Facile Assembly of Oppositely Charged Silver Sulfide Nanoparticles

Mar 15, 2015 - ... Textile & Clothing, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, China ... The large number of sulfur-related defects existi...
9 downloads 3 Views 4MB Size
Download PDF
Article pubs.acs.org/Langmuir

Facile Assembly of Oppositely Charged Silver Sulfide Nanoparticles into Photoluminescent Mesoporous Nanospheres Lianjiang Tan,*,† Shuiping Liu,‡ Qinglai Yang,† and Yumei Shen*,† †

Shanghai Center for Systems Biomedicine, Key Laboratory of Systems Biomedicine, Ministry of Education, Collaborative Innovation Center of Systems Biomedicine, and Bio-ID Center, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, China ‡ Key Laboratory of Eco-Textiles, Ministry of Education and College of Textile & Clothing, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, China S Supporting Information *

ABSTRACT: Inorganic mesoporous materials have been attracting increasing attention during the past decade. In the present work, photoluminescent Ag2S nanospheres with mesoporous structures were prepared by assembling Ag2S nanoparticles with opposite charges in aqueous phase. Without structure-directing templates, mesoporous Ag2S with well-ordered face-centered cubic superlattice structures and high specific surface area was obtained. The mesoporous Ag2S nanospheres had the same crystal phase as their precursors Ag2S nanoparticles. Different from their near-infrared emitting precursors, the mesoporous Ag2S nanospheres exhibited cyan emission under ultraviolet excitation. The large number of sulfur-related defects existing in the mesostructures is most likely responsible for the photoluminescence. This work provides new insights into fabricating photoluminescent mesostructured materials via scale-up strategy.

1. INTRODUCTION Three-dimensional (3D) mesoporous materials have received tremendous attention owing to their high specific surface area, uniform pore structure, and tailorable surface functionalities for potential applications including adsorption, storage, separation, transportation, catalysis, drug delivery, optical devices, and so on.1−7 Silica, carbon, metal oxides, and metals are the most popular precursors for mesoporous materials. Templateassisted methods,8 which include the use of soft templates (e.g., surfactants, polymers)9,10 and hard templates (mesoporous carbon or silica)11,12 that play a pivotal role in directing the formation of mesoscale structures, are usually employed to build these mesoscale structures. Although mesoporous materials with well-organized pore structure and high porosity can be readily prepared, they inevitably inherit the structure of the templates, which limits the design and development of new mesostructures and related properties. In the past years, template-free approaches have been reported for synthesizing mesoporous materials. Choi’s group successfully synthesized crystalline meso-TiO2 possessing enhanced photocatalytic activity for H2 production without the use of templates.13 The mesoporous TiO2 was prepared by packing colloidal TiO2 nanoparticles (NPs) densely through a facile process. Cao’s group reported template-free hydrothermal synthesis of nanoembossed mesoporous LiFePO4 microspheres using Fe3+ salt as raw material,14 which could meet the requirements of high-performance lithium-ion batteries. © XXXX American Chemical Society

While intensive studies have been focused on the synthesis of silica- and metal oxide-based mesoporous materials, preparing mesoporous semiconductor metal chalcogenides still remains a challenge due to the unique chemical complexity of these materials. Their mesostructures tend to break down when subjected to hydrolysis or heating. Li’s group prepared 3D mesoporous Ag2S and Ag2Se spheres by assembling Ag2S or Ag2Se NPs with the assistance of surfactants.15 The obtained mesoporous materials have highly ordered architectures, and the researchers concluded that the synthesis approach is independent of the chemical compositions of building blocks; however, they focused only on the synthesis methods and material structures without investigating the mesostructurerelated properties of the meso-Ag2S and Ag2Se. Moreover, organic solvents were used as the solvent and surfactant, which was adverse to pollution control and cost reduction. Other studies on silver chalcogenide mesoporous materials are rarely reported, attributable to the difficulty in processing such highmelting and insoluble materials. Herein, a facile strategy for preparing mesoporous Ag2S nanospheres is described, which was achieved by assembly of oppositely charged Ag2S NPs in aqueous phase and subsequent calcination at appropriate temperatures. Specifically, 3-mercapReceived: December 25, 2014 Revised: March 13, 2015

