Materials Science and Engineering B Tuning the Internal Structures Of

Materials Science and Engineering B 166 (2010) 14–18

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Materials Science and Engineering B

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Tuning the internal structures of CdSeS nanoparticles by using different selenium and sulphur precursors

Xianfeng Chen a,∗, John L. Hutchison a, Peter J. Dobson b, Gareth Wakeﬁeld c a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK b Oxford University Begbroke Science Park, Sandy Lane, Kidlington, Oxford OX5 1PF, UK c Oxonica Materials Ltd., 7 Oxford University Begbroke Science Park, Oxford OX5 1PF, UK article info abstract

Article history: Alloyed CdSeS nanoparticles (NPs) with composition gradient and homogeneous internal structures have Received 11 May 2009 been successfully prepared in aqueous solution by using different selenium and sulphur precursors. Received in revised form The as-prepared NPs form a homogeneous internal structure when sodium selenide and sodium sul- 16 September 2009 phide were used as selenium and sulphur precursors, respectively. Gradient internal structures were Accepted 23 September 2009 obtained for alloyed CdSeS NPs prepared by using sodium selenosulphate and sodium thiosulphate as selenium and sulphur precursors, respectively. The method takes advantage of the fact that different sele- Keywords: nium/sulphur sources release selenium/sulphur ions at different rates or have different reaction rates of Internal structure Nanoparticles selenium/sulphur toward cadmium. Alloyed © 2009 Elsevier B.V. All rights reserved. CdSeS

1. Introduction this paper, different selenium and sulphur precursors were used to tune the internal structures of the alloy NPs. We will show that II–VI colloidal semiconductor nanoparticles (NPs) have the initial NP optical properties are independent of the amount of attracted great interest for different applications [1–9], because sulphur precursor when using sodium selenosulphate and sodium the properties of these NPs can be tuned by their sizes [10]. thiosulphate as precursors, suggesting the formation of a binary However, the tuning of electronic, optical, and magnetic properties CdSe core in NPs. In comparison, the NP optical properties are by changing the particle size may cause problems in some appli- heavily dependent on the amount of sulphur precursor during cations [11]. In order to overcome these problems, a new class the whole synthesis process, indicating the formation of a ternary of alloyed semiconductor NP has been studied [11–18], because CdSeS NPs. By this tuning method, same size of CdSeS NPs can these alloyed NPs provide a way for continuous tuning of quantum have different band gaps. Alternatively, different size of CdSeS NPs confinement and hence the effective band gap without changing can have same band gaps. Therefore, the CdSeS NPs can be tuned the particle size. For example, the band gap of CdSeS NPs can to satisfy the different requirements of various applications. be adjusted by varying the selenium or sulphur concentration [15]. Herein, we report a simple method to tune the band gap 2. Experimental details of 3-mercaptopropionic acid (3-MPA) stabilized CdSeS alloy NPs by varying not only the selenium or sulphur concentration but 2.1. Materials also their internal structures. 3-MPA has been widely used for stabilizing semiconductor and noble metal nanocrystals and also Cadmium acetate (Cd(OAc)2·2H2O), sodium hydroxide, in the biomedical-related studies based on nanocrystals [19–22]. 3-mercaptopropionic acid, sodium sulphide nonahydrate, The carboxyl groups in 3-MPA can stabilize nanocrystals and and sodium thiosulphate (Na2S2O3) were obtained from also provide water solubility of nanocrystals. In addition, 3-MPA Sigma–Aldrich. Sodium selenide was purchased from Alfa- stabilized nanocrystals can be easily self-assembled into thin films Aesar. 0.02 M sodium selenide solution was prepared in a nitrogen by layer-by-layer methods due to their strong negative charge [23]. glove box. 0.02 M sodium selenosulphate (Na2SeSO3) solution was Therefore, we have chosen 3-MPA as the surfactant of the NPs. In formed by refluxing 0.5 g sodium sulphite and 0.05 g selenium powders in 33 ml water at 80 ◦C for 5 h. A trace amount of unreacted selenium powder was removed using a Whatman GD syringe filter ∗ (pore size, 1 ␮m). The prepared colourless sodium selenosulphate Corresponding author. E-mail address: [email protected] (X. Chen). solution was stored under dry nitrogen for later use.

