Photoluminescence of Colloids of Pristine ZnAl Layered Double

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Photoluminescence of Colloids of Pristine ZnAl Layered Double Hydroxides Zhuang Zhang, Guangming Chen,* and Kongli Xu Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China S Supporting Information *

ABSTRACT: Colloids of pristine ZnAl layered double hydroxides (LDHs) were found to display an unexpected photoluminescence (PL) phenomenon disparate from the corresponding LDH solids. First, ZnAl LDHs with nitrate in the interlayer were prepared by a hexamethylenetetramine hydrolysis method. Their colloids were obtained by subsequent delamination in formamide at room temperature. Interestingly, an unexpected PL phenomenon was observed for the colloids, different from that of the relevant solid powder. The Zn/Al ratio and concentration did not change the Stokes shift. However, they were found to be the major factors in the emission intensity. Numerous surface defect sites and interaction of LDH with formamide were believed to be the main reasons.

1. INTRODUCTION Layered double hydroxides (LDHs), a class of anionic clays, have attracted considerable interest mainly because of their unique structure with variable metal cations in layer constituents as well as different exchangeable anions in galleries,1 and versatile potential applications such as catalysts,2 adsorbents,3 drug release,4 and functional fillers in polymerbased nanocomposites.5−7 Very recently, photofunctions have become one major focus for the reports of solid film and powder of LDHs.4,8−10 Because of their numerous advantages, such as being easy to prepare, stable in air, interchangeable anions, and incorporable host layers, LDHs have become an excellent precursor to develop novel photofunctional materials. One effective method is to intercalate photofunctional anions into the LDH hosts, which has been reported to enhance the luminescence efficiency, the photostability, and thermal stability.8 On the other hand, LDHs with photofunctional platelets were achieved by doping functional metal cations such as rare earth ions into the LDH lattice.4,9 Furthermore, the above two strategies were combined; that is, the doped rare earth cations in LDH lattice and the intercalated organic anions were accomplished in one LDH.10 Surprisingly, to date, little attention has been paid to the photofunctions of the pristine LDHs themselves, especially the colloids of the pristine LDHs, which may also significantly affect the photofunctions of the corresponding LDH-based hybrids or materials. Delamination or exfoliation into colloids containing their single nanoplatelets is a key topic in the research of LDHs.11,12 In many cases, because of the inaccessibility to the inner surfaces, LDHs are required to be dramatically exfoliated to maximize their properties.11,12 Therefore, great efforts have been taken for LDH exfoliation and characterization of the resultant colloids.13−20 The strategy of LDH exfoliation to colloids shows a notable superiority to prepare polymer nanocomposites,21−23 or use as shell building blocks for construction of novel layer-by-layer (LBL) multilayered films.24,25 Moreover, the LDH colloids containing nanoscale © 2013 American Chemical Society

positively charged platelets, which have an exceedingly high two-dimensionality with single molecule or a few molecules’ thickness, usually exhibit unconventional and intriguing physicochemical properties that differ from those of corresponding bulk lamellar systems.2 In this paper, we explore the photoluminescence (PL) of the colloids for the pristine LDHs. Being one of the most frequently used types, especially in the photofunctional LDHs, ZnAl LDHs were adopted as a typical case in the present investigation. The effects of Zn/Al ratio and concentration on the PL for the colloids of the pristine ZnAl LDHs were taken into account.

