The present study reports on the introduction of various nanocatalysts containing nickel (Ni) nanoparticles (NPs) embedded within TiO2 nanofibers and TiO2 microparticles. Typically, a sol-gel consisting of titanium isopropoxide and Ni NPs was prepared to produce TiO2 nanofibers by the electrospinning process. Similarly, TiO2 microparticles containing Ni were prepared using a sol-gel syntheses process. The resultant structures were studied by SEM analyses, which confirmed well-obtained nanofibers and microparticles. Further, the XRD results demonstrated the crystalline feature of both TiO2 and Ni in the obtained composites. Internal morphology of prepared nanofibers and microparticles containing Ni NPs was characterized by TEM, which demonstrated characteristic structures with good dispersion of Ni NPs. In addition, the prepared structures were studied as a model for hydrogen production applications. The catalytic activity of the prepared materials was studied by in situ hydrolysis of NaBH4, which indicated that the nanofibers containing Ni NPs can lead to produce higher amounts of hydrogen when compared to other microparticles, also reported in this paper. Overall, these results confirm the potential use of these materials in hydrogen production systems.
In recent years, due to concerns about global warming and the depletion of fossil fuels from the natural reservoirs, the utilization of various other sources of energy had been intensively investigated by scientific society. There are various means of obtaining energy from the natural and artificial resources. Among the various forms of energy, hydrogen has become one of the most promising future energy means of harvesting. However, the production of this important source by direct water splitting without any byproducts is one of potential alternatives to hydrogen fuel for future energy supply [
Similarly, the strategies to use the metal-catalyst in presence of NaBH4 for accelerating the rate of hydrogen production had recently been accomplished. These various metal-catalysts include the platinum (Pt) [
The electrospinning technique has attracted considerable attention due to the production of fibers with diameters that range from the micrometer to the nanometer size [
The present work presents the fabrication of various kinds of nanocatalysts forms and their capability to produce hydrogen. The fabricated nanoforms were fabricated as pure TiO2 nanofibers, modified TiO2 nanofibers containing Ni nanoparticles (NPs) and TiO2 microparticles containing Ni NPs. These prepared nanocatalysts have been intensively studied and well characterized with various states of the art techniques. After characterization, the efficiency of these materials was tested for the production of hydrogen through in situ hydrolysis of NaBH4.
Poly(vinyl acetate) (PVAc, Mw = 500,000 g/mol) was obtained from Sigma Aldrich, USA. Titanium (IV) isopropoxide [Ti(Iso)], 98% assay was purchased from Junsei Co. Ltd., Japan. Nickel nanopowder <100 nm, 99.9% pure was purchased from Aldrich, USA.
The morphology of the obtained nanocatalysts was analyzed utilizing a JEOL JSM-5900 scanning electron microscope, JEOL Ltd., Japan. The phase and crystallinity of the nanofibers and microparticles was investigated using an X-ray diffractometer (XRD, Rigaku Co., Japan) with Cu K
The electrospinning process was utilized to produce TiO2 nanofibers containing Ni NPs. Typically, a sol-gel was prepared by mixing Ti(Iso) and PVAc (20 wt%, in DMF) with a weight ratio of 2 : 3. Thereafter, a few drops of acetic acid were added until the solution became transparent under stirring. To fabricate the sol-gels containing Ni NPs, a step by step methodology was adopted. Briefly, 0.5 g of nickel NPs, were added into a previously prepared transparent solution of Ti(Iso)/PVAc; the solution was subsequently homogenized under stirring for 10 minutes. A high voltage power supply (CPS-60 K02V1, Chungpa EMT Co., Republic of Korea), capable of generating voltages up to 60 kV, was used for electrospinning the sol-gels. The solution to be electrospun was supplied through a plastic syringe attached to a capillary tip, which contained a copper pin to connect to the positive electrode (anode) in the high power supply. The electrospinning system was completed through the attachment of the negative electrode (cathode) to a grounded metallic collector. The nanofibers were deposited on rectangular collector covered with thin sheet of aluminum foil, equipped with heating system having temperature of 40°C, which helps to remove the residual solvents after the fiber lands on collector (Scheme
The schematic illustration of a simple electrospinning spinning apparatus: (1) dc power supply (2) syringe, and (3) collector.
