Morphology-Controlled Syntheses and Functionalization of Trivacant Silicotungstate Nano-materials

LIU Ning WANG Jia-Xin WU Ya-Ning CHEN Yu-Hao WANG Guan ZHANG Dong-Di

(College of Chemistry and Chemical Engineering,Henan Key Laboratory of Polyoxometalate Chemistry,Henan University,Kaifeng,Henan 475000,China)

Abstract:Two novel nano-materials M3SiW9(M=Cu（Ⅱ）,Co（Ⅱ）)based on polyoxometalate have been successfully synthesized by chemical precipitation method using trivacant Keggin-type silicotungstate as precursor.Cu3SiW9exhibited extremely rare nanowhisker morphology in polyoxometalate-based nano-materials and Co3SiW9simultaneously showed uniform nanosphere morphology.Control experiments were implemented to indicate the morphologies of M3SiW9could be tuned by doping different transition metals.Furthermore,fluorescein sodium was employed as dopant to endow nanowhiskers with an emission peak at 510 nm.Moreover,the nanowhiskers were also used as carriers to combine with CdS quantum dots to improve the photocatalytic property.The results show that when the mass ratio of Cu3SiW9to CdS was 1∶1,the hydrogen production efficiency of the composite was about 10 times that of pure CdS quantum dots.This also confirms that the presence of polyoxometalate can effectively inhibit the recombination of CdS quantum dot photogenerated electrons and holes,thereby greatly improving the efficiency of photolysis of water to produce hydrogen.

Keywords:polyoxometalate;nanomaterials;photoluminescence;photolysis of water;hydrogen production

0 Introduction

As a fascinating species of inorganic compounds,polyoxometalates(POMs)have a wide range of applications because of their attractive properties in catalysis,magnetism,medicine,and materials science[1-5].POM-based nano-materials(PNMs)as a subclass of POM clusters,have been attracting much attention because of their unique properties in comparison with traditional single crystalline POM compounds[6-9].The application of nanotechnology in the preparation of PNMs has been a new route in recent years,and many researchers have been making great efforts to investigate these materials.Various PNMs with diverse morphologies and properties have been reported until now[10-12].In 2003,vesicles built from wheel-shaped Mo154were found in aqueous solution by Liu′s group[13].From then on,the unique solution behaviors of these“blackberry-like”structures were demonstrated in water and/or other polar solvents[14-17].After that,a novel class of microporous single crystals formed by self-assembly of(NH4)3PW12O40nanocrystallites was reported and the concept of“Sponge Crystal”was presented by Inumaru[18-19].Mizuno′s group addressed the all-inorganic dodecatungstophosphates nanocrystallites and cubic cesium hydrogen silicododecatungstate with anisotropic morphology in the following years[20-21].In 2011,Cronin et al.found a general approach to the fabrication of POM tubular architectures by osmotically driven crystal morphogenesis method[22].Two years later,Pang and co-workers obtained a series of uniform rhombic dodecahedral nanocrystals based on phosphomolybdate which would be used as an effective antibacterial agent[23].Chattopadhyay et al.prepared Mn-based heteropolytungstate microsphere by solvothermal method[24].For the past few years,our group has been working on the synthesis and functionalization of POM-based nano/micro-materials by chemical precipitation or hydrothermal methods.Surface modification and doping strategy were employed to endow silicotungstate microtubes with unique property[25].Then,phosphotungstate hollow spheres and phosphomolybdate microspindle were successfully prepared sequentially,both of which exhibited photoluminescence performance[26-27].Recently,we found that the shape and photoluminescence of the CeF3nanocrystals could be finely tuned by changing the component and amount of the POMs[28].

PNMs have attracted much interest in recent years for their prospects in various fields.Nevertheless,in comparison with the achievements of traditional single crystal POM compounds,PNM chemistry remains less explored to date.There are two major reasons for the few progresses in current situation.Firstly,the precursors in the syntheses of PNMs are saturated POMs which limit the variety of components.Secondly,the morphologies of PNMs are not rich.At present,only few different nano/micro structures of PNMs have been reported,such as polyhedron[10],rod[11],tube[29],wire[30]and spheres[18].Thus,how to create novel PNM morphologies by using vacant POM precursors attracts our interest.From this perspective,trivacantα-SiW9came to our attention.As an important precursor,α-SiW9has been widely used to construct many different POM mo-lecular structures.These compounds exhibit diverse properties in catalysis,fluorescence and magnetism[21-33].The employment ofα-SiW9and the other starting materials would prepare new PNMs with novel morphologies and properties.

