Coverage θH Research Articles

Facet-dependent electrocatalytic water splitting reaction on CeO2: A DFT + U study

Ceria has attracted considerable interest for the high-temperature electrocatalytic water splitting reaction (WSR) in solid oxide electrolysis cells (SOECs) due to the large active surface area enabled by its accessibility to ions, electrons and gas molecules. The mechanism of the WSR on the (1 1 1) facet of ceria has been studied extensively and hydroxyl decomposition is widely reported as the rate-determining step. However, WSR on other most-often exposed low-index surfaces like CeO2(1 1 0) and CeO2(1 0 0) remains unclear, especially the effect of high temperature on the formation of hydroxyls with different hydrogen coverage (ϴH) and reaction pathways. After identifying stable electron localization on cerium by using density functional theory corrected for on-site Coulomb interactions (DFT + U), we found that the formation of reaction intermediates such as oxygen vacancies, hydroxyls and vacancy-hydroxyl mixed phases on CeO2(1 0 0) is much more stable than that on CeO2(1 1 0) and CeO2(1 1 1). The higher stability of hydroxyls on CeO2(1 0 0) inhibits hydroxyl decomposition into H2, as compared to the reaction on the (1 1 0) and (1 1 1) facets, leading to the WSR on CeO2(1 1 0) and CeO2(1 1 1) 10 ~ 100 times faster than the reaction on CeO2(1 0 0) at temperature (T) < 950 K. In addition, we investigated the WSR via the formation of hydroxylated ceria with different coverage ϴH and found that the most efficient reaction pathway is also strongly facet-dependent, with the WSR on CeO2(1 0 0) and CeO2(1 1 1) preferentially proceeding on their overly hydroxylated surfaces (ϴH > 1, beyond one monolayer), while ϴH of the most active hydroxylated CeO2(1 1 0) decreased with the increase of temperature because of the unstable formation of highly hydroxylated CeO2(1 1 0) at high temperature. Our studies shed more light on the facet sensitivity of electrocatalysis on ceria.

Vibrational Study of the Hydrogen Adsorption on the Missing Row Platinum (110) Surface

The hydrogen vibration was investigated to analyze its affect on the hydrogen adsorption on the missing row platinum surface (H/Pt(110)-(1\(\times\)2) model) in the ultrahigh vacuum (UHV). The density functional theory (DFT) combined with the approximation oscillation of the hydrogen atom on the surface was used. When the hydrogen coverage ΘH on the surface is 100% (\(\Theta\) = 1 ML), and taking into account the vibrational effect, the bond formed at the edge of the first layer (short bridge) is the most stable site. The vibrational effect on the adsorption model H/Pt(110)-(1\(\times\)2) is significant.

Triggering basal plane active sites of monolayer MoS2 for the hydrogen evolution reaction by phosphorus doping

Monolayer MoS2 has attracted much attention due to its catalytic performance in water splitting. However, the low electronic conductivity and limited number of active catalytic sites of monolayer MoS2 limit the hydrogen production efficiency. In this work, the effect of phosphorus doping on the electronic characteristics and catalytic activity of the hydrogen evolution reaction in monolayer MoS2 was studied using density functional theory. A semiconductor to conductor transformation occurs in monolayer MoS2(1-x)Px for x larger than 0.25. The Gibbs free energy is greatly reduced from 2.18 to 0.03 eV for the adsorption of hydrogen on monolayer MoS0.5P0.75. The Gibbs free energy for hydrogen atom adsorption on monolayer MoS0.5P0.75 is between 0.03 and 0.16 eV with hydrogen coverage θH = 1/12–7/12 ML. The Gibbs free energy is close to zero, indicating that phosphorus doping can trigger the basal plane active sites on monolayer MoS2; thus, this work provides a new design for the improvement of the catalytic activity of two-dimensional transition metal dichalcogenide-based catalysts by phosphorus doping.

