Shell Nanoparticles for Enhanced and Sustainable Photocatalytic

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Durian-Shaped CdS@ZnSe Core@mesporous-Shell Nanocrystals for Enhanced and Sustainable Photocatalytic Hydrogen Evolution Zichao Lian, Masanori Sakamoto, Yoichi Kobayashi, Naoto Tamai, Jun Ma, Tsuneaki Sakurai, Shu Seki, Tatsuo Nakagawa, Mingwei Lai, Mitsutaka Haruta, Hiroki Kurata, and Toshiharu Teranishi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00789 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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The Journal of Physical Chemistry Letters

Durian-Shaped

CdS@ZnSe

Core@Mesoporous-

Shell Nanoparticles for Enhanced and Sustainable Photocatalytic Hydrogen Evolution Zichao Lian,† Masanori Sakamoto,*,‡ Yoichi Kobayashi,| Naoto Tamai,§ Jun Ma,# Tsuneaki Sakurai,# Shu Seki,# Tatsuo Nakagawa,※ Mingwei Lai,‡ Mitsutaka Haruta,‡ Hiroki Kurata,‡ and Toshiharu Teranishi*,‡ †

Department of Chemistry, Graduate School of Science, Kyoto University, Gokasho, Uji, Kyoto

611-0011, Japan. ‡

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

|

Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1

Noji-higashi, Kusatsu, Shiga 525-8577, Japan §

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1

Gakuen, Sanda, Hyogo 669-1337, Japan. #

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Nishikyo-ku, Kyoto 615-8510, Japan.

ACS Paragon Plus Environment

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Optical Instruments Division, Unisoku Co., Ltd., Kasugano 2-4-3, Hirakata, Osaka 573-0131,

Japan. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]

ABSTRACT: In artificial photosynthesis, the establishment of design guidelines for nanostructures to maximize the photocatalytic performance remains a great challenge. In contrast to the intense research into band-offset tuning for photocatalysts, the relationship between nanostructures and photo-induced carrier dynamics has still been insufficiently explored. Herein, we synthesized durian-shaped CdS@ZnSe core@mesporous-shell nanoparticles (d-CdS/ZnSe NPs) and investigate the carrier dynamics in photocatalytic hydrogen evolution. The cocatalystfree d-CdS/ZnSe NPs exhibited high photocatalytic activity for H2 evolution (14.8% apparent quantum yield at 420 nm) and excellent stability (maintaining 80% activity after 72 h) under visible light irradiation (> 422 nm). The transient absorption measurement and flash photolysis time-resolved microwave conductivity unveiled that the ultra-long-lived charge separation (> 6.2 ms) and swift hole transfer to the surfaces of ZnSe shell (11 ns) contribute the high catalytic activity and stability. The present work provides a novel insight for designing nanoparticulate photocatalysts with optimized performance.

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Solar energy conversion by photocatalytic water splitting is a powerful strategy to solve problems related to the energy crisis and environmental pollution.1-4 The photocatalytic activities of homogeneous nanoparticulate photocatalysts based on semiconductor nanoparticles (NPs) can be comparable with those of the currently dominant inhomogeneous systems such as photocatalyst sheets, photoelectrodes, and powder photocatalysts.5-7 In addition, the versatile tunability of the physical properties and geometrical structures of semiconductor NPs provides a route for exploring the favorability of different nanostructures for various photocatalytic reactions.8-11 Although a variety of nanostructures have been developed,12-16 the design of nanostructures for maximum photocatalytic activity remains a great challenge. Among the complicated combination of factors determining the photoactivity of semiconductor photocatalysts, both (i) the photo-induced carrier dynamics and (ii) the transport of carriers and substrates at the catalyst surfaces are highly important. To control the photoinduced carrier dynamics, the fabrication of heterostructures with a type-II band alignment has been adopted to achieve efficient charge separation, thus improving the photocatalytic activity.17-