A

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 1. Schematic Illustration of the Synthetic Procedure of Mesoporous Ag2S Nanospheres

Figure 1. (a) TEM micrograph and (b) high-resolution TEM micrograph of MPA-capping Ag2S NPs. (c) TEM micrograph and (d) high-resolution TEM micrograph of MPa-capping Ag2S NPs. (e) TEM micrograph, (f) high-resolution TEM micrograph, (g) SAED pattern, and (h) EDX spectrum of mesoporous Ag2S nanospheres calcinated at 350 °C. (the NPs concentration of both solutions was 10 mg/mL) were mixed under stirring, kept for different periods (20, 40, and 60 min) at room temperature. Stirring is aimed to keep the reaction solution homogeneous and to prevent the formed Ag2S aggregates from gathering. Once the stirring was halted, the resultant colloidal solution was centrifuged, filtered, washed with ethanol, and vacuum-dried at 40 °C, producing Ag2S aggregates. The Ag2S aggregates were then heated to different temperatures (250, 300, and 350 °C) with a heating rate of 10 °C/min in a furnace under an argon atmosphere. At each target temperature, the samples were kept for 2 h to obtain mesoporous Ag2S nanospheres. The final nanosphere production yield against the Ag2S NPs was 81% when the calcination temperature was 350 °C. 2.4. Characterization. Sample micrographs were recorded on a JEM-2100 transmission electron microscope (TEM, JEOL, Japan) at 200 kV. Samples were suspended in ethanol, fully dispersed by ultrasonic wave, and deposited on an amorphous carbon-coated copper grid prior to observation. The JEM-2100 TEM equipped with energy-dispersive X-ray (EDX) spectrometry was also used for the EDX analysis and selected area electron diffraction (SAED). Scanning electron micrographs (SEMs) were recorded using a JSM-7401F fieldemission scanning electron microscope (JEOL, Japan). Samples were Pt-coated at a sputtering rate of 1.5 kV/min prior to observation. Fourier transform infrared spectroscopy (FTIR) tests were carried out on a Spectrum 100 FTIR spectrometer (PerkinElmer, US). Powder wide- and small-angle X-ray diffraction (XRD) patterns were collected using a D/max-2200/PC X-ray diffractometer (Rigaku, Japan) fitted with nickel-filtered Cu Kα radiation. The data were collected at 0.02° intervals with counting for 0.2 s at each step. Adsorption−desorption isotherms of N2 were measured at −196 °C with an ASAP 2010 M+C surface area and porosimetry analyzer (Micromeritics, US) after degassing the samples. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB 250 X-ray photoelectron spectrometer (Thermo Scientific, US) with nonmonochromatic Al Kα X-ray (1486.6 eV). The analyzer was operated at 20 eV pass energy with an energy step size of 1 (full spectra) and 0.1 eV (high-resolution

topropionic acid (MPA)-capping Ag2S NPs and 3-mercaptopropylamine (MPa)-capping Ag2S NPs were first synthesized in aqueous solutions, respectively. The two types of Ag2S NPs are oppositely charged, and they tend to assemble in aqueous solution via electrostatic interaction between the ionized carboxyls and the protonated aminos. The resultant Ag2S aggregates were then calcinated to produce mesoporous Ag2S nanospheres (Scheme 1). Densely packed Ag2S NPs constituted the mesoporous nanospheres, which are photoluminescent (PL) under ultraviolet (UV) irradiation.

