Advanced Composites and Hybrid Materials

, Volume 1, Issue 2, pp 389–396 | Cite as

Hexagonally patterned mixed surfactant-templated room temperature synthesis of titania–lead selenide nanocomposites

Research Article

Abstract

Materials science is becoming a more and more important influencer in electronics, as new synthesis methods and new materials are consistently coming to fruition. In particular, templated synthesis schemes offer unique material options, various alignments, and micro- to nanoscale control over morphology. Surfactant and co-surfactant templating, further, offers the ability to synthesize composite materials via phase separation. Currently, nanoscale manipulation of sophisticated functional materials typically requires energy-intensive or time-intensive processes. The present study illustrates the use of a room temperature synthesis of hexagonally patterned lead selenide-titania nanocomposites, utilizing a versatile mixed surfactant-templating approach. We have found that the level of control of the simple bi-surfactant system presented illustrates the tunability of the micro- and nanostructure. The current system also utilizes a room temperature synthesis—not energy intensive—and the kinetics of the titania precursor reaction with water are extremely fast—not time intensive. Furthermore, while simple, this elegant templated synthesis strategy for creating highly organized composite materials has wide applications beyond the one currently reported, including photocatalysis, photonic crystals, sensors, among others. We anticipate our templated synthesis to be a starting point for more sophisticated nanoelectronic devices. For example, the pores can be impregnated with a variety of nanoparticles or many of the same nanoparticles can be synthesized concurrently and be well dispersed within the template. Furthermore, the templated system presented makes use of titania but can be easily adapted for other metal oxide or ceramic systems by simply changing the precursor.

Graphical abstract

Keywords

Titania Lead selenide Mixed surfactant-template Nanocomposite Hexagonally patterned Room temperature synthesis 

1 Introduction

N-type semiconductor titania (TiO2) has a multitude of unique optical and electronic properties that make it an ideal candidate for application in photovoltaics [1, 2, 3, 4], photocatalysis [5, 6], pigments [7, 8], and cosmetic products [9, 10]. Titania has three distinct crystalline phases, specifically anatase, brookite, and rutile. Although rutile titania has a smaller band gap (3.0 eV) than that of anatase (3.2 eV) [11], it is the anatase form of titania that is more suitable for electronics due to its densely packed crystal structure. This dense crystal structure allows for molecular orbitals to overlap and a high density of states in the conduction band, permitting fast electron injection and increased electron mobility [12]. These properties make titania an ideal candidate for use as an electron-accepting material for a variety of applications.

For optimization in devices, soft templating using surfactants is employed to gain high surface area to volume ratios. Surfactants are attractive for purposes of soft templating due to their ability to form thermodynamically stable structures, as well as highly organized self-assembled micro- and nanostructures [13, 14, 15, 16, 17]. The self-assembled surfactant titania support to be used here has been developed and characterized, and has been found to exhibit a hexagonally aligned pore structure, high interconnectivity, and an extremely high surface area to volume ratio for maximum loading. This structure would be ideal to direct current along the uniaxially aligned pore walls. Wide band gap and electron-accepting titania has been widely used in photovoltaic and sensing materials for a number of years [18, 19, 20] and has shown electrical conductivity, strong charge carrier separation, exciton generating properties at ultraviolet wavelengths, a low reflectance, and strong absorbance in the ultraviolet region ranging into the visible region [18, 21].

Systems composed of oil, water, and amphiphilic surfactants have been proven to form stable, highly organized nanostructures when in the presence of water. The self-assembled surfactant nanostructures are known to form lamellar layers of oil and water, micelles, liposomes, cylinders of one phase dispersed throughout the bulk phase, or disordered/ordered bi-continuous networks [13]. The high interconnectivity of an organized bi-continuous network is an ideal soft template to pattern a TiO2 structure due to high interconnectivity, high surface area to volume ratios, and high loading of nanoparticles uniformly throughout the structure.

Here, a soft-template synthesis technique is demonstrated for the fabrication of a highly ordered nanoporous anatase titania support loaded with nanorods. This route will allow for a high loading of the nanorods, as well as facilitate electron transfer from the nanorod conduction band to the TiO2 conduction band, producing an efficient nanocomposite heterostructure. To illustrate the support’s effectiveness, the titania is loaded with concurrently synthesized lead selenide—to emulate an all inorganic solar cell, as a proof-of-concept.

2 Materials and methods

2.1 Chemicals

Isooctane, dioctyl sulfosuccinate sodium salt (AOT), titanium (IV) isopropoxide (TIP), tri-n-octylphosphine (TOP), lead acetate trihydrate, selenium powder, oleic acid, n-tetradecylphosphonic acid, hexane, and diphenyl ether were purchased from Sigma-Aldrich. Lecithin (L-α-phosphatidylcholine) was purchased from Fisher Scientific. All chemicals were used as received without further purification.

