Supporting Information. Revealing the Size Effect of Platinum Cocatalyst for Photocatalytic

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1 Supporting Information Revealing the Size Effect of Platinum Cocatalyst for Photocatalytic Hydrogen Evolution on TiO2 Support: A DFT Study Dong Wang,, Zhi-Pan Liu,*, Wei-Min Yang*, State Key Laboratory of Green Chemical Engineering and Industrial Catalysis, SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai , China Collaborative Innovation Center of Chemistry for Energy Material, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Key Laboratory of Computational Physical Science (Ministry of Education), Department of Chemistry, Fudan University, Shanghai , China * Corresponding authors: zpliu@fudan.edu.cn; yangwm.sshy@sinopec.com 1. Calculation pre-testing results Table S1 Pre-tests on TiO 2 (101) slab layers in terms of H adsorption (E adh ) and the electron transfer energies (ΔE) from the bulk region to the surface. The calculation of ΔE is examined by two approaches, namely (i) introducing an extra ( photo-generated ) electron into the system; (ii) introducing an extra H atom on the bottom layer of TiO 2 slab. In approach (ii), the H atom was used to supply the excess electron in a charge neutral system, which produces a protonated O 2c and an additional electron trapped at a specific Ti ion. Unit: ev. Slab layers H adsorption (E adh ) ΔE O 2c Ti 5c introducing 1e introducing 1H 3 layers layers

2 Figure S1 The calculated PDOS of bulk anatase TiO 2 using HSE06 functional. All the states below 0 are occupied. The s, p and d orbitals are shown in black, red and blue color, respectively. 2. Geometry and electronic results. Table S2 The calculated energies of 14 selected structures from the stabilized AIMD simulations for three Pt/TiO 2 composites with large Pt clusters. The most stable structure is indicated in bold. Energy unit: ev. Candidates Pt 8/TiO 2 Pt 13/TiO 2 Pt 19/TiO

3 Figure S2 AIMD simulation trajectory as well as the obtained optimal structures for (b) quasi two-pt-layer model of Pt 8/TiO 2(101), (c) two-pt-layer model of Pt 13/TiO 2(101) and (d) three-pt-layer model of Pt 19/TiO 2(101) composites. The equilibration for the structure occurs roughly after ~6 ps (~8 ps for the Pt 13/TiO 2) in AIMD simulation as indicated by the black arrow. The optimized one-pt-layer model of Pt 5/TiO 2(101) is also shown in (a). Both the side view (I) and top view (II) of the optimal Pt/TiO 2 structures are illustrated, and all the Pt atoms are indexed with Arabic numbers. The O and Ti anchoring sites in contact with Pt cluster are highlighted in yellow and light blue colours, respectively. 3

4 Table S3 Bader charge analyses for optimal Pt/TiO 2 composites. Models Total positive charges Averaged positive charges on the Pt cluster e on each Pt atom e Pt 5/TiO Pt 8/TiO Pt 13/TiO Pt 19/TiO Figure S3 Computed DOS for the Pt 5, Pt 8, Pt 13, Pt 19 clusters in gas phase with their structures being fixed at the same with that on TiO 2. The E f position is aligned to zero and indicated by the dash line. Generally, the electronic structure becomes more smooth and richer as the cluster size goes bigger. Although the DOS of Pt 5 shows many intermittent localized states in the region ~1 ev below the E f, no noticeable gap is found around the E f. Therefore, all of the four Pt clusters show metallic characters with continuous DOS extending across the E f. 4

