pH-Controlled interfacial assembly and disassembly of highly luminescent

pH-Controlled interfacial assembly and disassembly of highly luminescent blue emitting Zn x Cd 1Àx S/dodecylamine complexesLinrui Hou,Caifeng Wang,Li Chen,Su Chen *State Key Laboratory of Material-Oriented Chemical Engineering,College of Chemistry and Chemical Engineering,Nanjing University of Technology,No.5Xin Mofan Rd.,Nanjing 210009,PR Chinaa r t i c l e i n f o Article history:Received 1April 2010Accepted 27May 2010Available online 1June 2010Keywords:Zn x Cd 1Àx S NCs Dodecylamine Quantum dots Blue emittingInterfacial assembly Interfacial disassemblya b s t r a c tWe present a robust strategy for assembling and disassembling of highly luminescent blue emitting Zn x Cd 1Àx S/dodecylamine (DDA)complexes via a facile water/chloroform interface.Firstly,thioglycolic acid (TGA)-stabilized ternary Zn x Cd 1Àx S nanocrystals (NCs)were synthesized in aqueous solution.And then,relying upon the facile water/chloroform interfacial platform,such NCs are assembled into the flake-like microstructures under the direction of DDA molecules when the pH of the water phase is adjusted into an optimal acidic range.Simultaneously,the NCs are transferred from the water into the chloroform phase due to the electrostatic interactions between carboxyl groups and amino of DDA mol-ecules.Interestingly,by reversibly adjusting the pH of the water phase into an optimal basic range,the flake-like Zn x Cd 1Àx S/DDA complexes are disassembled into separate NCs and DDA,and these NCs are reversibly transferred back into the water phase.The photoluminescence (PL)quantum yield (QY)of Zn x Cd 1Àx S/DDA complexes after interfacial assembly is improved to 55%from 6%.Particularly,a QY of about 22%still retains for the Zn x Cd 1Àx S NCs even after the interfacial disassembly.Ó2010Elsevier Inc.All rights reserved.1.IntroductionThe doped semiconductor NCs have attracted considerable interests because of their wide applications in diverse fields,such as biological labels [1],light-emitting devices (LEDs)[2]and nano-scale spintronic devices [3].To date,several semiconductor ternary alloyed NCs,such as Cd x Hg 1Àx S [4],Cd x Hg 1Àx Te [5],Cd x Zn 1Àx S [6],Pb 1Àx Mn x Se [7],Cd 1Àx Zn x Se [8]and Zn x Cd 1Àx S [9]have been successfully synthesized.To further extend their desirable applica-tions,it is of great importance to assemble NCs into multi-dimen-sional functional architectures,because controlled organization of ternary NCs into ordered structures allows for the coupling of the optical properties of individual NCs [10].Recently,the water/oil interface has been as an ideal platform for mediating the shape change of NCs and further affecting their collective properties [11–13].Wang and his coworkers transferred trioctylphosphine oxide-functionalized CdSe NCs from CHCl 3to water via a biphasic ligand exchange process and the resulting end product presented an increased PL in comparison with the original parent NCs [14].By making full use of a photocurable oil/water interface,Benkoski and his coworkers assembled CdTe NCs into irregular networks with a sponge-like appearance [15].In the assembly process of NCs,the nature of the bonding between NCs determines stabilityand dynamics of a NC aggregation,and even their collective prop-erties.For this reason,it is especially important to select suitable methods for the linkage of NCs to produce nanostructured aggre-gates,or clusters.Accordingly,different bond types,such as cova-lent [16–22],vander waals [23],coordinative [24–26]and hydrogen bond [27],have been applied.For these nanostructured aggregates,the fixed irreversible functions were obtained.In this work,different from all above bond types,we smartly re-port a flake-like Zn x Cd 1Àx S/DDA complex material with a novel reversible functions based on electrostatic interactions between the negative Zn x Cd 1Àx S NCs and the protonated DDA.As the con-trolled interfacial self-assembly of NCs can only take place in the water/oil interfaces via a slow interfacial diffusion process,it is vi-tal to properly