Charge-Transfer Dynamics Promoted by Hole Trap States in CdSe

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Charge Transfer Dynamics Promoted by Hole Trap States in CdSe QDs-Ni Photocatalytic System 2+

Yun Ye, Xiuli Wang, Sheng Ye, Yuxing Xu, Zhaochi Feng, and Can Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05061 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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The Journal of Physical Chemistry

Charge Transfer Dynamics Promoted by Hole Trap States in CdSe QDs-Ni2+ Photocatalytic System Yun Ye,†,‡,§ Xiuli Wang,†,§ Sheng Ye,† Yuxing Xu,†,‡ Zhaochi Feng,† and Can Li*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

§

These authors contributed equally to this work.

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ABSTRACT Manipulation of the photoinduced hole dynamics is a key strategy to improve the H2 evolution efficiency in the QDs-based photocatalytic systems. The ultrafast hole trapping by surface states of the QDs is beneficial to electron transfer but retards the trapped hole transfer. It deserves to investigate whether ultrafast hole trapping is beneficial to the photocatalytic H2 evolution activity. We employed two types of CdSe QDs, QDs-1 and QDs-2, with tuned surface hole trap states to investigate the effect of ultrafast hole trapping on charge transfer dynamics in the photocatalytic system. QDs-1 possesses higher density of surface hole trap states than QDs-2. Compared with QDs-2, the transfer dynamics of free electrons in QDs-1 to the proton reduction catalyst―Ni2+ are more promoted by hole trapping, characterized by the transient absorption spectroscopy (TAS) and photoluminescence (PL) techniques. Interestingly, the free hole transfer from QDs-1 to the sacrificial reagent―ascorbic acid (AA) was also improved more. Moreover, the surface defects of the QDs serve as binding sites for Ni2+ and AA, further promoting the electron transfer and hole removal dynamics, respectively. Our results illustrate that the ultrafast hole trapping increases the H2 production activity in the actual CdSe QDs-Ni2+ photocatalytic system.

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INTRODUCTION Semiconductor quantum dots (QDs) are promising materials for efficient solar energy conversion due to their precise controllable and surfactant-assisted synthetic methods, surface tunability and other favorable optical and electronic properties.1,2 Of particular interest is their role in capturing incident photons to induce charge separation in the photocatalytic systems due to their large absorption cross-section.3 The coupling of semiconductor QDs with molecular or metal catalysts has proven to be effective H2 production systems.4,5 The efficiency of these light-driven reactions mainly depends on the interfacial electron and hole transfer processes from the excited QDs. Most of the mechanistic studies on these photocatalytic systems were focused on the electron dynamics, and have been committed to attribute the high H2 production activity to the ultrafast electron transfer rates. Actually, the hole dynamics are equally important in the photocatalytic systems, because hole removal is the prerequisite step for the effective proton reduction by photoinduced electrons. In order to improve the hole removal dynamics in the photocatalytic H2 production systems, most researchers devote to search better hole scavengers with larger hole transfer driving forces.6,7 Simon et al. proposed a new efficient hole transport mechanism by using ethanol scavenger in combination with a suitable redox couple •OH/―OH at high pH conditions.8 Zamkov et al. reported that the chemical etched CdSe/CdS NRs show an efficient H2 production rate ~3-4 fold higher than un-etched ones, demonstrating that to expose the hole trapping domains to electron donors can promote the hole removal efficiency.9 With direct evidences, this result 3



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illustrates that controlling hole trapping is an effective strategy to manipulate the photoinduced hole transfer dynamics. For semiconductor QDs in aqueous solution of photocatalytic systems, there are large amount of surface trap states, especially for the QDs with smaller particle size. The surface trap states can serve as hole trapping centers.2,10 Hole trapping plays a crucial role in the carrier dynamics of photocatalytic systems. Firstly, hole trapping can generate long-lived charge separated states, and then promotes the electron transfer processes. Wu et al. have reported that the charge separated state is surprisingly long-lived (~1.2 ± 0.6 µs) due to the trapping of photogenerated holes in CdS NRs (~0.7 ps). By introducing Pt on the tips of CdS, electrons can be dissociated from the trapped holes and transfer rapidly to the Pt tip (~3.4 ps).11 Secondly, the trapped holes are much difficult to transfer to hole scavengers than the free holes. Tarafder et al. have shown that the valence band (VB) hole transfer from CdSe/CdS NRs to the surface bond tethered ferrocene (Fc-hex-SH) is on a timescale of 790 ps,12 while Wu et al. reported that the trapped holes in CdS NRs can be extracted to the surface adsorbed hole acceptor, phenothiazine (PTZ), with a time constant of ~3.6 ns.13 Therefore, hole trapping can promote electron transfer but retard the hole transfer. It deserves to investigate the complex roles of hole trapping to reveal its overall effect in the QDs-based photocatalytic systems. In this work, CdSe QDs are selected to study the roles of hole trapping, because the QD surface properties can be readily tailored by colloidal synthesis and the surface hole trap states can be easily generated. Specifically, as the particle size 4


