Cationic Ligand Protection A Novel Strategy for One-Pot Preparation of Narrow-Dispersed Aqueous CdS

https://www.360docs.net/doc/e49257347.html,/Langmuir

?2009American Chemical Society

Cationic Ligand Protection:A Novel Strategy for One-Pot Preparation of Narrow-Dispersed Aqueous CdS Spheres

ChunLei Wang,Hao Zhang,Zhe Lin,Xi Yao,Na Lv,MinJie Li,HaiZhu Sun,JunHu Zhang,and

Bai Yang*

State Key Laboratory of Supramolecular Structure&Materials,College of Chemistry,Jilin University,

Changchun130012,People’s Republic of China

Received March25,2009.Revised Manuscript Received June2,2009

In order to prepare the building blocks with middle refractive index for fabricating colloidal crystals,a new strategy of cationic ligand protection(CLP)was developed for one-pot preparation of narrow-dispersed CdS spheres with tunable sizes(94-303nm)and good aqueous dispersibility.The key of CLP strategy is controlling the aggregation behavior of small CdS nanocrystals(NCs)via automatic control of the ligand modification condition at different reaction stages, which is realized by selecting gradual decomposition of thioacetamide(TAA)as the anionic precursor and cetyltrimethylammonium bromide(CTAB)cations as the ligands for anions.Accordingly,at the initial stage of the reaction when TAA decomposes incompletely,the surface atoms of the CdS NCs are mainly Cd cations,leaving less anionic sites for the coordination of ligands.The poor ligand modification condition of CdS NCs still leads to their aggregation.Along with the decomposition of TAA,S anions and cationic ligands gradually dominate the surface of CdS aggregates,providing the aggregates sufficient protection,and thus,they are stably dispersed in water.The current CLP strategy simultaneously involves the processes of NC formation and NC assembly and therefore promotes the one-pot preparation of NC aggregates.The as-prepared CdS spheres possess uniform size,good water-dispersibility, and relative higher refractive index and thus can be applied for fabricating colloidal crystals.

Introduction

In recent years,semiconductor nanocrystals(NCs)have at-tracted great fundamental and technical interest because of their potential applications in light-emitting diodes,lasers,solar cells, and etc.1-3The basis of various applications is the preparation of NCs with high quality.Among numerous physical and chemical routes of NC preparation,the colloidal chemistry method is the best one for synthesis of NCs with controllable sizes,shapes, composition,and properties.4-6The colloidal method usually employs two types of materials,namely,the precursor and the ligand.The former is used to create the core of NCs,and the latter is used to prevent the aggregation of NCs.Through selection of various precursors and ligands,NCs with different shapes and functions can be prepared.The colloidal method also makes it possible to synthesize NCs alternatively in the aqueous solution or nonaqueous media.Recently,great progresses have been made on the basis of nonaqueous synthesis.The growth mechanisms of NCs have been gradually recognized,which further promoted the development of various synthesis strategies.7-9Up to now,NCs with various shapes,for instance,rod,rice,tadpole,branched shape,and so on,can be prepared via different synthesis strate-gies.10-12Currently,the emphasis on synthetic chemistry has become the design of NC aggregates with complex structure and composition.In2006,Peng’s group reported a limited ligand protection(LLP)strategy for one-pot preparation of hydropho-bic semiconductor nanoflowers.13,14The key to LLP is controlling the molar ratio of precursors and ligands,allowing for insufficient protection of NCs but adequate protection of the resulting aggregates.Such an LLP strategy simultaneously involves the processes of NC formation and NC assembly and thus can directly prepare NC aggregates.Unlike the traditional SiO2and polystyrene spheres which have relative low refractive indices (1.45for SiO2and1.59for polystyrene spheres),15-17semicon-ductor NC aggregates possess much higher refractive indices,18,19 making them ideal materials for fabricating photonic crystals with tunable and/or switchable band structures.20,21

*To whom correspondence should be addressed.E-mail:byangchem@jlu. https://www.360docs.net/doc/e49257347.html,.Telephone:+86-431-85168478.Fax:+86-431-85193423.

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Article Wang et al.