2. EXPERIMENTAL SECTION 2.1. Materials. Silver nitrate (AgNO3, 99.9%), sodium sulfide (Na2S, 98%), MPA (99%), and MPa (99%) were all purchased from Sigma-Aldrich. Quinine sulfate (98%), sodium hydroxide (NaOH, 97%), and nitric acid (HNO3, 97%) were provided by Aladdin. Millipore water was used throughout. All chemicals and reagents were used as received without further purification. 2.2. Synthesis of Ag2S Nanoparticles. Ag2S NPs were synthesized based on the protocol designed by us. Typically, 0.1 mmol of AgNO3 and 0.15 mmol of MPA or MPa were dissolved in 20 mL of water in a three-necked flask at room temperature under nitrogen flow. After stirring for 3−5 min, 1 mL of NaOH solution or HNO3 solution (1 M) was added and stirred for 2 to 3 h. Thereafter, 1 mL of Na2S solution (0.1 M, 50 °C) was quickly injected into the above solution, which was allowed to react under vigorous stirring at 50 °C for 2 h. When the reaction was finished, the resultant colloidal solution was cooled to room temperature and was dialyzed against water for 24 h to remove unreacted molecules and ions. MPA-capping and MPa-capping Ag2S NPs dispersed in water were then obtained, respectively. 2.3. Preparation of Mesoporous Ag2S Nanospheres. The aqueous solutions of as-synthesized Ag2S NPs with opposite charges B

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(031) facets and so on, typical of α-Ag2S crystals (Figure 1g). EDX analysis shows the element types in the nanospheres, including Ag, S, and Cu (from copper grid) (Figure 1h). The atomic ratio of Ag to S calculated based on EDX data was 1.86 (Table S1, SI), close to the stoichiometry of Ag2S. XPS spectra of the Ag2S NPs (Figure S5, SI) indicate that the oxidation state of Ag was univalent in the NPs.16 The O peak or N peak confirms the types of capping ligands around the Ag2S NPs. The full XPS spectrum of the mesoporous Ag2S nanospheres shows similar valence states of the Ag and S to those of the Ag2S NPs (Figure 2a). The absence of N signal or O signal

spectra). Binding energy calibration was based on C 1s at 284.6 eV. The size distribution of the samples were determined by a Nano ZS90 particle size and zeta potential analyzer (Malvern, U.K.) based on dynamic light scattering (DLS) at a scattering angle of 90°. Thermogravimetry (TG) was performed on a Q5000IR thermogravimetric analyzer (TA, US) under N2 flow from room temperature to 400 °C at a heating rate of 10 °C/min. Absorption spectra were recorded by a Lambda 35 UV−vis spectrophotometer (PerkinElmer, US), background corrected for any water contribution. PL emission and excitation spectra were obtained on a LS 55 luminescence spectrometer (PerkinElmer, US), applying an excitation wavelength of 365 nm. Photoluminescence lifetime was measured on a QM/TM/IM time-resolved fluorescence spectrophotometer (PTI, US) at 25 °C. The sample was excited by the 380 nm light with a pulse duration of 5 ns. The fluorescence decay data were fitted using a biexponential model.

3. RESULTS AND DISCUSSION 3.1. Morphology and Chemical Composition. TEM micrographs (Figure 1a,c) show that the synthesized MPAcapping and MPa-capping Ag2S NPs had spherical profiles and a narrow size distribution. The average diameter of the two types of Ag2S NPs determined by DLS was 4.7 ± 1.1 and 4.5 ± 1.0 nm, respectively (Table 1 and Figure S1, SI). The zeta Table 1. Parameters Obtained from DLS Measurements for the Two Types of Ag2S NPs NP sample

hydrodynamic diameter (nm)

polydispersity index

zeta potential (mV)

MPA-capping MPa-capping

4.7 ± 1.1 4.5 ± 1.0

0.08 0.10

−36.7 ± 1.2 33.8 ± 1.5

Figure 2. (a) Full XPS spectrum of the mesoporous Ag2S nanospheres. (b) Ag 3d signals and (c) S 2p signal recorded for the mesoporous Ag2S nanospheres.

