Monitoring Transport Behavior of Charge Carriers in a Single CdS

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

Monitoring Transport Behavior of Charge Carriers in a Single CdS@CuS Nanowire via In-Situ Single-Particle Photoluminescence Spectroscopy Mingshan Zhu, Chunyang Zhai, Sooyeon Kim, Mamoru Fujitsuka, and Tetsuro Majima J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01517 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 9, 2019

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The Journal of Physical Chemistry Letters

Monitoring Transport Behavior of Charge Carriers in a Single CdS@CuS Nanowire via In-Situ Single-Particle Photoluminescence Spectroscopy Mingshan Zhu,† Chunyang Zhai,§ Sooyeon Kim,‡ Mamoru Fujitsuka,‡ and Tetsuro Majima*‡ †

‡

School of Environment, Jinan University, Guangzhou 510632, P.R. China

The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

§

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P.R. China

ABSTRACT: Examination of the spectral and kinetic characteristics of charge carriers recombination

on

nanostructured

semiconductor by photoluminescence (PL) play a significant role in the understanding photocatalytic process. Here, with in-situ single-particle PL technique, we studied the transport behavior of charge carriers in individual one-dimensional

(1D)

core-shell

structures of CdS@CuS nanowires. Through the PL intensity changes in the singleparticle PL spectroscopy, effective interfacial electron transport along the interface of CdS and CuS was observed, which contributes to the significant improvement (i.e. 13.5fold increase) of photocatalytic H2 production than that for pure CdS nanowires. The present study provides a visual experimental evidence for understanding restraining the charge carriers recombination in the semiconductor. 1 ACS Paragon Plus Environment

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Nanostructured semiconductor photocatalysts have been widely developed for use in producing clean and renewable chemical fuels such as hydrogen (H2) from water.1−7 As the photocatalytic efficiency of a nanostructured catalyst is directly affected by the transport and recombination of photo-generated charge carriers at or near the surface, it is of great importance to develop a method to effectively monitor and characterize such process. Generally, the photoluminescence (PL) signals of semiconductor photocatalysts result from the recombination of photo-generated charge carriers.8,9 Nowadays, owing to their high sensitivity and nondestructive characters, the techniques of PL have been extensively used to investigate the electronic, optical and photochemical properties of the semiconductor materials.9 Despite significant efforts have been done with PL to elucidate the photochemical properties of semiconductor materials (such as the efficiency of charge carrier trapping and transport),9 the objects of most studies are the bulk powders. It is very difficult to show the intrinsic PL of nanostructured semiconductors because of their individual anisotropy. Single-particle spectroscopy is a powerful tool for exploring the structural and kinetic features of photocatalysis.8,10-16 For example, the 2010 paper by Lupton’s group published in Science investigated the effect of particle morphology on energy transfer and carrier relaxation across a CdSe/CdS heterojunction by using single-particle PL spectroscopy.15 This technique for singleparticle PL detection has advantages superior to the conventional ones that rely on the bulk sample, providing us opportunities such as the ultimate high selectivity and sensitivity and possible observations of the properties hidden in ensemble measurements.12,13 In addition, when pulsed lasers source is used, a time-correlated single-photon counting (TCSPC) system in PL experiment is possible to reconstruct a PL decay curve for the sample in the excited state.12,13 Accordingly, the interfacial charge transfer process and kinetics on individual nanostructured 2 ACS Paragon Plus Environment

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materials can be in-situ probed at a time on single-particle PL spectroscopy. Although deep mechanistic insight into the fundamental of heterogeneous catalysis was developed, advanced characterization methods as well as proper in-situ reaction cells still get great interesting.17 However, the in-situ microscopic observations of the interfacial charge transfer process on individual nanostructured materials by single-particle PL technolgy to understand the photocatalytic kinetics remain poorly explored. One-dimensional (1D) cadmium sulfide (CdS) nanowires have been intensively studied in visible-light-driven solar energy conversion applications, although the high recombination rate of photo-generated charge carriers leads to their relative low activity.18-21 An appropriate co-catalyst accommodates the photogenerated charge carriers and provides designated redox reaction sites effectively. The use of lost-cost CuS as a co-catalyst for photocatalytic water splitting has been received attentions.22-26 Moreover, the large different solubility constant (ksp) of CdS (8.0×10–27) and CuS (6.3×10–36) at room temperature,27,28 makes it possible for Cu2+ ions to replace Cd2+ ions in the CdS structure, since the formation of CuS is thermodynamically favorable (ΔG420 nm). Effective interfacial electron transport along the interface of CdS and CuS contributes to the significant improvement (i.e. 13.5-fold increase) 3 ACS Paragon Plus Environment


of photocatalytic H2 production than that for pure CdS nanowires. This present study provides important information to understand the behavior of charge transport and dynamics in heterostructure nanomaterials. It is also demonstrated that single-particle PL technique provides a great probing tool to photophysical/chemical processes on nanostructures.