In parallel with nonaqueous synthesis routes,aqueous methods have also been developed to prepare semiconductor NCs.22-24 The great advantage of hydrophilicity and diverse surface func-tionalities of aqueous NCs facilitate their conjugation with other ions,molecules,polymers,or biomolecules in various applica-tions.25,26Recently,the explicit structure and the growth mechan-ism of aqueous NCs have been revealed,27-30which promotes the preparation of aqueous nanorods and nanotubes.31-33In order to obtain aqueous aggregates with desirable structures and sizes,the two-step strategy is usually adopted,which contains the prepara-tion process of aqueous NCs and the assembly process of preformed NCs.34-41For instance,the preformed CdTe NCs can form microhexagonal columns,free-floating sheets,and pearl-necklace aggregates by means of self-assembly or oriented attachment.31-33Unfortunately,up to now,there are few effec-tive methods for preparing aqueous aggregates in a single pot.In this work,we develop a novel aqueous cationic ligand protection (CLP)strategy for one-pot preparation of spherical CdS aggre-gates.This CLP strategy simultaneously involves the processes of NC formation and NC assembly and hence is able to prepare aqueous aggregates in one pot.Through CLP strategy,CdS spheres with tunable sizes(94-303nm),narrow size distribution, and good aqueous dispersibility are obtained.The as-prepared CdS spheres are applied for fabricating colloidal crystals,since the relative higher refractive index of CdS(2.37)than that of tradi-tional SiO2and polystyrene spheres(1.45for SiO2and1.59for polystyrene spheres)15-17makes it possible for fabricating colloi-dal crystals with tunable and/or switchable band structures.

Experimental Section

Materials.All materials used in this work were analytical reagents.Cadmium acetate(Cd(Ac)2),cetyltrimethylammonium bromide(CTAB),cetyltrimethylammonium chloride(CTAC), sodium dodecyl sulfate(SDS),and acetic acid(HAc)were purchased from the Beijing Chemical Factory,China.Thioace-tamide(TAA)was purchased from Aldrich.Octadecyl-p-vinyl-benzyldimethylammonium chloride(OVDAC)was synthesized according to a previous work.25

Synthesis of CdS Spheres.The mixture of Cd(Ac)2and CTAB solutions was adjusted to pH5.2by using0.1mol/L HAc before the addition of TAA.The total concentration of Cd(Ac)2in solution was2.27?10-2mol/L,and the molar ratio of Cd(Ac)2/TAA/CTAB was4:4:1.To prepare CdS spheres,the crude solution was heated in an80°C oven for2.5h,and then the solution was cooled to room temperature(25°C).Scanning electron microscopy(SEM)was used to analyze the size and size distribution of CdS spheres.At least30CdS spheres were measured for each sample to estimate the size distribution.CdS solution was directly used for SEM and TEM measurements without any purification.

In order to monitor the growth process of CdS spheres,the crude solution was heated at40°C,and aliquots of the sample were taken at different intervals for transmission electron micro-scopy(TEM)measurements.

For fabricating colloidal crystals with CdS spheres,a puri-fication process was https://www.360docs.net/doc/e49257347.html,ly,the preformed CdS spheres were first separated by centrifugation and then redis-persed in water.CdS colloidal crystal chips were obtained by transferring drops of the purified CdS solution to Si or SiO2 substrates by pipet,and then dried at room temperature.SEM and transmission spectra were employed to measure the colloidal crystal chips.

Characterization.SEM images were recorded with a JEOL FESEM6700F electron microscope with a primary electron energy of3kV.TEM and selected area electron diffraction (SAED)were recorded by using a JEOL-2010electron micro-scope with an acceleration voltage of200kV.The resultant CdS solution was directly used for SEM and TEM characterization without any filtering or size-selective precipitation.X-ray powder diffraction(XRD)investigation was carried out by using a Sie-mens D5005diffractometer.X-ray photoelectron spectroscopy (XPS)was investigated through a VG ESCALAB MK II spectro-meter with Mg K R excitation(1253.6eV).The binding energy calibration was based on C1s at284.6eV.XRD and XPS measurements were carried out with the powder of CdS.To obtain such powder,CdS spheres were separated from the solu-tion by centrifugation and then dried in vacuum.Transmission spectra were recorded with a Shimadzu3100UV-vis near-infrared spectrophotometer.

