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Significant increase in bandgap and emission efficiency of In2O3 quantum dots by size-tuning around 1 nm in supermicroporous silicas Takafumi Suzuki, Hiroto Watanabe, Taiki Ueno, Yuya Oaki, and Hiroaki Imai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04181 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017
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Title Significant increase in bandgap and emission efficiency of In2O3 quantum dots by size-tuning around 1 nm in supermicroporous silicas
Author list Takafumi Suzuki,a Hiroto Watanabe,*b Taiki Ueno,a Yuya Oaki,a and Hiroaki Imai*a
a. Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. b. Tokyo Metropolitan Industrial Technology Research Institute, 2-4-10 Aomi. Koto-ku, Tokyo 135-0064, Japan.
E-mail:
[email protected] Abstract The size of In2O3 quantum dots (QDs) is tuned from 0.57 to 1.80 nm by using supermicroporous silicas (SMPSs) as a template. The bandgap energy and photoluminescence quantum yields of In2O3-QDs increase remarkably when their size decreased below 1 nm.
Main Text Introduction Indium oxide (In2O3) is an n-type semiconductor with a bandgap of Eg = 3.75 eV (direct) or 2.61 eV (indirect).1 It has been used as a transparent conductor for solar cells,2–4 flat-panel displays,5,6 and sensors.7,8 Therefore, numerous studies on the properties and synthesis of the semiconductor oxide have been reported.
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Tiny particles of semiconductor materials with a size below 10 nm (quantum dots (QDs)) display specific chemical and physical properties that differ from those of the bulk owing to quantum size effects.9–11 Meng et al. synthesized In2O3 nanoparticles ~5 nm in diameter by a solvothermal method with In(OH)3.12 However, they only observed a small difference in the visible absorption edge, which indicates a weak quantum size effect owing to their large particle size. Zhou et al. produced In2O3 nanoparticles by using mesoporous silica as their templates and observed a relatively large bandgap energy (Eg = 3.7 eV) 13 However, the relationship between the bandgap energy and the particle size was not revealed. Park et al. controlled the size of In2O3 nanoparticles from 4 to 8 nm by thermal decomposition of the In(acac)3 precursor in the presence of a stabilizing surfactant 14 Hsieh et al. tuned In2O3 nanoparticles from 1.8 to 5 nm by sputtering deposition 15 They discussed quantum size effects with energy shifts in the photoluminescence (PL) emission. However, it is difficult to discuss the quantum size effect from the variations of emission energies because the PL of nano-sized In2O3 would be affected by the presence of various defects. Therefore, the correlation between the particle size and the quantum size effect remains unclear. Since the Bohr radius of In2O3 is reported to be around 1.3 ~ 2.5 nm12, 13, fine size control of In2O3 nanoparticles is required for investigation. However, the tunable production of around 1 nm nanoparticles has been quite difficult. We synthesized semiconductor QDs using supermicroporous silicas (SMPSs) as templates.16, 17, 18
The size of the QDs was strictly tuned below 2 nm by changing the pore size of the
SMPSs. The remarkable expansion of the bandgap of QDs was observed owing to strong quantum size effects. Since the Bohr radius of In2O3 is around 1.3 ~ 2.5 nm, this method is suitable for preparation of In2O3-QDs. In the present study, the size of the In2O3-QDs was strictly tuned from 0.57 to 1.80 nm in diameter by changing the pore size of the SMPSs as templates and the amount of precursor solution impregnated into the SMPSs. We observed remarkable expansion of the bandgap of In2O3-QDs from 2.75 to 3.91 eV by decreasing the ACS Paragon Plus Environment
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size of the In2O3-QDs. Moreover, the PL quantum yields notably increased when their size decreased below 1 nm.
Experimental Solvent-free synthesis of supermicroporous silicas (SMPSs) Alkyltrimethylanmonium chloride (bromide) (CnTAC(B)), n = 4-18, n: carbon number of alkyl chain), tetraethyl orthosilicate (TEOS), and pH 2.0 HCl aq. were mixed with 0.2: 1.0: 4.0 mole ratio, respectively. After stirring the mixture at 298 K until the gelation, the products were dried at 333 K for 24 h and calcined at 873 K for 3 h. The samples are denoted as CnPS (n = 4-18, n: carbon number of alkyl chain)
Characterizations of SMPSs The nitrogen adsorption-desorption isotherms were obtained at 77 K with 3 Flex (Micromeritics) using samples pretreated under a vacuum at 433 K for 5 h. The specific surface area and total pore volume were calculated by the BET method. The pore size distribution was calculated by the BJH and GCMC methods. For the Grand Canonical Monte Caro (GCMC) method, the BEL-Master software was used for analyses. The analytical models of GCMC methods were as follows: pore models, oxygen-exposed surface with a cylindrical pore structure; adsorbate, nitrogen; peak assumption, Gaussian model with a single peak.
