A novel, cost effective, sonochemical-hydrothermal technique was used for the deposition of nanosized anatase titanium dioxide (TiO2) onto single wall carbon nanotubes (SWCNTs). This technique is described and the characterization of the synthesized TiO2-SWCNTs is reported. The characterization techniques employed include scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). From the characterization the size and morphology of the synthesized TiO2 nanoparticles (deposited on the SWCNTs) are reported. Furthermore, it is demonstrated that the created TiO2 nanoparticles are chemically attached to the SWCNTs. Also, an important correlation between calculated TiO2 crystal size and the red shifts in the lowest Raman TiO2 (E.g.) predominate peak is reported. The synthesized TiO2-SWCNTs have potential for large scale production and application in a variety of new technologies such as clean energy power generation devices, electrical storage devices, photocatalysts, and sensors.
The benefits of coating nanosized titanium dioxide (TiO2) onto the surface of carbon nanotubes (CNTs) is of great scientific interest as nanoparticles often exhibit different physical and chemical properties, as the size of the material becomes smaller and smaller, relative to their bulk material counterparts. This interesting shift in particle properties is largely due to the large surface area and surface to volume ratio of the material, which dominates the contributions made by the small bulk of the material [
Titanium dioxide coated carbon nanotubes (TiO2-CNTs) are currently being developed and considered for many applications, where nanoparticle (TiO2) are presently used, such as sensors, photovoltaics, and photocatalyts. The beneficial pairing of nanoparticle TiO2 with CNTs, or more particularly the pairing of TiO2 with single-wall carbon nanotubes (SWCNTs), is the enhancement of the many unique properties of unmodified nanosized TiO2. SWCNTs possess excellent mechanical properties, large surface areas, one dimensional electron transport properties, and allow for surface chemical modifications to control the type of bonds that can be formed with TiO2. Additionally, the TiO2-SWCNT interface results in a unique quantum effect to provide trap states for electrons. This is an useful technique, known as the Schottky barrier, where there is a space–charge separation region that functions to increase recombination times for electron–hole pairs. [
Various methods of synthesizing TiO2-CNTs have been reported. Generally, most of these processes are either time consuming, cumbersome, expensive, lack the control for the deposition TiO2, and/or produce inferior results. A list of some of these processes include sol-gel [14-18], sol [19,20], hydrothermal [21,22], solvothermal [
The advantages of using the novel sonochemicalhydrothermal technique to prepare titanium dioxidecoated carbon nanotubes (TiO2-CNTs) include low cost and readily available: 1) manufacturing equipment (sonicator, autoclave, and furnace); and 2) chemical precursors (aqueous titanium(III) sulfate and carbon nanotubes). Additionally, the technique is easy and relatively quick, producing chemically attached, uniformly dispersed, TiO2 encapsulated CNTs.
The synthesis of the TiO2-SWCNTs started by loading 10 mg of SWCNTs into a 50 mL conical bottom polypropylene centrifuge tube. Next, 20 ml of 0.38 M titanium(III) sulfate solution was added. The centrifuge tube with the SWCNTs and solution was then placed in a test tube rack under a sonicator probe horn that is mounted on a ring stand. The sonicator used is a Qsonica ultrasonic cell disruptor, model Q125 by Misonix, equipped with a 1/8" diameter probe and operating at a frequency of 20 kHz. The probe tip was immersed into the solution and a power of 1 W/mL was utilized. Samples were exposed for set amount of times to achieve desired amount of TiO2 deposition. After sonification was complete the tube containing the synthesized TiO2-SWCNT in titanium(III) solution was centrifuged at 3500 RPM for 1 hour. The tube was then removed from the centrifuge and the liquid supernatant was decanted from the TiO2- SWCNTs that were dropped to the bottom of the container. The TiO2-SWCNTs were then washed by adding 20 mL of deionized water and exposing to ultrasound sonication for a duration of 3 minutes at a power of 1 W/mL to re-suspend the TiO2-SWCNTs. The tube was then re-centrifuged at 3500 for 1 h, the liquid decanted and replaced with clean deionized water, and then resuspended by sonication for 3 minutes at a power of 1 W/mL. This wash procedure was repeated 2 more times. The TiO2-SWCNTs were stored in water in the centrifuge tubes until ready for characterization.
It was determined by Raman analysis that an amorphous form of TiO2 was deposited on the surface of the SWCNTs via this sonochemical process. It is of interest to convert amorphous (poorly crystallized) forms of deposited TiO2 to their corresponding ordered anatase crystal structure. For this conversion, a hydrothermal calcination process was used. Here, 10 mg of the TiO2-SWCNT reaction product and 10 mL of deionized water were placed in 23 mL Teflon® lined, high pressure stainless steel digestion bomb, utilized as an autoclave. The autoclave was loaded into a furnace for 8 h at 250 degrees C to achieve hydrothermal calcination of the TiO2-SWCNT product. Upon completion the finished autoclaved samples were cooled to ambient and stored in the deionized water.