Results and Discussion

One-Pot Preparation of Aqueous CdS Spheres by CLP Strategy.Through selection of cationic ligands,CdS spheres with uniform size and good dispersibility were prepared(Fig-ure1).By proper control of the reaction conditions,the sizes of CdS spheres could be adjusted from94to303nm.Typically,low reaction temperature and short reaction time benefited the formation of CdS spheres with small sizes.The detailed size and size distribution of CdS spheres at different reaction times are shown in Figure S1in the Supporting Information.With the prolonged reaction time,the size of CdS spheres increased gradually.XRD results(Figure2)of CdS spheres showed a wurtzite structure of the hexagonal bulk CdS.The diffraction peaks at24.9,26.6,28.1,43.8,47.9,and51.9°,respectively, corresponded to the(100),(002),(101),(110),(103),and(112) faces of hexagonal CdS(JCPDS card no.77-2306).Although the intensity of the(002)face was abnormal owing to the slightly anisotropic growth of CdS NCs along the(002)face in the presence of CTAB,TEM images indicated that CdS NCs were generally in the form of spherical particles(Figure3).Moreover, the broad diffraction peaks in the XRD pattern were a typical characteristic of nanomaterials,implying the as-prepared CdS spheres were composed of CdS NCs.According to the Debye-Scherer formula,the diameter of CdS NCs was calculated as7nm by using the diffraction peak at the(110)face.The SAED pattern exhibited broad diffuse rings(Figure2,inset),indicating the polycrystalline nature of the colloids.Obviously,the observed

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Wang et al.Article

sub-micrometer sized CdS spheres were not single crystals but the assemblies of small CdS NCs.

In order to investigate the formation process of CdS spheres in CLP strategy,aliquots of the sample were taken at different reaction periods.Moreover,for better observation of the whole evolution process of CdS spheres,a low reaction temperature (40°C)was adopted to decrease the reaction rate.From Figure 3,it can be observed that the color of the solution changed from colorless to light-green after heating for a short time.Simulta-neously,the TEM image showed that NCs had diameters of 1-10nm (Figure 3a).With the prolonged reaction time,some aggregates with diameters of several tens of nanometers pre-sented,meanwhile the solution color changed to green-yellow (Figure 3b).Further thermal treatment resulted in a yellow solution with bad light transmission,and moreover,the sizes and amount of the aggregates gradually increased (Figure 3c and d).Similar results were also observed for the CdS spheres reacted at 80°C (Figures S2and S3in the Supporting Informa-tion).Obviously,according to the results of TEM,the evolution process of CdS spheres was proved via NC assembly.Moreover,the formation process of CdS spheres was also reflected by the photoimages in Figure 3and UV -vis spectra in Figure S4in the Supporting Information.Since the intensity of scattering light related to the radius of particles in solution,the transparency of the solution reflected the size of CdS spheres.As shown in Figure 3,CdS solution showed good transparency at the initial stage of the reaction (Figure 3a),implying only small NCs (1-10nm)formed in solution.With the growth of CdS spheres,the light scattering become more serious,leading to the decreased transparency of CdS solution (Figure 3c and d).Overall,the current CLP strategy simultaneously contained the processes of NC formation and NC assembly,and thus was available to prepare aggregates in one-pot.

XPS was also used to quantitatively measure the surface composition of CdS spheres (Figure 4).Because Br atoms could only come from CTAB,the dramatically increased Br content in the reaction process actually suggested the alteration of ligand modification conditions at different reaction stages.As we know,CTAB is a surfactant with both a hydrophilic head and hydro-phobic tail.When CTAB was dissolved in acid solution,the Br atom would be dissociated from CTAB,making the left CTA +act as the actual cationic ligand.In the current work,when the hydrophilic head (CTA +)coordinated with the S anions on the CdS surface,the hydrophobic tail would direct to the solution,making CTA +capped CdS spheres hydrophobic.This

was

Figure 1.SEM images of CdS spheres by heating at (a)40°C

for 1.5h,(b)40°C for 5h,(c)80°C for 2.5h,and (d)80°C for 4h.The size distribution (inset)was obtained by statistics of 30CdS spheres.The concentration of Cd(Ac)2was kept at 2.27?10-2mol/L,whereas the concentration of CTAB was 5.7?10-3mol/L for (a -c),and 3.4?10-3mol/L for (d).Scale bars are 500nm.The diameters of CdS spheres are,respectively,94,142,220,and 303nm in (a -

d).

