The inherent instability of CH3NH3PbX3 remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3 have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH3NH3PbX3 perovskite crystals and their effects on the stability of perovskite materials.Keywords: CH3NH3PbBr3; density functional theory; STM simulations; surface defects;

•Os/Si has a thermodynamically favorable hydrogen adsorption free energy.•Os/SiNW composites show a low Tafel slope of −24 mV dec−1.•The performance of Os/SiNW catalysts exceeds Pt/C at high overpotentials.The development of highly efficient electrocatalysts for hydrogen evolution reaction is a fundamental undertaking of the hydrogen economy. Herein, we investigated the electrocatalytic performance of M/Si (M = Os, Rh, Pt, Pd, Re, Ru, Au or Ag) nanocomposites for hydrogen evolution reaction. The results show that Os/Si nanocomposites exhibit the best catalytic efficiency with a negligible onset overpotential (−25 mV), a small Tafel slope of −24 mV dec−1 and remarkable long-term stability. Of most importance, at a current density of the typical industrial production (−1000 mA cm−2), the energy conversion efficiency of the Os/Si nanocomposite is 29.3% higher than that of the commercial 40 wt% Pt/C. The density functional calculations reveal that such outstanding catalytic activity of the Os/Si catalyst arises from the thermodynamically more favorable hydrogen adsorption free energy (ΔGH* = −0.03 eV) at the osmium/silicon interfaces than that on platinum (ΔGH* = −0.09 eV) or osmium (ΔGH* = −0.26 eV).Os/SiNW exhibits a thermodynamically favorable hydrogen adsorption free energy of −0.03 eV endowing its excellent electrocatalytic activity and stability for HER, which even surpasses 40 wt% Pt/C at overpotentials above −60 mV.Download high-res image (103KB)Download full-size image

Hydrogen production with the aid of electrocatalysis is a critical component for several developing clean-energy technologies. Such a renewable energy depends heavily on the choice of cheap and efficient catalysts for hydrogen evolution, which has still been a challenge until now. In this work, the theoretical calculation indicates that Rh–Ag–Si ternary catalysts exhibit more active hydrogen evolution performance than Pt–Ag–Si because the migration activation energies of H atoms from Rh(111) to Si are lower than those from Pt(111) to Si via the Ag surface. This simulation was confirmed by the experimental results: Rh–Ag/SiNW (or Pt–Ag/SiNW) catalysts were prepared by directly reducing Rh (or Pt) and Ag ions with Si–H bonds. The Rh–Ag/SiNW-2 with the optimal mass ratio of 2.3 : 23.4 : 74.3 (Rh : Ag : Si) exhibited a lower Tafel slope (51 mV dec−1) and a larger exchange current density (87.1 × 10−6 A cm−2) than the Pt–Ag/SiNW. In addition, the mass activity of Rh–Ag/SiNW-2 at an overpotential of 0.2 V (11.5 mA μgRh−1) is 12.0, 5.0 and 3.3 fold higher than that of Rh–Ag (0.96 mA μgRh−1), Pt–Ag/SiNW (2.3 mA μgPt−1) and 40 wt% Pt/C (3.5 mA μgPt−1) catalysts, respectively. Moreover, the Rh–Ag/SiNW nanocatalysts had good stability in acidic media. The results presented herein may offer a novel and effective methodology for the designing of cost-efficient and environmentally friendly catalysts for electrochemical fields.

Hydrogen evolution by electrocatalysis is an attractive method of supplying clean energy. However, it is challenging to find cheap and efficient alternatives to rare and expensive platinum based catalysts. Pt provides the best hydrogen evolution performance, because it optimally balances the free energies of adsorption and desorption. Appropriate control of these quantities is essential for producing an efficient electrocatalyst. We demonstrate, based on first principles calculations, a stepwise designed Rh-Au-Si ternary catalyst, in which adsorption (the Volmer reaction) and desorption (the Heyrovsky reaction) take place on Rh and Si surfaces, respectively. The intermediate Au surface plays a vital role by promoting hydrogen diffusion from the Rh to the Si surface. Theoretical predictions have been explored extensively and verified by experimental observations. The optimized catalyst (Rh-Au-SiNW-2) has a composition of 2.2:28.5:69.3 (Rh:Au:Si mass ratio) and exhibits a Tafel slope of 24.0 mV·dec–1. Its electrocatalytic activity surpasses that of a commercial 40 wt.% Pt/C catalyst at overpotentials above 0.19 V by exhibiting a current density of greater than 108 mA·cm–2. At 0.3 V overpotential, the turnover frequency of Rh-Au-SiNW-2 is 10.8 times greater than that of 40 wt.% Pt/C. These properties may open new directions in the stepwise design of highly efficient catalysts for the hydrogen evolution reaction (HER).