2. EXPERIMENTAL SECTION 2.1. Materials. Zn(NO 3 ) 2 , Al(NO 3) 3, NaNO 3 , and hexamethylenetetramine (HMT) were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the reagents were of analytical reagent (A.R.) pure grade, and were used without further purification. Distilled water was used in the synthesis procedure. 2.2. Synthesis of ZnAl LDH Samples. All of the ZnAl LDHs with nitrate anions as counterions in the interlayer space were synthesized using an HMT hydrolysis method. The total metal cation concentration was 0.01 mol L−1, and the molar ratio of HMT/Al was controlled to be 1.5. In brief, Zn(NO3)2, Al(NO3)3, HMT, and NaNO3 were dissolved in distilled water first. Under nitrogen protection, the aqueous solution was refluxed at 100 °C for 10 h with continuous stirring. After being aged, the products were filtered, washed with distilled water, and finally dried at 50 °C in vacuum. 2.3. Preparation of ZnAl LDH Colloids. The colloids were prepared by delaminating the above ZnAl LDHs in Received: Revised: Accepted: Published: 11045

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Figure 1, a series of intense and sharp diffraction peaks characteristic of ordered two-dimensional (2D) alignment structure appear. The characteristic reflection positions, the corresponding distances, and the refined lattice parameters derived from the XRD patterns are illustrated in Table 1. The basal spacing for the synthesized ZnAl LDH particles is 0.89 nm, and the two weak diffraction peaks around 60.0° and 61.0° result from the characteristic reflections of (110) and (113) planes. Furthermore, the lack of any impurity peaks confirms the high purity of the products. These XRD results are in very good agreement with previous investigations.28 Figure 2 shows the FTIR spectra of the ZnAl LDHs. The broad absorption band ranged from 3750 to 2700 cm−1 is ascribed to stretching modes of the structural hydroxyl groups of the LDH platelets and the interlayer water molecules. The characteristic absorption band of nitrate anions at 1384 cm−1 demonstrates the success of the synthesis of ZnAl LDHs with nitrate occupying in the interlayer.26−28 In addition, a weak band at 830 cm−1 occurred as well due to the NO3− stretching vibration mode.29 As for the absorption bands at 552 and 448 cm−1 in all spectra, they could be assigned to the M−O−M lattice vibrations of the octahedral sheets.29 Subsequently, the pristine ZnAl LDHs were exfoliated in formamide.13−16 In Figure 3a, a highly transparent and stable colloid with typical Tyndall phenomenon can be clearly seen, suggesting that the ZnAl LDHs had been greatly exfoliated. Moreover, the featureless XRD pattern and lack of any sharp peaks (Figure 3b) for the centrifuged gel-like product of the colloid (the relatively wide peak in the range 15°−40° was due to the presence of formamide) provide additional proof of the dramatic exfoliation in the pristine ZnAl LDH colloid. The TEM image further confirms the significant exfoliation. In Figure S2 in the Supporting Information, the exfoliated nanosheets show very faint but homogeneous contrast, revealing their ultrathin and uniform thickness. 3.2. Photoluminescence of ZnAl LDHs. To shed light on the PL phenomenon and mechanism, the PL emission spectra of the solid powder and exfoliated colloid for the pristine ZnAl LDH were first compared in Figure 4. Distinct PL phenomenon can be observed for both samples. Generally, the numerous surface defects have been regarded as the origin of the luminescence in the range of 350−550 nm (2.2−3.5 eV) for nanocrystals.30 Furthermore, these numerous surface defects in the pristine ZnAl LDH nanolayers would facilitate the recombination process of excited electrons and holes, which perhaps resulted in the obvious PL phenomenon observed for the here-reported pristine ZnAl LDHs. Interestingly, the two curves shown in Figure 4 are quite different in shape, which suggests that the PL emission mechanism may be disparate between the solid and the colloid of the pristine LDH. Compared to the bulk solid powder sample, the colloid of the pristine ZnAl LDH displays emission bands between 400 and ∼600 nm, composed of an overlap of bands of blue and purple lights with maxima at around 417 and 441 nm, respectively. Because of the molecular interaction of LDH with the organic solvent of formamide and the increased surface defects resulting from the significant exfoliation, the larger Stokes shift contributed more significantly to the colloid than to the solid.31 Indeed, similar photoluminescence phenomena have been reported for other oxide nanoparticles. For example, the shape of the emission spectra is obviously different between the dry powder samples32 and the aqueous suspension33,34 of zinc oxide (ZnO) nanoparticles. Moreover, the aqueous suspension

