Keywords

1 Introduction

Inorganic nanosheets are utilized for converting or harvesting energy. The modern world is reliant on the use of energy in various forms, including photo, electrical, chemical, potential, mechanical, nuclear, and thermal energy. Efficient interconversion among these energy forms is crucial to the functioning of today’s society. This chapter will focus on the conversion of photoenergy into other forms of energy using inorganic nanosheets. Inorganic nanosheet-based photoenergy conversion materials have an advantage due to their unique properties, such as highly crystalline two-dimensional surfaces, high aspect ratios, and the separation of the front and back sides by atomically thin surfaces. Inorganic nanosheet-based photoenergy conversion has been reported in various fields, and this chapter briefly reviews the conversion of photoenergy into chemical, electrical, and mechanical energy, as well as other photoenergies (wavelength conversion or light harvesting).

2 Conversion of Photoenergy into Chemical Energy

Photocatalysis, the general process of converting photoenergy into chemical energy, has been extensively investigated since Fujishima and Honda first reported photocatalytic water splitting using a titanium dioxide (TiO2) semiconductor [1]. TiO2 photocatalysis has been developed for use in both organic pollutant degradation and hydrogen generation via water splitting.

Among various oxide semiconductor photocatalysts [2], TiO2 has proven the most suitable for widespread use in solar energy conversion and environmental applications due to its chemical inertness, strong oxidizing power, low cost, and long-term stability. Moreover, exfoliated titanium oxide nanosheets have received considerable attention [3].

As exfoliated nanosheets in suspension provide almost infinite available spaces for guest species to homogeneously adsorb the nanosheet surfaces, nanosheet-based photocatalysts are expected to have unique characteristics.

The photocatalytic decomposition of harmful organic compounds in water or air is a promising process for environmental remediation and has been studied using titania nanosheet photocatalysts under UV irradiation. For example, the nanocomposite prepared by pillaring Ti0.91O 0.36–2 nanosheets with TiO2 nanoparticles via an exfoliation-reassembly process efficiently decomposed organic pollutants such as methylene blue and 4-chlorophenol under UV light irradiation [4].

Photocatalytic water splitting for hydrogen generation using titania nanosheets has also been reported. As the conduction band minimum position of layered titanate is only slightly higher than the redox couple potential of H+/H2, titania nanosheets do not provide strong reduction for water splitting. For example, nitrogen-doped titanate exhibited apparent visible-light absorption up to 450 nm [5], whereas its performance in water splitting for hydrogen generation was nearly negligible. Osterloh and co-workers systematically studied the change in the optical, vibrational, electronic, and photocatalytic properties of multi-layered titanates A2Ti4O9 (A = K, H, TBA) [6]. The catalytic activity of all titanates was limited by the potential of the conduction band edge, which is too low for efficient H2 evolution in a neutral solution (Fig. 14.1). With the assistance of cocatalysts, such as Pt metal, Ti0.91O 0.36−2 exhibited a moderate hydrogen production yield. Loading with Pt shifted the band edge of all titanates toward oxidizing potentials (Fig. 14.1), which was attributed to Fermi level equilibration across the titanate-platinum interface.

Fig. 14.1
figure 1

Band gap potentials of various layered titanate and Ti4O9 2− nanosheets determined by optical and photoelectrochemical measurements. Experimental data for TiO2 P25 nanoparticles are included for comparison. Reprinted with permission from Ref. [6] (copyright 2010, American Chemical Society)

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Layered niobate is one of the other examples of a nanosheet-based photocatalyst, although Nb2O5 is not a photocatalyst under UV irradiation [7]. Chemical exfoliation of K4Nb6O17 and the similarly structured Dion–Jacobsen phase KCa2Nb3O10 produces 1 nm thin single-crystalline nanosheets or scrolls. The crystalline nanosheets and their Pt and IrO2-modified derivatives showed photocatalytic activity in water splitting [7–9].