potential distribution curves and the mean zeta potential values suggest that the two types of Ag2S NPs had opposite electric charges with similar quantity (Figure S2, SI and Table 1), which is a necessary premise for the electrostatic assembly of the Ag2S NPs. FTIR spectra of the as-synthesized MPA-capping and MPa-capping Ag2S NPs were recorded to verify the ligands bound to the surface of the NPs (Figure S3, SI). For the MPAcapping Ag2S NPs, the absorption bands at 3420 and 1710 cm−1 are ascribed to the stretching vibration of OH and CO in carboxyl groups. For the MPa-capping Ag2S NPs, the bands at 3390 and 3185 cm−1 are attributable to the N−H stretching vibration in NH2 groups, and the band at 1320 cm−1 is typical of C−N stretching. The FTIR results confirmed that the capping ligands at the surface of the Ag2S NPs were MPA and MPa. High-resolution TEM micrographs (Figure 1b,d) reveal lattice fringes of the Ag2S NPs with the lattice spacing of 0.23 and 0.24 nm, respectively, which were assigned to the (−112) facets characteristic of monoclinic α-Ag2S phase. Typical TEM and high-resolution TEM micrographs of mesoporous Ag2S nanospheres were demonstrated (Figure 1e,f). As the products of assembling the Ag2S NPs, the mesoporous Ag2S nanospheres were dozens of nanometers in diameter. Under larger magnification, we can see well-organized structures of the nanospheres with the Ag2S NPs stacking together compactly. Furthermore, the morphology of the Ag2S aggregates was observed by TEM (Figure S4, SI). The Ag2S NPs were found to aggregate rather than be away from each other. Compared with the mesoporous Ag2S nanospheres, the Ag2S NPs stacked together less closely, which also indicates the significance of calcination in producing well-structured nanospheres. SAED pattern of the mesoporous Ag2S nanospheres displayed diffraction rings assigned to (111), (220), and

indicates volatilization of the capping ligands during the calcination process for the Ag2S NPs. The high-resolution XPS spectra show more readily observed signals of Ag to S (Figure 2b,c). The assembly time needed for the Ag2S NPs to form Ag2S aggregates was examined because it may affect the size of resultant Ag2S nanospheres. After the Ag2S NPs were synthesized, an assembly process was conducted, during which the Ag2S NPs were attracted to one another by their opposite charges. The subsequent calcination process resulted in Ag2S nanospheres composed of tightly packed Ag2S NPs. It was found that the assembly time exerted significantly influences the size of the produced mesoporous Ag2S nanospheres. TEM and high-resolution TEM micrographs indicate that the diameter of the Ag2S nanospheres increased with the increase in assembly time, yet the ordered structures did not change (Figure 3a−d). It is evident that the nanospheres formed at longer assembly time contain more packed Ag2S NPs. We examined the size distribution of the mesoporous Ag2S nanospheres based on DLS. The size distribution profile shifted to larger values as the assembly time increased (Figure 3e), in good accordance with the TEM observation. Besides, the size distribution curves at the midpoint (10 min) and end point (20 min) of the assembly as well as after calcination were shown (Figure 3f). During the assembly, the number of individual Ag2S NPs decreased, and the Ag2S aggregates with larger dimension gradually formed, as reflected by the DLS curves. The size of the mesoporous Ag2S nanospheres was a little smaller than that of the Ag2S C

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. (a,c) TEM micrographs and (b,d) high-resolution TEM micrographs of mesoporous Ag2S nanospheres with different average sizes. Assembly time: (a,b) 40 min; (c,d) 60 min. (e) Size distribution of the mesoporous Ag2S nanospheres formed for different assembly times. (f) Size distribution of the samples at assembly time of 10 and 20 min as well as after calcination.

