Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered

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C: Energy Conversion and Storage; Energy and Charge Transport

Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered Layerby-Layer Assembly Toward Multifarious Photoredox Organic Transformation Ming-Hui Huang, Xiao-Cheng Dai, Tao Li, Yu-Bing Li, Yun-Hui He, Guangcan Xiao, and Fang-Xing Xiao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered Layer-by-Layer Assembly Toward Multifarious Photoredox Organic Transformation Ming-Hui Huang, a Xiao-Cheng Dai, a Tao Li, a Yu-Bing Li, a Yunhui He,b Guangcan Xiao,b Fang-Xing Xiao* a a. College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China. b. Instrumental Measurement and Analysis Center, Fuzhou University, Fuzhou, 350002, People’s Republic of China. Email: [email protected]

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Abstract Semiconductor quantum dots (QDs) have garnered tremendous attention by virtue of their substantial light harvesting and conversion efficiency, large number of active sites, unique quantum size confinement and multiple exciton generation effects. In this regard, recent years have witnessed their wide-spread applications in photocatalysis. Nonetheless, intrinsic disadvantages of QDs including unfavorable photostability, ultra-fast charge recombination rate, and sluggish kinetic of charge carriers retard the construction of high-efficiency QDs-based photocatalysts for solar energy conversion. Thus far, in-depth investigation on the visible-light-driven photo-redox organic transformation over QDs has not yet been exhaustively explored and corresponding photocatalytic mechanisms remain elusive. In this work, selecting cadmium selenide (CdSe) as a quintessential category of semiconductor QDs, we demonstrated a facile, green, easily accessible and rather efficient electrostatic self-assembly strategy to conspicuously boost the versatile photoredox performances of CdSe QDs toward selective organic transformation under visible light irradiation by intimately integrating with graphene (GR) via judicious surface charge tuning. In this scenario, intrinsically negatively charged CdSe QDs and surface-modified positively charged GR were utilized as the building blocks for spontaneous electrostatic self-assembly buildup, which gives rise to well-defined CdSe QDs-GR ensembles. More intriguingly, ligands capped on the CdSe QDs surface enable the alternate layer-by-layer (LbL) assembly of CdSe QDs and GR forming three-dimensional (3D) spatially multilayered heterostructures. Furthermore, it was significant to unveil that such self-assembled CdSe QDs-GR nanocomposites exhibit remarkedly enhanced and multifunctional photoredox performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and selective reduction of nitroaromatics to amino compounds under visible light irradiation, which far exceeds pristine CdSe QDs counterpart which exhibits almost negligible photoactivities. This can be ascribed to the pivotal role of GR for conspicuously capturing and shuttling electrons from band-gapphotoexcitation of CdSe QDs, intimate interfacial contact between the building blocks, enlarged specific surface area stemming from seamless GR encapsulation and intercalation, along with the unique ligandtriggered LbL assembly integration mode between CdSe QDs and GR, hence synergistically reducing the recombination rate and prolonging the lifetime of charge carriers. Furthermore, photoredox mechanisms of CdSe QDs-GR ensemble were elucidated. It is anticipated that our work would afford an efficacious avenue to finely modulate the charge transport over QDs for solar energy conversion.


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1. Introduction Solar energy has been attracting enormous attention in the past few decades upon the outbreak of global energy crisis.1–6 Among various solar-related techniques, photocatalysis has been deemed as a promising alternative to maximize the utilization of solar energy since the first report of photoelectrochemical (PEC) water splitting by Fujishima and Honda in 1972.7 From then on, a large variety of semiconductors have been explored for photocatalysis in a myriad of fields including water splitting, carbon dioxide reduction, selective organic transformation and environmental remediation.8–13 Despite the endeavors made in the past few decades, seeking for ideal semiconductors with favorable bandgap and solar conversion efficiency is still far from satisfactory. Among the diverse photocatalysts currently being explored for photocatalysis, of particular note is semiconductor-based quantum dots (QDs),14 which are defined as the nanocrystals with size twice less than the Bohr radius of excitons in the bulk materials.15 Due to ultra-small size, QDs are featured by many merits in comparison with bulk counterparts, such as quantum size confinement effect,16 multiple exciton generation effect, and tunable bandgap with size,17 thus making them promising alternatives for widespread opto-electronic applications, e.g., light-emitting diodes,18 laser diodes,19 photovoltaics,20 and biological labeling.21 Particularly, QDs consisting of elements from II-VI groups demonstrate great potential in photocatalysis, among which CdSe QDs have triggered a renaissance of great interest by virtue of size-dependent physicochemical properties. Recent years have witnessed a cornucopia toward the fabrication of CdSe QDs-based photocatalysts by integrating with second semiconductors (e.g., TiO2, ZnS and CdS) or noble metals (e.g., Au, Ag, Pd and Pt) for constructing hybrid photocatalysts.16 In spite of the advancements, development of high-efficiency CdSe QDs-based photocatalysts is retarded by the unfavorable stability of CdSe QDs when it was exposed to long-time light irradiation or air due to high surface energy, thereby resulting in large agglomeration of QDs and thus remarkably reduced specific surface area and number of reactive sites, which accelerates considerable deactivation in photoactivities.22–29 Moreover, fast recombination of photoinduced charge carriers over CdSe QDs need to be solved to maximumly prolong the lifetime of charge carriers for various photocatalytic reactions. To 3 ACS Paragon Plus Environment


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solve this dilemma, multifarious strategies have been explored including nanostructure engineering, elemental/non-elemental doping, or constructing heterostructures with other semiconductors with suitable band alignment.30–39 In particular, integration of metal chalcogenides (MCs) QDs with carbon materials has been deemed to be an efficacious route to surmount the drawback of MCs QDs given the merits of carbon materials especially for capturing and shuttling electrons.39,40 Thus far, various gorgeous carbon materials have been investigated including carbon nanotubes, graphene quantum dots, and graphene (GR) etc. Noteworthily, GR, as an allotrope of carbon composed of layers of carbon atoms tightly packed into a 2D honeycomb network, has been well-established as an excellent nanoscale building block for developing carbon/semiconductor photocatalysts considering the virtues of GR for boosting light absorption, increasing specific surface area, and prolonging the lifetime of photoinduced charge carriers by smoothly directing photoelectrons to the active sites.13,41–46 In this regard, it is expected that rational integration of CdSe QDs with GR in an appropriate fashion would benefit monodisperse distribution of CdSe QDs and simultaneously, help to modulate the directional charge separation/transfer for playing their cooperativity. Up to date, synergistic interaction of CdSe QDs and GR along with the corresponding photocatalytic mechanism in triggering the selective photoredox organic transformation has not yet been probed.47,48 Furthermore, photoredox catalysis occurring over CdSe QDs-GR ensembles has seldomly been systematically investigated. Besides, it should be stressed that construction of CdSe QDs-GR nanocomposites in previous works are primarily focused on the conventional methods such as hydrothermal/solvothermal methods, chemical precipitation or other wetchemistry routes which normally involve relatively complex synthetic procedures or harsh experimental conditions. Till now, there is still lack of a facile, green and easily accessible synthetic strategy to achieve rapid and intimate integration of CdSe QDs and GR under ambient conditions by merely tuning their surface charge properties in fabricating CdSe QDs-GR nanocomposites. Herein, a high-efficiency charge transfer channel was judiciously constructed over 3D CdSe QDs-GR nanocomposites by closely integrating CdSe QDs with GR nanosheets via a rather simple, green and layerby-layer (LbL) self-assembly strategy based on the pronounced electrostatic interaction, in which tailor4 ACS Paragon Plus Environment

