conjugated polymer nanocomposites

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated polymer nanocomposites

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Yaping Huang, Harrone Muhammad Sohail and Jun Lu Beijing University of Chemical Technology, Beijing, P.R. China

12.1

Fabrication and assembly of LDHs/conjugated polymer nanocomposites

12.1.1 Introduction 12.1.1.1 Conjugated polymers The majority of polymers (Zhang et al., 2017a; Knauert et al., 2010; Kularatne et al., 2012), such as broadly used commodity materials, polypropylene, polyethylene, polystyrene, and poly(ethylene terephthalate), have similar electrical and optical properties, which possess no mobile charges and no photoabsorption in the UV spectral region, and they are electric insulators and colorless. In fact, there is one particular class of polymers with quite different properties; they can be electric semiconductors or conductors and interact with light, and are denominated as conjugated polymers (CPs), having been discovered in the 1970s and awarded the Nobel Prize in chemistry in 2000. Conjugated polymers are a class of artificial synthesized polymers containing the delocalized π electrons in the main chain with the sp or sp2 hybridization carbon atoms like polythiophene, polyaniline, polypyrrole, polyvinylene, polyacetylene, etc. (Facchetti, 2011; Tennyson et al., 2010; Gibson et al., 2012). These are polymer photoelectric materials with metallic or semiconductive electric properties, concomitant with the processability and mechanical properties of polymers. The energy and charge can be transferred along the main chain of a large delocalized π electron conjugated system for rapid conduction at the applied voltage and/or photoexcitation, and some conjugated polymers are insulators or wide band gap semiconductors in their eigenstates, only after doping or chemical modification being transformed into a doping semiconductor or even a conductor. Conjugated polymers have strong light-capturing ability and can be used to amplify fluorescence sensoring signals, which play an increasingly important role in disease diagnosis and biological monitoring. As a new generation of

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00012-4 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

photoelectric materials, conjugated polymer was compatible for flexible optoelectric devices, such as electronic papers, foldable display and wearable devices, and was the model system for flexible electronics.

12.1.1.2 LDHs/conjugated polymer nanocomposites Conjugated polymers have demonstrated their distinctive properties in electronic conductivity and light-emitting properties in a variety of applications (Gutierrez et al., 2015; Wang et al., 2015; Zhong et al., 2016; Leclerc et al., 2016), the synthesis and fabrication process of conjugated polymer devices may be subject to inherent physical constraints such as insolubility and poor thermo/photostability of these conjugated polymers with rigid rod main chains. Although the conjugated polymers with improved solubility can be synthesized by a chemical synthetic method, these methods are complicated and high-cost procedures and affect their optical performance, and the modified conjugated polymers can only be dispersed in hazardous organic solvents for further processing. A variety of methods were attempted to try to solve the solubility problems of conjugated polymers in the application. Layer-by-layer (LbL) assembly, proposed by Iler (1966) in 1991 originally reported that the charged solid substrate was immersed into the colloidal solution containing the species with opposite charge deposited alternately, so as to obtain the ultrathin film (UTF) of colloidal species. This UTF preparation technology, based on the interaction between the oppositely charged species with an electrostatic interaction as the driving force, did not attracted much attention, until 1997, when Decher (1997) proposed the electrostatic alternating layers of assembly technology, and utilized this technology in the preparation of polyelectrolytes and organic smallmolecule UTFs, which aroused wide attention. The LbL technology for assembling and preparing UTFs is very facile, for instance, the polyanions and polycations are assembled on a positively charged substrate to form a UTF. The positively charged substrate is first immersed in the polyanionic solution, after standing for some time, the substrate is washed with water, the surface physically adsorbed polyanions are then removed and dried; and then the substrate is immersed in polycationic solution, left to stand for some time, then rinsed with water, thus completing one cycle of the polyanion/polycation assembly, with the above steps repeated in order to obtain multilayer (polyanion/polycation) UTF. In recent years, it has been found that LbL assembly has many advantages in the fabrication of functional thin films (Joseph et al., 2015; Yao et al., 2011; Yang et al., 2014), such as abundant film-forming species, facile operation and low cost, and the potential synergistic effect between assembled members. This method has been widely used in polymer thin film. The nanocomposite thin films based on LDHs have become a new emerging class of nanocomposites (Ma and Takayoshi, 2015b; Ma and Sasaki, 2015a), which have received extensive attention. In addition to polyelectrolytes, other materials such as organic small molecules, organic/inorganic nanoparticles, microcapsules, biomacromolecules, etc. can be assembled with LDHs into the UTFs by suitable driving forces. LDH nanosheets are ideal building blocks for the preparation of LDH-based films by LbL assembly techniques. Recently, researchers have

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focused on the preparation of LDH nanosheets by effective exfoliation in polar solvent-like formides (Naik et al., 2011; Gordijo et al., 2007; Naik and Vasudevan, 2011; Zhao et al., 2011; Wongariyakawee et al., 2012), which has laid a solid foundation for the construction of nanocomposite films by the LbL method. LDH crystallites have a high charge density in hydroxide layers, so their exfoliation is a challenge. Adachi-Pagano et al. (2000) first reported LDH exfoliation, the delamination behavior of dodecyl sulfate intercalated ZnAl-LDHs was investigated in different solvents. Hibino and Jones (2001) studied amino acid anion-intercalated MgAl-LDHs in formamide to achieve exfoliation. To avoid the use of toxic formamide, they further developed an environmentally friendly LDH exfoliation method, which used lactate-intercalated LDHs in water for several days, where the plate LDH nanosheet colloidal solution can be obtained (Hibino and Kobayashi, 2005). The above-mentioned several exfoliation methods are required to modify the interlayer microenvironment of LDHs with organic species, and there is no direct evidence that the obtained LDH nanosheets are monolayers, most of which are formed by several layers of LDH hydroxides. Sasaki (Ma et al., 2015) used wellcrystalline MgAl-NO2 3 LDHs without organic anion intercalation as precursors to obtain the exfoliation in formamide. The resulting LDH nanosheets had typical two-dimensional morphology, and the transverse dimension was at the micron level. In addition, the Sasaki group (Liang et al., 2010; Liu et al., 2007) have successfully achieved the exfoliation of transition metal LDHs (such as CoNi, CoAl, NiAl, etc.), and the resulting nanosheets provide novel functional composite films with a very good structure/functional unit for further application in optical, electrical, magnetic, catalytic, and adsorptic fields. Because LDHs are a typical anionic layered compound, the chemical composition of the hydroxide layers, the species, and amount of interlayer anion are controllable, and thus they show many unique physical and chemical properties (Hibino and Kobayashi, 2005). LDH nanosheets and conjugated polymer can be assembled by LbL method in an assembly process that is simple and convenient. The composition of conjugated polymer with LDH nanosheets by LbL assembly cannot only solve some application shortcomings of conjugated polymers in optoelectronic devices, but can also optimize the performance of optoelectronic devices to some extent. In this chapter, firstly, according to the interaction between the LDH nanosheets and the CP layer, we highlight the development of assembly technology for LDH/CP nanocomposites. Secondly, the luminescent and optoelectric properties and some applications based on LDH/CP nanocomposite UTFs are reviewed.