A sol-gel was prepared by mixing Ti(Iso) and PVAc (20 wt% in DMF) with a weight ratio of 2 : 3, respectively. Thereafter, a few drops of concentrated acetic acid were added under stirring to afford a transparent solution, to which 0.5 g of nickel NPs were added. Hereafter, these solutions were homogeneously mixed under stirring for 10 minutes. However, instead of electrospinning as previously described for fabrication of nanofibers, this solution was dried under vacuum at 80°C for 48 h to completely remove the solvents. The obtained solid materials were finely ground and sintered in air at 600°C for 1 h with a heating rate of 5°C/min. After the sintering process, the samples were further subjected to fine grinding to additionally reduce their size.
All the samples were investigated for their catalytic activity for the in situ hydrolysis of NaBH4. Typically, 50 mg of all the sample combinations were placed in a specially designed tight sealed flask which contained 50 mL of a distill water containing 50 mg of NaBH4 at constant temperature (25°C). The catalytic performances were compared for the hydrogen production from hydrolysis of NaBH4. Briefly, the reaction proceeded at a stirring rate of (1000 rpm) and the amount of hydrogen generated over time was measured immediately after all the components were added through the water displacement method [
In these experiments, PVAc was used as a binder for making the sol-gels to get good viscous solutions, so as to have the appropriate bending instability during the electrospinning process. After getting the nanofiber mats from electrospinning of the sol-gels, the obtained nanofiber mats were dried, and further subjected to frying in a furnace to remove the polymer binder (i.e., PVAc). In this context, (Figure
SEM images of the obtained catalysts after the calcinations process. Images of the pristine TiO2 nanofibers (a), images of the TiO2 nanofibers containing Ni NPs (b), and TiO2 microparticles containing Ni NPs (c).
Figure
Representation of this novel strategy to fabricate TiO2 nanofibers containing Ni NPs.
SEM results of the TiO2 microparticles containing Ni NPs are shown in (Figure
SEM images with EDX analysis. The EDX area of the pristine TiO2 nanofibers and its corresponding data (a), the EDX area of the modified TiO2 nanofibers containing Ni NPs and its corresponding data (b), the EDX area of the modified TiO2 microparticles containing Ni NPs and its corresponding data (c).
It is well known, that TEM can be utilized to differentiate between two different materials in regards to their different crystalline patterns. Therefore, to investigate the crystalline features of the prepared materials, TEM images were obtained and presented in (Figure
TEM images of nanofibers and microparticles after calcination process. Pristine TiO2 nanofibers in low magnification (a), the HRTEM images of the former figure (b). The low magnification images of the TiO2 nanofibers containing Ni NPs (c), the HR-TEM images of the corresponding former figure. The low magnification images of the TiO2 microparticles containing Ni NPs (e), the HR-TEM images of the corresponding figure (f).
As shown in (Figure
XRD results of the different nanocomposites.
Figure
Hydrogen production of the by various nanocomposites at 26°C in pH 7.4 distilled water to demonstrate the highest hydrogen production.
To practically investigate that nanofibers do have higher surface to volume ratio than that of microparticles, which in turn can put more light, the surface area of both fibers and microparticles has been measured by using Brunauer-Emmett-Teller (BET) technique (ASAP 2010, Micromeritics, Norcross, GA). It is noteworthy to mention that exact amount of 0.5 g of Ni NP was used to mix with sol-gel to fabricate nanofibers and microparticles, therefore one would assume that two combinations would produce same results. However, from those tests, it was observed that microparticles containing Ni NPs had surface area of 13.6765 ± 0.675 m2/g. The pure TiO2 nanofibers had a surface area of 15.3456 ± 0.1751 m2/g, and the nanofibers containing Ni NPs had a surface area of 23.2782 ± 0.1961 m2/g, respectively. These results are in accordance with our previous reports [
In conclusion, we were able to fabricate three different types of nanomaterials through combination of sol-gel and electrospinning process. Electrospinning of a colloid comprised of Ti(Iso) and Ni NPs produced ceramic nanofibers that contained attached Ni NPs and partially captured NPs. The SEM instrument was used to find out the morphologies of nanomaterials after the electrospinning and calcinations of sol-gel. SEM equipped with EDX technique was used to differentiate between pristine and Ni-loaded nanofibers and microparticles composites. TEM images were used to determine the appearance of Ni NPs over the individual nanofibers. All the materials obtained were evaluated for their catalytic activity towards hydrogen production, resulting in the nanofibers containing Ni NPs being the most promising materials for this application was due to high surface to volume ratio of nanofibers containing Ni NPs resulted to produce the highest production of hydrogen.
This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0006693). F. A. Sheikh and J. Macossay are thankful for partial financial support for this work from NIH-NIGMS-NIA Grant no. 1SC2AG036825-01.