Toward Facet Engineering of CdS Nanocrystals and Their Shape

Article pubs.acs.org/JPCC

Toward Facet Engineering of CdS Nanocrystals and Their ShapeDependent Photocatalytic Activities Xixi Wang, Maochang Liu,* Zhaohui Zhou, and Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 24, 2015 | http://pubs.acs.org Publication Date (Web): August 21, 2015 | doi: 10.1021/acs.jpcc.5b07370

S Supporting Information *

ABSTRACT: Controlling the shape or morphology of semiconductor nanocrystals is central to their enhanced physical and chemical properties. Herein, using CdS as a model photocatalyst, we demonstrate that the crystal habit of a visible-light-active semiconductor can be quantitatively controlled through synthesis kinetics. Growth rate control of {0001} facets (r1) and {101̅1} facets (r1′) of CdS nanocrystals was achieved by simply employing a syringe pump, which enables us to finely tune the crystal shape from nanocones, to nanofrustums, and further to nanoplates. These shape-controlled samples, showing altered proportions of {0001} to {1011̅ } facets, were used to investigate the crystal-facet dependence of solar hydrogen production. The results indicate that CdS nanoplates with the largest {0001} facets showed the highest photocatalytic activity. This work not only advances our knowledge on the growth mechanism of semiconductor crystals but also illustrates a robust method to targeted crystal design of semiconductors toward optimizing their associated catalytic activities.

1. INTRODUCTION

mechanisms reported for the evolution of CdS crystals with different shapes are usually incomplete. Herein, with the use of a simple syringe pump to quantitatively control the addition of the metal precursor, thus adjusting the reaction kinetics, we demonstrate that singlecrystal CdS of various shapes, from nanocone to nanofrustum, to nanoplate, could be obtained from a one-pot synthesis. The corresponding designed growth pattern involving diffusion of growth monomers driven by chemical potential has been revealed. Moreover, these nanoplates enclosed with {0001} facets have shown the highest activity for solar H2 generation from an aqueous solution containing sodium sulfide after evaluating the facets-dependent photocatalytic properties of these CdS nanocrystals. This work thus provides a powerful means to semiconductor synthesis with rational shapes and morphologies toward their associated catalytic applications.

Being regarded as a promising way to contribute solving the global energy crisis, climate change, and environmental pollution, semiconductor-based photocatalytic water splitting has received increasing attention due to its ability to produce clean and sustainable hydrogen energy.1−8 Among all reported semiconductor photocatalysts, CdS has been extensively studied due to its broad absorbance of visible light and suitable conduction band position for water reduction.9−14 Catalytic activity of semiconductors is directly dependent on the arrangement of surface atoms and thus to the type of crystallographic planes.15−19 Therefore, shape control and facets engineering of CdS have recently been investigated.20−25 In solar hydrogen production, reaction with CdS as the photocatalyst, {0001} facets are usually found to be more reactive because of their higher surface energy.21,22,26 Owing to the efforts of many research groups, it is now possible to prepare CdS crystals with various morphologies, including spheres,27,28 rods,9,29,30 and hexagons,12,31 etc. Despite the availability of such samples, there still lacks of a systematic study of the growth mechanism of such nanocrystals and their morphology−efficiency relationships. In general, most of the CdS shape-controlled syntheses are hydrothermal/ solvothermal-based which require high temperature/pressure conditions that are difficult to follow in situ. On the other hand, it is also difficult to conduct the same synthesis under two completely different growth conditions. The reason is that the number and nature of nuclei and seeds, and growth potential, generally controlled by reaction kinetics, may be significantly altered under different reaction conditions. As a result, the © XXXX American Chemical Society

2. EXPERIMENTAL SECTION Chemicals and Materials. All reagents were analytical grade and used without further purification. Sodium sulfide (Na2S·9H2O), sodium sulfite anhydrous (Na2SO3), and cadmium acetate (Cd(CH3COO)2·2H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethylene glycol (Tetra EG) was offered by Alfa Aesar, A Johnson Matthey Company. The water used in experiments are deionized water with a resistivity of 18.2 MΩ·cm. Received: July 30, 2015 Revised: August 19, 2015

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DOI: 10.1021/acs.jpcc.5b07370 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article