2.2 Lead selenide nanorod synthesis

Aligned lead selenide (PbSe) nanorods were synthesized following an existing solvothermal method [3, 22, 23] with minor modifications. Under dry nitrogen, a 1.0 M stock solution of TOPSe is prepared by adding 0.786 g of selenium to 10 mL of TOP while continuously stirred for 2 h at 50 °C. This is followed by dissolving 0.76 g of lead acetate trihydrate and 2 mL of oleic acid into 10 mL of diphenyl ether. The solution is then heated for 30 min at 150 °C under nitrogen through a bubbler to form lead oleate in situ. The lead oleate solution was then cooled to 60 °C where 4 mL of 0.167 M TOPSe is added to the solution. This solution is referred to as lead oleate-TOPSe solution.

In a separate vial under dry nitrogen, 0.2 g of n-tetradecylphosphonic acid is added to 15 mL of diphenyl ether and heated to 250 °C under vigorous stirring. The lead oleate-TOPSe solution is then injected into the solution of n-tetradecylphosphonic acid in diphenyl ether. As the mixture cools, the solution becomes cloudy—indicating the formation of PbSe nanorods. This final solution is heated for 50 s at 250 °C under the same vigorous stirring conditions. The solution is then cooled to room temperature. Once cooled, it is mixed with 31 mL of hexane. The PbSe nanorod solution is prepared beforehand, and can later be centrifuged and resuspended in a variety of different solvents—such as chloroform, water, or tetrahydrofuran—when ready for use.

2.3 Surfactant-templated nanocomposite synthesis

Synthesis of the surfactant-templated titania support follows an existing synthesis route [17] with minor modifications. This system incorporates the use of two surfactants: anionic AOT and a zwitterionic phospholipid lecithin. When dissolved in an organic phase (i.e., isooctane) and a calculated amount of deionized water (Wo, ratio of moles of water to AOT) is gradually added, the formation of a bi-continuous gel-like state with crystalline order will form [13]. Depending upon the Wo, the molar ratio of water to AOT, either cylindrical or lamellar structures will form (Fig. 1). Titania is formed through the hydrolysis and condensation of TIP. Since TIP is organic soluble and has limited solubility in water, formation of TiO2 will occur at the oil-water interfaces, therefore using the surfactant assemblies as structure-directing agents.
Fig. 1

Phase behavior of the AOT-lecithin system illustrating the transition from reverse hexagonal (H) to the lamellar (L) phase with increasing water content and temperature; reprinted with permission from [13]: copyright 2008 American Chemical Society

In a vial, TIP is added in a 2:1 volumetric ratio with isooctane. To this solution, 0.8 M of anionic AOT and 0.4 M of zwitterionic lecithin are added and vigorously mixed until completely dissolved. In a separate vial, lead selenide nanorods are centrifuged to remove the hexane and resuspended in deionized water. A calculated amount of the aqueous lead selenide nanorod solution corresponding to the desired Wo (ratio of moles of water to AOT) is then injected into the TIP/isooctane/AOT/lecithin solution and vigorously mixed using a vortex mixer. Immediately after addition of the aqueous phase, a white precipitate is observed indicating formation of titania. The samples are then dried at 60 °C for 24 h and then calcinated to remove any trace amounts of organics and surfactants, and to also take the TiO2 from its amorphous phase to its anatase crystalline phase. Calcination starts at 400 °C with an increase of 50 °C every 30 min until the temperature reaches 550 °C where it is held for 4.5 h. A fine white powder is obtained, where it is then washed with deionized water and centrifuged several times and dried again at 60 °C for 24 h.

2.4 Material characterization

Thin sections of the powdered nanocomposite were prepared for transmission electron microscopy (TEM) by embedding the particles in an epoxy resin and curing overnight. Using a MT2-B DuPont Ultramicrotome, the samples were then cut into 70–90-nm-thick slices using a diamond knife. To characterize the nanocomposite structure, TEM images were attained from a JEOL JEM 2100 instrument operated at 200 kV using 200-square mesh copper grids with a formvar carbon film. Elemental analysis of the samples was done by electron dispersive X-ray spectroscopy (EDX), which was also performed on the JEOL instrument using an Oxford INCA system.

