Flexible, Freestanding, and Functional SiO2 Nanofibrous Mat for Dye

based flexible sensors, electronics, energy storage devices, and catalytic ... For example, fabrics coated with TiO2 may degrade the organic dye pollu...
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Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Flexible, Freestanding, and Functional SiO2 Nanofibrous Mat for DyeSensitized Solar Cell and Photocatalytic Dye Degradation Fan Zheng† and Zhengtao Zhu*,†,‡ †

Nanoscience and Nanoengineering Program, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States ‡ Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States S Supporting Information *

ABSTRACT: In this work, we investigate the SiO2 nanofibrous nonwoven mat prepared by electrospinning as a porous and high-temperature durable substrate for preparation of the freestanding, flexible, and multifunctional composites. The neat SiO2 nanofibrous nonwoven mat (denoted as SiO2−NF) and the SiO2 mat functionalized with gold nanoparticles (denoted as Au@SiO2−NF) were readily prepared by electrospinning of spin dope containing precursors. Subsequently, a patterned layer of TiO2 nanoparticles was impregnated and transferred into the SiO2 or Au@SiO2−NF nonwoven mats. The freestanding composite mats of TiO2 nanoparticles and electrospun SiO2−NF or Au@SiO2−NF (denoted as TiO2−NP/ SiO2−NF or TiO2−NP/Au@SiO2−NF, respectively) were demonstrated for dye-sensitized solar cells (DSSCs) and photocatalytic dye degradation. By controlling the amount of TiO2, composite mats with only partially filled TiO2 nanoparticles on one side were used as photoanode and spacer in DSSCs; the device had an efficiency of 5.31%. Incorporation of Au nanoparticles in the photoanode (TiO2−NP/Au@SiO2−NF) improved the device performance. The thermally durable and freestanding TiO2−NP/SiO2−NF or TiO2−NP/Au@SiO2−NF was also used as readily recyclable and regeneratable material for effective photodegradation of the methylene blue in aqueous solution. KEYWORDS: thermally durable substrate, electrospinning, SiO2 nanofiber, dye-sensitized solar cell, photocatalytic dye degradation

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INTRODUCTION

Electrospinning is a simple and versatile method to prepare fibrous nonwovens of one-dimensional (1D) polymers, metals, ceramics, carbonaceous materials, and composites with diameters ranging from tens of nanometers to several micrometers.11−14 A typical electrospinning setup contains three components: a high dc voltage (usually in the range 5−40 kV) applied to a spinneret, a spinneret containing a spin dope, and an electrically grounded conductive collector placed at a certain distance (known as the “gap distance”) away from the spinneret. Under the dc voltage, when the electrostatic force overcomes the surface tension and the viscoelastic forces of the polymer droplet, a jet is ejected and travels straight from the spinneret. The jet then starts to bend and forms helical loops; the loop diameter increases as the length of a jet elongates by thousands of times in a period of 50 ms or less. This high elongational rate can effectively mix the different components in a spin dope and evaporate the solvent, resulting in the formation of a randomly overlaid nanofibrous nonwoven mat on the collector.

Three-dimensional and porous materials are desirable for many applications such as flexible/bendable energy storage/harvesting devices and environmental remediation.1,2 The porous structure and high surface areas enable these materials to serve as templates/substrates for a variety of functional materials. For example, cellulose fiber papers, although inherently nonconductive, can provide mechanical strength and flexibility for making conductive composite papers of conducting polymers, carbon nanotubes, and graphenes for electrochemical energy storage devices.3−5 Commercially available paper has been demonstrated as a substrate/template for flexible electronics and supercapacitors by simply coating/writing the conductive materials (e.g., the silver ink or carbon nanotubes) on paper.6,7 Textile fabrics, which are composed of fibers/yarns processed into wovens and knits by the weaving and knitting techniques, respectively, are also examples of the readily available porous materials. By incorporation of functional materials in the fabrics, textile-based flexible sensors, electronics, energy storage devices, and catalytic supports can be developed.8,9 For example, fabrics coated with TiO2 may degrade the organic dye pollutant due to the photocatalytic property of TiO2 under UV light.10 © XXXX American Chemical Society

Received: December 9, 2017 Accepted: February 26, 2018 Published: February 26, 2018 A

DOI: 10.1021/acsanm.7b00316 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