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made negatively charged CdSe QDs and surface modified positively charged graphene oxide (GO+) precursor were utilized as the building blocks. Attracted by the substantial electrostatic attractive force, negatively charged CdSe QDs were spontaneously and uniformly assembled on the positively charged GR framework with intimate interfacial contact. Furthermore, it was unveiled that spatial spontaneous LbL assembly between CdSe QDs and GR was initiated giving rise to spatially multilayered CdSe QDs-GR ensembles. More intriguingly, such self-assembled 3D CdSe QDs-GR nanocomposites demonstrated versatile and markedly enhanced photo-oxidation performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and photoreduction performances toward selective reduction of nitroaromatics to corresponding amino aromatics under visible light irradiation at ambient conditions, substantially exceeding pristine CdSe QDs counterpart which exhibited negligible photoredox activities under the same conditions. The origins accounting for the outstanding photoredox activities of CdSe QDs-GR nanocomposites are attributed to the construction of ultra-fast electron transfer channel endowed by seamless GR encapsulation that favors substantial light absorption, efficient charge separation and transport. Finally, photocatalytic mechanism of CdSe QDs-GR nanocomposites in versatile selective photoredox organic transformation was elucidated.

2. Experimental section 2.1 Materials Cadmium chloride (CdCl2·2.5H2O), sodium borohydride (NaBH4), sodium hydroxide (NaOH), mercaptoacetic acid (MAA), ethylenediamine (C2H8N2), ethanol (C2H6O) and graphite powder were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Selenium (Se) powder, 4nitroaniline (4-NA), 3-nitroaniline (3-NA), 2-nitroaniline (2-NA), 1-bromo-4-nitrobenzene, 4-nitrophenol (4-NP), 3-nitrophenol (3-NP), 2-nitrophenol (2-NP), 4-nitrotoluene, 4-nitroanisole and nitrobenzene were obtained from Aladdin Industrial Corporation (Shanghai, China). N-ethyl-N’-(3-dimethylaminopropyl) carbodiimide methiodide (EDC, 98%), Deionized water (DI H2O, Millipore, 18.2 MΩ·cm resistivity), benzyl alcohol (BA), p-methylbenzyl alcohol (p-MBA), p-methoxybenzyl alcohol (p-MOBA), pfluorobenzyl alcohol (p-FBA), p-chlorobenzyl alcohol (p-CBA), p-nitrobenzyl alcohol (p-NBA), 5 ACS Paragon Plus Environment


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cinnamyl alcohol (CA), and 3-methyl-2-buten-1-ol were obtained from Alfa Aesar (China) Chemicals Co., Ltd. All the materials above were used directly without further purification. 2.2 Preparation of negatively charged CdSe QDs CdSe QDs were synthesized by an aqueous synthesis method reported previously with some modifications.28,29 Specifically, 2 mmol of CdCl2·2.5H2O was dissolved in 200 mL of DI H2O and deaerated with N2 bubbling in a three-necked flask for 1 h. After that, 230 μL of MAA as a stabilizer was added into the above solution and pH value of the mixture was carefully adjusted to 11 with 1 M NaOH aqueous solution. Subsequently, oxygen-free NaHSe aqueous solution was prepared by dissolving 0.6320 g of NaBH4 in 10 mL of DI H2O under N2 bubbling, into which 0.2106 g of Se powder was added and stirred at a low speed for 2 h in an ice bath. 5 mL of the freshly prepared NaHSe aqueous solution was then quickly injected into Cd2+ aqueous solution under vigorous stirring and an orange-red solution was obtained and then it was refluxed in 333 K for 4 h. After cooling to room temperature, CdSe QDs aqueous solution was precipitated by adding into equal volume of ethanol with vigorous stirring and the precipitate was separated by centrifugation and dried in vacuum at 313 K. 2.3 Preparation of positively charged graphene oxide (GO) Pristine negatively charged GO nanosheet was synthesized by a modified Hummer’s method and detailed procedures were provided in Supporting Information (Appendix 1). Preparation of positively charged GO+ was referred to a previously published work.49 Specifically, 50 mg of GO was dissolved in 100 mL of DI H2O with ultrasonication for several hours. Then, 0.625 g of EDC and 5 mL of ethylenediamine were added into 50 mL of GO aqueous suspension, followed by uninterruptedly stirring at ambient conditions for 12 h. The resulting suspension was dialyzed (MWCO, 12-14KD, Spectra/Por) for 3 days to remove the residual EDC and ethylenediamine. 2.4 Self-assembly of CdSe QDs-GR nanocomposites Different volumes (3, 15, 30, and 45 mL) of positively charged EDC-modified GO (0.2 mg/mL) aqueous solution corresponding to weight percentage of 1, 5, 10, 15 % in the CdSe QDs-GR nanocomposites were 6 ACS Paragon Plus Environment