Figure 4. (a) XRD patterns of the Ag2S aggregates before calcination (bottom) and the mesoporous Ag2S nanospheres (top). (b) TG curve of the Ag2S aggregates; (inset) FESEM micrographs showing the mesoporous Ag2S nanospheres calcinated at 250 and 350 °C, respectively. (c) N2 adsorption−desorption isotherms and (d) pore-size distribution of the mesoporous Ag2S nanospheres calcinated at different temperatures. Assembly time for all samples was 20 min.

showed monoclinic α-Ag2S phase (JCPDS card no. 14-0072), with the diffraction peaks at 2θ = 28.8, 41.0, 43.7, and 53.5°, corresponding to (111), (031), (220), and (−213) facets, respectively. These results agreed well with the SAED diffraction rings. Moreover, the broadening of the diffraction

aggregates due to closer stacking and fusion of the NPs resulting from the calcination. 3.2. Structure of Mesoporous Ag2S Nanospheres. XRD patterns of the Ag2S aggregates and the mesoporous Ag2S nanospheres were recorded (Figure 4a). The Ag2S aggregates D

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 5. (a) UV−vis absorption and (b) PL emission of the MPA-capping and MPa-capping Ag2S NPs. PL emission lifetime results for the (c) MPA-capping Ag2S NPs and (d) MPa-capping Ag2S NPs.

Figure 6. (a) UV−vis absorption and (b) PL emission of the mesoporous Ag2S nanospheres calcinated at different temperatures. (inset) PL image of a quartz cuvette containing mesoporous Ag2S nanospheres. (c) PL emission lifetime results for the mesoporous Ag2S nanospheres based on biexponential fitting: τ1 = 14.6 ± 1.52 ns, τ2 = 54.6 ± 5.1 ns. (d) Photostability of the mesoporous Ag2S nanospheres in comparison with quinine sulfate under irradiation of a 100 W mercury lamp. The excitation wavelength for the PL emission and PL lifetime measurements was 365 nm.

peaks indicates the nanocrystalline nature of the Ag2S aggregates.17 The XRD pattern of the mesoporous Ag2S nanospheres showed the same crystal phase, but the diffraction

peaks were much sharper and narrower compared with the Ag2S aggregates. The significant difference in peak width before and after calcination is ascribed to dramatic increase in the E

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

make them disperse in water prior to the optical measurements. The mesoporous Ag2S nanospheres demonstrated greatly different optical properties from their precursors. A new absorption band appeared at 363 nm for the samples calcinated at 350 °C in addition to the NIR absorption, which was not observed for the Ag2S NPs. The band edge appeared at ∼415 nm. To facilitate comparison, we show UV−vis−NIR spectra of the Ag2S NPs and the mesoporous Ag2S nanospheres in the same graph (Figure S7, SI). The difference between the absorption spectra of the Ag2S NPs and the mesoporous Ag2S nanospheres was that the nanospheres had UV absorption. As the calcination temperature increased, the absorption of the nanospheres exhibited a slight red shift to 370 nm and then to 378 nm. The PL excitation spectrum shows two broad peaks centered at 272 and 382 nm, respectively (Figure S8, SI), indicating that the nanospheres could be easily excited by UV light. In the PL emission spectra of the three samples, an emission peak was centered at ∼480 nm under excitation of 365 nm light. The calcination temperature seemed to exert little effect on the emission wavelength of the nanospheres. Cyan emission could be observed from the nanospheres under excitation (Figure 6b, inset). It should be noted that the NIR emission of the Ag2S NPs was not observed for the nanospheres because the crystal size increased beyond the exciton Bohr radius. The PL decay data based on biexponential fitting show an emission lifetime of 54.6 ± 5.1 ns (Figure 6c). The other shorter characteristic time may originate from the defects at the pore surface of the nanospheres. The PL emission mechanism of mesostructured Ag2S has never been reported. Nevertheless, color centers related to oxygen vacancies have been found to cause optical absorption and PL emission in mesoporous ZrO222 and SiO2.23 Similarly, we believe that the sulfur-related defects account for the optical absorption and PL emission of the mesoporous Ag2S nanospheres discovered in this work. The mesostructures with high specific surface area favor the occurrence of sulfur vacancies. Additionally, a large amount of surface defects exist on the pore internal surfaces of the Ag2S nanospheres, which could lead to the formation of new energy levels in the band gap.22 We finally evaluated the photostability of the mesoporous Ag2S nanospheres, which were subjected to continuous irradiation of a mercury lamp at a power output of 100 W. Their PL intensity during the irradiation was monitored, with that of quinine sulfate also traced for comparison (Figure 6d). The nanospheres retained 80% of the original PL intensity after being irradiated for 3 h, while the quinine sulfate was nearly photobleached under the same conditions. These results suggest excellent photostability of the mesoporous Ag2S nanospheres.