Figure 2.XRD and SAED (inset)patterns of CdS spheres.The standard bulk hexagonal CdS is shown on the

bottom.

Figure 3.TEM images and the corresponding optical photo-graphs of CdS spheres with different reaction times:(a)0.5h,(b)1.5h,(c)2.5h,and (d)16h.Scale bars are 100nm.Reaction temperature was 40°

C.

Figure 4.XPS data of CdS spheres after reaction of 1h (top)and

2.5h (bottom)at 80°C.From left to right,the spectra are Br 3d,S 2p,and Cd 3d levels,respectively.

Article Wang et al.

apparently opposite with the experimental result,since CdS spheres had good aqueous dispersibility.The most possible case was the formation of CTAB double https://www.360docs.net/doc/e49257347.html,ly,another layer of CTAB adsorbed on CTA +capped CdS spheres,making the hydrophilic head direct to the solution and the hydrophobic tail interact with the tail of CTA +through hydrophobic -hydro-phobic interaction.42-45Obviously,in the current CLP strategy,the cationic ligands played a key role in the assembly process of CdS aggregates.Experimentally,when no CTAB or insufficient CTAB (Figure 5a and b)was used,we could only obtain CdS aggregates with irregular shapes.In contrast,CdS spheres with uniform size and good dispersibility could be prepared at the same condition except for the addition of sufficient CTAB (Figures 5c and 1).Besides,other types of cationic ligands,for instance,CTAC and OVDAC (Figure 5d and e),were also able to prepare quasi-spherical CdS aggregates.While anionic ligands,for in-stance,SDS,could only obtain some nanoobjects with irregular sizes and shapes (Figure 5f),suggesting the key role of cationic ligands in the assembly process of CdS aggregates.Notably,unlike the works of Matijevi c et al.that reported the preparation of CdS spheres without the assistance of ligands,46the cationic ligands in the current CLP strategy were necessary.Such differ-ence might attribute to the different reaction conditions adopted in each other.Moreover,compared with the undispersed CdS spheres without ligand modification,the current CdS spheres prepared by the CLP strategy had excellent aqueous dispersibility and multitudinous surface functional sites,which were essential for the subsequent assembly and composite processes in various applications.

On the basis of the aforementioned discussion,it could be easily comprehended that the key of CLP strategy was the automatic control of the aggregation behavior of small CdS NCs.Because CTAB could only coordinate with S anions in acid solution,at the initial stage of the reaction when only parts of TAA decomposed,the surface atoms of CdS NCs were dominated with Cd cations,leaving less anionic sites for coordinating with cationic ligands.Consequently,the insufficient ligand modification condition of CdS NCs led to their aggregation.Along with the decomposition

of TAA,S anions and ligands would be enriched on the surfaces of CdS aggregates,making them dispersible in solution.Appar-ently,the aggregation behavior of CdS NCs was automatically controlled by the alteration of ligand modification conditions at different reaction stages,which was realized by selection of gradually decomposed TAA as the anionic precursors 47and CTAB as the cationic ligands for the anions.It should be emphasized that the current CLP strategy adopted aqueous media and cationic ligands,providing a novel aqueous strategy for one-pot preparation of NC aggregates.The current CLP strategy was totally different from the LLP strategy which employed anionic ligands and nonaqueous media.13,14

Influence of Experimental Variables on the Size and Structure Evolution of CdS Spheres.Because CdS spheres formed and grew via the assembly of CdS NCs,the size of CdS spheres should be determined by the relative amount of CdS aggregates and the CdS NCs remaining in solution.From another point of view,the formation process of CdS spheres could also be considered as the growth process of CdS aggregates with CdS NCs as the “monomers”of CdS spheres and small aggregates (Figure 3b)as the “nuclei”of CdS spheres.In other words,CdS spheres grew at the cost of consuming CdS NCs.In fact,such a viewpoint is reflected by Figures S2and S3in the Supporting Information.As could be seen,the amount of CdS NCs decreased dramatically as the reaction proceeded,whereas the amount of CdS spheres increased with the prolonged reaction time,implying the growth of CdS spheres by consuming CdS NCs.Though the concepts of monomers and nuclei were typically for colloids and NCs,7-9they were also available in the current work because the as-prepared CdS spheres belonged to the category of colloids.Unlike the molecular monomers in the field of NCs,the “mono-mers”in the growth process of CdS spheres were small CdS NCs,and the growth process of CdS spheres should be determined by the relative ratio of “monomers”/“nuclei”.