12.1.2 Fabrication and assembly of LDH/CP nanocomposites With the development of electrostatic assembly technology, scientists have a deep understanding of the mechanism of electrostatic assembly, the structure of UTF, and the interaction between the components (Joao and Joao, 2014). The LDH/CP nanocomposite UTF can be fabricated conveniently by the LbL assembly method (Wongariyakawee et al., 2012). The assembled species with positive-charged LDH nanosheets can be polyanions, small anions, neutral molecules, metal complexes,

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and even cations. The assembled guest species include the polymer, biomacromolecules (protein, DNA, and RNA), inorganic metal nanoparticles, and semiconductor quantum dots and inorganic nanosheets (e.g., graphene). Researchers employed characterization techniques like UV-visible spectroscopy, fluorescence spectra, X-ray diffraction patterns (XRD), circular dichroism (CD) spectra, etc. to confirm the success of the LbL assembly. The morphology and structure of the LDH/CP nanocomposite UTFs is shown to prove this assembly by microscopic techniques like transmission scanning electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). For example, the morphology and supramolecular structure of luminescent UTFs were studied by small-angle X-ray diffraction patterns (SAXRD) and SEM techniques. The SAXRD pattern can illustrate whether the prepared films were ordered and periodic perpendicular to the substrate. Additionally, the periodic structure of UTFs was also verified by SEM, AFM, and fluorescence microscopy technique. The SEM images can show the thickness of UTFs, thus, calculating the UTF average thickness of one bilayer which is congruent with the SAXRD results, and if the film thickness was increased approximately linearly with the bilayer numbers (n), it illustrates films were ordered and periodic perpendicular to the normal of the substrates and that the film growth was linear. Other test data like XPS, NMR, IR spectroscopy, etc. can be used as supporting information to confirm LbL assembly. Through different testing measurements, the ordered and periodic perpendicular UTFs can be confirmed and the fabrication and assembly of LDH/CP nanocomposites by LbL assembly is feasible. The scheme of LbL assembly, the morphology and supramolecular structure of UTFs are documented in various places as following.

12.1.2.1 Layer-by-layer assembly method based on electrostatic interaction Conventional LbL assembly was suitable for polyanion with polycations or LDH nanosheet assemblies. The formation of a stable multilayer composite UTF is mainly the entropy effect of polyanion substitution and the charge reversal process. 2 The polyanions replace the small inorganic anions (e.g., CO22 3 ; NO3 , etc.) adsorbed on the LDH surfaces to form a stable polyanion adsorption layer, that is, a polyanion substitution was a thermodynamic entropy-increasing spontaneous process. At the same time, excessive negative charge also causes the polyanion adsorption layer to be negatively charged, and the charge polarity on the surface is reversed, to facilitate the subsequent assembly of the positively charged LDH nanosheets and multilayer formation. It is the first time that Sasaki’s group (Li et al., 2005) prepared the (LDH nanosheets/PSS)n multilayers using the electrostatic LbL assembly method. In order to improve the bonding force between the film and the substrate, the treated substrate (e.g., Si wafer or quartz glass) was soaked in polyetherimide (PEI) solution for 20 min, and rinsed in water, then transferred into poly (p-styrenesulfonate) (PSS) solution for 20 min and then water for rinsing, so that the substrate

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preassembled a layer of polymer. The substrates were then alternately assembled in the LDH nanosheet colloidal solution and PSS solution to finally get the multilayer thin film. The (MgAl-LDHs/PSS)n multilayers fabricated by the LbL assembly technique can be confirmed by UV-Vis absorption spectroscopy, which shows the linear absorption augmentation for PSS with the assembly cycle. This implied that the (MgAl-LDHs/PSS)n UTF has almost the same amount of assembly per cycle. The XRD characterization indicates the stacking order of the UTFs with a period in the normal direction of UTFs. Coronado and Martı´-Gastaldo (2013) fabricated PSS/ NiAl-LDH UTFs. The magnetic (PSS/LDH)n UTFs were performed according to the LbL method based on the presence of electrostatic attractive interactions between the corresponding charged components. The magnetic data of magnetic (PSS/LDH)n UTFs confirmed the effective transfer of the magnetic properties of the bulk LDH to the self-assembled film that displays glassy-like ferromagnetic behavior. LDH nanosheets can not only be used as components of hybrid films but also serve as a separator for separating the intercalated anions. To some extent, this achieves a long-range ordered arrangement of polyanions. Yan et al. (2009) combined the conjugated polymer, sulfonated poly(p-phenylene) (APPP) with LDH nanosheets mounted on a quartz glass substrate surface using the electrostatic LbL assembly method (Fig. 12.1). The results show that LDH/APPP UTF has an inorganic organic hybrid quantum well structure; and the rigid LDHs can effectively separate the APPP anions, thus avoiding the red/blue shift of the luminescence of

Figure 12.1 (A) The chemical formula of APPP; (B) the representation of one sheet of MgAl-LDH; and (C) LbL assembly process for (APPP/LDH)n UTF (Yan et al., 2009).

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APPP caused by the π π interaction between the polymer chains. For the (APPV/ LDH)n (n 5 3 30) UTFs, its UV-Vis absorption and fluorescence intensity were increased approximately linearly with the bilayer numbers (n) shown in Fig. 12.2, which can illustrate that the assembled films were ordered and periodically perpendicular (Yan et al., 2009). Based on the electrostatic interaction between the positively charged LDH nanosheets and polyanions, researchers have achieved a number of polyanions and LDH nanosheet LbL fabrication assemblies of LDH/CP nanocomposites such as (APPV/LDH)n UTFs (Fig. 12.2) (Yan et al., 2009), (SPT/ LDH)n UTFs (Yan et al., 2011b) (Fig. 12.3), etc. Based on the electrostatic LbL assembly method, the organic polyanion can be used as a carrier for small functional molecules due to their hydrophobic interaction, to achieve small anions, small cations, and even neutral small molecules assembled with LDH nanosheets to obtain the LDH/CP@small molecule nanocomposites. The assembly process of LDH nanosheets with polyanion@small molecules is shown in Fig. 12.4. As the LDH nanosheets are positively charged, the principle of charge balance determines that it can only be assembled with the anionic guests by electrostatic interaction, which greatly restricts the development of LDH layered functional materials. The realization of LDHs and cationic functional molecules is also a challenge. By using the coassembly method, polyanion can be used as the carrier to construct the cation@polyanion/LDH UTFs. More LDH/CP nanocomposites with various functional properties can be fabricated based on the electrostatic interaction between LDH and polyanions. The steps of the program were as follow: Firstly, the small cation functional guest can be adsorbed on the polyanion backbone, and the polyanion negative charge balanced partly, forming (small cation@polyanion) pairs.

