Biomolecule-Assisted Hydrothermal Synthesis and Optical Properties of Cu7S4Nanotubes

GUO Pei-Zhi HAN Guang-Ting WANG Bao-Yan ZHAO Xiu-Song,2

(1Laboratory of New Fiber Materials and Modern Textile,the Growing Base for State Key Laboratory,School of Chemistry,Chemical Engineering and Environmental Sciences,Qingdao University,Qingdao 266071,Shandong Province,P.R.China;2Department of Chemical and Biomolecular Engineering,National University of Singapore,4 Engineering Drive 4,Singapore 117576)

Biomolecule-Assisted Hydrothermal Synthesis and Optical Properties of Cu7S4Nanotubes

GUO Pei-Zhi1,*HAN Guang-Ting1WANG Bao-Yan1ZHAO Xiu-Song1,2

(1Laboratory of New Fiber Materials and Modern Textile,the Growing Base for State Key Laboratory,School of Chemistry,Chemical Engineering and Environmental Sciences,Qingdao University,Qingdao 266071,Shandong Province,P.R.China;2Department of Chemical and Biomolecular Engineering,National University of Singapore,4 Engineering Drive 4,Singapore 117576)

Cu7S4nanotubes were synthesized using a biomolecule DL-methionine-assisted hydrothermal method.The morphology and phase of the products can be controlled by adjusting the reaction parameters such as synthesis temperature,reaction time and the molar ratio of the reagents.We found that uniform polycrystal Cu7S4nanotubes with diameters of 100-600 nm and lengths of 40-100 μm can be controllably synthesized at 200℃when the molar ratio of Cu(NO3)2to DL-methionine in the synthesis system is 1∶2.Similar Cu7S4nanotubes can be obtained from D-or L-methionine systems.The bandgap energy of the Cu7S4nanotubes was measured to be about 2.88 eV,a remarkable blue shift in comparison with that of bulk Cu7S4(2.0 eV).We discussed the relationship between the products and the functional groups in the amphiphilic biomolecules.On the basis of our experimental data,we proposed that the Cu7S4nanotubes were formed versus a self-sacrificing template mechanism.

Hydrothermal synthesis;Cu7S4nanotube;Methionine;Biomolecule

Since the discovery of carbon nanotubes in 1991[1],nanoscale tubular structures of inorganic solids have attracted a great deal of research attention in terms of synthesis,assembly,and exploration of applications because of their unique physical and chemical properties[2].High-temperature processes,such as arcdischarge evaporation and vapour deposition,have been used in the synthesisofinorganic nanotubesasthe traditionalmethods[3-4].Additionally,tubular materials can also be synthesized from layered or pseudolayered structures in the absence[5]or presence[6]of a template.Hydrothermal synthesis is a soft-chemical processthat can be used to prepare advanced materials[7],in which the hydrothermal conditions,such as the concentrations of precursors,synthesis temperature,and synthesis time,can be controlled easily.This method has recently been used to synthesize unique inorganic nanotubes,such as Dy(OH)3nanotubes[8].

Copper sulfides,as one of the important semiconductor chalcogenides,have received great interest owing to their potential applications in solar cell,fast-ion conduction and catalysis[9-10]. Generally,sulfide nanostructures are synthesized based on the interactions between metal ions and S2-ions or with the assistance of the Cu(I)-complex formation or the sulfur sources,such as sulfur,sulfides,thio-alcohols and thio-ketones[9-22].For example,high symmetric 18-facet polyhedron nanocrystals of Cu7S4can be synthesized using thiourea[11].Snowflake-like CuS nanostructures are fabricated with the assistance of Cu(I)-cysteine complex formation by a L-cysteine-assisted hydrothermal method[13], and polycrystal lead chalcogenide nanotubes are formed based on the Pb(II)-cysteine complex template,in which Pb(II)ions are coordinated with the—COOH groups[21].Recently,copper sulfide nanotubes have been synthesized in the case of the template orsolvothermalapproaches[6,12,18-19],however,the synthesis of such nanotubes usually contains multi-step processes.It remains challengeable to synthesize pure tubular chalcogenides solids through a simple one-step approach[18-20].

