Zinc-Blende CdS Nanocubes with Coordinated Facets for

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Zinc-Blende CdS Nanocubes with Coordinated Facets for Photocatalytic Water Splitting Yangyang Zhang, Li-Li Han, Changhong Wang, Wei-Hua Wang, Tao Ling, Jing Yang, Cunku Dong, Feng Lin, and Xi-Wen Du ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03212 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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Zinc-Blende CdS Nanocubes with Coordinated Facets for Photocatalytic Water Splitting Yangyang Zhang, † Lili Han, † Changhong Wang,

‡

Weihua Wang,*, § Tao Ling, † Jing Yang, †

Cunku Dong, † Feng Lin*, ┴and Xiwen Du*, † †School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China. ‡School of Science, Tianjin University of Technology, Tianjin 300384, People’s Republic of China §Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, Nankai University, Tianjin 300071, People’s Republic of China. ┴Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, United States.

ABSTRACT: In order to develop catalysts that are efficient and stable under aggressive catalytic conditions, we detail a synthetic approach to produce zinc-blende CdS nanocubes (NCs), a metastable CdS polymorph that is terminated by coordinated facets. The hydrogen generation activity of these CdS NCs (ca. 11.6 mmol·g-1·h-1) was nearly four (4) times higher than that of wurtzite CdS nanoparticles (ca. 2.7 mmol·g-1·h-1) and twice higher than that of irregularly shaped zinc-blende CdS nanoparticles (ZB-NPs) (ca. 5.9mmol·g-1·h-1). Furthermore, the NCs also

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showed much improved long-term performance compared to these controlled photocatalysts. Finally, the DFT calculation and site-specific Au photo-deposition demonstrate that the improved activity and stability are attributable to the enhanced charge-flow steering and coordinated facet terminations.

KEYWORDS: metastable polymorph, zinc-blende, facets engineering, charge-flow steering, photocatalytic water splitting

Renewable hydrogen has been considered a clean energy carrier because it can potentially solve the pollution problems and fast growing CO2 emissions caused by the consumption of fossil fuels.1-7 One promising path towards the clean generation of hydrogen is photocatalytic water splitting, where a semiconductor is excited by the sunlight, generating electrons and holes that drive the decomposition of water into hydrogen and oxygen, respectively.8, 9 Since nothing other than sunlight is consumed in these reactions, photocatalytic water splitting is a green process and has attracted broad attention.10,11 Among the photocatalytic materials such as TiO2, 12 ZnO13 and GaP, 14 cadmium sulfide (CdS), with a narrow bandgap (ca. 2.4 eV) and suitable redox potentials, is one of the typical semiconductors for photocatalytic water splitting. However, the photocatalytic H2 generation activity of the bare CdS still needs to be improved.15-17 A great challenge in photocatalytic water splitting for CdS materials is to develop a unique structure that allow for efficient electron-hole separation under light irradiation,18preferably visible light. Admittedly, the photocorrosion of CdS materials during catalytic reactions is another roadblock to widespread applications,19 especially in aqueous media containing oxygen under light irradiation.20


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One possible way towards high catalytic performance is to endow common catalysts with novel phase structure, and this strategy has been demonstrated effective in many cases such as oxidative dehydrogenation, (amm)oxidation, photodegradation, electrooxidation or hydrogen generation.21-25 For instance, iron powder with some amount of retained austenite (i.e. metastable crystalline phase), was reported to have the active zero valent iron providing electrons to break chemical bonds during the degradation of azo dyes.26In contrast to stable α-Bi2O3, metastable βBi2O3 was highly reactive for photocatalytic degradation of organic dyes (for example, rhodamine B) in aqueous solution.27 In comparison with common hexagonal phase, ruthenium nanoparticles with metastable face-centered-cubic phase exhibited much lower activation energy for the hydrolysis of ammonia-borane and the generation of H228. As for CdS material, the common stable phase is hexagonal wurtzite, while metastable zinc-blende phase can be obtained when the particle size is reduced less than 5 nm, and the zinc-blende CdS nanoparticles can decompose methylene blue efficiently.29 Nevertheless, the ultrafine CdS nanoparticles suffer serious charge recombination and are prone to aggregation or dissociation under working conditions.30 Carefully designed surface termination facets of catalysts can promote the electron-hole charge separation. Li et al. reported that electrons and holes tend to accumulate separately on the {010} and {110} facets of bismuth vanadate (BiVO4).31 Another typical example is titanium dioxide (TiO2), where different facets of anatase TiO2 exhibit different electronic structures and facilitate the transfer of photogenerated electrons and holes to {101} facets and {001} facets, respectively.32 Controllable synthesis of CdS materials with different morphologies such as nanosheets,33 nanorods,34 nanoparticles35 or nanoclusters30 have been reported, but controlling their termination crystal facets has seldom been discussed thoroughly in the context of


