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Cite this: DOI: 10.1039/c4cc08380f Received 23rd October 2014, Accepted 1st December 2014 DOI: 10.1039/c4cc08380f
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Controllable luminescence of layered rare-earth hydroxide composites with a fluorescent molecule: blue emission by delamination in formamide† Qingyang Gu, Feifei Su, Shulan Ma,* Genban Sun and Xiaojing Yang
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We demonstrate the first example of exfoliated layered europium hydroxide (LEuH) composites with a fluorescent molecule that exhibit blue luminescence. Co-quenching in the solid state, blue emission (440 nm) in formamide, and green emission (514 nm) in water–formamide were found, for which delamination and swollen states account for the versatile luminescence behaviors.
Host–guest hybrids such as composites of inorganic matrices and organic chromophores can exhibit superior physicochemical characteristics and unique properties due to synergies of host and guest components.1 Inorganic hosts normally provide stabilization and protection, while chromophore guests provide optical functions such as color and fluorescence properties. Inorganic hosts can also render insoluble pigments from soluble dyes.2 Materials with a twodimensional (2D) layered structure3 are a large class of functional organized systems, characterized by tunable interlayer volume and variable interlayer guests. The arrangement of luminescent guests can be tuned within the 2D matrix, which facilitates the modulation of luminescence.4,5 Luminophore-layered hybrid materials with negatively-charged 2D layers have been extensively investigated.6–14 Layered double hydroxides (LDHs) are one type of 2D matrix with positively-charged layers showing various applications in catalysts,15 two-dimensional nanoreactors16,17 and adsorbents.18–21 LDH–chromophore hybrids have presented functions such as enhanced photostabilization in comparison with individual components.22 Recently, layered rare-earth hydroxides (LRHs), a new intriguing family of 2D materials structurally similar to LDHs, have attracted increasing attention,23–31 due to the excellent properties arising from lanthanide elements. 8-Hydroxy-pyrene-1,3,6-trisulphonate (abbr. HPTS) is a wellknown non-toxic water-soluble fluorescent molecule, which is widely used in various fields such as fluorescence probes for Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, chemical compositions, FT-IR, TG-DTA spectra and SEM images, XRD patterns, and the excitation spectra of samples. See DOI: 10.1039/c4cc08380f
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many applications due to its unique fluorescent properties, excellent photostability, high absorbance and high quantum yield. Normally HPTS in water exhibits two emissions, one is blue corresponding to a protonated form (ROH, Scheme S1, ESI†) and the other is green related to a deprotonated form (RO ).32 The pKa of HPTS decreases from 7.4 in the ground-state33,34 to 0.4 in the excited state.33 The ROH of HPTS in this excited state will ionize quickly releasing H+ to turn into RO form, which emits green emission by radiative transition to the ground state. So the blue emission of HPTS cannot be observed when the pH value of aqueous solution is greater than 3.0.35 Through structural adjustment or microenvironment changes, pure emission may be obtained.36 Much work involving fluorescence molecule intercalated LDHs had been reported,2,37–39 including HPTS–LDH hybrids that exhibit interesting luminescence in the solid state.35 However, there has been a lack of studies focusing on tunable or purified luminescence of HPTS, which restricts its application as color display material. With regard to luminescent materials, a prerequisite for applications is the development of transparent ordered films; unfortunately, such films have been less studied than solution and powder systems.40,41 Moreover, most of the reports related to film materials focus on LDHs possibly because of their easier delamination. Delaminated LRH nanosheets would be a very important class of functional 2D nanoscale materials42–45 as building blocks for assembly of ultrathin films for optical devices. Whereas LRH delamination remains great difficulty, due to strong affinity between interlayer anions and host layers. Normally delamination of LRHs needs a pre-intercalation using longchain sufactants such as dodecyl sulfate (DS)42,43 and the delamination degree is usually low,42,45 which limits subsequent assembly. So a rapid and facile delamination of LRHs is highly desired. Herein we prepare LRH (R = Eu, Gd) composites co-intercalated with HPTS and an anion surfactant OS (1-octane sulfonic acid sodium). Tunable luminescence of the composites is realized through the synergistic effect of LRH layers and HPTS with different states. Facile one-pot delamination of the composites is achieved when dispersed in formamide (FM), which contributes to blue emission. Co-precipitation and ion-exchange methods at various pH values were performed to prepare the LEuH composites (for experimental
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Fig. 1 (A) XRD patterns of the NO3–LEuH precursor (a) and LEuH composites obtained by co-precipitation (b) and ion-exchange at pH values of 2.9 (c), 4.0 (d), 5.3 (e), 7.3 (f), 10.3 (g). (B, C) Scheme showing different arrangements of gallery species.