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gradually added into 10 mL of negatively charged CdSe QDs (6 mg/mL) aqueous solution under vigorous stirring for 1 h forming precipitate which was centrifuged and transferred to a Teflon-lined stainless-steel autoclave (50 mL) containing 15 mL of DI H2O and 5 mL of ethanol. Hydrothermal treatment was then conducted at 373 K for 12 h and the samples were washed with ethanol and DI H2O and finally dried in vacuum at 313 K. 2.5 Characterization Zeta potential (ξ) measurements were performed by dynamic light scattering analysis (ZetasizerNano ZS90). Crystal structure was studied by X-ray diﬀraction (XRD, X’Pert Pro MPD, Philips, Holland) using Cu Kα as the radiation source under 40 kV and 40 mA. Morphologies of the samples were probed by field-emission scanning electron microscopy equipped with an energy-dispersive spectroscopy (FESEM, EDX, Philips XL-30, Philips, Holland). Transmission electron microscopy (TEM) and high-resolution (HR) TEM, EDX images were collected on a JEOL-2010 with an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were recorded on a TJ270-30A infrared spectrophotometer (Tianjin, China). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a photoelectron spectrometer (ESCALAB 250, Thermo Scientific, America), for which binding energy (B.E.) of the elements was calibrated by the B.E. of carbon (284.60 eV). UV-vis diﬀuse reflectance spectra (DRS) (Varian Cary 500 UV-vis spectrophotometer, Varian, America) were obtained using BaSO4 as the reflectance background ranging from 250 to 800 nm. Brunauer-Emmett-Teller (BET) specific surface areas were determined on a Quantachrome Autosorb-1-C-TCD automated gas sorption analyzer. Photoluminescence (PL) spectra were collected on a Varian Cary Eclipse spectrometer. Raman measurement was carried out on a Raman spectroscopy (Dxr-2xi, Thermo Scientific, America) with scans taken on an extended range from 0 to 3000 cm-1. 2.6 Photoelectrochemical (PEC) measurements PEC measurements were carried out on an electrochemical workstation (CHI660E workstation, CHI Shanghai, Inc.) with conventional three-electrode configuration using Pt foil & Ag/AgCl electrode as 7 ACS Paragon Plus Environment


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counter & reference electrode and 0.5 M Na2SO4 aqueous solution (pH=6.69) as the electrolyte. The working electrode was prepared on fluorine-dope tin oxide (FTO) glass that was cleaned by sonication in ethanol for 30 min and dried at 313 K. The boundary of FTO glass was protected using Scotch tape. 5 mg of the sample was dispersed in 0.5 mL of ethanol by sonication to get slurry which was spread onto the pretreated FTO glass. After air drying, the Scotch tape was unstuck and the uncoated part of the electrode was isolated with nail polish. The exposed area of the working electrode was 1 cm2. Finally, the working electrode was vertically dipped into the electrolyte and irradiated with visible light (λ>420 nm) (PLS-SXE 300D, Beijing Perfect Light Co. Ltd, China). The working electrodes were irradiated by visible light from a 300 W Xe arc lamp (PLS-SXE300D, Beijing Perfectlight Co. Ltd, China) equipped with a cut-off filter (λ>420 nm). Potentials of the electrode were calibrated against the reversible hydrogen electrode (RHE) based on the following formula: ERHE=EAg/AgCl+0.059pH+E°Ag/AgCl, with E°Ag/AgCl =0.1976V at 25℃ Transient photocurrent response (i.e., I-t) was collected under visible light irradiation at a bias of 1.23 V vs. RHE. Electrochemical impedance spectra (EIS) were measured on an IM6 electrochemical station (Interface 1000E, Gamry, American) with an amplitude of 10 mV in the frequency range of 105 kHz to 0.1 Hz in a K3[Fe(CN)6] aqueous solution (5 mM, pH=7). 2.7 Selective photocatalytic oxidation performances Selective photocatalytic oxidation of aromatic alcohols to corresponding aldehydes was carried out as follows: a mixture consisting of 8 mg of catalyst, 0.1 mmol of alcohol and 1.5 mL of BTF saturated with molecular oxygen was transferred into a 10 mL Pyrex glass bottle. After stirring for 1 h in the dark to obtain an evenly-dispersed suspension with adsorption-desorption equilibrium, the glass bottle was then irradiated by a 300W Xe arc lamp (PLS-SXE300D, Beijing Perfectlight Co. Ltd, China) for a certain time with a UV-CUT filter (λ>420 nm) as the visible light source. After reaction, the mixture solution was centrifuged at 12000 rpm to remove the catalyst and the supernatant was analyzed with a gas chromatograph (SHIMADZU GC-2014C). Conversion of alcohols, yield of aldehydes, and selectivity of 8 ACS Paragon Plus Environment

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the oxidation reaction were calculated by the following formulas(1-3):43 Conversion (%) =

[

Yield (%) = Selectivity (%) =

] × 100%

C0 - Calcohol C0 Caldehyde C0

× 100%

Caldehyde C0 - Calcohol

× 100%

(1)

(2) (3)

where C0 is the initial concentration of alcohols, Calcohol and Caldehyde are the concentration of alcohols and aldehydes after visible light irradiation. 2.8 Selective photocatalytic reduction performances Selective photocatalytic reduction of nitroaromatics to corresponding amides was carried out as follows: 10 mg of catalyst and 40 mg of Na2SO3 were added into 30 mL of nitroaromatics aqueous solution (5 ppm), which is saturated with N2 bubbling at ambient conditions. After vigorously stirring in dark for 1 h to reach the adsorption-desorption equilibrium, the glass vessel was irradiated by visible light (λ>420 nm). 3 mL of the solution was taken out at a given time interval (2 min) and centrifuged at 12000 rpm for 10 min to completely remove the catalysts and the supernatant was analyzed by a Varian Cary 50 UV-vis spectrophotometer. Photoactivity of the sample was defined by the following formula (4), Conversion (%) =

C0 - C C0

× 100%

(4)

where C0 represents the initial concentration of nitroaromatics and C is the concentration after a certain time visible light irradiation.

3. Results and discussion



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Scheme 1. Schematic flowchart for fabrication of CdSe QDs-GR nanocomposites. The flowchart for fabricating CdSe QDs-GR nanocomposites was illustrated in Scheme 1. More specifically, positively charged GO (GO+) was prepared by anchoring amine groups (-NH2) on the surface of negatively charged GO (GO-) via an EDC-mediated reaction between the carboxylic groups (and/or epoxides) on the planar surface of GO and ethylenediamine (Scheme S1). As reflected by Fig. S1, substantial positively charged surface of GO+ was verified by zeta potential (Fig. S1b) result within a wide pH profile (2~12) and this indicates surface charge property of GO- precursor has been successfully reversed from negatively charged to positively charged through a facile wet-chemistry method. Moreover, UV-absorption (Fig. S1a) spectrum reveals that GO+ was partially reduced to GR after EDC modification and the result is in line with FTIR results (Fig. S3) of GO- and GO+. Detailed characterizations of pristine GO- precursor were provided in Fig. S2. Noteworthily, tailor-made MAA-capped CdSe QDs are featured by ultra-small size (ca. 4 nm, Fig. S4a) with negatively charged surface owing to the deprotonation of carboxyl groups (-COOH) capped on the surface and this can be evidenced by zeta potential result (Fig. S4b). As a result, oppositely charged surfaces of CdSe QDs and GO+ colloidal suspension renders them suitable building blocks for electrostatic self-assembly, wherein negatively charged CdSe QDs can be uniformly and spontaneously anchored on the positively charged GO+ framework based on the substantial 10 ACS Paragon Plus Environment

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electrostatic interaction at ambient conditions. It should be emphasized that spontaneous alternate electrostatic self-assembly between CdSe QDs and GO+ can occur in terms of their pronounced oppositely charged surfaces, thus yielding well-defined 3D CdSe QDs-GR ensembles after thermal reduction.