crystal size caused by calcination. The XRD results reveal that the calcination procedure plays a key role in producing mesoporous nanospheres after assembly of the Ag2S NPs via electrostatic interaction. TGA was performed for the Ag2S aggregates (Figure 4b). The TG curve exhibited one weightloss step from 178 to 387 °C, signifying the combustion and volatilization of the capping ligands. The sample morphology at two specific temperatures was observed by field-emission scan electron microscopy (FESEM) (Figure 4b, inset). There was little difference in the surface morphology and size of the samples at 250 and 350 °C, indicating that the main structures had been formed as the temperature increased to 250 °C. N2 sorption analysis is a powerful tool for characterizing porous materials and was thus carried out to investigate the mesoporous characteristics of the Ag2S nanospheres. The N2 adsorption/desorption isotherms of the nanospheres calcinated at different temperatures show typical type IV curves (Figure 4c). The corresponding Brunauer−Emmett−Teller (BET) surface areas were 44, 58, and 63 m2 g−1 for the nanospheres calcinated at 250, 300, and 350 °C, respectively. The Barrett− Joyner−Halenda (BJH) pore-size distribution curves suggest average pore sizes of 7.3, 6.7, and 6.5 nm for the three samples (Figure 4c). Li’s group found that their Ag2S mesoporous colloidal spheres possessed a face-centered cubic (fcc) superlattice structure.15 It is assumed that our mesoporous Ag2S nanospheres also have the fcc structures; then, the pore size in the nanospheres can be estimated using eq 1 if the Ag2S NPs are regarded as spherical15 r≈

3

3 ⎛ 16π ⎞ ⎜64 − ⎟R ⎝ 16π 3 ⎠

(1)

where R is the radius of the Ag2S NPs constituting the nanospheres. According to this equation, the radius of the pore r ≈ 1.41 R. Comparing the sizes of the Ag2S NPs and the mesopores previously mentioned, we believe that the Ag2S NPs in the mesoporous Ag2S nanospheres stack in fcc superlattice. Small-angle XRD, which is a useful technique for characterizing nanoscaled superlattice, was performed for the nanospheres to further validate this speculation (Figure S6, SI). Two diffraction peaks at 3.1 and 6.2° can be assigned to (422) and (844) facets, respectively, in fcc superlattice structure.15 The N2 adsorption results indicate that calcination at a higher temperature led to more perfect hierarchical mesostructures with higher surface area. 350 °C was taken as a standard calcination temperature in this work. 3.3. Optical Properties. The near-infrared (NIR) PL properties of Ag2S nanocrystals have been widely reported.18−20 As the precursors of the mesoporous Ag2S nanospheres, the optical properties of the Ag2S NPs should be investigated. Both the MPA-capping and MPa-capping Ag2S NPs showed an absorption band at ∼800 nm and a symmetric emission peak at ∼980 nm (Figure 5a,b). The full width at half-maximum (fwhm) of the emission peak was as small as 65−70 nm, signifying a narrow size distribution of these NPs. PL decay data of the two types of Ag2S NPs fitted by biexponential model indicated a PL lifetime of and 583.6 and 653.0 ns, respectively (Figure 5c,d), which were similar to those of Ag2S quantum dots reported previously.21 The absorption and PL emission spectra of the mesoporous Ag2S nanospheres calcinated at different temperatures were recorded (Figure 6a,b). Because the nanospheres were not as hydrophilic as their precursors, ultrasonication was applied to

