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Hexagonal@Cubic CdS Core@Shell Nanorod Photocatalyst for Highly Active Production of H2 with Unprecedented Stability

Kui Li, Min Han, Rong Chen, Shun-Li Li, Shuai-Lei Xie, Chengyu Mao, Xianhui Bu, Xue-Li Cao, Long-Zhang Dong, Pingyun Feng,* and Ya-Qian Lan*

Dr. K. Li, Prof. M. Han, Dr. R. Chen, Prof. S.-L. Li, Mr. S.-L. Xie, Dr. X.-L. Cao, Mr. L.-Z. Dong, Prof. Y.-Q. LanJiangsu Key Laboratory of Biofunctional MaterialsSchool of Chemistry and Materials ScienceNanjing Normal UniversityNanjing 210023, ChinaE-mail: yqlan@njnu.edu.cnDr. K. LiSchool of Materials Science and EngineeringUniversity of JinanJinan 250022, ChinaDr. C. Mao, Prof. P. FengDepartment of ChemistryUniversity of CaliforniaRiverside, CA 92521, USAE-mail: pingyun.feng@ucr.eduProf. X. BuDepartment of Chemistry and BiochemistryCalifornia State UniversityLong Beach, CA 90840, USA

DOI: 10.1002/adma.201601047

synthesized and shown to split water into H2 and O2 with much enhanced activity over those with single phase structure alone. Similar phenomena were observed in BiVO4

[8] and CaTa2O6[9]

nanoparticle phase junctions. However, significant challenges were encountered when this strategy was applied to CdS. One study found that hexagonal-cubic CdS nanocrystals phase junc-tion actually showed poorer photocatalytic activity than that of pure hexagonal and cubic phase CdS nanocrystals, which was attributed to grain boundaries in the phase junction.[10] Hoffmann and co-workers circumvented this issue by inserting Pt between core–shell hexagonal-cubic CdS nanoparticles phase junction, which helped suppress the surface states of hexagonal CdS core and improved the photocatalytic H2 production rate by a factor of two.[11] Still, the challenge remains as to how to fabricate noble metal-free high-quality CdS nanostructure-based phase junction.

Here we report a highly effective low-cost strategy based on the integration of hexagonal-cubic core–shell architecture with nanorod morphology. It takes into consideration advantages of core–shell structures in passivating surface defects of quantum dots,[1a,b,12] as well as advantages of 1D nanorods in shortening the electron diffusion length and increasing the light absorp-tion.[13] Notably, the formation of hexagonal II–VI nanorod nanocrystals was reported earlier.[14]

The synthesis of core–shell type concentric CdS nanorod phase junctions (NRPJs) composed of hexagonal core and cubic shell is based on a facile one-pot hydrothermal method, i.e., direct treatment of cadmium nitrate and thiourea precursor solution at proper temperature with the variable Cd/S precursor molar ratio and reaction time (Scheme 1). With the optimized Cd/S precursor molar ratio and reaction temperature, ultrathin cubic shell forms over the hexagonal core. In contrast, pure hex-agonal CdS nanorods are generated in sulfur-rich environment. The obtained concentric CdS NRPJs with the ultrathin cubic shell (2.4 nm) show extremely high H2 production rate with a value of 742.5 μmol h−1 (494 times higher than that of pure hexagonal CdS nanorods) and unprecedented photocatalytic stability over 400 h, which is much longer than previously reported metal sulfide-based photocatalysts. Significantly, even under aerobic condition, the CdS NRPJs can continuously operate for over 100 h with H2 production activity superior to that of pure hexagonal CdS nanorods in vacuum by 108 times.