Fig. 1. (a) XRD patterns, (b) UV-vis diffuse reflectance spectra (DRS) with corresponding (c) transformed plots based on the Kubelka-Munk function vs. photon energy for CdSe QDs and CdSe QD-GR nanocomposites; (d) Raman spectra, (e) FTIR spectra and (f) N2 adsorption-desorption isotherms of CdSe QDs and CdSe QDs-1%GR nanocomposite with pore size distribution in the inset. Our research begins with the characterizations of CdSe QDs-GR nanocomposites. X-ray diffraction (XRD) was utilized to probe the crystal structures of the samples. As shown in Fig. 1a, blank CdSe QDs show broad diffraction peaks at 2θ values of 25.39°, 42.13°, 49.87°, 67.65° and 76.52°, which are ascribed to the (111), (220), (311), (331), and (422) crystal planes of cubic CdSe (JCPDS: 88-2346). XRD patterns of CdSe QDs-GR nanocomposites are analogous to pristine CdSe QDs except that peak intensity of CdSe QDs ingredient increases with increasing the percentage of GR, indicating compact GR encapsulation did not change the lattice structure of CdSe QDs but rather promote the directional stacking of CdSe QDs. Similar results have also been reported in previous works.13,43,44 Note that no peaks assignable to GR were 11 ACS Paragon Plus Environment


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observed in the XRD patterns of all these CdSe QDs-GR nanocomposites, which might be due to the relatively low loading amount of GR or possibly, its characteristic peaks were shielded by the main diffraction peaks of CdSe QDs. Optical properties of the samples were probed by UV-vis diffuse reflectance spectra (DRS). As displayed in Fig. 1b, blank CdSe QDs demonstrate substantial light absorption in the visible region with absorption band edge extending to ca. 720 nm, implying CdSe QDs possess excellent light harvesting capability. Apparently, light absorption of CdSe QDs-GR nanocomposites with different GR loading percentage was greatly enhanced within the same wavelength profile in comparison with pristine CdSe QDs, indicating GR is beneficial for reinforcing the light absorption of CdSe-GR nanocomposites. The transformed plots based on the Kubelka-Munk function vs. the energy of light were displayed in Fig. 1c, by which bandgaps (Eg) of the samples can be roughly estimated to be ca. 1.86, 1.84, 1.81, 1.79 and 1.82 eV, corresponding to pure CdSe QDs and CdSe-GR nanocomposites with GR percentage of 1, 5, 10, and 15 %, respectively. The results imply that Eg values of CdSe QDs-GR nanocomposites gradually decrease with GR encapsulation, which is in faithful agreement with previous reports.13,43,45 Raman spectroscopy and Fourier transformed infrared spectra (FTIR) were utilized to disclose the structure of the samples. As displayed in Fig. 1d, Raman spectrum of CdSe QDs shows an obvious peak at ca. 202 cm-1 that corresponds to the vibration mode of resonantly excited longitudinal optical (LO) phonon, along with the appearance of resonant excitation of the first (2LO) overtone at ca. 404 cm-1, which are attributed to the cubic phase CdSe QDs, consistent with XRD results (Fig. 1a). These two peaks with almost the same position and intensity were also observed in the Raman spectrum of CdSe-1%GR nanocomposite, which verifies successful integration of CdSe QDs in the nanocomposite via electrostatic interaction. Besides, another two peaks at 1349 and 1561 cm-1 ascribing to the D and G bands which correspond to the local structural disorders or defeats (sp3 hybridization) and sp2 hybridized carbon in GR were also observed in the Raman spectrum of CdSe-1%GR nanocomposite. Hence, Raman results strongly evidence successful integration of CdSe QDs with GR in the nanocomposites via electrostatic self12 ACS Paragon Plus Environment

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assembly. Consistently, as manifested in Fig. 1e, two peaks at 2924 & 2846 cm-1 and the peak at 1630 cm-1 in the FTIR spectrum of blank CdSe QDs are attributed to the methylene (-CH2-) and carboxyl (C=O) groups from the MAA ligands capped on the surface. Notably, the peak corresponding to -CH2- group can still be observed in the FTIR spectrum of CdSe QDs-1%GR nanocomposite albeit the intensity decreases to some extent and this is mainly caused by the GR encapsulation which shields the peak intensity. Besides, a pronounced peak at 1643 cm-1 arising from C=C groups was clearly seen, strongly corroborating the encapsulation of CdSe QDs with GR. It should be emphasized that C=O groups of CdSe QDs were shielded by the C=C groups in the FTIR spectrum of CdSe QDs-1%GR nanocomposite, thus making direct differentiation of C=O groups from C=O groups rather difficult. Consequently, Raman and FTIR results provide convincing evidences on the combination of CdSe QDs and GR in the nanocomposite via electrostatic self-assembly strategy. Specific surface area and porosity of the samples were probed by Brunauer-Emmett-Teller (BET) measurements. As shown in Fig. 1f, blank CdSe QDs shows nearly no absorption-desorption behavior while CdSe QDs-1%GR nanocomposite displays type-IV isotherm with a typical H3 hysteresis loop according to the IUPAC classification,50 which is the feature of mesoporous materials possessing slit shape resulting from the aggregation of plate-like particles, as verified by the pore size distribution in the inset. Based on which, specific surface areas of blank CdSe QDs and CdSe QDs-1%GR nanocomposite were determined as 0.4821 and 131.4820 m2·g-1, respectively. Obviously, CdSe QDs-1%GR nanocomposite exhibits remarkably increased specific surface area in comparison with blank CdSe QDs. The remarkably enlarged specific surface area of CdSe QDs-1%GR nanocomposite is predominantly attributed to the seamless encapsulation of CdSe QDs with GR which is characteristic of large lateral size with large specific surface area. The larger specific surface area of CdSe QDs-1%GR nanocomposite relative to CdSe QDs is in favor of providing more surface active sites, facilitating reactant molecules transportation, boosting light harvesting by multiple scattering, thereby synergistically contributing to the significantly enhanced photoactivities which will be discussed in following part.