4. CONCLUSIONS In summary, PL mesoporous Ag2S nanospheres have been successfully prepared by assembly of oppositely charged Ag2S NPs and then calcination. The assembly time for the Ag2S NPs to form Ag2S aggregates determines the size of the resultant nanospheres. The temperature at which the Ag2S aggregates are calcinated affects the mesostructures and the optical absorption of the nanospheres. The mesoporous Ag2S nanospheres had well-organized structures, high specific surface area, and identical crystal phase as their precursors. The sizes of the mesopores in the nanospheres and those of the precursors Ag2S NPs reflected that the nanospheres possessed fcc superlattice structures. Cyan emission can be observed from the nanoF

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(12) Jiao, F.; Hill, A. H.; Harrison, A.; Berko, A.; Chadwick, A. V.; Bruce, P. G. Synthesis of Ordered Mesoporous NiO with Crystalline Walls and A Bimodal Pore Size Distribution. J. Am. Chem. Soc. 2008, 130, 5262−5263. (13) Lakshminarasimhan, N.; Bae, E.; Choi, W. Enhanced Photocatalytic Production of H2 on Mesoporous TiO2 Prepared by Template-Free Method: Role of Interparticle Charge Transfer. J. Phys. Chem. C 2007, 111, 15244−15250. (14) Qian, J.; Zhou, M.; Cao, Y.; Ai, X.; Yang, H. Template-Free Hydrothermal Synthesis of Nanoembossed Mesoporous LiFePO 4 Microspheres for High-Performance Lithium-Ion Batteries. J. Phys. Chem. C 2010, 114, 3477−3482. (15) Wang, D.; Xie, T.; Peng, Q.; Li, Y. Shape Control of CdSe Nanocrystals with Zinc Blende Structure. J. Am. Chem. Soc. 2008, 130, 4016−4022. (16) Tan, L.; Wan, A.; Zhao, T.; Huang, R.; Li, H. Aqueous Synthesis of Multidentate-Polymer-Capping Ag2Se Quantum Dots with Bright Photoluminescence Tunable in a Second near-Infrared Biological Window. ACS Appl. Mater. Interfaces 2014, 6, 6217−6222. (17) Du, Y.; Xu, B.; Fu, T.; Cai, M.; Li, F.; Zhang, Y.; Wang, Q. NearInfrared Photoluminescent Ag2S Quantum Dots from a Single Source Precursor. J. Am. Chem. Soc. 2010, 132, 1470−1471. (18) Zhang, Y.; Hong, G.; Zhang, Y.; Chen, G.; Li, F.; Dai, H.; Wang, Q. Ag2S Quantum Dot: A Bright and Biocompatible Fluorescent Nanoprobe in the Second Near-Infrared Window. ACS Nano 2012, 6, 3695. (19) Jiang, P.; Tian, Z. Q.; Zhu, C. N.; Zhang, Z. L.; Pang, D. W. Emission-Tunable Near-Infrared Ag2S Quantum Dots. Chem. Mater. 2012, 24, 3−5. (20) Tan, L.; Wan, A.; Li, H. Ag2S Quantum Dots Conjugated Chitosan Nanospheres toward Light-Triggered Nitric Oxide Release and Near-Infrared Fluorescence Imaging. Langmuir 2013, 29, 15032− 15042. (21) Tan, L.; Wan, A.; Li, H. Synthesis of near-Infrared Quantum Dots in Cultured Cancer Cells. ACS Appl. Mater. Interfaces 2014, 6, 18−23. (22) Mondal, A.; Zachariah, A.; Nayak, P.; NayakJ, B. B. Synthesis and Room Temperature Photoluminescence of Mesoporous Zirconia with a Tetragonal Nanocrystalline Framework. J. Am. Ceram. Soc. 2010, 93, 387−392. (23) Carbonaro, C. M.; Ricci, P. C.; Anedda, A. Thermal Quenching Properties of Ultraviolet Emitting Centers in Mesoporous Silica. Phys. Rev. B 2007, 76, 125431−125436.