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photocatalytic water splitting. A recent study showed that wurtzite CdS nanoplates with dominating {0001} facets exhibited the highest photocatalytic activity compared to those terminated by other facets.36 So far, zinc-blende CdS structure with high energy facets has never been fabricated because this metastable phase was believed to be unstable at the particle size larger than 5 nm.37,38 We hereby propose to construct CdS nanostructure with metastable zinc-blende phase and unique facets as an active photocatalyst. We first synthesized PbS nanocubes and then transformed them into CdS nanocubes by a cation exchange method. The CdS nanocubes possess metastable zinc-blende phase and are enclosed with {100} and {111} facets. The DFT calculation suggests that the energy difference between {100} and {111} facets facilitate the charge separation, and the experimental measurement confirms that zinc-blende CdS nanocubes is more efficient for hydrogen generation in photocatalytic water splitting than CdS nanoparticles with either wurtzite phase or zinc-blende phase. The present study provides a synthetic strategy to metastable zinc-blend CdS materials and sheds light on the path towards tunable CdS crystal facets for enhanced water splitting reactions. RESULTS AND DISCUSSION The synthesis of single-crystalline cubic lead sulfide (PbS) cubic structures is highly repeatable and their intrinsic cubic crystal structure determines the thermodynamically favorable formation of single crystalline nanocubes, thus the obtained PbS cubic-structures are ideal morphologycontrolled templates for generating cubic-shaped CdS nanoparticles. Figure 1a shows a typical TEM image of the cube-shaped PbS nanoparticles with a particle size of approximately 15 nm. Electron diffraction pattern (inset of Figure 1a) confirms the presence


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of cubic crystal structured PbS nanoparticles, since the powder rings corresponding to the facecentered-cubic structure were observed. Through the HRTEM image (inset of Figure 1a), lattice fringes with interspacings of 0.297 and 0.210 nm can be clearly determined, corresponding to the (200) and (220) planes of PbS crystal, respectively. These observations indicate that the cubic PbS crystals were single-crystalline and predominantly enclosed by six nominal {100} facets. The X-ray powder diffraction (XRD) analysis also confirms the high crystallinity of these nanocubes, as characterized by the diffraction peaks from face-centered-cubic PbS (PDF#050592, space group: Fm-3m (225), Z=4) (Figure 1e and Figure S1c).The EDS(Figure S1d) confirms the presence of only Pb and S in the crystals. Note that Cu in the spectrum originated from the copper TEM grid. Template-assisted synthesis represents a straightforward route to obtain nanoparticles with controllable morphology. The combination of precise controls of size, morphology and phase control makes the PbS nanocubes ideal templates for the synthesis of cubic CdS nanoparticles through a cation exchange method. The TEM images (Figures 1b-d) together with high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) and HRTEM images clearly show that as the temperature was elevated from 200 °C to 260 °C, the PbS core decreased whereas the CdS shell thickness increased, but such change did not interrupt the shapes or uniformities of the initial nanocrystal templates. XRD patterns (Figure 1e) also indicate the presence of growing CdS phase composition with increasing CdS shell thickness. The relative peak heights of the PbS/CdS core/shell sample and positions are influenced by the CdS shell on the PbS core. Taking the most obvious peaks as an example, pure bulk CdS zinc-blende (PDF#10-0454, space group: F-43m (216), Z=4) has a higher peak intensity for the (111) reflections (26.5 degrees) compared to the (200) reflections (30.8 degrees) whereas pure PbS bulk phase (rock-salt structure) has a higher