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decreases, the properties of the surface atoms begin to dominate the entire particle, and the dangling bonds or vacancies on the surface will generate trap states.14-19 Moreover, the hole trap states constitute the majority surface trap states in CdSe QDs.20,21 Hence we synthesized 1.8 nm-CdSe QDs with tailored concentrations of surface hole trap states and studied the roles of hole trapping in charge transfer dynamics in the CdSe QDs-Ni2+ photocatalytic system. We demonstrated that the ultrafast hole trapping in the QDs not only accelerates the conduction band (CB) electron transfer rate, but also promotes the VB hole removal efficiency.

EXPERIMENTAL SECTION Synthesis of CdSe QDs. CdSe QDs were prepared according to the literatures 22 and 23 with slight modifications. The synthesis began with the addition of Se powder into 100 mL Na2SO3 aqueous solution to produce the colorless transparent Na2SeSO3 solution-Se precursor for use. On the other hand, CdCl2•2.5H2O and 3-mercaptopropionic acid (MPA) were dissolved in 190 mL deionized water, and the pH value of the solution was adjusted to 9.0 by the addition of NaOH solution to obtain the Cd precursor. After degassed and recharged with argon for several times for both precursors, 10 mL Se precursor was extracted and injected into the argon saturated Cd precursor solution. The mixture was heated to boiling and refluxed to control the growth of CdSe QDs under argon atmosphere in a three-neck round-bottom flask. After the reaction, the mixture was extracted from the reaction solution, then subsequently precipitated with isopropyl alcohol, and followed by centrifugation. The final product was redispersed in deionized water for use. At last, 5



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we obtained two desired types of MPA-capped CdSe QDs by different reaction times. The QDs were named as QDs-1 and QDs-2 with the growth time of 2 h and 4 h, respectively. And both the QDs were extremely stable during storage at room temperature in the dark. Characterization. X-ray powder diffraction (XRD) patterns of powder CdSe QDs were recorded using Rigaku D/Max-2500 powder diffractometer. UV-vis absorption spectra were recorded with a JASCO V650 spectrophotometer. Transmission electron microscopy (TEM) images of CdSe QDs were taken by Tecnai G2 Spirit (FEI Co.) operated at an accelerating voltage of about 100 kV. X-ray photoelectron spectra were obtained on an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo) equipped with a monochromatized Al Kα X-ray (hν = 1486.6 eV) as the excitation source. All the binding energies were calibrated using the hydrocarbon C 1s peak with the binding energy of 285.0 eV present in all spectra as a standard.24 Steady-state and time-resolved photoluminescence (PL) spectra were collected on a FLS920 fluorescence spectrometer (Edinburgh Instruments). A 450 W Xe lamp and a picosecond (ps) pulsed diode laser (406.8 nm) with pulse width of 64.2 ps were used as the excitation source for the steady-state and time-resolved PL spectra, respectively. Nanosecond transient absorption spectroscopy (ns-TAS) was performed using a LP920 laser flash photolysis spectrometer (Edinburgh Instruments). Femtosecond transient absorption spectroscopy (fs-TAS) was based on a regenerative amplified Ti:sapphire laser system from Coherent (800 nm, 35 fs, 6 mJ/pulse, and 1 kHz repetition rate), nonlinear frequency mixing techniques and the Helios 6


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spectrometer (Ultrafast Systems LLC). The excitation wavelengths for ns- and fs-TAS measurements were 355 and 350 nm, respectively. All experiments were performed at room temperature.