The Journal of Physical Chemistry C Preparation of CdS. This process was directed by a mini mechanical pump. In detail, 40 mmol of Na2S was first dissolved in 40 mL of Tetra EG in a 100 mL three-necked flask and then heated to 250 °C for over 10 min to evaporate water of crystallization in Na2S. The whole heating process was carried at N2 protected atmosphere. Then 2.5 mmol of Cd(CH3COO)2 was dissolved in 20 mL of Tetra EG by continuous magnetic stirring. Afterward, the solution was pumped into Na2S solution at a rate of (i) one-shot injection (fast injection), (ii) 5 mmol/h (moderate injection), (iii) 0.5 mmol/h (slow injection), and (iv) 0.25 mmol/h (very slow injection). The whole heating processes were under N2 protecting. After 5 h, the obtained products were centrifuged at 8000 rpm, washed by acetone and deionized water for several times, and dried at 343 K for 10 h in a vacuum oven. Photocatalytic Reaction. Photocatalytic reactions of hydrogen production by water splitting were conducted in a gas-closed system with a side irradiation Pyrex cell. An aluminum alloy shell was employed outside the Pyrex cell to reflect and gather the visible light illuminated by Xe lamp. Besides, a water cycling system was used to maintain the reaction temperature. Photocatalyst powder (50 mg) was dispersed by a magnetic stirrer in solution (180 mL) containing 0.35 mol/L Na2S and 0.25 mol/L Na2SO3. After being evacuated by N2 gas for over 10 min, the photocatalysts were irradiated by visible light (λ ≥ 420 nm) through a cutoff filter from a 300 W Xe lamp for 5 h. The amount of H2 gas was determined using a gas chromatograph (Bruker GC-450) or directly via drainage. Apparent quantum efficiencies (AQE) were measured using 425 nm band-pass filters and an irradiation meter and were defined by the equation

formation of CdS nuclei and growth monomers. The reaction kinetics is simply controlled by adjusting the injection rate of Cd2+. Imaging stable nuclei with polyhedral spherical shape, and a relatively high temperature for better crystallization and hexagonal phase formation of CdS, the seeds started for growth of CdS are supposed to be enclosed with {0001} and {101̅1} facets. Figure 1 shows a schematic of the growth process of CdS

Figure 1. Schematic illustration of (a) the major steps involved in the growth process and (b) the formation of a CdS nanocone, a CdS nanofrustum, and a CdS nanoplate, simply controlled by adjusting Cd2+ injection rate.

AQE = 2NH/NP where NH is the number of evolved H2 molecules and NP is the number of incident photons. Instrumentation. The X-ray diffraction (XRD) patterns were obtained from a PANalytical X’pert MPD Pro diffractometer using Ni-filtered Cu Kα irradiation (wavelength 1.5406 Å). The crystallite morphologic micrograph was determined on a field emission scanning electron microscope (SEM) of JSM-7800F (Japan) at an accelerating voltage of 3 kV and a transmission electron microscope (TEM) of FEI Tecnai G2 F30 S-Twin transmission electron microscope at 300 kV. An OXFORDMAX-80 energy-dispersive X-ray detector (EDX) which was mounted in the above TEM was used to conduct elemental analysis. K−M curves were obtained based on the UV−vis absorption of the samples, which were determined by a Hitachi U-4100 UV−vis−near-IR spectrophotometer. Brunauer−Emmett−Teller (BET) surface areas were determined by the N2 adsorption isotherms conducted in the Micromeritics ASAP 2020 plus instrument.

monomers that leads to the formation of CdS crystals with different crystal habits. For a typical growth process, two procedures occur: first, CdS growth monomers diffuse from bulk solution and reach the surface of a crystal nucleus/seeds (stagnant solution); second, monomers deposit at a certain surface planes of the nucleus/seeds, and then surface reconstruction occurs through surface diffusion.35,36 As shown in Figure 1a, after Cd2+ is being injected, CdS monomers would first deposit on either {0001} facets (step 1) or {101̅1} facets (step 1′), decided by the surface energies of each crystal facets and the chemical potential generated by the concentration of growth monomers. The final crystal habit is indeed adjustable by simply regulating the growth rates of the monomers on {0001} facets (r1) and {1011̅ } facets (r1′), as shown in Figure 1b. It is also worth pointing out that the deposited clusters may also undergo surface diffusion process (step 2) to make the crystal grow into more mature ones.36 The surface energy of {0001} facets is significantly higher than {101̅1} facets.22,26 The growth can then readily be controlled by changing the diffusion potential of solution monomers,37−40 through the kinetics of addition of Cd2+, as aforementioned. During this diffusion-controlled growth, it can be predicted that CdS crystals terminated with different facets shall be regulated by varying the ratio of r1 to r1′. Indeed, when the Cd2+ solution is rapidly injected, a large amount of CdS monomers will be generated, leading to a significantly high chemical potential of bulk solution (Pbulk). CdS monomers therefore rapidly diffuse into the diffusion sphere (the stagnant solution) to conduct growth. Meanwhile, as shown in Figure 2a,

