Effect of Surface Trap States on Photocatalytic Activity of

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Surface Trap States on Photocatalytic Activity of Semiconductor Quantum Dots Haoyang Zou, Chunwei Dong, Suyu Li, Chan Im, Mingxing Jin, Shiyu Yao, Tian Cui, Wenjing Tian, Yi Liu, and Hao Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01206 • 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

Effect of Surface Trap States on Photocatalytic Activity of Semiconductor Quantum Dots Haoyang Zou,b Chunwei Dong,b Suyu Li,c Chan Im,e Mingxing Jin,c Shiyu Yao*,a,b Tian Cui,a,d Wenjing Tian,b Yi Liu,b Hao Zhangb

a

College of Physics, Jilin University, Changchun 130012, PR China

b

State Key Laboratory of Supramolecular Structure and Materials, Jilin University,

Changchun 130012, P. R. China. c

Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, P. R.

China. d

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R.

China. e

Department of Chemistry, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul

05029, Korea

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ABSTRACT Semiconductor quantum dots (QDs) are promising photocatalysts for water splitting due to the large specific area, but the influence of surface trap states on the photocatalytic activity of QDs is still not fully understood yet. To answer this question, CdSe QDs with the same morphology, diameter, crystal structure and energy level are prepared following a hydrazine hydrate (N2H4) promoted synthesis strategy and conventional hydrothermal synthesis method. Through various characterizations and analysis, it is found that the conventional hydrothermal synthesized CdSe QDs (H-CdSe QDs) have a high concentration of Cd-involved shallow electron trap states, which seriously hinder the charge separation and transfer between CdSe and cocatalysts. In contrast, the N2H4 promoted synthesis strategy provides an energy-saving, low-cost, and facile pathway to eliminate the surface shallow electron traps, ensuring efficient charge separation and H2 production in CdSe QDs. As a result, the N2H4-promoted synthesized CdSe QDs (N-CdSe QDs) produce 44.5 mL (1998 µmol) H2 in 7 h, roughly 1.6 times higher than that of H-CdSe QDs (27.5 mL, 1236 µmol). Since surface trap states are widespread existed in semiconductor QDs, it is believed that our study provides valuable guidance on the design and preparation of QDs for photocatalysis.


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INTRODUCTION Since the first report on realizing artificial water splitting by photoelectrocatalysis (PEC) technology in 1972, the efficient utilization of solar energy for photocatalytic H2 production from water has been viewed as one of the appealing future strategies for solving the global energy shortage and environmental pollution issues simultaneously.1-8 Thousands of scientists have devoted tremendous efforts to explore various materials including organic dyes, metal complexes, metal oxides and conjugated polymer etc. as visible-light-driven photocatalysts over the past decades.9-14 Among these materials, semiconductor quantum dots (QDs) exhibit great potentials and promising performances in harvesting light for H2 production due to their large extinction coefficients over a broad spectral range and high surface-to-volume ratios.15-29 Despite enormous progresses have been achieved, the H2 production efficiencies of QDs are still limited. One of the main challenges lies in the charge separation and subsequent transfer to photocatalytic active sites for participating the photoreduction reactions. Because of the sizes located in nanoscale, QDs always possess a huge number of surface atoms compared with their bulk counterpart. These surface atoms usually induce the formation of various trap states on the surface of QDs.30-31 Although previous works have declared that the surface traps have influences on the separation, transfer, and recombination processes of photogenerated electrons and holes, the influence result (positive/negative) and the influence mechanism on the photocatalytic activity of QDs are still ambiguous and contradictory. Many people believed that the surface traps improve the separation efficiency 3 ACS Paragon Plus Environment


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of photogenerated electrons and holes in QDs, enhancing their photocatalytic efficiency.32-33 However, other people thought that the photoelectron conversion efficiency of QDs reduced by both the radiative and nonradiative recombination due to the existence of abundant surface traps.34-38 Therefore, understanding the correlation between the surface trap states and charge separation and transfer processes is crucial for improving the photocatalytic activity of QDs. Thanks to the strong reducibility of hydrazine hydrate (N2H4), which can efficiently prevent the oxidization of reactants and surface ligands, the N2H4-promoted growth of aqueous CdSe QDs at room temperature is an energy-saving, low-cost, and facile pathway to synthesize CdSe QDs with high quality.39-42 The comparison between N2H4-promoted synthesized CdSe QDs (N-CdSe QDs) and conventional hydrothermal synthesized CdSe QDs (H-CdSe QDs) offers us an opportunity to deeply understand the influence of surface trap states on the photocatalytic activity of QDs.43, 44 In this article, we firstly synthesized N-CdSe and H-CdSe QDs with the same size, morphology, crystal phase and energy level. Then, we evaluated the photocatalytic activities of N-CdSe and H-CdSe QDs. N-CdSe QDs exhibited a 1.6 times higher photocatalytic activity than that of H-CdSe QDs. Finally, through analyzing the surface trap states of N-CdSe and H-CdSe QDs by steady-state and transient-state photoluminescence (PL) spectra and femtosecond transient absorption (TA) spectra, we found that the shallow electron trap states on the surface of CdSe QDs seriously impede the charge separation and transfer between CdSe QDs and cocatalyst, thus hinder the H2 production. Since the dangling bonds are widespread existed on the surface of QDs, our study provides valuable guidance on the design and preparation of QDs for photocatalysis. 4 ACS Paragon Plus Environment