In this article,uniform polycrystal Cu7S4nanotubes are controllably synthesized through a one-step hydrothermal method using Cu(NO3)2and amphiphilic biomolecule DL-methionine as the precursors.Methionine is selected for the synthesis of metal sulfide nanostructures because the sulfur atom in the molecule can coordinate with metal ions[23-24].Furthermore,the effect of D-and L-methionine as the reagents on the formation of sulfide nanostructures has also been studied.

1 Experimental

All chemicals,including Cu(NO3)2·3H2O(Sinopharm Chemical Reagent Company,SCRC),sulfur(SCRC),DL-,D-and L-methionine[CH3S(CH2)2CH(NH2)COOH](SCRC),3,3-thiodipropionic acid(TCI),L-(-)-cystine(TCI)and alcohols(SCRC),are of analytical grade and used without further purification.The double-distilled water is used in all the cases.In a typical synthesis,aqueous Cu(NO3)2solution(0.2 mol·L-1,15 mL)and aqueous DL-methionine solution(0.2 mol·L-1,15 mL)are mixed under magnetic stirring at room temperature.After 5 min,the mixture is then transferred to a 40-mL teflon-lined autoclave,and placed in an oven at 200℃for 24 h.The synthesis procedures are similar to other samples,in which the molar ratios of the reagents,reaction temperature,and time are 1∶1 or 1∶2,140-200℃,and 0.5-24 h,respectively.The solid products are collected by filtration,washed seperately with distilled water and ethanol for several times,and then dried in an oven at 60℃for 6 h.In addition,the molar ratio of Cu(NO3)2to methionines in the reagents is abbreviated as the Cu/S ratio unless otherwise specified.

The X-ray photoelectron spectrum is measured with a VG Scientific ESCALAB 220-IXL spectrometer using Al Kaas the excitation source(1486.6 eV).UV-visible absorption,FTIR and photoluminescent spectral measurements are performed using an 8453 UV-Visible,a Nicolet 5700 and a Perkin-Elmer′s LS-50 Fluorescence spectrophotometer,respectively.X-ray powder diffraction(XRD)measurements are determined using a Bruker D8 advanced X-ray diffractometer equipped with graphite monochromatized Cu Kαradiation(λ=0.15418 nm)and Ni filter.Scanning electron microscopy(SEM)images are taken with a JSM-6390LV scanning electron microscope.Transmission electron microscopy(TEM)and high resolution transmission electron microscopy(HR-TEM)images are obtained with a JEM-2000EX and a JEM-2100 transmission electron microscope,respectively.

2 Results and discussion

Morphology-controlled synthesis of the products can be obtained through changing experimental parameters.Fig.1(A,B) show the SEM images of the samples synthesized from the systems with the Cu/S ratio of 1∶1 and 1∶2 at 200℃for 24 h.It can be seen that Cu7S4nanoparticles and nanotubes with diameters in the range of 300-1000 nm and lengths scales in the range of 10-50 μm are formed for the system with the Cu/S ratio of 1∶1(Fig. 1A).However,uniform Cu7S4nanotubes almost without nanoparticles are observed with diameters in the range of 100-600 nm and lengths extended to 40-100 μm(Fig.1B)when the Cu/S ratio is changed to 1∶2.Furthermore,TEM image of the samples as used in Fig.1B confirms the formation of the Cu7S4nanotubes (Fig.1C).Several sets of electron diffraction(ED)spots from different nanocrystals in Cu7S4nanotubes can be observed from the inset in Fig.1C.One set of the ED spots can be well indexed to the monoclinic Cu7S4phase for the(1600)planes,indicating that the nanocrystals aggregate into nanotubes with a highly oriented [100]crystallographic axis.The d-spacings as well as the bright regions depicted in the HRTEM image of Cu7S4nanotubes shown in Fig.1D indicate that the nanotubes are composed of small Cu7S4nanocrystals with a preferred growth direction of [100].

The effect of synthesis temperature on the morphology of the products has also been investigated.Fig.2A shows that needlelike nanostructures and irregular nanoparticles are formed at 140℃for the systems with the Cu/S ratio of 1∶1,while tubular and wire-like structures are observed when the synthesis temperature was raised by 20℃(Fig.2B).With the temperature further increased to 180℃,nanoparticles and tubular nanostructures of Cu7S4phase can be obtained similar to those of Cu7S4samples in Fig.1A.