details see ESI†). The composite prepared by co-precipitation (Fig. 1A-b) has a basal spacing (dbasal) of 2.45 nm, equal to that by ion exchange at pH = 2.9. Other composites (Fig. 1A-(c–g)) via ionexchange give decreased dbasal (from 2.02 to 1.95 nm) with pH increase, attributed to an increased affinity of guests to layers originating from the increasing deprotonation degree. Based on the LRH thickness of 0.65 nm,29 the 2.45 nm dbasal corresponds to a gallery height of B1.80 nm (= 2.45 0.65), meaning a bi-layered arrangement (Fig. 1B), considering the HPTS size of B1.01 0.75 0.50 nm3 and the OS height of B1.04 nm. Other composites with gallery heights of 1.30–1.37 nm (= 1.95/2.02 0.65) mean a monolayer arrangement (Fig. 1C). The area per unit charge (Scharge) is usually used to explain intercalation structure.29,46,47 For the 2.45 nm dbasal with 2 charged HPTS, the Scharge of 0.25 nm2 (= 1.01 0.50/2) is larger than that of the LRH layer (0.22 nm2),29 which means a bi-layered fashion.46,47 For the dbasal of 1.95–2.02 nm with 3 charged HPTS, the Scharge is 0.17 nm2 (= 1.01 0.50/3), being smaller than that of the LRH layer, so a mono-layered orientation is reasonable.46,47 The composite obtained by co-precipitation and ion-exchange at pH = 2.9 has the highest HPTS loading (Table S1, ESI†). SO3 vibrations48 appearing at 1171/1053 cm 1 (Fig. S1, ESI†) verify the composite formation. TG-DTA (Fig. S2, ESI†) indicates an elevated thermal stability (B100–200 1C) of organic species after intercalation. SEM shows obvious aggregation for the composite obtained by co-precipitation (Fig. 2A), while those by ion-exchange (Fig. 2B, Fig. S3, ESI†) reveal hexagonal morphology with columnar or flowerlike aggregates resembling the LEuH precursor (Fig. S3a, ESI†). In the solid state, pure HPTS powder (Fig. 2C-a) shows no any emission because of aggregation.35 For LEuH, emissions at 580– 698 nm assigned to 5D0–7FJ radiative-relaxational transitions of Eu3+ are observable (Fig. 2C-b). After forming composites, neither emissions of layer Eu3+ nor emissions of HPTS are observable (Fig. 2C-c), no matter what excitation wavelengths of 395 nm (for Eu3+) or 375 nm (for HPTS) are used. The marked quenching of layer Eu3+ emission
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Fig. 2 SEM images of LEuH composites obtained by (A) co-precipitation and (B) ion-exchange at pH = 7.3. (C) Solid emission spectra of (a) HPTS, (b) LEuH and (c) the LEuH–HPTS composite (the inset shows the photograph of the powder under 365 nm UV irradiation). Tyndall light scattering of colloidal suspensions of the LEuH composite in (D) FM and (E) water–FM. (F) BNU characters in blue color written using colloidal suspension (D).
