Facile Template-Free Fabrication of Aluminum-Organophosphorus

Article pubs.acs.org/JPCC

Facile Template-Free Fabrication of Aluminum-Organophosphorus Hybrid Nanorods: Formation Mechanism and Enhanced Luminescence Property Yawen Huang, Jiajun Ma, Junxiao Yang,* Ke Cao, and Zhongyuan Lu State Key Laboratory Cultivation Base for Nonmetal Composite and Functional Materials & School of Material Science and Technology, Southwest University of Science and Technology, Mianyang, 621010, People’s Republic of China S Supporting Information *

ABSTRACT: Recently, much effort has been directed toward fabrication of metalorganophosphorus hybrids with microporous, fibered, layered, and open structures to obtain desired mechanical, optical, electric, and catalytic properties. In this work, aluminum−phosphorus hybrid nanorods (APHNRs) with regular morphology were prepared by a template-free hydrothermal reaction of aluminum hydroxide with diphenylphosphinic acid (DPPA). Structure characterization of APHNRs by Fourier transform infrared spectroscopy, laser Raman spectroscopy, and X-ray diffraction demonstrate a structure with aluminophosphate main chains and phenyl pendant groups, which enable self-assembly into nanorods. The reaction conditions and the structures of phosphinic acids appear to have a significant impact on the morphology and size of nanorods. Moreover, the evolution of morphology and structure assembly during the forming process of APHNRs, as monitored by SEM and XRD, reveal a decomposition-assembly propagation process where the driving force of assembly is attributed to π−π stacking interactions between phenyl pendant groups. APHNRs show a significant increase in light emission relative to pure DPPA due to their compact structure resulting from the π−π stacking interaction. Detailed investigation revealed that photoluminescence was remarkably amplified by enhancing the compactness of APHNRs.

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INTRODUCTION Luminescent inorganic−organic hybrids have attracted increasing attention in the fields of advanced flat panel displays, biological and chemical labels, light phosphor powder, and so on1−5 due to their attractive and novel properties that are not easily attainable in either organic or inorganic alone. Typical inorganic−organic hybrids consist of rare earths and organic ligands that offer luminescence properties and coordinating sites, respectively. It has been well-documented that the characteristics of rare earths play a crucial role in determining their luminescence property.6−8 Contrastively, the role of organic ligands, especially the luminescent organic ligands, in tailoring luminescence properties was rarely studied. A number of studies have indicated that hybrids containing luminescent organics can achieve completely different electronic states due to the diversity and tailorability of organic ligands.9,10 In addition, the luminescence property is substantially changed11,12 when supramolecular interaction is introduced between luminescence organic units.11,12 However, related studies on supramolecular interaction have been limited to small organic molecules. Alongside the rapid development of nanoscience and nanotechnology, increased effort was put into the controlled preparation of inorganic−organic hybrid nanostructures, in particular, 1D nanostructures, to search for potential applications in light emission devices.13−15 Template assistance method is a versatile route to form nanostructure of hybrids16−18 but usually results in the introduction of impurities into the resulting materials. Alternatively, the supramolecular © 2012 American Chemical Society

interactions between the organic ligands in inorganic−organic hybrids of suitable architecture could stimulate self-assembly, possibly leading to a variety of well-defined nanostructures. Therefore, this methodology could possibly achieve supramolecular structures and nanostructures, with interesting luminescence properties, simultaneously in hybrids based on luminescent organics. However, to the best of our knowledge, the preparation of inorganic−organic nanostructured hybrid via self-assembling methods has not been reported. Self-assembly of organic ligands in hybrids depends significantly on the architecture of hybrids, supramolecular interactions between organic ligands, and the structure of organic ligands. First, the architecture of hybrids must provide a platform for inducing supramolecular interactions between the side organic ligands and for self-assembly. The inorganic− organic hybrid macromolecules that possess inorganic polymeric backbone and organic side groups could be ideal candidates to provide the strong supramolecular interaction between pendant organics. We noted that aluminum organophosphorus hybrids (APHs),19−23 in particular, aluminum phosphates, 21 possess this type of architecture. In a representative work, Florjańczyk et al21 have reported the preparation of organically modified aluminophosphates hybrid fibers by reacting boehmites with phosphoric acid mono- and Received: May 14, 2012 Revised: October 4, 2012 Published: October 4, 2012 22518

dx.doi.org/10.1021/jp304663a | J. Phys. Chem. C 2012, 116, 22518−22525

The Journal of Physical Chemistry C

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Figure 1. FTIR of APHNRs, ATH, and DPPA (a), Raman characterization of APHNRs (b), TG curve of APHNRs and DPPA (c), and XRD pattern of APHNRs (d).