reduction, and hydrothermal process) or by directly adding nanoparticles or precursors into the spin dope for electrospinning. For example, the electrospun silica nanofiber surface was modified by self-polymerization of polydopamine (PDA) and electroless plating of silver using glucose as a reducing agent.27 As another example, calcination of the electrospun hybrid fibers from tetraethyl orthosilicate (TEOS), poly-[3(trimethoxysily)propyl methacrylate] (PMCM), and silver nitrate (AgNO3) at 600 °C resulted in porous silica fibers with silver nanoparticles.28 Our group has explored the electrospun SiO2 nanofibers as a supporting component for flexible devices. Dual-spinneret electrospinning followed by the pyrolysis in air at 650 °C was used to prepare a freestanding/flexible hybrid mat consisting of crystalline ZnO nanofibers and amorphous SiO2 nanofibers.29 The flexible ultraviolet (UV) sensors based on the electrospun ZnO/SiO2 hybrid mat showed excellent sensitivity and reproducibility/reversibility under flat and bent conditions. A flexible hybrid mat consisting of anatase-phased TiO2 nanofibers and structurally amorphous SiO2 nanofibers was also prepared by dual-spinneret electrospinning followed by pyrolysis; the freestanding TiO2/SiO2 fibrous mat was then impregnated with binder-free TiO2 nanoparticles and sintered at 450 °C to form a flexible composite photoanode for dyesensitized solar cells (DSSCs).30 The device achieved a power conversion efficiency of 6.74 ± 0.33% on FTO/glass substrate. The hybrid TiO2/SiO2 nanofibrous mat impregnated with TiO2 nanoparticles was further explored as an innovative anode for a flexible lithium ion battery.17 In all these examples, the SiO2 nanofibers provide the essential mechanical support for the flexibility; the TiO2 nanoparticles provide the high surface area required for the electrochemical activity and dye loading in the devices, and the nanofibers of TiO2 or ZnO may facilitate the charge transport of the photogenerated electrons. In this work, we investigate the SiO2 nanofibrous nonwoven mat prepared by electrospinning as a porous and hightemperature durable substrate for preparation of the freestanding, flexible, and multifunctional composites. The neat SiO2 nanofibrous nonwoven (denoted as SiO2−NF) was prepared via electrospinning of TEOS/PVP precursor nanofibers followed by a high-temperature pyrolysis. A flexible SiO2 nanofibrous nonwoven mat functionalized with gold nanoparticles (denoted as Au@SiO2−NF) was prepared by adding HAuCl4 in the spin dope of TEOS/PVP. Subsequently, the SiO2 or Au@SiO2−NF nanofibrous nonwoven mats were used as a template to impregnate a layer of TiO2 nanoparticles prepared by doctor-blading, followed by calcination at high temperature to obtain the freestanding composite mats of TiO2 nanoparticles and electrospun SiO2−NF or Au@SiO2−NF (denoted as TiO2−NP/SiO2−NF or TiO2−NP/Au@SiO2− NF, respectively). In TiO2−NP/SiO2−NF and TiO2−NP/ Au@SiO2−NF, the TiO2 nanoparticles provided functionality of the mats for DSSC and photocatalytic dye degradation. By controlling the amount of TiO2 nanoparticles during doctorblading, composite mats only partially filled with TiO2 nanoparticles on one side were used as photoanode and spacer in DSSCs. The devices based on this unique integrated photoanode and spacer had an efficiency of ∼5.31%. Furthermore, incorporation of Au nanoparticles in the photoanode enhanced the device performance, presumably due to the plasmonic effects. The thermally durable composite mats of TiO2 nanoparticles and SiO2 nanofibers were also used

The electrospun nonwoven textiles possess properties (including high specific surface area, high aspect ratio, high porosity, and light weight) that are promising for various applications.15 In energy conversion/storage devices, charge generation, transport, collection, and diffusion often require large surface areas and good electrical properties, and an electrospun nonwoven fibrous mat may shorten the path of carrier transport and enhance the carrier collection ability by functioning as an electron expressway in the axial direction.16 Furthermore, the electrospun nanofibrous mats have large apparent pore size (in the scale of μm), which may provide a 3D template for preparation of functional nanomaterials.17−19 For example, TiO2 nanoparticles can be easily and abundantly embedded into an electrospun nanofibrous mat to form a composite anode electrode in a flexible lithium ion battery.17 Another potential application of electrospun nanofibrous nonwoven is the photocatalytic degradation of organic pollutants. In wastewater treatment, photocatalytic degradation is considered an effective technology.20 In the common photocatalytic degradation method, powders such as TiO2 nanoparticles are used as photocatalysts for oxidation of the organic pollutants. Removal and reactivation of the powders after photocatalytic reaction require refiltering the nanosized catalyst, which can be a time-consuming and costly process. Freestanding polymeric electrospun nonwovens, which have porous and high surface areas, provide a good template to immobilize the photocatalysts and can be readily recycled after the photodegradation. For example, a freestanding polyacrylonitrile-based (PAN-based) nanofibrous mat containing TiO2 particles prepared by electrospinning was evaluated for easy operation of the photodecomposition of dye rhodamine B under UV light.21 TiO2 nanoparticles immobilized on PAN nanofiber mats were also demonstrated as a flexible and recyclable photocatalyst for phenol degradation.22 The electrospinning process requires the spin dope to have a certain viscosity to form a stable spinning jet. For electrospun nanofibers of functional metal oxides (e.g., TiO2, ZnO), polymer is added along with the precursor of oxide to increase the viscosity of the spin dope, and postspinning pyrolysis at high temperature is required to remove the polymer and to convert precursor into oxide. These polycrystalline nanofibers of metal oxides are fragile; therefore, they often cannot form a flexible and freestanding fibrous mat and have limited usage in flexible/bendable devices. One exception is the amorphous SiO2 nanofibrous nonwoven prepared by electrospinning. Typically, electrospun SiO2 nanofibers are obtained by electrospinning the spin dope containing the aqueous solution (sol−gels) of the alkoxide precursors [e.g., tetraethyl orthosilicate (TEOS)] and the carrying polymers [e.g., poly(ethylene oxide), polyvinylpyrrolidone (PVP), and poly(vinyl alcohol)] followed by pyrolysis.23−26 Because of the amorphous nature of the SiO2 nanofibers, electrospun SiO2 nanofibrous mat is mechanically flexible and can retain its physical and chemical properties at high temperatures (≤800 °C). The good mechanical flexibility and high-temperature durability of the SiO2 nanofibrous nonwoven make the material an ideal substrate or template for preparation of functional materials and/or flexible devices that require high processing temperatures not accessible for polymer-based nonwoven and substrate. Generally, the functionalization of the SiO2 electrospun fibers can be achieved by a post-treatment of the asprepared electrospun fibers (including surface treatment, in situ B