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EXPERIMENTAL SECTION Chemicals. Cadmium chloride hydrate (CdCl2 2.5H2O, 99%), selenium powder (-100 mesh, 99.5+%), 3-mercaptopropionic acid (MPA, 99%), Sodium borohydride (NaBH4, >96%) and nickel chloride hexahydrate (NiCl2 6H2O, 99%) were purchased from Sigma Aldrich. Sodium hydroxide (NaOH, 99%) and hydrazine hydrate (N2H4 H2O, 85%) were obtained from Aladdin. All these chemicals were used as received. Synthesis of CdSe QDs. Aqueous CdSe precursors were obtained by injecting 0.6ml NaHSe solution (1M) to 150ml N2 saturated CdCl2 and MPA mixed solution at pH 10. The concentration of Cd2+ in the precursor solution was 20 mM with 1:1.5:0.2 molar ratio of Cd2+/MPA/HSe-. For the hydrothermal method, the precursor solution was refluxed at 100 oC for 8 h. For the N2H4 promoted method, the precursor solution was mixed with the same amount of N2H4 H2O and stored for 8 h. After that, the CdSe QDs solution was centrifuged for 10 min at 6000 rpm in the presence of isopropyl alcohol. The obtained CdSe QDs were washed with alcohol and centrifuged in the same condition. The washing process was repeated twice. Then the CdSe QDs were dried in the vacuum oven at room temperature over night. Finally the CdSe QDs were dissolved in deionized water. Photocatalytic H2 production. CdSe QDs and NiCl2 were dissolved in 25 mL deionized water with concentrations of 1.03× 10-5 M and 2.1× 10-4 M. 25 mL of isopropyl alcohol was added into the above solution with vigorous stirring. And then the pH was adjusted to 13. After that, the solution was degassed by evacuating for 20 min and then irradiated under a 300W Xe lamp (CEAULIGHT, CEL-HXF300) with a glass filter to cut off light shorter than 5 ACS Paragon Plus Environment


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400 nm. The generated H2 was measured by a thermal conductivity detector (TCD) in a gas chromatograph (CEAULIGHT, GC7290). Characterization.

Transmission

electron

microscopy

(TEM)

and

high-resolution

transmission electron microscopy (HRTEM) were conducted by a JEM-2100F electron microscope. (X-ray diffraction) XRD patterns were obtained by a PAN alytical B.V. Empyrean diffractometer. Ultraviolet–visible (UV-Vis) absorption spectra were carried out by a Lambda 800 UV-vis spectrophotometer. Steady-state photoluminescence (PL) spectra were implemented with a Shimadzu RF-5301 PC spectrophotometer. Transient-state PL spectra were conducted by an Edinburgh FLS980 Spectrometer. Ultraviolet photoelectron spectroscopy (UPS) measurement was investigated by a PREVAC UPS System. X-ray photoelectron spectroscopy (XPS) was performed by using a VG ESCALAB MKII spectrometer. The composition of CdSe QDs was obtained by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, OPTIMA 3300DV, Perkin Elmer). Femtosecond transient absorption (TA) spectra were measured by using a pump-probe system (Libra-USP-HE, Coherent Libra; AvaSpec-1650F-USB2, AVANTES). RESULTS AND DISCUSSION N-CdSe and H-CdSe QDs are synthesized following the strategies reported by the previous works.42-44 As shown in Figure 1a and 1b, both N-CdSe and H-CdSe QDs are spherical with the average diameter of 2.8 nm. X-ray diffraction (XRD) patterns of N-CdSe and H-CdSe QDs exhibit three diffraction peaks at 25.32º, 42.10º, and 49.74º, corresponding to the (111), (220), and (311) crystallographic facets of CdSe with zinc blende structure (JCPDS card No. 6 ACS Paragon Plus Environment

