In Situ Encapsulation of Ultrasmall CuO Quantum Dots with Controlled

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In Situ Encapsulation of Ultra Small CuO Quantum Dots with Controlled Band-Gap and Reversible Thermochromism Yuzhen Ge, Zameer Hussain Shah, Cui Wang, Jiasheng Wang, Wenxin Mao, Shufen Zhang, and Rongwen Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09578 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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ACS Applied Materials & Interfaces

In Situ Encapsulation of Ultra Small CuO Quantum Dots with Controlled Band-Gap and Reversible Thermochromism Yuzhen Ge, Zameer Hussain Shah, Cui Wang, Jiasheng Wang, Wenxin Mao, Shufen Zhang, Rongwen Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China

ABSTRACT：Silica encapsulated ultra small CuO QDs (CuO@SiO2) were synthesized by reverse-microemulsion. The CuO QDs with sizes ranging from 2.0 to 1.0 nm with corresponding band gaps of 1.4 to 2.6 eV were prepared simply by varying the concentration of Cu2+ precursor. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV-vis spectroscopy. The CuO@SiO2 composite displayed reversible thermochromism which resulted from the strong electron-phonon coupling of ultra small CuO in the confined space of SiO2 and the enhanced band-gap shift in visible light region depending on temperature. Besides, the as synthesized CuO@SiO2 was found to be highly stable for reversible thermochromism due to the micropore structure of silica matrix and local confinement of the QDs.

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KEYWORDS： Quantum dots, Band-Gap, CuO, Reverse-microemulsion, Thermochromism INTRODUCTION Quantum dots (QDs) are of significant importance owing to their matchless characteristics.1-3 The electronic and optical properties of the QDs can be easily tuned by varying their band gaps simply via changing QD size.4 The band gap is inversely related to the size of QDs. For example, the band gap of CdS can be tuned from 2.5 to 4.5 eV as its size is varied from the macroscopic crystal to the molecular regime.5-7 Similarly, the temperature shift of the excitonic transition energy becomes stronger with the decrease of QDs size.5, 8, 9 These properties make QDs as great candidates for applications like probing, solid lighting, and sensing.10-12 In particular, the ultra small QDs with the diameter under 2 nm, also known as the magic size QDs,13 showed unique features and potential applications in optics and electronics.13-17 There have been several reports on the synthesis of magic size QDs. Normally, these ultra small QDs were synthesized by wet-chemical method in water-phase18 or hot injection method in organic-phase under the protection of special ligands to prevent the aggregation of the ultra small particles.19-20 Meanwhile, the presence of ligand usually changes the properties of QDs significantly and limits their applications in many fields.16,

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Therefore, feasible synthesis

without the protection of ligand is required to preserve the unique properties of the ultra small QDs. Silica is chemically inert, biocompatible, and optically transparent. These properties make silica as an ideal support for QDs.22-25 However, there have been few reports on the properties and synthesis of silica supported magic size QDs. Velarde-Ortiz et al.26 reported CuO QDs embedded in a silica sol-gel derived matrix. They employed special dendrimer which contained amine-type ligands as a macrochelating agent. Watanabe et al.27 also synthesized WO3 QDs with diameter around 1 nm by the method of impregnation. They firstly prepared super-micropores


