Coffee-Ground-Derived Quantum Dots for ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Coffee-Ground-Derived Quantum Dots for Aqueous Processable Nanoporous Graphene Membranes Huan Xu,†,§ Lan Xie,‡ and Minna Hakkarainen*,† †

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Stockholm 10044, Sweden Department of Polymer Materials and Engineering, College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China § College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡

S Supporting Information *

ABSTRACT: Carbon-based quantum dots (QDs) with ultralow dimensions and controllable surface chemistry have unique properties appealing to diverse applications. Here, we disclose a high-throughput transformation of spent coffee grounds into uniform QDs assembled by few-layer graphene oxide nanosheets, employing a microwave-assisted strategy under aqueous reaction conditions. Given the low dimensions (30 nm) and high structural integrity, the highly oxygenated QDs exhibited excellent dispersibility in water with tunable fluorescence. The structural attributes of QDs conferred excellent affinity to graphene nanosheets, permitting aqueous processing of nanoporous graphene membranes applicable to removing a broad spectrum of water pollutants ranging from organic compounds to heavy metals while sustaining a high flux rate. The proposed “trash-to-treasure” strategy opens up new possibilities for aqueous processing of nanoporous graphene membranes with great potential in the environmental field. KEYWORDS: Biobased nanodots, Aqueous processing, Graphene membrane, Nanopores, Water purification

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acids30 and aromatic compounds31 during the bottom-up synthesis, such as electron-beam lithography32 and microwaveassisted pyrolysis.33 In addition to the environmental issues and high costs caused by many of these precursors,34 the existing manufacturing methods are usually time-consuming with large energy footprints and specific technical demands.35 This brings to light five essential factors that are of special relevance for the development of QDs: (1) low environmental impact for QD precursors and fabrication techniques,36 (2) well-controlled dimensions of underlying sheet units to assemble high-quality QDs, (3) selectivity toward adequately exfoliated few-layer sheets, (4) structural integrity to provoke optoelectronic effects, and (5) high throughput production within affordable processing time. This is still a challenge.37,38 Another interesting aspect concerns the possibility to realize direct functionalization of QDs during the preparation process to afford high activity for further decoration of molecular architectures and functional groups in the pursuit of ondemand functionality. Stimulated by the challenges in developing functional QDs, we recently established a microwave-assisted hydrothermal (MAH) approach, including the decomposition and carbonization of cellulose under microwave irradiation followed by

INTRODUCTION The intrinsic physiochemical properties of nanostructured carbon materials (e.g., carbon nanotubes and graphene nanosheets), together with the capability to create robust interactions with various species and matters,1−4 opens up a rich set of promising applications from energy devices to photoelectrical elements, biological electronics, and water purification.5−7 Broadening the family of nanocarbons to uncover unprecedented properties and functionalities,8 as exemplified by the burgeoning of new nanocarbon derivatives such as graphene nanoribbons,9 carbon nanothreads,10 and carbon quantum dots (QDs),11−14 is of high interest. Featuring ultralow dimensions and extensive functional groups, carbon QDs have shown great promise to trigger tunable photoluminescence in a broad range, combined with unique surface activity appealing to energy, environmental, and biomedical applications.15−18 Recent progress reveals the QD-enabled approach to aqueous processing of layered graphene membranes,19 the fabrication of which were normally limited to the use of ionic surfactants20 or specific techniques such as electrophoretic deposition.21 The quest to develop multifunctional QDs focuses increasingly on the synthesis of surface- and size-controlled QDs.22,23 A variety of carbon-containing precursors have been activated to produce QDs, including graphene sheets24 and carbon fibers,25,26 using a top-down strategy (e.g., laser ablation27 and hydrothermal degradation28,29), and amino © 2017 American Chemical Society

Received: March 2, 2017 Revised: April 2, 2017 Published: April 17, 2017 5360

DOI: 10.1021/acssuschemeng.7b00663 ACS Sustainable Chem. Eng. 2017, 5, 5360−5367

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ACS Sustainable Chemistry & Engineering