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Fig. 2. (a) TEM and (b) HRTEM images of CdSe QDs-1%GR nanocomposite with corresponding SAED pattern in the inset, (c-d) FESEM images with corresponding (e) EDX and (f-h) elemental mapping results. Morphologies and microstructures of the samples were explored by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Fig. 2a exhibits the TEM image of CdSe QDs-1%GR nanocomposite, which demonstrates a clear boundary with CdSe QDs evenly and intimately distributed on the whole GR framework. Moreover, close interfacial integration of CdSe QDs with GR can be reflected by the HRTEM image (Fig. 2b) of CdSe QDs-1%GR nanocomposite, wherein the lattice fringe is determined to be ca. 0.352 nm corresponding to the (111) crystal facet of cubic CdSe QDs. Selected area electron diffraction (SAED) pattern of CdSe QDs-1%GR nanocomposite in the inset of Fig. 2b suggests the single-crystallite structure of the nanocomposite. Panoramic FESEM image of CdSe QDs-1%GR nanocomposite was displayed in Fig. 2(c-d) which also reveal intimate encapsulation of CdSe QDs by GR nanosheets forming nanoporous morphology with clear boundary, which is in faithful agreement with TEM result (Fig. 2a). Besides, elemental mapping results of CdSe QDs-1%GR 14 ACS Paragon Plus Environment

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nanocomposite in Fig. 2(f-h) demonstrate clear signals of Cd, Se and C elements accounting for weight percentage of 51.89, 26.48 and 21.63 %, as unveiled by the energy dispersive X-ray (EDX) result (Fig. 2e). It should be emphasized that the C signal arises from GR scaffold, once again confirming integration of CdSe QDs with GR in the nanocomposites.

Fig. 3. (a) Survey spectrum and high-resolution (b) Cd 3d, (c) Se 3d, (d) C 1s, and (e) O 1s spectra of (I) blank CdSe QDs and (II) CdSe QDs-1%GR nanocomposite. X-ray photoelectron spectroscopy (XPS) was utilized to probe the composition and elemental chemical valence states of the samples. Survey spectra of blank CdSe QDs and CdSe QDs-1%GR (Fig. 3a) reveal the co-existence of Cd 3d, Se 3d, and C 1s elements. As shown in the high-resolution Cd 3d spectrum of CdSe QDs-1%GR (Fig. 3bII), the peaks at 405.14 eV and 411.94 eV corresponding to Cd 3d5/2 and Cd 3d3/2 are assigned to Cd2+ specie and this agrees with the high-resolution Cd 3d spectrum of blank CdSe QDs. Nevertheless, red-shift in the binding energy of high-resolution Cd 3d spectra was observed for CdSe QDs-1%GR in comparison with blank CdSe QDs (Fig. 3bI), implying GR 15 ACS Paragon Plus Environment


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encapsulation exerts profound influence on the electronic structure of CdSe QDs. Analogously, highresolution Se 3d spectrum (Fig. 3cII) of CdSe QDs-1%GR nanocomposite demonstrates a substantial peak at 54.25 eV corresponding to Se2- specie, which is in line with the high-resolution Se 3d spectrum of CdSe QDs (Fig. 3cI). Furthermore, similar red-shift of binding energy was also observed in the highresolution Se 3d spectrum of CdSe QDs-1%GR relative to blank CdSe QDs, which might be ascribed to the electronic interaction-induced charge transfer between CdSe QDs and GR. As displayed in Fig. 3dII, high-resolution C 1s spectrum of CdSe QDs-1%GR exhibits three peaks at 286.05, 287.45 and 288.65 eV corresponding to the oxygen-containing functional groups of C-OH, C-O-C & C=O and HO-C=O, respectively. Notably, intensity of these three peaks concurrently decreases and simultaneously, peak intensity at 284.60 eV corresponding to sp2 hybridized carbon (C-C) increases in comparison with highresolution C 1s spectrum of GO precursor (Fig. S2d). The result strongly evidences the removal of oxygen-containing functional groups and sufficient reduction of GO precursor to GR in the CdSe QDs1%GR nanocomposite. Consistently, removal of oxygen-containing functional groups in the CdSe QDs1%GR nanocomposite can also be evidenced by its high-resolution O 1s spectrum (Fig. 3eII) which manifests much lower peak intensity than that of blank CdSe QDs. For specific comparison, chemical bond species vs. binding energy for blank CdSe QDs and CdSe QDs-1%GR nanocomposite were summarized in Table S1. It should be stressed that sufficient reduction of GO to GR in the nanocomposites is of paramount important to harness the excellent electronic conductivity of GR for boosting charge transfer in photocatalytic reactions.


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Fig. 4. Photocatalytic performances of blank CdSe QDs and CdSe QDs-GR nanocomposites with different weight percentage of 1, 5, 10, and 15% toward selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation (λ>420 nm) for 3h. Photoactivities of CdSe QDs-GR nanocomposites were evaluated by selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD) under visible light irradiation (λ>420 nm) at ambient conditions. As manifested in Fig. 4 and Fig. S5, conversion, yield and selectivity of CdSe QDs-GR nanocomposites toward selective oxidation of BA to BAD are closely associated with the weight percentage of GR. Specifically, photoactivity remarkably increases with increasing the GR percentage to 1% and then it gradually decreases upon further increasing the percentage to 15%. Nevertheless, it should be noted that conversion, yield and selectivity of all these CdSe QDs-(1, 5, 10, 15%) GR nanocomposites always markedly exceed pure CdSe QDs which demonstrates negligible photoactivity under the same conditions. Notably, conversion of BA and yield of BAD over CdSe QDs-GR nanocomposites decrease when GR percentage exceeds 1% and the result can be ascribed to the fact that light absorption of CdSe QDs is seriously shielded by the excess GR encapsulation, thus leading to relatively inferior photocatalytic performances. The optimal sample was determined as CdSe QDs-1%GR nanocomposite with selectivity approaching to 100%. The substantially boosted photoactivities of CdSe QDs-1%GR nanocomposite relative to blank CdSe QDs can be attributed to the integrated reasons including improved separation of photogenerated electron-hole pairs, intimate interfacial integration mode, enhanced adsorption to reactants 17 ACS Paragon Plus Environment


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and large specific surface area endowed by GR encapsulation, which will be specifically probed in the latter part. Among which, interfacial contact between CdSe QDs and GR plays a pivotal role in boosting the photoactivity of CdSe QDs-1%GR nanocomposite. This can be verified by the photoactivity of CdSe QDs-1%GR- counterpart wherein CdSe QDs were directly assembled with pristine negatively charged GO- without EDC modification and no electrostatic interaction acting as the main driving force exists between CdSe QDs and GR. As shown in Fig. S6a, with the same GR weight percentage, CdSe QDs1%GR- always shows markedly inferior photoactivity in comparison with the optimal CdSe QDs-1%GR sample, strongly indicating rational combination of CdSe QDs with GR via electrostatic self-assemble is essential to afford intimate interfacial contact which contributes to the considerably boosted photoactivity.