may be ascribed to the significant nonradiative relaxation channels provided by high-energy vibration of –OH of HPTS.49,50 The absence of HPTS emission suggests that certain energy transfer exists in the system, in view of the close excitation wavelengths of 395 nm for Eu3+ and 375 nm for HPTS. Due to the lower energy needed for Eu3+ excitation, the provided energy may be absorbed preferentially by Eu3+, meanwhile the high-energy vibration of –OH of HPTS competes this energy, thus possibly causing a co-quenching effect (Scheme 1A). Luminescence behaviors of HPTS and composites in various solvents are investigated. Fig. 3A shows the emission spectra of colloidal suspensions of samples in pure FM (testing conditions: 0.02 g HPTS or composites were dispered into 10 mL of FM by mechanical shaking for 0.5 h). HPTS in FM (Fig. 3A-g) has two emissions, one is blue (453 nm) and the other is green (520 nm), similar to that found in water. All composites, however, display one strong blue emission centered at B440 nm with a shoulder at around 454 nm. Because a surfactant can reduce the fluorophore aggregation and improve the fluorescence, the surfactant OS was used to help to intercalate and dilute/isolate the HPTS molecules so
Scheme 1 The LEuH composite showing versatile luminescence behaviors under different states: co-quencing in the solid state (A), blue luminescence in the delaminated state in FM (B), and green luminescence in the swollen state in water–FM (C).
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Fig. 3 Emission spectra of LEuH–HPTS composites in (A) FM and (D) water– FM: by co-precipitation (a) and ion-exchange at pH values of 2.9 (b), 4.0 (c), 5.3 (d), 7.3 (e), 10.3 (f), and free HPTS (g). (G) HPTS in water solutions, and LGdH– HPTS composite colloidal suspensions in (H) FM and (I) water–FM. Photographs under (B, E) daylight and (C, F–I) UV irradiation at 365 nm.
as to prevent the aggregation. Moreover, OS as an anionic sufactant should not markedly affect the emission of HTPS,51 so herein the luminescence change of HPTS would be mainly attributed to the LEuH layer. For HPTS luminescence, the inhibition of the excited state proton transfer (ESPT) process will favor blue emission.35 Compared with the solid state, an exfoliated state of LEuH layers in FM may account for the inhibition effect of the ESPT process, which would contribute to the observable blue luminescence. When the composites were dispersed in FM for only 30 min, transparent colloidal suspensions were formed, which revealed obvious Tyndall light scattering (Fig. 2D). Using the colloidal suspensions, we write the initials BNU of our university (Beijing Normal Unversity) on paper that can be clearly observable due to the bright blue color (Fig. 2F). As is known, FM is a highly polar solvent normally used in LDH delamination,52 which can hardly be performed by water. For delamination of the LDH system,52,53 the carbonyl group (–CQO) of FM acting as a highly polar solvent has a strong interaction with the hydroxyl slabs of LDH layers to form a large number of hydrogen bonds, thus massive FM will be introduced into the gallery.53 Meanwhile, the –NH2 end of FM may not bond strongly to interlayer anions, thus producing a loosely stacked swollen phase with tens to hundreds of nanometers following the FM added amount.53–55 Water addition would restore the interlayer hydrogen bonding network and gallery spacing.53 With mechanical shaking or ultrasonic treatment imposing a transverse sliding force on the swollen phase, the host layers will come apart resulting in delamination. In LRHs, the delamination process would be similar. A high degree of interlayer expansion of LEuH was found with addition of a large volume of FM.42 But the delamination of LRHs normally needs pre-intercalation using long-chain sufactants such
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as DS42,43 and the delamination degree is low.42,45 Now we achieve the delamination facilely by one-pot dispersion of the composites in FM. As shown in Scheme 1B, in the delaminated state, the positivelycharged LEuH layers would provide an unique arrangement and microenvironment for HPTS anions that adhere to LEuH layers, and the strong electrostatic interactions between them make the deprotonation of HPTS via ESPT difficult. Roy et al. found that slow ESPT from HPTS to acetate occurs within a cationic micelle (cetyltrimethyl ammonium bromide).33 Compared with that in the solid state, the confinement effect by LEuH layers for HPTS in the delaminated state is markedly weakened and the electrostatic interaction becomes dominant. Moreover, the entered FM molecules would interact with HPTS molecules by formation of strong intermolecular hydrogen bonds that prevent the ESPT process of the ROH form of HPTS, thus also facilitating the blue emission. A synergistic effect of