Switch in Relative Stability between cis and trans 2-Butene on Pt(111) as a Function of Experimental Conditions: A Density Functional Theory Study

The adsorption of cis and trans 2-butenes on Pt(111) has been studied as a function of hydrogen coverage θH by means of calculations based on density functional theory (DFT) with the inclusion of dispersion forces. All hydrogen coverages have been considered, from 0 to 1.00 monolayer (ML). For each case, the di-σ and π adsorption geometries of the olefins have been compared at a surface coverage of θC4H8 = 0.11 ML. Calculations of the Gibbs free energies of these systems have identified the most stable 2-butene isomer (cis or trans) as a function of coverage, temperature, and pressure. In particular, focus was placed on two sets of conditions, namely, one with a pressure of 10–7 Torr, a temperature of 80 K, and a gas ratio (PH2/Pbutene) of 25, similar to the conditions used in surface-science studies, and a second with a pressure of 1 bar, a temperature range of 300–400 K, and a gas ratio (PH2/Pbutene) of 10, similar to catalytic hydrogenation conditions. With all selected functionals (PW91, PBE-TS, and optPBE), di-σ bonding was found to be the most stable for both isomers of 2-butene and for all hydrogen coverages except for θH = 1.00 ML. At low pressures, 2-butene is physisorbed at low temperatures (≤125 K with PBE-TS and ≤90 K with optPBE); however, when the temperature increases, coadsorption of the butene with 6 H atoms becomes the most stable configuration of the system (θH = 0.67 ML), and finally, 2-butene desorbs around 380 K, as estimated with PBE-TS (or around 325 K with optPBE). Interestingly, a switch in stability was observed with hydrogen coverage, from the adsorbed trans isomer being the more stable for θH < 0.44 ML to the adsorbed cis isomer becoming the more stable at higher hydrogen coverages, in agreement with the cis–trans isomerization behavior previously reported for this system. At high pressures, the behavior is similar, but with transitions occurring at higher temperatures. 2-Butene is physisorbed until the temperature reaches 250 K, and desorbs above 500 K. At hydrogenation reaction temperatures (between 300 and 500 K), a hydrogen coverage of roughly half a monolayer was calculated (0.66 and 0.44 for 300 and 500 K, respectively). Our results confirm that dispersion effects must be included to properly describe the 2-butene and hydrogen coadsorption on Pt(111), as PW91 predicts that 2-butene is never adsorbed on the platinum surface. On the other hand, DFT calculations including dispersion forces such as PBE-TS or optPBE afford a good understanding of catalytic systems under both ultra-high-vacuum conditions and catalytic hydrogenation conditions. For this system, the PBE-TS results are in good agreement with experiments: they correctly reproduce the coverage in hydrogen and the configuration of the 2-butene adsorbate (cis–trans isomer).

Adsorption of Hydrogen at the GaN(0001̅) Surface: An Ab Initio Study

Ab initio calculations are used to determine the basic physical properties of hydrogen adsorption at the N-terminated GaN(0001̅) surface. It was shown that the Fermi level is pinned at the valence band maximum (VBM) for hydrogen coverage θH = 0.75 ML above which the Fermi level is shifted to the conduction band minimum (CBM). Application of the electron counting rule (ECR) indicates that for θH = 0.75 ML, the Fermi level is not pinned, located in the bandgap. Ab initio calculations confirmed that hydrogen adsorption energy depends on the Fermi level at the surface as predicted and explained earlier (Krukowski et al. J. Appl. Phys. 2013, 114, 143705 and Krukowski et al. J. Appl. Phys. 2013, 115, 043529). The adsorption energy contains electron transfer contribution, below θH = 0.75 ML to VBM and above that coverage to CBM; thus it is ΔEads(H) ≅ 5.8 eV and ΔEads(H) ≅ 3.1 eV for a single H atom for these coverage ranges, respectively. Accordingly, the H2 equilibrium pressure at the GaN(0001̅) surface varies from very low for θH < 0.75 ML, about 10–20 bar, to a fraction of a bar for θH > 0.75 ML.