2.2. Synthesis procedures and Na2S2O3 were chosen as the selenium and sulphur precursors, respectively. The Cd:Se ratio was determined to be 12:1 for the syn- A solution of 0.24 mmol of Cd(OAc)2·2H2O and 0.58 mmol of thesis of all NPs shown in Fig. 1. To prepare alloyed CdSeS NPs, two 3-MPA in 48 ml of demineralized water was adjusted to pH 9.3 initial precursor ratios of Se to S, 1:1 and 1:3, were used in experi- with 1 M NaOH solution. The solution was placed in a three-necked ments. It should be noted that the initial ratio of Se to S might not flask fitted with a septum and valves and was de-aerated with be the component ratio of S to Se in the resultant CdSeS alloyed N2 bubbling for 1 h. Under vigorous stirring, different amounts of NPs. It is hard to conclude the structure of CdSeS NPs by HRTEM mixed solution of sodium selenide and sodium sulphide, or sodium because of the closeness of the CdS and CdSe lattice constants. The selenosulphate and sodium thiosulphate were injected. The solu- electron energy loss spectroscopy (EELS) and X-ray photoelectron tion was then refluxed at 100 ◦C for different times. It takes about spectroscopy (XPS) are not helpful in this area [12]. Because the sur- 20 min for all reaction systems to be heated to 100 ◦C. factant contains sulphur, inductively coupled plasma (ICP) analysis cannot determine the composition of sulphur in alloyed NPs, either. 2.3. Characterisation of the CdSeS NPs Therefore, the structure of alloyed CdSeS NPs is mainly concluded from their optical properties. Experimentally, the optical proper- The UV–vis absorption and photoluminescence (PL) of the ties of colloidal semiconductor NPs are size dependent [24]. The as-prepared NPs were recorded using a double-beam UV-Vis spec- absorption and PL spectra shift to lower energy as the semiconduc- trometer (Perkin-Elmer Lambda 19) and a PL spectrometer (Hitachi, tor NP size increases. In addition to this, the absorption spectra and FL-4500), respectively. All absorption and PL spectra were mea- PL spectra of CdSe NPs shift to lower energy due to the surrounding sured without any post-preparative size separation. The quantum of a CdS shell or to higher energy if CdSeS alloy forms in the NPs efficiency (QE) of NPs was determined by comparison with the [25–29]. Therefore, the absorption and PL spectra of semiconductor value of the QE of Rhodamine 6G in ethanol at a concentration of NPs can be used as probes for the formation process of CdSeS NPs. ∼10−7 M (QE = 95%). The size distribution and crystalline structure As stated above, generally, it is believed that the formation of of the as-prepared NPs were determined by high-resolution trans- CdSeS alloy NPs would result in a blue shift of absorption and PL mission electron microscopy (HRTEM). The NPs were supported on spectra because of the larger band gap energy of CdSeS compared to an amorphous carbon film and the images recorded at 400 kV in pure CdSe [29]. However, the following important features can be a JEOL 4000EX microscope. X-ray diffraction (XRD) patterns were found from Fig. 1 by comparing the optical properties of CdSe and taken by a Philips PW1710 diffractometer using Cu K␣ radiation. CdSeS NPs, which may not be in accordance with this hypothesis. In order to see the difference clearly, some optical properties of CdSe 3. Results and discussion and CdSeS NPs are summarised in Table 1. Firstly, the PL peak positions for CdSeS NPs red-shift faster in the early stage (say first 25 h) 3.1. Using Na2SeSO3 and Na2S2O3 as the Se and S precursors, respectively and then slower afterwards than those for CdSe NPs. Secondly, the PL QE of CdSeS NPs is much higher than that of CdSe NPs in the early Fig. 1 shows the PL spectra of CdSe and alloyed CdSeS NPs stage. For example, the PL QE of CdSeS (initial Cd:Se:S = 12:1:1) NPs prepared at different initial precursor ratios of Se to S. Na2SeSO3 is about as high as 2.74 times of that of CdSe NPs after 16 h refluxing

Fig. 1. The PL spectra of (a) CdSe (initial Cd:Se:S = 12:1:0), (b) CdSeS (initial Cd:Se:S = 12:1:1) and (c) CdSeS (initial Cd:Se:S = 12:1:3) NPs and the absorption spectra of CdSe and CdSeS NPs prepared by different precursor ratios after 20 min reﬂuxing. The Se and S precursors are sodium selenosulphate and sodium thiosulphate, respectively. (d) From bot- tom to top, the absorption spectrum corresponds to the CdSeS NPs prepared from initial S:Se ratio of 0, 1, 3, 6 and 9, respectively. The initial Cd:Se ratio is 12:1 in all experiments. 16 X. Chen et al. / Materials Science and Engineering B 166 (2010) 14–18

Table 1 The optical properties of CdSe and CdSeS NPs prepared by using sodium selenosulphate and sodium thiosulphate as Se and S precursors, respectively.