The phases of the products were verified by the XRD measurement.Fig.3(A,B)show XRD patterns of the solid products synthesized at 200℃from the systems with the Cu/S molar ratios of 1∶1 and 1∶2,respectively.It is clear that pure Cu7S4phases (JCPDS,No.23-0958)are obtained for these two samples.The broadening of the diffraction peaks indicates the formation of small crystals of Cu7S4phase.The yield of all Cu7S4products is estimated over 90%.Pure Cu7S4phase can also be formed withthe temperature down to 180℃(Fig.3C).As it can be seen from Fig.3D,the content of the Cu7S4phase is decreased with the temperature fell by 20℃and other crystalline phases,such as CuS (JCPDS,No.78-2122)and Cu2S(JCPDS,No.12-0227)can be observed.When the synthesis temperature is changed to 140℃, multi-crystalline phases are also obtained,as shown in Fig.3E, and the Cu7S4phase is almost unobservable.

Fig.2 SEM images of the products synthesized from the systems with the Cu/S ratio of 1∶1 at 140℃(A)and 160℃(B)for 24 h

In addition,the intermediate products collected at different synthesis time for the systems with the Cu/S ratio of 1:2 are carefully characterized.During the experimental process,it is seen that a small quantity of blue complexes is formed immediately after the addition of DL-methionine into aqueous Cu(NO3)2solution at room temperature.Only pure blue products are collected after 0.5 h reaction,which indicated that the coordinated complexes are completely formed in such a hydrothermal condition.Fig.4 displays the FTIR spectra of all the intermediate products in the regions of 3700-2600 and 1700-400 cm-1.From the spectra of DL-methionine and blue products(Fig.4(A,B)),it can be observed that the bands at 2945 and 2917 cm-1assigned to asymmetric stretching vibration of CH3—and—CH2—[25],respectively,are unchanged,while the changes in the intensities of these peaks are attributed to the coordination between Cu(II) ions and methionine molecules.However,the vibration band at 3433 cm-1in Fig.4A ascribed to symmetric stretching of—NH2group is varied to 3240 cm-1(Fig.4B)and the band at 1655 cm-1in Fig.4A ascribed to the deformation vibration of—NH2groupis disappeared in Fig.4B,indicating that the—NH2groups are coordinated with Cu(II)ions[25].The band at 1582 cm-1attributed to asymmetric stretching of COO-group(Fig.4A)is changed to 1618 cm-1in Fig.4B,denoting that COO-groups are coordinated with Cu(II)ions,which will be further confirmed by the XPS results.With the time elongated further,the spectra change greatly.As shown in Fig.4(C,D)for the samples synthesized after 1 and 3 h reaction,the bands at 3240,1618 and 1152 cm-1are almost disappeared,and new absorption peaks at 1630 and 1088 cm-1appeared,indicating that organic groups are gradually vanished[25].On the other hand,the morphology of the intermediate products is also changed with the reaction time elongated.It is clear that nanorod and nanowire structures can be obtained after 1 h reaction(Fig.5A),while tubular nanostructures marked with an arrow in Fig.5B in addition to nanowires can be observed with the time of 3 h(Fig.5B).Finally,uniform Cu7S4nanotubes are obtained when the hydrothermal synthesis is held for 24 h (Fig.1B).

Fig.3 XRD patterns of the products synthesized from the systems with Cu/S ratios of 1∶1(A)and 1∶2(B)at 200℃as well as with Cu/S ratio of 1∶1 at 180℃(C), 160℃(D),and 140℃(E)for 24 h

Fig.4 FTIR spectra of DL-methionine(A),and the products synthesized from the systems with Cu/S ratio of 1∶2 at 200℃for 0.5 h(B),1 h(C),3 h(D),and 24 h(E)

Fig.5 TEM images of the intermediate products synthesized from the systems with Cu/S ratio of 1∶2 at 200℃for 1 h(A)and 3 h(B)