RESULTS AND DISCUSSION Characterization of hole trap states. Two types of CdSe QDs, QDs-1 and QDs-2, were prepared by the same synthetic method. The powder XRD patterns of QDs-1 and QDs-2 are shown in Figure 1. The pronounced peaks of (111), (220) and (311) are the characteristic of cubic zinc-blende phase structure of CdSe QDs (JCPDS No.19-0191). QDs-1 and QDs-2 have almost the same particle size of 1.8 nm estimated from the theoretical equation by using the measured wavelength of their first absorption peaks shown in Figure 2(left).25 Also TEM images of them indicate a good dispersion and uniform size distribution, especially for QDs-2 (Figure S1).The size of the QDs decides the weight percent of surface atoms. The proportion of the surface atoms reach up to 80% at this QD size. Thus the surface defects dominate in both QDs-1 and QDs-2, including dangling bonds of Cd and Se, and other existing species. On the other hand, the QDs disperse stably in water by linking MPA ligands to passivate the surface and avoid deposition. The sorption of MPA ligands also introduces surface defects.19,26 Therefore, the different preparation time mainly affects the crystallization degree of the QDs with the same particle size. Combined with the fact that surface atoms dominate in the QDs, the different optical and carrier properties of QDs-1 and QDs-2 should be mainly due to their different surface states. Since the surface defects of the QDs are determined by the undercoordinated 7



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surface atoms, and the surface stoichiometry strongly affects the trapping distribution of photoinduced charge carriers in the QDs, it is indispensible to figure out the surface properties. XPS is a feasible technique to distinguish surface from inner core atoms of the QDs, because of their distinct electron binding energies.27-30 By taking a ratio of the integrated XPS peaks for the surface and core atoms respectively, the ratio of surface to inner atoms can be determined. As summarized in Table 1, we obtained the surface composition and stoichiometry with proper XPS peak analysis (Figure 3a, Figure 3c, Figure S2a, Figure S2c). Except for the regular Cd and Se species in the surface and interior of QDs-1 and QDs-2, S specie was also observed due to the involvement of MPA in the growth of CdSe QDs. The majority of hole trap states are supposed to originate from the dangling bond orbitals related to the surface Se and S atoms.27,28,31 Busby et al. proved that the hole trapping rate will increase by reducing Cd surface fraction.32 Thus, the atomic percentages of Cd surface atoms in QDs-1 (0.38) and QDs-2 (0.42) calculated from the integrated peak areas fitted by Cd, Se and S signals in XPS demonstrate that QDs-1 with less Cd surface atomic ratio has more surface trapped holes. The surface defects in CdSe QDs could be trapping centers for the photogenerated electrons or holes. PL is a technique with high sensitivity. A wide variety of PL decay dynamics of colloidal QDs have been reported,33-35 and it gives direct information of both the free and trapped carriers. Then we use PL to further characterize the surface states of CdSe QDs.36 Figure 2(right) gives the PL spectra of QDs-1 and QDs-2. They both show a narrow band-edge emission and a broad, lower 8


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energy trap-related emission, with different relative emission intensities. The enhanced trap-related emission and decreased band-edge emission for QDs-1 are direct consequences of the charge carrier trapping processes. In very small QDs, almost all trap-related emission comes from the surface PL.15 Vokhmintcev et al. attributed the trap-related emission to the formation of numerous surface defects, which act as hole trapping centers upon the surface capping of MPA ligands.2 Thus, the surface states of QDs-1 are likely to be dominated by surface hole trap states, and the trapped holes may constitute the majority of the surface trapped carriers for both QDs-1 and QDs-2. We used the fs-TAS to further confirm the trap states dominated carrier dynamics. As shown in Figure 4a and Figure S3a, TA spectra of QDs-1 and QDs-2 both show a photoinduced transient bleach signal and a much weaker and broader transient absorption signal at the wavelength longer than 500 nm. The double peak of the bleach signal is attributed to the increasing population of the 1S electron states, 1S(e)-1S3/2(h) and 1S(e)-2S3/2(h), respectively.37 Here we investigate the bleach signal at longer wavelength and attribute it to the population of the CB electrons, with negligible contribution from the VB holes.38 The absorption signal can be assigned to the surface trapped holes in the QDs.11 The transient absorption signal amplitude of QDs-2 is smaller than that of QDs-1 (Figure S3b), suggesting less surface hole trapping density in QD-2, in agreement with the PL results. In Figure 4b, the bleach signals of both QDs-1 and QDs-2 recover by ~10% within the first 100 ps. If the surface electron trapping process dominates, more significant bleach recovery should 9