As can be seen from the reports,nearly all of these nano/micro structures are prepared by using saturated POMs as starting materials.Thus,it is expected that novel PNMs could be formed using vacant POMs as precursors.In this report,we explored the synthesis and functionalization of two silicotungstate nanomaterials M3SiW9(M=Cu（Ⅱ）,Co（Ⅱ）),using Keggin-type trivacant POMs as the starting materials.Interestingly,the material exhibited extremely rare nanowhisker morphology in PNM when Cu（Ⅱ）ions were used as dopants.Meanwhile,Co3SiW9showed uniform nanosphere morphology.Control experiments indicated the shape of M3SiW9could be changed by using different transition metals(TMs).Furthermore,fluorescein sodium(FS)and CdS quantum dots(QDs)were used as functional materialsto combine with Cu3SiW9nanowhisker,respectively.The two nano-composites FS-Cu3SiW9and CdS/Cu3SiW9exhibited photoluminescence(PL)and photocatalytic properties.Therefore,this work not only demonstrates a new strategy to control the morphologies of PNMs,but also enrich the diversity of PNMs.

1 Experimental

1.1 Materials and instruments

All chemicals were reagent-grade and used without further purification.Na10[SiW9O34]·18H2O was synthesized according to reference[34]and identified by IR spectrum.CdS QDs was prepared on the basis of reference[35]and characterized by X-ray diffraction(XRD).The XRD was obtained on a Bruker D8 Advance instrument with CuKαradiation(λ=0.154 18 nm)in a 2θrange of 10°～60°at 293 K.The generator current and voltage during the XRD tests were 40 mA and 40 kV,respectively.The scanning electron microscope(SEM)image and energy dispersive X-ray(EDX)spectrum were identified by a JSM-7610F scanning electron microscopy with an acceleration voltage of 10 kV.IR spectra were recorded on an Avatar 360 Fourier transform infrared(FTIR)spectrophotometer using KBr pel-lets in a range of 4 000～450 cm-1.The X-ray photoelectron spectra(XPS)were collected using a PHI 5000 Versa Probe(UlVAC-PHI).Inductively coupled plasma optical emission spectroscopy(ICP-AES)experiments were performed on a Perkin-Elmer Optima 2100DV optical emission spectrometer.Electrospray ionization mass spectrometry(ESI-MS)routine spectra were carried out with a Bruker MTQⅢ-QTOF.The experiments were performed with the negative ion mode in acetonitrile solvent by direct infusion with a syringe pump with a flow rate of 5 μL·min-1.The Netzsch STA449F5 thermo-gravimetric analyzer was used for the thermal analysis in nitrogen dynamic atmosphere(60 mL·min-1)at a heating rate of 5 K·min-1and 10.6 mg powder of Cu3SiW9was thermally treated.The PL spectra were collected by a Hitachi F-7000 fuorescence spectrophotometer.The PL lifetime measurement was performed on an Edinburgh Instruments FLS980 spectrophotometer.UV-Vis diffuse reflectance measurements were recorded on a HITACHI UH4150 spectrometer.Mott-Schottky measurements were tested on a CHI 760E electrochemical workstation(Shanghai Chenhua Instrument Co.,China)in a three-electrode electrochemical cell using a 0.1 mol·L-1Na2SO4.