Fig.6 XPS spectrum of the blue intermediate products

To further clarify the formation of blue complex between DL-methionine and the Cu(II)ions,an XPS measurement is performed.As shown in Fig.6,the binding energies are observed at 284.8(C 1s),399.4(N 1s),530.7 and 531.5(O 1s),163.3(S 2p) and 934.4 eV(Cu 2p).These indicate that the Cu(II)ions are coordinated with the N,S and O atoms in methionine molecules, and thus the Cu(II)-methionine complex is formed[13,26-27].The binding energy of O 1s is observed as two peaks,indicating that the two oxygen atoms in carboxyl groups are not in the same conjugated environment and an octahedral coordination compound is suggested to be formed[26].Furthermore,a quantitative analysis shows that the composition ratio of methionine molecules to Cu(II)ions is 1.86∶1.Considering the fact that one methionine molecule has a sulfur atom,an—NH2group and a carboxyl group,it can be suggested that an octahedral Cu(II)-methionine complex is predominately formed through one Cu(II) ion coordinated with two biomolecules[23,28-29].

Fig.7A shows the UV-visible absorption spectrum of Cu7S4nanotubes.The spectrum demonstrates that the absorption peak of the Cu7S4nanotubes in the region of 360-520 nm as well as the width of the absorption band is associated with the dispersion of the nanocrystal size.According to the Scherrer formula, the size of the Cu7S4nanocrystal is about 8-11 nm.Taking the maxima in the absorption spectrum as the corresponding band gap,the determined band gap of uniform Cu7S4nanotube samples is about 2.88 eV.This shows a blue shift of ca 0.88 eV compared to that of 2 eV for bulk Cu7S4phase[10],indicating that the quantum size effect of small Cu7S4nanocrystals assembled in the nanotubes is existed.Fig.7B shows the photoluminescent spectrum of Cu7S4nanotubes.Under an excitation wavelength of 431 nm,Cu7S4nanotubes display three main emission peaks at 485, 529,and 588 nm.As the exact mechanism for explaining the nature of photoluminescence emission for copper sulfide nanostructures remains controversial,prior literature suggests that the nature of the emission spectrum depends on the morphology and structure of the sample itself as well as the solvent effect[12,30-31]. For instance,nanotubes composed of Cu7S4nanowire evincestrong emission peaks at 412 and 454 nm[12]while Cu7S4nanocrystals show a emission peak at 419 nm[30].Nevertheless,the excitonic emission shown in Fig.7B herein indicates that as-prepared Cu7S4nanotubes are likely of optical quality compared with those samples with stacking fault defects showing poor photoluminescence[32].

Fig.7 UV-Vis absorption(A)and photoluminescence(B) spectra of uniform Cu7S4nanotubes as used in Fig.1B

The functional groups in methionine molecules such as the—COOH,—NH2and C—S—C groups,are confirmed to play crucial roles in forming such uniform nanotubes.In our experiments,sulphur and tetrahydrothiophene are also used,but no tubular or wire-like structures are formed.When 3,3-thiodipropionic acid is used,strip like structures and nanoparticles can be obtained(Fig.8A),while fine particles are formed from L-(-)-cystine systems(Fig.8B).Based on these results,a possible formation mechanism of Cu7S4nanotubes can be suggested,as illustrated in Fig.9.Methionine derivatives are reported to be able to form six-or four-membered chelate rings through the nitrogen,sulfur,and oxygen atoms with metal ions[26,33-34].During the formation of Cu7S4nanotubes,an octahedral complex with a sixmembered chelate ring is formed firstly through the coordination between one Cu(II)ion and the N,S and O atoms of two biomolecules.These results are different from those of L-cysteine systems,in which Cu(I)-cysteine complex is formed under similar conditions[13,27].Although other reports showed that Bi(III) ions could not coordinate with methionine,but with L-cysteine, in relative high temperature[35].In our results,it is found that Cu(II) ions can coordinate with methionine molecules even at room temperature.Furthermore,the existence of C—S—C group in methionines could not promote the formation of Cu(I)-complex[27].

