORR overpotentials of 0.42, 0.44, 0.50, and 0.56 V, respectively, while Fe is identified as the most active catalyst for OER with an overpotential of 0.44 V. To understand the catalytic trends, we looked at the electronic structure through the metal $d-$band centers, the C$^{468}$ $p_z-$band centers, and the corresponding orbital charge populations. The band centers did not give a simple polynomial dependence on the overpotentials, though they did point to favorable electronic ranges for the best catalysts. The $d-$orbital charge population of the encapsulated metal, however, correlated most clearly with activity-especially for OER-yielding volcano-type plots. From these, B/Fe/B emerges as the best OER catalyst, while B/Mn/B lies closest to the ORR optimum. The $p-$orbital population at the active carbon site also captures the main trends, albeit less strongly. Overall, these results show that straightforward electronic descriptors can predict catalytic behavior in metal-encapsulated biphenylene bilayers and guide the search for efficient catalysts where the carbon framework itself drives the reactivity.

cond-mat.mtrl-sci↗

arXiv2026

Metal-Coordination Effects on the Stability and ORR/OER Activity of Layered Organometallic Single-Atom Catalysts: A Theoretical Study

Organometallic layered materials have emerged as promising single-atom catalysts for oxygen reduction and evolution reactions, but their practical use has been limited by insufficient electrochemical stability. Here, we present a density functional theory study clarifying the relationship between catalytic activity and stability in organometallic single-atom catalysts with metal-N$_4$ (MN$_4$) and metal-O$_4$ (MO$_4$) coordination. We compare graphene-embedded MN$_4$ motif and phthalocyanine-like frameworks with MO$_4$-coordination frameworks, including M$_4$(OHPTP)$_2$ and M$_3$(HHTP)$_2$ (M = Mn, Fe, Co, Ni, Cu, Zn). Stability is assessed by surface Pourbaix analysis, while activity is evaluated using the computational hydrogen electrode method. MN$_4$ systems show competitive overpotentials but suffer strong pH-dependent instability. In contrast, MO$_4$ frameworks exhibit enhanced robustness across wide pH ranges while maintaining good catalytic performance. A proposed stability descriptor enables direct comparison across systems, identifying MO$_4$ coordination structures, particularly M$_4$(OHPTP)$_2$ (M = Zn, Co) as optimal for balancing activity and stability in practical electrocatalysis.

cond-mat.mtrl-sci↗

arXiv2018

On the accuracy of the HSE hybrid functional to describe many-electron interactions and charge localization in semiconductors

Hybrid functionals, which mix a fraction of Hartree-Fock (HF) exchange with local or semilocal exchange, have become increasingly popular in quantum chemistry and computational materials science. Here, we assess the accuracy of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to describe many-electron interactions and charge localization in semiconductors. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain the accurate ground-state spin densities of the negatively charged (SiV)$^-$ and the neutral (SiV)$^0$ silicon-vacancy center in diamond, and of the cubic silicon carbide (3C-SiC) with an extra electron. We compare our DMC results with those obtained with the HSE functional and find a good agreement between both methods for (SiV)$^-$ and (SiV)$^0$, whereas the correct description of 3C-SiC with an extra electron crucially depends on the amount of HF exchange included in the functional. Also, we examine the case of the neutral Cd vacancy in CdTe, for which we assess the performance of HSE against the many-body \emph{GW} approximation for the description of the position of the defect states in the band gap.

cond-mat.mtrl-sci↗

arXiv2017

Self-compensation in phosphorus-doped CdTe

We investigate the self-compensation mechanism in phosphorus-doped CdTe. The formation energies, charge transition levels, and defects states of several P-related point defects susceptible to cause self-compensation are addressed by first-principles calculations. Moreover, we assess the in uence of the spin-orbit coupling and supercell-size effects on the stability of AX centers donors, which are believed to be responsible for most of the self-compensation. We report an improved result for the lowest-energy configuration of the P interstitial (P$_\text{i}$) and find that the self-compensation mechanism is not due to the formation of AX centers. Under Te-rich growth conditions, (P$_\text{i}$) exhibits a formation energy lower than the substitutional acceptor (P$_\text{Te}$) when the Fermi level is near the valence band, acting as compensating donor. While, for Cd-rich growth conditions, our results suggest that p-type doping is limited by the formation of (P$_\text{Te}$-V$_\text{Te}$) complexes.