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peak intensity for the (200) reflections (30.1 degrees).39 For a core/shell structure with a constant overall particle size, a thicker shell layer would increase its overall composition with respect to the amount of PbS in this heterostructure. The XRD results show exactly an increasing (111) intensity while the (200) peak decreases as the CdS shell thickness increases. In addition, (200) peak of the original PbS nanocrystals gradually decreases, and finally disappears, indicating the complete transformations from PbS nanocrystal templates to CdS. Electron diffraction pattern illustrated in Figure 1d also proves that the CdS nanocubes are zinc-blende which is consistent with the XRD patterns. EDS spectra (Figure 1f) of the products at different reaction temperatures show the gradual evolution of PbS to CdS, where the relative concentration of Cd and Pb in the samples is listed in Table S1. UV-vis absorption spectra in Figure S2 also show the transition of chemical composition, that as the fading away of PbS core the absorption in the longer wave band weakened.


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Figure 1. Evolution from PbS nanocubes to CdS nanocubes. TEM images of (a) PbS nanocubes and the products obtained at (b) 230°C, (c) 245°C and (d) 260°C. The SAED patterns and HRTEM images are inserted in (a) and (d), and HAADF-STEM images are inserted in (b) and (c). (e) and (f) are the XRD patterns and EDS of the products at various reaction temperatures, respectively.


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In order to investigate the unique properties that CdS NCs (nanocubes) may exhibit because of their structural and morphological characteristics, we compared the properties of irregularly shaped zinc-blende CdS nanoparticles (ZB-NPs) and wurtzite CdS nanoparticles (WZ-NPs) with them. All of the CdS samples were in situ deposited with same amount of Ni(OH)2 as detailed in the Experimental Section. According to the binding energies of Ni 2p3/2 (855.7 eV)40 and the EELS mapping of Ni element (shown in Figure S3), Ni(OH)2 was formed on the surface of CdS NCs. Firstly, we examined the activity of Ni(OH)2 decorated CdS NCs and ZB-NPs toward photocatalytic hydrogen evolution in the water splitting (Figure 2a). Despite the same crystal structure (Figure S4) and similar energy band gaps concluded from their UV–vis absorption spectra (Figure S5), the H2-generationof the NCs is obviously greater than that of the ZB-NPs. Figure 2b shows that the CdS NCs provided a high H2 evolution rate of 11.64 mmol·g-1·h-1, which was almost 2-fold higher than that of the ZB-NPs (5.85 mmol·g-1·h-1). The difference in the specific surface area was also taken into account, which was done by normalizing the H2 evolution rate to the BET surface areas of the two materials (shown in Figure 2c). Higher photocatalytic activity of CdS NCs can be seen more manifestly, that is 363.8 umol·m-2·h-1 for the CdS NCs (nonacubes) and 75.9 umol·m-2·h-1 for the ZB-NPs. Figure 2d displays a comparison of the recyclability of Ni(OH)2 decorated CdS NCs and ZB-NPs in the hydrogen generation reaction. Although both samples witnessed a rise during the initial hours likely due to the formation of cocatalyst,41 they exhibited dramatic difference after around 3 hours. The photocatalytic H2-generationof CdS NCs almost maintained a constant in 9 hours (from 3h to 12h), and even increase slightly. Between 12h and 18h, the rate of evolved H2 started to fall slightly but was still superior to that of ZB-NPs. The photocatalytic activity reduced because the


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specific morphology with coordinated facets began to transform into near-sphere particles (Figure S6). In contrast, the ZB-NPs experienced a noticeable drop of H2 generation after approximately 9 hours. The photocatalytic performance of ZB NPs in Figure 2(a) had some differences with that in Figure 2(d). ZB NPs were metastable without specific morphology so their activity could reduce during long term continuous measurement (Figure 2(a)). However, for the recycle experiment, the chamber was thoroughly purged with argon between every recycles. It might alleviate the loss of the activity especially for the metastable ZB NPs at the initial several cycles (Figure 2(d)). Then, we investigated the photocatalytic properties of CdS NCs (Nanocubes) and wurtzite CdS nanoparticles (WZ-NPs). The XRD patterns and TEM images of them can be found in Figure S4, and their size are almost on the save level. From Figure 2a and the transformation in the left part of the bar chart (Figure 2b), CdS NCs performed much better than WZ-NPs (2.7 mmol·g-1·h-1). In consideration of the specific surface areas, we also compare their activities by normalizing the H2 evolution rate to the BET surface areas of the two materials, and they are 363.8 umol·m-2·h-1 and 45.5 umol·m-2·h-1, respectively.