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silica matrix with different structure directing agents, then used it as size-controlled template. CuO QDs were also fabricated by this method.28 Beside these two methods, a facile self-catalytic approach based on multifunctional precursor molecules for the synthesis of Eu2O3@mesoporous silica was recently proposed by Cui et al.29 Of all the mentioned methods, either special precursors and designed templates or several preparation steps were needed for the synthesis of silica supported ultra small QDs. Therefore, it is worthwhile to develop simple new ways of assembling silica supported magic size QDs. Now, we report a facile one-pot method to synthesize silica encapsulated ultra small CuO QDs with controlled average sizes by reverse-microemulsion. The increase in the band gap of CuO QDs was observed by decreasing the size of CuO QDs. This was attributed to the strong quantum confinement effect. The band gap of CuO QDs was tuned from 1.4 to 2.6 eV by controlling size of CuO QDs in the range of 2.0 to 1.0 nm (the band gap of bulk CuO is 1.3 eV30). The QDs size was found to be related to the Cu2+ precursor concentration. More interestingly, the color of CuO@SiO2 composite was observed to change gradually and reversibly from light blue (20 °C) to green (500 °C) with the change of temperature, which makes the as prepared CuO@SiO2 a potential candidate as thermochromic material in the fields of high temperature thermal indicators, solar protection, and warning signals.31-33 The thermochromism of CuO@SiO2 composite was ascribed to the shift of the absorption edge in the visible region which results from the strong electron-phonon coupling in the confined space and enhanced band gap shift of ultra small CuO QDs depending on temperature.28, 34 For comparison, silica encapsulated ZnO QDs with the average size of 1.7 ± 0.2 nm, 2.6 ± 0.3 nm, and 3.4 ± 0.4 nm were also synthesized. Due to the wide band gap of ZnO (3.35 eV at room temperature35), no obvious band gap shift was observed in the visible region, which means no thermochromism.


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EXPERIMENTAL SECTION Materials. Polyoxyethylene (20) cetyl ether (Brij-58) was purchased from Acros. Tetraethyl orthosilicate (TEOS), ammonium hydroxide (NH3·H2O, 25%–28%), cyclohexane, isopropanol (IPA), were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). Copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) was purchased from Shantou Xilong Chemical Factory Guangdong. Zinc (II) nitrate hexahydrate (Zn(NO3)2·6H2O) was purchased from Tianjin Dibo Chemical Factory. All chemicals were of analytical grade and used without further purification. Deionized water (18.2 MΩ) was used in all experimental processes as needed. Synthesis of CuO@SiO2. The procedure is similar to previous work of our lab36, typically, a 50 mL two-necked round-bottomed flask with 15 mL of cyclohexane and 3.36 g of Brij-58 (3 mmol) was heated to 50 °C by water bath and dissolved under stirring, then 0.40 mL of Cu(NO3)2·3H2O aqueous solution with different concentrations was added to the transparent microemulsion. After 1 h of stirring, 1.0 mL of ammonium hydroxide was added dropwise and 0.5 h later, 2 g of TEOS was added to the system. To make sure the complete hydrolysis of TEOS another 2 h were needed, after which 15 mL of IPA was added to demulsify the microemulsion. The mixture was collected and centrifuged at 6000 rpm for 10 min. The precipitate was washed with 20 ml of IPA once and dried at 100 °C for 10 h. After calcined at 500 °C for 2 h under air stream, the final products were obtained. The concentration of Cu(NO3)2·3H2O aqueous solution we used in this paper were 0.12 M, 0.25 M, 1.0 M, 2.0 M, and the product we got were named as Cu-0.12, Cu-0.25, Cu-1.0 and Cu-2.0, respectively. The procedure of synthesis of ZnO@SiO2 was similar to that of CuO@SiO2, the concentration of Zn(NO3)2·6H2O aqueous solution was 0.5 M, 1.0 M, 2.5 M, and 4.0 M; the respective products were named as Zn-0.5, Zn-1.0, Zn-2.5 and Zn-4.0.