Figure 1. Synthetic roadmap to convert coffee grounds to QDs. (a−c) The MAH process was combined with oxidation to yield uniform QDs from coffee grounds. The photographs show (a) fresh coffee grounds from a household coffeemaker, (b) dot precursor suspensions after the MAH process, and (c) aqueous suspensions containing oxidized QDs, in which the structural features are delineated by the illustrative cartoons. (AAO, Waterman, Germany) using a typical filtration assembly (Sigma-Aldrich, Germany), followed by three-time washing with water to remove the water-soluble QDs.19 QDs were adequately washed away, allowing the formation of porous membranes exclusively assembled from graphene sheets. The graphene membranes were dried under vacuum conditions for 24 h prior to the structural characterization and the evaluation of water purification efficiency. The same method was also used to prepare the counterpart from G-QD0 solutions, but it led to discrete powder flakes. Water Purification by Porous Graphene Membranes. With the same filtration technique being used, the efficiency of water purification was evaluated for the porous graphene membranes. The metal compounds and organic pollutant were separately dissolved in water (50 mL) at the same concentration (1 mg/mL) and then directly fed to the purification system equipped with a graphene membrane adhered to the AAO membrane. The porous feature of the graphene membranes allowed the application of a high flux rate up to approximately 1 L m−2 h−1. Scanning Electron Microscopy (SEM) Observation. An SE4800 SEM (Hitachi, Japan), operating at an accelerated voltage of 1 kV, was used to image the surface morphology of coffee ground particles, precursor powders, QDs, and graphene membranes before and after water purification. All the samples were sputter-coated with a 3.5-nm-thick gold layer prior to the SEM observations. Transmission Electron Microscopy (TEM) Observation. TEM was used to image the morphology of QDs and G-QD1. Droplets of QD suspension in ethanol or G-QD1 suspension in water were deposited onto a lacey carbon film 400 mesh copper TEM grid (Ted Pella, Inc.) and allowed to dry under ambient conditions prior to TEM imaging (Hitachi HT7700, 80 kV). High-Resolution TEM (HR-TEM). HR-TEM and live fast Fourier transform (FFT) images of QDs were recorded on a Hitachi H-8100 EM (Hitachi, 200 kV). The method of sample preparation was the same as the TEM part described above. Atomic Force Microscopy (AFM) Topography Characterization. The QDs dispersed on mica substrates were imaged using a Nanoscope Multimode 8 (Bruker AXS, Santa Barbara, USA) with a type E piezoelectric scanner. Images were acquired in tapping mode using RTESP Si cantilevers (Bruker Probes, Camarillo, USA) with a typical spring constant of 40 N/m. X-ray Diffraction (XRD) Measurements. XRD measurements were performed on a homemade laboratory instrument (Bruker NanoStar, CuKα-radiation) in the Crystallography Lab, Department of Molecular Biology and Biotechnology, University of Sheffield. The Xray beam with a wavelength of 0.154 nm was focused to a tiny area of 4 × 4 μm2, and the distance from sample to detector was fixed at 350 mm. The diffraction profiles were collected by an X-ray CCD detector (Model Mar345, a resolution of 2300 × 2300 pixels, Rayonix Co. Ltd., USA). Raman Spectroscopy. The Raman spectra of coffee grounds, the dot precursor, and QDs were recorded on a micro-Raman

oxidation with strong acids, to prepare graphene oxide-type QDs.29,39,40 Here, we explore the possibility of using spent coffee grounds as a widely available carbon source rich in aromatic and functional groups,41 potentially leading to higher yields and allowing complete oxidation even in dilute acid and thus facile control over the size and surface chemistry of QDs. These coffee-ground-derived carbon QDs with low dimensions and a high oxygenation degree are expected to be highly watersoluble and affinitive to graphene sheets, which could pave a route to greener aqueous processing of graphene membranes.

