First-Principles Study on Stability and HER Activity of Noble Metal

Subscriber access provided by UniSA Library

Article

First-Principles Study on Stability and HER Activity of Noble Metal Single-Atoms on TiO: The Effect of Loading Density 2

Fengshuang Han, Zhaohui Zhou, Xiaohai Zhang, Zhenxiong Huang, Mingtao Li, and Liejin Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11486 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

First-principles Study on Stability and HER Activity of Noble Metal Single-Atoms on TiO2: The Effect of Loading Density Fengshuang Han†, Zhaohui Zhou‡, *, Xiaohai Zhang†, Zhenxiong Huang†, Mingtao Li†, Liejin Guo†, * †

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China ‡ Chemical Engineering and Technology, School of Environmental Science and Engineering, and Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education, Chang’an University, Xi’an 710064, China Abstract The highly dispersed “single-atom (SA)” catalysts on oxide supports significantly alter the catalytic reaction activity and selectivity, and meanwhile save the catalyst utilization. However, preparation of SA catalysts remains a challenge up to now. An approach effectively evaluating the stability and activity is required. Herein, density functional theory (DFT) based first-principles calculations were performed to evaluate the stability and photocatalytic hydrogen evolution reaction (HER) activity of noble metal (NM = Ag, Au, Pd and Pt) SAs loaded on TiO2 support. The chemical potential based thermodynamic model was employed to estimate the stability of SAs; the capability to trap photoelectrons on surface and free energy of hydrogen adsorption were used to estimate the photocatalytic HER activity of SAs. Comparing to the (101) surface, the (001) surface is more feasible for preparation of NM SAs due to the “soft” structural character caused by incompletely saturated surface atoms. The stability of SAs on the (001) is getting better with the loading density lowering except Au SA. After deposition of NM SAs on the (001), the photoelectron was extracted from the subsurface to the surface around the NM sites, facilitating the proton adsorption and reduction process. The calculated free energy of hydrogen adsorption shows that the photocatalytic HER activity of NM SAs on the (001) changes moderately with the loading density, but is very different to those for the TiO2 clean (001) and bulk NM (111). Both stability and activity evaluations dictate that Pd SAs on the (001) is the most promising candidate catalyst for photocatalytic HER. 1. Introduction Titanium dioxide (TiO2) has many applications.1 One of them is solar photocatalytic splitting water, which has attracted extensive attention during the last several decades in light of fossil energy consumption and environment remediation.2-4 TiO2 is earth-abundant, nontoxic, and long-term stable.5 TiO2-based catalysts can act as photocathodes for hydrogen evolution reaction (HER) or photoanodes for oxygen evolution reaction (OER). It has been well demonstrated that HER occurred efficiently on TiO2-based catalysts under both 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ultraviolet and visible light irradiation.6-8 However, noble metal (NM) cocatalysts, generally in the form of nanoparticles, are often required to reduce the overpotential for hydrogen evolution since the conduction band minimum (CBM) of TiO2 is only slightly negative to the redox potential for H+/H2.9 Some shortcomings come out with deposition of NM cocatalysts on TiO2, such as the high fabrication cost due to scarce NM and light adsorption blocking by surface NM nanoparticles. Downsizing of the NM cocatalyst nanoparticles provides an approach to saving the NM use and meanwhile increasing the catalytic efficiency and selectivity, arising from good dispersity of active sites.10-11 As an extreme, the “single-atom (SA)” catalysts can maximize the NM atom efficiency.12-14 With the size of NM cocatalyst nanoparticles decreasing, unsaturated surface atoms are more likely to be exposed to reactants in the solution and thus will change dramatically the surface physical chemistry.10 However, it is not easy to prepare stable SA catalysts under realistic conditions since SAs tend to agglomerate on the surface. 15 Fortunately, both experimental and theoretical work has shown that SA catalysts can be fabricated successfully. For example, tuning temperature, pressure, reaction atmosphere, and support reducibility can improve production of SA catalysts.15-17 Two important aspects of SA catalysts, i.e., stability and reactivity are closely related to the interaction between SAs and supports.18 For example, the isolated Pt atom can be stabilized by the strong metal-support interaction, although Pt SAs or small Pt clusters continued growing to form nanoparticles;19 Pt SAs on N-graphene and α-MoC exhibit significantly enhanced catalytic activity in comparison to Pt/carbon and Pt/β-Mo2C catalysts, respectively.19-20 However, the metal-support interaction is greatly affected by many factors, such as surface dangling bonds, defects and reconstructions.21 Furthermore, the two properties of stability and reactivity are not necessarily correlated with each other for a SA catalyst.15 In this regard, the stability and reactivity should be evaluated separately for a given catalytic reaction in different conditions, and sometimes a compromise between them needs to be made.17 NM SA catalysts on TiO2 have shown enhanced photocatalytic HER.22-23 The isolated Pt atom on TiO2 anatase (101) shows the highest activity toward H2 evolution with respect to Pd, Rh, and Ru SA catalysts. It is well known that the TiO2 anatase (101) is the thermodynamic majority facet,24 while the minority (001) facet is more reactive.25 Density functional theory (DFT) calculation has indicated that the (001) has higher band edges than the (101),26 suggesting that the photogenerated electrons in the conduction band might be more active for HER,27-29 although a consensus is far from been reached about the most photoactive facet of TiO2.30-33 Therefore, deposition of NM SAs on the reactive TiO2 (001) facet (denoted as NM-SA-(001)) could be a new way to prepare efficient HER photocatalysts with lowered cost and enhanced efficiency. 2


Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As opposed to the investigation on NM SAs on TiO2-(101) (denoted as NM-SA-(101)), where the geometries of Pt adsorption or doping were carefully inspected through comparing the DFT modeling and experiment but the stability was not estimated,23 the stability and activity for photocatalytic HER of NM-SA-(001) (NM = Ag, Au, Pt and Pd) are evaluated here as a function of the NM loading density using the first-principles calculation. Increasing the loading density is desired for SA catalysts, but difficult in real preparing processes.16 The clean (001) and (101) surfaces are considered the support in this work. The stability evaluation is done by using the chemical potential based thermodynamic model, and the photocatalytic HER activity is evaluated through estimating the capability of photoelectrons trapping on surface and calculating the free energy of hydrogen adsorption.34 The results show that NM SAs can form on the (001) at low loading densities, but fail on the (101); Pd SAs on the (001) is a promising candidate catalyst for photocatalytic HER. 2. Computational Details For the clean TiO2 anatase (001), a slab of four O-Ti-O tri-layers and 20 Å vacuum region were used with the bottom two tri-layers fixed during the geometry optimization. For the clean TiO2 anatase (101), a slab of three O-Ti-O tri-layers was adopted with the middle tri-layer fixed and a vacuum region of the same size. The Ti5c-NM-O2c-Ti5c adsorption configuration, i.e., the NM SA breaking the Ti5c-O2c bond in the surface, was adopted on the (001), one stable geometry proposed by Sun et al. for Au SA adsorption on the (001),21 see Figure 1(a). Four NM loading densities were considered, i.e., θ = 1/4, 1/9, 1/16 and 1/25 monolayer (ML) which were modeled with four slabs of different cell size (2×2), (3×3), (4×4) and (5×5), respectively. Accordingly, the Brillouin zones were sampled with four gamma-centered grids, i.e., 4×4×1, 3×3×1, 2×2×1, and gamma-only point, respectively. Similarly, the O2c-NM-O2c adsorption configuration, i.e., the NM SA binding at a bridge site of two O2c atoms, was adopted on the (101) since it was reported by Zhang et al that the configuration was the most stable geometry for Pd SA on the (101),35 see Figure 1(b). Two NM loading densities were considered, i.e., 1/4 and 1/16 ML modeled with (2×2) and (4×4) slabs, respectively, and 4×4×1 and gamma-only k-point grids were used for Brillouin zone sampling. The shape of NM NPs was assumed to be truncated octahedra which were enclosed by face-centered cubic (FCC) (001) and (111) facets, similar to the Au NPs of Wulff construction reported by Barmparis et al.36 Both clean (111) and (001) facets were taken into account to evaluate the surface energy of NM NPs. (1×1) slabs were used to model the two facets with a 12 Å vacuum region and the slab thickness of six atomic layers with the middle two layers fixed, which converged the surface energy within 0.001 eV/Å2. For the surface energy calculation of (111) and (001) facets, the k-point grids were taken as 10×10×1 for Ag, Au and Pt and 14×14×1 for Pd. For hydrogen adsorption on the clean NM (111), the slab with (2×2) 3



Page 4 of 19

periodicity was used, which contains a 12 Å vacuum and the slab thickness of four atomic layers with the two bottom layers fixed. The k-point grid of 8×8×1 was adopted to calculate the free energy of hydrogen adsorption. Calculations were performed using the Vienna ab initio simulation package projector augmented wave potentials treating the valence-core interaction.

39

37-38

with

The valence

electronic wavefunctions were expanded in the plane wave basis set with 400 eV energy cutoff. The criterion of electronic convergence in the self-consistent field was 10-5 eV and the force convergence was set to less than 0.01 eV/Å. The generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE)40 was used to treat the exchange-correlation energy unless explicitly noted. An extra electron was added into NM-SA-(001) to simulate the trapping state of photogenerated electrons. The extra electron was compensated by a homogeneous background countercharge to maintain a neutral system. Because of the well-known “self-interaction error”41 in the standard DFT, some calculations were repeated with the HSE06 hybrid functionals42-43 and the PBE+U approach44 to validate the effectiveness of the PBE method. 3. Results and discussion 3.1. Geometry of NM SAs on (001) and (101) Figure 1(a) exhibits the optimized geometry of Ag-SA-(001) at the loading density of 1/9 which represents the typical adsorption configurations of Ti5c-NM-O2c-Ti5c. For the geometries of NM-SA-(001) at all loading densities considered here, see Figure S1 of Supporting Information. Ag and Au SAs fail to break the surface Ti5c-O2c bond until the loading density is lower than 1/4, while Pd and Pt SAs break the surface Ti5c-O2c bond at any loading density under consideration. The difference suggests that Pd and Pt SAs bind to the surface stronger than Ag and Au SAs. The breakage of surface Ti5c-O2c bonds gives rise to significant structural deformation (see Table S1 for the dramatic changes in surface bonds resulting from NM SAs deposition) which spreads along the surface far away from the binding sit. As evidence, the length of Ti5c-O2c bonds far away from the binding site at low loading densities still deviates noticeably from the Ti5c-O2c bond length of the clean (001). The NM-O2c bond length on the (001) follows an order of Ag > Au > Pd > Pt at any loading density, as shown in Table S1, reflecting a reverse binding strength for NM SAs to the surface. Figure 1(b) presents the optimized O2c-NM-O2c adsorption configuration for Ag-SA-(101) at 1/4 loading density. For other type of NM-SA-(101) at different loading densities, the optimized geometries were shown in Figure S2 of Supporting Information. Compared to the (001), no Ti5c-O2c bond breaking was observed on the (101) upon NM SAs deposition at any loading density, suggesting weaker interaction between NM SAs and the (101). NM SAs adsorption merely results in moderate structural deformation with respect to the clean (101), 4


Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as evidenced by the moderate surface bond changes shown in Table S2. The NM-O2c bond length on the (101) follows a slightly different order from that on the (001): Au > Ag > Pd > Pt, as shown in Table S2.