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Characterizations. Morphological studies were carried out by a JEOL JEM-2000 EX transmission electron microscopy (TEM) which was performed at room temperature using an accelerating voltage of 120 kV. The high-resolution transmission electron microscopy (HRTEM) images were performed on JEM-2100F (JEOL) using an accelerating voltage of 200 kV. FT-IR was performed on a Thermo Nicolet Nexus instrument. The XRD pattern was recorded on a Rigaku DMAX IIIVC X-ray diffractometer with Cu-Ka (0.1542 nm) radiation scanning from 10° to 80° (2 θ) at the rate of 6 °/min. X-ray photoelectron spectroscopy (XPS) was acquired by Thermo VG ESCALAB 250 with a Al-Kα X-ray source operating at 150 W (15 kV). The binding energies were calibrated using the C 1s peak at 284.6 eV, and the software XPS PEAK 4.1 was used for Curve fitting. UV-vis absorbance spectra were obtained by JASCO UV-550. The reflection spectra were measured by HITACHI U-4100 spectrophotometer. Specific surface area measurement and porosity analysis were characterized using a Quantachrome Autosorb-1MP surface area and pore size analyzer. The Cu content was determined by the inductively coupled plasma optical emission spectroscopy (Optima 2000 DV, Perkin Elmer, ICP-OES). Thermochromic reversibility test. The thermochromic reversibility of CuO@SiO2 composite was presented by the evolution of the UV-vis absorbance curves after 10 times of temperature cycling. The temperature program was set at a heating rate of 2.5 °C/min from 20 °C to 500 °C and maintained for 30 min, then cooled to room temperature in 3 h. RESULTS AND DISCUSSION


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Scheme 1. Illustration of the possible formation process of CuO@SiO2. The possible formation process is shown in Scheme 1. The light-blue reverse-microemulsion was formed after Cu(NO3)2·3H2O aqueous solution was added to the mixture of Brij-58 and cyclohexane. After the addition of ammonium hydroxide, the reverse-microemulsion turned dark blue due to the formation of ammoniacal copper (II) complex ([Cu(NH3)4]2+). The solution became homogenous after 30 minutes of stirring. The TEOS was introduced and hydrolyzed at the oil/water interface and made sol in the polar phase36. As the condensation proceeded, a large three-dimensional network structure formed around the surfactant and [Cu(NH3)4]2+ was encapsulated in-situ at the same time36,37. Afterwards, the demulsification by isopropanol transformed [Cu(NH3)4]2+ into Cu(NH3)4(OH)2.38 During the drying process, Cu(NH3)4(OH)2 converted into Cu(OH)2 due to the loss of ammonia. The FT-IR spectra of product before (Figure


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1A) and after (Figure 1B) drying confirmed this transformation. The peaks around 1461 cm-1, 1399 cm-1, and 1351 cm-1 are characteristic peaks of Cu(OH)2. During the calcination at 500 °C, the Cu(OH)2 was dehydrated to CuO. The disappearance of characteristic peaks of Cu(OH)2 and emergence of characteristic peaks of CuO around 620 cm-1 (Figure 1C) confirmed this transformation. Meanwhile, the peaks at around 471 cm-1 correspond to the bending vibration of Si-O-Si, the peaks at 800 cm-1 and 1099 cm-1 are attributed to the symmetric stretching vibration and asymmetric stretching vibration of Si-O-Si. The disappearance of the peak around 2925 cm-1 means the removal of surfactants.

Figure 1. FT-IR spectra of [email protected] A) before drying, B) after drying, and C) after calcination.


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Figure 2. Representative TEM images of A, a) Cu-0.12, B, b) Cu-0.25, C, c) Cu-1.0, D, d) Cu2.0 under different scale bar, and the corresponding particles size histograms of CuO(Calculated by 100 random particles from TEM).