Preparation of In2O3-QDs In2O3-QDs were prepared in the pores of SMPSs by impregnating 2.0 M HNO3 solution of In(NO3)3 (0.20 mol/dm3, the volume of impregnation is equal to the pore volume of SMPSs). Then, the samples were decompression dried to remove water from the pores. This procedure was repeated prescribed times. After stirring and decompression drying the powder, the ACS Paragon Plus Environment
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samples were calcined at 723 K for 3 h. The products are denoted as Inn×m (n = 6-18, n: carbon number of alkyl chain of CnTAC(B) in preparation of SMPSs; m = 1-5, m: the repeat number of the precursor impregnation). The bulk bcc-In2O3 prepared by calcination of In(NO3)3 powder at 723 K for 3 h.
Characterizations of QDs Field-emission transmission electron microscopes (FE-TEM) images were obtained using FEI TECNAI G2, and energy dispersive X-ray spectroscopy (EDX) was measured using Oxford Instruments X-MaxN 80T. The X-ray Photoelectron Spectrum (XPS) was measured using a PHI Quantera II (ULVAC Φ), without pre-spattering. The charge compensation of binding energy was done by using C1s peak position of surface contaminated carbon and confirmed by Si2p peak of SMPS. Ultraviolet-visible (UV-vis) spectra were obtained using JASCO V-670 equipped with an integrating sphere. Absolute PL quantum yields (PL-QYs) were measured using a Hamamatsu Photonics C9920-02G.
Results SMPSs were synthesized by a solvent-free method which was developed in our previous study.16, 17, 18 The pore sizes of SMPSs were controlled by changing carbon number (n) of alkyl chain of alkyltrimethylammonium chloride (bromide) CnTAC(B) (n = 6– 18,) which was used as porogens. (see Schemes S1 and Table S1 for experimental procedures, adsorption isotherms, textual properties for SMPSs) In2O3-QDs were prepared by impregnation of indium nitrate In(NO3)3 aqueous solution into the pores of SMPSs followed by calcination at 723 K. The sizes of the QDs were controlled by changing the pore size of SMPSs and the repeat number of precursor impregnation. The products are denoted as Inn×m (n = 6–18, n: carbon number of alkyl chain of CnTAC(B) used in preparation of SMPSs; m = 1–5, m: the repeat number of the precursor impregnation) The ACS Paragon Plus Environment
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presence of In2O3 in the SMPSs was confirmed by Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). Thus, we obtained In2O3-QDs in the SMPS pores. (see Schemes S2, and Table S2, Figure S1–S8 for experimental procedures, and characterization of SMPSs and In2O3-QDs). As shown in Figures 1(a–c), S6, and S7, transmission electron microscope (TEM) images indicate that In2O3-QDs were well dispersed in the SMPS. In2O3-QDs with a narrow size distribution were gained by using the SMPS pores as templates. As reported in our previous article,16 wormhole-like cylindrical pores are produced by the template of cylindrical micelles in supermicroporous silicas. We confirmed that spheroidal quantum dots are formed in the cylindrical pores from the TEM images. The XRD profiles and the lattice fringe of the TEM images (Figure S5 and S8) indicate the presence of body-centered cubic (bcc) structure of present In2O3-QDs. The size of the In2O3-QDs clearly increased with the increased SMPS pore size and repeat number of precursor solution impregnated into the SMPS pores. We succeeded in controlling the size of In2O3-QDs in the range of 0.57 to 1.80 nm.