The CdS NRPJs were fabricated by directly treating Cd(NO3)2 and thiourea precursor solution at 220 °C with properly con-trolled Cd/S molar ratios and reaction times. The X-ray diffrac-tion (XRD) patterns (Figure 1a) indicated that the CdS nanorods made from high content of S precursor (i.e., low Cd/S ratio

Using semiconductor-based photocatalytic technology for water splitting to produce H2 is an ideal way to produce clean solar fuels and remedying environmental problems.[1] For this pur-pose, the key lies in finding proper photocatalysts that can har-vest sunlight and efficiently separate photo-generated charge carriers. CdS has a suitable band gap (Eg = 2.4 eV) for visible light absorption and proper conduction band (CB) position for reducing water to produce H2,[2] making it a potential candidate for photocatalytic water splitting. However, its application is limited by problems such as rapid recombination of charge car-riers, as well as low photostability.[2b,c,3]

To improve photocatalytic efficiency and stability of CdS, various methods, for example, the integration of CdS with other semiconductors or carbon-based materials, have been studied,[2b,3a,c] resulting in the synthesis of a number of CdS-based hetero-nanostructured photocatalysts such as CdS-TiO2,[4] CdS-ZnS,[2b] CdS-MoS2,[2a,3a] CdS-WS2,[2a] and CdS-WO3.

[5] Still, photocatalytic activity of these composites for H2 evolution is low and the stability is far below 100 h. Clearly, new strategies to enhance photocatalytic performance of CdS-based photocata-lysts are desirable.

One strategy, fabrication of surface phase junctions by uti-lizing semiconductor polymorphism, has been shown to be useful in enhancing photocatalytic performance. For example, phase junctions based on rutile-nanoparticle-decorated anatase nanocrystals[6] or α-Ga2O3-nanodot-patched ß-Ga2O3

[7] were

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at 1:8) showed pure hexagonal phase via kinetics dominated pro-cess. As the amount of S supply was reduced, the nucleation rate decreased and the reaction changed from kinetics-dominated to

thermodynamics-dominated process, leading to the formation of hexagonal/cubic CdS phase junction. This trend was more obvious for samples prepared at 180 °C (Figure S1, Supporting Information), where the sample with the lowest S supply leads to the for-mation of pure cubic phase. Remarkably, CdS phase junction could be obtained in the temperature range from 180 to 240 °C by adjusting the Cd/S ratio (Figure S2, Sup-porting Information). To probe the forma-tion mechanism of CdS phase junction, NaOH and ethylenediamine (EDA) alkaline solutions were used to facilitate the dissocia-tion of thiourea. In contrast to the distinct hexagonal-cubic CdS phase junction (Cd/S = 1:0.5) in neutral deionized water, the CdS prepared with the same synthetic parameters (i.e., Cd/S ratio and reaction temperature and time) derived from alkaline solutions showed pure hexagonal phase which is attributed to the higher dissociation rate of thiourea

(Figure S3, Supporting Information).Scanning electron microscopy (SEM) (Figure S4, Supporting

Information) indicates that the microstructure of CdS changed

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Scheme 1. Schematic illustrating the formation process of concentric CdS NRPJs composed of hexagonal core and cubic shell.

Figure 1. XRD patterns of CdS NRPJs synthesized at 220 °C under (a) different Cd/S molar ratios and (b) different reaction time, respectively. c) The HRTEM image, (d) HAADF-STEM image, and (e) the corresponding selected area electron diffraction pattern in the interface region of an individual concentric hexagonal-cubic CdS NRPJ.

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from hexagonal prisms to tree-like morphology with increasing S supply. Simultaneously, the increasing thiourea concentra-tion leads to a gradual disappearance of the cubic CdS phase (Figure 1a), and a reduction of the photocatalytic H2 production rate (Figure S5, Supporting Information). Notably, the sample with pure hexagonal phase (Cd/S = 1/8) shows an extremely low hydrogen evolution rate. The samples prepared at 180 °C behaved similarly (Figure S6, Supporting Information), and the photocatalytic activities of the samples with the optimal ratio of Cd/S at temperatures up to 240 °C were shown in Figure S7 (Supporting Information). These results confirm the extremely important role of CdS phase junction in improving its photo-catalytic activity.