Figure 1. Optimized geometries (upper panel: side view, lower panel: top view) of (a) Ag-SA-(001), (b) Ag-SA-(101), (c) Ag double-atom cluster on (001), and (d) hydrogen adsorbed Ag-SA-(001). The NM loading density is 1/9 in (a) and (d), 1/4 in (b), and 2/9 in (c). The typical adsorption configuration is Ti5c-Ag-O2c-Ti5c for Ag-SA-(001) and O2c-Ag-O2c for Ag-SA-(101). O, Ti, Ag, and H atoms are depicted in red, light blue, dark green, and white, respectively. The differences between the (001) and (101) can be explained by the disparity in the bonding nature of the two clean surfaces. The (001) is deemed as a “soft” surface while the (101) is a “tough” surface, as evidenced by the average Ti5c-O2c bond length, 1.95 Å of the (001) and 1.86 Å of the (101) from Table S1 and S2, respectively. The (001) is featured by incompletely saturated surface atoms and easily strained lattice, which make the surface Ti5c-O2c bonds easy to break upon adsorption of external atoms. The “soft” character of the (001) helps to release the lattice strain induced by NM SAs through spreading the lattice strain far away from the binding site, and thus tends to wrap SAs on the surface as the loading density decreases. In contrast, the (101) is formed by more rigid lattice of saturated surface atoms. External atom adsorption generates small lattice strain and the lattice strain cannot be released and spread. Preparation of SAC requires strong interaction of SAs with the support. The widely spread structural deformation along the (001) induced by NM SAs deposition gives rise to a mobility barrier which prevents the neighboring NM SAs from diffusing and agglomerating on the surface. On the contrary, the (101) is hard to distort and thus accommodation of SAs is difficult but clustering is easy. As a result, it can be expected that NM SAs prefer to be formed on the (001) more than on the (101). 5



Page 6 of 19

3.2 Stability of NM SAs on (001) and (101) We used the chemical-potential-based thermodynamic model proposed by Liu et al.17 to evaluate the stability of NM SAs on TiO2 support. Scheme 1 illustrates how to stabilize NM SAs on TiO2. The chemical potential of NPs should be greater than that of SAs, µNP > µSA, a constraint condition imposed on the chemical potential similar to that used by Liu et al. This condition favors disintegration of NPs to SAs. We added also to the previous model 17 another constraint condition on the chemical potential which inhibits nucleation of NM clusters. The chemical potential of NPs should be smaller than that of double-atom (DA) clusters, µNP < µDA, which prevents the formation of NM clusters through further disintegrating NPs.

Scheme 1. Diagram of two constraint conditions on NM (NM = Ag, Au, Pd, and Pt) chemical potentials in order to prepare NM SAs on TiO2 support, i.e., µSA < µNP < µDA; µSA, µNP, and µDA denote the chemical potentials of NM SAs, NPs, and DA clusters, respectively. According to Gibbs-Thomson (G-T) relation, the chemical potential of NM NPs (µNP) can be expressed as µNP R=2Ωγme /R (1) Ω is the molar volume of bulk NM atom, γme is the surface energy of NM NPs, and R is the radius of NPs. Following the assumption that the NP surface is made up of 86% (111) and 14% (001),17 the NP surface energy is expressed as the average of all constituting facets γme = ∑ fi γi

(2)

fi is the percentage of a specific facet and γi is the surface energy of that facet. Table 1 lists the lattice constant of FCC NM, the surface energy of (111) and (001) facets, and the surface energy of NM NPs. The surface energy of Au (111) is very close to the value reported by Barmparis et al.,36 and the relative magnitude of the two facets of the four NMs is consistent with the previous report.45 As a consequence, µNP(R) = 1.65/R, 1.61/R, 2.62/R, and 2.98/R eV for Ag, Au, Pd, and Pt NPs, respectively. Liu et al. gave 1.62/R eV for the chemical potential of Au NP. Table 1. The lattice constant (a0) of the FCC NM (NM = Ag, Au, Pd, and Pt), the surface energy of (111) and (001) facets (γ(111) and γ(001)), and the surface energy of NM NPs (γme). Ag a0 (Å)

Au

Pd

Pt

4.162 4.173 3.954 3.976 2

γ(111) (eV/Å ) 0.045 0.043 0.083 0.091 6


Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


γ(001) (eV/Å2) 0.051 0.053 0.093 0.114 γme (eV/Å2)

0.046 0.044

0.85

0.095

The chemical potential of NM SAs (µSA) was approximated by the formation energy of the first NM atom on TiO2 support,

µSA ≈ ENM_1-TiO2 -ETiO2-ENM

(3)

E(NM_1-TiO2) and E(TiO2) are the slab energies with and without one NM adatom, and ENM is the bulk NM energy per atom. Negative µSA indicates strong binding of the NM atom to the support, and thus is preferable for the SAs formation. The chemical potential of the DA cluster (µDA) was defined as