PL peak position (nm) PL QE

CdSe (initial Cd:Se:S = 12:1:0) CdSeS (initial Cd:Se:S = 12:1:1) CdSe (initial Cd:Se:S = 12:1:0) CdSeS (initial Cd:Se:S = 12:1:1)

16 h 480 490 2.9% 8.0% 25 h 492 499 7.3% 11.9% 41 h 507 512 13.4% 7.8% 50 h 512 516 14.0% 4.8% 60 h 518 520 12.4% 3.1% 70 h 523 523 10.7% 2.9% at 100 ◦C. Thirdly, the surface trap emission of CdSeS NPs is lower formula estimates the size of single crystalline domains (having than that of CdSe NPs. Fourthly, the PL QE of CdSe NPs increases to periodic lattice) but not the actual size of the NPs. Therefore, it is the maximum intensity slowly and then gradually decreases while expected that the NP size obtained from XRD patterns is always the PL QE of CdSeS NPs increases to the maximum intensity quickly smaller than that estimated from HRTEM images [25]. Fig. 3a and b and then greatly drops. From these observations, it may be con- shows HRTEM images of CdSe and CdSeS NPs, respectively. The PL cluded that the resultant CdSeS NPs do not have a homogeneous spectra peaks for these two types of NPs are nearly at same posi- structure, but probably have a composition gradient structure. In tion (511–512 nm). However, the average sizes of CdSe and CdSeS other words, at the early stage, CdSeS NPs have a “core@shell” struc- NPs are about 2.5 and 3.4 nm, respectively. This comparison clearly ture, which makes the PL positions red-shift faster and the PL QE is shows that the NPs with different sizes can have the same PL peak higher. This hypothesis can be conﬁrmed by examining Fig. 1d. This position. ﬁgure shows the absorption spectra of CdSe NPs and alloyed CdSeS

NPs synthesised at different initial precursor ratios of sulphur to 3.2. Using Na2Se and Na2S as the Se and S precursors, respectively selenium. The precursor ratios of S:Se were 0, 1, 3, 6 and 9. The prepared NPs were collected immediately after the solutions were As we know, different selenium and sulphur sources release ◦ refluxed to 100 C and the absorption spectra were recorded. From selenium and sulphur ions, respectively, at different rates Fig. 1d, it can be seen that the first and second absorption peaks [11,22,30]. Therefore, the optical properties and internal struc- are almost at the same position for all NPs. Clearly, at this stage, ture (composition gradient or homogeneous) of CdSeS NPs can all NPs are almost binary CdSe NPs because their first absorption be adjusted by taking advantage of this. In order to produce peak should blue shift with the increased amount of added sulphur alloyed CdSeS NPs with homogeneous internal structure, sodium precursor if the CdSeS NPs are formed with a homogeneous struc- selenide and sodium sulphide were chosen as selenium and sul- ture. From these results, it is reasonable to conclude that the core phur precursors, respectively. At all stages (20 min, 1, 23, and 46 h is selenium rich and sulphur has a gradient of increasing concen- after refluxing), the absorption and PL spectra of as-prepared NPs tration from the core to the surface for the CdSeS NPs prepared showed blue shift with increasing ratios of S to Se. The specific by this method. The initial core is rich in selenium probably due values of spectra peak positions are summarised in Table 2. The to the faster release of selenium ions from Na2SeSO3 or the faster peak positions indicate a homogeneous structure in CdSeS NPs reaction rate of selenium toward cadmium [12]. As the free sele- when Na2Se and Na2S were used as precursors. Graphs plotting nium is being depleted from the reaction mixture and the sulphur the absorption peak positions of CdSeS NPs and S:Se ratios at given ions become increasingly available, CdS deposition becomes more times (20 min, 1, 23 and 46 h) have been drawn in Fig. 4. These important toward the growth of the NPs. Therefore, the outer layer is largely made of CdS and this layer acts as an encapsulating shell for the CdSe-rich core. But unlike the traditional core@shell NPs, this type of gradient alloyed NPs is prepared in a single step and does not have an abrupt boundary between Se-rich core and the S-rich shell. Fig. 2 shows the XRD patterns of CdSe and CdSeS NPs after 70 h refluxing. The broad peaks imply that the NPs are very small. The XRD pattern also confirms the crystalline cubic structure of all NPs, ruling out the possibility of a phase change in the alloyed NPs. Diffraction peaks shown in this pattern correspond to the (1 1 1), (2 2 0) and (3 1 1) plane reflections of cubic CdSe (JCPDS Card No. 19-0191). The particle size in each case is estimated using Debye-Scherrer’s formula (d = 0.9/B cos , where d is the particle diameter, the X-ray wavelength 0.1548 nm, ˇ is the full width at half maximum, and the scattering angle). The sizes estimated for the NPs prepared with Cd:Se:S precursor ratios of 12:1:0, 12:1:1 and 12:1:3 are 2.57, 2.33 and 2.73 nm, respectively. The as-prepared NPs were also observed under HRTEM. All HRTEM images of the NPs also confirm that these NPs have crystalline structure. The size of the NPs was also estimated based the on the measurement of about 100 NPs for each sample. The size for the NPs prepared with Cd:Se:S precursor ratios of 12:1:0, 12:1:1 and 12:1:3 is 3.83 ± 0.14 (mean ± SEM), 3.31 ± 0.21, and 3.47 ± 0.14 nm, respectively. Obviously, the NP size estimated from HRTEM is larger Fig. 2. XRD patterns for CdSe and CdSeS NPs prepared by using sodium selenosulphate and sodium thiosulphate as Se and S precursors, respectively. The ratios of than that estimated from the XRD patterns. This difference is com- Cd:Se:S are labelled. Vertical solid lines and dashed lines indicate the reflections of mon in the case of NPs. It is worth noting that the Debye-Scherrer’s bulk CdSe and CdS with cubic structure, respectively. X. Chen et al. / Materials Science and Engineering B 166 (2010) 14–18 17