3. RESULTS AND DISCUSSION Reaction kinetics has been determined by reaction velocity to facilitate a wide variety of chemical transformations via changing growth potential during crystallization. On the most basic level, crystal formation will start with generation of atoms, nucleation, and subsequent growth.32−34 We therefore began the synthesis with a solution containing the solvent and an excess amount of sulfur ions (S2−) and coupled with the use of simple syringe pump to inject cadmium ions (Cd2+) to the solution (Scheme S1). Because of the small solubility product constant Ksp of CdS, Cd2+ is rapidly reacting, leading to the B


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The Journal of Physical Chemistry C

diffusion (growth). Obviously as shown in Figure 2c, growth can only occur on {101̅1} facets. The resulting mature crystal habit shall be nanoplates. It should be also noted that accompanied by the continuous increasing percentage of {0001} facets, the chemical potential of the stagnant solution will be significantly elevated, leading to the termination of CdS monomers diffusing to the sphere and formation of diffusion equilibrium. In this stage, growth occurs only in the form of self-ripening driven by surface free energy. Finally, if Cd2+ injection rate is set to an even lower speed, only self-assembled small particles can be formed. The growth of different facets was therefore affected by the synergy of surface energy and chemical potential gradient. A series of experiments were then carried out accordingly. In practice, a tetraethylene glycol (Tetra EG) suspension containing Na2S was placed in a three-necked glass bottle (Scheme S1). A high temperature of 250 °C was employed to offer a favorable environment for hexagonal CdS growth.41,42 The Cd2+, as mentioned, was added using a syringe pump by fast injection (single-shot injection), or by moderate injection (5 mmol/h), or by slow injection (0.5 mmol/h). Figure 3a−c shows the corresponding transmission electron microscopy (TEM) images of the shape-controlled CdS nanocrystals with side views. As expected, CdS nanocones, nanofrustums, and nanoplates, with characteristic geometric side-top angles (Figure S1), i.e., ca. 120°, were successfully prepared with fast, moderate, and slow injection rate, respectively. When a nanoplate was observed from [0001] direction (top view), the representative hexagonal shape was clearly shown (Figure 3d). The corresponding scanning electron microscopy (SEM) images are shown in Figure S2. Notably, the final CdS crystals presented some smaller crystals adherent to larger ones. The formation of such small nanoparticles can be explained by the growth mechanism as shown in Figure 1. As we mentioned, the growth of CdS single crystals relies on CdS monomers, which shall first deposit onto the initial seeds and then diffuse throughout the crystal surface. During the process, monomers


Figure 2. Schematic diagram for the proposed growth mechanisms of (a) CdS nanocone, (b) CdS nanofrustum, and (c) CdS nanoplate. Dashed circle in each stage represents the interface between the bulk solution and the stagnant solution. Each interface constructs a diffusion sphere. Arrows indicate the directions of growth monomers’ diffusion. Pbulk, P1, and P2 indicate the chemical potentials of monomers in bulk solution, {0001} facets, and {101̅1} facets, respectively; ΔP1 indicates the potential gradient between Pbulk to P1; ΔP2 indicates the potential gradient between Pbulk and P2.

in this situation, the potential gradient (ΔP1) between Pbulk to the chemical potential of {0001} facets (P1) almost equals the potential gradient (ΔP2) between Pbulk to that of {101̅1} facets (P2), i.e., 0 ≪ ΔP1 ≈ ΔP2. However, in order to minimize the total surface free energy, CdS monomers deposited on {0001} facets shall be more favorable, resulting in the formation of CdS nanocones. The resulting products are indeed in accordance with the thermodynamic spontaneous reaction process. If the Cd2+ injection rate drops to a certain level, however, sufficient to maintain Pbulk a little higher than P1, CdS monomers shall remain to diffuse into the stagnant solution. In this case, the relationship of the potentials becomes 0 < ΔP1 < ΔP2 (Figure 2b). The monomers therefore get a larger possibility to deposit on {1011̅ } facets. On the other hand, by also considering the reduction of surface free energy of final products, both {0001} and {101̅1} will be subjected to growth, giving rise to the formation of CdS nanofrustums. Additionally, if Cd2+ is injected very slowly, the concentration of CdS monomers will drop to a level even lower than a critical value, leading the value of Pbulk smaller than P1, however, bigger than P2 to maintain the