Figure 1. Scheme of the formation of core-shell CdS@CuS heterostructure (a). TEM and HRTEM images of CdS nanowires (b) and CdS@CuS heterostructures (c~e). The insert in image b is the HRTEM image of CdS. (d) and (e) are the HRTEM images of the two circled areas correspondingly labeled in (c). The core-shell CdS@CuS heterostructures were obtained by using cation-exchange process (Figures 1a and S1). Firstly, 1D CdS nanowires were facile synthesized by solvothermal method.30 Scanning electron microscopy (SEM) images of the as-prepared CdS nanowires (Figure S2) show that the lengths of the CdS nanowires range from 3 to 10 μm, and the diameters from 50 to 80 nm. The fine structure of CdS nanowire was confirmed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Figure 1b). The insert of Figure 1b clearly shows lattice fringes with 0.34 nm, which is assigned to (002) facet of CdS.30 4 ACS Paragon Plus Environment

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To fabricate the heterostructure of CdS@CuS, a certain amount of Cu2+ (aq.) was added into the as-prepared 50 mg CdS nanowires aqueous dispersion (Figure S1). Figure S3 shows that the morphologies of series of products keep 1D structures while the lengths are turned to shorten when high amount of Cu2+ (above 1 mL) was introduced. The lengths of CdS@CuS-1 and CdS@CuS-2 are 1~5 μm. The different contents of CuS in the heterostructure of CdS@CuS were determined by energy dispersive X-ray spectroscopy (EDX, Figure S4 and summarized in Table S1. TEM images (Figure S5) clearly show core-shell structures of the as-prepared CdS@CuS nanowires, in which an increasing thickness of the CuS shell was observed in nanowires made with higher amount of Cu2+. In addition, different lattice fringes respectively corresponding to CdS and CuS were observed with HRTEM along the core-shell interface (Figure 1c~e). Furthermore, the EDX elemental mappings of Cd and Cu elements clearly show the different components in the core-shell structure of CdS@CuS (Figures S6a-d). Figures S6e-g show the EDX elemental line scans on different area in the Figure S6a. It can be eaily seen that the core and shell contents are the element of Cd and Cu, respectively. An EDX spectrum (Figure S6h) displays the existence of both elements of Cu and Cd in above selected HAADF-STEM area. These results give a solid evidence on the formation of core-shell structure of CdS@CuS. Pure CuS nanoparticles were synthesized simply by adding Cu2+ (aq.) into S2- (aq.) solution. The product was characterized by TEM, EDX, XRD, and UV–vis diffuse reflectance spectra (UV-DRS) (Figures S7 and S8). To identify the chemical components of in CdS@CuS, the X-ray photoelectron spectroscopy (XPS) spectra of Cd 3d and Cu 2p of samples were measured as shown in Figure S9. Characteristic peaks are detected at around 405.5 and 412.3 eV for Cd 3d 5/2 and Cd 3d3/2, and 932.8 and 952.5 eV for Cu 2p3/2 and Cu 2p1/2, respectively.31 The values of Cu 5 ACS Paragon Plus Environment


2p3/2 and Cu 2p1/2 peaks agree well with the values of Cu2+ states,31 revealing the generation of CuS in the heterostructure of CdS@CuS. The XRD patterns further reveal the component and crystallographic phase of the as– synthesized samples. Figure S10A shows that all peaks match well with the crystal faces of hexagonalwurtzite structure CdS (JCPDS 41-1049).32 Due to overlapping, weak crystallization and low atom ratio of CuS, the XRD patterns only show main characteristic peaks of CdS, whereas a small peak at 31.9o in the heterostructures was detected, which is assigned to (103) crystal planes of CuS (JCPDS 06-0464), indicating the formation of CuS in the heterostructures. The optical absorption properties of the as-prepared samples were investigated with UV-DRS. Figure S10B shows the absorption edge at ca. 527 nm. After growth of CuS, tailing absorption was observed in the visible region for series heterostructure of CdS@CuS. This is because the CuS has broad absorption in the visible range (Figure S8b). Similar phenomenon has been observed in other published system with hybridization of CuS.23 As mentioned in the introduction, the single-particle PL imaging technique is a powerful tool for studying the photogenerated charge carriers in individual particles. To give a visual information on PL properties of CdS and CdS@CuS heterostructure, we monitor the in-situ replacement reaction via single-particle confocal PL spectroscopy at aqueous environment. The confocal microscope system with a chamber was used to directly monitor the PL behavior of individual CdS nanowires interacting with Cu2+ (aq.) in the aqueous solution (Figure 2a). Firstly, CdS nanowires aqueous dispersion (0.1 mg mL-1) was spin-coated on the pre-cleaned cover glass, onto which a chamber was attached to form a reaction vessel. Before the addition of Cu2+ (0.01M), an area with seven representative CdS nanowires was chosen from low magnification dark-field micrograph (Figure 2b). 6 ACS Paragon Plus Environment