Table 2 The optical properties of CdSe and CdSeS NPs prepared by using sodium selenide and sodium sulphide as Se and S precursors, respectively (initial Cd:Se = 20:1 in all experiments).

First absorption peak position (nm) PL peak position (nm) PL QE

20 min 1 h 23 h 46 h 23 h 46 h 23 h 46 h

S:Se = 0 401 413 485 507 505 523 2.1% 1.9% S:Se = 1 395 408 469 490 494 513 1.3% 2.3% S:Se = 2 386 400 463 482 489 507 1.1% 1.2% S:Se = 5 355 369 444 464 471 489 0.3% 0.4% show a linear relationship in all cases and they demonstrate that the absorption peak moves constantly with the change of S:Se ratio and refluxing time during the synthesis of CdSeS alloyed NPs. This we believe is a clear indication of the formation of homogeneous CdSeS structure. Fig. 5 shows the XRD patterns for CdSe and CdSeS NPs after 46 h refluxing. The broad peaks imply that the NPs are very small. The XRD pattern also confirms the crystalline cubic structure of

Fig. 4. Graphs between the absorption peak positions of CdSeS NPs and S:Se ratios at given times. The Se and S precursors are sodium selenide and sodium sulphide, respectively. The initial Cd:Se ratio is 20:1.

Fig. 5. XRD patterns CdSe and CdSeS NPs prepared by using sodium selenide and sodium sulphide as Se and S precursors, respectively. The initial Cd:Se ratio is 20:1 in all experiments. Vertical solid lines and dashed lines indicate the reﬂections of bulk CdSe and CdS with cubic structure, respectively.

all NPs, ruling out the possibility of a phase change in the alloyed NPs. Diffraction peaks shown in this pattern correspond to the (1 1 1), (2 2 0) and (3 1 1) plane reﬂections of cubic CdSe (JCPDS Card No. 19-0191). The average size of these NPs calculated from the Debye-Scherrer’s equation is about 2.4 nm for all samples, but their emission has changed from 523 nm for CdSe NPs to 489 nm for CdSeS NPs prepared from an initial S:Se ratio of 5:1. This result demonstrates that the NPs having the same size can have very different emission peak position.