choose the capping ligands to render NCs interfa-cially active [13].Therefore,the oil-soluble DDA bearing long alkyl side chain is a good choice for directing the interfacial assem-bly of water-soluble NCs.By virtue of a facile water/oil interface,where the as-prepared TGA-stabilized Zn x Cd 1Àx S NCs in water were applied as the water phase and the DDA dissolved in CHCl 3was served as the oil phase,DDA can induce the solubility change of NCs between water and CHCl 3by simply adjusting the pH value of the water phase.Interestingly,when the pH of the water phase is adjusted into an optimal acidic range,the NCs can be assembled into the flake-like microstructures under the direction of DDA,and the PL QY of NCs after interfacial assembly is enhanced from 6%to 55%.Within an optimal basic range,the flake-like Zn x Cd 1Àx S/DDA0021-9797/$-see front matter Ó2010Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2010.05.090*Corresponding author.Fax:+862583172258.E-mail address:chensu@ (S.Chen).complexes can be disassembled into the separate Zn x Cd1Àx S NCs and DDA reversibly.In particular,the QY about22%still retains for the Zn x Cd1Àx S NCs even after the interfacial disassembly.2.Experimental2.1.ChemicalsCadmium chloride(CdCl2Á2.5H2O),zinc chloride(ZnCl2),thio-glycolic acid(TGA),sodium sulfide(Na2SÁ9H2O),sodium hydroxide (NaOH),dodecylamine(DDA)and chloroform(CHCl3)were of ana-lytical grade and used as received.2.2.Synthesis of the water-soluble Zn x Cd1Àx S NCsAll manipulations were performed in open-air environment. The synthetic procedure was similar to that reported in the litera-ture with a minor modification[28].For the preparation of Zn0.02Cd0.98S,0.559g(2.45mmol)CdCl2Á2.5H2O and0.007g (0.05mmol)ZnCl2were simultaneously dissolved in20ml deion-ized water.Then,0.461g(5mmol)TGA dissolved in10ml deion-ized water was added under vigorously stirring,followed by adjusting the pH value to7by dropwise addition of5M NaOH solution.And then,0.361g(1.5mmol)Na2SÁ9H2O dissolved in 20ml H2O was added dropwise into the above mixed solution un-der stirring.The reaction was carried out for additional6h at room temperature.A series of Zn x Cd1Àx S NCs with different molar ratios of Zn and Cd,namely Zn/Cd=0.01/0.99,0.04/0.96,0.06/0.94,0.08/ 0.92,and0.10/0.90(mol/mol),were synthesized under the same conditions as Zn0.02Cd0.98S,keeping the total molar weight of [Cd2++Zn2+]as2.5mmol.Finally,the pH value of the water-soluble Zn x Cd1Àx S NCs was adjusted into the acid range by dropwise addi-tion of TGA aqueous solution for the next step use.2.3.Interfacial assembly of Zn x Cd1Àx S NCsThe interfacial assembly of Zn x Cd1Àx S NCs was performed as fol-lows:a certain amount of DDA was dissolved in50ml chloroform as the oil phase,and the above synthesized Zn x Cd1Àx S in water was used as the water phase,along with adjusting pH value into the acid range(pH<7).And then the vessel containing two phase solu-tion was sealed and maintained at room temperature for several days.Finally,the lower layer containing Zn x Cd1Àx S/DDA complexes was separated and divided into two parts:one part was subse-quently precipitated by addition of ethanol,and isolated by centri-fugation;the other part was reserved for the next step use.2.4.Interfacial disassembly of Zn x Cd1Àx S/DDA complexesThe disassembly procedure of Zn x Cd1Àx S/DDA complexes was reversible to the one used for assembly of NCs.The above reserved solution containing Zn x Cd1Àx S/DDA complexes was still used as oil phase,and sodium hydroxide aqueous solutions(25ml)with dif-ferent pH values were applied as the water phase,and then the vessel containing two phase solution was sealed and maintained at room temperature for another several days.Finally,the upper layer containing Zn x Cd1Àx S NCs was separated and subsequently precipitated by addition of ethanol,and isolated by centrifugation.2.5.CharacterizationElemental composition for Zn and Cd was obtained by induc-tively coupled plasma atomic emission spectroscopy(ICP-AES)by using a Perkin-Elmer Optima2100DV spectrometer,and the x va-lue for Zn x Cd1Àx