To investigate the formation mechanism of hexagonal/cubic CdS phase junction, the CdS samples with the optimal Cd/S ratio (1:0.5) were fabricated with different reaction times at 220 °C and abbreviated as CSx (x = 1, 6, 12, 18, 25, and 45 h). The Cd2+ from completely dissolved cadmium nitrate is always sufficient, whereas the continuous reaction decreases the con-tent of thiourea, and causes the insufficient S2− (Scheme 1). Consequently, the cubic CdS formed due to the change from kinetics-dominated to thermodynamics-dominated pro-cess with the reduced amount of S2− in longer reaction time (Figure 1b). A similar trend was found in the samples pre-pared at 180 °C (Figure S8, Supporting Information). The SEM images (Figure S9, Supporting Information) indicate that CS1 exhibits hexagonal prism morphology, and CS12 and CS18 show similar morphology due to their very limited shell thick-ness. A further increase in shell thickness causes the disappear-ance of this morphology.

The high-magnification transmission electron microscopy (TEM) image of an individual CdS NRPJ (CS18) (Figure 1c) shows the nanorod morphology. As depicted in the high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image (Figure 1d), a resolved phase junction could be observed between the shell and core region. And the staking mode of the CdS lattice changed from the cubic phase (ABCABC) to the hexagonal phase (ABABAB). The electron diffraction pattern recorded on the interface (Figure 1e) shows streaks, indicating the polytype nature of the CdS NRPJ. The high-resolution transmission electron microscopy (HRTEM) was performed to further investigate the concentric CdS NRPJ and the effect of reaction time on the thickness of the cubic CdS shell. As shown in Figure 2a–d (for details, see Figure S10, Supporting Information), the pure hexagonal CdS (CS1) shows well-defined hexagonal CdS lattice. Notably, a cubic shell with a thickness of about 1.4 nm could be observed in CS12. With the reaction time increasing to 18 h, the cubic CdS shell thickness reaches to 2.4 nm. Further increasing the reaction time leads to an increase of the cubic shell thickness to nearly 5 nm. All these observations are consistent with the proposed mechanism of constructing hexagonal/cubic CdS core/shell phase junction via modulating the kinetic and thermodynamic parameters.

A systematic study of the effect of cubic CdS shell thickness on the photocatalytic H2 production activity (Figure 2e, details in Figure S11, Supporting Information) shows the extremely important role of cubic CdS shell for improving the photocata-lytic activity. The pure hexagonal CdS (CS1) shows very low H2 production rate of 1.5 μmol h−1. Fascinatingly, the formation of

the core/shell phase junction dramatically improved the photo-catalytic H2 production by 252 times even for CS6. The max-imal H2 production rate up to 742.5 μmol h−1 with an apparent quantum efficiency (QE) of 43% at 420 nm was obtained for CS18, upon further increasing the reaction time, likely due to the increased crystallinity of cubic CdS shell. This extraordinary H2 production rate is 494 times higher than that of the pure hexagonal CdS, and compares favorably with the previously reported CdS-based heterostructure (Table S1, Supporting Information).[2b,e,3a,c,15] This extremely high photocatalytic activity of CS18 in comparison with the pure hexagonal CdS may come from the passivation of surface trap states of the hexagonal CdS core, and the tunneling of the energetically favorable electrons and holes through the ultrathin cubic CdS shell as reported in CdSe/CdS and CdS/ZnS heterojunctions.[15f,16] Moreover, the CS18 exhibited much higher photocatalytic hydrogen produc-tion activity than that of cubic CdS (Figure S12, Supporting Information). However, further increasing the shell thickness through longer reaction time leads to a slight reduction of the photocatalytic activity. The effect of reaction time on photo-catalytic activity became much more pronounced for the sam-ples prepared at 180 °C (Figure S13, Supporting Information). We have further measured the H2 evolution of the optimal CdS nanorod phase junctions (CS18) under different wave-length of monochromatic light, and the H2 evolution plotted against wavelength of monochromatic light is exhibited in Figure S14 (Supporting Information). The hydrogen evolu-tion rate decreased with the increasing wavelength, and the H2 evolution action spectra coincided with the absorption edge of

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Figure 2. a–d) The HRTEM images and (e) photocatalytic H2 production rate (per 20 mg) of CS1, CS12, CS18, and CS25.