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be observed, since electrons in the excited QDs are usually trapped at a timescale from hundreds of femtosecond to tens of picosecond.39-41 There have been proved that the photoinduced holes in the semiconductor NRs and CdTe QDs are trapped in ~0.7-1.5 ps.11,42-44 Also the transient absorption signals of the trapped holes in QDs-1 and QDs-2 arise within one picosecond (Figure S3b), thus the hole trapping process of CdSe QDs should be on a picosecond timescale. Furthermore, Wuister et al. attributed the ultrafast hole trapping to the unpassivated surface dangling bonds induced by the MPA capping ligands.45 Therefore, the photogenerated holes in CdSe QDs can be trapped very fast, and the holes do constitute the majority of the surface trapped carriers. Promoted electron transfer from CdSe QDs to Ni2+ by hole trap states. It has been proved that hole trapping is an ultrafast process (~ps) and the holes constitute the majority of surface trapped carriers in CdSe QDs. Moreover, there is higher density of hole trap states in QDs-1 than QDs-2. Then we combined the QDs with Ni2+ catalyst to investigate the contribution of ultrafast hole trapping to the electron transfer dynamics of the excited QDs. Firstly, we consider the electron transfer from the aspects of thermodynamics. The bandgap of QDs-1 and QDs-2 is almost the same (~2.9 eV) due to the same particle diameter. The VB maximum of CdSe QDs has been found to be located at ~1.1 V vs NHE, nearly independent of the particle size.46 As a consequence, the CB of QDs-1 and QDs-2 (~ –1.8 V vs. NHE) is more negative than the redox potential of Ni2+/Ni0 couple (–0.25 V vs. NHE). Therefore the consecutive electron reduction by the excited QDs is thermodynamically favorable. 10


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Secondly, XPS technique is confirmed to be able to accurately monitor the changes in surface potential.47,48 The impact of added Ni2+ on the surface potential of the QDs is quantitatively determined by the induced chemical shift of the surface Cd 3d peak position (Figure 3a, Figure 3b, Figure S2a, Figure S2b). The comparison of chemical shifts of surface Cd 3d for QDs-1 and QDs-2 after the addition of Ni2+ are given in Table 2. Both the binding energy of QDs-1 and QDs-2 shifts to higher energy, with larger shift of QDs-1. The shift to high energy direction indicates the decreasing of electron amounts. The higher energy shift of QDs-1 suggests the more effective electron-taking process during XPS measurement. The more abundant ultrafast hole trapping in QDs-1 must be responsible for the more significant electron transfer behavior. Next, we investigate the electron transfer kinetics from the excited QDs to Ni2+ with ns-TAS. As shown in Figure 5, the initial bleach intensity of QDs-1 decreased with the continuous addition of Ni2+. The initial transient bleach intensity of the QDs mainly originates from CB electrons after excitation and is proportional to their population.49 Moreover, electron transfer process usually occurs on picosecond timescale,14,50-54 and it generally finishes on nanosecond timescale. Thus the decreased initial bleach intensity suggests that the CB electrons transfer from QDs-1 to Ni2+. For QDs-2, electron transfer was proved by fs-TAS when low concentration of Ni2+ was added to ensure the stability of solution (Figure S4). Although the initial bleach signal decreased, the recovery of the bleach signal is retarded by the added Ni2+ for QDs-1 on nanosecond timescale. According to the above XPS results, there 11