DOI: 10.1021/acsanm.7b00316 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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good filling of the TiO2 nanoparticles in the mat. A thin layer of PMMA was first spin-coated on a glass substrate, followed by doctorblading of the TiO2 paste. Right after the doctor-blading, the SiO2−NF mat was laid on the layer of the TiO2 paste. Pressed gently, the TiO2 paste was squeezed into the SiO2−NF mat. Subsequent sintering removed the PMMA layer, and the freestanding TiO2−NP/SiO2−NF mat was obtained. The amount of TiO2 nanoparticles loaded in the SiO2−NF mat was adjusted by controlling the thickness of the tape during doctor-blading. The TiO2−NP/Au@SiO2−NF composite mat was prepared following the same procedure. Morphological Characterization. A Zeiss Supra 40 variablepressure field-emission scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and a JEOL JEM2100 transmission electron microscope (TEM) were employed to characterize the morphological and structural properties of the nanofibers and nanofibrous mat. The average nanofiber diameter for each sample was obtained by measuring 50 randomly selected fibers from the SEM image using the ImageJ software. The BET surface area was measured using a Micromeritics Gemini surface area analyzer. Xray photoelectron spectroscopy (XPS) analyses were performed using a PHI Versaprobe 5000 scanning X-ray photoelectron spectrometer with an incident monochromated X-ray beam from the Al target (200 μm, 50 W, 15 kV). The step size of 0.4 eV was employed with rate of 25 ms step−1, and each peak was scanned 8 times. Assembly of DSSCs and Device Characterization. The photoanode of the patterned TiO2−NP/SiO2−NF (or TiO2−NP/ Au@SiO2−NF) composite mat was prepared on a clean FTO/glass substrate. The TiO2 layer was first prepared by doctor-blading of the diluted TiO2 paste, and then, the SiO2 nanofibrous mat was pressed into the TiO2 layer, followed by sintering at 450 °C for 45 min. After being sensitized by the N719 dye for 24 h, the photoanode and the counter electrode (FTO/glass with Pt catalyst) were sandwiched with a binder clip. Finally, the Iodolyte AN-50 electrolyte was injected into the cell, and an epoxy layer was applied to seal the edges of the cell. In a typical experiment, three identical devices were fabricated and characterized, and the average value and one standard deviation of the device parameters were reported. A DSSC device was also assembled using the TiO2−NP/SiO2−NF mat as a composite photoanode and titanium gauze as a flexible electrode. The titanium gauze was first coated with a compact layer of TiO2 by immersion in a TiCl4 aqueous solution at 70 °C for 45 min followed by sintering at 400 °C. The photoanode on the titanium gauze was fabricated following the same procedure as preparation of the TiO2−NP/SiO2−NF mat, while the titanium gauze was introduced between the PMMA layer and the TiO2 paste. After being sensitized in the N719 dye solution for 48 h, the fabricated photoanode on the titanium gauze was washed by ethanol to remove the excess dye. The flexible photoanode with the titanium gauze electrode was put on a glass slide, and was then sandwiched with the Pt-coated FTO/glass counter electrode. The iodide/triiodide (I−/I3−) electrolyte was introduced by capillary action, and the device was sealed to complete the DSSC fabrication. The DSSC performance was evaluated with a Keithley 2400 sourcemeter. A 150 W solar simulator (Newport Co.) was used to simulate 100 mW cm−2 sunlight. The light intensity was adjusted using a Hamamatsu S1133 reference cell calibrated by the National Renewable Energy Laboratory (Golden, Colorado). For the incident photon-to-current conversion efficiency (IPCE) measurement, a monochromator (Photon Technology International) was employed to generate monochromatic irradiation from a 300 W solar simulator (Newport Co.), and the intensity of the incident monochromatic irradiation was measured by a power meter (PM100A, Thorlabs) and a Si-based standard photodiode power sensor (S120VC, Thorlabs). A Keithley 2400 sourcemeter was used to measure the short-circuit photocurrent density of the device under the monochromatic irradiation. The IPCE, which corresponds to the external quantum efficiency, is given by