5 Figure S4 Simulated light absorption spectrum for TiO 2(101) surface, Pt 5/TiO 2, Pt 8/TiO 2, Pt 13/TiO 2, Pt 19/TiO 2 respectively using HSE06 functional DFT calculations. The insert shows experimental absorbance results for Pt/TiO 2 with different Pt loadings (w%), taken from ref. 6 in the text. There exists a wavelength shift of 75 nm for the epitaxial tangent value of TiO 2 (shown as dash lines). Regarding the light adsorption, it is currently difficult (for us) to determine the exact excitation orbitals. Nevertheless, we have simulated the light adsorption spectrum for these four Pt/TiO2 composites as well as for the TiO2(101) surface, via computing the frequency dependent dielectric matrix using hybrid DFT calculations. From Figure S4, one can see that pure TiO2 (black curve) exclusively adsorbs the UV light and shows negligible absorbance for the visible light (> 400 nm). After loading of Pt clusters with different particle size, it shows obvious visible light absorbance, extending from 320 to more than 800 nm, for all these Pt/TiO2 composites. The absorbance intensity is found positively correlated with the Pt loading content, agreeing well with experimental results (see the insert). Even a very small amount of Pt loading on TiO2, for example the Pt5 cluster in theoretical approach or the 0.5 wt% Pt in experiments, can significantly affects the electronic structure of the system and introduce evident absorbance in visible light region. It should also be noted that there exists a wavelength shift of 75 nm for the epitaxial tangent value of TiO2 (shown as dash lines in Figure S4), which is likely caused by the difference between the theoretical results focusing on the TiO2(101) surface and the experimental results on bulk TiO2 particles. This is also consistent with the results that the band gap of TiO2(101) surface (Figure 2 in the text) is slightly wider than that of bulk TiO2 (Figure S1). 5

6 Figure S5 Partial DOS projected on Pt n cluster (blue) and anatase TiO 2(101) surface (yellow-green) using HSE06 functional DFT calculations in four systems: (a) Pt 5/TiO 2; (b) Pt 8/TiO 2; (c) Pt 13/TiO 2; (d) Pt 19/TiO 2, respectively. In each system, the DOS for both the clean Pt n/tio 2 and that with a proton adsorbed on the Pt cluster are shown for comparison. The vertical dotted lines (I-IV) indicate the VBM (I), CBM (II) of TiO 2(101), the Fermi level (III) and the band minimum of unoccupied Pt states (IV) for the system. 6

7 3. Photoelectron transfer. Figure S6 The spin density plots (iso-value of e /Bohr 3 ) for the electron transfer process in the presence of proton adsorption (PPET) in four Pt/TiO 2 composites, respectively. In total 4 proton adsorption sites are considered as shown in Table 2 in the text, and in all cases the electron transfer features are quite similar and thus we only show one situation for brief illustration. Left panels: a trapped electron in the subsurface region of TiO 2; Right: delocalized electrons on Pt cluster. 7

8 Figure S7 The spin density plots of more samples of electron trapping in the subsurface region of TiO 2 in four Pt/TiO 2 composites respectively at the iso-value of

9 Clean surface site 1 site 2 Pt19/TiO2 Pt13/TiO2 Pt8/TiO2 Pt5/TiO2 Figure S8 The influence of water surroundings on the electron transfer energies from TiO 2 to the Pt cluster in four Pt/TiO 2 composites. Three situations are considered: (i) clean surface without proton adsorption (left); (ii) proton adsorption at site 1 (middle); (iii) proton adsorption at site 2 (right). Results are summarized in Table 2 in the text. 9

10 4. Surface adsorption of H atom Figure S9 The optimized H adsorption structures on various sites of the Pt 5/TiO 2(101) composite. The corresponding adsorption energies are shown in Table S4. Figure S10 The optimized H adsorption structures on various sites of the Pt 8/TiO 2(101) composite. The corresponding adsorption energies are shown in Table S4. 10

11 Figure S11 The optimized H adsorption structures on various sites of the Pt 13/TiO 2(101) composite. The corresponding adsorption energies are shown in Table S4. Figure S12 The optimized H adsorption structures on various sites of the Pt 19/TiO 2(101) composite. The corresponding adsorption energies are shown in Table S4. 11