µDA = ENM_2-NM_1-TiO2 -ENM_1-TiO2 -ENM

(4)

where E(NM_2-NM_1-TiO2) is the slab energy with two NM adatoms. Positive µDA dictates weak binding interaction between the two NM atoms, and also favors the formation of SAs. Figure 2 and 3 compares firstly the chemical potentials of NM NPs and SAs on the (001) and (101), respectively. At higher loading densities on the (001) such as 1/4 and 1/9 ML, µSA > µNP, indicating the NPs fail to disintegrate to form the SAs. With the loading density lowering, µSA decreases rapidly due to the greatly enhanced NM-TiO2 binding strength. Down to the loading density of 1/16, the NM atom is more stable in the form of SA than in the NP for Ag, Au, and Pd, and down to 1/25 for Pt. This observation suggests that the NM SAs on the (001) might be stabilized with the loading density ranging from 1/16 to 1/9 for Ag, Au, and Pd, and from 1/25 to 1/16 for Pt. However, at the two loading densities on the (101), µSA is much greater than µNP, and no trend for decrease in µSA was observed with the loading density lowering. Even though the clean (101) facet might stabilize the NM SAs at a very low loading density, it is useless since a too low loading density will result in a negligible number of active sites. It is concluded here that the clean (101) terrace is inappropriate to prepare the NM SA catalysts, consistent with the finding reported by Xing et al.23 Therefore, the geometries of NM DA clusters on the (101) were not considered any longer and the photocatalytic HER activity of NM-SA-(101) was not evaluated either. To determine the stability of NM SAs on the (001), the geometries and chemical potentials of DA clusters must be further explored. Figure 1(c) shows the optimized geometry of Ag DA cluster on the (001) at 1/9 loading density. For the geometries of all NM DA clusters on the (001) considered here, see Figure S3. Bonds between two NM atoms were clearly observed at all loading densities, similar to the adsorption geometry of Au DA on the (001) reported Sun et al.21 The chemical potentials of NPs and DA clusters were compared at the loading density of 1/16 and 1/25 which favors disintegration of NPs to form SAs (µSA < µNP). From Figure 2, µNP < µDA for Ag, Pd, and Pt, but µNP > µDA for Au. This observation 7



indicates that Ag and Pd SAs can form on the (001) at a maximum loading density from 1/16 to 1/9, and Pt SAs from 1/25 to 1/16, but Au SAs cannot be prepared at any loading density. The Au SAs would continue to grow in the presence of NPs and the origin can be attributed to the strong binding interaction between Au atoms. Therefore, the capability to prepare SA catalysts on the clean (001) facet follows the trend: Ag ≈ Pd > Pt > Au. In addition, the change with the loading density in the binding strength between two NM atoms is significantly less than that in the NM-TiO2 binding strength, especially at 1/16 and 1/25, see Figure 2. The observation can be explained by the fact that the binding strength between two NM atoms is sensitive to the metal bond formed instead of the loading density.

Figure 2. Chemical potentials of (a) Ag, (b) Au, (c) Pd, and (d) Pt SAs and DA clusters for formation of NM-SA-(001) through comparing with the chemical potentials of NM NPs as a function of their radii. The chemical potentials calculated with the HSE06 functional were compared with those calculated with the PBE method, as shown in Figure S4. The changing trend in the chemical potential with the loading density is very similar for both functionals, see Figure S4(a). However, the chemical potentials of NM SAs are overall smaller for the HSE06 functional, usually shifted to lower values by 0.4~0.7 eV at the loading density of 1/9, see Figure S4(b). The decrease in the chemical potential leads to the stable region of NM SAs moving towards slightly higher loading density, for example, the maximum loading density between 1/16 and 8


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1/9 for PBE but around 1/9 for HSE06 for Ag SAs on the (001). On the other hand, the PBE+U approach overestimated the chemical potentials, leading to the stable region of lower loading density. It also should be noted that the adsorption configuration adopted in this work is not the most stable geometry according to the study by Sun et al.21 Therefore, it is expected that the more stable adsorption configuration would increase the loading density which stabilizes the NM SAs on the clean (001).

Figure 3. Chemical potentials of (a) Ag, (b) Au, (c) Pd, and (d) Pt SAs at two loading densities for formation of NM-SA-(101) through comparing with the chemical potentials of NM NPs. 3.3 Photoelectron Trapping on (001) An extra electron was added into NM-SA-(001) to simulate the trapping state of a photoelectron. The difference charge density, defined as the difference in the electron charge density with and without the extra electron, was presented in Figure 4 to exhibit the spatial distribution of the trapped photoelectron. The PBE+U approach was used with an empirical parameter U = 4.2 eV on the Ti 3d orbitals (this U value has been justified in many publications33, 46-48), since the delocalization of charge density can be alleviated by the on-situ Coulomb correction. The photoelectron is trapped successfully at one Ti6c site in the subsurface of the clean (001) with the PBE+U approach, see Figure 4(a), in contrast to the delocalized electron charge density shown in Figure S5(a) derived with the PBE method. The 9



HSE06 hybrid functional can give almost the same electron charge distribution as that derived with PBE+U, and larger slab thickness does not change the electron charge distribution either, see Figure S6. Consistent with the result reported by Ma et al.,33 trapping of the photoelectron in the subsurface is very likely to generate an energy barrier which must be overcome upon the charge migration to the surface. This energy barrier suppresses the photoinduced reactivity of the clean (001).