formamide at room temperature for 72 h. In brief, the exfoliation of ZnAl LDHs was completed by abundant use of formamide. Typically, the desired amount (0.010, 0.020, 0.050, and 0.20 g) of ZnAl LDH powder material was mixed with 100 mL of formamide in a conical beaker, which was sealed airtight after purging with nitrogen gas. Then the mixture was agitated vigorously at room temperature by a mechanical shaker. After shaking for 2 days, the resulting translucent colloidal suspensions were obtained. 2.4. Characterizations. Powder X-ray diffraction (XRD) measurements were conducted on a Rigaku D/max 2400 diffractometer with Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 4° min−1. The Fourier transform infrared spectroscopy (FTIR) spectra were collected on a Perkin-Elmer System 2000 FTIR spectrophotometer after 64 scans within 4000−400 cm−1. Transmission electron microscopic (TEM) images were taken on a transmission electron microscope (JEOL JEM-1011). The PL excitation and emission spectra were recorded using an Hitachi F-4500 fluorescence spectrophotometer using a right angle configuration with a 150 W Xe lamp as the excitation source. In the measurements of the emission spectra, a cutoff filter was applied to avoid interference of reflected light. Other details for PL spectra are as follows: scanning speed 240 nm/min, slit 2.5 nm, and PMT voltage 400 V.

3. RESULTS AND DISCUSSION 3.1. Characterizations of ZnAl LDH Powders and Colloids. A series of ZnAl LDHs with nitrate anions as counterions in the interlayer space were synthesized using an HMT hydrolysis method. Their structures were characterized by XRD patterns (Figure 1) and FTIR spectra (Figure 2). In

Figure 1. XRD profiles of the ZnAl LDH solid powder samples with different Zn/Al ratios.

Figure 2. FTIR spectra of the ZnAl LDH solid powder samples with different Zn/Al ratios.

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Table 1. Characteristics Derived from XRD Patterns for the ZnAl LDH Solid Powders 003

006

110

113

lattice parameters

Zn/Al

2θ (deg)

d (nm)

2θ (deg)

d (nm)

2θ (deg)

d (nm)

2θ (deg)

d (nm)

a (nm)

c (nm)

2 3 4

9.840 9.900 9.940

0.898 0.893 0.889

19.840 19.900 19.940

0.447 0.446 0.445

60.160 60.200 60.220

0.154 0.154 0.154

61.120 61.200 61.220

0.152 0.151 0.151

0.307 0.307 0.307

2.694 2.678 2.667

Figure 3. (a) A photograph showing Tyndall phenomenon for the colloid of pristine ZnAl LDH (Zn/Al = 4); (b) XRD pattern of the centrifuged gel-like sample.

Figure 5. Effect of Zn/Al ratio on photoluminescent (a) excitation and (b) emission spectra for the colloids of the pristine ZnAl LDHs.

Figure 4. Photoluminescent emission spectra of pristine ZnAl LDH solid and colloid (Zn/Al = 4) excited at 380 nm.

exhibited higher emission wavelength than that of the dry samples for the ZnO nanoparticles. 3.3. Effect of Zn/Al Ratio on PL of ZnAl LDH Colloids. To gain insights into the unexpected PL phenomenon and molecular mechanism of the pristine LDH colloid, the effects of the surface charge density and the concentration of LDH nanolayers were further studied. The surface charge density of the ZnAl LDH is mainly decided by the Zn/Al ratio, and the increase of the ZnAl ratio results in a decrease of the surface charge density. Therefore, Zn/Al ratio was employed herein to study the effect of surface charge density. In Figure 5a, the PL excitation spectra display similar shape with a main band centered at 380 nm, possibly due to UV absorption of the ZnAl LDH crystal lattice. Moreover, because low layer charge density means a lesser amount of the intercalated anions (nitrate), which can reduce the dissipation in the energy-transfer process between the positive nanolayers and the intercalated anions, the ZnAl LDH colloid with high Zn/Al ratio shows higher intensities than that with low Zn/Al ratio (Figure 5a).30,31,35 As for the emission spectra (Figure 5b), the Zn/Al ratio has an obvious effect on the PL intensity. Clearly, the intensity increases with the Zn/Al ratio as well. Therefore, we conclude that the Zn/Al ratio is a key factor to the intensities of both the PL excitation and emission spectra. 3.4. Effect of ZnAl LDH Concentration on PL of Colloids. Figure 6 shows the concentration dependency for the PL emission spectra of the colloids of the pristine ZnAl LDHs.