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Figure 2. Scheme of the microscopic single-particle PL measurement for in-situ monitoring replacement reaction and interfacial electron transport (a). Low magnification dark-field micrographs of CdS nanowires before addition of Cu2+ (b). PL images of CdS nanowires after addition of Cu2+ at 0 min (c), 5 min (d), 15 min (e), 30 min (f), and 60 min (g). Representative PL spectra of the nanowire as numbered 3 in the PL images at various reaction times (h). For single-particle PL measurement, a pulsed-wave (PW) laser at 405 nm was used as the excitation beam. Figure 2c shows the PL image of the unreacted CdS. Figures 2d-g show the PL images of individual nanowires with reaction time from 5 min to 60 min after Cu 2+ addition. It’s easy to see that the intensities of individual nanowires decreased gradually with the increase of reaction time. The corresponding PL spectra were shown in Figure 2h and Figure S11. A typical 7 ACS Paragon Plus Environment


PL emission spectrum at ca. 516 nm was observed for pure CdS to give its bandgap (2.4 eV). Since the excitonic emissions are sharp and located near the absorption edge of the CdS nanowires (527 nm), the signals in the CdS@CuS are the intrinsic characters of PL emissions of CdS. PL intensity was quenched greatly quickly in the beginning, owing to the continuous replacement reaction on the surface of CdS, until after 30 min, the intensity dropping slowed down significantly. The average PL intensity was quenched around 79.5% comparing to pure CdS after 60 min replacement time. Meanwhile, the maximal PL signals slightly shifted to lower energy after ion-exchange reaction. We know that the CuS is formed as shell on the surface of CdS nanowires with an increase of the reaction time. The effective charge transfer from the core of CdS to the shell of CuS contributes to the slight PL emission peak shift. Moreover, the thickness of CuS increases with the reaction time, also resulting in a gradual red-shift emission peak of PL. These results indicate the chemical transformation of CdS during the reaction process. To confirm the successful transformation, we compared PL images of unreacted CdS and CdS after 60 min ion-exchange reaction, with optical excitation at 405 and 640 nm. As shown in Figures S12a-c, at the excitation wavelength of 405 nm, strong PL emssion signals were observed, while no signal was from the unreacted CdS nanowires under 640 nm excitation. Compared to unreacted CdS, a relatively weak PL emssion image was observed under 405 nm excitation (Figure S12e). However, PL emission from the ion-exchanged products is still observed at the excitation wavelength of 640 nm (Figure S12f), indicating the formation of CuS in the ion-exchanged products because only CuS can be excited. The PL emssion morphology in Figure S12f is similar to those in Figure S12e. This is owing to the similar basic morphology of products after ion-exchange reaction with CdS nanowires, as demonstreted by SEM and TEM 8 ACS Paragon Plus Environment

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images (Figures S2, S3, and S5). To make a visual comparsion, the emssion intensiy bar of CdS@CuS from 0 to 100 was shown in Figure S13. Next, the PL emission spectra at the wavelength of 600-740 nm for pure CdS and CdS@CuS under different wavelength excitation were shown in Figure S14. In the heterostructure of CdS@CuS, a broad and weak emission peak was observed around 680 nm both under 405 and 640 nm irradiation (only CuS can be excited), while no emission signal was detected in the range of 600-740 nm for pure CdS at both light excitation. The position of emission signal of CdS@CuS is similar to that for pure CuS (Figure S15), confirming the formation of CuS after ion-exchange process. On the other hand, compare to pure CuS, the spectral width of PL emission for CuS in the CdS@CuS is broader. This is because of (i) morphology effect: irregular nanoparticles for pure CuS (Figure S7a) compare with shell for CuS in the heterostructure of CdS@CuS (Figures 1 and S5); (ii) for CdS@CuS, the core component is CdS, which will also has effect on this PL spectrum broadening. To give more precise comparison on CdS and CdS@CuS heterostructure, the single-particle PL measurements were performed on pure CdS and as-prepared [email protected] nanowires at ambient condition. As shown in Figures 3a and c, typical CdS single-particle PL signals were observed in the selected area from the low magnification dark-field micrograph under excitation at 405 nm laser. For heterostructure of [email protected], the emission intensities of individual [email protected] appeared to be much lower under the same condition (Figures 3b and d). The corresponding PL spectra of individual particles (of both kinds) were collected in the Figure 3e, which showed significantly lower PL emission intensities from the CdS@CuS heterostructure than those from the CdS non heterostructure. The PL emission peak of CdS@CuS was at ca. 518 nm, which is at lower energy than the PL peak from CdS (i.e. 512 nm). Emission intensities from 9 ACS Paragon Plus Environment


different particles were collected in Table S2 and the average emission intensities of CdS@CuS heterostructures were quenched around 82.1 % comparing to those from pure CdS nanowires. The corresponding PL quantum yield (PLQY) of CdS@CuS is around 12.2%.33,34