3 Conversion of Photoenergy into Electrical Energy

Devices used to convert photoenergy into electrical energy are generally called photovoltaic devices. Several types of efficient and mature photovoltaic devices have been developed, and can be distinguished according to their content and structure. Well-established photovoltaic materials on inorganic bases include silicon, III-V and II-VI PN junctions, and copper-indium-gallium-selenium (CIGS) photovoltaic devices [2].

As described in the previous chapter, TiO2 is the most common semiconductor photocatalyst, but it is also widely used in photovoltaic devices [2, 3]. The rapid transport and transfer of photogenerated electrons in these TiO2 films is pivotal to improving the conversion efficiency from photoenergy to electrical energy. Generally, TiO2 nanoparticles with a diameter of 10–20 nm were used; hence, the obtained film was usually transparent with little light scattering. The incorporation of relatively large particles (above 100 nm) has been employed at light-scattering centers to increase the optical length of the film, and enhanced light harvesting has been demonstrated [10]. The use of two-dimensional nanosheets with sizes approaching 100 nm should have a similar effect. As a result, such materials are expected to exhibit high light-harvesting efficiency and fast charge carrier motion due to the 2D crystalline surfaces.

To this end, Haga and co-workers reported novel photoelectrochemical devices using titania nanosheets with Zn porphyrin molecules fabricated by layer-by-layer techniques (Fig. 14.2) [11]. Closely packed titania nanosheet (Ti0.91O2) monolayers on indium tin oxide (ITO), mica, and quartz surfaces strongly adsorbed cationic [5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrinatozinc]4+ (ZnTMPyP4+) via electrostatic interactions.

Fig. 14.2
figure 2

a Schematic illustration of graduated multilayer film with Ag paste on an ITO electrode, b photocurrent action spectra of [(ZnTMPyP4+/titania nanosheet)5/ITO] + Ag paste (circle) and (ZnTMPyP4+/titania nanosheet)5/ITO (square). Reprinted with a slight modification with permission from ref [11] (copyright 2007, American Chemical Society)

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The visible-light irradiation of this multilayer film on an ITO electrode in the presence of triethanolamine as an electron donor resulted in an anodic photocurrent. The quantum yield for photocurrent generation by this two-layer film, with the electrode structure ITO/titania nanosheet/ZnTMPyP4+, was estimated to be 8.63%. The titania nanosheet itself has a wide band gap (3.8 eV) due to the 2D quantum size effect. This caused vertical electron transport efficiency, through the alternately layered titania nanosheets and organic dyes such as ZnTMPyP4+ on the solid surface, to lower as the number of layers increased due to the succession of vertical energy barriers. To overcome this limitation, the use of a lateral interlayer connection of all titania nanosheets with Ag paste was also examined, resulting in a significant improvement in photocurrent density compared with the bare titania nanosheet-ZnTMPyP4+ system. Other types of inorganic nanosheets, such as Ca2Nb3O10 −, TiNbO5 −, Ti2NbO7 −, and Ti5NbO14 3−, have also been reported as novel photovoltaic materials [12].

4 Conversion of Photoenergy into Mechanical Energy

Photoresponsive mechanical materials are usually defined as having macroscopic-scale morphologies that are reversibly changed by photoirradiation. These materials are promising for applications in light-driven actuators and artificial muscles that are able to convert the photoenergy into mechanical energy. Many excellent photoresponsive mechanical materials have been reported, such as molecular single crystals [13], and liquid crystal polymers [14].

For this purpose, Inoue and co-workers focused on the use of inorganic nanosheets [15, 16]. They reported that upon photoirradiation of layered K4Nb6O17 intercalated with a polyfluoroalkyl azobenzene surfactant, a very large magnitude lateral movement (sliding) of the nanosheets was reversibly induced. Nanosheet sliding has been clearly observed in a cross section of layered hybrid film, as a result of concomitant reversible cis-trans isomerization of the azobenzene moiety. Using photoirradiation at 368 and 463 nm, the total reversible sliding motion of the layered hybrid film reached ~1500 nm in the lateral direction. Furthermore, the thickness of the hybrid film (~450 nm) was reversibly changed by up to ±18 nm as each interlayer space shrank and expanded due to photoisomerization of the intercalated azobenzene derivative. This work shows that nanosheet-based photoinduced mechanical materials have an advantage due to their large two-dimensional surfaces and high aspect ratios.