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19-0191) (Figure 1c). High-resolution transmission electron microscopy (HRTEM) images present the lattice spacings of 2.15 and 1.85 Å, consisting with the (220) plane and (311) plane of zinc blende CdSe QDs (insets in Figure 1a and 1b). Ultraviolet–visible (UV-vis) absorption spectra are presented in Figure 1d. N-CdSe QDs possess a sharper exciton peak (1S) than that of H-CdSe QDs, implying the presence of N2H4 greatly weakens the interparticle electrostatic repulsion, thus facilitating the kinetic growth of QDs with uniform size distribution.39, 45 Moreover, the band gap of N-CdSe QDs is estimated to be 2.49 eV according to the plot of (αhν)2 versus photon energy, which is 0.01eV red shifted compared with the band gap of H-CdSe QDs (2.48 eV). The surface stoichiometry of CdSe QDs is characterized by X-ray photoelectron spectroscopy (XPS). As shown in Table S1 and Figure S1, the surface Cd/Se ratio of N-CdSe and H-CdSe QDs are 0.92 and 1.32. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) analysis exhibits the same result: the Cd/Se ratio in N-CdSe QDs is lower than that in H-CdSe QDs (Table S2). This difference can be ascribed as follow: As to hydrothermal synthesis, the coordination interaction between Cd2+ and mercapto-ligands favors the formation of CdSe QDs with Cd-rich surface. In contrast, the presence of N2H4 efficiently prevents the oxidization of HSe- and mercapto-ligands due to their strong reducibility, promoting the growth of CdSe QDs with Se-rich surface.39,

41

Ultraviolet

photoelectron spectroscopy (UPS) measurement is performed to determine the band structure position of CdSe QDs (Figure S2). The valence (VB) and conduction band (CB) of N-CdSe and H-CdSe QDs are -3.39/-5.88 eV and -3.42/-5.90eV respectively. From the viewpoint of 7 ACS Paragon Plus Environment


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energy level alignment, both N-CdSe and H-CdSe QDs have the nearly same and suitable electronic band energies relative to normal hydrogen electrode (NHE) to realize the evolution of H2. Photocatalytic H2 production performances of N-CdSe and H-CdSe QDs are evaluated under visible-light irradiation. The H2 evolution rates of N-CdSe and H-CdSe QDs are measured in isopropyl alcohol aqueous solution (IPA/H2O, 1:1(v/v)) with the presence of NiCl2 formed Ni(OH)2 as cocatalyst (Figure S3).27, 46 During a long period of 7 h, a total volume of 44.5 mL (1998 µmol) H2 are generated by using N-CdSe QDs as the photocatalyst, which is roughly 1.6 times higher than using H-CdSe QDs instead (27.5 mL, 1236 µmol) (Figure 2). Because the sizes, morphologies, crystal structures and energy levels of N-CdSe and H-CdSe QDs are almost the same, it is reasonable to speculate that the distinct difference in photocatalytic activities between N-CdSe and H-CdSe QDs may derive from their various surface states. Based on previous reports, the microscopic picture describing the nature of QDs’ surface states has been achieved.47-52 Generally, the surface states of CdSe QDs can be classified into two species: On the one hand, the unpassivated Se sites on the surface of CdSe QDs can cause the formation of deep hole trap states30, 38, 53-55 with the fast emission decay channel.56 On the other hand, the surface Cd ions on CdSe QDs can serve as the shallow electron trap states31, 57, 58 with relatively long PL lifetime.56 In addition, the bandgap between CB and deep hole trap states is usually larger than that between CB and shallow electron trap states. Based on the theory above, the surface states of N-CdSe and H-CdSe QDs are analyzed by 8 ACS Paragon Plus Environment