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Figure 2. (a) Evolution of H2 versus time under illumination for Ni(OH)2 decorated CdS NCs (Nanocubes), irregularly shaped CdS NPs (ZB-NPs) and wurtzite CdS NPs (WZNPs) (b) Comparison of photocatalytic H2 generation activities of different samples with different morphologies. Left ones are mass activities (MA) with respect to the mass and the right is specific activities (SA) with respect to the surface areas of active materials, respectively. (c) N2 adsorption/desorption isotherms of the three samples. The specific surface areas of the Nanocubes, ZB-NPs and WZ-NPs are determined 32 m2·g-1, 77 m2·g1

and 60 m2·g-1, respectively (d) Recycle hydrogen generation property of three kinds of

nanoparticles with Ni(OH)2 decoration in 18h.


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We also studied the inﬂuence of the synthetic parameters of CdS NCs (Figure S7), observing that higher temperature may promote an efficient cation diffusion, so that the corresponding samples performed better photocatalytic H2-generation activity. A further increase in Cd2+ content during cation exchange caused an increase of the photocatalytic activity (Figure S8). Higher temperature and precursor concentration might play an important role in the reaction kinetics, leading to increased purity of CdS (Table S1). The CdS NCs annealed at 400°C were also applied to catalytic hydrogen generation after deposition of Ni(OH)2 (Figure 3a). The activity of treated CdS NCs was lower than that of CdS NCs without annealing. This difference is reasonable, because the superiority of CdS NCs is the special morphology with exposed {100}facets, and they were damaged after annealing. As shown in Figure S9, once dispersive CdS NCs became agglomerate and were no longer angular, which naturally led to loss of activity. The XRD peaks at 28.2°, 47.8°, and 58.3°verify the presence of wurtzite CdS (PDF#41-1049, space group: P63mc (186), Z=2) after annealing (Figure 3b). The general wisdom tells that the annealed CdS would have better photocatalytic performance because of the increased crystallinity. However, due to the loss of the unique cubic shape and exposed (100) surface, the annealed CdS NCs show much inferior performance (Figure 3a).


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Figure 3. (a) Rate of H2 evolution of irregularly shaped CdS NPs (ZB-NPs) and that of CdS NCs before and after annealing at 400°C (b) XRD patterns of irregularly shaped CdS (ZB-NPs) and that of CdS NCs before and after 40min annealing at 400°C Having the same zinc-blende phase, comparable particle size and similar absorption of light, CdS NCs and irregularly shaped CdS NPs performed distinct activities towards photocatalytic water splitting. This is likely attributed to the better charge-flow steering in the cubic ones. The density-theory (DFT) calculation and site-specific Au deposition experiment were used to confirm this hypothesis. DFT calculations were performed as implemented in plane-wave basis code Vienna ab initio Simulation Package (VASP).42The generalized gradient approximation (GGA) exchangecorrelation potential in the Perdew−Burke−Ernzerhof (PBE) form was employed.43The


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calculated lattice constant of bulk CdS is 5.941 Å, which is close to the experimental value of 5.818 Å. Figure S10 shows the band structure and partial density of states (PDOS) of bulk CdS calculated by using the HSE hybrid functional. A direct band gap of 2.41 eV was achieved (experimental optical bandgap: 2.4 eV). The PDOS shows that the valence band maximum (VBM) is mainly contributed by S-3p and Cd-4d orbitals, whereas the conduction band minimum (CBM) is mainly contributed by S-3p and Cd-5s orbitals. This indicates that the VBM is mainly constructed by the p-d hybridization between S-3p and Cd-4d orbitals. The CBM is mainly contributed by the s-p (S-3pand Cd-5s) hybridizations. Actually, in the absence of sharp corners, the CdS NCs could be described as truncated cubes rather than absolutely perfect cubes (Figure 1d). Thus, CdS NCs were mainly terminated by {100} facets while their truncated corners were terminated by {111} facets. To explore the electron transfer mechanism between {100} and {111} facets of CdS with both Cd and S terminations ((100)-Cd-term, (100)-S-term, (111)-Cd-term and (111)-S-term), their band edge alignments are calculated (Figure S11). As shown in Figures 4a-b, a slab model with 5 double layers is adopted. We name the atomic layers from bottom to top as layer 1 to layer 5. In the layer 1, the dangling bonds of S or Cd atoms are passivated by H atoms (H atoms belong to layer 1). During structural optimization, layer 1 and all H atoms are fixed. Local density of states (LDOS) indicates that layer 1 is successfully passivated and the gap states are mainly contributed by the surface layer (layer 5). By calculating the work function and band structures, we aligned the band edges of all the surfaces (Figure S12a). Based on the surface energy (Es) and attachment energy (Ea) of different surface models, {100} and {111} facets of zinc-blende CdS tend to terminated by cadmium atoms and sulfur atoms, respectively.44,45Thus, we propose that the couple of CdS (100)-Cd-term/CdS (111)-S-term is suitable to understand the experimental