Fig.8 SEM images of the products synthesized from 3,3-thiodipropionic acid(A)and L-(-)-cystine(B)systems with the Cu/S molar ratio of 1∶1 at 200℃for 24 h

Fig.9 Schematic illustration of the formation of the Cu7S4nanotubesSynthesis conditions as in Fig.1B

When the synthesis time prolongs to 1 and 3 h for the system with the Cu/S ratio of 1∶2,it is multi-crystalline phases such as CuS,Cu7S4,and Cu2S,that can be obtained based on the results of XRD patterns(data not shown).However,TEM results of these intermediates reveal that wire-like structures are gradually changed to tubular-like structures when the reaction time changed from 1 to 3 h.Further extending the synthesis time leads to the complete formation of Cu7S4nanocrystals,and thus nanowires of mixed phases can be considered as self-sacrificing templates for the nanotube formation(Fig.9).The formation process of the as-made Cu7S4nanotubes is similar to those of high symmetric Cu7S4polyhedron nanocrystals,in which cubic Cu2O nanostructures are first formed and Cu7S4phase are gradually formulated due to the addition of thiourea[11].It is also suggested that the nanotubes should be developed from the nanocrystalsaggregated together preferentially along a certain axis corresponding to the nature of monoclinic group and attached spontaneously through dipole-dipole attractions[36-37],thus making it more probable for nanocrystals to aggregate with their[100] crystallographic axis oriented in one direction.When the Cu/S molar ratio is changed to 1∶1,the experimental results(Fig.1A) show that the formation of these Cu7S4nanotubes is similar to those of pure nanotubes although the diameters of these nanotubes are somewhat larger than those of pure samples.And the excess of Cu(II)ions makes for the Cu7S4nanoparticle formation.With the Cu/S ratio up to 2∶1,however,the carboxyl group could coordinate with Cu(II)ionsleadingto the formation ofCuO phase[38].It is suggested that the Cu7S4phase formation is a thermodynamic process based on those results through changing reaction time and temperatures(Figs.3-5).Furthermore,it should be pointed out that similar Cu7S4nanotubes and results can also be obtained using D-or L-methionine instead of DL-methionine as the reagents(data not shown).

3 Conclusions

Uniform Cu7S4nanotubes are controllable synthesized through the methionine-assisted hydrothermal synthesis approach.The Cu(II)-methionine complex with the molar ratio of 1∶2(Cu(II) ions to methionine)is formed firstly via the coordination between Cu(II)ions and methionine molecules.With the reaction time elongated,nanowire structures containing multi-crystalline phases,such as CuS,Cu7S4and Cu2S,are formed,affording the self-sacrificing templates for the Cu7S4nanotube formation.The nanotubes are composed of small Cu7S4nanocrystals and the band gap energy is about 2.88 eV.This successful synthesis of Cu7S4nanotubes from alkyl sulphide systems may help to synthesize desired metal sulfide nanostructures through rational molecular design.

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Cu7S4纳米管的生物分子辅助水热合成与光学性质

郭培志1,*韩光亭1王宝燕1赵修松1,2

(1青岛大学化学化工与环境学院,纤维新材料与现代纺织国家重点实验室培育基地,山东青岛 266071;2Department of
Chemical and Biomolecular Engineering,National University of Singapore,4 Engineering Drive 4,Singapore 117576)

使用生物分子DL-甲硫氨酸辅助水热方法合成Cu7S4纳米管,产物的形貌与晶型可通过改变实验参数进行调控.研究表明,硝酸铜和DL-甲硫氨酸在反应开始时的配位比为1∶2,而且当反应物的摩尔比为1∶2和反应温度为200℃时可合成直径为100-600 nm、长度达40-100 μm的多晶Cu7S4纳米管.使用D-或L-甲硫氨酸均能得到类似Cu7S4纳米管.Cu7S4纳米管的禁带宽度为2.88 eV,与Cu7S4的块体材料相比有明显蓝移.基于实验研究结果,讨论了甲硫氨酸分子中的官能团与反应产物之间的联系并提出了Cu7S4纳米管的自牺牲模板法形成机理.

水热合成;Cu7S4纳米管; 甲硫氨酸; 生物分子

O649;O613.5;O614.12

Received:February 3,2010;Revised:April 17,2010;Published on Web:June 30,2010.

*Corresponding author.Email:pzguo@qdu.edu.cn;Tel:+86-532-83780378.

The project was supported by the National Natural Science Foundation of China(20803037),Doctoral Foundation of Shandong Province

(2007BS04022),Natural Science Foundation of Shandong Province(ZR2009BM013),and“Taishan Scholar”Program of Shandong Province,China.

国家自然科学基金(20803037),山东省博士基金(2007BS04022),山东省自然科学基金(ZR2009BM013)和“泰山学者”计划资助项目

ⒸEditorial office of Acta Physico-Chimica Sinica