NCs was the molar ratio of molar weight of[Zn2+]to total molar weight of[Cd2++Zn2+].Ultraviolet–visible(UV–vis) spectra were recorded on a Perkin-Elmer Lambda900UV–vis spec-trometer.Fluorescence spectra were recorded on a Varian Cary Eclipse spectrofluorometer with360nm laser beam as a light source,and the quantum yield was calculated by comparison with 9,10-DPA with quantum yield(QY)at95%.Transmission electron microscope(TEM)images were collected on a JEOL JEM-2100 electron microscope.High-resolution transmission electron micro-scopic(HRTEM)observation was performed with a JEOL JEM-2010 transmission electron microscope.The microstructures of the CdS/ DDA hybrids were observed by scanning electron microscopy (SEM)with a QUANTA200(Philips-FEI,Holland)instrument at 30.0kV.The X-ray diffraction(XRD)patterns were conducted on Bruker-AXS D8ADVANCE X-ray diffractometer with Cu K a radia-tion(k=0.1542nm)at a scanning speed of6°/min over2h range of10–80°.Fourier transform infrared(FT-IR)were recorded on a Nicolet6700FT-IR spectrometer and the samples were ground with KBr crystals,and the mixture was pressed into a pellet for IR measurement with32scans from4000to500cmÀ1at a resolu-tion of4cmÀ1.3.Results and discussionTGA-stabilized Zn x Cd1Àx S NCs herein were synthesized by the reaction of cadmium chloride,zinc chloride and sulfur ions in the presence of TGA as the organic ligand.TGA is a common ligand pro-viding solubility for NCs and simultaneously hindering the further growth of crystal grains,which is responsible for the formation of nanosized crystals.Recently,the color-tunable emission of Zn x Cd1Àx S NCs were prepared by changing constituent stoichiome-tries composition of NCs,however,the reaction had to be per-formed at relatively high temperature($310°C)[9].In our case, the preparation of Zn x Cd1Àx S NCs was done only at room temperature.Fig.1presents the XRD patterns of the TGA-stabilized Zn x Cd1Àx S NCs with varied x value(x=0.01$0.10).The considerable broad-ening of diffraction peaks reflects the formation of the smaller NCs.As shown in Fig.1,the characteristic XRD patterns of the al-loyed NCs with different Zn molar fractions all exhibit three prom-inent peaks,which can be indexed to the(111),(220)and(311) planes[29],indicating the cubic structure of the ternary alloyed NCs.Moreover,the diffraction peaks gradually shift toward low an-gle with increasing the Zn molar fraction in NCs.The gradual peak shifting of the NCs may also rule out the phase separation or sep-patterns of TGA-stabilized Zn x Cd1Àx S NCs:(a)x=0.01,0.06,(e)x=0.08and(f)x=0.10.L.Hou et al./Journal of Colloid and Interface Science349(2010)626–631627is indicative of the formation of alloyed Zn x Cd 1Àx S NCs [4,31,32].From Fig.2b,we have found that the PL intensity of NCs is strongly affected by Zinc molar fraction in NCs.The PL intensity increases gradually with an increase in zinc molar fraction and reaches a maximum value at x =0.06.Although controlling constituent stoi-chiometries in alloyed NCs can improve their PL properties to a cer-tain extent,the PL emission of TGA-stabilized Zn x Cd 1Àx S NCs is mainly dominated by broad trap emission.To further improve the PL properties,the initial TGA-stabilized Zn x Cd 1Àx S NCs solution with the highest PL intensity was applied as the water phase,and DDA in the CHCl 3was served as the oil phase to further tailor the surface structure change of the NCs.The strategy involves the selection of suitable experimental conditions,for this reason,we checked the influences of pH value,DDA concentrations and the interfacial reaction time on the PL properties of Zn x Cd 1Àx S/DDA complexes,respectively.Fig.3a shows the influences of pH value on the PL properties of Zn 0.06Cd 0.94S/DDA complexes.As the degree of protonation of DDA is altered with pH,pH value is a key factor deciding the occurrence of phase transfer.If the pH value of the water phase is adjustedheal surface traps participating in nonradioactive recombination processes,thus improve the PL properties of NCs [33–35].How-ever,at pH value lower than 5.8,the