The morphology of pristine SWCNTs and TiO2-SWCNTs (2 h sonication) were characterized by scanning electron microscopy (S 4800 SEM, Hitachi Co., Tokyo, Japan). The pristine SWCNT sample for this analysis was prepared by ultrasonic dispersion in isopropanol followed by deposition onto a conductive porous silver membrane. The TiO2-SWCNTs (stored in water from the synthesis step) were prepared for analysis by homogeneously dispersing in water followed by deposition onto a conductive porous silver membrane. A small sample was cut from this prepared composite and adhered via conductive tape to the microscope stage. Once the sample was staged it was then loaded into the microscope for analysis.
Sample spectra were obtained using a Thermo Scientific Nicolet Almega XR Dispersive Raman Spectrometer equipped with an Olympus BX-51 research microscope. Raman spectra analysis was conducted on samples from 100 to 4000 cm−1 with the laser operating at 532 nm at 100% of 150 mW with the beam going through the microscope equipped with a MPlain 10X BD objective.
X-ray diffraction (XRD) data was collected by using a Rigaku Miniflex, with CuKα radiation of 40 kV/4 mA, λ = 1.5406 Å. For pristine SWCNTs, the sample was prepared by deposition of SWCNTs on a silver membrane filter as described above for the SEM analysis. A small sample was cut from this prepared composite and the scan was ran from 2θ = 10˚ to 40˚. For the remaining samples (TiO2-SWCNTs), measured as dry powders, preparation consisted filling the powder sample dish and loading into the diffractometer. The collected scans were obtained from a range of 2θ = 20˚ to 75˚.
The first point of reference is
From the SEM image it is demonstrated that the SWCNTs show a superstructure that exhibits a high degree of entanglement between the bundles of tubes. This can be accounted for by the report that SWCNTs allow for remarkable Van der Walls interactions due to their smooth, uniform surfaces in close proximities. Thus, the majority of the tubes are bundled via direct van der Waals attractions along their entire lengths [
The Raman spectrum for the SWCNTs is shown in
As reported by Eklund, five peaks centered at around 1340 cm−1 (D mode), 1580 cm−1 (G mode), 2450 cm−1, 2680 cm−1 (G’ mode), and 3180 cm−1 are due to SWCNTs [
XRD was performed for SWCNTs deposited on a silver membrane filter as shown in
A single peak centered at 2θ = 38.36˚ is identified as silver from the membrane filter support and agrees with published literature [
spectrum is used for comparison against TiO2-SWCNTs spectrums.
The Raman spectra for P25 (a commercially produced nanosized TiO2 from Evonik) is shown in
The six Raman active modes, A1g + 2B1g + 3Eg, for anatase TiO2 that have been reported in the literature are detected at 144 cm−1 (E.g.), 197 cm−1 (E.g.), 399 cm−1 (B1g), 513 cm−1 (A1g), 519 cm−1 (B1g), and 639 cm−1 (E.g.),[
XRD of P25 nanoparticle was ran from 2θ = 20˚ to 75˚. Anatase TiO2 has diffraction peaks at 2θ = 25.18˚ (strongest peak), 37.78˚, 48.00˚, 53.89˚, 54.99˚, 62.57˚, 68.68˚, 70.15˚, and 75.01˚, corresponding to the reflections from 101, 004, 200, 105, 211, 204, 116, 220, and 215 crystal planes [
Here, the spectrum shows predominately anatase TiO2 with the necessary diffraction peaks at 2θ = 25.18˚ (strongest peak), 37.78˚, 48.00˚, 53.89˚, 54.99˚, 62.57˚, 68.68˚, 70.15˚, and 75.01˚. The remaining peeks are due to the rutile structure. The integrated intensities of the strongest peak of both anatase (Ia,101 at 25.4˚) and rutile (Ir,110 27.5˚) were used to calculate the percentage of rutile and anatase according to the formulas: Wr(%) = Ir/(0.8844xIa + Ir) × 100 and Wa(%) = 100 − Wr(%) [
This portion of the study was conducted in order to characterize and determine a profile for increasing sonication exposure times for the deposition of TiO2 onto SWCNTs. Sonochemical deposition times of 10 minute, 20 minute, 30 minute, and 2 h were performed and investigated.
From the micrograph microscopy image it is clear that the entangled superstructure and, clearly demostrates the nanosized TiO2 that has been successfully decorated on the SWCNTs. It is noted that homogeneous deposition of both agglomerates of small TiO2 particles and isolated grains appear to be deposited on the SWCNTs. This is in agreement with findings reported by Yao [
Figures 7-10 depict the Raman spectrum of the manu-
factured TiO2-SWCNTs from 90 cm−1 to 4000 cm−1, at deposition times of 10 minute, 20 minute, 30 minute, and 2 h exposures to sonication.