and formation mechanism of the resulting APH hybrid nanorod were investigated. Moreover, we found that the as-prepared hybrid displays highly amplified light emission intensity relative to pure DPPA. The effect of the π−π stacking interactions, the 1D nanostructure, and the length of nanorods on luminescence property was studied in detail.

diesters. The fact that this can be done without adding any directing agents confirmed that organic ligands may play an important role in forming the 1D structure. Second, the supramolecular interactions must enable the formation of controlled shapes and properties. In recent years, π−π stacking interaction (the edge-to-face or the face-to-face orientation) has drawn much attention.24 Research investigations show the importance and diversity of these interactions, which include the packing of aromatic molecules in crystals,25 tertiary structures of proteins26,27 and the aggregation of porphyrins in solution.28 In particular, face-to-face π−π stacking interactions, the subject of a number of investigations, play a vital role in the stabilization of supramolecular materials with controlled shapes and properties.29 Finally, apart from enabling facile reaction with ATH, the utilized organic ligands must enable π−π stacking interactions and offer the luminescence property. Phosphinic acids with aryl groups were generally used to complex with the main group and transition elements, resulting in insoluble, hydrophobic and thermally stable hybrids.30−32 In addition, the presence of aryl groups would induce π−π stacking interaction. Some phosphinic acids, typically diphenylphosphinic acid (DPPA), show weak light emission under exciting at UV radiation.33 In addition, DPPA has been employed as a coadsorbent, which can significantly improve the total energy conversion efficiency.34 In the study presented in this article we utilized DPPA to react hydrothermally with aluminum hydroxide (ATH), forming a structure with the aluminophosphonates backbone and phenyl side groups. By employing the face-to-face π−π stacking interaction between the phenyl groups in such structure, we constructed 1D nanorod structure without the assistance of templates. The structure, morphology, property,

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EXPERIMENTAL SECTION

Materials. Aluminum nitrate (Al(NO) 3·9H2O), urea (H2NCONH3), n-butanol, ethanol, acetone, and ether were purchased from Chengdu Kelong Reagent. DPPA was purchased from Yantai Gem Chemicals. Dibenzylphosphinic acid (DBPA) and bis(3-methoxy-3-oxopropyl) phosphinic acid (BMOPA) were synthesized according to the reported method.35 All reagents were used as received. Aluminum Hydroxide Precursor.36 Anhydrous aluminum nitrate (15.0 g, 0.04 mol) was dissolved in 13.5 mL of deionized water to give a clear solution. Then, urea (22.5 g, 0.375 mol) was added to the solution. The resulting mixture was stirred at 95 °C until the formation of a transparent gel. After washing with water and subsequent centrifugation, the gel was dispersed in 150 mL of n-butanol, followed by azeotropic distillation to remove residual water. A white powder was obtained after drying at 110 °C for 12 h. Aluminum-Organophosphorus Hybrid Nanorods (APHNRs). ATH precursor (0.39 g, 5 mmol) was dispersed in deionized water (7.5 mL) under ultrasonic. Then, DPPA (1.09 g, 5 mmol) was added to this solution (0.667 mol/L) with vigorous stirring for 30 min, and the fresh solution was transferred to a Teflon-lined stainless-steel autoclave. The autoclave was kept at 160 °C for 12 h in an electric oven. Afterward, the solution was cooled to room temperature and 22519



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filtered. The residues were washed with deionized water, ethanol, acetone, and ether, in that order, and then dried in vacuum oven at 60 °C for 12 h, giving white solid powders. Characterization. Fourier transform infrared (FT-IR) spectra were obtained on Perkin-Elmer Spectrum One spectrophotometer with a resolution of 2 cm−1. Raman spectroscopic measurements were performed on an inViaReflex confocal laser Raman spectrometer (Renishaw) with a resolution of 2 cm−1. Powder X-ray diffractometer (XRD) patterns were recorded at a Panalytical X’Pert Pro diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed on a TA SDT Q600 simultaneous DTATGA at a heating rate of 20 °C min−1. Scanning electron microscopy (SEM) of APHNRs was recorded on an S-4800 (Hitachi) field-emission scanning electron microscopy. Transmission electron microscopy (TEM) was recorded by an FEI (FEI Company) transmission electron microscope. A UV−vis spectrum was recorded in a 1-cm-length path quartz cell with a Shimadzu Spectronic UV3150 (Shimadzu, Japan) apparatus at a resolution of 0.5 nm. Photoluminescence was examined by fluorospectrophotometer (PL, PE LS55).