First,the influence of the reactant concentration was investi-gated.As shown in Figure 6,when the molar ratio of Cd(Ac)2/TAA/CTAB was kept as 4:4:1,the size of CdS spheres related to the concentration of Cd(Ac)2.The maximal size of CdS spheres presented at a Cd concentration of 1.14?10-3mol/L.Further increase of the reactant concentration inversely led to the decrease of sphere size.It was comprehensible,since the increase of reactant concentration would lead to the increased amount of both CdS NCs (“monomer”)and the small aggregates

(“nuclei”).

Figure 5.SEM images of CdS spheres with different concentrations of ligand:(a)without CTAB addition,(b)CTAB of 1.13?10-3mol/L,

(c)CTAB of 2.27?10-3mol/L,(d)CTAC of 5.7?10-3mol/L,(e)OVDAC of 5.7?10-3mol/L,and (f)SDS of 5.7?10-3mol/L.Scale bars are 1μm.

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Wang et al.Article

The final size of CdS spheres was determined by the ratio of “monomers”/“nuclei”.Apparently,the sample with a Cd con-centration of 1.14?10-3mol/L possessed the maximal ratio of “monomers”/“nuclei”,making the as-prepared CdS spheres have the maximal sphere size.

Second,the influence of the reactant ratio was investigated in Figure 7.When the concentration of Cd(Ac)2was 2.27?10-2mol/L (Figure 7a -d),the size of CdS spheres changed dramati-cally with the increased concentration of TAA.Moreover,we also compared the size of CdS spheres at a Cd(Ac)2concentration of 1.14?10-2mol/L (Figure 7e -h),and a similar trend was also found.This was easily comprehended,since the decomposition of TAA determined the content of S anions and ligands modification condition of CdS spheres,and thus affected the NC assembly process.Meanwhile,TAA also determined the production rate of CdS NCs,which directly related to the ratio of “monomers”/“nuclei”.Therefore,the concentration of TAA significantly affected the size of CdS spheres.

Third,the solution pH also affected the quality of CdS spheres (Figure 8).In acid solution,CdS spheres exhibited good dispersion.

This implied the key role of cationic ligands (CTA +)in controlling the assembly process of CdS spheres in the CLP strategy.In comparison,the concentration of CTA +reduced in basic solu-tion,making cationic ligands and the CLP strategy invalid,and hence macroscopic aggregates appeared.On the other hand,the production rate of CdS NCs also related to the pH of the solution.In acid conditions,the decomposition resultant of TAA was H 2S,which needed the deprotonation process before the reaction with Cd cations,thus benefiting the gradual aggregation of CdS NCs.While in the basic solution,the resultant was S 2-,which

could

Figure 6.SEM images of CdS spheres with Cd(Ac)2concentration of (a)2.27?10-3mol/L,(b)1.14?10-2mol/L,and (c)2.27?10-1mol/L.

Scale bars are 500nm.The molar ratio of Cd(Ac)2/TAA/CTAB was kept at

4:4:1.

Figure 7.SEM images of CdS spheres with different molar ratios of reactants.The size distribution (inset)was obtained by statistics of 30

CdS spheres.The concentration of Cd(Ac)2was 2.27?10-2mol/L in (a -d)and 1.14?10-2mol/L in (e -h).The concentration of TAA was 0.57?10-2mol/L in (a,e),1.14?10-2mol/L in (b,f),2.27?10-2mol/L in (c,g),and 4.54?10-2mol/L in (d,h).Scale bars are 1μm in (a -h).The corresponding concentration of Cd(Ac)2and TAA,together with the diameters of CdS spheres,is

plotted.

Figure 8.SEM images of CdS spheres prepared at pH of 5.2(a)

and 9.0(b).Scale bars are 500nm.