HPTS and LEuH layers with a delaminated state among FM may lead to the favorite blue luminescence. Meanwhile, strong electrostatic interaction of the LEuH layer with anionic HPTS is also useful in the modified release of the fluorescent molecule HPTS. All these advantages make this kind of hybrid a promising fluorescent material. The composite prepared by co-precipitation shows very weak emission (Fig. 3A-a), while those obtained by ion-exchange exhibit enhanced emission (Fig. 3A-(b–f)). The composite prepared at pH = 2.9 reveals the strongest intensity, possibly related to the highest HPTS loading (Table S1, ESI†). Under UV irradiation at 365 nm, free HPTS in FM displays a bluish-green color (Fig. 3C-g), while the composites exhibit a bright blue color (Fig. 3C-(b–f)). For the composite prepared by co-precipitation, even with relatively high HPTS content (Table S1, ESI†), the very weak emission may be attributed to its original aggregation. Though host layers of this composite can also achieve delamination, the poor delamination quality due to aggregation might weaken the emission. To compare the function of different layer ions, the LGdH layer was used to check the luminescence behavior. Under the same conditions of pH = 7.3 (for experimental details see ESI†), the as-formed LGdH–HPTS composite has XRD and FT-IR data (Fig. S4, ESI†) similar to LEuH–HPTS. The colloidal suspension of LGdH– HPTS in FM presents a 496 nm peak (Fig. 3H) belonging to blue-green emission. So various layer ions such as Eu3+ and Gd3+ generate different luminescence behaviors though under the same conditions. This highlights the synergistic effect of HPTS with the LEuH layer in a delaminated state which is beneficial to blue luminescence. Emission spectra of LRH composites in water–FM were measured to check the luminescence behavior. For testing, 0.02 g of the composite was firstly dispersed into 10 ml of FM for 0.5 h to get colloidal suspensions. Then 5 ml of aforementioned suspensions were taken out to mix with 20 ml water with different pH values. All composites are found to display green emission (514 nm) (Fig. 3D), similar to that found for free HPTS in water solutions (Fig. 3G). The intensity order of the composites is in agreement with that in FM. Under UV irradiation, all composites display a green color (Fig. 3F). LGdH–HPTS in water–FM also shows green emission (Fig. 3I). The different luminescence behaviors of the composites in FM and water–FM may originate from the various existential states of LRH layers. In water–FM, a restacked/swollen state of the LRH layers may be present. In the LDH system, the colloid
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of exfoliated nanosheets is not stable in water, and substitution of water for FM may lead the sheets to restack to a layered structure,56 that is, water addition would restore interlayer hydrogen bonding networks and gallery spacing.52 Thus, in the present water-involving case, the LRH nanosheets tend to form a restacked layered structure, however, due to the presence of the significant FM amount, a swollen state would exist as shown in Scheme 1C. The Tyndall light scattering of colloidal suspensions of the LEuH composite in water–FM (Fig. 2E) supports the swollen state. The vanishing of basal reflections of the colloidal aggregate in the XRD pattern (Fig. S5, ESI†) may indicate a swollen phase. In this state, there would be water layers adjacent to LRH sheets due to formation of hydrogen bonding networks between water molecules and hydroxide groups of the LRH layer. These water layers will isolate the HPTS molecules from LRH layers with a high degree of interlayer expansion, which would weaken the interactions between the LRH layers and HPTS/OS molecules. This would favor green luminescence. Different excitation wavelengths (Fig. S6, ESI†) show various chemical environments of HPTS contributing to the different luminescence behaviors. In summary, intercalation of a fluorescent molecule HPTS and a surfactant OS into LRH (R = Eu, Gd) produces composites exhibiting tunable fluorescence behaviors depending on the existential state and the layer ion type. In the solid state, the layer confinement effect and energy transfer result in co-quenching for Eu3+ and HPTS emissions. In the delaminated state in FM, a synergistic effect (electrostatic interaction between LEuH layers and anionic HPTS, hydrogen bonding between HPTS and FM, as well as energy transfer of Eu3+ and HPTS) leads to blue luminescence. Water addition restores a swollen phase that weakens the interactions of LRH layers with HPTS, which favors green luminescence. This study is expected to open a new field of LRHs for fabricating luminous film materials especially blue emission based on delaminated LRHs. This work is supported by the National Science Foundation of China 21271028, 51272030 and 21271001.