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EXPERIMENTAL SECTION

Materials. The spent coffee grounds (Arvid Nordquist, Sweden) were used as the carbon source. Metal compounds such as Co(Ac)2, CuCl2, and Fe2(SO4)3 were obtained from Fluka Inc., Sigma-Aldrich. The organic pollutants were modeled by aqueous 2,4-dichlorophenol (2,4-DCP) solution. All other chemical reagents of analytical grade were obtained from VWR, Germany, and were used as received. Preparation of QDs. An efficient and environment-respecting route was used to transform the coffee grounds to the QD precursor following the microwave-assisted hydrothermal (MAH) strategy devised in our group.29,42 The spent coffee grounds (10 g) were soaked in 50 mL of dilute H2SO4 solution (0.01 g/mL), followed by direct submission to microwave heating for 2 h (SynthWAVE, Milestone Inc., USA). The MAH reaction was conducted with a preset temperature of 180 °C and a pressure of 40 bar. The dark brown precursor product was filtrated from the solution and washed with distilled H2O (20 mL). The precursor was then oxidized in 10% HNO3 (∼20 mL), sequentially under sonication at 45 °C for 0.5 h and then with magnetic stirring at 90 °C for 0.5 h. The reaction was then cooled down and diluted by adding cold H2O. Light yellow QD powder of high expansion density was obtained after rotary evaporation of acidic H2O. It is worth stressing that 9.6 g of QDs could be synthesized from 10 g of dot precursor. Preparation of Graphene. Graphene nanosheets were prepared via the chemical reduction of graphene oxide precursor. As a first step, graphene oxide was synthesized from natural graphite via a modified Hummer’s method using NaNO3, H2SO4 and KMnO4.43,44 The pH of as-prepared graphene oxide aqueous solution was adjusted in the range of 10.0−10.5 by ammonia solution (28 wt % in water), followed by adding hydrazine solution (35 wt % in water) with a weight ratio to graphene oxide of 0.7 under gentle stirring. The hydrazine added solution was heated in a water bath (95 °C) for 1 h to allow sufficient reduction until it became black.45 The graphene nanosheets were filtrated and dried under vacuum conditions. QD-Assisted Fabrication of Porous Graphene Membranes. QDs were added to the graphene aqueous solutions (0.5 mg/mL) with graded QD/graphene weight ratios of 0 (G-QD0), 0.5 (G-QD0.5), and 1 (G-QD1), which was assisted by ultrasonication. The mixed GQD1 solutions (100 mL) were filtrated through a 100 nm membrane 5361


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Figure 2. Structural evolution from coffee grounds to QDs. (a−c) Digital images and (d−f) SEM micrographs of (a, d) dried coffee grounds, (b, e) dot precursor powder, and (c, f) oxidized nanodots showing the changes in monolithic states and microstructures.

Figure 3. Structural determination of coffee-ground-derived QDs. (a) TEM micrograph showing the small and uniform QDs dispersed in ethanol (0.5 mg/mL). (b) An individual QD revealing the origin from few-layer nanosized sheets, as depicted in the inset cartoon. (c) AFM height and (d) 3D height images of QDs showing the high uniformity. (e) Height profile measured along the line in c. (f) HR-TEM image of crystalline domains in QDs showing the lattice fringes with an in-plane spacing of 0.214 nm consistent with nanocrystalline graphite, as confirmed by the inset FFT pattern. (g) Raman spectrum of QDs displaying the distinct D and G bands located at 1358 and 1582 cm−1, respectively. (h) XRD profile recorded for QDs evidencing the structural attributes of graphitic lattices. (i) FTIR spectrum of QDs indicating the presence of oxygen-rich functional groups by the identification of H-bonded −OH, CO, and C−O groups, together with the existence of C−H and conjugated CC. (j) Fitted results of the XPS C 1s spectrum of QDs pointing out the functional groups connected to carbon atoms. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of coffee grounds, the dot precursor, and QDs were recorded

spectroscope (DXRxi, Thermo Scientific Instrument, USA) with a laser wavelength of 532 nm. 5362


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Figure 4. QD-assisted aqueous fabrication of nanoporous graphene membranes. (a) Photographs comparing the dispersion stability for the aqueous suspensions of G-QD0, G-QD0.5, and G-QD1. (b) TEM micrograph of G-QD1 after one-week standing showing the QDs were uniformly and closely adsorbed onto the surfaces of well-exfoliated graphene nanosheets, with few QDs traced in the bulk. The inset image shows the Tyndall effect indicating the system homogeneity. (c) Vacuum-assisted filtration technique to prepare graphene membranes from G-QD1 suspensions. (d) Digital images of graphene monoliths deposited on filter membranes: G-QD0 only led to discrete flakes, while an associating membrane was assembled from G-QD1. (e) SEM images of graphene membrane surface, showing highly porous and wrinkled topology for the exfoliated graphene sheets. (f) Crosssection SEM micrographs illustrating a thickness of around 200 μm.