1.2 Synthesis of M3SiW9(M=Cu,Co,Ni)

M(CH3COO)2·H2O(0.10 g,M=Cu,Co,Ni)was dissolved in 5 mL of CH3COONa·3H2O(0.50 mol·L-1,pH≈6.20)solution.After the solution was heated to 70℃,Na10[SiW9O34]·18H2O(0.48 g,0.17 mmol)was added and the mixture was reacted for 1 h.The obtained green solution was filtered to keep a clear condition.Then,0.25 g polyethylene glycol(Mr=8 000,referred as‘PEG-8000’for short)was added and fully mixed.Fol-lowing,240 μL saturated KCl solution was dripped into the as-prepared solution slowly to give a light green precipitate.The homogeneous mixture was stirred for another 6 h.Finally,the solid product(Cu3SiW9:green,Co3SiW9:reddish brown,Ni3SiW9:reddish brown)was collected by centrifugation and washed with water and ethanol to removed excess regents.The yields of Cu3SiW9,Co3SiW9and Ni3SiW9were 28%,33% and 35%,respectively(based on W).

1.3 Synthesis of Cu3SiW9microrod

Cu3SiW9microrod was prepared similar to Cu3SiW9nanowhisker,but the volume of saturated KCl solution was changed to 360 μL.Green solid product was obtained by centrifugation and washed with water and ethanol to removed excess regents.

1.4 Synthesis of FS doped Cu3SiW9nanowhisker

The synthetic method was similar to the synthesis of Cu3SiW9nanowhisker.Cu(CH3COO)2·H2O(0.10 g,0.50 mmol)was dissolved in 5 mL of CH3COONa·3H2O(0.50mol·L-1,pH≈6.20)solution.After the solution was heated to 70℃,Na10[SiW9O34]·18H2O(0.48 g,0.17 mmol)was added and the mixture was reacted for 1 h.After filtering,40 μL FS(8.85 mmol·L-1)was added slowly to the clear solution.Then,0.25 g PEG-8000 was added and 240 μL saturated KCl solution was dripped into the solution slowly to give a yellow precipitate.The homogeneous mixture was stirred for another 6 h.Finally,the yellow solid product was collected by centrifugation and washed with water and etha-nol to removed excess regents.

1.5 Synthesis of CdS QDs loaded Cu3SiW9photocatalyst and photoproduction of hydrogen

Na2S solution(160 mL,0.4mol·L-1)was added dropwise to the CdCl2(200 mL,0.4mol·L-1)solution under vigorous stirring.The mixture was stirred for 6 h,and left to stand for 12 h.Then,the precipitate was separated by filtering and dispersed into 80 mL water.Next,the resulting suspension was transferred to a 150 mL Teflon-lined autoclave for 6 d and kept at 200℃for 72 h.After slow cooling to room temperature,yellow product CdS QDs were harvested bycentrifugation,which were dried and saved for further using.

A mixture of as-prepared CdS QDs(0.288 g,0.002 mol)and Na2SO3(7.56 g,0.06 mol)were dispersed and dissolved in 100 mL of deaerated water,which was loaded in a 250 mL N2atmosphere protected three-neck-flask.In the next step,Na2S(7.02 g,0.09 mol)was added slowly,and 3 mL Ni(NO3)2·6H2O(0.14%,w/w)solution was injected with stirring.The mixture was stirred for 30 min under flowing N2protection.Then,Cu3SiW9nanowhisker(0.26 g)was added and stirred for another 30 min to give a nanocomposite photocatalyst.

The reaction of visible-light photocatalytic H2-production was carried out in a special quartz glass tube.The resulting homogeneous solution(60 mL)was transferred to the 120 mL reactor,and a 500 W Xenon lamp with a UV-cutoff filter(λ≥420 nm)was used as the visible light source.The lamp was positioned 10 cm away from the reactor and this position was fixed.During H2-production,the reactor was in an argon-flow environment and at constant ambient temperature by a circulating water jacket.

2 Results and discussion

Scheme 1 shows the schematic synthetic processes of Cu3SiW9nanowhisker and Co3SiW9nanosphere.During the procedure,TM ions were reacted with trivacantα-SiW9precursor under mild so-lution initially.Then,non-ionized surfactant PEG-8000 was employed to construct a closed capsule environment in a linear direction.The 1D nano-material would be crystallized in this situation.