(A)

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Figure 12.2 Characterization of (APPV/LDH)n (n 5 3 30) UTFs: (A) UV-Vis absorption spectra (the inset shows the absorbance at 207, 344 nm vs n), (B) fluorescence spectra (Yan et al., 2009).

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Figure 12.3 (A) Chemical formula of SPT; (B) representation of one monolayer of MgAllayered double hydroxide (MgAl-LDH) (dark pink: Al(OH)6 octahedra; green: Mg(OH)6 octahedra); (C) the assembly process of (SPT/LDH)n UTFs (Yan et al., 2011b).

Figure 12.4 The electrostatic assembly process of LDH nanosheets with small anionic/ cationic/small neutral molecule@polyanion pairs.

Then, the LDH nanosheets and the (small cation@polyanion) pairs are alternatively assembled to obtain LDH/CP UTFs. The relationship between small cation and polyanion can be compared to the parasitic relationship in an ecological system,

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that is, the small cation can be effectively parasitic on the surface of the polyanion, and the complex coexists within the interlayer of LDH nanosheets. Lu and coworkers assembled small tris (8-hydroxyquinolate-5-sulfonate) aluminum (III) (AQS32) anion with exfoliated MgAl-LDH nanosheets into ordered ultrathin films by employing the LbL assembly method (Li et al., 2011). The small anion, AOS32 together with polyanions such as one of poly(acrylic acid) (PAA), poly(styrene-4-sulfonate) (PSS), and poly [5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene vinylene] (PPV), co-assembled. alternatively with LDH nanosheets, respectively; and found immobilizing small anions into the interlayer of LDHs by the electrostatic force between the polyanions and LDH nanosheets. Liu’s group researching exfoliated LDHs and montmorillonite (MMT) nanosheets with opposite charges, found that they can be assembled to form an ordered composite film through an electrostatic interaction (Liu et al., 2014, 2015; Wang et al., 2014). With the electrostatic LbL assembly, the photoactive divalent cation bis(N-methylacridinium) (BNMA) and polyvinyl alcohol (PVA) were intercalated between the LDHs and the MMT layer. The assembly process of (MMT/ BNMA@PVA/LDHs/BNMA@PVA)n UTFs is shown in Fig. 12.5. The MMT/LDH thin film formed a kind of electronic microenvironment (EME), and it was found that the lifetime of the inserted fluorescent species can be enhanced by about 40 times (Yan et al., 2010). The exfoliated LDHs and MMT nanosheets with opposite charges can provide a homogeneous 2D microenvironment for chromospheres, and offer the inorganic rigid building blocks at the same

Figure 12.5 The assembly process of (MMT/BNMA@PVA/LDHs/BNMA@PVA)n UTFs: (A) a representation of MMT, pink: [AlO6] octahedron, green: [SiO4] tetrahedron, yellow: [MgO6] octahedron, (B) a nanosheet of MMT, (C) chemical formula of BNMA, (D) structure of BNMA, (E) chemical formula of PVA, (F) structure of PVA, (G) a representation of BNMA@PVA solution, (H) a representation of LDHs, pink: Al(OH)6 octahedra, green: Mg (OH)6 octahedra, (I) a nanosheet of LDHs, (J) the MTFs in one cycle (Liu et al., 2015).

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time. The scheme of BNMA with the interlayer between the MMT and LDHs monolayers is shown in Fig. 12.6. Yan et al. (2010) selected BNMA and polyvinyl sulfonate (PVS) as the model system, the BNMA@PVS pair in their mixed solution can be used as a whole to alternately assemble with exfoliated MgAl-LDH monolayers to obtain the (BNMA@PVS/LDHs)n UTFs. Furthermore, the multiple component luminescence adjustable UTFs can be fabricated, such as (APPP/LDH)n(BNMA@PVS/LDHs)m, (APPP/LDH)n(APPV/LDH)m, (BNMA@PVS/LDHs)n(APPV/LDH)m, and (BNMA @PVS/LDHs)m(APPV/LDH)q UTF with blue/green, blue/orange, red/blue, and red/green dual color luminescence, the assembly design is shown in Fig. 12.7 (Yan et al., 2011a). Block copolymer can form the micelle in water with hydrophobic core, which can contain the neutral organic small molecules with excellent optoelectric properties; these negative-charged block copolymer micelles can also be assembled with the positive charge of LDH nanosheets, to realize the small neutral molecules assembly with LDH. Li et al. (2012a) introduced the neutral bis (8-quinolinolato) zinc complex (Znq2) into block copolymer (poly(tert-butylacrylate-co-ethyl acrylate-co-methacrylate), PTBEM), and the negatively charged spherical micelles were alternately assembled with positively charged LDH nanosheets. The process for the fabrication of Znq2@PTBEM micelle and (Znq2@PTBEM/LDH)n UTFs is shown in Fig. 12.8. Li et al. (2015) introduced spiropyran (SP) into spherical micelles formed by PTBEM and alternately assembled with LDH nanosheets to form (SP@PTBEM/LDHs)n UTF with optical switching function. Qin et al. (2015) introduced the neutral dye molecule 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) into the block copolymer polystyrene- b-polyacrylic

Figure 12.6 The scheme of BNMA in an electronic microenvironment between the MMT and LDH monolayers. The thick green arrows indicate the EME’s direction. For the BNMA cations, the blue dotted lines show the repulsion of LDH nanosheets, and the red dashed lines show the attraction of MMT nanosheets, and between the LDHs and MMT nanosheets, there exist the electrostatic attractive interaction (dashed green arrows) (Wang et al., 2014).

Figure 12.7 (A) Representation of one monolayer of MgAl-layered double hydroxide (MgAl-LDH) (dark pink: Al(OH)6octahedra; green: Mg(OH)6 octahedra); the chemical formulae of: (B) APPP (blue luminescence), (C) BNMA@PVS (green luminescence), (D) APPV (orange luminescence), and (E) APT (red luminescence); (F) the typical procedure for assembling dual-color-emitting UTFs with blue/green, blue/ orange, red/blue, and red/green luminescence (Yan et al., 2011a).

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Figure 12.8 Schematic representation of the PTBEM, Znq2, and MgAl/LDH nanosheet and the process for fabrication of Znq2@PTBEM micelle and (Znq2@PTBEM/LDH)n UTF (Li et al., 2012a).

acid (PS-b-PAA). The anionic micelles were then assembled with LDH nanosheets alternately, shaped (PS-b-PAA@DCM/LDH)n UTFs that can be used to detect the common volatile organic compound (VOC) vapors.