5 Wavelength Conversion of Photoenergy and Light Harvesting

5.1 Theory of Förster Resonance Energy Transfer

Förster (or fluorescence) resonance energy transfer (FRET) is the transfer of photoexcitation energy from a donor fluorophore to an acceptor [17–19].

$${\text{A}}^{*} + {\text{B}} \to {\text{A}} + {\text{B}}^{*}$$
(14.1)

Here, * denotes the electrically excited state, A and B denote energy donor and energy acceptor molecules, respectively.

The reaction rate constant for FRET is expressed as shown in Eq. 14.2,

$$k_{\text{ET}} = \frac{{9\phi_{D} \kappa^{2} ln10}}{{128\pi^{5} n^{4} N_{A} \tau_{D} R^{6} }}\smallint f_{D} \left( \nu \right)\varepsilon_{A} \left( \nu \right)\frac{d\nu }{{\nu^{4} }}$$
(14.2)

where ϕ D is the fluorescence quantum yield of the donor, n is the refractive index of the medium, N A is Avogadro’s number, к 2 is the dipole orientation factor, τ D is the excited lifetime of the donor in the absence of an acceptor, R is the distance between the donor and the acceptor, and the integral part represents the spectral overlap between the absorption spectrum of the energy acceptor and fluorescence spectrum of the energy donor. As can be seen from Eq. 14.2, the energy-transfer rate constant is inversely proportional to the sixth power of R. A large spectral overlap integral between the absorption spectrum of the energy acceptor and the fluorescence spectrum of the energy donor is necessary for efficient energy transfer. Moreover, a parallel arrangement of the transition dipoles enhances the FRET efficiency and a perpendicular orientation prevents FRET (expressed as к 2 in Eq. 14.2) [20]. Generally, FRET phenomena are observable at donor–acceptor distances of around 20–60 Å when the spectral overlap is large enough [21–27].

Since FRET is a highly sensitive spectroscopic measurement technique for processes occurring on the nanometer and sub-nanometer scale, it has been widely used for qualitative and quantitative biochemical applications, such as DNA sequencing [28]. Moreover, a light-harvesting antenna in natural photosynthetic bacteria realizes highly efficient photoexcited energy-transfer processes for photosynthesis [29]. This has strongly encouraged researchers to study synthetic molecular or supramolecular FRET systems for artificial photosynthesis and artificial light-harvesting systems.

While research on FRET has been mainly focused on synthetic molecular systems, supramolecular biosystems (with DNA, peptides, etc.), or quantum dots, nanosheets have been well used in the twenty-first century [30]. The methodologies of using nanosheets can be categorized into two series: (1) the use of nonphotofunctional nanosheets as FRET reaction fields for organic guests, and (2) the use of photofunctional nanosheets as a FRET donor or acceptor. The advantages of such FRET research on 2D nanosheets are (i) effective control of к 2 according to the 2D reaction field [20], (ii) ease of handling the materials due to their large size (several tens to thousands of nm), (iii) additionally, the photophysical characteristics of organic guests such as fluorescence quantum yields or lifetimes are sometimes enhanced [31, 32] by adsorption onto atomically flat 2D surfaces. Recent results on 2-dimensional FRET utilizing nanosheets will be reviewed.

5.2 Nonphotofunctional Nanosheets as FRET Reaction Fields for Organic Guests

Typical, anionic charged-clay minerals are useful materials for nonphotofunctional nanosheets because of their chemical stability, optical transparency in the visible region, and ease of handling [33]. Cationic guest molecules can form stable complexes through electrostatic interactions (and through hydrophobic interactions when applied in aqueous solution).

From the various types of anionic clay minerals, saponite (of the smectite group) is the focus of this chapter due to its suitable charge density. The typical chemical formula of saponite clay is expressed as [(Si7.20Al0.80)-(Mg5.97Al0.03)O20(OH)4]−0.77(Na0.77)+0.77. The experiments presented here have been carried out using an aqueous suspension of exfoliated single saponite nanosheets at low concentrations.