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Figure 2 shows the representative TEM and HRTEM images of Cu-0.12, Cu-0.25, Cu-1.0, Cu2.0, the average particles sizes of CuO@SiO2 were 31.5 ± 1.5 nm, 25.2 ± 1.3 nm, 27.8 ± 1.6 nm and 27.3 ± 1.4 nm (the particles size histograms which calculated by 200 random particles from TEM shows in Figure S1), respectively. A slightly decreasing in SiO2 particle size can be observed with the increasing Cu2+ concentration. Besides, a large quantity of ultra small QDs with regular morphologies can be observed by HRTEM in all the products. The size distribution histogram of the obtained CuO QDs (the CuO QDs are too small to be measured in Cu-0.12) shows the sizes of CuO QDs increased from 1.1 ± 0.12 nm (Cu-0.25), 1.4 ± 0.13 nm (Cu-1.0) to 2.0 ± 0.17 nm (Cu-2.0) as the increasing concentration of Cu(NO3)2·3H2O aqueous solution. We suppose the CuO QDs were formed by the in-situ dehydrated of Cu(OH)2 particles, the higher concentration of Cu2+) in the confined space of silica means the bigger Cu(OH)2 particles can be formed. Figure S2 shows the TEM images of Cu-1.0 before and after calcination, small clusters already formed before calcination which confirmed the existence of Cu(OH)2 particles. The XRD pattern of CuO@SiO2 presented in Figure S3, the broad peak under 40° is attributed to the amorphous silica. It is worth mentioning that no obvious diffraction peaks corresponding to CuO can be recognized from wide-angle XRD patterns due to the ultra small size of CuO.37, 39 Table 1 show the content of Cu in all CuO@SiO2 samples based on the ICP-OES analysis. The results indicate that the practical Cu content in CuO@SiO2 increase synchronously with the concentration of Cu2+ precursor. It is worth to mention that the practical Cu content are much close with the theoretical Cu content, more than 90% of the Cu2+ are encapsulated by the final product. Compared with the traditional impregnation method, the metal utilization are much improved by our method.


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Table 1 Content of Cu in Cu-0.12, Cu-0.25, Cu-1.0, Cu-2.0 based on the ICP-AES analysis.

Sample

Content of Cu (mg/L)

Practical Cu content in CuO@SiO2 (%)

Theoretical Cu content in CuO@SiO2 (%)

Encapsulating percentage (%)

1

Cu-0.12

4.111

1.00

1.06

94.6

2

Cu-0.25

7.275

1.77

1.86

95.4

3

Cu-1.0

25.57

6.56

7.05

93.0

4

Cu-2.0

58.69

14.3

15.2

94.0

For the ICP-AES analysis, 20 mg of product was dissolved with 1 mL HF, and transferred to 50 ml volumetric flask. Practical Cu content = Content of Cu based on ICP-AES×Volume of solution (0.05 L)×100% / Amount of CuO@SiO2 (0.02 g). Theoretical Cu content = Amount of Cu2+ precursor added/Total amount of CuO@SiO2. Encapsulating percentage of Cu = Practical Cu content in CuO@SiO2/Theoretical Cu content in CuO@SiO2.

The elemental composition and chemical status of the surface compositions of Cu-1.0 were analyzed by XPS. Figure 3A shows the full range of the XPS spectrum for Cu-1.0 in which Cu, O, C, Si are observable. The presence of C originates from chamber contamination in the XPS equipment. Figure 3B, C show the narrow scan spectra for Cu 2p and O 1s, respectively. In the narrow scan spectrum of Cu 2p, there are two peaks with binding energies at 934.9 eV and 954.9 eV due to Cu 2p3/2 and Cu 2p1/2 in CuO.40 Meanwhile, intense satellite features at 942.9 eV and 963.1 eV can be ascribed to Cu 2p3/2 and Cu 2p1/2 in CuSiO3 which formed under high temperature. The formation of CuSiO3 also implied the close interation between CuO and SiO2


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in the confined space. Both peaks at 531.6 eV and 531.9 eV belong to O 1s, arising from CuO and SiO2, respectively. The peak of Si 2p with a binding energy of 103.3 eV corresponds to the binding state of Si in SiO2. These results indicate the presence of CuO in the silica matrix.

Figure 3. A) Survey XPS spectra of Cu-1.0, and narrow scan spectra for the elements of B) Cu, C) O. An obvious color change of CuO@SiO2 was observed during the annealing. Figure 4 shows the optical photographs of Cu-0.12, Cu-0.25, Cu-1.0 and Cu-2.0 at different temperatures. A distinct color change was observed from light blue (20 °C) to green (500 °C) in the silica encapsulated ultra small CuO QDs. Figure S4 shows the temperature-dependent color change of Cu-1, a gradual color change happened as the increasing of temperature. Besides, as the concentration of Cu(NO3)2·3H2O aqueous solution increased, the color of product became darker accordingly.