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are Cd, Se, and S dangling bonds on the surface of CdSe QDs. The Cd and Se vacancies and impurities result from S occupying the Se sites should be the main surface defects. The adsorption mode for Ni2+ onto the QD surface supposed to be the case that Ni2+ bonding in Cd vacancy. Thus the adsorbed Ni2+ may firstly modify the surface of QDs-1 by reducing Cd vacancy. We consider this modification effect could be the reason causing a longer lifetime of the bleach signal. Therefore, the surface defects in the QDs can not only act as hole trap states to promote electron transfer from the QDs to Ni2+, but may also contribute to the surface modification by providing the more easily adsorption sites for Ni2+. To further prove the above views and obtain more information about the influence of added Ni2+ on the surface of QDs-1 and QDs-2, PL techniques were employed. The emission spectra of QDs-1 and QDs-2 with the addition of Ni2+ are shown in Figure 6. With 50 µM Ni2+ added, the PL emission intensities are enhanced (Figure 6a) and lifetimes increased (Figure S5a, Figure S5b) for QDs-1. The emission peaks of QDs-1 show an obvious redshift upon the addition of Ni2+, while the peak shapes remain unchanged. These results suggest that the firstly added Ni2+ acted as surface passivation for QDs-1 by adsorption, and reduced the nonradiative pathway without introducing new defects.15 The redshift is probably due to the formation of patches of a Ni-Se or Ni-S shell on the surface of QDs-1.29,55,56 Thus the added Ni2+ may also forms a tiny part of the exciton confinement and shapes the band-edges of QDs-1.57,58 For QDs-2, no enhanced PL emission was observed in the presence of Ni2+ (Figure 6b). The redshift of the band-edge emission also appears but at higher 12


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concentration of the added Ni2+, with negligible shift of the trap-related emission. These results should be due to the fewer amounts of surface defects, especially Cd vacancies in QDs-2. The PL emission quenching (Figure 6a, Figure 6b) and the reduction of PL lifetimes (Figure S5) for QDs-1 and QDs-2 at higher Ni2+ concentrations demonstrate the existence of electron transfer processes. Then, we plotted PL lifetimes (τ0/τ) vs. concentrations of the added Ni2+ (Figure 6c, Figure 6d) to quantitatively compare the electron transfer dynamics of QDs-1 and QDs-2. And the longest PL average lifetime of the QDs is named as τ0 regardless of the presence of Ni2+. Here, we employ the approximate exponential decay fitting (equation S1, equation S2) to obtain the PL lifetimes of QDs-1 and QDs-2 with and without the added Ni2+. As shown in equation S5, the rate constant of electron transfer (kET) is proportional to the PL quenching rate constant (kq). Obviously, QDs-1 shows larger quenching efficiency for the band-edge emission, while QDs-1 and QDs-2 exhibit almost the same quenching dynamics for the trap-related emission. The quenching of the band-edge emission in the presence of Ni2+ was caused by the free electron transfer from CB of the QDs to the surface adsorbed Ni2+, demonstrating that QDs-1 with more surface hole trap states has better free electron transfer rate, 6.5 times larger than that of QDs-2. For the trap-related emission, its shortened lifetime was a result of the trapped electron transfer process. The similar quenching indicates that the trapped electrons show almost the same promoted transfer rates. For CdSe QDs, the amount of trapped electrons should be limited since the holes constitute the majority of the surface trapped carriers. 13



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Therefore, the total electron transfer of QDs-1 is more efficient than that of QDs-2. Role of hole trap states in hole transfer from CdSe QDs to AA. In the CdSe QDs-Ni2+ photocatalytic system, the sacrificial reagent is an indispensable part, since hole removal by the sacrificial electron donors is a limiting step in the photocatalytic H2 generation cycle.6 Ascorbic acid (AA), as the powerful and most commonly used sacrificial reagent in aqueous solution, was chosen to investigate the influence of controlling surface hole trap states in CdSe QDs on the hole transfer dynamics. Thermodynamically, it is feasible for AA to remove holes from the excited QDs, since the VB potential of QDs-1 and QDs-2 (~1.1 V vs NHE) is more positive than the redox potential of AA (-0.066 V vs NHE).59 From the perspective of dynamics, the retarded bleach recoveries of QDs-1 and QDs-2 suggest AA extracted holes from the excited QDs (Figure S6). The recoveries were slowed down slightly because of the low added concentrations of AA for the stability consideration. The influence of added AA on the surface properties of CdSe QDs was investigated by PL techniques. As shown in Figure 7a and Figure 7b, the trap-related emissions of QDs-1 and QDs-2 display continuous redshift and unchanged peak shapes with the increasing concentrations of AA. The redshift of the band-edge emission in QDs-1 appears immediately with the addition of AA (25 µM), while it is only observed at higher AA concentration (100 µM) for QDs-2. Besides, the redshift of PL emissions in QDs-2 is greater than QDs-1 with 100 µM AA added. Considering the possible surface adsorption sites in CdSe QDs, AA molecule can bond to the surface dangling Cd with the group -OH, even more it can take the place of the native 14