decrease of PL intensity is observed,probably because the Zn x Cd 1Àx S NCs are partially decom-posed in strong acidic medium.In this case,the pH value of 5.8is much desirable for the assembly of NCs with DDA.Fig.3b presents the dependence of the PL intensity of Zn 0.06Cd 0.94S/DDA complexes on DDA concentrations.Upon the addition of DDA in the chloro-form phase,the NCs are spontaneously transferred from the water to the chloroform phase.Furthermore,the PL intensity of Zn 0.06Cd 0.94S/DDA complexes increases with the increase of DDA concentrations and reaches the maximum at [DDA]=0.125M.With increasing DDA concentration,the surface traps are gradually healed.However,with excessive DDA in chloroform,besides elec-trostatic force,lots of DDA can be adsorbed on the NC surface via Vander Waals force to result in the surface disorder of NCs,and these new sites may serve as traps and prohibit radioactive recom-bination of electrons and holes.Fig.3c shows the PL spectra of Zn 0.06Cd 0.94S/DDA complexes as a function of reaction time as well as the corresponding parent TGA-stabilized Zn 0.06Cd 0.94S NCs.In Fig.2.(a)UV–vis absorption and (b)PL spectra of TGA-stabilized Zn x Cd 1Àx S NCs.PL spectra of Zn 0.06Cd 0.94S/DDA complexes at different effect factors:(a)pH ([DDA]=0.125M,interfacial reaction time:3d);(b)DDA concentrations (pH interfacial reaction time:3d)and (c)interfacial reaction time (pH =5.8,[DDA]=0.125M).adjusting the pH value into the basic range,the TGA-stabilized Zn 0.06Cd 0.94S NCs were slowly transferred back to the water phase passing across the water/oil interface due to the deprotonation of DDA.In the controlled experiment,all the reactions were per-formed under a still environment and were achieved via a slow diffusion-controlled process,and the phase transfer can be control-lable through tuning the reaction time and the pH in water phase.For the reversible transfer process,we specially investigate the influences of pH value and secondary interfacial reaction time on the PL properties of secondary TGA-stabilized Zn 0.06Cd 0.94S NCs (Fig.4).As observed from Fig.4a,the increase of pH value enhances the PL intensity of secondary TGA-stabilized Zn 0.06Cd 0.94S NCs.The PL enhancement effect can be attributed to the increase of the numbers of NCs transferring back into the water because of the detachment of DDA from NC surfaces.When the pH value is up to 10,the PL intensity obtains its maximum,and then again de-creases.The gradual drop may possibly result from the partial floc-culation of NCs under the stronger basic environment.As shown in Fig.4b,the PL intensity increases with prolonged reaction time and reaches a maximum value when reaction time proceeds 5d.After-wards,the PL intensity starts to decrease with reaction time,pos-sibly due to the appearance of dislocations and new traps with further detachment of DDA.The phase transfer and reversible phase transfer of Zn x Cd 1Àx S NCs between the water and chloroform phases was confirmed by FT-IR investigations.Fig.5a shows the FT-IR spectrum of TGA-sta-bilized Zn 0.06Cd 0.94S NCs.Strong absorption peaks at 1380and 1560cm À1(v COO À)reveal the existence of the alkaline carboxyl groups on the surfaces of Zn 0.06Cd 0.94S NCs,and there is no absorp-tion band attributing to the S-H vibrations,indicating that the TGA molecules are surely bond on the NC surface.As shown in Fig.5b,be-sides the existence of a new characteristic absorption peak at 1630cm À1(v NH ),the peaks at 1380and 1557cm À1(v COO À)still ex-ist in Zn 0.06Cd 0.94S/DDA complexes,furthermore,there is no char-acteristic peaks (from 1680to 1670cm À1)attributing to acylamide,suggesting the other interactions rather than covalent bond between the alkaline carboxyl of CdS NCs and amino of DDA.In fact,CdS NCs capped by TGA contains negative carboxyl group,while DDA molecules contain positive charge at the unchanged,indicating the unchanged size of NCs during the assembly and disassembly processes.Fig.6b shows the corre-sponding PL spectra of Zn 0.06Cd 0.94S NCs,Zn 0.06Cd 0.94S/DDA com-plexes and secondary Zn 0.06Cd 0.94S