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Figure 1. (a–c) Typical TEM images of In2O3-QDs (Inn×m). (d) The relationship between the size of In2O3-QDs, the pore size of the SMPSs, and the number of impregnations of the precursor of In2O3-QDs. (n = 6–18, n: carbon number of alkyl chain of CnTAC(B) used in preparation of SMPSs; m = 1–5, m: the repeat number of the precursor impregnation)
As shown in Figure 2(a), the absorption edge of In2O3-QDs shifted to the short wavelength side as the size decreased. The blue shift of the absorption edge indicates that the bandgap of In2O3-QDs depended on their size owing to quantum size effects. The bandgap energy values Eg of bulk and In2O3-QDs were calculated from Tauc plots (Figure 2(b)). The Eg values of bulk In2O3 was calculated as 2.58 eV which corresponds to the indirect gap of bcc-In2O3.19 For the In2O3-QDs, the Eg values were increased with decreasing the particle size and controlled within a range of 2.75 to 3.91 eV by tuning the size of the In2O3-QDs.
Figure 2. (a) The diffuse reflectance UV-vis spectra. (b) Tauc plots of Inn×m.
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Figure 3 shows the relationship between the size of the In2O3-QDs and the bandgap energy. The remarkable increase of the bandgap energy of In2O3-QDs was observed by decreasing the In2O3-QDs size to below 1 nm. Moreover, the variation in the In2O3-QDs bandgap energy can be fitted by the Brus equation.20 The reduced mass of the electron and hole (µ) for In2O3 was calculated to be 3.89 m0 with a relationship between the size and the bandgap energy (Figure S9).
Figure 3. The relationship between the particle size of In2O3-QDs and the bandgap energy. The size dependency of the bandgap energies has not been clear because the size control of In2O3-QDs around 1 nm was difficult. In the present study, the In2O3-QDs sizes were tuned to around 1 nm, and we revealed a strong correlation between the size and the bandgap energy. Similar to previous reports,12–15 we observed intense blue-green luminescence from the In2O3-QDs in SMPSs under ultraviolet (250 nm) irradiation. Bulk In2O3 or SMPSs showed no PL under UV irradiation. The PL of the In2O3 was known as a defect related emission, and their emission energies were varied with their structure and nature of the defect states.12-15, 21 The PL spectra of a different size of In2O3-QDs are shown in Figure 4 and S10 (Excitation
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wavelength = 250 nm). All the samples exhibit two PL peaks at 450 and 470 nm, with a shoulder around 540 nm which were similar to the emission of the In2O3 nanowire.21 In the present In2O3-QDs, peak shifts and their proportion varied slightly with the decreasing size of In2O3-QDs, while they showed strong quantum size effects in their absorption spectra.
Figure 4. The relationship between the particle size of In2O3-QDs and the absolute PL quantum yields. The inset shows the PL spectra of In2O3-QDs (Excitation wavelength = 250 nm)
The PL emissions of the semiconductors are generally classified into two categories: the near band-edge (NBE) emission22 and the defect-related (DR) one.23 The luminescence wavelength of the NBE emission is sensitive to the quantum size effect, while that of the DR emission is ascribed to the state of the defects. The present In2O3-QDs show no remarkable PL peak shifts with the size variation. Thus, the PL of the present In2O3-QDs was attributed to defects related emission, such as oxygen or indium vacancies, which come from the spatially localized energy states. Furthermore, we found that the PL intensity of the In2O3-QDs was highly enhanced by decreasing their size. To elucidate this phenomenon, we examined absolute PL quantum yields measurements of the In2O3-QDs and summarized our findings in Figure 4. The PL quantum yields increased ACS Paragon Plus Environment
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remarkably with the decreasing particle size of In2O3-QDs in the sub-nanometer region. This size-related enhancement of the PL quantum yields is similar to the case of doped semiconductor systems, i.e., Tb-doped Y2O3 QDs.24 These results revealed that the particle size reduction in In2O3-QDs has a strong influence on the emission efficiency.
Conclusion In this study, the size-controlled In2O3-QDs were synthesized in the pores of SMPSs. The size of the In2O3-QDs was tuned from 0.57 to 1.80 nm by changing the pore size and the repeat number of precursor impregnation into the SMPSs. A remarkable expansion of the bandgap of In2O3-QDs from 2.75 to 3.91 eV was observed in the sub-nanometer region owing to strong quantum size effects. Furthermore, the PL quantum yields of In2O3-QDs were highly enhanced by decreasing their size, especially in the sub-nanometer region.
Acknowledgements This work was partially supported by Kato Foundation for Promotion of Science and Grant-in-Aid for Scientific Research (No. 22107010) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology.
Associated Content Supporting information is available free of charge via the Internet at http://pubs.acs.org/.
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