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MPA ligand.2 Thus we can speculate that the two adsorption modes should exist at the same time, and the directly bond mode is easier and shows lower effect on the redshift. For QDs-1 with more surface defect sites, the bond mode dominants, while the replacement of MPA ligand by AA places a great role in QDs-2. On the other hand, at the first 25 µM AA added, the enhanced PL emissions and prolonged lifetimes (Figure S7) of QDs-1 suggest that the added AA decreased the contribution of nonradiative decay to the overall excited state lifetime. It has been revealed that the native capping ligands will develop a steric barrier, hindering the diffusion of the donor molecules to the QD surface and thereby inhibiting the charge transfer dynamics.6,8 So the intimate contact of AA with the QDs removes the obstacle and is beneficial to the hole transfer processes.2 Hole transfer dynamics are often obtained from PL quenching of the QDs with the added hole acceptor. The plots of PL lifetime (τ0/τ) with the added AA concentrations (Figure 7c, Figure 7d) are analyzed to compare the hole transfer rates of QDs-1 and QDs-2. The definition method of τ0 is the same as that mentioned above with Ni2+. With low concentrations of AA added (50 µM for QDs-1 and 100 µM for QDs-2), the band-edge emission lifetimes of QDs-1 and QDs-2 barely change, but the trap-related emission lifetimes are prolonged continuously with the increasing added AA due to the reduced nonradiative pathway caused by the surface modification. Moreover, the increasing trend of the trap-related emission lifetimes of QDs-1 is much more than that of QDs-2, probably due to the more exposed and readily accessible surface defect sites in QDs-1. 15



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After the modification stage, the hole extracting from CdSe QDs with high concentrations of added AA begins to dominate. For the band-edge emission, the hole removal processes become apparent after the addition of 50 µM AA, while it takes more than 100 µM for the trap-related emission. It should be mentioned that the native surface ligand, MPA, also acts as a sacrificial electron donor to remove holes in the QDs.60 Because the oxidation potential of MPA ligands (~0.81 V vs NHE) is much more positive than AA,59 the added AA should definitely be an effective electron donor, evidenced by the acceleration of PL decays. The shortened lifetimes of the band-edge and trap-related emissions with the added AA are attributed to the free and trapped hole transfer, respectively. The more effective acceleration of the band-edge emission decay suggests the ultrafast hole trapping promotes the free hole transfer from the VB of QDs-1 to the attached AA, and the free hole transfer rate of QDs-1 is 6.7 times greater than QDs-2. On the other hand, for the trap-related emission, QDs-2 shows a 4.8 times faster decay rate. It is reasonable due to the lower density of hole trapping states in QDs-2, and in view of this, the efficient transfer of trapped holes will not make an effective contribution to the overall charge separation. In combination of the free and trapped holes, the total hole transfer of QDs-1 should be better than that of QDs-2. This result indicates that the hole trap states in the QDs are beneficial to not only the electron transfer but also the hole transfer processes. Role of hole trap states in actual photocatalytic system. Since the ultrafast hole trapping benefits both the electron and hole transfer from the excited QDs, we now turn to the actual photocatalytic H2 production system composing of CdSe QDs, 16


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50 µM AA and Ni2+ with various concentrations. By plotting the PL lifetimes (τ0/τ) with Ni2+ concentrations in the presence of 50 µM AA (Figure 8), the effect of hole trap states on electron transfer in the actual photocatalytic system are investigated. As discussed above, the added AA will not only play a certain role in surface modification to reduce the nonradiative pathway, but also further promote the charge separation by the removal of free holes in the excited QDs (Figure S8). Then, the added Ni2+ can extract electron efficiently (Figure S8, Figure S9). Amazingly, the quenching efficiency of QDs-1 by the added Ni2+ is larger than that of QDs-2 for both the free and trapped charge carriers. These results demonstrate that the hole trap states promote the total charge transfer dynamics in the photocatalytic H2 production systems, indicating a positive correlation between the ultrafast hole trapping and proton reduction efficiency. To verify the conclusion, we performed a brief comparison of the H2 generation activities of the QDs-1/Ni2+ and QDs-2/Ni2+ photocatalytic systems with AA as the sacrificial reagent (Figure S10), and QDs-1 has better H2 production performance than QDs-2 under the same conditions. Lian and coworkers have confirmed that the ultrafast trapping of VB holes leads to long-lived CB electrons bound to the trapped holes.60 As illustrated in Figure 9, we attributed the promoted electron (⑤) and hole transfer (⑥) rates in the excited QDs to the ultrafast hole trapping behavior (②). By intentionally creating plenty of hole trap states on the surface of CdSe QDs, the free electron and hole transfer efficiency can be improved notably by suppressing recombination of CB electrons and VB (③) or trapped (④) holes. The promoted transfer behavior of the trapped electron is not 17