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Cite this: DOI: 10.1039/c4cc08380f Received 23rd October 2014, Accepted 1st December 2014 DOI: 10.1039/c4cc08380f
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Controllable luminescence of layered rare-earth hydroxide composites with a fluorescent molecule: blue emission by delamination in formamide† Qingyang Gu, Feifei Su, Shulan Ma,* Genban Sun and Xiaojing Yang
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We demonstrate the first example of exfoliated layered europium hydroxide (LEuH) composites with a fluorescent molecule that exhibit blue luminescence. Co-quenching in the solid state, blue emission (440 nm) in formamide, and green emission (514 nm) in water–formamide were found, for which delamination and swollen states account for the versatile luminescence behaviors.
Host–guest hybrids such as composites of inorganic matrices and organic chromophores can exhibit superior physicochemical characteristics and unique properties due to synergies of host and guest components.1 Inorganic hosts normally provide stabilization and protection, while chromophore guests provide optical functions such as color and fluorescence properties. Inorganic hosts can also render insoluble pigments from soluble dyes.2 Materials with a twodimensional (2D) layered structure3 are a large class of functional organized systems, characterized by tunable interlayer volume and variable interlayer guests. The arrangement of luminescent guests can be tuned within the 2D matrix, which facilitates the modulation of luminescence.4,5 Luminophore-layered hybrid materials with negatively-charged 2D layers have been extensively investigated.6–14 Layered double hydroxides (LDHs) are one type of 2D matrix with positively-charged layers showing various applications in catalysts,15 two-dimensional nanoreactors16,17 and adsorbents.18–21 LDH–chromophore hybrids have presented functions such as enhanced photostabilization in comparison with individual components.22 Recently, layered rare-earth hydroxides (LRHs), a new intriguing family of 2D materials structurally similar to LDHs, have attracted increasing attention,23–31 due to the excellent properties arising from lanthanide elements. 8-Hydroxy-pyrene-1,3,6-trisulphonate (abbr. HPTS) is a wellknown non-toxic water-soluble fluorescent molecule, which is widely used in various fields such as fluorescence probes for Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, chemical compositions, FT-IR, TG-DTA spectra and SEM images, XRD patterns, and the excitation spectra of samples. See DOI: 10.1039/c4cc08380f
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many applications due to its unique fluorescent properties, excellent photostability, high absorbance and high quantum yield. Normally HPTS in water exhibits two emissions, one is blue corresponding to a protonated form (ROH, Scheme S1, ESI†) and the other is green related to a deprotonated form (RO ).32 The pKa of HPTS decreases from 7.4 in the ground-state33,34 to 0.4 in the excited state.33 The ROH of HPTS in this excited state will ionize quickly releasing H+ to turn into RO form, which emits green emission by radiative transition to the ground state. So the blue emission of HPTS cannot be observed when the pH value of aqueous solution is greater than 3.0.35 Through structural adjustment or microenvironment changes, pure emission may be obtained.36 Much work involving fluorescence molecule intercalated LDHs had been reported,2,37–39 including HPTS–LDH hybrids that exhibit interesting luminescence in the solid state.35 However, there has been a lack of studies focusing on tunable or purified luminescence of HPTS, which restricts its application as color display material. With regard to luminescent materials, a prerequisite for applications is the development of transparent ordered films; unfortunately, such films have been less studied than solution and powder systems.40,41 Moreover, most of the reports related to film materials focus on LDHs possibly because of their easier delamination. Delaminated LRH nanosheets would be a very important class of functional 2D nanoscale materials42–45 as building blocks for assembly of ultrathin films for optical devices. Whereas LRH delamination remains great difficulty, due to strong affinity between interlayer anions and host layers. Normally delamination of LRHs needs a pre-intercalation using longchain sufactants such as dodecyl sulfate (DS)42,43 and the delamination degree is usually low,42,45 which limits subsequent assembly. So a rapid and facile delamination of LRHs is highly desired. Herein we prepare LRH (R = Eu, Gd) composites co-intercalated with HPTS and an anion surfactant OS (1-octane sulfonic acid sodium). Tunable luminescence of the composites is realized through the synergistic effect of LRH layers and HPTS with different states. Facile one-pot delamination of the composites is achieved when dispersed in formamide (FM), which contributes to blue emission. Co-precipitation and ion-exchange methods at various pH values were performed to prepare the LEuH composites (for experimental
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Fig. 1 (A) XRD patterns of the NO3–LEuH precursor (a) and LEuH composites obtained by co-precipitation (b) and ion-exchange at pH values of 2.9 (c), 4.0 (d), 5.3 (e), 7.3 (f), 10.3 (g). (B, C) Scheme showing different arrangements of gallery species.
