We investigated the oxidation of formaldehyde, one of the major indoor air pollutants, into CO2 and H2O over Fe atoms trapped in defects on graphene by first-principles based calculations. These trapped Fe atoms are not only stable to withstand interference from the reaction environments but are also efficient in catalyzing the reactions between coadsorbed O2 and formaldehyde. The oxidation of formaldehyde starts with the formation of a peroxide-like intermediate and continues by its dissociation into η1-OCHO coadsorbed with an OH radical. Then, the adsorbed OCHO undergoes conformational changes and hydride transfer, leading to the formation of H2O and CO2. Subsequent adsorption of O2 or formaldehyde facilitates desorption of H2O and a new reaction cycle initiates. The calculated barriers for formation and dissociation of the peroxide-like intermediate are 0.43 and 0.40 eV, respectively, and those for conformation changes and hydride transfer are 0.47 and 0.13 eV, respectively. These relatively low barriers along the reaction path suggest the potential high catalytic performance of trapped Fe atoms for formaldehyde oxidation.

We performed a first-principles based investigation on the potential role of Au atoms stabilized by defects on graphene in ethylene epoxidation. We showed that the interactions between the Au atoms and vacancies on graphene not only make the Au atomic diffusion a 2.10 eV endothermic process, but also tune the energy level of Au-d states for the activation of O2 and ethylene and promote the formation and dissociation of the peroxametallacycle intermediate. The catalytic cycle of ethylene epoxidation is initiated with the formation of a peroxametallacycle intermediate by the coadsorbed ethylene and O2, through the dissociation of which an ethylene epoxide molecule and an adsorbed O atom are formed. Then, gaseous ethylene reacts with the remnant O atom directly for the formation of another ethylene epoxide molecule. The desorption of ethylene epoxide is facilitated by the subsequent adsorption of O2 or ethylene and a new reaction cycle initiates. The calculated energy barriers for the formation and dissociation of the peroxametallacycle intermediate and the regeneration of Au sites are 0.30, 0.84 and 0.18 eV, respectively, and are significantly lower than those for aldehyde formation. These findings suggest the potential high catalytic performance of these Au atoms for ethylene epoxidation.

Taking CO oxidation as a probe, we investigated the electronic structure and reactivity of Pt atoms stabilized by vacancy defects on hexagonal boron nitride (h-BN) by first-principles-based calculations. As a joint effect of the high reactivity of both a single Pt atom and a boron vacancy defect (PtBV), the Pt–N interaction is −4.40 eV and is already strong enough to prohibit the diffusion and aggregation of the stabilized Pt atom. Facilitated by the upshifted Pt-d states originated from the Pt–N interaction, the barriers for CO oxidation through the Langmuir–Hinshelwood mechanism for formation and dissociation of peroxide-like intermediate and the regeneration are as low as 0.38, 0.10 and 0.04 eV, respectively, suggesting the superiority of PtBV as a catalyst for low temperature CO oxidation.

We addressed the potential catalytic role of Pt-embedded graphene in CO oxidation by first-principles-based calculations. We showed that the combination of highly reactive Pt atoms and defects over graphene makes the Pt-embedded graphene a superior mono-dispersed atomic catalyst for CO oxidation. The binding energy of a single Pt atom onto monovacancy defects is up to −7.10 eV, which not only ensures the high stability of the embedded Pt atom, but also vigorously excludes the possibility of diffusion and aggregation of embedded Pt atoms. This strong interfacial interaction also tunes the energy level of Pt-d states for the activation of O2, and promotes the formation and dissociation of the peroxide-like intermediate. The catalytic cycle of CO oxidation is initiated through the Langmuir–Hinshelwood mechanism, with the formation of a peroxide-like intermediate by the coadsorbed CO and O2, by the dissociation of which the CO2 molecule and an adsorbed O atom are formed. Then, another gaseous CO will react with the remnant O atom and make the embedded Pt atom available for the subsequent reaction. The calculated energy barriers for the formation and dissociation of the peroxide-like intermediate are as low as 0.33 and 0.15 eV, respectively, while that for the regeneration of the embedded Pt atom is 0.46 eV, indicating the potential high catalytic performance of Pt-embedded graphene for low temperature CO oxidation.