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Furthermore, because the accumulation of holes can provoke the photo-corrosion of

semiconductor photocatalysts, the type-II band alignment enhances the photo-stability. From the viewpoint of nano-structural design based on the carrier dynamics and the transport of carriers and substrates at catalyst surfaces, we proposed the hierarchical core@mesoporous-shell as a promising structure with significant advantages. In the core–shell motifs, the mesoporous shell with a high specific surface area facilitates charge transfer to reactants at the solid-liquid interface, making it an ideal architecture to prevent charge recombination (i.e., back reaction). Herein, we report the synthesis and the structure-specific photocatalytic activities of the “durian-shaped” type-II hierarchical CdS@ZnSe core@mesoporous-shell NPs (d-CdS/ZnSe NPs) for the first time. Pristine CdS NPs, with a sharp, size-tunable optical transition with a direct bandgap of > 2.4 eV, show high light-harvesting ability, and a suitably positioned conduction band for the H2 evolution reaction (HER).20, 21 However, the low catalytic activity and poor stability of CdS have limited its applicability in photocatalytic HER.22 The d-CdS/ZnSe NPs showed much-enhanced photocatalytic H2 evolution efficiency (apparent quantum yield: 14.8% at 420 nm) compared with the pristine CdS NPs. Furthermore, it maintained around 80% of the photocatalytic activity of d-CdS/ZnSe NPs even after 72 h under the presence of the sacrificial reagent (Na2S–Na2SO3). Through direct observation of the carrier dynamics of dCdS/ZnSe NPs by transient absorption spectroscopy (TAS) and photo-conductivity measurements, the pathways of the carriers which is responsible for the enhanced and sustainable H2 evolution were revealed. The present results could serve as a powerful guideline for the design of nanoparticulate photocatalysts with optimized performance. The d-CdS/ZnSe NPs were synthesized by the seed-mediated method, using CdS NPs as seeds. The detailed synthetic procedures are presented in Supporting Information (Figure S1). Figure 1a


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shows a transmission electron microscopy (TEM) image of the d-CdS/ZnSe NPs with uniform core@mesoporous-shell structure.23 The length and width of the d-CdS/ZnSe NPs are 55.1 ± 6.7 nm and 40.1 ± 4.3 nm, respectively. As shown in Fig. 1b, the X-ray diffraction (XRD) patterns indicate that the d-CdS/ZnSe NPs are composed of wurtzite ZnSe (w-ZnSe, Joint Committee on Powder Diffraction Standards (JCPDS) no. 15-0105) and wurtzite CdS (w-CdS, JCPDS no. 772306) phases. The CdS NPs are surrounded by several lattice fringes of ZnSe with different directions, indicating the high crystallinity of the mesoporous ZnSe shells (Fig. 1c). As can be seen in Fig. 1d from the enlargement of the red and orange rectangles in Fig. 1c, the lattice spacings were 0.33 nm and 0.34 nm, which were assigned to the w-ZnSe (002) plane and w-CdS (002) plane, respectively, from the intensity profiles of the lattice fringes in Fig. 1f. In addition, as displayed in Fig. 1e, the fast Fourier transform (FFT) patterns further support the formation of both phases. The high-angle annular dark field–scanning TEM (HAADF-STEM)–energy dispersive X-ray spectrometry (EDS) mapping further confirmed the core–shell structure of the d-CdS/ZnSe NPs (Fig. 1g).


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Figure 1. (a) Representative TEM image of d-CdS/ZnSe NPs, scale bar: 50 nm. Inset (lower left): cartoon of the d-CdS/ZnSe NPs with the CdS core and the ZnSe shell, (b) XRD patterns of CdS NPs and d-CdS/ZnSe NPs, (c) HRTEM image of d-CdS/ZnSe NPs, (d) enlargement of the dashed green rectangle in (c), (e) FFT patterns of the dashed blue rectangle in (d) from the direction, (f) Intensity profiles of the lattice fringes in the red and orange rectangular regions in (d), (g) HAADF-STEM-EDS elemental mapping images of d-CdS/ZnSe NPs, scale bar: 20 nm. To evaluate the photocatalytic properties of d-CdS/ZnSe NPs in the HER, they were dissolved in water via ligand exchange (see Supporting Information for details). The photocatalytic