Figure 3. Low magnification dark-field micrographs of CdS (a) and [email protected] (b). PL images of individual CdS (c) and [email protected] (d) nanowires from the selected areas in the dark-field micrographs respectively in (a) and (b). PL spectra of the corresponding individual CdS and [email protected] nanowires as numbered in the PL images (e). Representative PL decay profiles of CdS (No. 5) and [email protected] (No. 3’) with the excitation at 405 nm (f). Generaly, the PL quenching is very typical and powerful characterization for demonstrating the electron and/or energy transfer between two components in the heterostructures.9 The band positions of CdS and CuS are calculated according to UV–visible diffuse reflectance spectra modified by Kubelka–Munk function and Mott–Schottky plots (Figure S16). Thus, a heterojunction is expected at their interface in the heterostructure of CdS@CuS (Figure S17), which the photogenerated electrons from CdS are easily injected to adjacent CuS, contributing to the improvement of charge separation in CdS.22-24 The efficient reduce of the recombination of 10 ACS Paragon Plus Environment

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electron-hole in the heterostructure resulting in the high-efficiency PL quenching in the CdS@CuS heterostructure. On the other hand, the existence of sulfur vacancies at the surface are considered based on the EDX results (Table S1) and HRTEM images (Figure S18). Moreover, after ion-exchange reaction, Cu2+ dopants are acted as recombination centers. These surface vacancies and dopant element are acted as traps states, which are also well-known to the band gap recombination.35,36 Accordingly, the effiective charge separation between CdS and CuS together with the traps states contributes to the high-efficiency PL quenching in the CdS@CuS heterostructure. To probe the dynamics of photogenerated carriers in samples, time-resolved PL decays were measured as shown in Figure 3f. The corresponding fitted PL decays are summarized in Table 1. Firstly, the time-resolved PL decays of CdS show short-lifetime (τ1: 0.77 ns (75%)) and longlifetime (τ2: 3.6 ns (25%)) components. These lifetimes suggest that the electron–hole recombination responsible for PL is originated from different shallow trap states (probably with different trap depths) near the CB bottom that act as the PL emission centres.37 For the heterostructure of [email protected], due to the existance of traps states (surface vacancies and Cu2+ dopant) after ion-exchange, the occupation of short-lifetime component (τ1: 0.16 ns) of PL emission increased to 99%. In addition, the decrease of τ1 in the heterostructures is attributed to the formation of new recombination pathways at Cu centers.36 The long-lifetime (τ2: 3.0 ns) is explained by the filling of shallow trap states in the CB bottom of CuS. Generally, in the heterostructure system, the average lifetime is used to evaluate the efficiency of photogenerated electron–hole separation.37 The PL decay average lifetimes (av) of pure CdS and [email protected] are 1.5 and 0.16 ns, respectively. Such 9.4 times shorten average lifetime in the [email protected] indicates that the excited carriers are very easy to be transferred from CdS to the adjacent CuS 11 ACS Paragon Plus Environment


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from the shallow traps, suppressing the recombination of the electrons and holes through PL emission.38 We also assign that the long-lifetime and short-lifetime components are the photogenerated electrons by radiative and non-radiative recombination pathways wth localized holes, respectively.39 The rates of electron transfer process by radiative and non-raditive pathway are roughly estimated to 5.6×107 and 5.0×109 s-1, respectively (Table 1). Table 1. Kinetic parameters of PL and FTDRS lifetimes (τ) of CdS and [email protected] heterostructures under 405 and 400 nm excitation, respectively.

Sample

τ1

τ2

τav

kr-ET (s-1)

kn-ET (s-1)

CdS (PL)

0.77 ns (75%)

3.6 ns (25%)

1.5 ns

―

―

[email protected] (PL)

0.16 ns (99%)

3.0 ns (1.0%)

0.16 ns

5.6×107

5.0×109

CdS (FTDRS)

34.1 ps (94.8%)

251 ps (5.2%)

45.4 ps

21.7 ps (96.8%)

123 ps (3.2%)

24.9 ps

[email protected] (FTDRS) k r-ET and kn-ET are the rates of electron transfer process by radiative and non-radiative pathways, respectively. The rate of electron transfer process is calculated according equation: k 

1

 CdS @ CuS 0.5



1

 CdS

.