Fig. 5. Time-online photocatalytic performances of blank CdSe QDs and CdSe QDs-1%GR nanocomposite toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes 18 ACS Paragon Plus Environment

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under visible light irradiation (λ>420 nm) for 4 h, including (a) benzyl alcohol, (b) p-methylbenzyl alcohol, (c) p-methoxybenzyl alcohol, (d) p-fluorobenzyl alcohol, (e) p-chlorobenzyl alcohol, (f) p-nitrobenzyl alcohol, (g) cinnamyl alcohol, and (h) 3-methyl-2-buten-1-ol, along with (i) selective photooxidation reaction model under the current experimental conditions. Apart from BA (Fig. 5a), photocatalytic performances of CdSe QDs-1%GR nanocomposite toward selective oxidation of other aromatic alcohols to corresponding aldehydes were also carried out under the same conditions. As illustrated in Fig. 5, CdSe QDs-1%GR nanocomposite always exhibits the conspicuously enhanced conversion and yield in comparison with blank CdSe QDs toward selective oxidation of p-methylbenzyl alcohol (Fig. 5b), p-methoxybenzyl alcohol (Fig. 5c), p-fluorobenzyl alcohol (Fig. 5d), p-chlorobenzyl alcohol (Fig. 5e), p-nitrobenzyl alcohol (Fig. 5f), cinnamyl alcohol (Fig. 5g), and 3-methyl-2-buten-1-ol (Fig. 5h), thereby reflecting highly efficient and versatile photoactivities of CdSe QDs-1%GR nanocomposite for selective organic transformation under visible light irradiation. Noteworthily, photoactivity of blank CdSe QDs is rather low and in some cases it is negligible, which is mainly due to its ultra-fast recombination rate of photoinduced electron-hole pairs, therefore giving rise to extremely short lifetime of charge carriers. Simultaneously, it is worth noting that CdSe QDs might be directly oxidized by holes which further deteriorates the photoactivity of blank CdSe QDs. On the contrary, electrons photoexcited from CdSe QDs in the CdSe QDs-1%GR nanocomposite can be efficiently captured by GR nanosheets in terms of its gorgeous electronic conductivity. In this way, subsequently, electrons are involved in the generation of various active species (e.g., hydroxyl radicals, superoxide radicals) which contributes to the markedly improved photoactivities. Moreover, adsorption capability of blank CdSe QDs and CdSe QDs-1%GR toward different alcohols was evaluated. As shown in Fig. S7, CdSe QDs-1%GR nanocomposite only presented a trifle better adsorptivity toward reactant and the result indicates the superior photoactivities of CdSe QDs-1%GR to blank CdSe QDs are not primarily stemmed from different adsorption capability but rather other reasons such as prolonged lifetime of photoinduced charge carriers, which will be systematically probed in the latter part.



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Fig. 6. Conversion of BA and yield of BAD over CdSe QDs-1%GR nanocomposite with adding different scavengers under visible light irradiation (λ>420 nm) for 4 h. To ascertain the primary reactive species involved in the selective oxidation of alcohols over CdSe QDs-1%GR nanocomposite, a series of controlled experiments were thus carried out. To this end, benzoquinone (BQ), tert-butyl alcohol (t-BuOH), potassium persulfate (K2S2O8) and phenol (PhOH) were separately added into the reaction system to serve as scavengers for quenching superoxide radicals (·O2-), hydroxyl radicals (·OH), photogenerated electrons (e-) and holes (h+), respectively. As manifested in Fig. 6, photoactivity of CdSe QDs-1%GR nanocomposite performed in N2-saturated BTF solvent was remarkably reduced compared with the original O2-saturated BTF, indicating oxygen bubbling of BTF is indispensable and dissolved oxygen plays an important role in boosting the photocatalytic performance. This result is understandable as dissolved oxygen is beneficial for producing various active species such as superoxide radicals (·O2-), hydroxyl radicals (·OH), and hydrogen peroxide (H2O2). When PhOH was added into the reaction system, conversion and yield of CdSe QDs-1%GR nanocomposite sharply decreases, implying hole makes a substantial contribution in trigger the selective photocatalytic oxidation of BA to BAD. As well, a similar inhibition effect was also observed with the addition of K2S2O8 which acts as electron scavenger, suggesting electron also play a crucial role in contributing to the significantly enhanced photocatalytic performance of CdSe QDs-1%GR nanocomposite. This can be ascribed to the 20 ACS Paragon Plus Environment

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fact that adding K2S2O8 in the reaction system for quenching electrons retards the generation of ·O2and ·OH active species. The correlation of electrons with the formation of ·O2-, ·OH and H2O2 active species can be demonstrated by the following formulas (5-7). Consistently, photoactivity of CdSe QDs1%GR nanocomposite decreases when BQ or t-BuOH was added into the reaction system, persuasively evidencing both ·O2- and ·OH are responsible for the selective photocatalytic organic transformation process. Bases on the above analysis, contributing role of these active species follows the order of h+>·OH>e->·O2-. O2 + e- → ·O2-

(5)

·O2- + ·O2- + H+ → H2O2 + O2

(6)

H2O2 + ·O2- → ·OH + OH- + O2

(7)



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Fig. 7. Photocatalytic performances of blank CdSe QDs and CdSe QDs-1%GR nanocomposite toward selective reduction of nitroaromatics under visible light irradiation (λ>420 nm) with the addition of Na2SO3 for quenching photogenerated holes and N2 bubbling at ambient conditions including (a) 4-NA, (b) 3-NA, (c) 2-NA, (d) 4-bromo-1-nitrobenzene, (e) 4-NP, (f) 3-NP, (g) 2-NP, (h) 4-nitrotoluene, (i) 4nitroanisole, and (j) nitrobenzene together with the (k) typical reaction model under the current experimental conditions. Besides the selective photocatalytic oxidation of aromatic alcohols to corresponding aromatic aldehydes as aforementioned, selective photocatalytic reduction performances of CdSe QDs-1%GR nanocomposite under visible light irradiation were also probed. It should be stressed that photoreduction 22 ACS Paragon Plus Environment