as readily recyclable and regeneratable materials for effective photodegradation of the methylene blue in aqueous solution. In comparison with our previous work of the flexible hybrid mat achieved via dual-spinneret electrospinning,30 in which the fragile nature of the polycrystalline TiO2 nanofibers in the hybrid mat may be problematic for the long-term stability of the flexible devices, the results reported here have several distinguished advantages. (1) The flexible devices based on the neat SiO2 nanofibrous nonwoven mat infused with nanoparticles and other functional groups may overcome the problem caused by the fragile TiO2 and ZnO nanofibers. (2) A thin polymeric sacrificial layer used during the infusion and transfer printing leads to a more viable and robust process for functionalization of the freestanding nanofibrous mat. (3) The transfer and functionalization process would enable design of a SiO2 nanofibrous nonwoven mat as both flexible support of the TiO2 nanoparticle anode and device separator simultaneously. The process may significantly simplify fabrication of the devices (e.g., lithium ion battery and DSSC) that need an insulating separator between two electrodes. (4) The freestanding, hightemperature durable, and functionalized composite mats would allow readily recyclable and regeneratable photoactive materials for continuous use in photodegradation.

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EXPERIMENTAL SECTION

Materials. Tetraethyl orthosilicate (TEOS), chloroauric acid trihydrate (HAuCl4·3H2O), polyvinylpyrrolidone (PVP, Mw = 1 300 000), methylene blue, and N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and ethanol were purchased from SigmaAldrich Chemical Co (St. Louis, MO). The FTO glass (F-doped SnO2, 8 Ω/□) was provided by Hartford Glass Co. The titanium gauze (100 mesh woven of 0.05 mm diameter wires) was purchased from Alfa Aesar. The platinum precursor (Platisol T), N719 dye, and the electrolyte (Iodolyte AN-50 electrolyte) were purchased from Solaronix. Nanocrystalline TiO2 paste (18NR-T) with average particle size of 20 nm was provided by Dyesol. Parafilm was purchased from Fisher Scientific. All chemicals were used as received without further purification. Preparation of Electrospun SiO2 and Au@SiO2. The nanofibers of SiO2 and Au@SiO2 (denoted as SiO2−NF and Au@SiO2−NF, respectively) were prepared by electrospinning followed by pyrolysis. For the spin dope of SiO2−NF, 1.5 g of TEOS was mixed with 0.6 g of diluted HCl solution (prepared from 25 mL of H2O and 3 drops of 6 M HCl solution) and 0.4 g of ethanol followed by stirring for 1 h; then, the above TEOS solution was added to the solution containing 1 g of PVP and mixture solvents of 6.5 g of DMF and 1.3 g of DMSO. Subsequently, the spin dope was stirred for 5 h. For the spin dope of Au@SiO2−NF, 8.6 mg of HAuCl4·3H2O was added to the solution of PVP, and the calculated weight percent of Au nanoparticles in the Au@SiO2−NFs was 1.0 wt %. During electrospinning, a positive dc voltage of 10 kV from an ES30P high-voltage power supply (Gamma High Voltage Research, Inc.) was applied to the spin dope through a stainless-steel needle. The feed rate was set at 0.8 mL h−1 by using a KDS 200 syringe pump (KD Scientific, Inc.). The electrospun precursor nanofibrous mat was then collected on a laboratory-built rotating drum (with diameter of ∼25 cm) covered with aluminum foil. The as-spun nanofibrous mat was kept in ambient condition for 24 h to allow for complete hydrolysis/ condensation of the SiO2 precursor; thereafter, the mat was heated to 500 °C at the ramp rate of 1 °C min−1 and held at 500 °C for 5 h with constant airflow in a tube furnace (Lindberg 54453) to obtain SiO2− NF or Au@SiO2−NF. Fabrication of TiO2−NP/SiO2−NF and TiO2−NP/Au@SiO2− NF Composite Mats. An electrospun SiO2 nanofibrous mat (SiO2− NF mat) with thickness of 50 μm was used in the fabrication of the TiO2−NP/SiO2−NF composite nanofibrous mat. Diluted Dyesol TiO2 paste (paste/ethanol = 3:1, weight ratio) was used to obtain

IPCE (%) = C

1240 (eV nm) × Jsc λΦ

× 100%

DOI: 10.1021/acsanm.7b00316 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic of electrospinning setup. (b) Schematic illustrating the formation process of the TiO2−NP/SiO2−NF and TiO2−NP/Au@ SiO2−NF nanofibrous composite mats. (c) SEM image of the SiO2 fibers. (d) Histogram of the diameter distribution of the SiO2 fibers. (e) SEM image of the Au@SiO2 fibers. (f) Histogram of the diameter distribution of the Au@SiO2 fibers. in which Jsc (mA cm−2) is the short-circuit photocurrent density obtained under the monochromatic irradiation, and λ (nm) and Φ (mW cm −2 ) are the wavelength and the intensity of the monochromatic light, respectively.31 Photodegradation of Methylene Blue by the TiO2−NP/ SiO2−NF and TiO2−NP/Au@SiO2−NF Mats. A TiO2−NP/SiO2− NF (or TiO2−NP/Au@SiO2−NF) composite mat (size ∼3 × 5 cm2) was placed in a glass bottle filled with 100 mL of aqueous solution of methylene blue (MB) with a concentration of 5.0 mg L−1. The mass of TiO2 nanoparticles in the composite mat was 60 mg. The UV irradiation upon the MB solution was provided by a 150 W solar simulator (Newport Co.). The absorbance of the MB solution was collected by a UV−vis spectrophotometer (HP 8452A Diode-Array spectrophotometer) during the degradation reaction to evaluate the degradation rate of MB. After the MB solution became colorless, the degradation was considered completed. The composite mat was regenerated by heating the mat at 500 °C for 0.5 h to remove organic pollutant adsorbed on the mat during the degradation of MB for further photodegradation experiment.