Ultraviolet-visible-near infrared spectroscopy (UV-Vis-NIR) was performed on a Perkin Elmer Lambda 900 Spectrophotometer to determine and confirm band gap energies of the individual materials used in the nanocomposite heterostructure synthesis. TEM and EDX were also performed on the PbSe nanorods for structural characterization as well as elemental analysis. Photoconductive characterization of the nanocomposites was carried out on a fabricated circuit (Fig. 2) using a Fluke 8808a multimeter and bench-top power supply.
Fig. 2

Photoconductive experimental testing setup

3 Results and discussions

3.1 Bi-surfactant templated nanoporous titania

Formation of the soft-templated titania support from the TIP/isooctane/AOT/lecithin/water system produces a white powder. This powder is known to contain nanopores on the order of 20–250 nm in diameter depending upon the calculated Wo of the sample [17]. These sectioned TEM images of the powdered nanocomposite show this hexagonal pattern arrangement as indicated by the soft-template phase diagram (Fig. 1). Figure 3a–e represents thin-sectioned TEM images of the powdered nanocomposite at different Wo values. Breakage is observed in some pore walls (indicated by black arrows) due to solvent removal from the drying process as well as TiO2 calcination. This observed breakage shows that the porous TiO2 template displays high interconnectivity throughout the structure along with high surface area to volume ratios for maximum PbSe loading. Figure 4 displays the PbSe nanorods synthesized in situ and then resuspended in deionized water.
Fig. 3

Thin-sectioned TEM micrographs of soft template titania support of a, b Wo = 130; c Wo = 90; d, e Wo = 110 exhibiting hexagonal porous structure with high interconnectivity (black arrows); electron diffraction (f) of the nanocomposite indicating the successful phase change from amorphous to anatase TiO2; EDX (g) of the pore walls indicates the deposition and presence of PbSe (orange and green arrows) as well as the presence of titania (white arrows)

Fig. 4

TEM micrographs of lead selenide nanorods under a low and b high magnification; EDX (c) of the lead selenide nanorods indicating the presence of both elements (Pb—white arrows, Se—orange arrow) as well as phosphorous (green arrow) due to the use of TOP as the capping agent

3.2 Use as a photovoltaic material

Lead selenide semiconducting nanostructures are ideal for photovoltaic sensitizing due to their optoelectrical properties [24, 25], as well as their ability to exhibit multiple exciton generation (MEG) [26, 27, 28, 29, 30]. Hence, this can make these structures more sensitive to a wider range of the electromagnetic spectrum, making them ideal for photovoltaics and other applications. Strong charge carrier separation between the generated electrons and holes must be achieved to prevent recombination so that quantum efficiency can be maximized.

Ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy was conducted on the PbSe nanostructures and the titania precursor solution. This analysis was done to confirm the band gaps of both the PbSe and TiO2 solutions used on the NC synthesis. Figure 5a, b displays the absorbance data from which the associated material band gaps were calculated.
Fig. 5

Plots of UV-Vis-NIR absorbance data for both a PbSe and b titania with their calculated corresponding band gap energies

The associated band gap energies of the materials being analyzed can be easily calculated by use of the following equation: \( {E}_g=\raisebox{1ex}{$h\ c$}\!\left/ \!\raisebox{-1ex}{$\lambda $}\right. \), where E g is the band gap energy, h is Planck’s constant, c is the speed of light, and λ is the wavelength. PbSe absorbance data (Fig. 5a) yields two very distinct peaks at wavelengths of 1153 and 1410 nm. The peak at 1153 nm corresponds to the capping agent, TOP, used in the PbSe synthesis. This was confirmed by conducting UV-Vis-NIR spectroscopy on TOP alone. Furthermore, the wavelength corresponding to the second peak is that of the lead selenide nanostructures. By use of the band gap energy equation, in which Planck’s constant and the speed of light are known, the band gap energy is easily calculated to be 0.87 eV. Measurement of titania’s band gap energy is slightly different than that of PbSe since its absorbance data does not form a clear peak. It is at the absorption edge of the curve in Fig. 5b (indicated by the black arrow) where the wavelength is measured [31]. The corresponding wavelength at the absorption edge equates to 377 nm, by applying the band gap equation (above) titania’s associated band energy is that of 3.29 eV.

To determine if the fabricated nanocomposite exhibits photovoltaic properties, a photoconductive circuit was constructed as previously reported [28, 32] with minor modifications (Fig. 2). Figure 6 represents the resulting current density—voltage (I–V) curves showing nanocomposite responses to light with varying Wo concentrations.
Fig. 6

I–V curves depicting the photogenerated current density of nanocomposite powders with varying Wo concentrations when exposed to a visible-IR light source for 30 s at a a 15-cm distance from the light source and b 25-cm distance from the same light source; these results were validated c, d by reproduction of the same experiments

