Grantee Research Project Results
Final Report: Plasmon Sensitized TiO2 Nanoparticles as a Novel Photocatalyst for Solar Applications
EPA Grant Number: R829603Title: Plasmon Sensitized TiO2 Nanoparticles as a Novel Photocatalyst for Solar Applications
Investigators: Chumanov, George
Institution: Clemson University
EPA Project Officer: Aja, Hayley
Project Period: July 1, 2002 through June 30, 2005 (Extended to June 30, 2006)
Project Amount: $320,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Nanotechnology
Objective:
Efficient conversion of sunlight into electrical and/or chemical energy is of great technological importance for modern society and future generations. One attractive possibility for utilization of solar energy is based on the ability of small semiconductor particles to function as photocatalysts promoting various oxidation and reduction reactions under sunlight. Titanium dioxide (TiO2) is the most promising material for such applications because it is an efficient, environmentally friendly, and relatively inexpensive photocatalyst. However, wide technological usage of this photocatalyst is largely hindered by the fact that ultraviolet light that does not constitute significant fraction of solar spectrum is required for its activation. Any improvement of photocatalytic efficiency of TiO2 by shifting its optical response from UV to the visible spectral range will have profoundly positive effect.
The main objective of the research was to synthesize and test a novel photocatalyst that consists of small silver or gold nanoparticles covered with a thin TiO2 shell. Silver and gold nanoparticles are very efficient systems for the interaction with visible light due to the excitation of plasmon resonances. It is expected that, due to the coupling of plasmon resonances in the core with the electron-hole pair generation in the shell, these hybrid Ag/Au TiO2 nanoparticles will exhibit photocatalytic activity in the visible spectral range thereby utilizing more efficiently solar energy.
Summary/Accomplishments (Outputs/Outcomes):
Synthesis of Silver Nanoparticles
A new method was developed. The nanoparticles were synthesized by slow reduction of aqueous silver oxide solution by hydrogen gas under pressure. The reaction can be terminated at any time, providing particles of a desired size to better match with solar spectrum. Typically, 3 hours of the reduction results in 90-100 nm particles as determined from transmission electron microscopy images. Subsequently, several centrifugation and filtering steps were applied to remove residual chemicals and to obtain particles with narrow size distribution. A unique feature of the nanoparticles prepared by this method is that they do not have any extraneous ions except hydroxy species associated with the metal surface. The cleanness of the particle surface is an important requirement for the photocatalytic function of this system. The particles were characterized with a variety of techniques including spectroscopy, Raman scattering, electron microscopy, atomic force microscopy, and X-ray diffraction. For the first time, the absorption and scattering spectra and efficiencies were separately measured for a wide selection of different particles of sizes ranging from 20 to 150 nm. Based on these results, silver nanoparticles of 50-60 nm diameter were identified as the most suitable for photocatalytic applications as they exhibit the highest efficiency (10 fold) for the interaction with light in the visible spectral range. The results are summarized in Evanoff and Chumanov, 2004a (1); Evanoff and Chumanov, 2004b (2); and Kumbhar, et al., 2005 (3).
Measuring Distance Dependence of the Local Field Near Ag Nanoparticles
It is well known that optical excitation of plasmon resonances in metallic nanoparticles results in the local electromagnetic field around the particles that is enhanced compared to the incident field. The distribution of this field around the particles depends on the particle parameters, such as size, shape, optical constants, and surrounding dielectric medium. It is important to know this distribution for designing of plasmon enhanced photocatalysts. The distribution has been previously modeled theoretically, and we addressed this issue experimentally by measuring spectral shifts of the plasmon resonance as a function of the thickness of a silica layer around silver nanoparticles. Different thickness silica layers around particles were synthesized using the sol-gel chemistry. The observed red spectral shifts occur because of the increase of the local dielectric function near the particle as the thickness of the silica layer increased. At some limiting thickness, no further spectral shift was noted, and this thickness was interpreted as being equal to the distance to which the local field extends from the surface of the nanoparticles. The result was consistent with theoretical models and the other study where self-assembled monolayers were used to change the local dielectric function. The results are published in Evanoff and Chumanov, 2005 (4).
WO3 – Modified Ag Nanoparticles
In order to understand processes that take place at the metal semiconductor interface and are important for catalytic properties of hybrid photocatalysts, we have fabricated and studied Ag nanoparticle arrays coated with tungsten oxide layers. Tungsten oxide is a known electrochromic material that undergoes a reversible redox process to form reduced tungsten oxide of mixed valency. During this process the color, the electric conductivity, and, consequently, the dielectric function of the oxide change. We discovered that this change can be used for modulating the plasmon frequency of the coupled arrays. The modulation took place via the damping of the plasmon resonance. The damping was attributed to increase of the imaginary part of the dielectric function due to the double injection of Li+ ions and electrons into the WO3 matrix. Nevertheless, 10-fold improvement of the modulation depth and the response time was achieved with this approach as compared to tungsten oxide alone.
