Photoluminescence of colloids of pristine MgAl layered double hydroxides

Abstract

We present an unexpected photoluminescence (PL) phenomenon for colloids of pristine MgAl layered double hydroxides (LDHs). Moreover, the PL spectra displayed little dependency on LDH platelet size, excitation wavelength or colloid concentration. The possible mechanism was discussed mainly in terms of specific surface areas, numerous surface defects and excited electron–hole recombination.

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In recent years, layered double hydroxides (LDHs) have received increasing attention due to their interlayer anion exchangeability, multi-functions, versatile constituents and wide potential applications in catalysts,^1a,b bionanotechnology,^1c hydrogels,^1d and functional nano-fillers of polymers,^1e–g etc. Very recently, a few reports have focused on the photofunctions or optical properties for solid film or powder of LDHs intercalated by fluorophore,^2a sensitiser^2b or ultraviolet (UV) absorbent^2c anions. The second way to obtain LDHs with photofunctions is to construct crystalline structures doped with or containing rare earth ions, such as europium anions (Eu³⁺).³ Very recently, we report tunable photoluminescence (PL) by combination of host lattice introduction of Eu³⁺ and interlayer intercalation of photoluminescent organic anions, coumarin-3-carboxylate.⁴ Unfortunately, the intrinsic optical properties of the undoped pristine LDH nanoparticles without any surface modification are far from clear to date.⁵

Exfoliation or delamination of LDHs to achieve colloids containing their nanoparticles is an important issue in various applications including photoactive or electroactive devices and polymer based nanocomposites.^1f,g,6 Compared with other layered inorganic solids such as clays, LDHs are much more difficult to be exfoliated due to the strong attractions among adjacent nanolayers.⁷ Fortunately, the LDH with nitrate as counter-anion (LDH_NO₃) has been reported to be conveniently exfoliated in formamide by ultrasonic treatment at room temperature.⁸ Subsequently, an aqueous dispersion of LDH was reported to obtained by intercalation of isethionate anions,^1d,9 which exhibited high stabilities towards heating, flowing shear forces and acid/alkali.^1d Additionally, an acid–salt anion exchange method was developed to de-intercalate carbonate ions from LDH_CO₃.¹⁰ Very recently, we report a direct decarbonation procedure wherein the LDH_CO₃ can be directly transformed into LDH_NO₃ by one step.¹¹ However, in a literature survey, the intrinsic properties of the pristine LDH colloids, such as photophysics, have received little attention. Thus, in-depth and systematic investigations are urgently needed.

The aim of this study is to explore the photophysics such as PL of the colloids containing pristine LDH nanoparticles. Because the LDH containing Mg and Al in the octahedral nanosheets (MgAl LDH) is perhaps the most extensively studied type, it is adopted in this investigation. Furthermore, the effects of LDH platelet size, colloid concentration and exited wavelength were taken into account, and the possible mechanism was discussed briefly.

In order to shed light on the effect of LDH lateral sizes on the photophysics, LDHs with different lateral sizes were synthesized via urea, urea + sodium hydroxide (NaOH) and urea + hexamethylenetetramine (HMT) methods.^11a The detailed structural characterizations including powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic techniques were reported elsewhere.^11a The morphological measurements by scanning electron microscopic (SEM) images revealed that the average lateral sizes of the synthesized MgAl LDH_CO₃ particles were 2.87 ± 0.47 μm (urea method), 0.86 ± 0.18 μm (urea + NaOH method) and 0.47 ± 0.11 μm (urea + HMT method), respectively. Moreover, the three MgAl LDH_CO₃ particles were directly decarbonated into MgAl LDH_NO₃ by HNO₃–NaNO₃ exchange process without obvious damage.^11a

After the direct decarbonation, the obtained MgAl LDH_NO₃ particles were exfoliated in formamide at room temperature (ESI†). The exfoliation was confirmed by direct observation with transparent dispersion with Tyndall effect and XRD analysis. As shown in Fig. 1A, the MgAl LDH_NO₃ dispersion was highly transparent and almost colourless. Neither precipitation nor large particles could be recognized. Furthermore, a laser beam was sent through the sample contained in a glass beaker and was strongly scattered, characteristic of Tyndall effect resulting from the scattering and reflection of the greatly exfoliated MgAl LDH_NO₃ in the colloids. Then, the colloids were centrifuged, and the sediments of viscous gel-like samples were collected for XRD measurements. In Fig. 1B, all of the MgAl LDH_NO₃ colloids displayed featureless XRD patterns. The original intense and sharp peaks, characteristic of ordered alignment of the MgAl LDH_NO₃ layers,^11a disappeared. And the broad peak between 15° and 40° (Fig. S1†) was due to the presence of formamide. Therefore, on the basis of these results, we conclude that the MgAl LDH_NO₃ colloids were obtained, which consisted of uniformly dispersed MgAl LDH_NO₃ nanoparticles. The molecular mechanism for the exfoliation may be ascribed to the high polarity of formamide and the strong hydrogen bonding between formamide and the LDH host.^8b

Fig. 1 (A) Photograph and Tyndall phenomenon of a typical MgAl LDH_NO₃ colloid (5 g L⁻¹) containing their exfoliated nanoparticles. (B) XRD patterns of the gel-like samples centrifuged from the exfoliated MgAl LDH_NO₃ in formamide, wherein the MgAl LDH_NO₃ samples were obtained after the HNO₃–NaNO₃ exchange of the MgAl LDH_CO₃ synthesized via (a) urea, (b) urea + NaOH, and (c) urea + HMT method, respectively.