Generally, the detectable limit of the PL technology is nanosecond, it’s difficult to give more information on surface defects and trap-states on exciton recombination. To give more clear information on ultrafast electron transport dynamics between CdS and CuS, femtosecond timeresolved diffuse reflectance spectroscopy (FTDRS) measurements of CdS and [email protected] 12 ACS Paragon Plus Environment

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were performed under 400-nm laser excitation. As shown in Figure 4a, a broad transient absorption band was observed for CdS in the near-infrared wavelength region (820~1180 nm), which is assigined to be the signal of trapped electrons.40,41 Similar to pure CdS, the sample of [email protected] has a broad absorption band as well (Figure 4b). The concentration of electrons (transient absorption intensity at 1070 nm) for both samples decreased via the charge recombination in a multi-exponential fashion at a time period of 0–300 ps. Note that, at 200 ps after the excitation, the concentration of charges for pure CdS decreased to 14% of the initial value while [email protected] decreased to 5.7%. The faster decrease of the concentration of electrons in [email protected] indicates that electron decay process is accelerated in [email protected] compared with that in pure CdS. To evaluate electrons decay kinetics, the time profiles of transient absorption were fitted by two-exponential functions according to A= A0 +

in which A and i refer to the

amplitudes and lifetimes of ith components, respectively (Figure 4c). The lifetimes with a short (τ1) and a long (τ2) lifetimes are observed in both samples and summarized in Table 1. Generally, transient absorption lifetime in the picosecond domain may reflect the electron dynamics associated with the electron trap states that are energetically located within the bandgap.42,43 Firstly, for pure CdS, two components are in the picosecond domain, which reflect that these electrons are located within the bandgap and are rapid trapped via nearsurface recombination. Consider the dominate occupation of lifetimes in PL, an interfacial electrons transport between CdS and CuS are main pathway for contributing PL quenching. Secondly, similar to PL decay, after introduced CuS, the av of CdS@CuS (24.9 ps) from FTDRS is much shorter than pure CdS (45.4 ps). The shorten lifetimes in the heterostructure of CdS@CuS suggest that the adjacent

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CuS will be acted the trap sites, which provide an additional electron-transfer channel (from the conduction band of CdS to CuS) apart from the electron-trapping states of CdS.42

Figure 4. Observed during TDR spectroscopic measurement for CdS (a) and [email protected] (b) after irradiation of 400 nm laser flash. Time profiles of normalized transient absorption at 1070 nm (c). To further demonstrate the effective interfacial electron transport between CdS and CuS in the CdS@CuS heterostructure, photocurrent responses and electrochemical impedance spectroscopy (EIS) were carried out. Figure S19 shows that both electrodes display rapid and repeatable on/off 14 ACS Paragon Plus Environment

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cycles of photocurrent densities when they were used as a working electrode with on/off visible light irradiation. The average photocurrent density from CdS@CuS (0.96 mA cm–2) was 6.4 times of that from pure CdS (0.15 mA cm–2). The absorbed photons turn to be charges with light excitation and the higher charge separation efficiency in the heterostructure resulted in higher photocurrent densities.40,44 In addition, EIS spectra of the above electrodes in dark and with light irradiation were measured (Figure S20). Generally, the diameter of semicircle arc (DSA) is used to evaluate the rate of interfacial electron transport, which the smaller DSA means the faster electron mobility.45,46 The results show that CdS@CuS heterostructure has the smallest DSA under light irradiation. This further confirms the improved charge separation efficiency in the CdS@CuS heterostructure than in the CdS nanowires only. Efficient evolution of H2 through photocatalytic technology via solar light holds tremendous promise for clean energy.47 Generally, an efficient charge separation/transfer is a crucial step in the photocatalytic process.48 It has been well recognized that CuS can work as an electron sink and cocatalyst to promote the separation and transfer of photogenerated electrons from other semiconductor to reduce water to H2.22-24 As a proof-of-concept application, we evaluated the photocatalytic activities for H2 evolution under visible light irradiation (>420 nm), with Na2S and Na2SO3 as the sacrificial reagents. As shown in Figure 5a, while only 0.43 μmol H2 was detected (71.9 μmol g-1 h-1) with CdS nanowires, an increase to 13.5 times (i.e. 5.83 μmol, 971 μmol g-1 h-1) of H2 was detected with CdS@CuS core-shell nanowires. The apparent quantum efficiency at 420 nm is around 11.2%. As known, the most efficient H2 evolution cocatalyst is noble metal Pt nanoparticles. The present catalytic activity is even higher than Pt as cocatalyst in the Pt-CdS (3.0 wt%, detail information in Figure S21) under similar condition, suggesting that



an efficient charge transport process in CdS@CuS contributes to the highly efficient photocatalytic H2 production.