Considering different metal ions would have an impact on the growth of nanostructure,a series of TM ions(Cu（Ⅱ）,Co（Ⅱ）,Ni（Ⅱ）)were used.When Cu（Ⅱ）ion was introduced,a novel whisker shape was discovered.After that,Co（Ⅱ）ion was used to instead Cu（Ⅱ）ion.Fortunately,the morphology was changed to sphere.Following,Ni（Ⅱ）ion was employed to prepare Ni3SiW9by using the same procedure.Nevertheless,amorphous powders were afforded(Supporting information,Fig.S1).These control experiments indicate TM ions play an important role in the formation of the products.In other words,the shape of SiW9nano-materials could be tuned by doping different TMs.

Scheme 1 Synthetic strategy of M3SiW9(M=Cu（Ⅱ）,Co（Ⅱ）)materials

Apart from exploring the influence of TM ions in reaction system,great efforts were also put into the dosage of KCl in the formation of nanostructure.In general,240 μL saturated KCl solution was added to the mixture to form nanowhisker shape.Under a similar procedure,the amount of saturated KCl solution was changed to 360 μL.Unexpected phenomenon was observed,the nanowhisker was transformed to microrod.As shown in Fig.1a and 1b,the Cu3SiW9microrods exhibited uniform shape.The average diameter and length were 2.37 and 11.68 μm,respectively.Therefore,the amount of saturated KCl solution is also a significant condition in the growth of nanostructure.

In addition,we also explored the effects of different stirring time on nanowhisker shape.Typical-ly,uniform nanowhiskers were formed after stirring 6 h in the last step.Under a similar procedure,the stirring time was changed to 4 or 8 h.As shown in Fig.1c,when the stirring time was 4 h,the whisker morphology was not isolated.Subsequently,the stirring time was increased to 8 h(Fig.1d),broken nanowhiskers were observed.These evidences prove that the stirring time play an important role in the formation of nanowhiskers.Moderate stirring time would help staring materials to assemble into uniform shape.Otherwise,the self-aggregation of novel morphology would be obstructed or destroyed.

Fig.2 shows a typical SEM micrograph of the Cu3SiW9nanowhiskers.From the SEM micrograph,uniform nanowhiskers could be observed clearly.The average diameter of these nanowhiskers was about 440 nm and the average length wasca.21.3 mm.One-dimensional PNMs are rarely reported in comparison with zero-dimensional morphologies[36],especially the nanowhisker shape is seldom observed in POM chemistry.Furthermore,according to the previous literatures,the whisker morphology is always formed by organic materials,such as polysaccharide and cellulose[37-38].Thus,this finding may be of particular importance in material science.EDX analysis for the nanowhisker was also recorded,which clearly exhibited the corresponding components of Cu3SiW9(Fig.S2).

Fig.1 SEM images of Cu3SiW9microrods(a,b);SEM images of Cu3SiW9nanowhisker prepared by stirring for 4 h(c)and 8 h(d)

Fig.2 SEM images of Cu3SiW9nanowhiskers

Interestingly,when Co(CH3COO)2·4H2O was used to replace Cu(CH3COO)2·H2O,the nanowhisker shape was changed to nanosphere.Fig.3 shows the typical SEM images of the Co3SiW9nanospheres.The average diameter of the nanosphere was about 550 nm,according to the statistical 100 particles.Compared to the PW12nanocrystallites(186～318 nm)and PMo12rhombic dodecahedral nanocrystals(ca.200 nm),which were prepared by Mizuno et al.and Pang et al.respectively[20-23],the size of Co3SiW9nanosphere was larger than these previous structures.It is suspected that the different sizes might be related to the synthesis conditions or compositions.

Moreover,ICP-AES experiments were performed on a Perkin-Elmer Optima 2100DV optical emission spectrometer to estimate the contents of Na,K,Cu/Co and W in these materials.The results confirm the compositions of these materials(Table S1).It is worth pointing out that ICP-AES results are consistent with EDX results(Fig.S2 and S3).The atomic ratio ofnCu/nWornCo/nWfor these materials was about 1∶3.These results show that saturated Keggin type silicotungstates arebuilding blocks.Therefore,according to the atomic ratio of heterometal-to-W,the two materials are named Cu3SiW9and Co3SiW9for short.