Figure 4. Photoelectron charge distribution calculated with PBE+U for (a) clean (001), (b) Ag-, (c) Au-, (d) Pd-, and (e) Pt-SA-(001) at 1/9 loading density. Yellow (green) color denotes electron charge accumulation (depletion). The iso-surface value is set to 0.0012 e/bohr3. After deposition of NM SAs, the photoelectron was extracted from the sublayer to the outmost layer with the charge density mainly around the NM and O atoms, as shown in Figure 4(b-d). PBE exhibits qualitatively similar electron charge distribution, see Figure S5(b-d). DOS and Bader charge49 analyses reveal the role of NM SAs in the photoelectron trapping. Figure 5(a) shows that the CBM of clean (001) was composed of the subsurface Ti6c states rather than the surface Ti5c states, explaining why the photoelectron cannot be trapped on the clean (001) surface. After deposition of NM SAs, unoccupied electronic states appear in the band gap, as illustrated by the DOS peaks between the highest occupied state and the CBM in Figure 5(b-e). These states are primarily contributed by hybridization of NM and Ti5c orbitals and can be effective electron acceptors. This result is supported further by Bader charge analysis calculated with PBE+U, which indicates that the NM SAs are positively charged and about 0.39, 0.01, 0.15, and 0.02 electrons are transferred from the NM SAs to TiO2 for Ag, Au, Pd, and Pt, respectively. Upon the photoelectron trapped, excess charges of about -0.42, -0.35, -0.26, and -0.34 electrons are localized around Ag, Au, Pd, and Pt SAs, respectively, rationalizing the result of difference charge density. DOS (see Figure S6) and Bader charge (before photoelectron trapped: 0.27, 0.05, 0.23, and 0.08; after photoelectron trapped: -0.29, -0.38, -0.18, and -0.30) calculated with PBE show qualitative agreement with the results presented with PBE+U. NM SAs on the (001) facet can pump photoelectrons to the 10


Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst surface, facilitating the subsequent proton adsorption and electron transfer process of HER.

Figure 5. Total and local DOS calculated with PBE+U for (a) clean (001), (b) Ag-, (c) Au-, (d) Pd-, and (e) Pt-SA-(001) at 1/9 loading density. Ti5c and Ti6c of panel (a) represent the DOSs contributed by surface 5-coordinated Ti atoms and subsurface 6-coordinated Ti atoms, respectively, while Ti5c of panel (b-e) represents the DOS of one surface 5-coordinated Ti atom bonding with the NM atom. For clarity, the DOSs of NM and Ti5c of panel (b-e) are magnified by 20 times, and all the TiO2 valence bands which are mainly composed of O 2p states are aligned roughly to 0 eV. The vertical short dashed line in each panel denotes the highest occupied state in each case. 3.4 Free Energy of Hydrogen adsorption on (001) 11



Page 12 of 19

The activity of electrocatalytic hydrogen evolution reaction on solid electrodes can be estimated by calculating the free energy change of hydrogen adsorption on the catalyst surface, ∆GH*.34 According to Sabatier principle, the intermediate hydrogen binding strength facilitates both H* adsorption and desorption on the catalyst surface. When the electrode potential is at 0 V with respect to NHE, the ∆GH* should be very close to zero.

50

At the

operation condition of TiO2 photocathodes, the electrode potential is considered equivalent to the CBM potential, i.e., -0.05 V vs. NHE. Taking the electrode potential into consideration, Xing et al. gave the optimum ∆GH* of -0.05 eV for the thermodynamic HER model.22 At the standard condition, i.e., pH = 0, pH2 = 1 bar and T = 298 K, ∆GH* is calculated according to ∆GH* = ∆EH* + ∆EZPE - T∆S + eU

(5)

∆EH* is the hydrogen adsorption energy 1 2

∆EH* =EH-NM_1-TiO2 - ENM_1-TiO2 - E(H2 )

(6)

where E(H-NM_1-TiO2) is the energy with H* adsorption and E(H2) is the energy of an H2 molecule. ∆EZPE is the change in zero-point energy (ZPE) before and after H* adsorption. These values are approximately equal to 0.04 eV as derived on the metal Cu electrode.50-51 We also calculated ∆EZPE = 0.038 eV for hydrogen adsorbed Ag-SA-(001). However, ∆EZPE calculated on non-metal element electrodes are quite different.52-54 The entropy contribution, T∆S for an hydrogen molecule is derived from NIST-JANAF Thermochemical Tables55 and equal to -0.2 eV. The applied potential U = -0.05 V is equivalent to the CBM potential of TiO2. To sum up, ∆GH* = ∆EH* + 0.19 eV. The geometry of hydrogen adsorbed on Ag-SA-(001) at 1/9 loading density was presented in Figure 1(d) and all the geometries of hydrogen adsorbed NM-SA-(001) were shown in Figure S8. The NM-H bonds tend to form along the (001) surface since the H atom is attracted simultaneously by the NM and Ti5c atoms. Figure 6 shows the free energy of hydrogen adsorption on NM-SA-(001) at the standard condition with a bias of -0.05 eV with respect to NHE, as well as those on TiO2 clean (001) and NM (111) surface for comparison. The TiO2 clean (001) and NM (111) can be deemed as zero and infinitely large loading densities, respectively. Comparing ∆GH* on the three surfaces shown in Figure 6, several features can be concluded. Firstly, ∆GH* on NM-SA-(001) decreases overall with the loading density lowering from 1/4 to 1/25. However, the variation magnitude is much smaller (for example, ∆GH* almost converges at the loading density of 1/16) than that in the chemical potential of NM SAs, suggesting that the hydrogen adsorption on NM-SA-(001) is less sensitive to the loading density. Secondly, ∆GH* on NM-SA-(001) follows the order of Pd > Pt ≈ Ag > Au and is basically proportional to the photoelectron charge accumulated around the NM SA sites. The correlation can be understood by the fact that the surface containing larger amount of photoelectron charge can attract more strongly the proton from the solution and 12


Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


offer more electron charge to form bonds with the adsorbed proton. Thirdly, the H atom binds to the NM SAs much stronger than to TiO2 clean (001), as demonstrated by the significantly reduced ∆GH*, indicating that NM SAs improve much the HER activity with respect to the TiO2 clean (001). Finally, the binding strength of the H atom to the NM SAs supported by TiO2 (001) surface changes a lot compared to the corresponding NM (111) surface, exhibiting some novel HER catalysis of the supported NM SAs which will be discussed in detail below.