Figure 6. Concentration dependence of intensity for photoluminescent emission spectra of the colloids of the pristine ZnAl LDHs. The inset shows the intensity data at 441 nm at various concentrations.

Distinctly, the intensity increases with the concentration. A possible explanation is described in the following. It is wellknown that nanomaterials have high specific surface areas (SSAs), which inevitably lead to the occurrence of numerous defects on the surface of nanomaterials. Thus, a lot of traps beneficial to the generation of luminescence appear.35,36 As for the present pristine ZnAl LDH colloids, the pristine LDHs were greatly exfoliated. As a result, the colloids should possess high SSAs, and tremendous amounts of surface defects should occur. Compared to the colloid at low concentration, the ZnAl LDH colloids at high concentration thus should have more 11047

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(7) Fu, P.; Xu, K.; Song, H.; Chen, G.; Yang, J.; Niu, Y. Preparation, Stability and Rheology of Polyacrylamide/Pristine Layered Double Hydroxide Nanocomposites. J. Mater. Chem. 2010, 20, 3869. (8) Latterini, L.; Nocchetti, M.; Aloisi, G. G.; Costantino, U.; De Schryver, F. C.; Elisei, F. Structural, Photophysical, and Photochemical Characterization of 9-Anthracenecarboxylate−Hydrotalcite Nanocomposites: Evidence of a Reversible Light-Driven Reaction. Langmuir 2007, 23, 12337. (9) Gao, X. R.; Hu, M.; Lei, L. X.; O’Hare, D.; Markland, C.; Sun, Y. M.; Faulkner, S. Enhanced Luminescence of Europium-doped Layered Double Hydroxides Intercalated by Sensitiser Anions. Chem. Commun. 2011, 47, 2104. (10) Gunawan, P.; Xu, R. Lanthanide-Doped Layered Double Hydroxides Intercalated with Sensitizing Anions: Efficient Energy Transfer between Host and Guest Layers. J. Phys. Chem. C 2009, 113, 17206. (11) Xu, Z. P.; Stevenson, G. S.; Lu, C.-Q.; Lu, G. Q.; Bartlett, P. F.; Gray, P. P. Stable Suspension of Layered Double Hydroxide Nanoparticles in Aqueous Solution. J. Am. Chem. Soc. 2005, 128, 36. (12) Gordijo, C. R.; Leopoldo Constantino, V. R.; de Oliveira Silva, D. Evidences for Decarbonation and Exfoliation of Layered Double Hydroxide in N,N-Dimethylformamide−Ethanol Solvent Mixture. J. Solid State Chem. 2007, 180, 1967. (13) Adachi-Pagano, M.; Forano, C.; Besse, J.-P. Delamination of Layered Double Hydroxides by Use of Surfactants. Chem. Commun. 2000, 91. (14) Hibino, T. Delamination of Layered Double Hydroxides Containing Amino Acids. Chem. Mater. 2004, 16, 5482. (15) Hibino, T.; Kobayashi, M. Delamination of Layered Double Hydroxides in Water. J. Mater. Chem. 2005, 15, 653. (16) O’Leary, S.; O’Hare, D.; Seeley, G. Delamination of Layered Double Hydroxides in Polar Monomers: New LDH-acrylate Nanocomposites. Chem. Commun. 2002, 1506. (17) Yan, Y.; Liu, Q.; Wang, J.; Wei, J.; Gao, Z.; Mann, T.; Li, Z.; He, Y.; Zhang, M.; Liu, L. Single-step Synthesis of Layered Double Hydroxides Ultrathin Nanosheets. J. Colloid Interface Sci. 2012, 371, 15. (18) Wu, Q. L.; Olafsen, A.; Vistad, O. B.; Roots, J.; Norby, P. Delamination and Restacking of a Layered Double Hydroxide with Nitrate as Counter Anion. J. Mater. Chem. 2005, 15, 4695. (19) Hou, W.; Kang, L.; Sun, R.; Liu, Z.