Figure 5. Photocatalytic H2 production from water with different catalysts under visible light irradiation (a). Effect of ratio of CuS in CdS@CuS on photocatalytic H2 production rate under visible light irradiation (b). The influence of the different amount of CuS in the heterostructure of CdS@CuS on photocatalytic H2 production was investigated. Figure 5b shows that [email protected] with 10 at% of CuS displayed optimal H2 production. The further increase of CuS results in a decrease of photocatalytic efficiency. It is found that the electron transfer efficiency is not correlated to the rate of H2 generation after the optimized ratio of CuS. This is because the process of photocatalytic H2 generation not only depends on electron transfer rate but also relies on recombination losses and significant photons.49 The CuS may act as charge recombination centers at high content.23 Moreover, the increase of CuS results in the decrease of CdS at the same time, leading less of photons to be excited from CdS. The different wavelength irradiation and stability on H2 production of CdS@CuS were further investigated. For details, see in Figures S22, S23 and corresponding description. 16 ACS Paragon Plus Environment

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In conclusion, 1D core-shell CdS@CuS heterostructures were facile synthesized by cationexchange process. Based on the change of PL signal observed with our single-particle PL spectroscopic measurement, we were able to in-situ visually monitor the ion-exchange process from CdS to CdS@CuS on individual structures. Moreover, information from time-resolved PL decays allowed us to determine the kinetics of interfacial photogenerated electron transfer from CdS to CuS. The efficient interfacial photogenerated charge transfer on the CdS@CuS heterostructures is assumed to contribute to the observed 13.5 folds increase of photocatalytic H2 production under visible light comparing with that using CdS nanowires. The present results provide a visual experimental evidence for understanding restraining the charge carriers recombination in the semiconductor. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, figures and corresponding discussions of additional supporting experimental data. AUTHOR INFORMATION Corresponding Author Email: [email protected] (T.M.). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT



This work has been partly supported by Grant-in-Aid for Scientific Research (project 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.

REFERENCES (1) Chen, X.; Shen, S.; Guo, L.; Mao, S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503-6570. (2) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev.2015, 44, 5148-5180. (3) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting. Chem. Soc. Rev. 2014, 43, 7787-7812. (4) Xie, G.; Zhang, K.; Guo, B.; Liu, Q.; Fang, L.; Gong, J. R. Graphene-based materials for hydrogen generation from light-driven water splitting. Adv. Mater. 2013, 25, 3820-3839. (5) Zhu, M.; Cai, X.; Fujitsuka, M.; Zhang, J.; Majima, T. Au/La2Ti2O7 Nanostructures Sensitized with Black Phosphorus for Plasmon‐Enhanced Photocatalytic Hydrogen Production in Visible and Near‐Infrared Light. Angew. Chem. Int. Ed. 2017, 56, 2064-2068. (6) Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets. J. Am. Chem. Soc. 2011, 133, 10878-10884. (7) Zhang, K.; Dai, Y.; Zhou, Z.; Jan, S. U.; Guo, L.; Gong, J. R. Polarization-induced saw-tooth-like potential distribution in zincblende-wurtzite superlattice for efficient charge separation. Nano Energy 2017, 41, 101-108. (8) Empedocles, B.; Neuhauser, R.; Shimizu, K.; Bawendi, M. Photoluminescence from single semiconductor nanostructures. Adv. Mater. 1999, 11, 1243-1256. (9) Jing, L. ; Qu, Y.; Wang, B.; Li, S.; Jiang, B.; Yang, L.; Fu, W.; Fu, H.; Sun, J. Review of 18 ACS Paragon Plus Environment

Page 18 of 23

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity. Sol. Energy Mater. Sol. Cells 2006, 90, 1773-1787. (10) Tachikawa, T.; Ochi, T.; Kobori, Y. Crystal-Face-Dependent Charge Dynamics on a BiVO4 Photocatalyst Revealed by Single-Particle Spectroelectrochemistry. ACS Catal. 2016, 6, 2250-2256. (11) Lou, Z.; Kim, S.; Zhang, P.; Shi, X.; Fujitsuka, M.; Majima, T. In Situ Observation of Single Au Triangular Nanoprism Etching to Various Shapes for Plasmonic Photocatalytic Hydrogen Generation. ACS Nano 2017, 11, 968-974. (12) Tachikawa, T.; Majima, T. Single-molecule, single-particle fluorescence imaging of TiO2-based photocatalytic reactions. Chem. Soc. Rev. 2010, 39, 4802-4819. (13) Tachikawa, T.; Majima, T. Single-molecule, single-particle approaches for exploring the structure and kinetics of nanocatalysts. Langmuir 2012, 28, 8933-8943. (14) Zheng, Z.; Majima, T. Nanoplasmonic photoluminescence spectroscopy at single‐particle level: sensing for ethanol oxidation. Angew. Chem. Int. Ed. 2016, 55, 2879-2883. (15) Borys, N. J.; Walter, M. J.; Huang, J.; Talapin, D. V.; Lupton, J. M. The role of particle morphology in interfacial energy transfer in CdSe/CdS heterostructure nanocrystals. Science 2010, 330, 13711374. (16) Hatazaki, S.; Sharma, D. K.; Hirata, S.; Nose, K.; Iyoda, T.; Kölsch, A.; Lokstein, H.; Vacha, M. Identification of short- and long-wavelength emitting chlorophylls in cyanobacterial photosystem i by plasmon-enhanced singleparticle spectroscopy at room temperature. J. Phys. Chem. Lett. 2018, 9, 6669-6675. (17) Weckhuysen, B. M. Preface: recent advances in the in-situ characterization of heterogeneous catalysts. Chem. Soc. Rev. 2010, 39, 4557-4559. (18) Zhai, T.; Fang, X.; Li, L.; Bando, Y.; Golberg, D. One-dimensional CdS nanostructures: synthesis, properties, and applications. Nanoscale 2010, 2, 168-187. (19) Li, H.; Wang, X.; Xu, J.; Zhang, Q.; Bando, Y.; Golberg, D.; Ma, Y.; Zhai, T. One-dimensional CdS nanostructures: A promising candidate for optoelectronics. Adv. Mater. 2013, 25, 3017-3037. 19 ACS Paragon Plus Environment