leads to a severe destruction of ATH-layered crystalline structure. The reflections corresponding to intercalated structure do not exist. Hence, the structure of APHNRs was not simply a grafting or intercalating structure. In addition, a strong reflection at 2θ = 7.4° (d = 12.0 Å) in the XRD pattern supported the existence of an Al(DPP)3 bridging structure because the spacing of 12.0 Å was inconsistent with the diameter of the polymeric chain with Al(DPP)x repeating units.21 In conjunction with previous work,20 the hybridized structure of APHNRs that we suggested was mainly built from polymeric chains with repeating units of Al(DPP)x, where octahedrally coordinated Al atoms were bridged by several diphenylphosphate (DPP) ligands. Absorption bands corresponding to such ligands were, indeed, observed in FT-IR. Moreover, two weak reflections at 2θ = 22.0−25.0° with dspacing of 0.387 and 0.382 nm were observed (Figure 1d), which implied the presence of the face-to-face π−π stacking interactions of aryl groups.45−48 More importantly, the reflection at 2θ = 7.4° implies a kind of layered or fibrous morphology. Such morphology was confirmed by SEM and TEM. Figure 2a−c shows typical SEM and TEM images of ATH and APHNRs. It can be clearly seen that after hydrothermally reacting with DPPA, ATH agglomerates were transformed into APH nanorods. These nanorods exhibit high monodispersity and a rod-like morphology with an average diameter of ∼50 nm. Such nanorod morphology was

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RESULTS AND DISCUSSION Structure and Morphology of APHNRs. The crystal structure of ATH consists of layers of aluminum octahedral with hydroxyl groups on either side wherein hydrogen bonds link the layers together.37,38 The lack of exchangeable cations such as Na+ causes a completely different modification process as compared with montmorillonite (MMT) and lactate dehydrogenase (LDH). As demonstrated in early works,39,40 ATH can react with organophosphorus compounds via acid− base condensation reaction, consequently forming Al−O−P bonds that are stable to heat and hydrolysis. The structure of APHNRs was characterized by FT-IR. As shown in Figure 1a, the adsorption bands of PO−H rocking at 971 cm−1 and Al−OH stretching at 1061 cm−1 were greatly decreased or nearly disappeared. Moreover, P−O−Al adsorption bands appeared, showing a considerable wavenumber shift as compared with P−O−H and Al−OH adsorption bands. It is reasonable to conclude that these observations indicate the formation of P−O−Al covalent bonds24,37,38 and a highly hybrid structure of APHNRs. Further indications of the structure were provided by Raman spectroscopy (Figure 1b). Several new bands appeared in the range of 1000−1400 cm−1, assignable to the vibrations of the C−C skeleton, P−C skeleton, and the PO2 unit.41 Furthermore, the asymmetric and symmetric P−O vibration bands of O−P−O located near 800 cm−1 and an intense Al−O−P band at 617 cm−1 were observed,42,43 suggesting the formation of Al−O−P−O−Al repeating units.44 These analyses from FT-IR and Raman spectrum revealed the existence of bidentate PO2 structure in APHNRs. In theory, the formation of hybrid structure would lead to high thermal stability. This was confirmed by the TGA results of APHNRs and DPPA. From the TGA curves (Figure 1c), it can be clearly seen that APHNRs showed a significantly higher Td relative to DPPA. The crystalline structure of APHNRs was examined by powder X-ray diffraction (XRD) (Figure 1d). As is well known, XRD spectrum of ATH displays a typical peak at 2θ = 18.3°, which is ascribed to a hydroxide layered structure with a very narrow basal spacing of 0.485 nm.38 However, we found that this peak was absent in the XRD spectrum of APHNRs. This indicates that the reaction of DPPA with ATH

Figure 2. SEM image of ATH (a) and APHNRs (b), TEM images of APHNRs (c), HRTEM image of APHNRs (d), and EDX of APHNRs (e). 22520



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Figure 3. Growth process of APHNRs recorded by SEM: hydrothermal treatment for 30 min (a), 2 h (b), 6 h (c), and 12 h (d).