microscopically identifiable beads were observed for both SiO2 and Au@SiO2 nanofibers. The surface of the SiO2 nanofibers was smooth, whereas the Au@SiO2 nanofibers had slightly rougher surface. The average diameter of the SiO2 nanofibers was ∼140 nm with a relatively wide size distribution, and the Au@SiO2 nanofibers had average diameter of ∼120 nm and similar size distribution as the SiO2 nanofibers. The result suggested that adding of chloroauric acid in the spin dope led to slightly thinner fibers, likely due to the improved electrical conductivity of the spin dope, which reduced the surface tension of the spin dope and increased stretching of the spinning jet.32,33 The SEM images also showed that the nanofibrous mats had porous structures, in which the nanoparticles could be readily infused to add functionality. TEM images (Figure S1) show that the SiO2 nanofibers were amorphous in nature, and the Au nanoparticles in the Au@SiO2 had sizes of ∼10 nm. TiO2−NP/SiO2−NF and TiO2−NP/Au@SiO2−NF Composite Mats. For the polymer nanofibers, further functionalization is often limited because of the choice of solvents and the processing temperature. For example, infusion and adhesion of metal oxide nanoparticles on the nanofibers often require removing the organic components and improving the nanoparticle connectivity through high-temperature sintering (e.g., >400 °C), which is not compatible with the polymeric nanofibers or substrates. The SiO2−NF and Au@SiO2−NF mats may overcome these limitations and serve as a flexible and high-temperature durable template for further functionalization. Figure 2a shows the schematic of preparation of a freestanding and flexible TiO2−NP/SiO2−NF composite mat using a pattern transfer process. A PMMA scarifying layer was first prepared by spin-coating; the TiO2 layer was deposited and patterned on the PMMA layer by doctor-blading of the TiO2 paste. After a SiO2−NF mat was pressed into the TiO2 layer, the sample was heated at 500 °C to remove the PMMA layer as well as the organic binders in the paste; as a result, a freestanding composite mat filled with TiO2 nanoparticles (denoted as TiO2−NP/SiO2−NF) was obtained. This freestanding TiO2−NP/SiO2−NF mat was mechanically flexible, as

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RESULTS AND DISCUSSION Mechanically Flexible Electrospun SiO2−NF and Au@ SiO2−NF Mats. The SiO2−NF and Au@SiO2−NF nonwoven mats were obtained by electrospinning followed by pyrolysis at 500 °C. Figure 1a shows the typical setup for electrospinning. The spin dopes containing TEOS or TEOS/HAuCl4·3H2O and PVP were used to prepare the precursor nanofibrous mats; the as-prepared nanofibrous mats were kept at ambient condition for sol−gel formation of SiO2 before pyrolysis. During pyrolysis, PVP polymer and HAuCl4 were decomposed, and the SiO2 nanofibers and Au nanoparticles were formed. The nominal weight percent of Au in the nanofibrous mat was 1 wt %. As shown in Figure 1b, after electrospinning and pyrolysis, the obtained SiO2−NF and Au@SiO2−NF nonwoven mats were freestanding and mechanically flexible; these nonwoven mats were further functionalized by infusion with TiO2 nanoparticles. Representative SEM images of the SiO 2 nanofibers and the Au@SiO2 nanofibers are shown in Figure 1c,e, respectively, and the corresponding diameter distributions of the SiO2−NF and Au@SiO2−NF mats are shown in Figure 1d,f, respectively. Randomly overlaid nanofibers without D

DOI: 10.1021/acsanm.7b00316 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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that the bottom part of the mat was not filled by TiO2 nanoparticles. A similar pattern-transferring process was also used to fabricate the TiO2−NP/Au@SiO2−NF mat for DSSC and photocatalytic dye degradation applications. The morphology of the TiO2−NP/Au@SiO2−NF mat was similar to that of the TiO2−NP/SiO2−NF mat. The surface areas and pore size of the nanofibrous mats are shown in Figure S4. The BET surface areas of the SiO2−NF mat and the Au@SiO2−NF mat were 12.42 m2 g−1 and 12.50 m2 g−1, respectively. With impregnation of the TiO2 nanoparticles, the TiO2−NP/SiO2−NF mat had a surface area of 57.97 m2 g−1, and the TiO2−NP/Au@SiO2−NF mat had the surface area of 70.11 m2 g−1. The possible chemical binding in the TiO2−NP/Au@SiO2−NF mat was investigated using XPS spectroscopy, and the result is shown in Figure S5. The elements of the TiO2−NP/Au@SiO2−NF mat were mainly Ti and O, suggesting that TiO2 nanoparticles were impregnated onto the Au@SiO2−NF nanofibers uniformly during the process. Note that XPS only reveals the elemental information on the surface (up to 5 nm depth) so that only a trace amount of Si and no Au elements were observed in the XPS spectrum. Dye-Sensitized Solar Cells Based on Integrated Photoanode and Spacer. The controlled filling method resulted in both a functional layer of TiO2 nanoparticles and an insulating layer of SiO2 nanofibers in the freestanding nanofibrous mat, which was used as an integrated photoanode and spacer in DSSCs; the side of the mat containing the TiO2 nanoparticles served as a photoanode, whereas the other side of the mat with no TiO2 nanoparticles served as the spacer between the photoanode and the counter electrode. As shown in Figure 3a, the side of the SiO2−NF mat partially filled with TiO2 nanoparticles was in contact with the FTO glass. After being immersed in N719 dye solution, the areas of the SiO2−