spheres under UV excitation, which is most likely ascribed to the existence of numerous sulfur-related defects in the mesostructures. This work provides new insights into scale-up production of PL mesostructured materials, which may find application in photoelectronic devices.

■

ASSOCIATED CONTENT

S Supporting Information *

Size distribution, zeta-potential, FTIR spectra and XPS spectra of the Ag2S NPs; TEM image of the Ag2S aggregates; smallangle XRD and PL excitation spectrum of the mesoporous Ag2S nanospheres; and UV−vis−NIR spectra of the Ag2S NPs and the mesoporous Ag2S nanospheres. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*L.T.: Tel: 86-21-34204561. E-mail: [email protected]. *Y.S.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (grant no. 51403125), the Fundamental Research Funds for the Central Universities (no. JUSRP1042), and the Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20130073120087).

■

REFERENCES

(1) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films. ACS Nano 2013, 7, 4042−4049. (2) Wu, H.; Kong, D.; Ruan, Z.; Hsu, P.-C.; Wang, S.; Yu, Z.; Carney, T. J.; Hu, L.; Fan, S.; Cui, Y. A Transparent Electrode Based on a Metal Nanotrough Network. Nat. Nanotechnol. 2013, 8, 421−425. (3) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Hierarchically ordered oxides. Science 1998, 282, 2244−2246. (4) Zlotnikov, I.; Werner, P.; Blumtritt, H.; Graff, A.; Dauphin, Y.; Zolotoyabko, E.; Fratzl, P. Mesoporous Silica: A Perfectly Periodic Three-Dimensional Protein/Silica Mesoporous Structure Produced by an Organism. Adv. Mater. 2014, 26, 1681−1687. (5) Tian, B.; Liu, J.; Dvir, T.; Jin, L.; Tsui, J. H.; Qing, Q.; Suo, Z.; Langer, R.; Kohane, D. S.; Lieber, C. M. Self-Adjusted Synthesis of Ordered Stable Mesoporous Minerals by Acid-Base Pairs. Nat. Mater. 2012, 11, 986−994. (6) Corma, A.; Atienzar, P.; García, H.; Chane-Ching, J.-Y. Hierarchically Mesostructured Doped CeO2 with Potential for SolarCell Use. Nat. Mater. 2004, 3, 394−397. (7) Zheng, H.; Huang, Z.; Che, S. Mesostructured Chitosan−Silica Hybrid As a Biodegradable Carrier for a pH-Responsive Drug Delivery System. Dalton Trans. 2012, 41, 5038−5044. (8) Wan, Y.; Yang, H. F.; Zhao, D. Y. ’Host-Guest’ Chemistry in the Synthesis of Ordered Nonsiliceous Mesoporous Materials. Acc. Chem. Res. 2006, 39, 423−432. (9) Maclachlan, M. J.; Coombs, N.; Ozin, G. A. Non-Aqueous Supramolecular Assembly of Mesostructured Metal Germanium Sulphides from (Ge4S10)4-Clusters. Nature 1999, 397, 681−684. (10) Tian, B.; Liu, X.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Self-Adjusted Synthesis of Ordered Stable Mesoporous Minerals by Acid-Base Pairs. Nat. Mater. 2003, 2, 159−163. (11) Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. Mesoporous Mn2O3 and Mn3O4 with Crystalline Walls. Adv. Mater. 2007, 19, 4063−4066. G

DOI: 10.1021/la5049979 Langmuir XXXX, XXX, XXX−XXX