In another work, we studied the direct injection/withdrawal of electrons into/from silver nanoparticles by potential-induced changes of the local dielectric environment. The arrays were prepared on indium tin oxide transparent electrodes and differential spectroelectrochemical measurements were undertaken. These experiments reveal only plasmon shift without damping, and the results were interpreted as potential-induced changes in the particles’ local dielectric function that does not have a large imaginary part. In order to explore analytical value of this behavior, we attempted to utilize for DNA hybridization assay. Whereas the adsorption of a single-stranded oligonucleotide on the surface of the arrays resulted in well-pronounced changes in differential spectra, the addition of the complementary strand led to only weak, somewhat irreproducible changes. The described results were published in Evanoff and Chumanov, 2005 (4) and Daniels and Chumanov, 2005 (7).
Preparation of Titania-Coated Silver Nanoparticles
The experience gained with tungsten oxide was utilized for coating Ag nanoparticles with titania. A technique based on sol-gel chemistry of alcoxides was developed to synthesize hybrid metal/semiconductor nanoparticles. In the process of developing of this technique, different titanium alcoxides with various concentrations as well as a number of catalysts and capping agents were tested. The biggest obstacle that needed to be overcome relates to finding condition for the condensation of a titania precursor on the surface of silver nanoparticles without inducing their aggregation. In a typical coating reaction, silver nanoparticles are transferred into water/ethanol (20:80 v/v) solution to obtain a specific optical density. Titanium (IV) n-butoxide is added in the amount that depends on the concentration of silver particles and the desired thickness of the titania layer. For example, a suspension of silver nanoparticles with optical density 8 requires 0.05 mM titanium (IV) n-butoxide to yield 20 nm thick titania layer. The hydrolysis reaction is allowed to proceed for an hour under vigorous stirring. Addition of 10 ml of water/ethanol mixture and immersing the reactor in an ice-bath initiates the condensation process. The reaction is further stirred for an hour in the ice-bath to ensure complete condensation of the amorphous titania around the silver nanoparticles. The resulting solution is centrifuged and washed several times with ethanol to remove residual chemicals and free titania nanoparticles. Finally, the coated particles are redispersed in water for further characterization. By increasing the concentration of titanium (IV) n-butoxide while keeping the concentrations of water and silver nanoparticles constant in the overall reaction mixture, titania layers of different thickness were obtained around the particles. The results were published in Chumanov and Kumbhar, 2004 (5).
Hydrothermal Treatment
Aqueous suspensions of titania-coated silver nanoparticles were sealed under vacuum in quartz tubes. The tubes were then introduced into pressurized stainless steel bombs and heated for various times at different temperatures. The hydrothermal treatment was explored for the purpose of converting amorphous titania resulted from the solgel reaction to a crystalline anatase form that is known the most photochemically active form of titania. The complete conversion to anatase was achieved at 350°C for 4 hours in an autoclave. Special care was exercised to prevent oxidation of silver nanoparticles. This was achieved by making a reducing environment in the reaction vessel. The coated nanoparticles were characterized by a number of spectroscopic techniques, electron microscopy, and X-ray diffraction. The results related to coating of silver nanoparticles with titania and converting it to the anatase form are published in Chumanov and Kumbhar, 2004 (5).
Photochemical Experiments
Photocatalytic activity of aqueous solutions of the hybrid nanoparticles was tested in the decolorization of sulforhodamine-B. The dye concentration was maintained at 10-7 M for all the photocatalytic reactions. The photocatalytic measurements were performed in a 50 ml Pyrex photoreactor at room temperature and atmospheric pressure in a dark room. The reactor was placed at a fixed distance of 10 cm from the lamp housing. Laser light with wavelengths 520 nm and 568 nm corresponding to the resonance maxima of silver nanoparticles were used to see the effect of charge transfer and the enhanced local electromagnetic field. Also, a 150 W Xe lamp was employed as a visible light source. UV cutoff filter was placed between the irradiation source and the photoreactor to eliminate radiation below 420 nm while using the 150 W Xe lamp light source. The hydrothermally treated titania particles degraded the sulforhodamine-B dye molecules confirming the charge transfer from the excited state of the dye molecule to the conduction band of titania, which in turn results in its decolorization, suggesting the sulforhodamine-B undergoes degradation through the dyesensitization mechanism.
However, the hydrothermally treated titania-coated silver nanoparticles did not show any improved photocatalytic activity as compared to titania alone when excited into Plasmon resonance. Moreover, when excited into the band gap of titania, the observed photocatalytic activity of the hybrid nanoparticles was actually smaller than that of titania alone. The tests were performed with sulforhodamine-B. The reduced activity was explained in terms of electrical shortening that reduced life time of the photoinduced electron/hole pairs. The silver core acts as an acceptor for both photogenerated electrons and holes leading to their efficient recombination. This process competes with the electron transfer to sulforhodamine-B molecules, thereby greatly reducing the efficiency of their reduction.