4. Conclusion

In summary, alloyed CdSeS NPs with a composition gradient Fig. 3. HRTEM images of (a) CdSe NPs (initial Cd:Se:S = 12:1:0) having average size and homogeneous internal structures have been successfully pre- 2.5 nm and (b) CdSeS NPs (initial Cd:Se:S = 12:1:3) having average size 3.4 nm. The PL pared by using different selenium and sulphur precursors. The spectra peaks for these two types of NPs are nearly at same position (511–512 nm). The Se and S precursors are sodium selenosulphate and sodium thiosulphate, respec- as-prepared NPs form a homogeneous internal structure when tively. sodium selenide and sodium sulphide were used as selenium 18 X. Chen et al. / Materials Science and Engineering B 166 (2010) 14–18 and sulphur precursors, respectively. Gradient internal structures [8] X. Chen, J. Hutchison, P.J. Dobson, G. Wakeﬁeld, J. Mater. Sci. 44 (2009) 285. were obtained for alloyed CdSeS NPs prepared by using sodium [9] Y. Wang, Z.Y. Tang, M.A. Correa-Duarte, I. Pastoriza-Santos, M. Giersig, N.A. Kotov, L.M. Liz-Marzan, J. Phys. Chem. B 108 (2004) 15461. selenosulphate and sodium thiosulphate as selenium and sulphur [10] D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (2001) precursors, respectively. 207. The method takes advantage of the fact that different sele- [11] R.E. Bailey, S.M. Nie, J. Am. Chem. Soc. 125 (2003) 7100. [12] E. Jang, S. Jun, L. Pu, Chem. Commun. 24 (2003) 2964. nium/sulphur sources release selenium/sulphur ions at different [13] X.H. Zhong, M.Y. Han, Z.L. Dong, T.J. White, W. Knoll, J. Am. Chem. Soc. 125 rates or have different reaction rates of selenium/sulphur toward (2003) 8589. cadmium. These results demonstrate that composition and inter- [14] X.H. Zhong, Y.Y. Feng, W. Knoll, M.Y. Han, J. Am. Chem. Soc. 125 (2003) 13559. nal structure of NPs can also be used to tune the optical properties [15] Z.B. Pi, L.Y. Wang, X.K. Tian, C. Yang, J.H. Zheng, Mater. Lett. 61 (2007)4857. [16] L.A. Swafford, L.A. Weigand, M.J. Bowers, J.R. McBride, J.L. Rapaport, T.L. Watt, of alloyed semiconductor NPs without changing the particle size. S.K. Dixit, L.C. Feldman, S.J. Rosenthal, J. Am. Chem. Soc. 128 (2006) 12299. Further research need to be done to investigate if this approach can [17] Y. Wang, Y.B. Hou, A. Tang, B. Feng, Y. Li, J. Liu, F. Teng, J. Cryst. Growth 308 be widely used for tuning the internal structures in making other (2007) 19. [18] Y.G. Zheng, Z.C. Yang, J.Y. Ying, Adv. Mater. 19 (2007) 1475. alloy NPs. [19] J. Aldana, Y.A. Wang, X. Peng, J. Am. Chem. Soc. 123 (2001) 8844. [20] W.J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S.C. Williams, R. Acknowledgement Boudreau, M.A. Le Gros, C.A. Larabell, A.P. Alivisatos, Nanotechnology 14 (2003) R15. [21] M.N. Kalasad, M.K. Rabinal, B.G. Mulimani, Langmuir, in press, We thank Oxonica Materials Ltd. (Oxford, UK) for funding. doi:10.1021/la901798y. [22] A.L. Rogach, A. Kornowski, M.Y. Gao, A. Eychmuller, H. Weller, J. Phys. Chem. B 103 (1999) 3065. References [23] M.Y. Gao, C. Lesser, S. Kirstein, H. Mohwald, A.L. Rogach, H. Weller, J. Appl. Phys. 87 (2000) 2297. [1] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) [24] A.P. Alivisatos, Science 271 (1996) 933. 2013. [25] B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, [2] W.C.W. Chan, S.M. Nie, Science 281 (1998) 2016. K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101 (1997) 9463. [3] Y. Yin, A.P. Alivisatos, Nature 437 (2005) 664. [26] H.B. Bao, Y.J. Gong, Z. Li, M.Y. Gao, Chem. Mater. 16 (2004) 3853. [4] E.A. Weiss, R.C. Chiechi, S.M. Geyer, V.J. Porter, D.C. Bell, M.G. Bawendi, G.M. [27] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 Whitesides, J. Am. Chem. Soc. 130 (2008) 74. (1997) 7019. [5] Q. Zhao, P.A. Graf, W.B. Jones, A. Franceschetti, J. Li, L.W. Wang, K. Kim, Nano [28] M.A. Malik, P. O’Brien, N. Revaprasadu, Chem. Mater. 14 (2002) 2004. Lett. 7 (2007) 3274. [29] I. Mekis, D.V. Talapin, A. Kornowski, M. Haase, H. Weller, J. Phys. Chem. B 107 [6] V.I. Klimov, Annu. Rev. Phys. Chem. 58 (2007) 635. (2003) 7454. [7] S.C. Erwin, L.J. Zu, M.I. Hafter, A.L. Efros, T.A. Kennedy, D.J. Norris, Nature 436 [30] X.F. Chen, J.L. Hutchison, P.J. Dobson, G. Wakeﬁeld, J. Colloid Interface Sci. 319 (2005) 91. (2008) 140.