As seen in Fig. 6, the I–V curves depict a clear photogenerated response to light exposure. The clear increased trend from application of a positive indicative of typical photoconductive activity. Consistent increase in photogenerated current is observed in Fig. 6a, c. It is known that in both the visible and IR regions, PbSe has the ability to undergo multiple exciton generation (MEG upon the material band gap. So photocurrent is expected to be observed under this type of exposure, as well as amplified photocurrents with increasing PbSe concentrations (i.e., higher Wo values). The consistency increased photocurrent trend is due to the application of an external bias. In a photovoltaic circuit, no external bias is present. Therefore, once the depletion region reaches equilibrium, electron transfer across the heterojunction is inhibited by Coulomb forces and the photogenerated current threshold is reached. Application of an external bias induces the formation of an electric field across the device setup. In forward bias mode (0–5 V), a decreased shift in the energy barrier associated between the titania and PbSe conduction bands occurs. When coupled with strong cathode charge attraction from forward bias application, Coulomb forces are overcome in the depletion region allowing for increased photocurrent extraction with increasing voltage bias. Alternatively, reverse bias mode (0 to − 5 V) causes an increased shift in the energy barrier height associated between the two adjacent conduction bands. This marked increase in barrier height coupled with Coulomb force repulsion inhibits any photocurrent generation. With this, photocurrent generation under reverse bias mode seems evident around − 4 V (Fig. 6a, c). This is due to what is called a reverse bias breakdown voltage. At higher voltages, the electric field at the junction accelerates the electrons present. When a substantial kinetic energy equal to that of the energy gap or greater is attained, multiple electron-hole pairs are created, otherwise known as impact ionization. The resultant charge carriers sustain enough energy from the electric field to cross the energy barrier and surpass the Coulomb forces present in the depletion region [27].

A notable difference is observed in photogenerated current by increasing the distance from the light source to the nanocomposite. Since photons have a tendency to scatter [33], increasing the distance from the light source to the target gives rise to this phenomena occurring. This trend is clearly observed by comparison of Fig. 6a to b and c to d. Photon scattering inhibits photocurrent generation due to a marked decrease in photons actually striking and being absorbed by the nanocomposite. Decreasing the amount of photons absorbed will consequently decrease the amount of electron-hole pairs created leading to a decrease in photocurrent.

4 Conclusions

Templated synthesis schemes offer unique material options, various alignments, and micro- to nanoscale control over morphology. Surfactant and co-surfactant templating, further, offers the ability to synthesize composite materials via phase separation. This work presents the synthesis and characterization of a soft-templated titania–lead selenide nanocomposite by the use of surfactants. This highly ordered nanoporous support provides high surface area to volume ratios for maximum loading of PbSe nanorods within the nanocomposite pores, which is ideal in terms of photovoltaic applications. We have found that the level of control of the simple bi-surfactant system presented illustrates the tunability of the micro- and nanostructure. The current system also utilizes a room temperature synthesis—not energy intensive—and the kinetics of the titania precursor reaction with water are extremely fast—not time intensive. Furthermore, while simple, this elegant templated synthesis strategy for creating highly organized composite materials has wide applications beyond the one currently reported, including photocatalysis, photonic crystals, sensors, among others. By application, a narrow band gap semiconductor, such as PbSe, strong absorbance, and multiple exciton generation, is exhibited in both the visible and infrared regions of the electromagnetic spectrum. Photoconductive characterization of nanocomposites with varying Wo concentrations confirms photocurrent generation when exposed to light, classifying the material as having photovoltaic properties. We anticipate our templated synthesis to be a starting point for more sophisticated nanoelectronic devices. For example, the pores can be impregnated with a variety of nanoparticles or many of the same nanoparticles can be synthesized concurrently and be well dispersed within the template. Further optimization of the nanocomposite structure would entail a high surface area planar heterojunction between the two materials to provide stronger charge carrier separation. This device design would expect to yield higher photogenerated currents. Furthermore, the templated system presented makes use of titania but can be easily adapted for other metal oxide or ceramic systems by simply changing the precursor.

Notes

Acknowledgements

The authors would like to thank Prof. Emerit. Anthony Nunes for his helpful discussions. SEM and TEM micrographs for the graphical abstract are reprinted from preliminary research with permission from references [13, 17].

Funding information

The authors would like to recognize the NASA-funded Rhode Island Space Grant Consortium, the University of Rhode Island Transportation Center, and the Honda Initiation Grant for their financial support of this project.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Stephanie R. Aceto
    • 1
  • Yang Lu
    • 2
  • Radha Narayanan
    • 3
  • David R. Hesket
    • 4
  • Evan K. Wujcik
    • 2
    • 5
  • Arijit Bose
    • 1
  1. 1.Laboratory of Soft Colloids & Interfaces, Department of Chemical EngineeringThe University of Rhode IslandKingstonUSA
  2. 2.Materials Engineering And Nanosensor [MEAN] Laboratory, Department of Chemical and Biological EngineeringThe University of AlabamaTuscaloosaUSA
  3. 3.Department of ChemistryThe University of Rhode IslandKingstonUSA
  4. 4.Department of PhysicsThe University of Rhode IslandKingstonUSA
  5. 5.Department of Civil, Environmental, and Construction EngineeringThe University of AlabamaTuscaloosaUSA

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