We addressed the electronic structure of Cu atoms embedded in hexagonal boron nitride (h-BN) and their catalytic role in CO oxidation by first-principles-based calculations. We showed that Cu atoms prefer to bind directly with the localized defects on h-BN, which act as strong trapping sites for Cu atoms and inhibit their clustering. The strong binding of Cu atoms at boron vacancy also up-shifts the energy level of Cu-d states to the Fermi level and promotes the formation of peroxide-like intermediate. CO oxidation over Cu atoms embedded in h-BN would proceed through the Langmuir–Hinshelwood mechanism with the formation of a peroxide-like complex by reaction of coadsorbed CO and O2, with the dissociation of which the a CO2 molecule and an adsorbed O atom are formed. Then, the embedded Cu atom is regenerated by the reaction of another gaseous CO with the remnant O atom. The calculated energy barriers for the formation and dissociation of peroxide complex and regeneration of embedded Cu atoms are as low as 0.26, 0.11 and 0.03 eV, respectively, indicating the potential high catalytic performance of Cu atoms embedded in h-BN for low temperature CO oxidation.

Quantum size effect (QSE) is of central importance in nanoscience. For semiconductors, it is generally perceived that the QSE raises the band gap of a nanocrystal by effectively increasing the kinetic energies of electrons and holes. Using first-principles calculations and Si nanofilms as a test case, we investigated the impact of surface passivation of nanocrystals on the QSE and extended the view of the traditional QSE, as depicted in the classic effective mass model, to include the quantum boundary effect. We showed that the band gap of Si nanofilms is critically affected by the passivation species at the surface, which could result in not only the commonly observed increase in band gap but also a decrease with respect to the bulk value. The Si nanofilms can have a band gap that is virtually the same as that of bulk Si when film thickness is less than 2 nm. The new understanding of the QSE opens a new degree of freedom in engineering the electronic and optoelectronic properties of nanomaterials.

We studied the electronic structure of MPd12 (M = Fe, Co, Ni, Cu, Zn, Pd) nanoparticles deposited on graphene substrates and their reactivity toward O adsorption, which are directly related to the catalytic performance of these composites in oxygen reduction reaction, by first-principles-based calculations. We found that the alloying between M and Pd can enhance the stability of nanoparticles and promote their oxygen reduction activity to be comparable with that of Pt(111). The defective graphene substrate can provide anchoring sites for these nanoparticles by forming strong metal–substrate interaction. The interfacial interaction can contribute to additional stability and further tune the averaged d-band center of the deposited alloy nanoparticles, resulting in strong interference on the O adsorption. As the O adsorption on these composites is weakened, the oxygen reduction reaction kinetics over these composites will also be promoted. These composites are thus expected to exhibit both high stability and superior catalytic performance in oxygen reduction reaction.

The impact of graphene substrate–Pd nanoparticle interaction on the O, OH, and OOH adsorption that is directly related to the electrocatalytic performance of these composites in oxygen reduction reaction (ORR) has been investigated by first-principles-based calculations. The calculated binding energy of a Pd13 nanoparticle on a single vacancy graphene is as high as −6.10 eV, owing to the hybridization between the dsp states of the Pd particles with the sp2 dangling bonds at the defect sites. The strong interaction results in the averaged d-band center of the deposited Pd nanoparticles shifted away from the Fermi level from −1.02 to −1.45 eV. Doping the single vacancy graphene with B or N will further tune the average d-band center and also the activity of the composite toward O, OH, and OOH adsorption. The adsorption energies of O, OH, and OOH are reduced from −4.78, −4.38, and −1.56 eV on the freestanding Pd13 nanoparticle to −4.57, −2.66, and −1.39 eV on Pd13/single vacancy graphene composites, showing that the defective graphene substrate will not only stabilize the Pd nanoparticles but also reduce the adsorption energies of the O-containing species to the Pd particle, and so as the poisoning of the ORR active sites.