Figure 3. (a−c) TEM images of CdS nanocone, nanofrustum, and nanoplate crystal, respectively. (d) TEM image of CdS nanoplate from top view. (e, f) HRTEM of a CdS nanocrystal from side view and top view, respectively. (g) SAED pattern from [0001] direction. (h, i) EDX mapping of a CdS nanoplate. (j) Powder XRD patterns of CdS nanocrystals of various shapes. (k) Optical properties of CdS nanocones, nanofrustums, and nanoplates. C


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The Journal of Physical Chemistry C may also undergo self-assembly and form small nanoparticles. On the other hand, the observation of such nanoparticles provided an experimental proof of our hypothesis of the monomer-based growth. Table S1 and Figure S3 summarize some statistical data, including the size, the distribution, the shape purity, and specific area of CdS nanocones, CdS nanofrustums, and CdS nanoplates. The specific surface areas were first calculated based on these statistic data, which showed a little deviation from the BET surface areas. Such difference should be a result of the mentioned nanoparticles. The threedimensional model of a typical CdS single crystal was also established (Figure S4) for better understanding the structure. The microstructure and elemental distribution of the nanoplate were further analyzed by high-resolution TEM (HRTEM) and energy dispersive X-ray (EDX) mapping. As shown in the HRTEM images (Figure 3e,f), the fringes with lattice spacing of 0.669, 0.358, and 0.315 nm can be indexed to the {0001}, {101̅0}, and {101̅1} planes,25,28,29,43−45 respectively, of a hexagonal close packed (hcp) lattice. The well-resolved, continuous fringes in the same orientation, together with the selected area electron diffraction (SAED) pattern of sharp spots (Figure 3g), indicate the single-crystal nature of the nanocrystal. Elemental mapping analysis of Cd and S in Figure 3h,i revealed that the nanoplate are homogeneous. Moreover, the morphology transition from nanocone to nanoplate was also in consistent with the XRD analysis with the observation of slightly increased (0002) diffraction intensities. However, no notable difference was detected in their UV−vis absorption properties (Figure 3k). For a better fundamental understanding of the mechanism involved in the growth process of these nanocrystals, the crystal habit evolution at different stages of the synthesis was investigated. We only focused on the two extreme cases shown in Figure 3a,c, which involved the fast and slow addition of Cd2+, respectively. As shown in Figure S5, CdS nanocones were formed within the first hour. Afterward, they grow into larger ones. This “defocusing of size distribution” process is thermodynamically controlled,37 which further confirmed our aforementioned hypothesis. As a comparison shown in Figure S6, CdS nanoparticles were first formed as seeds for growth at the first hour during the nanoplates formation. They subsequently transformed into nanosheets after 2 h and then turned into well-shaped hexagonal nanoplates. Interestingly, similar ripening process to reduce the surface energy, as expected, was also observed when the reaction proceeded for 6 h. Besides, when Cd2+ injection rate was adjusted to an even lower level, i.e., 0.25 mmol/h, the resultant product was composed of small nanoparticles without showing further growth, even when the reaction lasted for 12 h (Figure S7). Photocatalytic hydrogen production under visible light was carried out in a Na2S/Na2SO3 aqueous solution and irradiated by a 300 W Xe lamp coupled with a 420 nm cutoff filter.46 Figure 4 shows the time-coursed H2 evolution property and the corresponding initial H2 generation rates. CdS nanoplates showed the highest activity for hydrogen production. The H2 generation rate over the nanoplates reached 1.61 mmol gCdS−1 h−1, which was 2.3 and 1.6 times that over nanocones and nanofrustums, respectively. The superiority of the nanoplates was further confirmed by the specific activities (inset of Figure 4b) when the specific surface areas calculated by the statistical data of each sample (see Table S1) were taken into account for each samples in their hydrogen production ability. The activity

Figure 4. Visible-light-driven H2 evolution from an aqueous solution containing 0.35 M Na2S, 0.25 M Na2SO3, and 50 mg of CdS catalyst. (a) Time courses of photocatalytic hydrogen production of CdS nanocones, nanofrustums, and nanoplates. (b) Average photocatalytic hydrogen production rate of CdS nanocones, nanofrustums, and nanoplates. Inset in (b) shows corresponding specific activities that were normalized with respect to their calculated surface areas. All the tests were repeated at least three times to ensure the reproducibility, which also gave the mean rates of H2 evolution.