12.1.2.2 Layer-by-layer assembly method based on hydrogen bond interactions In addition to employing the electrostatic interaction between LDH nanosheets and polyanions to fabricate the LDH/CP UTF system, other weak interactions, such as hydrogen bonds, van der Waals forces, hydrophobic interactions, and so on can also be used as a driving force for the assembly of LDH/CP UTFs. Besides electrostatic assembly, the LDH nanosheets and CP molecules can also be interacted by hydrogen bonding due to the abundant hydroxyl groups in the LDH layers and water molecules within the interlayers to prepare a composite thin film. Neutral conjugate polymer with OH, NH2 groups can be selected and assembled with LDH nanosheets to form a LDH/CP composite UTF; the assembly process diagram is shown in Fig. 12.9. Han et al. (2011) fabricated (PVA/LDH)n UTF with the LbL assembly method based on hydrogen bond LbL assembly; polyvinyl alcohol (PVA) is an electrically neutral polymer that cannot be assembled by electrostatic forces with LDHs, whereas both PVA and LDHs are rich in OH group, suggesting a hydrogen bond interaction is possible between them, which has been investigated by infrared spectroscopy and molecular dynamics simulations. Huang et al. (2009) fabricated (PVA/MMT/PVA/LDH)n with LbL hydrogen bonding assembly. Based on the multilayer assembly driving force of hydrogen bonds between LDH and neutral polymer, the more neutral polymer can be used as a carrier to

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Figure 12.9 The assembly process diagram of LDH nanosheets and neutral polymer interaction based on hydrogen bonding interaction (Han et al., 2011).

realize the coassembly of the neutral small molecules with the LDH nanosheets. Chen et al. (2010) fabricated (PVA@GO/LDH)n UTFs with the LbL assembly method based on the hydrogen bonding interaction. Li et al. (2012b) assembled the MgAl-LDH nanosheets with a (polyvinyl carbazole (PVK)/perylene) pair on the basis of hydrogen bonding interaction to fabricate the (PVK@ perylene/LDHs)n UTFs on the quartz substrate. A schematic diagram of the LbL assembly procedure is shown in Fig. 12.10. The contained N atoms in PVK exerted the hydrogen-bonding interaction with the OH group on the LDH layer. This method has some universality, and many molecules can be assembled based on the hydrogen-bonding interaction with LDH nanosheets to form various UTFs, and the hydrogen-bonding-based assembly cycle can be repeated up to 50 times, being comparable with electrostatics assembly. Furthermore, the neutral complex Ir (F2ppy)3 can be mixed with (PVK) to form the (Ir(F2ppy)3@PVK) pair, which can be alternately assembled with the LDH nanosheets to form the (Ir(F2ppy)3@PVK/ LDH)n UTFs for the detection of VOCs (Qin et al., 2014). This method utilizes the hydrogen-bonding interaction between the neutral PVK with carbazole groups and LDH nanosheets with numerous hydroxyl groups to realize the encapsulation of neutral (Ir(F2ppy)3) complex into the interlayers of LDH nanosheets. The assembly process is shown in Fig. 12.11. Qin et al. (2016) fabricated (Alq3@DCM@PVK/LDH)n and (Ir(F2ppy)3@DCM@PVK/LDH)n nanocomposite UTFs with a 2D cascade

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Figure 12.10 Schematic representation of the fabrication process of the (PVK@perylene/ LDH)n UTFs, the molecular structure of perylene and PVK (top): (A) the pretreated quartz substrate, (B) Mg-Al LDH nanosheets, and (C) perylene@PVK complex (Li et al., 2012b).

Figure 12.11 Coassembly of the (Ir(F2ppy)3@PVK/LDH)n UTFs (Qin et al., 2014).

FRET process. Using the PVK polymer as a carrier, neutral molecules, Alq3, Ir (F2ppy)3 and DCM were successfully encapsulated within LDH layers to investigate the 2D cascade FRET process, the assembly process was shown in Fig. 12.12. The morphology and supramolecular structure of the luminescent UTFs were studied for the PVK@DCM/LDHs (F1), PVK@Alq3@DCM/LDHs (F2), and PVK@Ir (F2ppy)3 @DCM/LDHs (F3) UTFs. As shown in Fig. 12.13, the SAXRD showed that the Bragg peak at 2θ 5 1.34 , 1.37 , and 1.32 , indicated the periodic spacings about 6.58, 6.44, and 6.68 nm, corresponding to F1, F2, and F3 films, respectively, that is the UTFs had an ordered periodic structure along the film, normally up to 25 bilayers. The top-view SEM images showed the UTFs were smooth and the side-view SEM images showed the thickness of F1, F2, and F3 UTFs were about 122, 119, and 124 nm (n 5 18), respectively, Thus, the UTF average

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Figure 12.12 (A) The scheme of the one- and two-step cascade FRET process, (B) molecular structures of the donor and acceptor, and (C) schematic representation of the fabrication of thin films process (Qin et al., 2016).

Figure 12.13 (Left) Small-angle XRD patterns for the UTFs-25: (a) F1, (b) F2, and (c) F3 UTFs. (Right) The structural and morphological characterization of F1, F2, and F3 UTFs-18: (A) top-view, (B) side-view SEM images, (C) fluorescence microscopy image, and (D) AFM images (Qin et al., 2016).

thickness of one bilayer can be calculated to be 6.78, 6.62, and 6.89 nm, which is congruent with the SAXRD results. The luminescent UTFs show a homogeneous bright orange color under a fluorescence microscope (Fig. 12.13, right, C),

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demonstrating that the fluorophores are distributed uniformly throughout the UTF. The AFM images of the fabricated UTFs illustrate that the root-mean-square (rms) roughness was 8 nm, and the roughness of F3 UTFs increased gradually from 5.40 to 12.75 nm as the n increased from 9 to 18, indicating a relatively smooth surface of these UTFs.

12.1.2.3 Layer-by-layer assembly method based on van der Waals forces Zhang et al. (2016) have illustrated the assembly of neutral conjugated polymer and LDH nanosheets based on van der Waals forces. Poly[(9,9-dihexylfuoreny-2,7diyl)-co-(9-ethylcarbazole-2,7-diyl)] (PFH-Ec), poly(9,9-n-diylhexyl-2,7-fuorenealt-9-phenyl -3,6-carbazole) (PFPC), poly(3-hexylthiophene-2,5-diyl)(P3HT) poly (2,5-di(2’-ethyl- hexyl)phenylene-1,4-ethynylene) (PPE), poly(9,9-n-dihexylfluorene-2,7-diyl)(PHF), polyphenylene vinylene (PPV), and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (Fig. 12.14) were assembled with LDH nanosheets, respectively, and the UV absorption and luminescence spectroscopy of these LDH/ CP UTFs are consistent with those of the corresponding neutral conjugated polymers, and the absorbance increases linearly with increasing number of assembly cycles. It was found that the PPE, PPV, PHF, and P3HT were neutral conjugated polymers without polar groups, such as OH, NH2, etc., and the electronegative heteroatoms such as O, F, and N. Therefore, it is speculated that the assembly driving force of these LDH/CP UTFs should be different from the electrostatic interaction and hydrogen bonding; there may be weak van der Waals force (such as polarization, deformation, dipole, etc.). It is possible that the interaction between the positively charged LDH nanosheets and the delocalized π electrons on the main chain of the conjugated polymer play an indispensible role in the LbL assembly for these CH3

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Figure 12.14 The molecular formulae of conjugated polymer assembly with LDH nanosheets through van der Waal’s forces (Zhang et al., 2016).