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The reasons for the darker color does not only lies in the bigger particles size of CuO QDs, but also the higher content of Cu in product which can be confirmed by the ICP results (Table 1). Figure 5A shows the UV-vis absorbance spectra of Cu-0.12, Cu-0.25, Cu-1.0 and Cu-2.0 at room temperature. The relatively weak absorbance from 400 nm to 500 nm demonstrates the bluish color of product. A broad signal of around 750 nm is attributed to d–d transitions of Cu (II) surrounded by oxygen in CuO. The strong absorbance peak from 200 nm to 400 nm is attributed to the enlarged band gap of CuO QDs and the different absorption band edge also means the different band gap of CuO QDs. Figure S5 shows the reflectance spectra of CuO@SiO2, the broad peaks from 400-550 nm demonstrate the bluish color which are consistent with the absorbance spectra. The band gap energy (Eg) of CuO QDs was calculated from a photon-energy (hυ) dependent (Ahυ)1/2 obtained from the UV-vis absorbance spectra according to the Tauc plot41 in Figure 5B, where A is the absorbance and hυ is the photon energy. The intersection point of horizontal ordinate and the straight black line represents the band gap energy of CuO QDs. As the particles size was increased, the band gap of CuO QDs changed from 2.6 eV in [email protected] to 2.4 eV in [email protected], 1.8 eV in [email protected], and 1.4 eV in [email protected]. The size dependent band gap of CuO QDs attributed to the confined electron motion, also known as quantum confinement effect, and the thermochromism of CuO@SiO2 composite was ascribed to the shift of the absorption edge in the visible region which results from the strong electron-phonon coupling in the confined space and enhanced band-gap shift depending on temperature. 28, 34


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Figure 4. Optical photographs of Cu-0.12, Cu-0.25, Cu-1.0, Cu-2.0 at different temperature: A, B, C, D at 20 °C, a, b, c, d at 500 °C.


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Figure 5. A) UV-vis absorbance spectra and B) Tauc plots of Cu-0.12 (a), Cu-0.25 (b), Cu-1.0 (c), Cu-2.0 (d). The reusability of material has significant importance for the practical applications. Metal/silica systems are known to easily form silicates at high temperature. As proved by XPS previously, CuSiO3 was produced during the annealing, but the formed CuSiO3 does not change the optical properties of our materials. Figure 6A and B show the TEM images of Cu-2.0 before and after 10 times of cycling with a heating rate of 2.5 °C/min from 20 °C to 500 °C, and only tiny difference can be observed. The average size of CuO QDs slightly increased from 1.9 ± 0.20 nm to 2.0 ± 0.18 nm (Figure S6), which means no aggregation of CuO QDs during the cycling (As the materials in Figure 2D and Figure 6A were separately synthesized, the particle size of CuO QDs


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show tiny difference). Figure 6C shows the evolution of the UV-vis absorbance curves of Cu-1.0 after 10 times of cycling and no obvious difference can be observed in the curves which means good stability of reusing and practically potential use as high temperature thermal indicators, solar protection, and warning signals. As far as we know, the mostly reported inorganic thermochromic materials were based on phase transition42-44 and ligand-to-metal charge transfer with temperature45. However, most of them were irreversible and no literature have been reported about thermochromic materials which can stand the temperature over 500 °C. The outstanding thermal stability of CuO@SiO2 arises from the micropore of the materials. The nitrogen adsorption−desorption isotherms implies the micropore structure with a large surface area of 117 m2 g-1 and an average pore size of 0.6 and 0.9 nm (calculated by the conventional Horvath – Kawazoe model and shown in the inset of Figure 6D). Different from the general impregnation way of preparing ultra small QDs which the size of QDs are smaller than or equal to the pore size of supporters27, the size of QDs synthesized by our method are larger than the pore size of SiO2. The smaller size of QDs than pore size usually implies the potential aggregation of QDs and the bigger size of QDs than pore size means the local confinement of the QDs by silica matrix from each other, which means stable QDs.