(20) Zhai, C.; Sun, M.; Zhu, M.; Zhang, K.; Du, Y. Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires. Int. J. Hydrogen Energy 2017, 42 5006-5015. (21) Jang, J.; Joshi, U.; Lee, J. Solvothermal synthesis of CdS nanowires for photocatalytic hydrogen and electricity production. J. Phys. Chem. C 2007, 111, 13280-13287. (22) Zhang, L. J.; Xie, T. F.; Wang, D. J.; Li, S.; Wang, L. L.; Chen, L. P.; Lu, Y. C. Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties. Int. J. Hydrogen Energy 2013, 38, 11811-11817. (23) Zhang, J.; Yu, J.; Zhang, Y.; Li, Q.; Gong, J. R. Visible light photocatalytic H2-production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer. Nano Lett. 2011, 11, 4774-4779. (24) Zhang, L.; Jiang, T.; Li, S.; Lu, Y.; Wang, L.; Zhang, X.; Wang, D.; Xie, T. Enhancement of photocatalytic H2 evolution on Zn0.8Cd0.2S loaded with CuS as cocatalyst and its photogenerated charge transfer properties Dalton Trans. 2013, 42, 12998-13003. (25) Markovskaya, D. V.; Cherepanova, S. V.; Saraev, A. A.; Gerasimov, E. Y.; Kozlova, E. A. Photocatalytic hydrogen evolution from aqueous solutions of Na2S/Na2SO3 under visible light irradiation on CuS/Cd0.3Zn0.7S and NizCd0.3Zn0.7S1+z Chem. Eng. J. 2015, 262, 146-155. (26) Chen, X.; Zhang, W.; Li, Y.; Song, Q.; Dong, L. Fabricate globular flower-like CuS/CdIn2S4/ZnIn2S4 with high visible light response via microwave-assisted one–step method and its multipathway photoelectron migration properties for hydrogen evolution and pollutant degradation. ACS Sustainable Chem. Eng. 2016, 4, 6680-6688. (27) Wang, J.; Feng, H.; Chen, K.; You, T. Seeded-synthesis and characterization of CdS hexagonal nanoplates. Mater. Lett. 2013, 104, 87-90. (28) Dean, J. A. Lange's Handbook of Chemistry, 15th ed. NewYork: McGraw-Hill Inc, 1999 (Section 8, Table 8.6). (29) Lubeck, B.; Han, T.; Gash, A.; Satcher, Jr., J.; Doyle, F. Synthesis of mesostructured copper sulfide 20 ACS Paragon Plus Environment