It is well known that organic molecules tend to aggregate and/or segregate on inorganic surfaces, mainly because of hydrophobic interactions and Van der Waals interactions between the organic molecules [34, 35]. Since H-aggregation and irregular aggregation drastically decrease the excited-state lifetime, efficient photochemical reactions such as FRET had been considered difficult in clay/dye complexes.

Uniform molecular adsorption on saponite surfaces was realized in the pioneering work by Takagi and Inoue in 2002 (Fig. 14.3) [36, 37]. Systematic experiments using numerous cationic porphyrin derivatives showed that a porphyrin molecule, which has a specific molecular structure, can be adsorbed on clay surfaces without concomitant aggregation, even under high-density conditions. Such nonaggregated alignment results from distance matching due to guest—host Coulombic interactions between the positively charged dye molecules and the negatively charged-clay surface. This effect was termed the “size-matching effect.” The average center-to-center intermolecular distance was determined to be 2.4 nm under saturated adsorption conditions based on a hexagonal array. This value is most interesting from the viewpoint of photochemistry, because the interaction between the transition moments of adjacent porphyrin molecules (aggregation) is negligible; however, interactions in the excited state are possible. Thus, efficient photochemical reactions such as FRET are expected.

Fig. 14.3
figure 3

Schematic representation of a saponite/porphyrin supramolecular complex. The guest porphyrins adsorb on the saponite surface as monomers without aggregation through the “size-matching effect.” Reprinted with permission from Ref [38] with a slight modification (copyright 2011, American Chemical Society)

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FRET between two different kinds of porphyrins (m-TMPyP(D) and p-TMPyP(A)) was investigated by steady-state and time-resolved fluorescence measurements [38]. A decrease in donor fluorescence and an increase in acceptor fluorescence were observed as the dye loading increased (Fig. 14.4 left). Clear isoemissive points were observed, as indicated by the arrows in the figure.

Fig. 14.4
figure 4

Left. The fluorescence spectra for m-TMPyP(D)/p-TMPyP(A)/saponite complexes. The thick lines are the individual fluorescence spectra of the m-TMPyP(D)/saponite and p-TMPyP(A)/saponite complexes. The arrows indicate isoemissive points. Right. The energy-transfer efficiencies at various loading levels and ratios of porphyrins. Reprinted with permission from Ref. [38] (copyright 2011, American Chemical Society)

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In Fig. 14.4 right, the energy-transfer efficiencies (η ET) are plotted against the porphyrin loading levels. η ET increased with increasing porphyrin loading levels, because the average intermolecular distance between the porphyrins decreased. Notably, the maximum η ET reached ~100% at higher acceptor ratios and dye loadings. The mechanism of the FRET process has been investigated by time-resolved fluorescence spectroscopy.

In spite of the advantages of saponite–porphyrin systems, a limitation is that anionic clay nanosheets could only be used as hosts for cationic adsorbent molecules. Neutral molecules, such as aromatics, do not adsorb on these surfaces. A new strategy has been developed to overcome this problem. The strategy involved a supramolecular approach in which a cationic organic host system, which included a neutral guest molecule, was adsorbed on anionic saponite surfaces [39, 40].

A cavitand containing eight ammonium groups (octaamine, OAm [41]) was used as the host for neutral organic molecules (Fig. 14.5). Strong Coulombic attractions between the ammonium groups and the anionic saponite anchored OAm to the nanosheets. Since OAm forms capsular assemblies with neutral organic guest molecules, energy transfer between two OAm capsules, one containing a donor molecule (pyrene, 2) and the other encapsulating an acceptor molecule (2-acetylanthracene, 3), adsorbed on saponite surfaces was investigated [40]. The efficient FRET between 2@OAm2 16+ (pyrene encapsulated within two OAm8+ species) and 3@OAm2 16+ adsorbed on saponite nanosheets was demonstrated by monitoring the fluorescence intensities and lifetimes. With increased loading levels of guests, the fluorescence intensity of donor 2 decreased, while that of acceptor 3 increased (Fig. 14.5 right). This suggested the possibility of FRET in the current system. η ET was determined to be 85%. As expected, owing to the decreased intermolecular distance, η ET increased with the loading level. By the detailed time-resolved fluorescence measurement, FRET rate constant was calculated to be 1.4 × 107 s−1.