First Principle Based Simulation of Cyclic Voltammogram: Bromide Adsorption on Pt(111) Surface

Introduction: Adsorption of halide anions on the electrode surface is fundamental phenomena and important in the various applications, such as the synthesis of the shape-controlled nano-particles. Cyclic voltammetry is the most important experiment for understanding the surface reactions. However it is sometimes difficult to assign the origin of peak positions and/or shapes of the voltammogram. Comparison of the experimental results with the simulated voltammograms might be helpful for assigning and clarifying of the atomistic behavior. In this study, we focus on the bromide adsorption on the Pt(111) surface, which is complicated process because Br- adsorbs simultaneously with the desorption of H+. The transition of Br-adsorption structure during voltammetry has been discussed, but is still unclear. We simulated voltammogram based on the density functional theory (DFT) calculations and discussed the peak positions, shape, and adsorption/desorption processes. Method: In this study, following two redox reactions were considered,H+ (aq) + * + e -<--> H* (1),Br - (aq) + * <--> Br* + e - (2),where * indicates the Pt(111) surface. The adsorption free energies (ΔGads) for these reactions were calculated by DFT. (√3x√3)-Br, (3x3)-Br, and (√3x√3)-H adosprtion structures were considered by using the slab models. The surface coverage (θ) dependence of the ΔGads are also calculated. Methodology of the voltammogram simulation has been disscussed by some research groups [1-3]. We adopted a slimilar approach to their works. The θBr and θH were calculated at each potential steps from the ΔGads(θ) under the steady-state approximation. The currents were evaluated by the variation of the θBr and θHduring potential sweep.All DFT calculations were performed by the “interface” program package [4]. The RPBE exchange-correlation functional, the norm-conserving pseudopotential, and numerical basis sets were used. Results and discussion: Figure 1 shows the calculated θH and θBr dependence of the ΔGads. This result describes the repulsive interaction between adsorbed atoms, for example, the ΔGads increases with the increase of θBr in the case of θH=0.33. The simulated voltammograms by using these ΔGads values are shown in Fig. 2. Although the peak shape is broad, the peak positions are in good agreements with the experimental one [5]. Figure 3 shows the calculated surface coverage θH and θBr. The θBr gradually increases from 0.1V, which corresponds the (√3x√3)-Br- adsorption. The θBr exceeds 0.33 around 0.3V, which means that the adsorption structure of Br-gradually changes from (√3x√3) to (3x3). Therefore, it is indicated that the shoulder peak around 0.3V in Fig. 2 is originated from the transition to (3x3)-Br adsorption structure.

Ultra-highly doped Si1−xGex(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics

Si 1−x Ge x (001) layers doped with B concentrations CB between 2×1016 and 2×1021 cm−3 were grown on Si(001)2×1 at Ts=500–700 °C by gas-source molecular-beam epitaxy (GS-MBE) from Si2H6, Ge2H6, and B2H6. Secondary-ion mass spectrometry measurements of modulation-doped structures demonstrate that B doping has no effect on the Ge incorporation probability. Steady-state B and Ge surface coverages (θB and θGe) were determined as a function of CB using in situ isotopically tagged temperature-programmed desorption. Results for Si0.82Ge0.18 layers grown at Ts=500 °C show that θGe remains constant at 0.63 ML while the bulk B concentration increases linearly up to 4.6×1020 cm−3, corresponding to saturation coverage at θB,sat=0.5 ML, with the incident precursor flux ratio ξ=JB2H6/(JSi2H6+JGe2H6). B is incorporated into substitutional electrically active sites over this entire concentration range. At higher B concentrations, CB increases faster than ξ and there is a large decrease in the activated fraction of incorporated B. The B segregation enthalpy during Si0.82Ge0.18(001) growth is −0.42 eV, compared to −0.53 and −0.64 eV during Si(001):B and Ge(001):B GS-MBE, respectively. Measured segregation ratios rB=θB/xB, where xB is the bulk B fraction, range from 15 to 500 with a temperature dependence which is consistent with equilibrium segregation. Film deposition rates RSiGe(CB) decrease by up to a factor of 2 with increasing CB⩾5×1019 cm−3, due primarily to a B-segregation-induced decrease in the dangling bond density. The above results were used to develop a robust model for predicting the steady-state H coverage θH, θB, θGe, and RSiGe as a function of ξ and Ts.