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results and corresponding schematic diagram is shown in Figure 4c. The VBM and CBM of CdS(100)-Cd-term are ~1.24 eV higher than CdS (111)-S-term case, which demonstrates that the excited electrons will transfer from {100} to {111} facets and excited holes will transfer reversely (Figure 4c). Therefore, the coordinated (111)-S-term and (100)-Cd-term facets can promote the charge separation significantly.

Figure 4. (a) Side view of Cd terminated (001) surface slab model and LDOS of each layer (b) Side view of S terminated (111) surface slab model and LDOS of each layer (c) schematic diagram of band edge alignments for {100} and {111} facets of Zinc-Blende CdS and (d) TEM picture and High-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) image of Au-CdS clearly show that the particles of Au are mostly photo-deposited on the corners of the nanocubes. Namely, the


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photogenerated electrons are readily available for the reduction reaction on the {111} facets. We then validated the directional charge-flow in the CdS NC by the site-specific Au photodeposition using the same light source as that used in the photocatalytic water splitting (i.e., 300W Xe lamp). The as-prepared samples are denoted as Au-CdS. The photo-reduction of metal ions (equation 1) can be described as follows： + 3 → (1) Figure 4d displays dark corners in the TEM image and bright corners for the structure of hetero-nanocubes by using high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM). Together with Figure S13a it is interesting to note that the gold particles are selectively photo-deposited at the corners and the photo-reduction deposition suggests the accumulation of photo-generated electrons on the {111} facets. EDS spectrum (Figure S13b) demonstrates presence of gold species. In addition, The Au4f XPS spectrum indicates the formation of Au0 (inserted in Figure S13b). Based on the binding energies of Au 4f in XPS, the Au 4f 7/2 and Au 4f 5/2 peaks located at 84.0 eV and 87.5 eV indicating that Au is in the metallic state. HRTEM images of Au-CdS captured from different crystal orientation and models of them are shown in Figure S14. All these results unambiguously reveal that the photogenerated electrons and holes tend to accumulate on the {111} facets and {100} facets,31 respectively, which results in the reduction and oxidation reactions taking place on the corresponding corners and (100) facets (Figure 4c). Such nature of the CdS nanocubes is believed to play an important role in promoting the photocatalytic properties in the water splitting.


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The site-specific photo-deposition of platinum particles was performed to further confirm the above conclusion. TEM image and corresponding EDS spectrum (Figure S15) clearly show that the particles of Pt are mostly photo-deposited on the {111} facets (i.e., corners) of the nanocubes, that is, the photogenerated electrons tend to migrate to the {111} facets for selective metal reduction. In consideration of the surface chemistry, Au particles were deposited on CdS NCs by thermal reduction under a dark condition. In contrast, the Au nanoparticles were loaded randomly on the surface of CdS NCs both {100} facets and {111} facets. It is possible that the unique surface chemistry of hydrogen bond formation contributes to the activity of the catalyst. Here, chargeflow steering was believed to be the major cause for improved photocatalytic activity. The result of gold deposition by thermal reduction is shown in Figure S16. CONCLUSION In summary, we have taken a mature synthetic approach to synthesize high-quality, monodispersed zinc-blende CdS nanocubes with exposed coordinated facets, that are {100} facets and corner-truncated {111} facets. The unique morphology and structure of the cubic-CdS photocatalysts led to enhanced photocatalytic H2generation activity (both mass and surface area specific activities) and stability compared to the irregularly shaped zinc-blende CdS nanoparticles and wurtzite CdS. Both experimental and DFT studies clearly demonstrated that the zinc-blende CdS nanocubes ensured well steered charge flow and allowed for the transfer of photogenerated electrons and holes to the {111} facets (i.e. corners) and {100} facets, respectively. Such an oriented accumulation of electrons and holes enhanced the separation of charge carriers and increased the photocatalytic activity toward H2 generation by water splitting.