The hydrogen evolution reaction (HER) is a fundamental electrochemical reaction to produce hydrogen gas from the electrolysis of water, and has been regarded as an attractive solution to many energy challenges. In the past few decades, platinum has been widely demonstrated to split water into hydrogen gas at high reaction rates and low overpotentials. Nevertheless, decreasing the loading of Pt in the designed electrocatalysts is very significant. However, with low Pt loading, it is challenging to maintain excellent catalytic performance. Here we report a Pt–Au–Si ternary nanocomposite as a highly active catalyst for the HER. The optimal composition of Pt–Au–Si was determined to be 7.2 : 31.3 : 61.5 (Pt : Au : Si, mass ratio) with a Tafel slope of only 24 mV dec−1. To be specific, the mass activity of the Pt–Au–SiNW-2 catalyst was 6.5 fold that of a 40 wt% Pt/C catalyst at an overpotential of 60 mV. More importantly, the cathodic current density even surpassed 40 wt% Pt/C when the overpotential was larger than 0.17 V (the corresponding current density was 108 mA cm−2). The outstanding catalytic performance might be derived from the synergy of the respective properties of the three components: the fast hydrogen adsorption rate on Pt, the quick migration of the adsorbed hydrogen atoms on Au and the rapid hydrogen evolution rate on Si. This research opens up a novel strategy to prepare highly efficient catalysts for electrochemical hydrogen evolution reactions.

The demand for catalyst with higher activity and higher selectivity is still a central issue in current material science community. On the basis of first-principles calculations, we demonstrate that the catalytic performance of the Pd–TiO2 hybrid nanostructures can be selectively promoted or depressed by choosing the suitable shaped Pd and TiO2 nanocrystals. To be more specific, the catalytic activities of Pd nanoparticles enclosed by (100) or (111) facets can be promoted more significantly when dosed on the TiO2(001) than on TiO2(101) under irradiation. Such theoretical prediction has then been further verified by the experimental observations in which the Pd(100)–TiO2(001) composites exhibit the highest catalytic performance toward the activation of oxygen among all the other shaped hybrid nanostructures. As a result, the selection of facets of support materials can provide an extra tuning parameter to control the catalytic activities of metal nanoparticles. This research opened up a new direction for designing and preparing catalysts with enhanced catalytic performance.Keywords: interaction; O2 activation; Palladium; synergy; titania;

Recently, borophene was reported to be produced on silver surfaces. We employed density functional theory and electronic transport calculations to investigate the stabilities, electronic structures and transport properties of borophene nanoribbons. The stability of a borophene nanoribbon increases with its width and only the lined-edged borophene nanoribbons are stable in the free-standing form. Such anisotropic stability dependence is ascribed to the large scale delocalization of π electrons along the boron rows. In particular, all line-edge borophene nanoribbons undergo edge reconstructions, in which the out-of-plane bulking edge atoms are reconstructed to form quasi planar edge structures. Such edge reconstructions have not yet been reported, which is critical for the formation of boron nanostructrues. Subsequent electronic transport calculations based on a non-equilibrium Green’s function indicated that line-edge borophene nanoribbons exhibit low-resistivity Ohmic conductance. Our results indicated that the line-edge borophene nanoribbons present remarkable properties (high thermal stabilities, Ohmic conductance with low electrical resistivity and good rigidities) and are promising for applications as one-dimensional electrical connections in compact nanoscale circuits.

Late transition metals, such as Rh, Ir, Pd and Pt, have a strong tendency to form a square-planar 16-electron complex. Although this feature has been widely used in organometallics to develop homogeneous catalysts, a single-atom heterogeneous analogue has not yet been reported. In this work, we show that a 16-electron complex may act as an important transition state in the CO oxidation over a single Pt atom supported by a MoS2 monolayer (Pt/MoS2). The catalytic oxidation reaction prefers to start with the Langmuir–Hinshelwood (L–H) reaction, where the CO and O2 molecules are first co-adsorbed on the Pt atom, then cross a small barrier of 0.40 eV to form a square-planar 16-electron intermediate state, and subsequently the first CO2 is released. The activation barrier of the following Eley–Rideal (E–R) reaction is only 0.23 eV. The superior catalytic reactivity of the Pt/MoS2 surface can be explained by the quantum confinement effect of the Pt-5d orbitals and the stability of the square-planar 16-electron transition state. In addition, MoS2 may serve as a defect-free two dimensional anchoring substrate for Pt atomic adsorption. It provides not only a very large surface-to-volume ratio, but also a well-defined structure with a uniform distribution of anchoring points. The square-planar 16-electron intermediate state of the L–H reaction, together with the new MoS2 anchoring substrate, may provide a new opportunity for the design of single-atom catalysts on two-dimensional surfaces.