All the Raman spectra reveal the SWCNTs peaks but are absence of crystalline TiO2, as noted by comparison with
Here the spectrum shows no discernible peaks that would be expected for a crystalline material, as noted by comparison with the XRD spectrum for P25 as seen in
Figures 12-15 depict the Raman spectrum of the synthesized TiO2-SWCNTs from 90 cm−1 to 4000 cm−1, at
deposition times of 10 minute, 20 minute, 30 minute, and 2 h exposures to sonication, followed by hydrothermal
calcination treatment.
All the Raman spectra displayed the necessary peaks for anatase TiO2 crystals and SWCNTs showing they are both present in the samples. This is illustrated by comparison with the spectrum obtained for P25 as shown in
This is also the case for the other measured TiO2-SWCNT
samples.
All the samples show spectral peaks that are broadened and/or shifted with respect to P25, with the 10 minute sample showing the largest shift. For the 10 minute sample, the largest shift differences are noted at the highest and lowest E.g. mode. The highest mode has blue shifted (peak at 631.58 cm−1) while the lowest mode (E.g.) has red shifted by approximately Δ16 cm−1 (from 147.20 cm−1 for P25 anatase to 160.68 cm−1). A similar shift was reported by Bersani et al. who attributed it to phonon confinement caused by the decrease in the crystal size of the anatase TiO2 [
Here the compiled Raman spectrum from 90 cm−1 to 220 cm−1 for the synthesized TiO2-SWCNTs at deposition times of 10 minute, 20 minute, 30 minute, and 2 h exposures to sonication with hydrothermal calcination treatment is shown. The spectrum is normalized for the large TiO2 (E.g.) peaks clustered around 147 cm−1 to 161 cm−1 with the spectrum of p25 added for reference. With the grain size of P25 equal to a measured 20.4 nm, it is clear that the samples have relatively smaller sized TiO2 particles. The decrease in size trend corresponds to a decrease in sonication exposure time. In addition to the shift in peaks, an increase in band asymmetry and broadening are observed as the length of sonification time decreases. The broadening of the peaks is reported to result from strain gradients in systems where TiO2 is being chemically anchored to CNTs. These strain effects can extend several nm into materials [
width at half maximum (FWHM) for the samples at the lowest E.g. mode is: 10 minute = 49.1 cm−1, 20 minute = 41.8 cm−1, 30 minute = 41.2 cm−1, 2 h = 39.2 cm−1, P25 = 25.4 cm−1.
The XRD spectra for the hydrothermally treated samples, including 10 minute, 20 minute, 30 minute, and 2 h are presented in Figures 18-21.
Here the scans were ran from 2θ = 20˚ to 75˚. The spectrums show the characteristics of predominately anatase TiO2 with diffraction peaks at 2θ = 25.18˚ (strongest peak), 37.78˚, 48.00˚, 53.89˚, 54.99˚, 62.57˚, 68.68˚, 70.15˚, and 75.01˚. The average crystal diameter for each sample was calculated from the largest peak by the Scherrer’s equation as described previously. These values are reported in
It is clear from the data that the size of TiO2 decreases as the length of deposition time is reduced. The size of the deposited TiO2 versus the observed Raman red opshift, for the lowest mode (E.g.), for the samples evaluated, is also depicted in
In this report a novel sonochemical technique and subsequent hydrothermal annealing process, utilized to synthesize crystalline anatase TiO2-SWCNTs was disclosed. This method is low cost and uses readily available equipment and chemical precursors. Additionally, the process is easy and relatively quick, producing chemically attached, uniformly dispersed, TiO2 encapsulated CNTs. These main advantages of the sonochemicalhydrothermal technique generally provide a superior
synthetic route as compared with other processes that are either time consuming, cumbersome, expensive, lack the control for the deposition TiO2, and/or produce inferior results. The TiO2-SWCNTs were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. From the characterization the size, ranging from 8.4 to 20.3 nm, anatase and rutile morphology of the synthesized TiO2 nanoparticles were reported. Furthermore, it was postulated that the created TiO2 nanoparticles are chemically attach to SWCNTs. This is supported by the Raman spectra where in addition to the observed red shift in lowest TiO2 E.g. Raman peak, an increase in band asymmetry and broadening are also observed. An important correlation between calculated TiO2 crystal size and the red shifts in the lowest Raman TiO2 (E.g.) predominate peak was reported. The synthesized TiO2-SWCNTs have potential for application in a variety of new technologies such as clean energy power generation devices, electrical storage devices, photocatalysts, and sensors.
This work was partially supported by AFRL/Clarkson Aerospace Corp Minority Leaders Program, TSU 10- S567-012-02C2. The authors would also like to thank Dr. Tineke Berends, at Houston Community College, for use of the XRD and her assistance with collection of the XRD data.