The formation of the passive oxide surface layer accounts for the superior biocompatibility of NiTi alloy based implant materials. However, the usage NiTi alloy is limited by the long-term release of biotoxic Ni ion from the bulk, facilitated by the formation of defects and vacancies in the surface oxide layer during conventional processing. To aid the improvement of the biocompatibility of NiTi alloys, extensive first-principles based calculations were performed to uncover the microscopic mechanism for the temperature controlled oxidation of NiTi alloy. We show that the oxygen adsorption and diffusion on the NiTi surface are the elementary steps for the formation of a surface oxide layer. Oxygen will bind strongly with the alloy surface and even induce surface reconstruction, and the adsorption energy can be as high as −6.14 eV. The requested surface diffusion for formation of TiO2 surface terminations is thermodynamics driven, but the corresponding kinetics is strongly affected by temperatures. These result in formation of TiO at low temperatures and TiO2 at elevated temperatures.

We investigated the oxidation of formaldehyde, one of the major indoor air pollutants, into CO2 and H2O over Fe atoms trapped in defects on graphene by first-principles based calculations. These trapped Fe atoms are not only stable to withstand interference from the reaction environments but are also efficient in catalyzing the reactions between coadsorbed O2 and formaldehyde. The oxidation of formaldehyde starts with the formation of a peroxide-like intermediate and continues by its dissociation into η1-OCHO coadsorbed with an OH radical. Then, the adsorbed OCHO undergoes conformational changes and hydride transfer, leading to the formation of H2O and CO2. Subsequent adsorption of O2 or formaldehyde facilitates desorption of H2O and a new reaction cycle initiates. The calculated barriers for formation and dissociation of the peroxide-like intermediate are 0.43 and 0.40 eV, respectively, and those for conformation changes and hydride transfer are 0.47 and 0.13 eV, respectively. These relatively low barriers along the reaction path suggest the potential high catalytic performance of trapped Fe atoms for formaldehyde oxidation.

We performed a first-principles based investigation on the potential role of Au atoms stabilized by defects on graphene in ethylene epoxidation. We showed that the interactions between the Au atoms and vacancies on graphene not only make the Au atomic diffusion a 2.10 eV endothermic process, but also tune the energy level of Au-d states for the activation of O2 and ethylene and promote the formation and dissociation of the peroxametallacycle intermediate. The catalytic cycle of ethylene epoxidation is initiated with the formation of a peroxametallacycle intermediate by the coadsorbed ethylene and O2, through the dissociation of which an ethylene epoxide molecule and an adsorbed O atom are formed. Then, gaseous ethylene reacts with the remnant O atom directly for the formation of another ethylene epoxide molecule. The desorption of ethylene epoxide is facilitated by the subsequent adsorption of O2 or ethylene and a new reaction cycle initiates. The calculated energy barriers for the formation and dissociation of the peroxametallacycle intermediate and the regeneration of Au sites are 0.30, 0.84 and 0.18 eV, respectively, and are significantly lower than those for aldehyde formation. These findings suggest the potential high catalytic performance of these Au atoms for ethylene epoxidation.

We addressed the potential catalytic role of Pt-embedded graphene in CO oxidation by first-principles-based calculations. We showed that the combination of highly reactive Pt atoms and defects over graphene makes the Pt-embedded graphene a superior mono-dispersed atomic catalyst for CO oxidation. The binding energy of a single Pt atom onto monovacancy defects is up to −7.10 eV, which not only ensures the high stability of the embedded Pt atom, but also vigorously excludes the possibility of diffusion and aggregation of embedded Pt atoms. This strong interfacial interaction also tunes the energy level of Pt-d states for the activation of O2, and promotes the formation and dissociation of the peroxide-like intermediate. The catalytic cycle of CO oxidation is initiated through the Langmuir–Hinshelwood mechanism, with the formation of a peroxide-like intermediate by the coadsorbed CO and O2, by the dissociation of which the CO2 molecule and an adsorbed O atom are formed. Then, another gaseous CO will react with the remnant O atom and make the embedded Pt atom available for the subsequent reaction. The calculated energy barriers for the formation and dissociation of the peroxide-like intermediate are as low as 0.33 and 0.15 eV, respectively, while that for the regeneration of the embedded Pt atom is 0.46 eV, indicating the potential high catalytic performance of Pt-embedded graphene for low temperature CO oxidation.