NCs.The PL spectrum of the parent CdS NCs shows a broad peak centered at 552nm,mainly originating from the broad emission of the recombination of sur-face electron and hole (QY ca.6%).Compared with the PL of Zn x Cd 1Àx S NCs,the PL peaks of Zn x Cd 1Àx S/DDA complexes present the blue-shift of the maximum band position and the narrowing of the peak widths at half-maximum after interfacial assembly,along with enhancement on the PL intensity (QY ca.55%).The reduction of surface traps with the help of the DDA modification via the interfacial assembly should be responsible for the corre-sponding PL blue-shift and enhancement of PL intensity [38,39].When the NCs were reversibly transferred back to the water phase,the relatively strong QY about 22%is still obtained.The PL change can be further confirmed by the digital photographs shown in the inset of Fig.6b.The pH-adjusted electrostatic interactions between the nega-tively charged TGA-stabilized Zn 0.06Cd 0.94S NCs and DDA not only can mediate the solubility of NCs between water and CHCl 3,but secondary TGA-stabilized Zn 0.06Cd 0.94S NCs at different effect factors:(a)pH (secondary interfacial reaction time:5d)and spectra of (a)TGA-stabilized Zn 0.06Cd 0.94S NCs,complexes (pH =5.8,[DDA]=0.125M,interfacial secondary Zn 0.06Cd 0.94S NCs (pH =10,secondary L.Hou et al./Journal of Colloid andHalas et al.[40]and Osterloh et al.[41],where the NCs are directly attached onto the silica spheres via electrostatic interactions.In our case,the resultingflake-like structure arises from the self-assembly of NCs under the direction of DDA,which is elucidated in Scheme1and Fig.7c.The structural change probably results from the dipolar–dipolar interactions among the NCs[42,43].As shown in Scheme1,when the pH of the water phase is adjusted to the acidic range,DDA is protonated in the biphasic interface, thus the TGA-stabilized Zn0.06Cd0.94S NCs are dragged from the water phase to the biphasic interface by DDA due to the electro-static interactions between the negative Zn x Cd1Àx S NCs and the protonated DDA.The NCs driven under dipolar–dipolar interaction rearrange orderly in the biphasic interface to obtainflake-like structure.From the HRTEM of a Zn0.06Cd0.94S/DDAflake(Fig.7c), NCs with differently oriented lattice planes remain in the nanoflake,which further confirms the formation offlake construct-ing from the NCs.Most interestingly,when the pH value of the water phase is adjusted into the modest basic range,the NCs are transferred back to the water phase from the oil phase again due to the disappearing of electrostatic interactions between NCs and DDA,and theflake-like structures are simultaneously disassem-bled into separate NCs(Scheme1).As shown in Fig.7d,the rela-tively uniform dispersion NCs with the diameter of ca.4nm still are retained in the secondary TGA-stabilized Zn0.06Cd0.94S NCs. 4.ConclusionWe have demonstrated a robust strategy to induce the assem-bly of TGA-stabilized NCs with DDA to theflake-like aggregationspectra of(1)TGA-stabilized Zn0.06Cd0.94S NCs,(2)Zn0.06Cd0.94S/DDA complexes(pH=5.8,[DDA]=0.125M,interfacial0.06Cd0.94S NCs(pH=10,secondary interfacial reaction time:5d).Insets of(b)indicate the corresponding digital TGA-stabilized Zn0.06Cd0.94S NCs(2)Zn0.06Cd0.94S/DDA complexes and(3)secondary TGA-stabilized Zn0.06Cd0.94S NCs taken underZn0.06Cd0.94S NCs,(b)SEM and(c)HRTEM of Zn0.06Cd0.94S/DDA complexes(pH=5.8,[DDA]=0.125M,Cd0.94S NCs(pH=10,secondary interfacial reaction time:5d).via a water/oil interface by adjusting the pH value of the water phase into the optimal acidic range and the QY of NCs after self-assembly is improved to55%from6%.Importantly,when the pH value of the water phase is readjusted into the optimal basic range, theflake-like aggregations again disassemble into the separate NCs via the water/oil interface.Moreover,the NCs are transferred back into the water phase,and QY about22%still retains for the NCs. 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Schematic illustration of the interfacial assembly and disassembly processes of theflake-like Zn x Cd1À。