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on a PerkinElmer Spectrum 2000 spectrometer (PerkinElmer Instrument) with 16 scans at a resolution of 4 cm−1. X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS C 1s and O 1s spectra were collected using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer using a monochromated Al Kα source operated at 150 W. An analyzer pass energy of 160 eV for acquiring wide spectra and a pass energy of 20 eV for individual photoelectron lines were used. The surface potential was stabilized by the spectrometer charge neutralization system. The binding energy (BE) scale was referenced to the C 1s line of aliphatic carbon, set at 285.0 eV. Processing of the spectra was accomplished with the Kratos software. Powder samples of coffee grounds, the precursor, and QDs were gently pressed into a pellet directly on a sample holder using a clean Ni spatula. Energy Dispersive X-ray Spectrometry (EDS) Microanalysis. The elemental compositions for the graphene membranes after metal and micropollutant removal were mapped on an EDS detector (XMaxN, Oxford Instruments, UK) linked to the S-4800 SEM. The working voltage for EDS mapping was 20 kV.

RESULTS AND DISCUSSION Figure 1 describes the hypothesis on the mechanisms underlying the structural evolution from coffee grounds to ultrasmall and uniform QDs, sequentially involving (1) decomposition and carbonization of coffee grounds into a dot precursor consisting of fine carbon sheets and (2) oxygenation of the dot precursor in dilute acid to cut and exfoliate the sheets so as to obtain few-layer nanosheets constituting QDs.25 The hypothesis on the structural transformation of coffee grounds was preliminarily examined by SEM observations. Starting from the rough and hollow crude coffee ground particles (Figure 2a,d), the MAH process enabled the transformation to a fine dot precursor through decomposition and polymerization/carbonization (Figure 2b,e).42,46 The dot precursor was composed of highly stacked sheets featuring tight interlaminar attractions and wide dimensional distribution ranging from a few nanometers to several micrometers (Figure 2e). The intercalation of oxygen functional groups during the oxidation treatment led to uniform QDs with an expanded 5363


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Figure 5. Application of nanoporous graphene membranes for water purification. (a) Illustration of the filtration method to evaluate the water purification efficacy for the nanoporous graphene membranes, as delineated by the turbidity distinction between the feed and filtrated Cocontaminated solutions. (b) SEM micrograph of a Co-adsorbed graphene membrane showing the nanosized metal particles, as spatially imaged by (c) EDS elemental mapping. (d) SEM image of an Fe-filtrated membrane describing creation of nanochannels in the graphene wrinkles essential for high-rate flux.

development of carbon nanodots, these methods normally involve high technological requirements or carbon sources of high cost or toxicity.22,51 Here, turning crude biomass waste to value-added QDs under aqueous reaction conditions enables an economical and scalable strategy to multifunctional nanodots.30,52,53 The basic sheet units underlying the QDs were structurally attributed to nanosized graphene oxide (Figure 3f−h). HRTEM and FFT images of QDs indicated that the crystalline domains consisting of a hexagonal unit cell in a honeycomb network were featured by distinct lattice fringes with an inplane spacing of 0.214 nm, corresponding to the (100) plane of nanocrystalline graphite (Figure 3f).54 The presence of graphitic domains was further elucidated by the Raman spectrum of QDs showing the disordered D band (related to edge distortion and structural defects) at 1358 cm−1 and the ordered G band (indicative of the sp2 carbon structure) at 1582 cm−1 (Figure 3g).55,56 The large G to D intensity ratio (IG/ID), as high as 1.4, offered evidence for the extensive graphitization in QDs.38 Measurements of intersheet spacing by X-ray diffraction (XRD) confirmed the coexistence of a (002) facet of graphite, (001) lattice plane of graphene oxide, and disordered carbons (Figure 3h).57 This is in line with Fourier transform infrared spectroscopy (FTIR) characterization clearly identifying the skeletal carbons (C−H), conjugated carbons (CC), and oxygenated carbons (CO and C−O) in QDs (Figures 3i and S10).58−60 The oxygen functional groups in QDs were further explored by X-ray photoelectron spectroscopy (XPS) profiles, additionally confirming the coexistence of sp2 and sp3 carbons (Figures 3j and S11).61 The small size in combination with high oxygenation degree provided QDs with excellent dispersibility in water at high concentrations up to tens of grams per liter (Figure S11c). The excellent dispersibility of QDs in water and high affinity to carbon allotropes provided the possibility of aqueous processing of graphene nanosheets (Figure 4). The pristine graphene directly dispersed in water (0.5 mg/mL, G-QD0) showed poor dispersion stability with a large amount of precipitates after 1 week (Figure 4a). In contrast, the QDstabilized suspensions with QD/graphene weight ratios of 0.5 (G-QD0.5) and 1 (G-QD1) exhibited long-term dispersion stability. The QDs were regioselectively accommodated and