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steady-state and transient-state PL spectra. As shown in Figure 3, the steady-state PL spectrum of N-CdSe QDs exhibits a broad emission from 480 to 700 nm, which can be fitted into three peaks at 522, 568, and 629 nm. Correspondingly, three decay channels with lifetimes of 2, 13.67, and 87.83 ns are calculated by fitting the PL decay curve with multiexponential equation (Figure S4 and Table S3). Based on the emission wavelength and the lifetime, it is inferred that the PL peak at 522 nm belongs to the intrinsic emission of CdSe QDs. While the emissions at 568 and 629 nm derive from the radiative transition from shallow electron trap states and deep hole trap states to VB respectively. Similar to N-CdSe QDs, the PL spectrum of H-CdSe QDs also comprises three emissions with different lifetimes: the PL peaks at 530, 578, and 637 nm with lifetimes of 28.2, 153.7, and 4.6 ns are assigned to the intrinsic emission, shallow electron trap related emission, and deep hole trap related emission. By comparing the areas of PL peaks and percentages of lifetimes, it is noted that the intrinsic emission/trap emission ratio of N-CdSe QDs is lower than that of H-CdSe QDs, implying that the presence of N2H4 can suppress the formation of trap states. Furthermore, the contribution of shallow electron trap related emission for H-CdSe QDs is much greater than that for N-CdSe QDs, indicating the percentage of shallow electron traps in H-CdSe QDs is higher than that in N-CdSe QDs. The results above indicate that despite possessing the same categories of surface states, the percentage of various surface states in N-CdSe and H-CdSe QDs are totally different, and this difference may determine the photocatalytic activity of CdSe QDs.



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In order to verify our hypothesis, TA spectroscopy is employed to investigate the separation, transfer, and recombination processes of photogenerated charges in CdSe QDs. As shown in Figure 4a and 4b, the TA spectra of N-CdSe and H-CdSe QDs show strong negative exaction photobleaching (PB) feature at 480 nm, corresponding to the intrinsic S0-S1 transitions of CdSe. Additionally, both N-CdSe and H-CdSe QDs have weak photoinduced absorption (PA) features at lower probe energies nearby the PB feature (Figure 4c). Based on previous reports, these PA features mainly arise from the excitonic and biexcitonic processes due to the existence of surface trap states, and these surface trap states are identified as the deep hole trap states.51, 59-64 However, different from N-CdSe QDs, H-CdSe QDs have an additional broad PA feature at 570 nm (Figure 4d). Since the intrinsic excited-state absorption features of CdSe QDs locate in the near-infrared/mid-infrared spectral ranges,65 the PA feature in the visible region (570 nm) should be the absorption of electrons in shallow electron traps. The TA kinetics associated with PA features of CdSe QDs are summarized in Figure S5. The analysis on the PB recovery by multiexponential fit further proves the existence of various surface states in CdSe QDs. As shown in Figure 5a, the recovery curve of N-CdSe QDs can be fitted by a biexponential equation, which demonstrates the presence of another state, corresponding to the deep hole trap state with PA feature at 505 nm, between CB and VB. Nevertheless, a triexponential equation is needed to fit the recovery curve of H-CdSe QDs (Figure 5b). Thus, there are two additional states existed between CB and VB of H-CdSe QDs, consisting with the appearance of PA features at 515 and 570 nm related to the deep hole trap state and shallow electron trap state. Interestingly, the shallow electron trap 10 ACS Paragon Plus Environment

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states are not observed in the TA spectrum of N-CdSe QDs. Associated with the areas of PL peaks and percentages of lifetimes in Figure 3 and Table S3, it may be attributed to the lower percentage of shallow electron trap states in N-CdSe QDs compared with H-CdSe QDs. The analysis results from TA spectra not only confirm the difference in surface states between N-CdSe and H-CdSe QDs, but also imply the negative effect of shallow electron trap states on the photocatalytic activity of CdSe QDs. To further clarify the effects of various surface states on the photocatalytic activity of CdSe QDs, the electron transfer dynamics from CdSe QDs to the cocatalyst Ni(OH)2 are investigated. Figure 5c shows the decay curve of N-CdSe QDs’ exaction PB feature at 475 nm with and without Ni(OH)2. Compared with pure N-CdSe QDs, the presence of Ni(OH)2 strongly accelerates the bleaching recovery. Nearly 53% of bleaching recovery is achieved within 2 ns for Ni(OH)2 absorbed N-CdSe QDs, which is much higher than that of N-CdSe QDs without Ni(OH)2. The fast recovery rate indicates the efficient electron transfer from CdSe QDs to Ni(OH)2, which is beneficial to the separation between photogenerated electrons and holes, ensuring the high photocatalytic activity of CdSe QDs. In contrast, H-CdSe QDs possess completely different electron transfer dynamics from CdSe QDs to Ni(OH)2 (Figure 5d). The adsorption of Ni(OH)2 does not increase the bleaching recovery rate of H-CdSe QDs but decrease it. This result indicates the presence of shallow electron trap states undoubtedly hinder the electron transfer from CdSe QDs to Ni(OH)2, largely suppressing the photocatalytic reaction on the surface of H-CdSe QDs.