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shown in the diagram due to its negligible amount. Furthermore, the surface defect sites in the QDs can be surface adsorption sites for the electron acceptor Ni2+ and sacrificial electron donor AA, which benefits electron and hole transfer processes by changing the QD surface potential and reducing the distance between the QD and AA, respectively.2

CONCLUSIONS In this work, we synthesized two types of CdSe QDs by intentionally controlling their surface hole trap states. The impact of ultrafast hole trapping on the photogenerated electron and hole transfer dynamics in CdSe QDs was investigated. In addition to the favorable charge separation, ultrafast hole trapping can significantly improve the transfer dynamics of both the free electrons and free holes in the QDs. Moreover, Ni2+ adsorbs on the surface defect sites of the QDs, which would change the QD surface potential, to be further beneficial to the electron transfer process. AA can directly attach to the proper surface dangling bonds of the QDs, and the intimate contact increases the hole removal efficiency. Thus the ultrafast hole trapping is beneficial to the overall charge separation and improves the proton reduction efficiency. The rational manipulation of hole trapping in semiconductor QDs is a promising strategy to control the photogenerated charge separation and transfer processes in the QDs-based systems, which allows to be widely applied in solar energy conversion, including mainly photocatalysis and photovoltaics.

18


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ASSOCIATED CONTENT Supporting Information Additional equations, tables and figures (PDF)

AUTHOR INFORMATION Corresponding Author *Phone: 86-411-84379070. Fax: 86-411-84694447. E-mail: [email protected].

Author Contributions §

Y. Y. and X. W. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology (No. 2014CB239400), National Natural Science Foundation of China (No. 21373209, 21633015 and 21621063) and Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB17020200). The authors thank Prof. Shengye Jin and Dr. Jing Leng for fs-TAS measurements.

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FIGURES Table 1. The stoichiometry and surface atomic ratios of QDs-1 and QDs-2. The surface-to-inner ratios of Cd, Se and S are calculated from the integrated peak area fitted by Cd, Se, and S signals in XPS. Surface atomic ratio

Sei/Cd ratio

Ses/Cd ratio

Se/Cd ratio

Si/Cd ratio

Ss/Cd ratio

S/Cd ratio

Cd

Se

S

QDs-1

0.254

0.078

0.332

0.138

0.312

0.452

0.382

0.124

0.494

QDs-2

0.274

0.178

0.452

0.108

0.249

0.357

0.420

0.242

0.338

QDs

Table 2. Chemical shift of surface Cd 3d for QDs-1 and QDs-2 after the addition of Ni2+. QDs

Surface Cd 3d5/2 (chemical shift)

Surface Cd 3d3/2 (chemical shift)

QDs-1

0.74

0.74

QDs-2

0.43

0.44

10

20

30

(311)

(220)

(111)

Intensity/a.u

QDs-1 QDs-2

40 50 2θ/degree

60

70

Figure 1. XRD patterns of CdSe QDs-1 and QDs-2. QDs-1 QDs-2

PL Intensity/a.u.

QDs-1 QDs-2

Absorbance

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


300

400

500 600 Wavelength/nm

700

800

Figure 2. UV-vis absorption (left) and PL spectra (right) of QDs-1 and QDs-2. The PL spectra were obtained with the same solution concentration. 25



(a) QDs-1

Cd 3d5/2

Cd 3d3/2

inner Cd

(c) QDs-1

surface Cd surface S inner Se S 2s

2+

(b) QDs-1/Ni

Se 3s

inner S

surface Se

414

412

410 408 406 Binding Energy/eV

402 230

404

229

228 227 226 225 Binding Energy/eV

224

223

Figure 3. XPS spectra and elemental analysis of Cd 3d in the absence (a) and presence (b) of Ni2+, Se 3s and S 2s (c) for QDs-1. The peak analysis of Cd 3d, Se 3s and S 2s shows well-defined inner and surface contributions. 0.3