structure in the monolith and a narrow size distribution for the nanostructures (Figure 2c,f). With the high-throughput production of 9.6 g of QDs from 10 g of dot precursor, it is apparent that the MAH method signifies an efficient and economically viable approach to gram-scale fabrication of exfoliated nanodots, laying down a cornerstone for easy and effective design of carbon-based nanostructures with wellcontrolled dimensions. Combining an ultrasonic bath in aqueous hydrazine hydrate for 0.5 h, high-temperature autoclave treatment for 6−8 h, and dialysis for 2 days, Pan et al. reported the fabrication route to nanodots from coffee grounds with a yield of 33%.47 Compared to the conventional hydrothermal route, our proposed MAH approach holds distinct advantages in terms of high efficiency and low environmental impact, as well as control over the quality of carbon sheets and the QD product. Figure 3 offers insights into the morphological features, structural attributes, and chemical composition of the coffeeground-derived QDs. TEM images show ultralow dimensions for the QDs ranging from several nanometers to tens of nanometers (Figure 3a), displaying a gradual increase in the average diameter from 6.2 to 30.4 and 84.6 nm as the QD concentration increased from 0.05 to 0.5 and 2 mg/mL (Figure S3). The variations in the dimensions with prominent concentration dependence, as well as the structural resolution of individual QDs (Figure 3b), implied that the QDs were essentially assembled by the nanosized graphene oxide sheets.5,48 This assertion was further evidenced by AFM measurements showing distinct gaps between the height and the diameter of QDs (Figure 3c−e). Specifically, the height of QDs was in a narrow range of 1−3 nm (Figure 3e,f), likely revealing the origin from the assemblies of 2−10 smaller nanosheets.49 The dot precursor was presumably composed of tightly stacked large sheets (Figure S2), which were amenable to further exfoliation and assembling to sheet-stacking QDs. The dimension analysis indicates that the coffee-ground-derived QDs (approaching a lateral size of 6 nm at 0.05 mg/mL) fall into the category of the smallest carbon nanodots by top-down synthetic approaches, having sizes comparable to inorganic nanodots such as cadmium selenide.50 Although bottom-up syntheses, such as crystallization and polymerization of small carbon molecules, have made an essential contribution to the 5364



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CONCLUSIONS The MAH method was utilized to decompose and carbonize crude coffee grounds to fine carbon sheets, which were readily oxidized and exfoliated to highly uniform QDs of low dimensions (30 nm) and high structural integrity. The rich surface traps and oxygen functional groups inherent to the underlying ultrasmall graphene oxide nanodots provided QDs with excellent dispersibility in water and excitation-dependent emissions. In addition to distinct photoluminescence, the water-soluble QDs enabled aqueous processing of graphene nanosheets to nanoporous membranes constructed of wellexfoliated and associated sheets. Featuring large active surfaces and numerous nanochannels, the nanoporous graphene membranes exhibited exceptional adsorption capabilities for heavy metal ions and organic compounds even at a high flux rate approaching 1 L m−2 h−1. The effective control over the underlying carbon sheet structure, together with the simplicity of the synthetic process and the potential amenability to surface functionalization, can further expand the application range of biomass-derived QDs beyond applications as nanolights and dispersive surfactants.