details see ESI†). The composite prepared by co-precipitation (Fig. 1A-b) has a basal spacing (dbasal) of 2.45 nm, equal to that by ion exchange at pH = 2.9. Other composites (Fig. 1A-(c–g)) via ionexchange give decreased dbasal (from 2.02 to 1.95 nm) with pH increase, attributed to an increased affinity of guests to layers originating from the increasing deprotonation degree. Based on the LRH thickness of 0.65 nm,29 the 2.45 nm dbasal corresponds to a gallery height of B1.80 nm (= 2.45 0.65), meaning a bi-layered arrangement (Fig. 1B), considering the HPTS size of B1.01 0.75 0.50 nm3 and the OS height of B1.04 nm. Other composites with gallery heights of 1.30–1.37 nm (= 1.95/2.02 0.65) mean a monolayer arrangement (Fig. 1C). The area per unit charge (Scharge) is usually used to explain intercalation structure.29,46,47 For the 2.45 nm dbasal with 2 charged HPTS, the Scharge of 0.25 nm2 (= 1.01 0.50/2) is larger than that of the LRH layer (0.22 nm2),29 which means a bi-layered fashion.46,47 For the dbasal of 1.95–2.02 nm with 3 charged HPTS, the Scharge is 0.17 nm2 (= 1.01 0.50/3), being smaller than that of the LRH layer, so a mono-layered orientation is reasonable.46,47 The composite obtained by co-precipitation and ion-exchange at pH = 2.9 has the highest HPTS loading (Table S1, ESI†). SO3 vibrations48 appearing at 1171/1053 cm 1 (Fig. S1, ESI†) verify the composite formation. TG-DTA (Fig. S2, ESI†) indicates an elevated thermal stability (B100–200 1C) of organic species after intercalation. SEM shows obvious aggregation for the composite obtained by co-precipitation (Fig. 2A), while those by ion-exchange (Fig. 2B, Fig. S3, ESI†) reveal hexagonal morphology with columnar or flowerlike aggregates resembling the LEuH precursor (Fig. S3a, ESI†). In the solid state, pure HPTS powder (Fig. 2C-a) shows no any emission because of aggregation.35 For LEuH, emissions at 580– 698 nm assigned to 5D0–7FJ radiative-relaxational transitions of Eu3+ are observable (Fig. 2C-b). After forming composites, neither emissions of layer Eu3+ nor emissions of HPTS are observable (Fig. 2C-c), no matter what excitation wavelengths of 395 nm (for Eu3+) or 375 nm (for HPTS) are used. The marked quenching of layer Eu3+ emission
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Fig. 2 SEM images of LEuH composites obtained by (A) co-precipitation and (B) ion-exchange at pH = 7.3. (C) Solid emission spectra of (a) HPTS, (b) LEuH and (c) the LEuH–HPTS composite (the inset shows the photograph of the powder under 365 nm UV irradiation). Tyndall light scattering of colloidal suspensions of the LEuH composite in (D) FM and (E) water–FM. (F) BNU characters in blue color written using colloidal suspension (D).