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Figure 6. TEM images of Cu-2.0 before A) and after B) 10 times of cycling, C) evolution of the UV-vis absorbance curves of Cu-1.0 after 10 times of cycling, D) nitrogen adsorption−desorption isotherms of Cu-2.0 (The inset figure shows the micropore size distribution calculated by Horvath-Kawazoe model).


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Figure 7. Representative TEM images of A) Zn-0.5, B) Zn-1.0, C) Zn-2.5, D) Zn-4.0, and the XPS narrow scan spectra for the elements of E) Zn, F) O. For comparison, silica encapsulated ZnO QDs with different sizes were also synthesized by the same method. Figure 7A-D shows the TEM images of Zn-0.5, Zn-1.0, Zn-2.5 and Zn-4.0, the average size of ZnO QDs in Zn-1.0, Zn-2.5 and Zn-4.0 were 1.7 ± 0.2 nm, 2.6 ± 0.3 nm, 3.4 ± 0.4 nm (Figure S9), respectively. Figure 7E shows the XPS narrow scan spectra for the elements of Zn, two peaks with binding energies at 1045.5 eV and 1022.5 eV due to Zn 2p1/2 and Zn 2p3/2


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in ZnO. As the element oxygen exists in both ZnO and SiO2, the narrow scan spectrum of O 1s (Figure 7F) can be split into two peaks with the binding energies of 532.7eV and 532 eV. These results indicate the presence of ZnO in the silica matrix. Figure 8A shows the powder UV-vis absorbance spectra of ZnO@SiO2, a gradual increasing in the absorbance can be observed from 700 nm to 300 nm for all samples. However, both the optical photographs of Zn-2.5 under 500 °C (Figure 8B, Figure S10) and 20 °C (Figure 8C) show light yellowish color and no thermochromism can be observed. The reason for no thermochromism of ZnO@SiO2 is the wide band gap of ZnO (3.35 eV at room temperature for bulk materials), as the excitation wavelength lies in UV region, no obvious band gap shift can be observed in visible region. In short, for the designing of thermochromism materials based on the band gap shift, two essential factors should be considered: 1) the ultra small size of QDs; 2) the suitable band gap of QDs.

Figure 8. A) Powder UV-vis absorbance spectra of Zn-0.5, Zn-1.0, Zn-2.5, Zn-4.0, and optical photographs of Zn-2.5 under B) 500 °C, C) 20 °C. CONCLUSIONS


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In summary, silica encapsulated ultra small CuO QDs with different sizes showing different band-gap were synthesized by a facile one-pot method. CuO@SiO2 was found to be remarkably stable during the thermochromic reversibility tests. This high stability was due to the local confinement of the QDs by silica matrix. For comparison, ZnO@SiO2 was also synthesized by the same method. As the ultra small CuO QDs (ZnO QDs) were formed by the dispersion of Cu2+ (Zn2+) attributed to the coordination of Cu2+ (Zn2+) with NH3 and the in situ dehydration of Cu(OH)2 (Zn(OH)2), we anticipate that this strategy can be exploited as a general method for the preparation of a series of ultra small QDs of transition metal (e.g., Ni, Co, Pd, Pt) ions which have some affinity for ammonia. AUTHOR INFORMATION Corresponding Author * Rongwen Lu E-mail:[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grans No.21176038, 21576044, 21536002), the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No.21421005), and Dalian University of Technology Innovation Team (DUT2013TB07). SUPPORTING INFORMATION


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The supporting information includes particle size histograms, XRD patterns, UV-vis reflectance spectrum for CuO@SiO2; TEM images of Cu-1.0 before and after drying; optical photographs of Cu-1.0 at different temperatures; particle size histograms of CuO before and after 10 times recycling; FTIR, XPS, optical photographs for ZnO@SiO2, and particle size histograms of ZnO. REFERENCES 1.

Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum dots. Science 1996,

271 (5251), 933-937. 2.