OCH3

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LDH/CP UTFs, and the specific reasons and evidence need to be further studied. There are also other issues worthy of further study for this assembly method, such as: how to balance the positive charge of the LDH hydroxides, the interaction between inorganic anions (NO32) existing in the interlayer and the neutral guest molecule.

12.1.2.4 Layer-by-layer assembly based on miscellaneous interaction Except the above method for the fabrication of LDH/CP UTFs, other interactions, such as magnetic-field-assisted assembly, and miscellaneous interaction including electrostatic, hydrogen bonding, and van der Waals force can also be used as a driving force for preparing LDH/CP UTFs. Shao et al. (2011) constructed the (CoFeLDH/MnTPPS)n UTFs through magnetic-field-assisted assembly, as shown in Fig. 12.15. Nowadays, biotechnology and nanotechnology are the two main trends in the development of science and technology, and the combination of these two fields has been paid more and more attention. The study of the analysis and identification of proteins and other biological small molecules is also significant (Han et al., 2011; Ji et al., 2008). Bioinorganic nanocomposites as the material basis of bionanotechnology have become the research focus. At present, the development of new biological/inorganic nanocomposites is relatively less commonly reported. In recent years, composite materials based on biomolecules and LDHs has also been of interest, with the development of LDH-based composites (Wu and Schanze, 2014; Chen et al., 2013; Mary-Ann et al., 2008; Bellezza et al., 2009; Vial et al., 2008; Le´a et al., 2006; Hu et al., 2013). For biomolecules, it is difficult to combine them with LDH nanosheets by conventional assembly methods without sacrificing their biological structure and functions. Positively charged LDH nanosheets with negatively charged biomolecules can be combined by the LbL assembly technique with miscellaneous interaction to obtain biomolecule/LDH composite UTFs. For

Figure 12.15 Schematic representation for the MFA LBL assembly of the (CoFe-LDH/ MnTPPS)n UTFs (Shao et al., 2011).

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complex biological macromolecules, such as proteins and nucleic acids, we can use their unique groups, based on electrostatic, hydrogen bonding, and van der Waals force to assemble LDH nanosheets, to fabricate a new kind of bioinorganic composite UTF functional material. Nucleic acids are mainly located in the cell nucleus, playing the role of storage and transmission of genetic information in protein replication and synthesis. Nucleic acids are not only the basic genetic carrier, but also play an important role in protein biosynthesis, individual growth, gene expression, and a series of major life phenomena. Nucleic acids were composed of nitrogenous organic bases, pentose, and phosphoric acids, and are a typical polyanion within hydrogen-bonding groups. Therefore, nucleic acid can be immobilized on the LDH interlayer to form a nucleic acid/inorganic composite UTF based on a variety of forces. For example, Shi et al. (2014) utilized MgAl-LDH nanosheets with DNA layers to prepare (DNA/LDH)n UTF. The (DNA/LDH)n UTFs were immersed in porphyrin (TMPyP) with chiral chromosphere solution, (TMPyP@DNA/LDH)n was obtained through the LbL method. These (TMPyP@DNA/LDH)n UTFs under different external conditions, had a binding arrangement between TMPyP and intercalated DNA in LDHs, resulting in different induced CD signals, so that the film can be used as a chiral optical switch. The characterization of (DNA/LDH)n UTF by LbL assembly was studied as shown in Fig. 12.16 by morphological characterization techniques like SEM, XRD, and AFM. Compared with exfoliated LDH nanosheets, the (DNA/LDH)n UTFs display a narrow, symmetric, and strong Bragg diffraction reflection at 2θ 5 3.98 , whose intensity is enhanced linearly along with an increase in bilayer number. The SEM image shows the average repeating distance is B2.58 nm, which is approximately consistent with the thickness augment per deposited cycle (B2.65 nm). DNA fragments and genetic units control basic biological function, such as the telomere which is a special junction of the eukaryotic chromosome end, and the human telomeres are essentially repeating nontranscribed sequences (TTAGGG) and some binding proteins to form a special structure. The telomere is closely related to cell apoptosis, cell transformation, and immortalization, and is the mitotic clock of cell life. When cells divide, DNA is replicated once, telomeres shorten the point, so telomere length reflects the cell copy history and replication potential. The greater the number of cell divisions, the more the telomere wears, and the more life is shortened; severely shortened telomeres are signals of cell aging. Recently, our research group has carried out the detection of different lengths of telomere fragments, based on the driving force of electrostatic and hydrogen bonding interaction, the fluorescent dyes, and long-chain telomere simulation nucleic acid were blended and assembled with LDH nanosheets to form a complex (fluorescent dyes @ ssDNA/LDHs)n UTF, the telomere fragments of its complementary sequence were tested to obtain the fluorescence changes for different lengths of telomere in the physiological condition, which show the potential application value in monitoring the shortening of telomeres. From the chemistry point of view, the protein is a class of organic macromolecules with a basic unit of more than 20 kinds of amino acids. There is some residual

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Figure 12.16 (A) XRD patterns of the exfoliated LDH nanosheets and the (DNA/LDH)n UTFs (n 5 10, 15, and 20). (B) Top view of the SEM image for the (DNA/LDH)20 UTF (inset: cross-section of the SEM image). (C) AFM image and (D) high-magnification SEM image of the (DNA/LDH)20 UTF (Shi et al., 2014).

within this amino acid containing carboxyl, amino, hydroxyl, mercapto, and other polar groups, which can be interacted with LDH nanosheets to prepare UTFs. Therefore, more protein molecules can be intercalated into the LDH nanosheet to obtain the protein/LDH UTFs. Hemoglobin (HB) and horseradish oxygenase (HRP) and NiAl-LDHs nanosheets were assembled by the LbL method; and (LDH/HB/ LDH/HRP)n UTFs were obtained (Kong et al., 2010). Green fluorescent protein (GFP) is a bioluminescent composed of 238 amino acids, which exhibited bright green fluorescence under UV light excitation. The immobilization of GFP is of significant interest for applications in biosensing due to its exquisite biological functions. However, there are some challenges for immobilization and application due to its vulnerable and sophisticated 3D structures. To keeping the 3D structure and biofunction of GFP, the LDH nanosheets from the colloidal mill method were employed to realize the assembly in the aqueous solution (Zhang et al., 2017b). Based on the special structure of fibrous silk fibroin (SF), and the interactions between SF and the CdTd QDs, the (CdTe QDs@SF/LDH)n UTF was fabricated,

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which showed the enhanced luminescence of CdTe QDs due to the β-sheet SF, compared with their solution counterpart. It has been found that these novel biomolecule/LDH UTFs by LbL assembly based on miscellaneous interactions have great potential applications in biological detection and sensoring.