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reactions in our current reaction system were carried out under ambient conditions in an inert atmosphere along with the addition of hole scavenger, which guarantees the photoelectron is the only active specie triggering the reduction reactions. More specifically, photoreduction performance of CdSe QDs-1%GR nanocomposite was first evaluated by anaerobic reduction of 4-nitroaniline (4-NA) to 4-phenylenediamine (4-PDA) in an aqueous phase under visible light irradiation (λ>420 nm) with the addition of Na2SO3 as the quencher for photogenerated holes and N2 purge under ambient conditions. UV-vis light adsorption spectra of 4-NA were monitored to unveil the conversion of 4-NA to 4-PDA with irradiation time. According to the UV-vis absorption spectra of 4-NA (Fig. S8) in the presence of CdSe QDs-1%GR nanocomposite under visible light irradiation, it is apparent that absorption peak intensity at 380 nm corresponding to 4-NA gradually decreases and concomitantly, a new absorption peaks at 320 nm corresponding to 4-PDA appears and the peak intensity increases with prolonging the irradiation time,13,45 suggesting progressive conversion of 4-NA to 4-PDA under visible light irradiation. Blank experiments show that no photoactivity was observed in the absence of catalyst or light irradiation under the same experimental conditions (Fig. S9), indicating it is indeed a light-driven photocatalytic process. The importance of interfacial contact between CdSe QDs and GR building blocks was verified in Fig. S6b which reveals CdSe QDs-1%GR exhibits much more enhanced photoactivity than CdSe QDs-1%GRcounterpart. As displayed in Fig. 7a, CdSe QDs-1%GR nanocomposite demonstrates markedly enhanced photoactivity under visible light irradiation toward selective reduction of 4-NA to 4-PDA compared with blank CdSe QDs. Alternatively, as displayed in Fig. 7(b-j), apart from 4-NA, CdSe QDs-1%GR nanocomposite once again exhibits considerably enhanced visible-light-driven photoactivities toward reduction of other nitroaromatics to corresponding amino compounds in comparison with blank CdSe QDs. To probe the reasons accounting for the significantly enhanced photoactivity of CdSe QDs-1%GR nanocomposite, adsorption experiments were performed (Fig. S10) and the result suggests that adsorptivity of catalysts has little effect on the photoactivity. In this regard, the superior photoactivities of CdSe QDs-1%GR nanocomposite are predominantly attributed to the integrated reasons including prolonged lifetime of photoinduced charge carriers assisted by intimate GR encapsulation for efficiently 23 ACS Paragon Plus Environment


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trapping electrons and enhanced light absorption.

Fig. 8. (a) Photoactivities of CdSe QDs-1%GR nanocomposite toward 4-NA reduction with and without adding K2S2O8 as electron scavenger, (b) cyclic photoreduction of 4-NA over CdSe QDs-1%GR nanocomposite under visible light irradiation (λ>420nm) with the addition of Na2SO3 for quenching photogenerated holes and N2 bubbling at ambient conditions. Action spectra of CdSe QDs-1%GR nanocomposite under monochromatic light irradiation toward (c) selective reduction of 4-NA (irradiation time: 10min) and (d) selective oxidation of BA (irradiation time: 3h). To highlight the crucial role of electrons in selective photoreduction reaction, control experiment by adding K2S2O8 as electron scavenger into the reaction system was performed. Fig. 8a shows that photocatalytic performance of CdSe QDs-1%GR nanocomposite toward 4-NA reduction was drastically reduced when K2S2O8 was added in the reaction system, strongly indicating electron indeed plays a pivotal role in initiating the selective photoreduction reactions. On the other hand, photostability is a paramount insect of catalyst for future potential applications. As revealed in Fig. 8b, CdSe QDs-1%GR nanocomposite demonstrates favorable photostability with no apparent photoactivity decay even after five successive cyclic reactions. The result is reasonable in terms of the hole quenching assisted by Na2SO3 during the reaction, thereby retarding the oxidation of CdSe QDs. Moreover, action spectra of CdSe QDs24 ACS Paragon Plus Environment

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1%GR nanocomposite toward photoredox organic transformation under different monochromatic light irradiation was systematically probed to disclose the wavelength region responsible for the conspicuously boosted photoactivities. As manifested in Fig. 8(c-d), a substantial peak ranging from 450 to 550 nm was clearly seen in the action spectra of CdSe QDs-1%GR nanocomposite toward either selective photoreduction of 4-NA or photooxidation of BA, which strongly corroborates the crucial role of CdSe QDs ingredient for substantial light harvesting and conversion in the CdSe QDs-1%GR nanocomposite.

Fig. 9. (a) On-off transient photocurrent responses, (b) LSV results (5mV·s−1), (c) open circuit potential decay, (d) electrochemical impedance spectroscopy (EIS) Nyquist plots, (e) photoluminescence (PL) spectra (excitation wavelength: 380 nm) and (f) Mott-Schottky results of blank CdSe QDs and CdSe QDs1%GR nanocomposite. Photoelectrochemical (PEC) measurements were performed to evaluate the fate of photoinduced electron-hole pairs and meanwhile, to ascertain the role of GR played in promoting the photoredox 25 ACS Paragon Plus Environment


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activities of CdSe QDs-GR nanocomposites. As displayed in Fig. 9a, on-off transient photocurrent response of CdSe QDs-1%GR nanocomposite is almost two times larger than blank CdSe QDs under visible light irradiation, suggesting charge separation over CdSe QDs-1%GR nanocomposite is much more efficient than blank CdSe QDs with the involvement of GR which is in favor of ultra-fast electron transfer. Consistently, linear-sweep voltammograms (LSV) results (Fig. 9b) suggest that photocurrent density of CdSe QDs-1%GR is always larger than blank CdSe QDs under the same conditions within a wide voltage profile, which implies more effective charge separation and longer lifetime of charge carriers over CdSe QDs-1%GR compared with CdSe QDs. Moreover, Fig. 9c shows the decay of open circuit potential of blank CdSe QDs and CdSe QDs-1%GR nanocomposite, wherein CdSe QDs-1%GR demonstrates much more prolonged electron lifetime and larger photovoltage compared with CdSe QDs, once again corroborating the more efficient separation of photogenerated electrons-holes pairs over CdSe QDs-1%GR. Besides, Electrochemical impedance spectroscopy (EIS) Nyquist analysis has been wellestablished as an efficacious technique to evaluate the charge transfer resistance in the interfacial region of work electrode and electrolyte. Normally, the smaller semicircular arc radius the more efficient charge separation.51 Consistently, as mirrored by Fig. 9d, semicircular arc radius in the Nyquist plot of CdSe QDs-1%GR nanocomposite is smaller than that in the Nyquist plot of blank CdSe QDs, verifying the crucial role of GR in improving the separation of electron-hole pairs. This speculation can be corroborated by photoluminescence (PL) technique which has been evidenced as an efficient tool to assess the separation efficiency of photoinduced charge carriers over semiconductors. It has been well-established that the lower PL intensity the more efficient separation of electron-hole charge carriers.52 Fig. 9e shows that PL intensity of CdSe-1%GR nanocomposites is much weaker than CdSe QDs, implying more efficient separation and longer lifetime of charge carriers over CdSe QDs-1%GR as a result of intimate GR encapsulation. Fig. 9f shows the Mott-Schottky plot of CdSe QDs-1%GR nanocomposite, by which flat band potential (Vfb) of CdSe QDs ingredient in the nanocomposite (1%) was determined to be ca. -0.63 V vs. Ag/AgCl, i.e., -0.43 V vs. NHE by the horizontal intercept of extrapolation along the linear part,53 by which the conductive band potential (VCB) was determined to be ca. -0.53 V vs. NHE.54 According to 26 ACS Paragon Plus Environment