Figure 6. The free energy of hydrogen adsorption on NM-SA-(001) at four loading densities at standard conditions (pH = 0, pH2 = 1 bar., and T = 298 K) and under a photogenerated bias U = -0.05 V. For comparison, ∆GH* on TiO2 clean (001) and NM (111) surface are also depicted. The HER activity of bulk NM described by ∆GH* on NM (111) surface is qualitatively consistent with the previous experimental and theoretical studies,50 following the order of Pt > Pd > Au > Ag. However, downsizing the bulk NM to SAs dramatically changes the HER activity. For Ag and Au, ∆GH* are negative values for SAs, while they are around 0.4 eV for bulk materials. Especially for Au SAs, ∆GH* is smaller than -1.0 eV, suggesting a very strong binding interaction between the H atom and the Au SA. Considering both the stability and HER activity of Au SAs, as well as the variation trend in ∆GH* from bulk to SAs, may explain why Au nanoparticles with size ranging 3-30 nm are most active in photocatalytic HER.56 According to the same reason we predict that it is likely to find a range of Ag nanoparticle catalysts which favor the photocatalytic HER instead of the Ag SA catalyst on the (001). On the other hand, the HER activity of Pt SAs is predicted to be worse than Pd SAs. The HER activity of Pt SAs is even worse than bulk Pt, in contrast to the report by Xing et al.,22 probably due to the fact that different TiO2 surfaces are adopted in two studies. Combining the stability evaluation and calculated free energy of hydrogen adsorption, Pd SAs at the loading density from 1/16 to 1/9 could be a very promising candidate catalyst for photocatalytic HER. Therefore, high dispersion of Pd catalysts might be a strategy to prepare efficient and 13



cost-effective TiO2-based HER photocatalysts. The HSE06 functional and PBE+U gave similar ∆GH* at 1/9 loading density, as shown in Figure S9. Changes of only 0.1-0.2 eV were observed in the free energy of hydrogen adsorption. Different approaches have negligible effect on ∆GH* primarily due to the fact that the surface structure is seldom affected by hydrogen adsorption. 4. Conclusions In summary, we performed a DFT based first-principles investigation on the thermodynamic stability and the photocatalytic HER activity of NM SAs loaded on TiO2 support. It was found that the TiO2 (001) facet is a better candidate to anchor NM SA catalysts than the (101) facet. The chemical potential of NM SAs on the (001) decreases dramatically with the loading density lowering, while it keeps almost constant on the (101). The difference originates from the bonding nature of the two surfaces, incomplete saturated (001) surface and fully saturated (101) surface, leading to much stronger binding interaction of NM SAs to the (001) than to the (101) and meanwhile much more sensitivity of the stability of NM SAs on the (001) to the loading density than the stability of NM SAs on the (101). As a consequence, NM SAs can be formed on the clean (001) at low loading densities whereas they cannot be prepared on the clean (101) according to the current calculations. Au SA catalyst on the (001) is an exception since Au SAs tend to bind with each other, leading to growth of small Au clusters. We further estimated the capability of trapping photoelectrons on surface by NM SAs. It was found that deposition of NM SAs on the (001) can effectively extract the photoelectron charge from the subsurface to surface around the NM SA sites, facilitating the subsequent proton adsorption and electron transfer process of HER, as evidenced by the photoelectron charge accumulation correlating well with the hydrogen adsorption strength. Finally, we calculated the free energy of hydrogen adsorption energy as a function of the NM loading density. ∆GH* converges almost at the loading density of 1/16. The photocatalytic HER activity of NM SAs on the (001) is totally different to those for the TiO2 clean (001) and bulk NM (111). We rationalized the size of Au nanoparticles with the highest activity in photocatalytic HER by using the results of the present investigation, and predicted that high dispersion of Pd catalysts might be a novel route to prepare HER-efficient and cost-effective TiO2-based photocatalysts from both the stability and activity evaluations. Associated Content Supporting Information Geometries of NM-SA-(001), NM-SA-(101), NM DA clusters on (001) and hydrogen adsorbed NM-SA-(001) at coverage from 1/4 to 1/25; photoelectron charge distribution of slabs with different thickness and DOS calculated with PBE for clean (001) and 14


Page 14 of 19

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NM-SA-(001); photoelectron charge distribution calculated with the HSE06 hybrid functional; Chemical potentials and free energies of hydrogen adsorption calculated with the HSE06 hybrid functional; the surface bond length of NM-SA-(001) and NM-SA-(101). Author Information Corresponding Authors *E-mail: [email protected] (Z.H.Z.); [email protected] (L.J.G.) Notes The authors declare no competing financial interest. Acknowledgements This work is supported by the Fundamental Research Funds for the Central Universities (xjj2013004) and the National Natural Science Foundations of China (No. 51323011, No. 51236007). References 1.

Diebold, U., The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53-229.

2.

Kudo, A.; Miseki, Y., Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev.