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As is well-known, a typical H2 production reaction of QDs usually undergoes the following primary processes: (1) photoabsorption and electron excitation; (2) charge separation and transfer; (3) H2O reduction and H2 production. In order to gain a clear insight into the influence of surface states on photocatalytic activity of CdSe QDs, energy band diagrams and photocatalytic processes are illustrated in Figure 6. In the case of N-CdSe QDs, the electrons are excited to CB under irradiation, leaving a positive charged hole in VB. Because of the low percentage of shallow electron trap states in N-CdSe QDs, the photogenerated electrons can transfer to Ni(OH)2 to participate the reduction of H2O spontaneously. In contrast, the photocatalytic process in H-CdSe QDs is quite different. Many photogenerated electrons are trapped by the shallow electron traps, which seriously hinder the electron transfer and subsequent reduction of H2O. The amount of Ni adsorbed on CdSe QDs after photocatalysis further support this perspective. Under light irradiation, the consumption of electrons on reducing H2O will increase the concentration of OH- around CdSe QDs. While the high concentration of OH- can facilitate the deposition of Ni2+ on CdSe QDs in the form of Ni(OH)2.66 From the results of XPS, it is clear to see that the percentage of Ni on N-CdSe QDs is much higher than that on H-CdSe QDs after photocatalysis, which strongly indicates that there are more electrons transfer to the surface of N-CdSe QDs to participate the photocatalystic reaction, while lots of electrons are trapped by the shallow electron traps in H-CdSe QDs (Table S4). CONCLUSION


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In summary, we investigate the effect of surface trap states on the photocatalytic activity of CdSe QDs. CdSe QDs with the same spherical morphology, diameter of 2.8 nm, and zinc blende structure are synthesized through N2H4-promoted and conventional hydrothermal methods. Based on the steady-state, transient-state PL spectra and TA spectra characterizations, two kinds of surface trap states, including shallow electron trap states and deep hole trap states, are discovered in both N-CdSe and H-CdSe QDs. The shallow electron trap states, trapping the photogenerated electrons and hindering their transfer to participate the photocatalytic reactions, play a negative effect on the photocatalytic activity of CdSe QDs. Due to the strong reducibility, the presence of N2H4 can efficiently suppress the formation of shallow electron trap states. Therefore, N-CdSe QDs exhibit a H2 production volume of 44.5 mL (1998 µmol) in 7 h, which is 1.6 times higher than that of H-CdSe QDs. The results suggest that the N2H4-promoted method is an energy-saving, low-cost, and facile synthesis method to improve the photocatalytic activity of QDs. Our study not only discloses the relationship between surface trap states and photocatalytic activity of QDs, but also provides valuable guidance on the design and preparation of QDs for photocatalysis.



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Figure 1. TEM and HRTEM images of (a) N-CdSe and (b) H-CdSe QDs. (c) XRD patterns exhibit zinc blende structure of N-CdSe and H-CdSe QDs. (d) UV-vis absorption spectra of N-CdSe and H-CdSe QDs.


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Figure 2. Photocatalytic H2 production of N-CdSe and H-CdSe QDs in the solution of IPA and water (IPA/H2O, 1:1(v/v)) at pH 13 under visible light irradiation for 7 h. This image cannot currently be display ed.



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Figure 3. PL spectra analysis of (a) N-CdSe and (b) H-CdSe QDs. Black solid lines represent the raw PL data, red dash lines represent the fit curve of PL data. Fit peaks for different states are individually displayed in blue dash-dot lines, solid lines and short dash lines.


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Figure 4. TA spectra of (a) N-CdSe and (b) H-CdSe QDs in aqueous solution at indicated delay times with 400 nm excitation. (c,d) Magnified TA spectra of photoinduced absorption features observed in panel a, b.



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Figure 5. TA kinetics associated with bleaching features of (a) N-CdSe and (b) H-CdSe QDs in aqueous solution (black dots) with the corresponding multi-exponential fitting curves (red solid lines). Comparison of TA kinetics associated with bleaching features of (c) N-CdSe and (d) H-CdSe QDs with and without Ni(OH)2.


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Figure 6. Energy band diagrams and photocatalytic mechanisms of (a) N-CdSe and (b) H-CdSe QDs with Ni(OH)2 as cocatalysts for H2 production.



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ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge. Supplemental data for material characterization (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail for S.Y.: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the China Postdoctoral Science Foundation (Grant No. 2017M611312), the National Key Research and Development Program of China (No. 2016YFB0401701), NSFC (Nos. 21773088, 51425303), JLU Science and Technology Innovative Research Team (2017TD-06) and the Special Project from MOST of China. We would like to thank Dr. Yuchao Hu and Dr. Yajun Gao for their expert advices.

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Effect of Surface Trap States on Photocatalytic Activity of

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