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via photo-absorption by the catalyst. More-over, the CS18 shows much larger hydrogen production than that of the hexagonal CdS sample at a monochromatic light of 420 nm.

The short lifetime is the most severe drawback of metal sulfide photocatalytic materials. It is highly desirable but chal-lenging to improve their photocatalytic sta-bility. Notably, the optimal CdS NRPJs (CS18) shows an excellent photocatalytic stability over 400 h (Figure 3a), which is much longer than that of hexagonal CdS (only about 10 h) (Figure S15, Supporting Information).[2b] The stable junction of the CdS NRPJs was further confirmed by XRD and HRTEM after 400 h reaction (Figure S16, Supporting Information). This extremely long lifetime makes CdS NRPJs the best sulfide photocata-lyst owing to the unique CdS core/ultrathin shell phase junction (Table S1, Supporting Information).[2a,b] As shown in Figure 3b, different atmospheres were used to fur-ther evaluate the photocatalytic activity of CS18. Under N2 atmosphere, the H2 evolu-tion rate is lower as the H2 is not easy to be released. Interest-ingly, the CdS NRPJs show considerable photocatalytic activity even under aerobic condition, indicating that the as-prepared CdS phase junction may be suitable for practical applications. The lower H2 evolution rate under aerobic conditions may be ascribed to the inhibition of reaction between photo-generated holes and sacrificial reagent by O2 and/or the backward reac-tion of H2 and O2 on the photocatalyst surface.[17] Notably, this value is still 108 times higher than that of the hexagonal CdS in vacuum. More significantly, the CS18 showed excellent photo-catalytic stability even under aerobic condition as confirmed by the stability test over 100 h (Figure 3c).

The X-ray photoelectron spectroscopy (XPS) valence band (VB) spectra indicate that the VB of the hexagonal CdS is 0.13 eV above that of cubic CdS (Figure 4a). Considering the larger bandgap of cubic CdS than its hexagonal counterpart,[11] as proved by the UV–vis reflection spectra (Figure 4b), the CB of the cubic CdS is calculated to be upshifted by 0.08 eV and the as-prepared CdS NRPJ shows the type I heterojunction.[11] The enhanced photocatalytic activity may come from the pas-sivation of surface trap states on hexagonal CdS, as reported in the CdS-ZnS and CdSe-ZnS core–shell structures.[11,18] For the hexagonal CdS nanorods, the photo-generated charge car-riers are easily transferred to abundant surface trap states where the electrons or the holes with lower energy are not readily available for photocatalytic reactions.[15f ] In CdS NRPJs, most surface states of the hexagonal CdS core are passivated via the cubic CdS shell, making the electrons and holes ener-getically favorable for photocatalytic reactions.[15f ] Thanks to the ultrathin shell thickness of CdS NRPJs, the confined electrons and holes with high energy in the core would tunnel through the cubic CdS shell to the external surface for reactions.[2b,18] Thus, the photo-generated holes could be rapidly consumed by the sacrificial reagent and the recombination probability of

electron/hole pairs would be greatly reduced. Further evidence comes from the steady-state photoluminescence (PL) and time-resolved transient-state photoluminescence (TRPL) spectra tests (Figure S16, Supporting Information), which can provide valuable information on the transfer of photo-generated charge carriers and their lifetime. Compared with that of pure hex-agonal CdS NRs, the emissions of CdS NRPJs (Figure S17a, Supporting Information) were remarkably quenched and the Stoke’s shifts were largely reduced, confirming that the surface trap states of hexagonal CdS cores are effectively passivated and the charge carriers are favorable to penetrate or tunnel across the ultrathin cubic CdS shell. That is to say, the electron–hole recombinations are greatly suppressed in CdS NRPJs, facili-tating to boost their photocatalytic performance. Additionally, the TRPL tests (Figure S17b, Supporting Information) reveal that the decay lifetime of charge carriers in CdS NRPJs (0.428 ns) is shorter than that of H-CdS (0.645 ns), implying that the lifetime of photo-generated holes in CdS NRPJs might be very short and rapidly tunneled from the core to the shell surface to be depleted by the sacrificial reagents in solution. While the photo-generated electrons may linger on the catalyst surface and reduce H2O to produce H2. It should be mentioned that the sample obtained with short reaction time may possess poor crystallization quality of cubic shell and increase the sur-face defect centers. On the other hand, shells that are too thick will inhibit the tunneling effect of charge carriers.[2b] So, CdS NRPJs with proper shell thickness and crystallinity exhibit the highest H2 production activity.