12.2

Optical and optoelectric properties of LDH/CP nanocomposites

12.2.1 Optical properties 12.2.1.1 Photostability of LDH/CP nanocomposites Due to the intrinsic shortcoming of conjugated polymers in the application, including aggregation quenching, poor solubility, short service life, relatively poor thermal or optical stability, and various conjugated polymers were designed and synthesized to overcome these drawbacks. In the LDH/CP nanocomposites, the UV-proof properties of the LDH hydroxide layers protect the intercalated conjugated polymer from photodegradation, and have good light-resistant performance. Yan et al. (2009) fabricated (APPP/LDH)n UTFs with blue luminescence by LbL assembly and found that (APPP/LDH)n UTFs have longer fluorescence lifetimes and a higher photostability for UV irradiation than the counterpart samples, APPP/polycation UTFs. The alternative assembly of APPP with LDH nanosheets results in ordered stacking to form LDH/CP nanocomposites with well-defined blue photoluminescence and prolonged fluorescence lifetimes, which confirms that the LDH monolayers improved the luminescence properties of APPP by avoiding the formation of π-π stacking structure. Moreover, the existence of LDH monolayer leads to higher UV photostability for the blue luminescence of APPP as shown in Fig. 12.17. The LDH monolayer is rigid and layered in comparison to flexible polyelectrolyte. Thus, alternative assembly of conjugated polymer with rigid LDH monolayers can result in new inorganic/organic hybrid UTFs, in which LDH monolayers can provide more constraints for reducing the π π stacking and suppressing thermal vibrations of polymers, and the nonradiative relaxation of excited states within polymer backbone. In addition, the presence of the inorganic layer can improve the photostability of CP. Yan et al. (2011b) fabricated luminescent ordered SPT/LDH UTFs by the LbL assembly method with good luminescent performance. The sulfonated polythiophene (SPT) polymer with red luminescence showed rather poor optical properties owing to low-band gap accompanied by strong nonradiative relaxation and π π stacking interactions in its solution. The luminescence properties of SPT/LDH polymer nanocomposites confirmed that LDH monolayers improved the luminescence properties of SPT by avoiding the formation of π π stacking of polymer backbones. Moreover, the existence of LDH monolayer leads to higher UV photostability for the red luminescence of SPT. The UV-resistant capabilities of the (SPT/ LDH)32 UTF are shown in Fig. 12.18.

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Figure 12.17 Decay of normalized maximal PL intensity IPL with time, normalized against the initial PL value; λex 5 365 nm to probe the UV irradiation resistance ability of (APPP/ PDDA)27 (black data points) and (APPP/LDH)27 UTF (red) irradiated under 344 nm UV light. Insets: photographs under UV light of (a) (APPP/PDDA)27 and (b) (APPP/LDH)27 UTF after the UV resistance experiment was finished (Yan et al., 2009).

Figure 12.18 The decay of the normalized maximal luminescence intensity with irradiation time (360 nm UV light) demonstrating the different UV-resistant capabilities of the (SPT/ LDH)32 UTF and the SPT drop-cast film (Yan et al., 2011b).

Based on the electrostatic interaction between LDHs and guest molecules, researchers achieved a number of LDH/CP nanocomposites with enhanced luminescence properties. The LDH/CP nanocomposite UTFs have an inorganic organic hybrid quantum well structure with the electric insulating LDH hydroxide layers as

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the energy barrier, to effectively separate the conjugated polymer within the 2D confinement environments, thus avoiding blue/red shift of luminescence based on π π interaction, therefore LDH/CP nanocomposites have improved luminescence properties. With the coassembly method, the polyanions can be used as a support to achieve a small cation@polyanion and electrically neutral organic fluorescent guest molecules/LDH UTFs. These luminescent UTFs have better photostability as well as fluorescent intensity.

12.2.1.2 Luminescence properties of LDH/conjugated polymer nanocomposites and applications According to the luminescence principles and our research conclusion, the criteria for getting good luminescence properties from LDH/CP nanocomposites can be summarized as follows: (1) the interlayered polymer or small molecules were dispersed homogeneously without concentration-quenching phenomenon. (2) The reversal of interlayer polarity from hydrophilic to hydrophobic ones was necessary for the homogeneous distribution of some neutral polymers and small molecules to achieve the good luminescence properties. (3) The interlayered two-dimensional confined space between the LDH hydroxide layers suppresses the vibration of the assembled molecule, which is favorable for enhancing the fluorescence properties of the polymer and small molecules. The LDH/CP nanocomposite UTFs realized the immobilization of fluorescent molecules, which is favored for the device application of these fluorescent molecules in light-emitting diodes, solar cells, and biological and chemical sensors. These immobilized conjugated polymers sometimes exhibited improved photostability, enhanced luminescence performance, and some novel luminescence properties. Yan et al. (2011b) fabricated an (SPT/LDH)n UTF showing red luminescence and reversible pH photoresponse. This UTF showed the polarized luminescence due to the incorporation of a photoactive polymer within a 2D interlayer of LDHs. They constructed the (Ir (F2ppy)3@PVK/LDH)n UTFs, which exhibited the fast, sensitive, and reversible response to common VOCs (Qin et al., 2016). The reversible luminescence response of this nanocomposite UTF is as shown in Fig. 12.19. They also constructed (PS-PAA@DCM/LDH)n UTFs, which exhibited solvatochromism luminescence in different solvent vapors, and can be applied in sensing for solvent polarity (Fig. 12.20) (Qin et al., 2015). Han et al. (2010) reported the preparation of reversible photoresponsive UTFs using a photoactive azobenzene polymer, poly{1 4[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1, 2-ethanediyl sodium salt} (PAZO) and exfoliated LDH nanosheets. In (PAZO/LDH)n multilayer films, azobenzene chromophores exhibited reversible trans cis photoisomerization. The isolation effect of LDH nanosheets imposes enough free volume for the photoisomerization of the azobenzene group in PAZO, accounting for its complete trans cis isomerization as well as high reversibility and reproducibility. These LDH/CP nanocomposites incorporated a photoactive moiety, providing an attractive and feasible methodology for

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Figure 12.19 Reversible luminescence response of (Ir(F2ppy)3@PVK/LDH)n UTFs for VOCs (Qin et al., 2014).

(A)

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Figure 12.20 Reversible photoluminescence response of the (PS-PAA@DCM/LDH)n UTFs to VOC polarity (Qin et al., 2015).

creating light-sensitive materials and devices with potential photo read/write capabilities; these UTFs can be potentially applied in the fields of optical coatings, photosensors, and optical information storage. The (LDH/HB/LDH/HRP)n UTF (Kong et al., 2010) modified electrode has good electrocatalytic behavior for substrate catechol, and has good reproducibility and storage stability. GFP/LDHs UTFs realize the immobilization of GFP (Zhang et al., 2017b), which was highly responsive to pH and could be detected in some small biological molecules. (QDs@SF/LDH) UTFs have enhanced fluorescence, longer lifetime, and larger quantum yield than QD aqueous solution, and displayed a fluorescence response to immune globulin. By introducing biomolecules with different functions between the LDH nanosheets, the biomolecules can be effectively protected without losing their biological function, and these biological/inorganic nanocomposites will be widely used in biosensing, bioidentification, biochemical analysis, biomarkers, bio-imaging, drug release, and so on.