12 Table S4 Adsorption energies of H atom (ΔG H) at various Pt sites on Pt 5/TiO 2, Pt 8/TiO 2, Pt 13/TiO 2 and Pt 19/TiO 2 composites, respectively. The H adsorption sites are the bridge sites involving two Pt atoms (Figure S9-S12). Two of the most reactive sites (the smallest exothermic ΔG H) on each Pt/TiO 2 composite are indicated in bold. Energy unit: ev. Sites Pt 5/TiO 2 Pt 8/TiO 2 Pt 13/TiO 2 Pt 19/TiO \ \ \ Results of another two Pt/TiO2 composites Here we provide results of two more models, the Pt7/TiO2 and Pt25/TiO2 composites, using the same calculation approach: 1. The quasi two-pt-layer Pt7/TiO2 model (determined by AIMD; Figure S13) is constructed to verify the superior activity of the quasi two-pt-layer structure in photocatalytic HER (not limited to the Pt8 cluster). One can see that the electron transfer energy (-0.15 ev for the IET and ev for the PPET) is less competent than the one-pt-layer Pt5/TiO2 but more exothermic than the multi-pt-layer models; whereas the H-H coupling barrier (Ea coup = 0.81 ev) is significantly lower than the one-pt-layer (1.16 ev) but close to the multi-pt-layers (~0.75 ev); all the results of the Pt7/TiO2 are quite similar with that of the Pt8/TiO2, reaching a good balance between the surface catalytic activity and electron transfer efficiency. Therefore, a quasi-two-layer structure with roof-like ridges in the second layer is theoretically predicted as the finest shape of Pt cocatalyst on TiO2 for photocatalytic HER. 2. The four-pt-layer Pt25/TiO2 model (AIMD; Figure S14) is constructed to confirm that the photocatalytic properties for supported multi-pt-layer nanoparticles basically converge as early as two Pt-layers. This is evident as the two-pt-layer Pt13/TiO2 and three-pt-layer Pt19/TiO2 always show quite similar results in both the surface catalysis and the electron transfer process. Here, by further investigating a four-pt-layer Pt25/TiO2 model, it was found that: i) for the electron transfer aspect, although the IET energy is slightly enhanced relative to the Pt13/TiO2 or the Pt19/TiO2 composites (-0.04 vs or 0.01 ev), the leading way of the PPET energy is very close to the two- or 12

13 three-pt-layer models (-0.30 vs or ev); ii) for the surface catalysis aspect, the H adsorption energy and also the Ea coup are found generally comparable among all the multi-pt-layer models. Therefore, we confirm the converged photocatalytic properties for multi-pt-layer nanoparticles ( 2 atomic layers). Figure S13 Summarized results for the Pt 7/TiO 2 composite. (a) AIMD simulation trajectory (~19 ps) as well as the obtained optimal structures. Both the side view (I) and top view (II) are illustrated, and the O and Ti anchoring sites in contact with Pt cluster are highlighted in yellow and light blue colours, respectively. (b) The spin density plots (iso-value of e /Bohr 3 ) for both the initial state (left) and the final state (right) during the electron transfer process. Both the IET and PPET energies are indicated. (c) Geometry structure for H adsorption at two reactive sites (site a and b) and the transition state of H-H coupling. Gibbs adsorption energies for the coup co-adsorption of two H atoms (H_ab), as well as the H-H coupling barrier E a, are listed. 13

14 Figure S14 Summarized results for the Pt 25/TiO 2 composite. (a) AIMD simulation trajectory (> 17 ps) as well as the obtained optimal structures. Both the side view (I) and top view (II) are illustrated, and the O and Ti anchoring sites in contact with Pt cluster are highlighted in yellow and light blue colours, respectively. (b) The spin density plots (iso-value of e /Bohr 3 ) for both the initial state (left) and the final state (right) during the electron transfer process. Both the IET and PPET energies are indicated. (c) Geometry structure for H adsorption at two reactive sites (site a and b) and the transition state of H-H coupling. Gibbs adsorption energies for the co-adsorption of two H atoms (H_ab), as well as the H-H coupling barrier E coup a, are listed. 14

15 6. Remarks on layer-dependent and size-dependent In this work, we aimed to figure out the size effect of deposited Pt particles from the view of the layer-dependent activity in Pt/TiO2 composites, originated from the charge redistribution at the metal/tio2 interface (see reasons in the fourth paragraph in the introduction section in the text). It is necessary to address that, firstly, we are theoretically focused on the layer-dependent trend rather than the exact particle size, which has too many candidate structures and is also hard to determine its global minimum in geometry. In this regard, our models are adequate enough that explicitly cover a full range of layered Pt nanoparticles from the smallest one-pt-layer to the four-pt-layer model. We can obtain the vertical height h of all these layered Pt clusters, including the finest structure of the quasi two-pt-layer model. Then, we propose to use a simplified model of a supported metal particle in a spherical segment (Figure 5 in the text) to present a rough estimation on the particle size of Pt cocatalyst on TiO2. In this way, we can convert the theoretical parameter h into experimental size d (diameter), and determine the optimal Pt size in experiments (section 3.5 in the text). Therefore, the word layer-dependent and size-dependent actually refer to the same idea in this work, but are explained from a different view of theoretical model for the former and experimental parameter for the latter. 15