-H. Exfoliation of Layered Double Hydroxides by an Electrostatic Repulsion in Aqueous Solution. Colloids Surf., A 2008, 312, 92. (20) Bellezza, F.; Nocchetti, M.; Posati, T.; Giovagnoli, S.; Cipiciani, A. Synthesis of Colloidal Dispersions of NiAl, ZnAl, NiCr, ZnCr, NiFe, and MgFe Hydrotalcite-like Nanoparticles. J. Colloid Interface Sci. 2012, 376, 20. (21) Li, B. G.; Hu, Y.; Zhang, R.; Chen, Z. Y.; Fan, W. C. Preparation of the Poly(vinyl alcohol)/Layered Double Hydroxide Nanocomposite. Mater. Res. Bull. 2003, 38, 1567. (22) Shan, X.; Song, L.; Xing, W.; Hu, Y.; Lo, S. Effect of NickelContaining Layered Double Hydroxides and Cyclophosphazene Compound on the Thermal Stability and Flame Retardancy of Poly(lactic acid). Ind. Eng. Chem. Res. 2012, 51, 13037. (23) Costantino, U.; Bugatti, V.; Gorrasi, G.; Montanari, F.; Nocchetti, M.; Tammaro, L.; Vittoria, V. New Polymeric Composites Based on Poly(ε-caprolactone) and Layered Double Hydroxides Containing Antimicrobial Species. ACS Appl. Mater. Interfaces 2009, 1, 668. (24) Guo, Y.; Xiao, Y.; Zhang, L.; Song, Y.-F. Fabrication of (Calcein−ZnS)n Ordered Ultrathin Films on the Basis of Layered Double Hydroxide and Its Ethanol Sensing Behavior. Ind. Eng. Chem. Res. 2012, 51, 8966. (25) Chen, D.; Huang, S.; Zhang, C.; Wang, W.; Liu, T. Layer-bylayer Self-assembly of Polyimide Precursor/Layered Double Hydroxide Ultrathin Films. Thin Solid Films 2010, 518, 7081. (26) Costantino, U.; Marmottini, F.; Nocchetti, M.; Vivani, R. New Synthetic Routes to Hydrotalcite-like Compounds − Characterisation

exfoliated nanoparticles even nanolayers and much more defect sites. As a result, these increased surface defect sites contributed greatly to the enhanced intensity of the PL emission spectra, as shown in Figure 6.

4. CONCLUSIONS In summary, the colloids of the pristine ZnAl LDHs were reported to display an unexpected PL phenomenon quite different from the corresponding bulk solid powder. The Zn/Al ratio and concentration did not change the Stokes shift of the colloids. Moreover, the PL emission intensity was found to depend mainly on the Zn/Al ratio and the concentration. The numerous surface defect sites due to the great exfoliation in the colloids of the pristine LDHs as well as the interaction of LDH with formamide may be the main reasons contributing to the PL emission. We believe our findings benefit the understandings of the intrinsic properties or functions of the pristine LDHs, and may open new avenues to construction of functional colloids and LDH-contained functional hybrids or composites.

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ASSOCIATED CONTENT

S Supporting Information *

XRD and TEM characterization of ZnAl_LDH. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 62561623. Fax: +86 10 62559373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 51073162). G. Chen acknowledges K. C. Wong Education Foundation, Hong Kong.

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REFERENCES

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