Fig.3 SEM images of Co3SiW9nanospheres

Scheme 2 Probable mechanism of morphology control

Significantly,the morphology of silicotungstate can be controlled by using different TMs.Considering the formation mechanism of M3SiW9nanostructures,it is not difficult to understand this phenomenon.As shown in Scheme 2,the probable mechanism could be described as follows:M3SiW9nanocrystallites are formed initially through Ost-wald′s process[39].Following,these nanocrystallites are aggregated together by loose aggregation[19].According to the reports of Liu et al.,cations play an important role in the formation of vesicles.The structural curvature could be changed by using different cations[14-17].Thus,the M3SiW9nanocrystal-lites would be aggregated through two different ways.When Cu2+ions are employed,small-curvature morphology is prepared which exhibits nanowhisker shape.In addition,large-curvature morphology is affected by Co2+ions,resulting to a nanosphere shape.

The title compounds Cu3SiW9and Co3SiW9were characterized by XRD(Fig.4).The XRD pattern of as-prepared pureα-SiW12(Fig.S4)was in accordance with those of title compounds.Especially,the diffractions peaking at 17.3°,27.9°,32.9°,34.6°,46.8°,53.2°and 55.3°are corresponded to the data of Cu3SiW9and Co3SiW9.The peaks at 17.3°,27.9°,32.9°and 34.6°were appropriately indexed to theα-SiW12at the positions of(122),(403),(134)and(060)planes[30,40-41](PDF No.70-1714).

Fig.4 XRD patterns of Cu3SiW9nanowhisker and Co3SiW9nanosphere

As shown in Fig.5a,IR spectra ofα-SiW9,Cu3SiW9nanowhiskers and Co3SiW9nanospheres were observed between 450 and 4 000 cm-1.The Cu3SiW9nanowhiskers and Co3SiW9nanospheres could be identified by four characteristic IR bands appearing at 1 007 cm-1(W-Ot),953 cm-1(Si-Oa),910 cm-1(W-Ob)and 794 cm-1(W-Oc),which is in accordance with the bulkα-SiW9[31,33].The th ermogravimetric(TG)curve(Fig.5b)of Cu3SiW9nanowhiskers gave a total weight loss of 23.17% in a range of 33～1 000 ℃.The weight loss of 4.33% during the first step from 33 to 127℃corresponds to the release of adsorbed water molecules.On further heating,the second weight loss of 4.38% between 127 and 258℃is approximately attributed to the removal of structural water molecules.The third weight loss of 14.46% from 258 to 1 000℃results from the decomposition of POM skeletons.

Fig.5 (a)IR spectra of α-SiW9,Cu3SiW9nanowhiskers and Co3SiW9nanospheres;(b)TG curve of Cu3SiW9nanowhiskers

Fig.6 XPS spectra of Cu3SiW9nanowhiskers:(a)Cu2p;(b)W4f

The Cu3SiW9nanowhiskers were also characterized by XPS.Using a Shirley background subtraction,the fitting curves are shown in Fig.6.The Cu2pshowed a series of obvious signals in XPS spectrum.Especially,the strong satellites centered at 942.5 and 962.8 eV indicate the existence of Cu2+ions[42-43],which are coordinated to oxygen atom of silicotungstate.According to the spectrum,different valence states of Cu element might be existed in this material.As depicted in Fig.6a,the peaks 1～3 centered at 932.75,934.55 and 935.83 eV are attributed to Cu（Ⅰ） oxide(11.55%),Cu（Ⅱ）oxide(69.04%)andCu（Ⅱ） acetate/hydroxide(19.41%),respectively[44-45].These results confirm that the Cu2+is the greatest portion in this material.The W4fspectrum exhibited two strong fitted peaks centered at 35.1 and 37.2 eV(Fig.6b),which are attributed to the 4f7/2and 4f5/2spin orbit of W6+ions in the silicotungstate[46-47],respectively.

It is worth pointing out that ICP-AES results are consistent with EDX results.In particular,these data could be used to conclude the atomic ratio of these materials.Integrating the results of IR,XRD,EDX and ICP-AES,the formulas Na3K[Cu3SiW9O34]and Na3K[Co3SiW9O34]are established forCu3SiW9nanowhiskers and Co3SiW9nanospheres,respectively.