2009, 38, 253-278. 3.

Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S., Semiconductor-based Photocatalytic Hydrogen

Generation. Chem. Rev. 2010, 110, 6503-6570. 4.

Roger, I.; Shipman, M. A.; Symes, M. D., Earth-abundant Catalysts for Electrochemical and

Photoelectrochemical Water Splitting. Nat. Rev. chem. 2017, 1, 0003. 5.

Fujishima, A.; Zhang, X.; Tryk, D. A., TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci.

Rep. 2008, 63, 515-582. 6.

Kho, Y. K.; Iwase, A.; Teoh, W. Y.; Mädler, L.; Kudo, A.; Amal, R., Photocatalytic H2 Evolution over

TiO2 Nanoparticles. The Synergistic Effect of Anatase and Rutile. J. Phys. Chem. C 2010, 114, 2821-2829. 7.

Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S., Increasing Solar Absorption for Photocatalysis with Black

Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. 8.

Zhang, K.; Wang, L.; Kim, J. K.; Ma, M.; Veerappan, G.; Lee, C.-L.; Kong, K.-j.; Lee, H.; Park, J. H., An

Order/Disorder/Water Junction System for Highly Efficient Co-catalyst-free Photocatalytic Hydrogen Generation. Energy Environ. Sci. 2016, 9, 499-503. 9.

Cao, M.; Tang, Z.; Liu, Q.; Xu, Y.; Chen, M.; Lin, H.; Li, Y.; Gross, E.; Zhang, Q., The Synergy between

Metal Facet and Oxide Support Facet for Enhanced Catalytic Performance: The Case of Pd–TiO2. Nano Lett. 2016, 16, 5298-5302. 10. Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P., Subnanometre Platinum Clusters as Highly Active and Selective Catalysts for the Oxidative Dehydrogenation of Propane. Nat. Mater. 2009, 8, 213-216. 11. Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Meyer, R. J.; Redfern, P. C.; Teschner, D.; Schlögl, R.; Pellin, M. J.; Curtiss, L. A.; Vajda, S., Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects. Science 2010, 328, 224-228. 15



12. Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T., Single-atom Catalysis of CO Oxidation Ssing Pt1/FeOx. Nat. Chem. 2011, 3, 634-641. 13. Vilé, G.; Albani, D.; Nachtegaal, M.; Chen, Z.; Dontsova, D.; Antonietti, M.; López, N.; Pérez-Ramírez, J., A Stable Single-Site Palladium Catalyst for Hydrogenations. Angew. Chem. Int. Ed. 2015, 54, 11265-11269. 14. Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T., Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Accounts Chem. Res. 2013, 46, 1740-1748. 15. Jones, J.; Xiong, H.; DeLaRiva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G.; Oh, S.; Wiebenga, M. H.; Pereira Hernández, X. I.; et al, Thermally Stable Single-atom Platinum-on-ceria Catalysts via Atom Trapping. Science 2016, 353, 150-154. 16. Liu, P.; Zhao, Y.; Qin, R.; Mo, S.; Chen, G.; Gu, L.; Chevrier, D. M.; Zhang, P.; Guo, Q.; Zang, D.; et al, Photochemical Route for Synthesizing Atomically Dispersed Palladium Catalysts. Science 2016, 352, 797-800. 17. Liu, J.-C.; Wang, Y.-G.; Li, J., Toward Rational Design of Oxide-Supported Single-Atom Catalysts: Atomic Dispersion of Gold on Ceria. J. Am. Chem. Soc. 2017, 139, 6190-6199. 18. Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F., A New Type of Strong Metal–Support Interaction and the Production of H2 through the Transformation of Water on Pt/CeO2(111) and Pt/CeOx/TiO2(110) Catalysts. J. Am. Chem. Soc.

2012, 134, 8968-8974.

19. Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B.; Li, R.; Sham, T.-K.; Liu, L.-M.; et al, Platinum Single-atom and Cluster Catalysis of the Hydrogen Evolution Reaction. Nat. Commun. 2016, 7, 13638. 20. Lin, L.; Zhou, W.; Gao, R.; Yao, S.; Zhang, X.; Xu, W.; Zheng, S.; Jiang, Z.; Yu, Q.; Li, Y.-W.; et al, Low-temperature Hydrogen Production from Water and Methanol Using Pt/α-MoC Catalysts. Nature 2017, 544, 80-83. 21. Sun, C.; Smith, S. C., Strong Interaction between Gold and Anatase TiO2(001) Predicted by First Principle Studies. J. Phys. Chem. C 2012, 116, 3524-3531. 22. Xing, J.; Jiang, H. B.; Chen, J. F.; Li, Y. H.; Wu, L.; Yang, S.; Zheng, L. R.; Wang, H. F.; Hu, P.; Zhao, H. J.; et al, Active sites on hydrogen evolution photocatalyst. J. Mater. Chem. A 2013, 1, 15258-15264. 23. Xing, J.; Chen, J. F.; Li, Y. H.; Yuan, W. T.; Zhou, Y.; Zheng, L. R.; Wang, H. F.; Hu, P.; Wang, Y.; Zhao, H. J.; et al, Stable Isolated Metal Atoms as Active Sites for Photocatalytic Hydrogen Evolution. Chem.– Eur. J. 2014, 20, 2138-2144. 24. Gong, X.-Q.; Selloni, A.; Batzill, M.; Diebold, U., Steps on Anatase TiO2(101). Nat. Mater. 2006, 5, 665-670. 25. Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q., Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638-641. 26. Li, Y.-F.; Liu, Z.-P.; Liu, L.; Gao, W., Mechanism and Activity of Photocatalytic Oxygen Evolution on Titania Anatase in Aqueous Surroundings. J. Am. Chem. Soc. 2010, 132, 13008-13015. 27. Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L., Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152-3153. 28. Yu, J.; Qi, L.; Jaroniec, M., Hydrogen Production by Photocatalytic Water Splitting over Pt/TiO2 Nanosheets with Exposed (001) Facets. J. Phys. Chem. C 2010, 114, 13118-13125. 29. Liu, G.; Sun, C.; Yang, H. G.; Smith, S. C.; Wang, L.; Lu, G. Q.; Cheng, H.-M., Nanosized Anatase 16


Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2 Single Crystals for Enhanced Photocatalytic Activity. Chem. Commun. 2010, 46, 755-757. 30. Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H.-M., On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem. Int. Ed. 2011, 50, 2133-2137. 31. Gordon, T. R.; Cargnello, M.; Paik, T.; Mangolini, F.; Weber, R. T.; Fornasiero, P.; Murray, C. B., Nonaqueous Synthesis of TiO2 Nanocrystals Using TiF4 to Engineer Morphology, Oxygen Vacancy Concentration, and Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134, 6751-6761. 32. Xu, H.; Reunchan, P.; Ouyang, S.; Tong, H.; Umezawa, N.; Kako, T.; Ye, J., Anatase TiO2 Single Crystals Exposed with High-Reactive {111} Facets Toward Efficient H2 Evolution. Chem. Mater. 2013, 25, 405-411. 33. Ma, X.; Dai, Y.; Guo, M.; Huang, B., Relative Photooxidation and Photoreduction Activities of the {100}, {101}, and {001} Surfaces of Anatase TiO2. Langmuir 2013, 29, 13647-13654. 34. Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z., Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem. Int. Ed. 2014, 53, 2-16. 35. Zhang, J.; Zhang, M.; Han, Y.; Li, W.; Meng, X.; Zong, B., Nucleation and Growth of Palladium Clusters on Anatase TiO2(101) Surface: A First Principle Study. J. Phys. Chem. C 2008, 112, 19506-19515. 36. Barmparis, G. D.; Remediakis, I. N., Dependence on CO Adsorption of the Shapes of Multifaceted Gold Nanoparticles: A Density Functional Theory. Phys. Rev. B 2012, 86, 085457. 37. Kresse, G.; Furthmuller, J., Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 38. Kresse, G.; Furthmuller, J., Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. 39. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758-1775. 40. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 41. Cohen, A. J.; Mori-Sánchez, P.; Yang, W., Insights into Current Limitations of Density Functional Theory. Science 2008, 321, 792-794. 42. Heyd, J.; Scuseria, G. E., Efficient Hybrid Density Functional Calculations in Solids: Assessment of the Heyd-Scuseria-Ernzerhof Screened Coulomb Hybrid Functional. J. Chem. Phys. 2004, 121, 1187-1192. 43. Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Angyan, J. G., Screened Hybrid Density Functionals Applied to Solids. J. Chem. Phys. 2006, 124, 154709. 44. Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., Electron-energy-loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505-1509. 45. Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J., The Surface Energy of Metals. Surf. Sci. 1998, 411, 186-202. 46. Jedidi, A.; Markovits, A.; Minot, C.; Bouzriba, S.; Abderraba, M., Modeling Localized Photoinduced Electrons in Rutile-TiO2 Using Periodic DFT+U Methodology. Langmuir 2010, 26, 16232-16238. 47. Ji, Y.; Wang, B.; Luo, Y., Location of Trapped Hole on Rutile-TiO2(110) Surface and Its Role in Water Oxidation. J. Phys. Chem. C 2012, 116, 7863-7866. 17



48. Wang, D.; Wang, H.; Hu, P., Identifying the Distinct Features of Geometric Structures for Hole Trapping to Generate Radicals on Rutile TiO2(110) in Photooxidation Using Density Functional Theory Calculations with Hybrid Functional. Phys. Chem. Chem. Phys. 2015, 17, 1549-1555. 49. Henkelman, G.; Arnaldsson, A.; Jonsson, H., A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comp. Mater. Sci. 2006, 36, 354-360. 50. Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U., Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23-J26. 51. Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Norskov, J. K., Computational High-throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909-913. 52. Deng, J.; Ren, P.; Deng, D.; Yu, L.; Yang, F.; Bao, X., Highly Active and Durable Non-precious-metal Catalysts Encapsulated in Carbon Nanotubes for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 1919-1923. 53. Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z., Hydrogen Evolution by a Metal-free Electrocatalyst. Nat. Commun. 2014, 5, 3783. 54. Zhou, Z.; Han, F.; Guo, L.; Prezhdo, O. V., Understanding Divergent Behaviors in the Photocatalytic Hydrogen Evolution Reaction on CdS and ZnS: a DFT Based Study. Phys. Chem. Chem. Phys. 2016, 18, 16862-16869. 55. Chase, M. W., Jr., NIST-JANAF Themochemical Tables, . In J. Phys. Chem. Ref. Data, Monograph 9 [Online] 1998; pp. 1-1995. 56. MurdochM; Waterhouse, G. I. N.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; LlorcaJ; IdrissH, The Effect of Gold Loading and Particle Size on Photocatalytic Hydrogen Production from Ethanol over Au/TiO2 Nanoparticles. Nat. Chem. 2011, 3, 489-492.

18


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

19


First-Principles Study on Stability and HER Activity of Noble Metal

Recommend Documents