Brus, L. Electronic Wave Functions in Semiconductor Clusters: Experiment and Theory.

J. Phys. Chem. 1986, 90 (12), 2555-2560. 3.

Xia, Y. Editorial: Are We Entering the Nano Era? Angew. Chem. Int. Ed. 2014, 53 (46),

12268-12271. 4.

Kim, J. Y.; Voznyy, O.; Zhitomirsky, D.; Sargent, E. H. 25th Anniversary Article:

Colloidal Quantum Dot Materials and Devices: A Quarter-Century of Advances. Adv. Mater. 2013, 25 (36), 4986-5010. 5.

Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.;

Eychmueller, A.; Weller, H. CdS Nanoclusters: Synthesis, Characterization, Size Dependent Oscillator Strength, Temperature Shift of the Excitonic Transition Energy, and Reversible Absorbance Shift. J. Phys. Chem. 1994, 98 (31), 7665-7673. 6.

Brus, L. E. Electron-Electron and Electron-Hole Interactions in Small Semiconductor

Crystallites: The Size Dependence of the Lowest Excited Electronic State. J. Chem. Phys. 1984, 80 (9), 4403-4409. 7.

Satoh, N.; Nakashima, T.; Kamikura, K.; Yamamoto, K. Quantum Size Effect in TiO2

Nanoparticles Prepared by Finely Controlled Metal Assembly on Dendrimer Templates. Nat.


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanotechnol. 2008, 3 (2), 106-111. 8.

Chin, P. T. K.; Donega, C. D. M.; Bavel, S. S.; Meskers, S. C. J.; Sommerdijk, N.;

Janssen, R. A. J. Highly Luminescent CdTe/CdSe Colloidal Heteronanocrystals with Temperature-Dependent Emission Color. J. Am. Chem. Soc. 2007, 129 (48), 14880-14886. 9.

Narayanaswamy, A.; Feiner, L. F.; Meijerink, A.; van der Zaag, P. J. The Effect of

Temperature and Dot Size on the Spectral Properties of Colloidal InP/ZnS Core-Shell Quantum Dots. ACS Nano 2009, 3 (9), 2539-2546. 10.

Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan,

G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Quantum Dots for Live Cells, in Vivo Imaging, and Diagnostics. Science 2005, 307 (5709), 538-544. 11.

Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.-S.;

Fukumura, D.; Jain, R. K.; Bawendi, M. G. Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12 (5), 445-451. 12.

Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X.

Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum dots. Nature 2014, 515 (7525), 96-99. 13.

Bowers, M. J.; McBride, J. R.; Rosenthal, S. J. White-Light Emission from Magic-Sized

Cadmium Selenide Nanocrystals. J. Am. Chem. Soc. 2005, 127 (44), 15378-15379. 14.

Jose, R.; Zhelev, Z.; Bakalova, R.; Baba, Y.; Ishikawa, M. White-Light-Emitting CdSe

Quantum Dots Synthesized at Room Temperature. Appl. Phys. Lett. 2006, 89 (1) 013115-3. 15.

Newton, J. C.; Ramasamy, K.; Mandal, M.; Kumbhar, A.; Sardar, R. Low-Temperature

Synthesis of Magic-Sized CdSe Nanoclusters: Influence of Ligands on Nanocluster Growth and


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

Photophysical Properties. J. Phys. Chem. C 2012, 116 (7), 4380-4389. 16.

Rosson, T. E.; Claiborne, S. M.; McBride, J. R.; Stratton, B. S.; Rosenthal, S. J. Bright

White Light Emission from Ultrasmall Cadmium Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134 (19), 8006-8009. 17.

Pennycook, T. J.; McBride, J. R.; Rosenthal, S. J.; Pennycook, S. J.; Pantelides, S. T.

Dynamic Fluctuations in Ultrasmall Nanocrystals Induce White Light Emission. Nano lett. 2012, 12 (6), 3038-3042. 18.