Fig. 2 shows the room-temperature luminescent excitation (EX) and emission (EM) spectra for the three pristine MgAl LDH_NO₃ colloids containing their exfoliated nanoparticles. The EM spectra were monitored over the range 400–600 nm by excitation at 378 nm. All of the colloids exhibited similar profiles in EM spectra. Two well-resolved emission bands were distinct at 417 and 441 nm. The similar profiles and the same wavenumbers with intensity maxima in the EM spectra suggest that the origin of the PL phenomenon for the three MgAl LDH_NO₃ colloids may be the same. It is widely accepted that the luminescence in the range of 350–550 nm (2.2–3.5 eV) for nanocrystals results from the numerous surface defects.¹² One of the major characteristics of nanomaterials is their high specific surface areas (SSAs). The high SSAs inevitably lead to the occurrence of numerous defects on the surface of nanomaterials, which may act as traps beneficial to the generation of luminescence.¹³ As for the present MgAl LDH_NO₃ colloids, the pristine MgAl LDH_NO₃ were greatly exfoliated into their nanoparticles. As a result, the nanoparticles should possess high SSAs, and tremendous amounts of surface defects occurred. These numerous surface defects in the pristine MgAl LDH_NO₃ nanoparticles would further facilitate the recombination process of excited electrons and holes, which perhaps resulted in the obvious PL phenomenon observed for the here-reported pristine MgAl LDH_NO₃ colloids.

Fig. 2 Excitation (EX) and emission (EM) luminescent spectra for the colloids (5 g L⁻¹) composed of exfoliated MgAl LDH_NO₃ nanoparticles. The MgAl LDH_NO₃ were exfoliated in formamide after treatment by HNO₃–NaNO₃ exchange of the MgAl LDH_CO₃ synthesized by (a) urea + NaOH, (b) urea + HMT and (c) urea method, respectively.

In addition, the effects of excitation wavelength and LDH colloid concentration on the EM spectra were recorded. Fig. 3 and 4 clearly showed that the EM spectra of the pristine MgAl LDH_NO₃ colloids exhibited little dependence of excitation wavelength (360 nm in Fig. 3) or colloid concentration (2 g L⁻¹ in Fig. 4). In comparison with the EM spectra collected with excitation wavelength of 378 nm at colloid concentration of 5 g L⁻¹ (shown in Fig. 2), the shape or profile of the EM spectra was not changed. And even the wavelengths of the bands were not changed, either.

Fig. 3 Emission spectra excited at 360 nm for the colloids at 5 g L⁻¹ composed of exfoliated MgAl LDH_NO₃ nanoparticles. The MgAl LDH_NO₃ were exfoliated in formamide after treatment by HNO₃–NaNO₃ exchange of the MgAl LDH_CO₃ synthesized by (a) urea + NaOH, (b) urea and (c) urea + HMT method, respectively.

Fig. 4 Emission spectra excited at 378 nm for the colloids at 2 g L⁻¹ composed of exfoliated MgAl LDH_NO₃ nanoparticles. The MgAl LDH_NO₃ were exfoliated in formamide after treatment by HNO₃–NaNO₃ exchange of the MgAl LDH_CO₃ synthesized by (a) urea + NaOH, (b) urea and (c) urea + HMT method, respectively.

In summary, we report unexpected PL of pristine MgAl LDH_NO₃ colloids containing of their nanoparticles with three different lateral size dimensions. The exfoliation structure was demonstrated by photograph of transparent dispersion with distinct Tyndall phenomenon as well as broad and featureless XRD patterns. Very interestingly, these colloids were found to exhibit obvious PL phenomenon. The reason of the PL phenomenon might be due to the numerous surface defects and the recombination of excited electrons and holes. Moreover, the PL spectra exhibited little dependence on LDH platelet size, excitation wavelength or colloid concentration. These results help us to deepen our understandings toward the intrinsic optical properties especially photophysics of pristine LDHs, and may widen the applications of LDH colloids containing their nanoparticles.

Acknowledgements

The authors are grateful to National Natural Science Foundation of China (51073162). G. Chen acknowledges Youth Innovation Promotion Association, CAS.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Exfoliation and characterizations, XRD patterns. See DOI: 10.1039/c4ra01987c

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