hardly observed in previous studies related to organophosphorus-modified aluminum. The fine structure of APHNRs was visible by high-resolution transmittance electron microscopy (HRTEM) (Figure 2d). The inset of Figure 2d showed a corresponding fast Fourier transformation (FFT) image. An irregular ring-shaped diffraction demonstrated that the nanorod has an amorphous structure. Meanwhile, a small boehmite phase occurred (Figure S1 of the Supporting Information). These phenomena show good consistence with the analysis results from XRD. Figure 2e showed the EDX spectra of APHNRs, from which the main elements in APHNRs can be confirmed as C, O, Al, and P. Apart from the contribution of the carbon of the nanorods, the strong signals of carbon also originated from the covered carbon film in the TEM testing. Formation Mechanism. To better understand the formation process of these APHNRs, we recorded the morphology evolution of APHNRs by SEM and XRD, as shown in Figures 3 and 4. In the initial stage, the size of ATH agglomerates decreased markedly; meanwhile, ultra-small-sized nanoparticles or nanorods emerged (Figure 3a and Figure S2 of the Supporting Information). The decrease in size was indicative of the destruction of ATH crystal via the reaction with DPPA. The degree of reaction greatly affected the regularity of morphology. For example, a mixture of nanoparticles and nanorods with poor morphology was gained by ambient pressure reaction (Figure S3 of the Supporting Information). Figure 3b,c shows that the ultrasmall-sized nanorods and nanoparticles directionally propagated into nanorods, leading to increased length accompanied by a slight increase in diameter. Such directional propagation could not be possible on any surfactants. This may have originated from the unique structure of APHNRs. Such a structure was characteristic of a face-to-face π−π stacking interaction in the radial direction, thus leading to a constrained propagation and ordered architecture and, consequently, favoring a dominate propagation in the axial direction. The π−π stacking interaction in the radial direction was further evidenced by the observation

Figure 4. XRD patterns of APHNRs prepared from hydrothermal process for 30 min, 2 h, 6 h, and 12 h (a), d spacing distance, height and area % of the reflection peak at 2θ = 7.3 to 7.4° as a function of time (b), d spacing distance, and height and area % of the reflection peak at 2θ = 22.0−25.0° as a function of time (c).

that the intensity of the characteristic reflection of π−π stacking at 2θ = 22.0−25.0° increased with increasing diameter. Apart from its important role in controlling the propagation along the 22521



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Figure 5. Decomposition−assembly propagation processes to generate APHNRs.

Figure 6. SEM images of APHs produced under different reaction conditions: reaction of ATH with DPPA at 180 °C (a) and reaction of ATH with DPPA with the concentration of 0.167 mol/L (b).

radial direction, the π−π stacking interaction, which usually directs a supramolecular architecture, may lead to an ordered arrangement of polymeric chains into a close-packed columnar structure.49 Figure 3d shows that prolonging the reaction time to 12 h resulted in nanorods that mainly propagated along the axial direction. Meanwhile, the surface became smooth and the morphology became regular. The corresponding XRD data (Figure 4) clearly show that the peak intensity assigned to Al(DPP)x bridging structure increased. These results imply that the propagation of nanorods along the axial direction may be achieved via a reaction between terminal functionalities of nanorods, such as PO-H, PO, Al−OH, and so on, generating an Al(DPP)x bridging structure. We noted that the organic ligands showed significant influence on the morphology of APH materials. As demonstrated in previous studies, varying the structure of organic ligands results in a series of nanostructures, such as nanotubes, nanowires, and hollow spindles.50−53 In our experiments, each of BMOPA, DBPA, and DPPA were used to react with ATH. Only DPPA could generate the well-shaped nanorods. When using BMOPA, a type of long, thin, and leaflike morphology (Figure S4a of the Supporting Information) was generated under identical conditions. It is possible that the lack of building blocks enabling supramolecular interaction was responsible for the failure to generate nanorods. This result further supports the important role of π−π stacking interaction