cond-mat.mtrl-sci↗

arXiv2016

First-principles DFT + GW study of the Te antisite in CdTe

Formation energies, charge transitions levels, and quasiparticle defect states of the tellurium antisite $(\text{Te}_\text{Cd})$ in CdTe are addressed within the DFT${0.05cm}+{0.05cm}$\emph{GW} formalism. We find that $(\text{Te}_\text{Cd})$ induces a (+2/0) deep level at 0.99 eV above the valence band maximum, exhibiting a negative-U effect. Moreover, the calculated zero-phonon line for the excited state of $(\text{Te}_\text{Cd})^0$ corresponds closely with the $\sim$1.1 eV band, visible in luminescence and absorption experiments. Our results differ from previous theoretical studies, mainly due to the well-known band gap error and the incorrect position of the band edges predicted by standard DFT calculations.

cond-mat.mtrl-sci↗

arXiv2015

Noncovalent functionalization of carbon nanotubes and graphene with tetraphenylporphyrins: Stability and optical properties from ab-initio calculations

The stability, electronic and optical properties of single-walled carbon nanotubes (CNTs) and graphene noncovalently functionalized with free-base tetraphenylporphyrin (TPP) molecules is addressed by density functional theory calculations, including corrections to dispersive interactions. We study the TPP physisorption on 42 CNT species, particularly those with chiral indices ($n$,$m$), where $5 \leq n \leq 12$ and $0\leq m\leq n$. Our results show a quite strong $π$-$π$ interaction between TPP and the CNT surface, with binding energies ranging from 1.1 to 1.8 eV, where higher energies can be associated with increasing CNT diameters. We also find that the TPP optical absorptions would not be affected by the CNT diameter or chirality. Results for the TPP physisorption on graphene show a remarkable stability with binding energy of 3.2 eV, inducing a small redshift on the $π$-stacked TPP absorption bands. The strong graphene-TPP interaction also induces a charge transfer from TPP to graphene, indicating a $n$-type doping mechanism without compromising the graphene structure.

cond-mat.mtrl-sci↗

arXiv2015

Single- and double-wall carbon nanotubes fully covered with tetraphenylporphyrins: Stability and optoelectronic properties from ab-initio calculations

The optoelectronic properties of single- and double-wall carbon nanotubes (CNTs) noncovalently functionalized with tetraphenylporphyrins (TPPs) are addressed by dispersion-corrected ab initio calculations. Five CNT species with different chiralities were considered. We find that the most stable configurations are those where the CNTs are fully covered by TPPs, exhibiting binding energy of about 2~eV/TPP. The semiconducting CNT-TPP compounds show optical response characterized by a strong absorption associated to the TPP bands, with increasing intensity with the TPP concentration. In addition, molecular dynamic simulations show that the compounds would be stable at temperatures as high as 100$^{\circ}$C.

cond-mat.mtrl-sci↗

arXiv2003

Energetics of nitrogen incorporation reaction in SiO2

We study using first-principles calculations the energetics, structural and electronic properties of nitrogen incorporation in SiO2. We consider NO, NH, N2 and atomic N as the nitriding species interacting with a Si-Si bond of an otherwise perfect SiO2 network in order to simulate the nitrogen incorporation near Si/SiO2 interface regions. We find that all the species react with the Si-Si bond forming bridge structures with the Si atoms without dissociating, where NH and atomic N form the most stable structures. Concerning the electronic properties, our results show that the incorporated NH is the only structure which does not introduce trapping center at the interface. The structures involving NO and atomic N are acceptors, whereas that involving N2 may be either a donor or an acceptor. The hydrogen passivation of the electrically active centers is also discussed.

cond-mat.mtrl-sci↗