Article Wang et al.

directly react with Cd 2+,making the aggregation process too fast to be easily controlled.23

Applications of CdS Spheres.The current CLP strategy provided a novel method for one-pot preparation of aqueous CdS spheres.The uniformity of CdS spheres made them ideal building blocks for fabrication of colloidal crystals.Simply by the dryness of the purified solution (see Experimental Section),CdS spheres could be fabricated to colloidal crystals.As shown in Figure 9,the colloidal crystals of CdS spheres showed a face-centered cubic lattice.Moreover,multilayer structure of the assembling could be observed,reflecting the formation of three-dimentional colloidal crystals.Transmission spectra of the CdS colloidal crystal chips (Figure 9c)exhibited obvious diffraction peaks.The position of the diffraction peak (λmin )changed with the diameters of building blocks.For instance,λmin was,respectively,618and 689nm when CdS spheres with diameters of 180and 200nm were used.According to the Bragg equation,the refractive index of spherical building blocks could be calculated:48

λmin

?22 1=2R fn block 2te1-f Tn void 2

tsin 2θ

h i 1=2where R indicates the diameter of the spherical building blocks;f is the volume diffraction of the colloidal crystals;θrepresents the angle between the incident light beam and the normal to the surface of the crystal;and n block and n void are the refractive indices of the building blocks and the void between them in colloidal crystals.In the current work,sin θwas 0and n void was 1.0(the refractive index of air).f was assumed to be 0.7405according to the face-centered cubic lattice.48Therefore,the refractive index of CdS spheres was calculated as 2.37,which was similar to the reference value.18

As we know,the tunable refractive index of the building block was essential for fabricating photonic crystals with adjustable photonic band gaps.20,21Up to now,SiO 2and polystyrene spheres were the most popular building blocks,which had relative low refractive indices (1.45for SiO 2and 1.59for polystyrene).15-17Recently,Se@CdSe spheres with a high refractive index of 2.84were also reported as novel building blocks.21,35As for building blocks with a middle refractive index (1.59-2.84),there is still not

much work nowadays.Apparently,the current work provides a novel method for preparing building blocks with a middle refractive index (1.59-2.84).Moreover,the excellent aqueous dispersibility and multitudinous surface functional sites of CdS spheres facilitated their subsequent modification and assembling process in various applications.For instance,it was possible to prepare CdS composite spheres with SiO 2,TiO 2,or polystyrene layers,49which would be potentially applied in photonic crystals,photocatalysis,and photovoltaics.The corresponding works are also under way in our laboratory.

Conclusions

In the current work,a novel CLP strategy was originally developed for one-pot preparation of aqueous CdS spheres.By selection of TAA as the anionic precursors and CTAB as the cationic ligands for capping anions,the ligand modification condition of CdS particles could be controlled automatically at different reaction stages,which made the processes of NC formation and NC assembly simultaneously involved in the growth process of CdS spheres,thus benefiting the design of NC aggregates in one pot.The as-prepared CdS spheres showed uniform size and good water-dispersibility,facilitating their subsequent modification or composite process in various applica-tions.Moreover,the relative higher refractive index of CdS spheres than that of traditional SiO 2and polystyrene spheres also made it ideal for fabricating colloidal crystals with tunable and/or switchable band structures.

Acknowledgment.This work is supported by the NSFC (20534040,20704014,20731160002),the 973Program of China (2007CB936402,2009CB939701),the FANEDD of China (200734),the Program of Technological Progress of Jilin Province (20080101),and the Open Project of State Key Laboratory of Polymer Physics and Chemistry of CAS.

Supporting Information Available:SEM and TEM images of CdS spheres at different reaction times;estimated amounts of CdS nanocrystals and spheres at different reac-tion times;and UV -vis spectra of CdS spheres at different reaction times.This material is available free of charge via the Internet at

https://www.360docs.net/doc/e49257347.html,.

Figure 9.SEM images and transmission spectra of colloidal crystals fabricated with CdS spheres with diameters of (a)180nm and (b)200nm.The corresponding diffraction peak was 618nm (black solid line)for (a)and 689nm (red dash-dotted line)for (b).

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(49)Velikov,K.P.;Blaaderen,https://www.360docs.net/doc/e49257347.html,ngmuir 2001,17,4779.