anchored to the graphene surfaces, facilitating the exfoliation of graphene nanosheets and expansion of intersheet spacing (Figure 4b and S16), thus creating prerequisites for the formation of homogeneous QD-stabilized graphene suspensions. Upon adsorption of QDs onto graphene sheets, a set of stabilizing mechanisms including the creation of steric spacing and electrostatic repulsion were potentially introduced, permitting long-term stabilization of the graphene suspensions (Figure S15).19,62,63 Figure 4c illustrates that facile vacuum-assisted filtration of G-QD1, followed by washing three times with water to remove the QDs, offered a straightforward route to associated graphene membranes (weight of ∼50 mg, density of ∼0.07 g cm−3) with high structural integrity and uniformity. To the contrary, numerous large cracks were generated between the discrete flake fragments in the case of G-QD0 (Figures 4d and S17). The QD-processed graphene membranes were characterized by a thickness of approximately 200 μm and a highly porous topology both at the surface and in the interior (Figure 4e,f), providing a morphological analogue to the porous MoO2 nanosheets synthesized on nickel foam.64 Of interest is the generation of crumped thin graphene nanosheets spaced by numerous wrinkles that were probably created during the removal of QDs, contributing to the suppression of intersheet attraction-induced graphene stacks and inheriting the highly active surfaces of well-exfoliated graphene nanosheets. The nanoporous graphene membranes, featuring large active surfaces and rich wrinkles to store specific “particles” like electric charges and biological molecules, could have significant technological implications in diverse fields from energy devices to efficient absorbents.65−67 Recent scientific advancements have shown great promise for nanoporous graphene membranes in the rapidly growing area of water disinfection and desalination.68−70 Here, we evaluated the effectiveness of the aqueous-processed graphene membranes in removing heavy metal ions and organic compounds from water, as examined by direct filtration through the graphene membranes at a high flux rate approaching 1 L m−2 h−1 (Figures 5 and S19−S26). As demonstrated by the cases of Co, Fe, and 2,4-dichlorophenol pollutions, the large sheet surfaces, high surface activity, and rich nanochannels between crumped wrinkles of graphene contributed synergistically to rapid adsorption of metal ions (Figure 5b−d) and aromatic organic compounds (Figure S26), which is potentially further enhanced by the electrostatic forces in between. Figure 5b shows a large number of metal particles immobilized at the sheet surfaces, while the EDS microanalysis measured a prominent adsorption of 7 wt % Co in the filtrated graphene membrane based on the atomic weight proportions (Figures 5c and S20). Moreover, the uniform distribution of Co offered evidence for the accommodation of metal particles in the graphene wrinkles in addition to the deposition at the sheet surfaces. In particular, the generation of nanochannels likely facilitated the dynamics of water transport,71,72 accounting for the exceptionally high flux rate under atmospheric pressure while blocking pollutants (Figure 5d). As distinguished from the conventional solvent-processed graphene papers having condensed intersheet stacking,19,71,73 the nanoporous architectures in our graphene membranes can greatly enrich the active surfaces with wrinkled topology and enable effective sieving of specific particles at high flux rates, potentially providing a breakthrough strategy to advance the graphene-related membrane technologies.74−76

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00663. Experimental details; SEM images of coffee grounds and dot precursor; TEM and AFM images of QDs; Raman spectra; XRD profiles; EDS element analyses; FTIR spectra; XPS spectra; TGA curves of coffee grounds, dot precursor, and QDs; fluorescence properties of aqueous QDs; size and zeta potential measurements and TEM images of graphene/QDs; SEM images of G-QD0; UV− vis spectra and SEM images showing the removal efficiency of heavy metal and 2,4-dichlorophenol by graphene membranes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lan Xie: 0000-0003-0539-2077 Minna Hakkarainen: 0000-0002-7790-8987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.X. acknowledges financial support from the National Natural Science Foundation of China (21604016). The Swedish Research Council (VR) is acknowledged for financial support (contract grant number 2014-4091). H.X. is grateful for the financial support by the China Scholarship Council (CSC) for studying abroad. The authors are deeply indebted to Karin H. Adolfsson for generous help with experimental operation and valuable discussions.

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