Effects of H coverage on Ge segregation during Si1−xGex gas-source molecular beam epitaxy

The effects of H coverage θH on Ge segregation during Si1−xGex gas-source molecular beam epitaxy (GS-MBE) were investigated using D2 temperature programmed desorption (TPD). Si1−xGex films with x=0.01–0.30 were grown from Si2H6/Ge2H6 mixtures at Ts=450–800 °C, held at the growth temperature for 30 s, cooled to &amp;lt;200 °C, and then exposed to atomic deuterium until saturation coverage. D2 TPD spectra were fit using four peaks corresponding, in order of decreasing activation energy, to desorption from Si monodeuteride, Ge–Si mixed-dimer monodeuterides, Si dideuteride, and Ge monodeuteride. Steady-state Ge surface coverages were determined from the TPD data as a function of Ts and x. In contrast to solid-source MBE films grown in this temperature regime, Ge segregation during GS-MBE decreases with decreasing Ts due to the increasing H coverage. The results were well described by a model accounting for the Si/Ge site exchange and θH. The Ge segregation enthalpy varies from −0.28 eV at Ts⩾800 °C, where the steady-state hydrogen coverage θH approaches zero, to −0.10 eV at Ts⩽450 °C, where θH is nearly saturated.

Methane adsorption and hydrogen isothermal desorption kinetics on a C(001)–(1×1) surface

Methane (CH4) adsorption and isothermal hydrogen (H2) desorption kinetics on a C(001)–(1×1) surface have been investigated by observing the surface hydrogen coverage θH which is estimated from the electron-stimulated desorption yield of H+ ions. The time evolution of θH under CH4 exposure, in pressure and temperature ranges of 1×10−8–5×10−7 Torr and 600–800 °C, reveals that the CH4 adsorption is a first-order reaction. The isothermal H2 desorption is conducted in a temperature range of 1095–1270 °C. The time evolution of θH shows that the isothermal H2 desorption is also a first-order reaction. From Arrhenius plots of the reaction coefficients, the activation energies of CH4 adsorption and H2 desorption are evaluated to be 7.3±1.7 and 21±4.9 kcal/mol, respectively. Atomistic kinetics model for CH4 adsorption and H2 desorption are discussed, especially in connection with a recent finding of gas source molecular beam epitaxy of diamond (001) with pure CH4 without any hydrogen or oxygen radicals.

Phase Transitions in Chemisorbed Hydrogen Layers

AbstractA brief survey is given about hydrogen‐transition metal systems which exhibit adsorbed hydrogen phases with long‐range order that can undergo order‐order and order‐disorder transitions depending upon the surface coverage θH and the temperature, respectively. A distinction has to be made between genuine lattice gas phases in which the H atoms simply occupy lattice sites with identical local symmetry without affecting the positions of the underlying metal atoms and those chemisorbed H phases which involve a distortion of the surface geometry of the substrate leading to the well‐known surface reconstruction. The systems H/Ni(111), H/Pd(111), H/Pd(100) and H/Fe(110) belong to the first category whereas H/Ni(110) and H/Pd(110) represent examples of the latter category, i.e., that involves surface reconstructed phases. As far as they have been determined the phase diagrams of the ordered H phases will be described and interpreted in terms of the mutual H‐H interactions.