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EXPERIMENTAL METHODS Materials synthesis.The PbS NCs were obtained by a hot-injection method. The procedures were similar to a previously reported method in the literature.46Sulfur powder dissolved in oleylamine (OAm) was injected into the three-neck flask with PbCl2 dissolved OAm at 220 °C. The mixture was aged for 20 min, resulting in black colloidal solution. The characterization of the PbS nanocubes is presented in the Figure S1. PbS@CdS core/shell nanocubes and CdS nanocubes were synthesized following the procedures previously reported for PbTe@CdTe core/shell nanoparticles (NPs).47 Various amount of Cd (Ac)•3H2O was dissolved in a mixed solvent containing OA and ODE. The solution was injected to a 500mL three-neck flask (equipped with a magnetic stirring bar) with 25 mL of ODE suspension with PbS NCs (2.658 g/L). The obtained mixture reacted at different temperatures (200~300°C) under argon flow. Finally, the product was washed with ethanol several times and transferred hydrophobic nanocrystals to aqueous solution prior to further characterization (see Supplementary information). To synthesize irregularly shaped zinc-blende CdS NPs, 2 g (7.5 mmol) of cadmium acetate trihydrate and 0.2405 g (7.5 mmol) of sulfur powder were dissolved, each, in 75 mL of OA by stirring at 100°C for 15 min. The two solutions were mixed together in a 500-mL flask and reacted for 1 h at 180 °C under magnetic stirring and in Ar flow. The precipitate was retrieved by centrifugation and washed with ethanol and hexane twice. Wurtzite CdS NPs were prepared by hydrothermal method. Briefly, 0.14 mol Cd(NO3)2·4H2O was dissolved in 25mL deionized water. After being stirred vigorously for 30min 0.14 mol Na2S ·9H2O was added into the solution and stirred for another 30min forming a deep yellow


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precipitate. Hydrothermal treatment of these CdS sol was carried out in a Teflon-lined stainless steel autoclave of 40mL capacity and the autoclave was heated at 160°C for 12h. Then the autoclave container was removed and quenched to room temperature. The precipitate was rinsed with deionized water for twice and finally dried in a vacuum desiccator at 60 °C for 6 h.48 To study the charge steering experiment, in situ photodeposition at room temperature was performed. For Au-CdS (or Pt-CdS), 6 mg CdS powder and 5mg metal precursors HAuCl4 (or H2PtCl6) were mixed in 10 mL of 9:1 deionized water–ethanol mixture, and the suspension was then irradiated by a 300 W Xe lamp under continuous stirring. After photo-deposition, the suspension was washed with de-ionized water, and prepared for the subsequent characterization. Au nanoparticles growth onto the CdS NCs by thermal reduction was performed with same amount of precursors and solution system as the photodeposition. And the suspension was heated to 50°C for 1h without illumination. Characterization of materials. The crystal structures were investigated by X-ray diffraction (XRD, Rigaku D/max 2500v/pc). The morphology study (including TEM images, SAED patterns and HAADF-STEM images) was performed with a JEOL 2100F FEG transmission electron microscopy (TEM) equipped with a field-emission gun operated at 200 kV. The composition was analysed with an Oxford INCA energy dispersive spectroscopy (EDS) module attached to the TEM microscope. BET surface area was measured at 77 K with an ASAP 2020 physisorption analyzer (USA) using N2 as the probe gas. XPS data were acquired on a Perkin Elmer 5100 system with a non-monochromatic Al anode X-ray source. The UV-vis absorption spectra were recorded using a Hitachi U4100 UV–vis absorption spectrometer. Atomicresolution annular dark-field (ADF) images were performed with a 200 keV aberration-corrected