may be ascribed to the significant nonradiative relaxation channels provided by high-energy vibration of –OH of HPTS.49,50 The absence of HPTS emission suggests that certain energy transfer exists in the system, in view of the close excitation wavelengths of 395 nm for Eu3+ and 375 nm for HPTS. Due to the lower energy needed for Eu3+ excitation, the provided energy may be absorbed preferentially by Eu3+, meanwhile the high-energy vibration of –OH of HPTS competes this energy, thus possibly causing a co-quenching effect (Scheme 1A). Luminescence behaviors of HPTS and composites in various solvents are investigated. Fig. 3A shows the emission spectra of colloidal suspensions of samples in pure FM (testing conditions: 0.02 g HPTS or composites were dispered into 10 mL of FM by mechanical shaking for 0.5 h). HPTS in FM (Fig. 3A-g) has two emissions, one is blue (453 nm) and the other is green (520 nm), similar to that found in water. All composites, however, display one strong blue emission centered at B440 nm with a shoulder at around 454 nm. Because a surfactant can reduce the fluorophore aggregation and improve the fluorescence, the surfactant OS was used to help to intercalate and dilute/isolate the HPTS molecules so
Scheme 1 The LEuH composite showing versatile luminescence behaviors under different states: co-quencing in the solid state (A), blue luminescence in the delaminated state in FM (B), and green luminescence in the swollen state in water–FM (C).
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Fig. 3 Emission spectra of LEuH–HPTS composites in (A) FM and (D) water– FM: by co-precipitation (a) and ion-exchange at pH values of 2.9 (b), 4.0 (c), 5.3 (d), 7.3 (e), 10.3 (f), and free HPTS (g). (G) HPTS in water solutions, and LGdH– HPTS composite colloidal suspensions in (H) FM and (I) water–FM. Photographs under (B, E) daylight and (C, F–I) UV irradiation at 365 nm.
as to prevent the aggregation. Moreover, OS as an anionic sufactant should not markedly affect the emission of HTPS,51 so herein the luminescence change of HPTS would be mainly attributed to the LEuH layer. For HPTS luminescence, the inhibition of the excited state proton transfer (ESPT) process will favor blue emission.35 Compared with the solid state, an exfoliated state of LEuH layers in FM may account for the inhibition effect of the ESPT process, which would contribute to the observable blue luminescence. When the composites were dispersed in FM for only 30 min, transparent colloidal suspensions were formed, which revealed obvious Tyndall light scattering (Fig. 2D). Using the colloidal suspensions, we write the initials BNU of our university (Beijing Normal Unversity) on paper that can be clearly observable due to the bright blue color (Fig. 2F). As is known, FM is a highly polar solvent normally used in LDH delamination,52 which can hardly be performed by water. For delamination of the LDH system,52,53 the carbonyl group (–CQO) of FM acting as a highly polar solvent has a strong interaction with the hydroxyl slabs of LDH layers to form a large number of hydrogen bonds, thus massive FM will be introduced into the gallery.53 Meanwhile, the –NH2 end of FM may not bond strongly to interlayer anions, thus producing a loosely stacked swollen phase with tens to hundreds of nanometers following the FM added amount.53–55 Water addition would restore the interlayer hydrogen bonding network and gallery spacing.53 With mechanical shaking or ultrasonic treatment imposing a transverse sliding force on the swollen phase, the host layers will come apart resulting in delamination. In LRHs, the delamination process would be similar. A high degree of interlayer expansion of LEuH was found with addition of a large volume of FM.42 But the delamination of LRHs normally needs pre-intercalation using long-chain sufactants such
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as DS42,43 and the delamination degree is low.42,45 Now we achieve the delamination facilely by one-pot dispersion of the composites in FM. As shown in Scheme 1B, in the delaminated state, the positivelycharged LEuH layers would provide an unique arrangement and microenvironment for HPTS anions that adhere to LEuH layers, and the strong electrostatic interactions between them make the deprotonation of HPTS via ESPT difficult. Roy et al. found that slow ESPT from HPTS to acetate occurs within a cationic micelle (cetyltrimethyl ammonium bromide).33 Compared with that in the solid state, the confinement effect by LEuH layers for HPTS in the delaminated state is markedly weakened and the electrostatic interaction becomes dominant. Moreover, the entered FM molecules would interact with HPTS molecules by formation of strong intermolecular hydrogen bonds that prevent the ESPT process of the ROH form of HPTS, thus also facilitating the blue emission. A synergistic effect of HPTS and LEuH layers with a