Page 20 of 23

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by cation exchange and liquid-crystal templating. Adv. Mater. 2006, 18, 781-784. (30) Xu, D.; Liu, Z.; Liang, J.; Qian, Y. Solvothermal synthesis of CdS nanowires in a mixed solvent of ethylenediamine and dodecanethiol. J. Phys. Chem. B 2005, 109, 14344-14349. (31) Wagner, C. D.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy: a Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (32) Wei, R.-B.; Huang, Z.-L.; Gu, G.-H.; Wang, Z.; Zeng, L.; Chen, Y.; Liu, Z.-Q. Dual-cocatalysts decorated rimous CdS spheres advancing highly-efficient visible-light photocatalytic hydrogen production. Appl. Catal. B Environ. 2018, 231, 101-107. (33) Hsu, Y.-J.; Lu, S.-Y. Low temperature growth and dimension-dependent photoluminescence efficiency of semiconductor nanowires. Appl. Phys. A 2005, 81, 573-578. (34) van der Stam, W.; Geuchies, J. J.; Altantzis, T.; van den Bos, K. H. W.; Meeldijk, J. D.; Aert, S. V.; Bals, S.; Vanmaekelbergh, D.; de Mello Donega, C. Highly emissive divalent-ion-doped colloidal CsPb1–xMxBr3 perovskite nanocrystals through cation exchange. J. Am. Chem. Soc. 2017, 139, 40874097. (35) Wang, Y.; Meng, G.; Zhang, L.; Liang, C.; Zhang, J. Catalytic growth of large-scale single-crystal CdS nanowires by physical evaporation and their photoluminescence. Chem. Mater. 2002, 14, 17731777. (36) Plante, I. J.-L.; Teitelboim, A.; Pinkas, I.; Oron, D.; Mokari, T. Exciton quenching due to copper diffusion limits the photocatalytic activity of CdS/Cu2S nanorod heterostructures. J. Phys. Chem. Lett. 2014, 5, 590-596. (37) Li, X. G.; Bi, W.; Zhang, L.; Tao, S.; Chu, W.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Single-atom Pt as co-catalyst for enhanced photocatalytic H2 evolution. Adv. Mater. 2016, 28, 2427-2431. (38) Huang, L.; Yang, J.; Wang, X.; Han, J.; Han, H.; Li, C. Effects of surface modification on photocatalytic activity of CdS nanocrystals studied by photoluminescence spectroscopy. Phys. Chem. Chem. Phys. 2013, 15, 553-560. 21 ACS Paragon Plus Environment


(39) Berends, A. C.; Rabouw, F. T.; Spoor, F. C. M.; Bladt, E.; Grozema, F. C.; Houtepen, A. J.; Siebbeles, L. D. A.; de Mello Donega, C. Radiative and nonradiative recombination in CuInS2 nanocrystals and CuInS2-based core/shell nanocrystals. J. Phys. Chem. Lett. 2016, 7, 3503−3509. (40) Zhu, M.; Kim, S.; Mao, L.; Fujitsuka, M.; Zhang, J.; Wang, X.; Majima, T. Metal-free photocatalyst for H2 evolution in visible to near-infrared region: black phosphorus/graphitic carbon nitride J. Am. Chem. Soc 2017, 139, 13234-13242. (41) Yoshihara, T.; Katoh, R.; Furube, A.; Tamaki, Y.; Murai, M.; Hara, K.; Murata, S.; Arakawa, H.; Tachiya, M. Identification of reactive species in photoexcited nanocrystalline TiO2 films by widewavelength-range (400−2500 nm) transient absorption spectroscopy. J. Phys. Chem. B 2004,108, 3817–3823. (42) Bi, W.; Li, X.; Zhang, L.; Jin, T.; Zhang, L.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Molecular cocatalyst accelerating hole transfer for enhanced photocatalytic H2 evolution. Nat. Commun. 2015, 6, 8647. (43) Zhang, Q.; Luo, Y. Probing the ultrafast dynamics in nanomaterial complex systems by femtosecond transient absorption spectroscopy. High Power Laser Science and Engineering 2016, 4, e22. (44) Zhang, J.; Yu, J.; Jaroniec, M.; Gong, J. R. Noble metal-free reduced graphene oxide-ZnxCd1–xS nanocomposite with enhanced solar photocatalytic H2 production performance. Nano Lett. 2012, 12, 4584-4589. (45) Ouyang, T.; Ye, Y.-Q.; Wu, C.-Y.; Xiao, K.; Liu, Z.-Q. Heterostructures composed of N-doped carbon nanotubes encapsulating cobalt and β-Mo2C nanoparticles as bifunctional electrodes for water splitting. Angew .Chem. Int. Ed. 2019, 58, 4923-4928. (46) Zhu, M.; Zhai, C.; Sun, M.; Hu, Y.; Yan, B.; Du, Y. Ultrathin graphitic C 3N4 nanosheet as a promising visible-light-activated support for boosting photoelectrocatalytic methanol oxidation. Appl. Catal. B: Environ. 2017, 203, 108-115. (47) Su, J.; Wei, Y.; Vayssieres, L. Stability and performance of sulfide‑ , nitride‑ , and phosphide-based electrodes for photocatalytic solar water splitting. J. Phys. Chem. Lett. 2017, 8, 5228-5238. 22 ACS Paragon Plus Environment

Page 22 of 23

Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) Bai, S.; Jiang, J.; Zhang, Q.; Xiong, Y. Steering charge kinetics in photocatalysis: intersection of materials syntheses, characterization techniques and theoretical simulations. Chem. Soc. Rev. 2015, 44, 2893-2939. (49) Simon, T.; Carlson, M. T.; Stolarczyk, J. K.; Feldmann, J. Electron transfer rate vs recombination losses in photocatalytic H2 generation on Pt-decorated CdS nanorods. ACS Energy Lett. 2016, 1, 1137-1142.


Monitoring Transport Behavior of Charge Carriers in a Single CdS

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