Late transition metals, such as Rh, Ir, Pd and Pt, have a strong tendency to form a square-planar 16-electron complex. Although this feature has been widely used in organometallics to develop homogeneous catalysts, a single-atom heterogeneous analogue has not yet been reported. In this work, we show that a 16-electron complex may act as an important transition state in the CO oxidation over a single Pt atom supported by a MoS2 monolayer (Pt/MoS2). The catalytic oxidation reaction prefers to start with the Langmuir–Hinshelwood (L–H) reaction, where the CO and O2 molecules are first co-adsorbed on the Pt atom, then cross a small barrier of 0.40 eV to form a square-planar 16-electron intermediate state, and subsequently the first CO2 is released. The activation barrier of the following Eley–Rideal (E–R) reaction is only 0.23 eV. The superior catalytic reactivity of the Pt/MoS2 surface can be explained by the quantum confinement effect of the Pt-5d orbitals and the stability of the square-planar 16-electron transition state. In addition, MoS2 may serve as a defect-free two dimensional anchoring substrate for Pt atomic adsorption. It provides not only a very large surface-to-volume ratio, but also a well-defined structure with a uniform distribution of anchoring points. The square-planar 16-electron intermediate state of the L–H reaction, together with the new MoS2 anchoring substrate, may provide a new opportunity for the design of single-atom catalysts on two-dimensional surfaces.

Recently, borophene was reported to be produced on silver surfaces. We employed density functional theory and electronic transport calculations to investigate the stabilities, electronic structures and transport properties of borophene nanoribbons. The stability of a borophene nanoribbon increases with its width and only the lined-edged borophene nanoribbons are stable in the free-standing form. Such anisotropic stability dependence is ascribed to the large scale delocalization of π electrons along the boron rows. In particular, all line-edge borophene nanoribbons undergo edge reconstructions, in which the out-of-plane bulking edge atoms are reconstructed to form quasi planar edge structures. Such edge reconstructions have not yet been reported, which is critical for the formation of boron nanostructrues. Subsequent electronic transport calculations based on a non-equilibrium Green’s function indicated that line-edge borophene nanoribbons exhibit low-resistivity Ohmic conductance. Our results indicated that the line-edge borophene nanoribbons present remarkable properties (high thermal stabilities, Ohmic conductance with low electrical resistivity and good rigidities) and are promising for applications as one-dimensional electrical connections in compact nanoscale circuits.

Hydrogen production with the aid of electrocatalysis is a critical component for several developing clean-energy technologies. Such a renewable energy depends heavily on the choice of cheap and efficient catalysts for hydrogen evolution, which has still been a challenge until now. In this work, the theoretical calculation indicates that Rh–Ag–Si ternary catalysts exhibit more active hydrogen evolution performance than Pt–Ag–Si because the migration activation energies of H atoms from Rh(111) to Si are lower than those from Pt(111) to Si via the Ag surface. This simulation was confirmed by the experimental results: Rh–Ag/SiNW (or Pt–Ag/SiNW) catalysts were prepared by directly reducing Rh (or Pt) and Ag ions with Si–H bonds. The Rh–Ag/SiNW-2 with the optimal mass ratio of 2.3:23.4:74.3 (Rh:Ag:Si) exhibited a lower Tafel slope (51 mV dec−1) and a larger exchange current density (87.1 × 10−6 A cm−2) than the Pt–Ag/SiNW. In addition, the mass activity of Rh–Ag/SiNW-2 at an overpotential of 0.2 V (11.5 mA μgRh−1) is 12.0, 5.0 and 3.3 fold higher than that of Rh–Ag (0.96 mA μgRh−1), Pt–Ag/SiNW (2.3 mA μgPt−1) and 40 wt% Pt/C (3.5 mA μgPt−1) catalysts, respectively. Moreover, the Rh–Ag/SiNW nanocatalysts had good stability in acidic media. The results presented herein may offer a novel and effective methodology for the designing of cost-efficient and environmentally friendly catalysts for electrochemical fields.