in forming nanorods. It was also found that the use of DBPA under the same reaction conditions led to nanorods with poor regularity (Figure S4b of the Supporting Information). This could be ascribed to the presence of a methylene linkage in DBPA, which would lower the reactivity of DBPA relative to that of DPPA. Overall, these observations revealed that a high reactivity and π−π stacking interaction are necessary for forming a well-defined nanorod morphology. Therefore, the formation mechanism of APHNRs could be defined as a reaction-assembly propagation process, which was illustrated in Figure 5. In the beginning, DPPA intercalated into ATH, consequently destroying the crystalline structure and forming a polymeric chain with Al(DPP)3 repeating units. Simultaneously, such polymeric chains can assemble into ultrasmall nanorods via the π−π stacking interaction. Terminal functionalities of the titled nanorods would allow the continued propagation along the axial direction, extending the length of nanorods. Reaction Conditions. Normally, the strategy to design and control the morphology and orientation of crystallites involves adjusting the thermodynamics and kinetics of nucleation and growth of materials by controlling experimental parameters.54 As previously described, the reaction time plays a critical role in the formation of APHNRs. The influence of other factors, such as temperature and the concentration of phosphinic acid, on the growth of nanorod was studied systematically. We 22522



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conducted the reaction at room temperature, 160 and 180 °C, respectively. No nanorods were formed when the reaction was performed at room temperature. When the reaction temperature was elevated to 160 °C, APHNRs could be observed with long, smooth, and regular appearances (Figure 3d). Further elevating the temperature to 180 °C resulted in APHNRs with more regular morphology (Figure 6a). One possible explanation is that high temperature facilitated the reaction between ATH and DPPA and the destruction of ATH crystal, thus decreasing the size of nanoparticles. The reduction in size is favorable for the self-assembly of nanoparticles. Moreover, an increase in length was seen at higher temperature. It is reasonable to hypothesize that active groups like P−OH and PO could completely react with Al−OH between terminal functionalities of nanorods upon increasing the temperature or time; this appears to be particularly favorable to growth along low-index faces, like (010), (100), (110), and so on.55,56 The influence of DPPA concentration on morphology was also studied by comparing the morphology of nanorods generated at concentrations of 0.667 (Figure 3d) and 0.167 mol/L (Figure 6b). Clearly, the reaction at lower concentration produced APHNRs with longer and tenuous rod-like morphology. This may have resulted from a reduction in the nucleating number or in the initial number of nanorods as the concentration of DPPA was decreased. Photoluminescence Properties. UV−vis absorption spectra of APHNRs and DPPA (CH2Cl2 solution with a concentration of 0.125 g/L) are depicted in Figure 7. APHNRs

at 332 and 572 nm under excitating at 234 nm, whereas little or no emission was observed by ATH and AlOOH. The fluorescence peaks are attributed to the S1→S0 radiative transition (fluorescence) and the triplet state T1→S0 radiative transition (phosphorescence) of phenyl, respectively. Moreover, a weak emission at 425 nm was noted, which may be ascribed to the electron transition of Al3+ or charge-transfer transition between metal and organic ligand (MLCT).58 To explain this, the samples were subjected to radiation at 266 nm instead of 234 nm. The luminescence spectrum (Figure S5 of the Supporting Information) obtained clearly shows that the electron transition of pure Al3+ could not generate obvious emission peaks with excitations above 260 nm.59 If MLCT does exist, then this emission peak should remain. However, it can be seen that the emission peak of APHNRs at 425 nm disappeared. Hence, it can be concluded that MLCT hardly exists in APHNRs and the weak emission at 425 nm may be ascribed to the electron transition of Al3+. Overall, the role of Al3+ was merely to act as an inert atom to form complexes with DPP ligands. As compared with DPPA, APHNRs exhibited relatively broad and strong fluorescence peak. Moreover, this remarkable enhancement was highlighted by the change in fluorescence spectra corresponding to the formation process of APHNRs (Figure 8), from which one can see that the fluorescence intensity increased significantly when the reaction was carried out for 6 h and then showed a slight decrease by 12 h. Furthermore, we also investigated the fluorescence property of APHNRs prepared at different temperatures and concentrations of DPPA (ATH). Elevating the temperature or decreasing the concentration led to an enhancement in the intensity of fluorescence. Combining the results from FL (Figures 7 and 8), SEM (Figure 3), XRD (Figure 4), and nitrogen adsorption (Table S1 of the Supporting Information), we note several clues that would provide a reasonable explanation for the influence of reaction conditions on fluorescence property. First, the values of BET surface area of APHNRs (Table S1 of the Supporting Information) did not show significant changes at different reaction times, temperatures, or DPPA concentrations. Therefore, we think that the surface area may show only a minor impact on the property of luminescence. Second, a good correlation was noted between the changes in face-to-face π−π stacking and FL intensity. In other words, during the formation process of APHNRs, the XRD peak intensity assigned to face-to-face π−π stacking and FL intensity showed a similar tendency to increase at first and then to decrease slightly. In addition, both the maximum intensity of π−π stacking and FL appeared at 6 h. This correlation implies an important role of π−π stacking interaction in enhancing the intensity of FL. However, in this stage, the affection mode of π−π stacking interaction on luminescence properties remains unclear. We conjecture that the face-to-face π−π stacking interactions, combined with the polymeric backbone constructed by Al(DPP) x bridged structure at high coordinating saturation level, probably endow ligands with restricted movement and endow materials with reduced defects and enhanced regularity. This would result in a high level of inhibition in the nonradiative transition,60,61leading to an increase in the luminescence intensity of APHNRs. Nevertheless, further investigations are required to support this explanation. Third, when temperatures are elevated or concentrations are decreased, an enhancement in the fluorescence intensity was also observed, which shows good