Aboulaich, A.; Billaud, D.; Abyan, M.; Balan, L.; Gaumet, J. J.; Medjadhi, G.; Ghanbaja,

J.; Schneider, R. One-Pot Noninjection Route to CdS Quantum Dots via Hydrothermal Synthesis. ACS Appl. Mater. Interfaces 2012, 4 (5), 2561-2569. 19.

Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly

Monodisperse CdE (E = Sulfur, Selenium,Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115 (19), 8706-8715. 20.

Peng, X.; Wickham, J.; Alivisatos, A. P. Kinetics of II-VI and III-V Colloidal

Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions. J. Am. Chem. Soc. 1998, 120 (21), 5343-5344. 21.

Dolai, S.; Dutta, P.; Muhoberac, B. B.; Irving, C. D.; Sardar, R. Mechanistic Study of the

Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors. Chem. Mater. 2015, 27 (3), 1057-1070. 22.

Nann, T.; Mulvaney, P. Single Quantum Dots in Spherical Silica Particles. Angew. Chem.

Int. Ed. 2004, 43 (40), 5393-5396. 23.

Yang, Y. H.; Gao, M. Y. Preparation of Fluorescent SiO2 Particles with Single CdTe

Nanocrystal Cores by the Reverse Microemulsion Method. Adv. Mater. 2005, 17 (19), 2354-


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2357. 24.

Zhu, Y.; Li, Z.; Chen, M.; Cooper, H. M.; Lu, G. Q.; Xu, Z. P. SiO2@CdTe@SiO2 for

Cellular Imaging. Chem. Mater. 2012, 24 (3), 421-423. 25.

Aubert, T.; Soenen, S. J.; Wassmuth, D.; Cirillo, M.; Van Deun, R.; Braeckmans, K.;

Hens, Z. Bright and Stable CdSe/CdS@SiO2 Nanoparticles Suitable for Long-Term Cell Labeling. ACS Appl. Mater. Interfaces 2014, 6 (14), 11714-11723. 26.

Velarde-Ortiz, R.; Larsen, G. A Poly(propylene imine) (DAP-Am-64) Dendrimer as Cu2+

Chelator for the Synthesis of Copper Oxide Clusters Embedded in Sol-Gel Derived Matrixes. Chem. Mater. 2002, 14 (2), 858-866. 27.

Watanabe, H.; Fujikata, K.; Oaki, Y.; Imai, H. Band-Gap Expansion of Tungsten Oxide

Quantum Dots Synthesized in Sub-Nano Porous Silica. Chem. Commun. 2013, 49 (76), 84778479. 28.

Tamaki, H.; Watanabe, H.; Kamiyama, S.; Oaki, Y.; Imai, H. Size-Dependent

Thermochromism Through Enhanced Electron-Phonon Coupling in 1 nm Quantum Dots. Angew. Chem. Int. Ed. 2014, 53 (40), 10706-10709. 29.

Cui, F.; Cui, T. Self-Catalytic Synthesis of Metal Oxide Nanoclusters@Mesoporous

Silica Composites Based on Successive Spontaneous Reactions at Near Neutral Conditions. Chem. Commun. 2014, 50 (94), 14801-14804. 30.

Wu, D. X.; Zhang, Q. M.; Tao, M. LSDA+U Study of Cupric Oxide: Electronic Structure

and Native Point Defects. Phys. Rev. B 2006, 73 (23), 235206. 31.

Day, J. H. Thermochromism of Inorganic Compounds. Chem. Rev. 1968, 68 (6), 649-657.

32.

Bloomquist, D. R.; Willett, R. D. Thermochromic Phase Transitions in Transition Metal

Salts. Coord. Chem. Rev. 1982, 47 (1-2), 125-164.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

33.

Page 24 of 26

Gaudon, M.; Carbonera, C.; Thiry, A. E.; Demourgues, A.; Deniard, P.; Payen, C.; Letard,

J. F.; Jobic, S. Adaptable Thermochromism in the CuMo1-xWxO4 Series (0

In Situ Encapsulation of Ultrasmall CuO Quantum Dots with Controlled

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