Figure 2. (a) Schematic illustrating the fabrication procedure of the TiO2−NP/SiO2−NF composite mat. (b) Cross-sectional SEM image of the TiO2/SiO2 composite nanofibrous mat with fully filled TiO2 nanoparticles and the corresponding EDS spectra. (c) Cross-sectional SEM image of the TiO2/SiO2 composite nanofibrous mat with partially filled TiO2 nanoparticles and the corresponding EDS spectra.

shown in Figure 2a. Additionally, after 100 cycles of bending and release, the TiO2−NP/SiO2−NF mat remained mechanically robust and flexible without macroscopic cracks, as shown in the cross-sectional SEM image (Figure S3). The pattern-transferring process also enabled us to control the patterned area and penetration depth of TiO2 nanoparticles in the SiO2−NF. As shown in Figure 2a, the TiO2 nanoparticles were patterned as a 0.24 cm2 dot in the TiO2−NP/SiO2−NF composite mat and further sensitized by N719 dye. By adjustment of the thickness of the TiO2 nanoparticle layer, the depth of the TiO2 nanoparticles infused in the SiO2−NF mat could be controlled. In the SEM image of Figure 2b, the TiO2 nanoparticles were uniformly filled and attached to the SiO2 nanofibers. The EDS spectra in areas 1 and 2 showed similar composition of Si, Ti, and O, confirming the uniform filling of the TiO2 nanoparticles in the mat. In the crosssectional SEM image of Figure 2c, only the top part of a SiO2 nanofibrous mat was filled with the TiO2 nanoparticles. The partial filling was further confirmed by the EDS spectra of areas 3 and 4. In the spectrum of area 3, where the SiO2−NF mat was filled with TiO2 nanoparticles, the ratio of Si, Ti, and O was comparable with those in areas 1 and 2, indicating similar filling of TiO2 nanoparticles as in Figure 2b; on the other hand, the spectrum of area 4 showed very little amount of Ti, confirming

Figure 3. (a) Schematic structure of the DSSC device based on the integrated TiO2−NP/SiO2−NF composite photoanode and spacer. (b) J−V curves of the DSSCs. (c) Incident photon-to-current efficiency (IPCE) curves for the DSSCs. E

DOI: 10.1021/acsanm.7b00316 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Table 1. Performance of DSSCs with Different Types of Photoanodes under 100 mW cm−2 Simulated AM 1.5 Solar Light type of photoanode

efficiency (%)

Jsc (mA cm−2)

Voc (V)