Furthermore, the transient photocurrents of CS1, CS18, and CS25 showed that the samples (CS18&25) with phase junctions exhibit much higher photocurrents than that of pure hexag-onal sample (CS1) (Figure 4c), confirming the importance of cubic CdS shell in the process of charge carriers transfer. Con-sistent with H2 evolution tests, the highest photocurrent was

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Figure 3. a) The photocatalytic stability of CS18 in vacuum under visible light irradiation (≥420 nm). b) The photocatalytic H2 production activity of CS18 under different conditions. c) The photocatalytic stability of the CS18 under aerobic condition.

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obtained on CS18, owing to its optimum cubic CdS shell thick-ness for electron and hole tunneling and superior separation and transfer of charge carriers due to the passivated surface states.[17,19] The similar transient photocurrent responses from samples made with the different Cd/S ratios further confirmed the important role of CdS NRPJs in improving photocatalytic activity (Figure S18, Supporting Information). Furthermore, the electrochemical impedance spectra (EIS) analysis (Figure 4d) indicated that CS18 exhibited the smallest semicircle in the middle-frequency region, indicating its fastest interfacial charge transfer due to the well-modulated CdS NRPJs.

In summary, core–shell concentric CdS NRPJs composed of hexagonal core and cubic shell have been successfully syn-thesized via directly reacting Cd(NO3)2 and thiourea precursor solution at proper temperature with variable Cd/S molar ratio and reaction time. The concentric hexagonal-cubic CdS NRPJs with the ultrathin cubic shell (2.4 nm) exhibit extremely high photocatalytic H2 yield of 742.5 μmol h−1 (494 times higher than pure hexagonal CdS nanorods). Especially, such concentric CdS NRPJs also possess excellent photocatalytic stability over 400 h, much longer than previously reported metal sulfide-based pho-tocatalysts. Moreover, even under aerobic conditions, our con-centric CdS NRPJs can persistently operate for 100 h with the photocatalytic H2 yield superior to that of pure hexagonal CdS nanorods in vacuum by 108 times. The superior photocatalytic performance of such concentric hexagonal-cubic CdS NRPJs is attributed to the following two reasons: (i) the unique core–shell concentric nanorod geometric structure can facilitate charge transfer (tunneling) through the passivation of surface states and the enhanced tunneling of charge carriers; (ii) the presence of ultrathin cubic shell not only passivates the surface state of

hexagonal core but also reduces the recombination of charge carriers via enhanced tunneling. These findings illustrate that the photocatalytic activity and stability of CdS, and possibly other metal sulfide photocatalysts in general, can be greatly improved by elaborate phase junction engineering at nanoscale.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsK.L., M.H., and R.C. contributed equally to this work. This work was financially supported by NSFC (Nos. 21371099, 21271105, 21471080, and 21541007), the NSF of Jiangsu Province of China (Nos. BK20130043 and BK20141445), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Foundation of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, and NSF (DMR-1506661, P.F.).

Received: February 23, 2016Revised: June 27, 2016

Published online:

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Figure 4. a) Valence-band XPS spectra and b) UV–visible diffuse reflection spectra (inset shows the (αhν)2 vs hν curve) for pure cubic and hexagonal CdS samples. c) Transient photocurrent responses and d) Nyquist plots for the samples synthesized at 220 °C under different reaction time.