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12.2.1.3 Fluorescence resonance energy transfer (FRET) of LDH/CP nanocomposites Fluorescence resonance energy transfer (FRET) is an energy transfer phenomenon between two fluorescent species [energy donor/accepter (D/A)] that are close to each other, and when the emission spectrum of the energy donor overlaps with the absorption spectrum of the energy acceptor, and the spacing between the two molecules is less than 10 nm, a nonradioactive energy transfer named FRET occurs, in which the fluorescence intensity of the donor is much lower or quenched sometimes, while the fluorescence of the acceptor is greatly enhanced. Within the LDH/CP nanocomposite UTFs, an effective two-dimensional FRET process can be realized with the luminous efficiency of the UTF and the lifetime of the luminescence has been greatly improved. The neutral polymer-polyvinyl carbazole (PVK), neutral fluorescent small molecule 2 perylene, and LDH nanosheets were coassembled to obtain ((perylene@PVK)/LDH)n nanocomposite UTFs by the hydrogen bonding LbL assembly method. The effective 2D FRET process within the interlayers of LDHs was realized. This 2D FRET process can be interrupted by common VOC vapor, which can be used as a new VOC fluorescence sensor (Qin et al., 2014). The possible mechanism is shown in Fig. 12.21. Qin et al. (2014) coassembled the neutral polymer-polyvinyl carbazole (PVK), neutral phosphorescent small molecule 2 iridium metal complex (Ir (F2ppy)3), and LDH nanosheets to obtain (PVK@Ir (F2ppy)3)/LDH)n nanocomposite UTFs by the hydrogen bonding LbL assembly method. The study indicated that the PVK and Ir (F2ppy)3 molecules worked as energy donor and acceptor, respectively, and can realize effective 2D FRET process within the interlayers of LDHs, the luminous efficiency and lifetime of the phosphorescence of Ir (F2ppy)3 have been greatly improved, as shown in Fig. 12.22, as has the lifetime of the acceptor and donor in the UTF, as shown in Fig. 12.23. Due to the confinement effect of LDH, the spacing between PVK and Ir (F2ppy)3 was restricted to within less than 10 nm, which is beneficial to improve the FRET efficiency and the luminous efficiency of UTF. Compared to the traditional FRET system with a D-A pair, the multistep cascade FRET system has two or more D-A pairs and has more advantages: larger Stoke shift, stronger emission intensity, and lower detection limits. Three different FRET

Figure 12.21 A representation of the reversible luminescence response to the VOC stimulus based on the FRET process in (perylene@PVK/LDH)n UTFs (Li et al., 2012b).

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250 (PVK/LDH)25 UTFs 200

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Figure 12.22 Fluorescence spectra of (PVK/LDH)25 film excited at 294 nm, and (Ir (F2ppy)3@PVK/LDH)25 film excited at 294 and 434 nm (Qin et al., 2014).

Figure 12.23 (A) The luminescence decay curves of PVK (at 407 nm) of (PVK/LDH)25 (a); PVK (at 375 nm) (b); and Ir (F2ppy)3(at 471 nm) of UTF-25 (c) excited at 294 nm; inset is the magnified decay curve in the range of 0 50 ns. (B) The luminescence decay curves of Ir (F2ppy)3: in toluene solution (at 473 nm) excited at 434 nm (a); in Ir (F2ppy)3@PVK (8 wt %) drop-casted films (at 471 nm) (b); in UTF-25 (at 471 nm), both excited at 294 nm (c) (Qin et al., 2014).

luminescent films were successfully prepared based on the hydrogen-bonding LbL assembly method, (PVK@DCM/LDHs)n, (PVK@Alq3@DCM/LDHs)n, and (PVK@Ir(F2ppy)3@DCM/LDHs) (denominated as F1, F2, and F3 UTF, respectively), all of which exhibited the UTF prominent luminescence performance owing to the 2D cascade FRET process (Qin et al., 2016). The UV absorption and fluorescence spectra showed that the multistep FRET process was successfully realized in the UTFs. Those LDH/CP nanocomposite UTFs obtained a significant enhancement

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of light emission and extended lifetime of DCM dye. The fluorescence spectra and luminescence decay curves of DCM (557 nm) in three different FRET systems are shown in Fig. 12.24. Moreover, the F3 nanocomposite UTFs show fast, sensitive, and selective fluorescence signal patterns toward four common volatile organic compounds (VOCs) based on interfering with the 2D cascade FRET process, implying its potential application in the VOC selective sensing field (Fig. 12.25).

Figure 12.24 (A) Fluorescence spectra and (B) luminescence decay curves of DCM (557 nm) in three different FRET systems UTFs-18: F1 (curve a), F2 (curve b), and F3 (curve c), excited by 300 nm. Inset: pictures of the corresponding films under UV irradiation (DCM doping concentration is 6%, 4%, and 9% for F1, F2, and F3UTFs, respectively) (Qin et al., 2016).

Figure 12.25 The luminescence spectra of F3 film in different VOC vapors (the table is the output fluorescence signal of the film exposed to VOC vapors, ε is the solvent dielectric constant) (A); photographs taken under 365 nm UV irradiation (B) and the reversible fluorescence response (C) toward nitrobenzene vapor for eight consecutive cycles (Qin et al., 2016).

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12.2.2 Optoelectric properties 12.2.2.1 Photodetectors Photodetectors that convert incident light into electrical signals to probe the light (wavelength and intensity) are widely used in industrial and scientific fields such as optical communications, aeronautical engineering, and biological and environmental surveys. Photodetectors based on novel inorganic nanostructures have been widely investigated, and exhibit excellent optoelectronic performances. However, photodetectors based on inorganic nanostructures need some complicated preparation technology and expensive equipment, which greatly restricts their practical applications. Furthermore, most inorganic photodetector materials have fixed light absorption and a narrow spectral sensitivity, which would be suitable for only fixed or narrow-band light detection. LDH/CP nanocomposites based on organic molecules with various types, broadband absorption range and excellent flexibility, have become suitable candidates for new-generation photodetectors. Zheng et al. (2016) coassembled a CP electron donor, poly[N-9’-heptadecanyl2,7- carbazole-alt-5,5-(4’,7’-di-2-thienyl 2’,1’,3’-benzothiadiazole)](PCDTBT), and electron acceptor, poly(5-(2-ethyl-hexyloxy)-2-methoxycyano-terephthalylidene) (CN-PPV) with Mg2Al-LDH nanosheets by hydrogen bonding LbL assembly method to obtain a novel photodetector based on two-dimensional (2D) confined CP electron donor 2 acceptor coassembled (PCDTBT@CN-PPV/LDHs)n UTFs. The photodetection mechanism of this UTF is shown in Fig. 12.26. As a novel photodetector, The UTFs exhibit broad-range visible-light absorption, from 400 to 650 nm,

Figure 12.26 (A) Schematic illustration of the (CN-PPV@PCDTBT/LDHs)20 UTF photodetector; (B) energy-level alignment of PCDTBT and CNPPV within the Mg2Al-LDH nanosheets under light irradiation; and (C) the proposed 2D PCT mechanism scheme (Zheng et al., 2016).

Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated

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Figure 12.27 (A) I 2 V relationships of the (PCDTBT@CN-PPV/LDHs)20 UTFs excited by light with different intensities, ranging from 0.5 to 150 mWcm22; (B) corresponding fitting power law curve of photocurrent and light intensity (Zheng et al., 2016).

resulting from complementary absorption of PCDTBT and CN-PPV polymers. The fluorescence emission of the UTFs is completely quenched, implying the occurrence of a photo-induced charge transfer (PCT) process. The coassembled UTFs have a high photocurrent and on/off switching ratio (300 nA/B120), in contrast to those of the PCDTBT/CN-PPV drop-casting thin film (5.4 nA/B1.6); a fast response; a short recovery time (lower than 0.1 s); and excellent wavelength and light-intensity dependence. The PCT mechanism can be attributed to the formation of a 2D bulk heterojunction of the two polymers within the interlayers of the LDH nanosheets. Furthermore, the flexible UTFs on polyethylene terephthalate substrates are also fabricated, exhibiting excellent folding strength and electrical stability. The photoresponsive intensity dependence of (PCDTBT@CN-PPV/ LDHs)n UTFs is shown in Fig. 12.27.

12.2.2.2 Photocatalysis There exist numerous organic molecules with various HOMO/LUMO electron levels, which could form an electron donor acceptor (D-A) system when in contact. The photocharge transfer process will occur when the D-A system has suitable energy level alignments which endow this system with various electric, magnetic, and photocatalytic properties. Organic D-A charge-transfer complex salt showed excellent conductivity and magnetic properties. The D-A pairs based on organic polymers and small-molecule semiconductors have been widely used in organic feld-effect transistors (OFETs), organic photoconductors, and D-A type heterojunctions as organic solar cells. In these D-A systems, the photo-induced electron transfer (PET) process across a D-A interface was the key process, dominating the efficiency of photoelectric conversion. However, the D-A organic systems were rarely used as photocatalyts for PEC water-splitting, possibly due to the expensive price of D-A molecules and the photo-instability of most organic molecules.

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Zheng et al. (2015) fabricated the DAS-DNS/LDHs composite system by cointercalating the 4,4-diaminostilbene-2,2-disulfonate (DAS) and 4,4-dinitro-stilbene2,2-disulfonate (DNS) anions into Zn2Al LDHs. The DAS-DNS/LDHs composite exhibited broad UV-visible light absorption and fluorescence quenching, which was a direct indication of the photo-induced electron transfer (PET) process between the intercalated DAS (donor) and DNS (acceptor). The cointercalated DAS/DNS anions were orderly aligned within the interlayers and the HOMO/LUMO energy levels of the intercalated DAS and DNS anions were affected to match and couple as the electron donor and acceptor, respectively. The DAS-DNS/LDH composite was fabricated as the photoanode and Pt as the cathode. Under UV-visible light illumination, the enhanced photo-generated current (4.67 mA/cm2 at 0.8 V vs. SCE) was generated in the external circuit, and the photoelectrochemical water split was realized. Furthermore, this photoelectrochemical water-splitting performance had excellent crystalline, electrochemical, and optical stability. Therefore, this novel inorganic/organic hybrid photoanode exhibited potential application prospects in photoelectrochemical water-splitting. The proposed 2D PET mechanism scheme and the energy-level alignment illustrated in Fig. 12.28 indicate that the photo-generated electron would transfer from DAS to DNS anions upon illumination. The 2D PET process was realized within the interlayers, which resulted in charge separation in the photoanode, and the free 0 V vs vacuum level

–1 e–

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Figure 12.28 The proposed 2D PET mechanism scheme of photo-generated current of DASDNS/LDHs photoanode and the energy level alignment of DAS and DNS anions in Zn2AlLDH interlayers, E (H1/H2) 5 0 0.059 pH (vs. NHE), pH 5 6.8, E (O2/H2O) 5 1.23 0.059 pH (vs. NHE), pH 5 6.8.

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electrons reached the Pt cathode to reduce the proton to hydrogen, while O2 was evolved at the photoanode by H2O molecules oxidized by the hole of the photoexcited DAS, and completed the entire PEC water-splitting. It can be concluded that the 2D confinement effect within the LDH interlayers can provide a good photocharge transfer condition for the electron donor and acceptor, and effectively realize the generation and separation of photo-generated electrons and holes. This novel inorganic/organic composite photoanode exhibited potential application prospect in PEC water-splitting, and the flexibility of organic D/A pair selection and cointercalation design for this inorganic/organic hybrid composite paved a broad and promising way to develop a kind of new, low-cost, and simple layered PEC system for solar energy conversion and application.

12.3

Conclusions and outlook

LDHs as a typical anionic inorganic layered material and its hydroxide layers can be exfoliated to form LDH nanosheets, which provide a basis for the construction of composite multifunctional film materials with various species. Based on the electrostatic interaction, the assembly of functional polyanions and LDH nanosheets can be achieved, a coassembly of small anions/cationic and polyanionic blends with LDH nanosheets was developed to construct the more LDH/CP nanocomposite UTFs. As a kind of novel nanocomposite, LDH/CP thin-film materials exhibited good photostability, and luminescent or optoelectric properties. Various assembly methods were developed to fabricate these nanocomposite materials based on the exfoliation and assembly features of LDH compounds. Based on the traditional electrostatic LbL method, hydrogen bond, van der Waals interaction, and miscellaneous interactions were employed into the LbL assembly process to encapsulate the small organic anions, cations, or neutral molecules/polymer into the interlayers of LDH nanosheets, which extended the assembled guest members for this nanocomposite and many neutral conjugated polymers, protein, and DNA/RNA can be assembled into the interlayers by this modified LbL assembly method. This LDH/ CP nanocomposite showed novel luminescence properties like 2D FRET process and PET process, which can be successfully applied into VOC probing, photocatalysis, and photodetectors. Thus, this nanocomposite has realized the immobilization of molecules with good photofunctional performance in a solution state, and is compatible for the device-orientation application involving photofunctional molecules/polymers. Above all, it is in the ascendance for LDH/CP nanocomposite photofunctional materials, which exhibited an unexpected performance based on the synergistic effect arising from the host guest interaction within this composite material. This chapter has summarized the current research status of the LDH/CP nanocomposites, from which it can be witnessed that the LDH nanosheets played a significant role in the implementation of photofunction of the guest molecules, like a 2D “molecular container” at the nanometer scale. It must be admitted that more open problems

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Layered Double Hydroxide Polymer Nanocomposites

still exist and are worth solving in this field, such as the detailed interlayer structure, the nonelectrostatic assembly mechanism, and the 2D confinement effect. This is not only an opportunity for developing novel composite materials, but also a challenge for inorganic chemists to explore its functional potential and application scope as far as they can.

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