fill factor

TiO2 NPs (Dyesol) TiO2−NP/SiO2−NF TiO2−NP/Au@SiO2−NF

4.91 ± 0.12 5.31 ± 0.22 5.72 ± 0.18

10.75 ± 0.27 11.62 ± 0.41 12.52 ± 0.33

0.68 ± 0.01 0.68 ± 0.01 0.68 ± 0.01

0.67 ± 0.01 0.67 ± 0.01 0.67 ± 0.01

NF mat filled with TiO2 nanoparticles (as the photoanode) were sensitized, and the other areas of the SiO2−NF were used as a spacer between the photoanode and the counter electrode. Three devices were fabricated and tested; two devices used the partially filled TiO2−NP/SiO2−NF and TiO2−NP/Au@SiO2− NF mats as photoanode and spacer, and the third (control) device was fabricated using TiO2 nanoparticles as photoanode and parafilm as spacer. The J−V curves of the DSSCs are shown in Figure 3b, and the performance parameters of the devices are summarized in Table 1. The control device had efficiency of 4.91%, Jsc of 10.75 mA cm−2, Voc of 0.68 V, and fill factor of 0.67. The device with the composite photoanode of TiO2 nanoparticles infused in the SiO2−NF mat had efficiency of 5.31%, which was about 7% higher than that of the control device; the improved efficiency was largely due to the increased Jsc, which might be attributed to the light scattering of the SiO2 nanofibers.34,35 The efficiency was further improved in the device composed of TiO2−NP/Au@SiO2−NF with 1 wt % Au. The increase of Jsc with the Au nanoparticles in the photoanode would be likely related to the plasmonic effect on the improved light-harvesting.36 The results of the incident photon-toelectron conversion efficiency (IPCE) under short-circuit condition are shown in Figure 3c. The IPCE spectra had similar shape to the absorption of the N719 dye,37,38 and the IPCE curves for the DSSCs were in good agreement with Jsc values observed from J−V curves. The integrated TiO2−NP/SiO2−NF photoanode and spacer was assembled with a titanium gauze (100 mesh woven from 0.05 mm diameter wire with opening area of 64%) to demonstrate the high-temperature processability of the TiO2−NP/SiO2−NF mat with flexible electrode material. The structure of the device is shown in Figure 4a. The titanium gauze has low sheet resistance, good flexibility, high transmittance of light, and superior corrosion resistance.39 Because of the high-temperature durability of the composite photoanode and titanium gauze, the TiO2 nanoparticles could be sintered at 500 °C, which would significantly improve the connectivity among TiO2 nanoparticles and the resultant device performance.40 The inset of Figure 4b shows the cross-sectional SEM image of the titanium gauze with TiO2−NP/SiO2−NF. The J−V curve of the device is presented in Figure 4b. This device had the efficiency of 3.09%, Jsc of 7.23 mA cm−2, Voc of 0.70 V, and fill factor of 0.61. Compared to the devices in Table 1, the low efficiency of the flexible device was due to the reduced area of the titanium gauze. After correction for the opening area of the titanium gauze (64%), the efficiency of the device was 4.82%. Figure 4c shows the solar cell performance of a patterned photoanode. The inset of Figure 4c shows the SiO2−NF mat impregnated with seven TiO2 dot areas sensitized with N719 to make a patterned photoanode for DSSC. The device based on the patterned photoanode had efficiency of 3.74 ± 0.20%, Jsc of 9.76 ± 0.92 mA cm−2, Voc of 0.71 ± 0.01 V, and fill factor of 0.54 ± 0.03. The low efficiency was likely due to the increased series resistance of the FTO with increased area of the photoanode, indicated by the low fill factor of the device.

Figure 4. (a) Schematic structure of the DSSC device based on flexible TiO2−NP/SiO2−NF composite photoanode/spacer and titanium gauze. (b) J−V curve of the device based on flexible photoanode and electrode. Inset: cross-sectional SEM image of flexible photoanode/spacer and titanium gauze electrode. (c) J−V curve of the device based on the patterned TiO2−NP/SiO2−NF photoanode sensitized with N719. Inset: photo of the freestanding patterned TiO2−NP/SiO2−NF mat.

Photocatalytic Dye Degradation. For an assessment of the photocatalytic activity of the freestanding and hightemperature durable TiO2−NP/SiO2−NF composite mat, the decomposition of a widely used dye, methylene blue (MB), was investigated. Figure 5a shows the setup of the photodegradation. The TiO2−NP/SiO2−NF (or TiO2−NP/Au@ SiO2−NF) nanofibrous mat was placed in the MB solution. Under light irradiation of a 1.5 AM solar simulator, the MB solution decolored gradually. After the MB solution became colorless, which indicated that the degradation of MB was completed, the freestanding TiO2−NP/SiO2−NF (or TiO2− NP/Au@SiO2−NF) mat was picked up from the solution. The nanofibrous mat was readily regenerated for continuous use in photodegradation by burning out the residues of photodegradation at 500 °C. The UV−vis adsorption changes of MB in the presence of different nanofibrous mats under simulated sunlight irradiation (110 mW cm−2) are shown in Figure 5b−e. Three absorbance peaks of MB at 243, 286, and 664 nm were observed. The peaks at 243 and 286 nm are attributed to the absorbance of the π → π* transition, whereas the peak at 664 nm is attributed to the absorbance of the n → π* transition.41 During the degradation, photons were absorbed by TiO2 and used for oxidation of the dye, with the initial opening of the central aromatic ring and an F

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Figure 5. (a) Flowchart showing the setup of dye degradation experiment and recycling of the TiO2−NP/SiO2−NF and TiO2−NP/Au@SiO2−NF composite mats. UV−vis spectra of the MB solution taken at different times in the presence of (b) the SiO2 mat (control sample), (c) the TiO2− NP/SiO2−NF composite mat, (d) the regenerated TiO2−NP/SiO2−NF composite mat, and (e) the TiO2−NP/Au@SiO2−NF composite mat under the simulated solar light.

almost complete mineralization of carbon, nitrogen, and sulfur into CO2, NH4+, NO3−, and SO42−, respectively.42 For the control sample of the pristine SiO2−NF mat without the photocatalytic TiO2 nanoparticles, the absorbance of the MB solution dropped slightly after 4 h of irradiation (Figure 5b). On the contrary, the absorbance of MB decreased significantly during the 4 h exposure to the solar irradiation with the presence of the TiO2−NP/SiO2−NF composite mat in the solution (Figure 5c), indicating the photodegradation of MB by the TiO2 nanoparticles. On the basis of the decrease of the absorbance at 664 nm, the decolorization of the MB solution and photocatalytic degradation achieved an efficiency of 90% after 4 h of irradiation. The TiO2−NP/SiO2−NF composite mat was readily recycled and regenerated by burning out the organic residue at 500 °C. As shown in Figure S6, the XRD patterns of the regenerated composite mats were identical to those of the original composite mats, indicating complete removal of the residue during regeneration. The regenerated TiO2−NP/SiO2−NF composite mat showed comparable photocatalytic performance as compared to the freshly prepared mat, as demonstrated in Figure 5d. The dye degradation experiment was also carried out using TiO2−NP/Au@SiO2− NF, and the UV−vis adsorption changes of MB were shown in Figure 5e, which showed a similar result and slightly faster photodegradation as compared to TiO2−NP/SiO2−NF. Figure 6a and b show the natural logarithm of the concentration change (c/c0) with time under simulated and nature solar light, respectively. Here, c0 refers to the initial concentration of the MB solution before irradiation, and c refers to the concentration of MB at different times. On the basis of Beer’s law, the concentration is proportional to the absorbance of MB at 664 nm in the UV−vis spectra in Figure 5. The ln(c/c0) varies linearly with time, suggesting that the photocatalytic degradation of MB by TiO2 nanoparticles followed pseudo-first-order reaction kinetics. The pseudofirst-order rate constants, obtained by the linear fit of the ln(c/c0) versus time curves in Figure 6, are summarized in Table 2. The MB solution with the presence of the control