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Supporting Information

for Adv. Mater., DOI: 10.1002/adma.201601047

Hexagonal@Cubic CdS Core@Shell Nanorod Photocatalystfor Highly Active Production of H2 with UnprecedentedStability

Kui Li, Min Han, Rong Chen, Shun-Li Li, Shuai-Lei Xie,Chengyu Mao, Xianhui Bu, Xue-Li Cao, Long-Zhang Dong,Pingyun Feng,* and Ya-Qian Lan*

1

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Supporting Information

Hexagonal@Cubic CdS Core@shell Nanorod Photocatalyst for Highly Active

Production of H2 with Unprecedented Stability

Kui Li, Min Han, Rong Chen, Shun-Li Li, Shuai-Lei Xie, Chengyu Mao, Xianhui Bu, Xue-Li

Cao, Long-Zhang Dong, Pingyun Feng,* and Ya-Qian Lan*

[*] Dr. K. Li[+]

, Prof. M. Han[+]

, Miss R. Chen[+]

, Prof. S.-L. Li, S.-L. Xie, Dr. X.-L. Cao, L.-Z.

Dong, Prof. Y.-Q. Lan,

Jiangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Materials

Science, Nanjing Normal University, Nanjing 210023, China

E-mail: yqlan@njnu.edu.cn

[+] These authors contributed equally to this work.

Dr. K. Li

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

Dr. C. Mao, Prof. P. Feng

Department of Chemistry, University of California, Riverside, California 92521, United

States

E-mail:pingyun.feng@ucr.edu.

Prof. X. Bu

Department of Chemistry and Biochemistry, California State University, Long Beach,

California 90840, United States

2

Experimental Section

Sample Preparation. CdS nanorods were prepared using Cd(NO3)2·4H2O, CH4N2S as

precursors by one-step hydrothermal method. In a typical process, 0.33 mmol

Cd(NO3)2·4H2O and the corresponding ratios of Cd/S equal to 1:0.125, 1:0.25, 1:0.5, 1:1, 1:2,

1:4, 1:8 were dissolved in 10 mL distilled water. The solutions were subsequently transferred

into 12 mL teflon-lined autoclave and maintained for 18 h at different temperatures of 180,

200, 220 and 240 oC. The samples fabricated at 180 and 220

oC with the optimal ratios of

Cd/S were selected for further investigating the different reaction times of 1, 3, 6, 9, 12, 18,

25, 35 and 45 h on their microstructure and photocatalytic activity. The final yellow products

were rinsed three times with distilled water and ethanol respectively, and dried at 60 oC for

overnight in a vacuum oven to evaporate the solvent.

Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a D/max

2500 VL/PC diffractometer (Japan) equipped with graphite monochromatized Cu Kα

radiation (λ = 1.54060 Å). Corresponding work voltage and current is 40 kV and 100 mA,

respectively. The transmission electron microscopy (TEM) and high-resolution TEM

(HRTEM) images were recorded on JEOL-2100F apparatus at an accelerating voltage of 200

kV. The atomic structure of the CdS phase junction was characterized using an ARM-200CF

(JEOL, Tokyo, Japan) transmission electron microscope operated at 200 kV and equipped

with double spherical aberration (Cs) correctors. Surface morphologies of the phase junction

materials were examined by a scanning electron microscope (SEM, JSM-7600F) at an

acceleration voltage of 10 kV. The UV−Vis absorption and diffused reflectance spectra were

recorded using a Cary 5000 UV-Vis spectrometer (Viarian, USA) with BaSO4 as a

reflectance standard. Electrochemical impedance spectra (EIS) measurements were carried

out in three-electrode system and recorded over a frequency range of 500 kHz-200 MHz with

ac amplitude of 10 mV at 0.5 V. Na2S (0.1 M) and Na2SO3 (0.02 M) mixture solution was

used as the supporting electrolyte. XPS data were collected using an X-ray photoelectron

spectrometer (XPS) by a scanning X-ray microprobe (PHI 5000 Verasa, ULAC-PHI, Inc.)