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the Eg value (1.84 eV) determined by DRS result (Fig. 1c) and the formula Eg = VVB - VCB, valence band potential (VVB) of CdSe QDs was calculated to be 1.31 V vs. NHE. Furthermore, electron carrier density (ND) can be calculated by the following equation:55

() 1

-1

d 2 C 2 ND = [ ] 𝜀𝑟ε0e0 dV

( )

where εr denotes the dielectric constant of the semiconductor (5.8 for CdSe),56 ε0 denotes the permittivity of a vacuum (8.86 × 10-12 F·m-1), e0 is the electronic charge unit (1.6 × 10-19 C), and V is the potential applied to the electrode. As displayed in Fig. S11, ND values of blank CdSe QDs and CdSe QDs-1%GR were determined to be ca. 1.27 × 1020 and 1.55 × 1020 cm-3 respectively. Apparently, CdSe QDs-1%GR demonstrates larger ND relative to blank CdSe QDs which is in line with the previous conclusion.

Scheme 2. Schematic illustration of photocatalytic mechanism of CdSe QDs-GR nanocomposites. Based on the above systematic investigation, a feasible photocatalytic mechanism of CdSe QDs1%GR nanocomposite is proposed and vividly depicted in Scheme 2. More specifically, when CdSe QDs27 ACS Paragon Plus Environment


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GR nanocomposites was irradiated by visible light, CdSe QDs ingredient was instantly band-gapphotoexcited with electrons photoexcited from the VB to the CB, thus leaving holes in the VB. Subsequently, electrons in the CB of CdSe QDs were rapidly transferred to the neighboring intimately integrated GR layer owing to their favorable energy level alignment (Ef = -0.08 V vs. NHE),57 which efficiently inhibits the recombination and prolong the lifetime of photogenerated electron-hole charge carriers. Concerning the selective photocatalytic oxidation of aromatic alcohols, electrons captured by GR can directly combine with O2 molecules dissolved in the reaction system (BTF) to engender superoxide (·O2-) radicals [E°(O2/·O2-) = -0.284 V vs. NHE58] in terms of their suitable relative energy level position. Although hydroxyl radical (·OH) cannot be directly generated since the VB of CdSe QDs was not positive enough to oxide H2O [E°(·OH/H2O) = 2.38 V vs. NHE],59 they can be produced by partial transformation of ·O2- radicals by continuous reactions (formula 5-7). As a result, these in-situ formed ·O2- and ·OH radicals active species enable concurrent selective oxidization of aromatic alcohols to corresponding aromatic aldehydes. Alternatively, it should be particularly noted that holes in the VB of CdSe QDs might simultaneously participate in the direct oxidation of aromatic alcohols to aromatic aldehydes. On the other hand, with regard to the selective photocatalytic reduction reactions, electrons trapped by GR are directly involved in the photoreduction of aromatic nitro compounds to corresponding amino compounds, wherein electron is the only active specie without the formation of ·O2- and ·OH radicals because holes are completely quenched by hole scavenger (Na2SO3) with O2 totally expelled out of the reaction system via N2 bubbling. Apparently, seamless encapsulation of CdSe QDs with GR nanosheets is indispensable for attaining the significantly enhanced selective photoredox organic transformation performances. Aside from the improved separation and prolonged lifetime of charge carriers endowed by GR, GR encapsulation is also beneficial for reinforcing the transportation of aromatic alcohols and aromatic nitro compounds to the ideal active sites to some extent considering their structure compatibility, as reflected by the remarkably enlarged specific surface area of CdSe QDs-GR compared with blank CdSe QDs. Consequently, it is these integrated factors that synergistically contribute to the substantially enhanced photoredox performances of CdSe QDs-GR nanocomposite under visible light irradiation. 28 ACS Paragon Plus Environment

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4. Conclusions In summary, a facile, green, easily accessible and efficient self-assembly strategy has been developed for rational construction of CdSe QDs-GR nanocomposites at ambient conditions, by which negatively charged CdSe QDs was intimately anchored on the surface-modified positively charged GR framework based on the pronounced electrostatic attractive interactions. It was significant to unveil that seamless encapsulation of CdSe QDs with GR layer endows CdSe QDs-GR nanocomposites conspicuously enhanced photo-redox performances toward versatile selective organic transformation, including selective oxidation of aromatic alcohols to corresponding aldehydes and selective reduction of nitroaromatics to amino aromatics under visible light irradiation at ambient conditions. The origins accounting for the marked enhanced photoredox performances of CdSe QDs-GR nanocomposites are predominantly ascribed to the integrated reasons including substantially boosted separation of photogenerated charge carriers and remarkably enlarged specific surface area assisted by seamless GR encapsulation, thereby synergistically contributing to the superior and versatile photoactivities. Moreover, active species responsible for the photoredox organic transformation were systematically probed and corresponding photocatalytic mechanism was determined and elucidated. It is anticipated that our work would shed new insights for rational construction of GR-QDs ensembles via intelligent surface charge modulation for solar energy conversion.

Supporting Information Synthesis procedures of GO; Structure of positively charged GO suspension; Characterization of positively charged GO; Characterization of negatively charged GO; FT-IR of negatively charged GO and positively charged GO; Characterization of CdSe QDs; Chemical bond species vs. B.E. for different samples; GC spectra toward photooxidation of BA to BAD over CdSe QDs-1%GR nanocomposite under visible light irradiation (λ>420 nm); Photocatalytic performance of CdSe QDs-1%GR and CdSe QDs1%GR- nanocomposites toward selective oxidation of BA and reduction of 4-NA under visible light irradiation (λ>420 nm); Remaining fraction of alcohols after the adsorption-desorption equilibrium; UVvisible absorption spectra of 4-NA with visible light (λ>420nm) irradiation over CdSe QDs-1%GR 29 ACS Paragon Plus Environment


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nanocomposite; Control experiments of 4-NA photoreduction without catalyst or visible light irradiation; Remaining fraction of 4-NA after the adsorption-desorption equilibrium; Electron carrier density of CdSe QDs and CdSe QDs-1%GR nanocomposite.

Acknowledgements The support by the Award Program for Minjiang scholar professorship is greatly acknowledged. This work was financially supported by the National Natural Science Foundation of China (No. 21703038).

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TOC Graphic


Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered

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