Absorption

0.0

(b)

0.0

QDs-1 QDs-2

-0.3

-0.6

(a) QDs-1 1.1 ps 102 ps 1.02 ns 4.95 ns

-0.9 -1.2 -1.5 350

mDelta OD

-0.3 mDelta OD

Bleach

400


550

-0.6 -0.9 -1.2 -1.5

600

0

20

40 60 Time/ps

80

100

Figure 4. TA spectra of QDs-1 (a) and the bleach recoveries of QDs-1 and QDs-2 in the 100 ps timescale (b).

0 -2 mDelta OD

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

Page 26 of 29

-4 -6

2+

QDs-1/Ni 0 µM 100 µM 200 µM 300 µM

-8

-10 -12 -20

-10

0

10

100

1000

Time/ns

Figure 5. Transient bleach kinetics of QDs-1 with the added Ni2+ concentrations.

26


Page 27 of 29

(a) QDs-1/Ni

2+

2+

0 µM 50 µM 500 µM

8 4

3

(b) QDs-2/Ni

10

0 µM 50 µM 500 µM

PL Intensity/10

PL Intensity/10

4

4

2 1 5 nm

7 nm

0 410

500


6 4 2

550 600 650 700 750 6 nm

0 410

800

500


800

8

6 (c) Band-edge emission 5

(d) Trap-related emission QDs-1 QDs-2

7

QDs-1 QDs-2

6

4 τ0/τ

τ0/τ

5

3

4 3

2

2 1

1 0

500 1000 1500 2+ Concentration of Ni /µM

0

2000

500 1000 1500 2+ Concentration of Ni /µM

2000

Figure 6. PL spectral changes of QDs-1 (a) and QDs-2 (b) with the added Ni2+ concentrations. The inset of (b) is the enlarged part of trap-related emission. Graphs for the band-edge (c) and trap-related (d) emissions of QDs-1 and QDs-2 showing the change in their respective lifetimes with various Ni2+concentrations. 10

4

2.5 2.0

(b) QDs-2/AA

0 µM 25 µM 50 µM 100 µM

8

1.5 1.0 0.5 0.0 410

0 µM 25 µM 50 µM 100 µM

4

(a) QDs-1/AA

PL Intensity/10

3.0

PL Intensity/10

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


6 4 25 nm

2

550 600 650 700 750 5 nm

500

15 nm


800

0 410

8 nm

500


27


800


9 8

QDs-1 QDs-2

7

(d) Trap-related emission QDs-1 QDs-2

12

6

10

5

8

τ0/τ

τ0/τ

14

(c) Band-edge emission

4 3

6 4

2

2

1

0 0

50 100 150 200 250 Concentration of AA/µM

300

0

50

100 150 200 250 Concentration of AA/µM

300

Figure 7. PL spectral changes of QDs-1 (a) and QDs-2 (b) with the added AA. The inset of (b) is the enlarged part of trap-related emission. Graphs for the band-edge (c) and trap-related (d) emissions of the QDs-1 and QDs-2 showing the change in their respective lifetimes with various AA concentrations. 3.5

8

(a) Band-edge emission 3.0

QDs-1 QDs-2

7 6

2.5 τ0/τ

τ0/τ

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

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2.0

(b) Trap-related emission QDs-1 QDs-2

5 4 3

1.5

2 1.0

1 0.0

0.2 0.4 0.6 0.8 2+ Concentration of Ni /mM

1.0

0.0

0.2 0.4 0.6 0.8 2+ Concentration of Ni /mM

1.0

Figure 8. Graphs for the band-edge (a) and trap-related (b) emissions of QDs-1 and QDs-2 showing the change in their respective lifetimes with various Ni2+ concentrations in the presence of 50 µM AA.

Figure 9. Illustration of the photoinduced electron and hole relaxation and transfer processes in the CdSe QDs-Ni2+ photocatalytic system with AA as the sacrificial reagent. The nonradiative recombination pathways are not shown here. 28


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TOC Graphic

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Charge-Transfer Dynamics Promoted by Hole Trap States in CdSe

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