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dedicated scanning transmission electron microscopy (Hitachi HD2700C). Electron energy loss spectroscopy (EELS) data were acquired using a Gatan Tridiem spectrometer. Relative concentration of Ni(OH)2 deposited on CdS materials was determined by an Agilent 7700x ICPMS system. Performance Measurement. For the photocatalytic water splitting measurement, 6 mg of CdS materials was dispersed in 40 mL of 9:1 deionized water–ethanol mixture contained in a glass chamber, and 5.6g NaOH and 0.95mg NiCl2·6H2O were added resulting in a yellow solution. Ni(OH)2 was in situ deposited on the surface of all the CdS materials under an identical condition and it acted as the cocatalyst here. The relative content of Cd and Ni in the samples showed no major difference (shown in Table S2). In order to remove both the oxygen from the headspace of the reactor and that dissolved in the water, the chamber was thoroughly purged with argon, and then irradiated by a 300 W Xe lamp. The lamp-reactor distance and angle were fixed throughout the experiment. H2 evolution was monitored with a gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector, and each time 0.2 ml of gas phase inside the glass reactor was taken to the GC. The amount of H2 evaluated is expressed as the molar amount, while the evaluation of H2 formation rate is expressed as the molar amount evolved per hour of generation. Calculation Methods. Density-functional theory (DFT)calculations were performed as implemented in plane-wave basis code Vienna ab initio Simulation Package (VASP).

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kinetic-energy cut off of 500 eV was used. For surface calculations, a vacuum region of 15 Å was introduced in the periodic slab model to minimize the spurious interactions between top and bottom images. The Brillouin zone was sampled by (9×9×9) k-points for bulk CdS and (9×9×1) k-points for CdS surfaces. By using a conjugate gradient method, all atomic positions and lattice


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constants were optimized, where the total energy and atomic forces were minimized. The total energy difference between two electronic steps was less than 10−4 eV and the convergence criterion of Hellmann-Feynman force on each atom was 0.01 eV/Å during the ionic relaxation. For electronic structure calculations, the conventional density functional theory usually underestimates the band gap, and thus the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional was used.49, 50 The Hartree-Fock mixing parameter was set to 0.325 in HSE calculations. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. : Other related experimental methods, characterization of PbS nanocubes, different CdS materials and hetero-nanocubes (TEM images, XRD patterns, EDS spectra, XPS spectra, EELS mapping and ICP-MS measurements), comparisons (UV-vis absorption, chemical composition and photocatalytic activities) among samples prepared under different cation exchange conditions and details of calculation methods. (PDF)


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ACKNOWLEDGEMENTS This work was supported by the National Basic Research Program of China (2014CB931703) and the Natural Science Foundation of China (51671141, 51571149 and 51471115). F.L. gratefully acknowledges Virginia Tech Department of Chemistry startup funds. REFERENCES (1) Lubitz, W.; Tumas, W. Chem. Rev. 2007, 107, 3900-3903. (2) Gray, H. B. Nat. Chem. 2009, 1, 7. (3) Barber, J. Chem. Soc. Rev. 2009, 38, 185-196. (4) Nocera, D. G. Acc. Chem. Res. 2012, 45, 767-776. (5) Gu, J.; Yan, Y.; Krizan, J. W.; Gibson, Q. D.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B. J. Am. Chem. Soc. 2014, 136, 830-833. (6) Wang, M.; Chen, L.; Li, X.; Sun, L. Dalton Trans. 2011, 40, 12793-12800. (7) Wang, F.; Wang, W. G.; Wang, H. Y.; Si, G.; Tung, C. H.; Wu, L. Z. ACS Catal. 2012, 2, 407-416. (8) Gu, J.; Yan, Y.; Young, J. L.; Steirer, K. X.; Neale, N. R.; Turner, J. A. Nat. Mater. 2016, 15, 456-460. (9) Martin, D. J.; Reardon, P. J. T.; Moniz, S. J. A.; Tang, J. J. Am. Chem. Soc. 2014, 136, 12568-12571.


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Table of Contents Graphic (TOC)


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Zinc-Blende CdS Nanocubes with Coordinated Facets for

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