Fig. 14.5
figure 5

Structure of octaamine cavitand (OAm, 1), a plausible adsorption structure model of guest@OAm2 16+ on saponite surfaces under maximum adsorption conditions, and fluorescence spectra for the {2@OAm2 16+ + 3@OAm2 16+}∩clay complex and respective 2@OAm2 16+∩clay and 3@OAm2 16+∩clay complexes. Reprinted with permission from Ref. [40] with a slight modification (copyright 2013, American Chemical Society)

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As summarized above, supramolecular surface photochemistry has undergone significant development recently [37, 38, 42–44]. Recent developments and understandings of molecular attitudes on inorganic surfaces greatly encourage the application of these nanosheet–dye systems in photonics fields.

5.3 Photofunctional Nanosheets as FRET Donor/Acceptors

Some photofunctional nanosheets, such as graphene oxide or transition metal dichalcogenides, have been used as FRET donor or acceptors in combination with other organic dyes or quantum dots. In this chapter, FRET phenomena utilizing the excited state of nanosheet materials will be reviewed.

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms ordered in a two-dimensional honeycomb lattice [45]. Graphene oxide (GO) consists of a similar atomically thin structural lattice, but possesses oxygen-containing functional groups [46]. Consequently, GO possesses recombined electron–hole pairs localized within a small sp2 carbon domain embedded in a sp3 matrix, and displays photoluminescence properties. Due to its heterogeneous structure containing various densities and distributions of carboxyl or hydroxyl groups, predominantly arising from differences in synthesis processes, GO can be fluorescent over a broad range of wavelengths from the near IR to UV regions. Thus, GO can be used as a FRET donor or acceptor when its emission wavelength is properly controlled [47–49].

Dong and co-workers demonstrated the first efficient FRET between quantum dots (QDs) and GO for biosensing applications [50]. Specifically, CdTe QDs, conjugated with a molecular beacon (MB), were used to detect DNA concentration and sequence, as well as to detect proteins such as thrombin. By the surface modification of QDs with a MB, the FRET between QDs and GO and the strong interaction between the ssDNA of the MB loop structure and GO were combined to develop a novel sensitive and selective platform for fluorescence quenching detection of DNA. Upon recognition of the MB to the target, the increasing QD-GO distance and the weakened DNA-GO interaction significantly hindered the FRET, and thus, increased the fluorescence of the QDs (Fig. 14.6 left).

Fig. 14.6
figure 6

Left: Schematic representation of GO-induced fluorescence quenching of MB-QDs and the biosensing mechanism through FRET. Right Fluorescence spectra of MB-QDs after incubation with a target, b single-base mismatch stand, c three-base mismatch stand, and d no target, followed by addition of GO for 5 min. Inset: fluorescence intensity ratio F 0/F. Reprinted with permission from ref [50] (copyright 2010, American Chemical Society)

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The specificity of the MB-QD probe was studied using three kinds of DNA sequences, including a perfectly complementary target, a single-base mismatched strand, and a three-base mismatched strand. Specifically, the MB-QD probe differentiated between the perfectly complementary target, single-base mismatched strand (signal was 40% of that of the perfectly complementary target), and three-base mismatched strand (signal was 15%). Moreover, the quenching efficiency was much higher for MB-QDs than the typical efficiency seen in MB-based detection, which helped to improve the achievable sensitivity and dynamic range. These results demonstrated that the proposed approach could effectively detect the target with high specificity and had potential applications in single nucleotide polymorphism analysis.

Single- or several-layered transition metal dichalcogenides (TMDs) are another example of photofunctional–nanosheet-based FRET [51, 52]. TMDs are generally expressed as MX2, where M is a transition metal of groups 4–10 and X is a chalcogen. Exfoliation of the bulk TMDs into mono- or few-layer structures largely preserves their properties, and also leads to additional characteristics due to confinement effects.