Figure 7. UV−vis absorption (a) and PL spectra (b). The photoluminescence spectrum was recorded by exciting at 234 nm.

and DPPA showed similar UV−vis absorption peaks at 237, 261, 266, and 273 nm under exciting at 234 nm. These absorption bands are assigned to ππ* transition of localized phenyl units.57 In addition, UV spectrum of APHNRs shows an additional wide adsorption at the range of 250−600 nm, which is possibly originated from the absorbance of metal Al3+. The solid-state luminescence properties of DPPA and APHNRs have been examined at room temperature. As shown in Figure 7b, DPPA and APHNRs show pronounced emission centered 22523



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the decomposition of the ATH crystal structure into polymeric chains with Al(DPP)3 repeating unit structure. Simultaneously, the π−π stacking interactions between polymeric chains drive the chain to arrange into nanorods. This novel discovery opens up a new route to prepare a variety of inorganic−organic hybrid nanostructures. Possibly owing to their nanostructure, APHNRs show significant enhancement in luminescence efficiency relative to DPPA. The luminescence intensity was also highly enhanced by increasing the reaction time and temperature. This observation may be correlated with the increase in face-to-face π−π stacking interactions. To the best of our knowledge, this is the first report to demonstrate that it is possible to attain a high level of fluorescence by the self-assembly of certain weakly luminescent organic compounds. The unique structures and properties of these aluminum-organophosphorus hybrids make it possible to confer on them a wide range of morphology and other properties by utilizing a series of new ligands, leading to attractive biomedical, electrical, or optical functions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

HRTEM images of boehmite phase and fine crystallic structure of boehmite phase. Higher resolution SEM images of initial nanorods and ultrasmall sized nanoparticles in the initial stage. SEM image of samples by ambient pressure reaction, chemical structure of DBPA and BMOPA. Emission peak of APHNRs in CH2Cl2 solution by utilizing an excitation wavelength of 266 nm instead of 234 nm. BET surface area of APHNRs prepared under different reacting conditions. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NFSC-NSAF (Grant No. 10976024), China-NSFC (Grant No. 51107106), the Youth Foundation Incubation Programme of Sichuan Province (Grant No. 2011JQ0042), and the Applied Basic Research Foundation of Mianyang (Grant No. 10Y002-5). We thank Dr. Wenhai Wang and Yongjun Ma for SEM and TEM measurements, respectively.

Figure 8. PL spectra of APHNRs: influence of the reaction temperature (a), reaction time (b), and the concentration of solution (c) on the fluorescence intensity of APHNRs.

correlation with the improved face-to-face π−π stacking interaction.

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CONCLUSIONS New APHNRs were prepared by reacting ATH with DPPA using a facile template-free hydrothermal process. FT-IR and Raman measurements suggest that the organophosphorus species were incorporated into ATH via the formation of Al− O−P bonds, mainly producing the bridging bidentate structure. This reaction also leads to the destruction of the ATH crystalline structure. In addition, XRD measurements provided evidence of the presence of the face-to-face π−π stacking interactions of aryl groups. This interaction may provide one of the predominant driving forces to form the rod-like morphology. TGA proves the thermostability of these supramolecular hybrid structures, and SEM and TEM images of APHNRs demonstrate the nanorod morphology and an amorphous crystal structure. The formation process of APHNRs shows a decomposition-reformation mechanism. First, the reaction of ATH with phosphinic acids results in

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