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hydrogen evolution properties of d-CdS/ZnSe NPs were evaluated under the irradiation of a Xe lamp (λ > 422 nm) using Na2S–Na2SO3 as a hole scavenger. First, the nanostructure of dCdS/ZnSe NPs in the photocatalytic HER was optimized by varying the surface coverage of the CdS NPs by the ZnSe mesoporous shells. The rationale for this was that the HER proceeds on the surface of the CdS phases. The relationship between the surface coverage and the catalytic activity clarified the optimized structure (Figs. S3 and S4). This can be understood as a balance between the CdS surface exposure and the ZnSe shell growth, which at a certain point maximizes the efficiency of charge separation. Thus, hereafter, we focus on studying the d-CdS/ZnSe NPs showing the maximum photocatalytic activity. Figure 2a shows the time-dependent profiles of photocatalytic H2 evolution under visible light irradiation (λ > 422 nm) for 24 h. The H2 evolution rate of the d-CdS/ZnSe NPs is 119 µmol h−1 g−1, which is 8.40 or 408 times higher than that of the CdS NPs (14.1 µmol h−1 g−1) or ZnSe NPs (0.292 µmol h−1 g−1), respectively (Figs. 2b and S5). The apparent quantum yield (AQY) of the photocatalytic HER under irradiation by Xe light through the band-pass filter (i.e., action spectrum) was consistent with the absorption spectrum of d-CdS/ZnSe NPs (Fig. S6). The highest AQY was 14.8% at 420 nm. In addition, to examine the stability of the d-CdS/ZnSe NPs, three consecutive photocatalytic reaction cycles for 24 h each were carried out (Fig. 2c). Although a slight decrease of the photocatalytic activity, which might be attributed to photocorrosion, was observed in the third cycle,22 the d-CdS/ZnSe NPs maintained 80% photocatalytic activity in the HER after 72 h. Photocorrosion is typically the biggest problem for metal chalcogenide photocatalysts, and the present d-CdS/ZnSe NPs could be a model structure to solve this problem. After reaction, the morphology of the d-CdS/ZnSe NPs still showed the core–shell structure, without any significant


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changes (Fig. S7). These results show that the d-CdS/ZnSe NPs exhibit high photocatalytic performance and excellent stability in the HER.

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Figure 2. (a) Time-dependent photocatalytic activity of the d-CdS/ZnSe NPs in the HER under visible light irradiation (λ > 422 nm) in Na2S–Na2SO3 aqueous solution. (b) Comparison of H2 evolution rates for CdS NPs, d-CdS/ZnSe NPs, and ZnSe NPs. (c) Stability testing of photocatalytic H2 evolution for d-CdS/ZnSe NPs. To elucidate why the d-CdS/ZnSe NPs show high catalytic activity and superior stability, we measured the TA spectra of the NPs. Because TAS measurements enable direct observation of the photo-generated carriers (especially, electrons) in NPs, they allow us to identify the carrier dynamics that contribute to the activity and stability of the d-CdS/ZnSe NPs.


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Figure 3. (a) UV-vis-NIR absorption spectra of CdS NPs and d-CdS/ZnSe NPs in chloroform, (b) Energy diagram of the CdS/ZnSe interface: Eg1 = 2.46 eV for CdS and Eg2 = 2.88 eV for ZnSe. VB: valence band, CB: conduction band. Transient absorption spectra of (c) CdS NPs in hexane and (d) d-CdS/ZnSe NPs in chloroform in ps region upon 400-nm laser excitation (7 µJ/cm2). (e) Temporal decay profiles monitored at 490 nm upon 400-nm laser excitation in ps to ns region for CdS NPs and d-CdS/ZnSe NPs. Blue line indicates the best fitting. (f) Temporal decay profiles of the CdS NPs and d-CdS/ZnSe NPs probed at 500 nm upon 355-nm laser excitation (0.5 mJ/cm2) in the ns to ms region. Blue lines represent the best fitting.