The ESI-MS measurement has been found to be a useful analytic tool in studying the solution behavior of nano-sized clusters,which has been widely used to explore many types of POMs.Therefore,the ESI-MS spectrum of Cu3SiW9nanowhiskers in deionized water was performed in the negative ion mode,in order to confirm the identity of the cluster in the solution.As depicted in Fig.7,a series of peaks(m/z869.1,876.5 and 882.6)for-3 charged ions were observed in am/zrange of 865～887,which correspond to those peak positions for[NaK2Cu3SiW9O34Cl2·H2O]3-,[Na2K2Cu3SiW9O34Cl2OH]3-and [Na2K2Cu3SiW9O34Cl3]3-,respectively.The results reveal that the Keggin type POM retains their structural integrity in solution.

The research on PL property of PNMs is still lacked,which limits the practical applications,especially the PL property of vacant POM materials.FS is an environmentally friendly and inexpensive dye,which is widely used as a fluorescent tracer for many applications due to its excellent performance.From this perspective,FS was used as dopant to combine with Cu3SiW9nanowhiskers,and was expected to obtain functionalized nano-materials.Besides,in order to explore the fluorescent nature of FS-Cu3SiW9,gradually varied emission spectra were recorded.These gradually varied emission spectra could be used to indicate the component of FS-Cu3SiW9.According to the synthetic method of Cu3SiW9,twenty samples were prepared one by one.In the synthesis of first sample,FS was not used initially.Subsequently,the amount of FS was increased at a 10 μL interval between each sample.As depicted in Fig.8a,no emission peak was observed at the beginning of the test.With the increasing of FS,the FS-Cu3SiW9exhibited emission peak at 510 nm gradually.These results indicate the PL property of FS-Cu3SiW9is originated from FS entirely.

Fig.8b shows the PL lifetime measurement of FS-Cu3SiW9in distilled water.The PL decay curve of FS-Cu3SiW9was well fitted to single-exponentialI(t)=Aexp(-t/τ),whereAandτare the pre-exponential constant and the lifetime.The results and related parameters are illustrated in Table 1.According to the previous reports,the PL lifetime of FS is 4.26 ns[26].And the PL lifetime of FS-Cu3SiW9was 3.76 ns,which is shorter than the PL lifetimeTavof FS.There are several reasons for this phenomenon.First of all,the shortened PL lifetime might be aroused by concentration quenching after the FS was introduced into Cu3SiW9nanowhiskers[48].Secondly,the FS-Cu3SiW9would be interacted with water molecule by hydrogen bonding.The excitation energy may be transferred between the chromophoric center and its surrounding groups.Final-ly,Cu2+ions could be coordinated with FS molecules.This case would induce non-radiative pathways,leading to shortening of the PL lifetime[49-50].

Fig.8 (a)PL spectra of incremental doped FS in Cu3SiW9(from 0 to 200 μL,volume interval between each sample:10 μL);(b)PL decay curves of FS-Cu3SiW9(doped with 200 μL FS)measured at 510 nm with excitation at 360 nm

Table 1 Fitting parameters and PL lifetimes of FS and FS-Cu3SiW9

Fig.9 exhibits a comparison of the time profiles of the H2evolution in the presence of different photocatalysts.The photocatalytic performance of CdS QDs were improved obviously with the help of Cu3SiW9.As depicted in Fig.9,the amount of H2gener-ated by pure CdS QDs was about 45.06 μmol·g-1after 5 h.And in the meantime,the amount of H2generated by CdS/Cu3SiW9photocatalysts with different ratios of(1∶2.5,1∶0.5,1∶1.5 and 1∶1)were 39.98,150.34,202.14 and 248.31 μmol·g-1,respectively.When the mass ratio of CdS to Cu3SiW9was 1∶1,the amount of H2was more than 5.5 times that of pure CdS QDs.The photocatalytic activities for pure CdS was relative lower than the other photocatalysts.With the changing ofthe mass ratio of CdS to Cu3SiW9(in other words,the amount of POM was increased gradually),the photocatalyst showed much higher activities.When the CdS QDs was loaded on the same mass of Cu3SiW9nanowhiskers,the photocatalyst revealed the highest performance.However,with the increasing of Cu3SiW9nanowhiskers,the material showed lower activity.These results indicate that the visible-light photocatalytic activity of pure CdS QDs could be improved by loading on moderate amount of Cu3SiW9nanowhiskers.Moreover,silicotungstate plays as a key cocatalyst that promotes the photocatalytic reaction.