delaminated state among FM may lead to the favorite blue luminescence. Meanwhile, strong electrostatic interaction of the LEuH layer with anionic HPTS is also useful in the modified release of the fluorescent molecule HPTS. All these advantages make this kind of hybrid a promising fluorescent material. The composite prepared by co-precipitation shows very weak emission (Fig. 3A-a), while those obtained by ion-exchange exhibit enhanced emission (Fig. 3A-(b–f)). The composite prepared at pH = 2.9 reveals the strongest intensity, possibly related to the highest HPTS loading (Table S1, ESI†). Under UV irradiation at 365 nm, free HPTS in FM displays a bluish-green color (Fig. 3C-g), while the composites exhibit a bright blue color (Fig. 3C-(b–f)). For the composite prepared by co-precipitation, even with relatively high HPTS content (Table S1, ESI†), the very weak emission may be attributed to its original aggregation. Though host layers of this composite can also achieve delamination, the poor delamination quality due to aggregation might weaken the emission. To compare the function of different layer ions, the LGdH layer was used to check the luminescence behavior. Under the same conditions of pH = 7.3 (for experimental details see ESI†), the as-formed LGdH–HPTS composite has XRD and FT-IR data (Fig. S4, ESI†) similar to LEuH–HPTS. The colloidal suspension of LGdH– HPTS in FM presents a 496 nm peak (Fig. 3H) belonging to blue-green emission. So various layer ions such as Eu3+ and Gd3+ generate different luminescence behaviors though under the same conditions. This highlights the synergistic effect of HPTS with the LEuH layer in a delaminated state which is beneficial to blue luminescence. Emission spectra of LRH composites in water–FM were measured to check the luminescence behavior. For testing, 0.02 g of the composite was firstly dispersed into 10 ml of FM for 0.5 h to get colloidal suspensions. Then 5 ml of aforementioned suspensions were taken out to mix with 20 ml water with different pH values. All composites are found to display green emission (514 nm) (Fig. 3D), similar to that found for free HPTS in water solutions (Fig. 3G). The intensity order of the composites is in agreement with that in FM. Under UV irradiation, all composites display a green color (Fig. 3F). LGdH–HPTS in water–FM also shows green emission (Fig. 3I). The different luminescence behaviors of the composites in FM and water–FM may originate from the various existential states of LRH layers. In water–FM, a restacked/swollen state of the LRH layers may be present. In the LDH system, the colloid
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of exfoliated nanosheets is not stable in water, and substitution of water for FM may lead the sheets to restack to a layered structure,56 that is, water addition would restore interlayer hydrogen bonding networks and gallery spacing.52 Thus, in the present water-involving case, the LRH nanosheets tend to form a restacked layered structure, however, due to the presence of the significant FM amount, a swollen state would exist as shown in Scheme 1C. The Tyndall light scattering of colloidal suspensions of the LEuH composite in water–FM (Fig. 2E) supports the swollen state. The vanishing of basal reflections of the colloidal aggregate in the XRD pattern (Fig. S5, ESI†) may indicate a swollen phase. In this state, there would be water layers adjacent to LRH sheets due to formation of hydrogen bonding networks between water molecules and hydroxide groups of the LRH layer. These water layers will isolate the HPTS molecules from LRH layers with a high degree of interlayer expansion, which would weaken the interactions between the LRH layers and HPTS/OS molecules. This would favor green luminescence. Different excitation wavelengths (Fig. S6, ESI†) show various chemical environments of HPTS contributing to the different luminescence behaviors. In summary, intercalation of a fluorescent molecule HPTS and a surfactant OS into LRH (R = Eu, Gd) produces composites exhibiting tunable fluorescence behaviors depending on the existential state and the layer ion type. In the solid state, the layer confinement effect and energy transfer result in co-quenching for Eu3+ and HPTS emissions. In the delaminated state in FM, a synergistic effect (electrostatic interaction between LEuH layers and anionic HPTS, hydrogen bonding between HPTS and FM, as well as energy transfer of Eu3+ and HPTS) leads to blue luminescence. Water addition restores a swollen phase that weakens the interactions of LRH layers with HPTS, which favors green luminescence. This study is expected to open a new field of LRHs for fabricating luminous film materials especially blue emission based on delaminated LRHs. This work is supported by the National Science Foundation of China 21271028, 51272030 and 21271001.
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