with Al kα radiation and the C1s peak at 284.6 eV as internal standard. EIS data were

recorded using Electrochemical workstation (EC-lab, SP-150, VMP3-based instruments,

America) under a surface power density of about 0.1 mW/cm2. The PEC measurements were

performed with a Zahner PEC workstation (Zahner, Germany). All experiments were carried

out at room temperature using a conventional three-electrode system with a modified ITO

electrode (sheet resistance 20−25 Ω/square) with a geometrical area of 1.0 ± 0.1 cm2 as the

3

working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl electrode as the

reference electrode. All the CdS samples were dispersed in deionized water with a

concentration of 1.5 mg/mL and deposited on the ITO electrode and dried infrared light

irradiation. All of the PEC measurements were carried out under 505 nm of irradiation at a

constant potential of 0.5 V in 0.1 M Na2S + 0.02 M Na2SO3 mixed solutions. The steady-state

photoluminescence (PL) spectra were recorded on LS-50B (PERKIN-ELMER instruments)

with an excitation wavelength of 460 nm. And the time-resolved transient-state photo-

luminescence spectra were obtained on FLSP920 Edinburgh fluorescence spectrometer at

ambient temperature.

Photocatalytic Hydrogen Production. The photocatalytic H2-production experiments were

performed via a photocatalytic H2-production activity evaluation system (CEL-SPH2N,

CEAULight, China) in a 300 mL Pyrex flask, and the openings of the flask were sealed with

silicone rubber septum. A 300 W xenon arc lamp through a UV-cutoff filter with a

wavelength range of 420 ~ 800 nm, which was positioned 13 cm away from the reaction

solution, was used as a visible light source to trigger the photocatalytic reaction. The focused

intensity on the flask was ∼ 200 mW·cm−2

, which was measured by a FZ-A visible-light

radiometer (made in the photoelectric instrument factory of Beijing Normal University,

China). In a typical photocatalytic H2-production experiment, 20 mg of the as-prepared

photocatalyst was suspended in 100 mL of mixed aqueous solution containing Na2S (0.35 M)

and Na2SO3 (0.25 M). Before irradiation, the system was vacuumed for 5 min via the vacuum

pump to completely remove the dissolved oxygen and ensure the reactor was in an anaerobic

condition. A continuous magnetic stirrer was applied at the bottom of the reactor to keep the

photocatalyst particles in suspension during the experiments. H2 content was analyzed by gas

chromatography (GC-7900, CEAULight, China). All glasswares were carefully rinsed with

DI water prior to usage. The photocatalytic stability was performed in the same processing

parameters. The sacrificial regent was renewed every 25 h for the evaluation of the

photocatalytic stability in vacuum, while the sacrificial regent was renewed for every 10 h in

the aerobic condition. The apparent quantum efficiency (QE) of 20 mg catalyst for H2

evolution is measured using a 4208 nm band-pass filter. QE for H2 evolution are calculated

to 43 % according to the equation shown below:

4

Supporting Figures

Figure S1. Effect of different Cd/S ratios on the XRD patterns of the CdS samples prepared at

180 C.

Figure S2. XRD patterns of the samples with the optimal ratio of Cd/S at different

temperatures ranging from 180 to 240 C.

5

Figure S3. Comparison of the CdS samples synthesized at 220 C with Cd/S ratio of 1:0.5 in

(a) neutral deionized water (DIW) (b) EDA and (c) NaOH alkaline solutions. The alkaline

solutions could facilitate the dissociation of thiourea, and the corresponding samples show

pure hexagonal CdS phase. These results confirm the extremely important critical role of

thermodynamically and kinetically parameters on the formation of CdS phase junction.

Figure S4. SEM images of the CdS samples prepared with Cd/S ratio of (a) 1:0.5, (b) 1:1, (c)

1:2 and (d) 1:8 at 220 C.

Figure S5. Comparison of (a) the photocatalytic H2-Production rate and (b) the action spectra

of H2 evolution for the samples synthesized at 220 C with different Cd/S precursor molar

ratios.