Molybdenum disulfide (MoS2) is a widely known layered TMD. It is an indirect-band-gap semiconductor with an energy gap of ~1.29 eV in the bulk form (although it does not show virtual emission) and has also attracted interest as a photovoltaic and photocatalytic material. The band gap of MoS2 increases on decreasing crystal thickness below 100 nm due to quantum confinement effects, and the quantum yield drastically increases from the bulk form. In addition to the increase in its size, the nature of the band gap also changes from indirect to direct when the thickness reaches a single monolayer. Recent success in isolating MoS2 monolayers has allowed the observation of strong photoluminescence that can be attributed to the direct-gap electronic structure of monolayer MoS2 [53].

Prins and co-workers have recently reported systematic FRET phenomena from colloidal quantum dots to exfoliated monolayer and few-layer MoS2 [54]. They used CdSe/CdZnS core/shell QDs, whose emission spectrum overlaps with the direct excitonic absorption of MoS2. Monolayer and few-layer MoS2 nanosheets were deposited on Si/SiO2 substrates, and QDs were deposited by spin-coating to form a submonolayer that is uniformly ≤1-QD thick.

Figure 14.7a shows a bright-field optical micrograph of a MoS2 flake prior to the deposition of QDs. The different colors indicate that there are several thicknesses present. For the observed region in Fig. 14.7a, the thickness was identified by micro-Raman measurements as bulk-like thickness (>8 layers) down to monolayer thickness (depicted as numbers in the Figure). Fluorescence lifetime decays of QDs were observed by laser-scanning microscopy (Fig. 14.7b). The fluorescence lifetime of QDs located away from the flake was close to their native lifetime of 20.4 ns, while those of the QDs on top of the MoS2 were significantly shortened, indicating the strong quenching of the QD excited state by FRET to the MoS2 nanosheets. Interestingly, the lifetime was dependent on flake thickness, with the strongest QD quenching occurring for the thinnest regions of MoS2. The differences in QD lifetimes and calculated energy-transfer efficiencies on layered MoS2 with varying thicknesses can be expressed as 2D mapping images, as shown in Fig. 14.7b and c, respectively. As a result of systematic experiments, the energy-transfer rates corresponding to the different thicknesses ranging from 1 to 8 layers were determined with the energy-transfer rate in the bulk limit (Fig. 14.7d).

Fig. 14.7
figure 7

a Optical micrograph of a mechanically exfoliated MoS2 flake with indicated layer thicknesses of the different facets. b QD fluorescence lifetime map of the same flake with a saturated color scale to emphasize the lifetime variations within the flake area. c Energy-transfer efficiency map of the same flake. Scale bar is identical for all three panels. d Energy-transfer rate as a function of the number of MoS2 layers. The dashed line represents the energy-transfer rate to thick, bulk-like MoS2, and the circles indicate experimental data. Inset: schematic representation of the definition of the thickness (t) and distance (d) parameters. Reprinted with permission from Ref. [54] with a slight modification (copyright 2014, American Chemical Society)

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Similarly, organic nanosheets, such as graphitic carbon nitrides [55], hexagonal boron nitrides [56], synthetic 2D polymers [57], and supramolecular metal-complex nanosheets [58], are also promising as novel 2D FRET systems.

FRET is now a promising tool due to its distance dependence; k ET ∝ R −6 (Eq. 14.2), especially for biological nanoscale analyzes. Moreover, 2D FRET is a promising candidate for accomplishing artificial photosynthesis; an artificial light-harvesting system (which generally requires stepwise efficient FRET processes) and an artificial reaction center complex, a leading candidate in renewable energy production. Hence, nanosheet-based efficient FRET or other photophysical processes are greatly encouraged for future energy and environmental applications.

6 Summary

In this chapter, inorganic nanosheet-based photoenergy conversion systems have been briefly reviewed. Some representative works have already reported efficient photoenergy conversion as well as the effective utilization of unique advantages of two-dimensional nanosheets. Novel and more efficient photoenergy conversion systems using inorganic nanosheets with a help of molecules, nanoparticles or other kinds of inorganic nanosheets are highly desired in order to realize future sustainable society.