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Figure 3a shows the steady-state absorption spectra of CdS NPs and d-CdS/ZnSe NPs in chloroform. The band edge of the ZnSe phase can be clearly distinguished in d-CdS/ZnSe NPs, in contrast to the pristine CdS NPs. From the absorption spectra, the optical band gap energy was estimated to be 2.46 eV for CdS and 2.88 eV for ZnSe (Fig. S5), as illustrated in Fig. 3b. These values are consistent with the previous reports.24 Figures 3c and 3d show the TA spectra of the CdS NPs and the d-CdS/ZnSe NPs upon 400-nm laser excitation. The TA spectra of CdS NPs (Fig. 3c) shows photo-bleaching, caused both by state filling and a red-shifted absorption feature (XA1) at early delay time. XA1 decays swiftly (with a time constant of 550 fs) to cause the bleaching of the band edge exciton band (XB) and the absorption corresponding to the transition to the higher-energy exciton state (XA2). The XA1 feature at 508 nm can be assigned to hot excitons in the NPs.25 After hot-exciton relaxation (> 2 ps), the spectrum of the exciton-induced red-shift showed no further changes, and only the XB feature was observed in the TA spectra.26 The XB feature showed only slight decay within the time window of the instrument, indicating the long lifetime of the single exciton state at this excitation intensity, as shown in Fig. S8. In contrast to the CdS NPs, no emergence of the XA1 feature was observed in the TA spectra of the d-CdS/ZnSe NPs (Fig. 3d), indicating the fast dissociation of hot excitons in the CdS phases. Furthermore, the Stark effect of the local electric field also induced a red-shift of the XB feature for the d-CdS/ZnSe NPs.13 The increase of ∆A at an early time can be attributed to the state filling of the CdS cores by photo-excited electrons formed in the ZnSe phases. To investigate the charge separation and recombination in CdS NPs and d-CdS/ZnSe NPs, the kinetic profiles were explored from sub-ps to µs (Fig. 3e and 3f). The build-up dynamics of the photo-bleached XB of both CdS NPs and d-CdS/ZnSe NPs were monitored at 490 nm and analyzed using a non-linear least-squares iterative convolution method. For the CdS NPs, the time constant of build-up


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dynamics of XB was 160 fs, which can most probably be assigned to the internal relaxation process from the higher excited states. In contrast, two time constants were observed in these dynamics in d-CdS/ZnSe NPs. The fast component corresponded to the instrument response function ( 6.2 ms). The electrons migrate to the surface of the CdS cores and react with H2O to produce H2. Meanwhile, the holes in CdS core transferred to the ZnSe surface at the rate of 9.1 × 1011 s−1. The rate of hole transport from CdS core to surface of ZnSe shell is 5.6 × 106 times faster than the rate of annihilative recombination. Therefore, we concluded that key processes enhancing the HER is cascading hole transfer as follows: (i) photo-induced hole separation, (ii)


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the swift hole transfer to the surface of the d-CdS/ZnSe NP and (iii) hole extraction at solidliquid interface promoted by the high surface area of mesoporous ZnSe shells.

Figure 5. Schematic illustration of photo-induced carrier dynamics of a d-CdS/ZnSe NP in HER. Hierarchical d-CdS/ZnSe NPs, which were synthesized from the viewpoint of optimized design for carrier dynamics, exhibited excellent photocatalytic performance. The spectroscopic elucidation of the photo-induced carrier dynamics provided important insights into optimizing the nanostructure to maximize the photocatalytic functionality. The mesoporous hole-accepting layer, which enables the efficient cascading transport of the holes from the CdS cores to the scavengers at the rate of 9.1 × 1011 s−1, enhances the activity and sustainability of the dCdS/ZnSe NPs. The present results offer important guidance to the design of suitable nanostructures to realize highly efficient and sustainable photocatalytic hydrogen evolution.

ASSOCIATED CONTENT Supporting Information


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Supporting Information Available: Experimental methods, Figures, and corresponding discussions of additional supporting experimental data. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ORCID Masanori Sakamoto: 0000-0001-5018-5590 Toshiharu Teranishi: 0000-0002-5818-8865 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP16H06520 (Coordination Asymmetry) (T.T.) and JP17H05257 (Photosynergetics) (M.S.), SENTAN JST, and a JSPS Research Fellowship (17J09073). REFERENCES (1) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141-145. (2) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. (3) Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z.-X.; Tang, J. Visible-light Driven Heterojunction Photocatalysts for Water Splitting - a Critical Review. Energy Environ. Sci. 2015, 8, 731-759.


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Shell Nanoparticles for Enhanced and Sustainable Photocatalytic

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