6

Figure S6. Comparison of (a) the photocatalytic H2-Production rate and (b) the action spectra

of H2 evolution for the samples synthesized at 180 C with different Cd/S precursor molar

ratios.

Figure S7. The maximal a) photocatalytic H2-Production rate and b) action spectra of the H2

evolution of the CdS samples prepared with the optimal ratio of Cd/S at different

temperatures ranging from 180 to 240 C.

Figure S8. XRD patterns of the samples prepared at 180 C for different reaction time with a

constant Cd/S precursor molar ratio of 1:2.

7

Figure S9. SEM and HRTEM images of the samples with reaction time of (a) 1 h, (b) 12 h,

(c) 18 h and (d) 25 h.

Figure S10. HRTEM images of (a) CS1, (b) CS12, (c) CS18 and (d) CS25.

Figure S11. The influence of the thickness of cubic CdS shell layer on (a) the photocatalytic

H2-production rate and (b) the action spectra of H2 evolution for the samples synthesized at

220 C under different reaction times with the Cd/S precursor molar ratio of 1:0.5.

8

Figure S12. Comparison the photocatalytic hydrogen production performance of pure

hexagonal CdS, pure cubic CdS and the optimal CdS NRPJs (CS18).

Figure S13. Comparison of (a) the photocatalytic H2-Production rate and (b) the action

spectra of H2 evolution for the samples synthesized at 180 C under different reaction time

with the Cd/S molar ratio of 1:2.

9

Figure S14. The H2 evolution of the optimal CdS NRPJs (CS18) plotted against wavelength

of monochromatic light (blue star). The hydrogen evolution rate decreased with increasing

wavelength, and the H2 evolution evolution action spectra coincided with the absorption edge

of the CS18, suggesting the reaction proceeds via photo-absorption by the catalyst. Moreover,

the CdS NRPJs shows much larger hydrogen production (20.5 μmol/h) than that of the

hexagonal CdS sample (0.057 μmol/h) at a monochromatic light of 420 nm.

Figure S15. Photocatalytic cycling stability plots for photocatalytic H2 evolution by using

pure hexagonal phase CdS nanorods as the photocatalyst.

Figure S16. a) X-ray diffraction patterns of the CdS NRPJs (CS18) before and after the

irradiation for 200 and 400 h. The HRTEM images of the CdS NRPJs (CS18) (b) before and

(c) after irradiated for 400 h.

10

Figure S17. (a) The steady-state photoluminescence (PL) and (b) time-resolved transient-state

photoluminescence (TRPL) spectra for hexagonal CdS (H-CdS) and CdS nanorod phase

junctions (NRPJs) at ambient temperature, respectively.

Figure S18. Transient photocurrent responses of the samples synthesized at 220 C for 18 h

with the Cd/S precursor molar ratio of 1:0.5, 1:2, and 1:4, respectively.

11

Table S1. Comparison results of H2-production rate in the CdS based photocatalysts via water

splitting.

Photocatalyst

H2 evolution

Ref.(year) Activity

(mmol/h/g)

Improving

multiplea

Stability

(h)

Intensity (mW/cm

2)

CdS NRPJs 37.125 494 400 200 This work

CdS/Ni 63 N/A 80 h:

Decrease

20 %

200

1(2014)

ZnS/Zn1-

xCdxS/CdS 34.2 48 ≈20

200 2 (2015)

CdS/ZnS 0.792 56 >60 N/A 3 (2014)

CdS/MoS2 0.54 36 N/A N/A 4 (2008)

Zn1-xCdxS 7.42 24 N/A 180 5 (2013)

MoS2–

rGO/CdS 23.2 12.39 >12

63 6 (2014)

CdS/Cd 11.687 7.012 N/A N/A 7 (2014)

CdS/RGO 56 4.87 N/A 180 8 (2011)

CdS/g-C3N4 4.152 2.075 N/A 180 9 (2013)

CdS/ZnS N/A ≈2 N/A N/A 10 (2013)

aImproving multiple=H2-production rate of the optimal catalyst/H2-production rate of CdS.

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