order was also in agreement with that normalized to the BET surface areas (Figure S8). Moreover, when NiS cocatalyst was loaded on CdS nanoplates, the hydrogen production rate was further improved to 22.3 mmol gCdS−1 h−1. The apparent quantum efficiency of the loaded nanoplates reached 18% at 425 nm, which is comparable to most reported CdS photocatalysts, including the first report by Reber et al.47 This result quantitatively demonstrates the significance of crystal habit and surface facets control to improve efficiency. In principle, the density of surface undercoordinated atoms is usually used as a crucial criterion to predict photocatalytic activity of crystal facets.48,49 In our case, because the photoreduction reaction always occurs on metal atoms which construct the conduction band, the state and ratio of unsaturated three-coordinate Cd atoms (Cd3c) become the most significant parameters. As shown in Figure 5a, {0001}

Figure 5. Schematic of atomic structure of {0001}, {101̅0}, and {1011̅ } facets of hexagonal CdS. All atoms on {0001} surface are Cd3c; 1/2 atoms on {101̅0} are Cd3c; 2/3 atoms on {101̅1} are Cd3c.

facets possess 100% Cd3c atoms. {101̅0} facets, on the contrary, possess 50% Cd3c and 50% saturated four-coordinate Cd atoms (Cd4c) (Figure 5b). As a geometrical combination of {0001} and {101̅0} facets, {101̅1} facets have 2/3 Cd3c and 1/3 Cd4c atoms (Figure 5c). This notion theoretically explains the higher reactivity of these {0001} facets in comparison to {101̅1} facets. D



The Journal of Physical Chemistry C To further validate the rationale, first principles calculation was employed to calculate the density of state (DOS) of these specific facets and corresponding Gibbs free energy changes during hydrogen adsorption−desorption. Similarly, we focused on the two extreme facets, i.e., {0001} and {1010̅ } facets. For {101̅1} facets, the band levels and energy change should be a medium case. According to the DOS calculation, both CB and VB positions of {0001} facets are more negative than {101̅0} (see Figure S9a). Based on it, the band alignments of {0001}, {101̅1}, and {101̅0} facets were schematically illustrated in Figure S9b. The formation of such type II band alignment is well-known for charge separation.5 Specifically, photogenerated electrons transfer to {0001} facets for reduction while photogenerated holes accumulate on {101̅1} facets for oxidation. It is worthy pointing out that in hole scavenger contained reaction, hydrogen generation is the rate-limited step.50 In this case, the larger percentage of {0001} facets is believed useful for the enhancement of hydrogen production. In addition, {0001} facets should be more favored for hydrogen atoms adsorption and disproportion because of their smaller Gibbs free energy changes (see Table S2). All these factors, again, undoubtedly give the superiority of {0001} facets for hydrogen production from water in comparison to {101̅1} facets.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07370. Calculation details, computational method, statistical data of CdS samples, experimental setup, measured angles between two adjacent {101̅1} facets, indications of diameter and height, SEM images of CdS samples, nanocrystal model, growth process of CdS nanocones and nanoplates, SEM image of CdS with a very low injection rate, specific activities been normalized with respect to their calculated surface areas (PDF)

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The authors greatly acknowledge the funding support from the National Natural Science Foundation of China (No. 51236007, No. 51323011), China Postdoctoral Science Foundation (No. 2014M560769), and the China Fundamental Research Funds for the Central Universities. We also appreciate Dr. L. Vayssieres for helpful discussions and critical reading of the manuscript.

4. CONCLUSION In summary, by employing syringe pump, we provide a powerful means to the fabrication of facet-engineered CdS nanocrystals. The growth was demonstrated to be kinetically diffusion-controlled. Specifically, by simply changing the Cd2+ injection rate, the growth along ⟨0001⟩ and ⟨101̅1⟩ directions was readily regulated, leading to the tunable shape evolution of CdS nanocrystals, namely nanocone, nanofrustum, and nanoplate. The nanoplates with largest {0001} facets, as expected, exhibited the highest photocatalytic activity for hydrogen production in comparison to the nanocones and nanofrustums. This work contributes to the significant advancement of our knowledge on the growth mechanism of semiconductor nanocrystals and associated crystal habit dependence on their photocatalytic efficiency for solar hydrogen generation at large scale.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.L.). *E-mail: [email protected] (L.G.). Notes

The authors declare no competing financial interest. E


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The Journal of Physical Chemistry C

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Toward Facet Engineering of CdS Nanocrystals and Their Shape

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