Figure 6. Kinetics of photodegradation of MB in the presence of (a) different nanofibrous mats under simulated solar light, and (b) TiO2− NP/SiO2−NF and TiO2−NP/Au@SiO2−NF mats under natural sunlight. (c) Percentage of dye degradation after 2 h under simulated solar light in the presence of the regenerated TiO2−NP/SiO2−NF mat following the procedure in Figure 5a. The percentage of dye degradation is defined as (A0 − A)/A0 × 100%, in which A0 and A are the absorption of the methyl blue at 664 nm before and after 2 h of the photocatalytic degradation experiment, respectively.

sample (SiO2−NF mat) under light irradiation showed very little photodegradation. The freshly prepared and regenerated G

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TEM images of SiO2−NF and Au@SiO2−NF, and SEM image of TiO2−NP/SiO2−NF after 100 cycles of bending (PDF)

Table 2. Pseudo-First-Order Rate Constant for Reactions of MB Degradation type of nanofibrous mat

pseudo-first-order rate constant (h−1)

TiO2−NP/SiO2−NF, 1st TiO2−NP/SiO2−NF, 2nd TiO2−NP/Au@SiO2−NF TiO2−NP/SiO2−NF under natural sunlight TiO2−NP/Au@SiO2−NF under natural sunlight

0.53 0.45 0.60 0.66 0.81

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhengtao Zhu: 0000-0002-9311-2110 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

TiO2−NP/SiO2−NF mat showed comparable reaction rates, indicating the excellent regeneration of the photocatalytic properties of the nanofibrous mat by high-temperature treatment. Compared with the typical methods of photocatalytic degradation, in which the TiO2 nanoparticles or the polymer mesh infused with the TiO2 nanoparticles was suspended in solution, the freestanding TiO2−NP/SiO2−NF mat, in which the TiO2 nanoparticles were impregnated in the porous SiO2 nanofibrous mat, was readily recycled and regenerated for photodegradation.43 Figure 6c shows the percentage of dye degradation, under simulated solar light for 2 h as a function of regeneration times for the TiO2−NP/ SiO2−NF mat. After 10 cycles of reuse and regeneration (as shown in Figure 5a), the TiO2−NP/SiO2−NF mat was still highly effective in photocatalytic dye degradation. Additionally, by addition of the Au nanoparticles in the SiO2 nanofibers, the photodegradation efficiency was further improved in the TiO2− NP/Ag@SiO2−NF sample. This was because the Au nanoparticles could act as electron traps to facilitate the separation of photogenerated electron−hole pairs and promote the interfacial electron transfer process.44 The surface plasmon resonance of the Au nanoparticles under visible light irradiation might also play a role in enhancing the photocatalytic activity.45

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Aeronautics and Space Administration (NASA Cooperative Agreement NNX13AD31A). The authors would like to thank Professor Hao Fong and his group for their help with the electrospinning process. The authors would also like to thank Professor Lei Zhu for XPS measurement of our samples.

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CONCLUSIONS The SiO2 nanofibrous nonwoven mat was used as a porous and high-temperature durable substrate in preparation of the freestanding, flexible, and multifunctional composites. The neat SiO2−NF and the Au-nanoparticle-functionalized Au@ SiO2−NF were prepared by electrospinning. Freestanding and flexible TiO2−NP/SiO2−NF or TiO2−NP/Au@SiO2−NF then was formed by impregnation and patterned transfer of TiO2 nanoparticles followed by calcination at high temperature, in which the TiO2 nanoparticles provided functionality to the mat for DSSC and photocatalytic dye degradation. Composite mats only partially filled with TiO2 nanoparticles on one side were used as integrated photoanode and spacer in DSSCs, which had an efficiency of 5.31%. The device performance was further improved by incorporation of Au nanoparticles in the TiO2−NP/Au@SiO2−NF-based device, presumably due to the plasmonic effects. Additionally, the TiO2−NP/SiO2−NF or TiO2−NP/Au@SiO2−NF mats were used in effective photocatalytic degradation of methylene blue. Because of the freestanding and high-temperature durable nature, the composite mats were readily recyclable and regeneratable for continuous use in photodegradation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.7b00316. H

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