Fig.9 Visible-light photocatalytic activity(a)and photocatalytic hydrogen evolution(b)of pure CdS QDs and CdS/Cu3SiW9photocatalysts with different mass ratios of CdS to Cu3SiW9

The photocatalytic activity of CdS QDs has been enhanced obviously by the combination with Cu3SiW9nanowhiskers.In order to illustrate the mechanism of photocatalytic hydrogen production,additional measurements were performed to calcu-late the band edges in CdS QDs and Cu3SiW9.As can be seen from Fig.S5a,the optical reflectance spectrum measurements were performed,and the band gaps were calculated by the Kubelka-Munk(K-M)method[51-52].The band gaps of CdS QDs and Cu3SiW9nanowhiskers were approximately 2.3 and 2.8 eV,respectively.Also,Mott-Schottky measurements were recorded to determine the LUMO(CB)positions such that the band edges in CdS QDs and Cu3SiW9can be crudely inferred(Fig.S4b)[53-54].The LUMO(CB)locations of the CdS QDs and Cu3SiW9nanowhiskers were-0.81 and-0.42 V(vs NHE),respectively.

Fig.10 (a)Energy diagrams of HOMO(VB)and LUMO(CB)levels of CdS QDs and Cu3SiW9nanowhiskers;(b)Photocatalytic mechanism of CdS/Cu3SiW9

The detailed positions of band edges in CdS QDs and Cu3SiW9are shown in Fig.10a.Thus,it is clear to illustrate the mechanism of photocatalytic hydrogen production.This phenomenon should be attributed to the synergistic effects of the components(Fig.10b).On the one hand,it is worth noting that the separation of electrons-holes plays an important role in photoreaction.In theory,electron would transfer from the valence band(VB)to the conduction band(CB)through the photoexcitation of the CdS QDs.And then electron-hole pairs would be yielded[55].Although the charge separation occurs,the electron-hole pairs would be destined for recombination.On the other hand,POMs have the capability for accepting electrons.Thus,Cu3SiW9nanowhiskers could be used to efficiently suppress electron-hole recombination in CdS QDs[56].The electrons transferred from the CB of CdS QDs to the LUMO orbital of the Cu3SiW9nanowhiskers could occur.Based on these perspectives,such electron transfer enhances the separation of photo-induced holes and electrons,and thus the photocatalytic activities are improved significantly.

3 Conclusions

In summary,M3SiW9(M=Cu（Ⅱ）,Co（Ⅱ）)have been successfully prepared and fully characterized.In the first place,PNMs are almost constructed from saturated Keggin-type POMs.Trivacant silicotungstates are seldom used as precursors to form PNMs.In the second place,the morphology of M3SiW9could be controlled in the synthetic procedure by using different TMs.Especially,the one dimensional nanowhisker morphology is rarely reported in PNMs chemistry.Thus,the synthetic method reported in this work not only enriches the components and morphologies of PNMs,but also finds an effective way to control the structures of PNMs in nano size.Furthermore,M3SiW9nanowhiskers have been combined with FS and CdS QDs to achieve PL and photocatalytic property respectively.This method is an effective way to endow material with new function and developing new product.In brief,the strategy demonstrated in this report can be applied to control the shapes and functionalize polyoxometalate based nano-materials with different properties.

Acknowledgements:This work was supported by the National Natural Science Foundation of China(Grants No.21901061,U1504201).The authors are grateful to the Program for Science&Technology Innovation Talents in Universities of Henan Province(Grant No.19HASTIT004)and the Program for Innovation Teams in Science and Technology in Universities of Henan Province(Grant No.20IRTSTHN004).The authors also gratefully acknowledge Prof.XU Xiang-Xing,Dr.ZHANG Chao,Dr.SUN Ming-Jun,Mr.LI Yan-Kai and Ms.HOU Li-Jie for the measurements and helpful discussion.

Supporting information is available at http://www.wjhxxb.cn