Iron-Doped Titania Nanoparticles
To circumvent the problem of electrical shorting, a thin dielectric spacer layer between the silver core and titania coating was developed. Enhancement of photocatalytic properties in such hybrid nanoparticles is expected to take place via enhanced local field associated with plasmon excitation in silver core. Two issues should be resolved in order to test this hypothesis. First, for the efficient energy transfer the band gap absorption in titania should overlap with the plasmon resonance, which is in the visible spectral range. Second, to optimize the thickness of the spacer layer, it is necessary to measure how far the local field propagates from the surface of silver nanoparticles. The first issue was addressed by synthesizing metal-doped titania, which is known to be capable of exhibiting photocatalytic activity more in the visible spectral range. Among various metal ions, doping with iron (III) has been widely investigated because of its unique electronic structure and its size that closely matches that of titanium (IV). The energy level of the Fe4+/Fe3+ couple is just above the titania conduction band and the energy level of the Fe3+/Fe2+ couple is just above the valance band. A modified sol-gel process was developed to synthesize iron-doped titania photocatalyst, and the photocatalyst was extensively characterized by a variety of analytical techniques. This titania was further tested for photocatalytic activity and sulforhodamine-B reduction was observed when excited with visible light. These results are published in Kumbhar and Chumanov, 2005 (6).
Plasmon Coupling in 2D Arrays of Ag Nanoparticles
While working with hybrid nanoparticles for photocatalytic applications, we discovered a novel phenomenon that is expected to lead to the development of novel methods for utilization of solar energy. Self-assembly of Ag nanoparticles prepared by the hydrogen reduction method on PVP-modified surfaces results in 2D arrays. The interparticle distance in these arrays can be controlled by varying the self-assembly time and the concentration of the nanoparticles in the suspension. It is important to accomplish the self-assembly from suspensions of low ionic strength (free of ions) to ensure a thick double layer around the nanoparticles and, consequently, long range electrostatic repulsion. This prevents the particles from forming clusters on the surface. We observed a new phenomenon, which is the near-field plasmon coupling in 2D arrays of Ag nanoparticles when the interparticle distances were comparable to the particles’ diameter. The coupling was characterized by dramatic changes in the extinction spectra resulted in an intense sharp peak in the blue spectral range. We studied the dependence of this coupling on the interparticle distance by immobilizing the arrays into polydimethylsiloxane resin, as described above. The immobilized arrays were stretched along one and two axes with simultaneous measurements of the extinction spectra. Extremely sharp dependence of spectral shape was observed: a 50 percent increase in interparticle distance led to complete plasmon decoupling and the extinction spectra became identical to those of free nanoparticles in suspension. Such sharp dependence as well as results from polarization-dependent and angular-dependent absorption measurements allowed us to conclude the quadrupolar character of the coupling. The coupling takes place by virtue of the overlapping local electromagnetic fields associated with the electron oscillation in the nanoparticles. The sharp peak corresponds to a new plasmon mode characterized by coherent electron oscillations in neighboring particles and represents a cooperative interaction of many nanoparticles with light. The strong dependence of the frequency of this peak on the dielectric medium and species adsorbed on the metal surface makes these arrays amenable for sensing applications. These results are detailed in Chumanov and Malynych, 2005 (8) and Malynych and Chumanov, 2006 (9).
Conclusions:
Methods for synthesis of several different hybrid Ag/semiconductor nanoparticles were developed. Plasmonic hybrid nanoparticles, specifically silver/titania have shown promises as improved solar photocatalyst for many environmental applications. In addition, a novel phenomenon was discovered that is coherent plasmon coupling in 2D arrays of Ag nanoparticles. This phenomenon will have a major impact on the development of advanced methods for utilization of solar energy, such as photovoltaics and optical antennas. All relevant results were published in 11 peer-reviewed journal articles and 14 selected presentations at national and international meetings.
References:
- Evanoff Jr. DD, Chumanov G. Size-controlled synthesis of nanoparticles: 1. “Silver-only” aqueous suspensions via hydrogen reduction. Journal of Physical Chemistry B 2004;108(37):13948-13956.
- Evanoff Jr. DD, Chumanov G. Size-controlled synthesis of nanoparticles: 2. Measurement of extinction, scattering, and absorption cross sections. Journal of Physical Chemistry B 2004;108(37):13957-13962.
- Kumbhar AS, Kinnan MK, Chumanov G. Multipole plasmon resonances of submicron silver particles. Journal of the American Chemical Society 2005;127(36):12444-12445 [communication].
- Evanoff Jr. DD, Chumanov G. Synthesis and optical properties of silver nanoparticles and arrays. ChemPhysChem, 2005;6(7):1221-1231.
- Kumbhar A, Chumanov G. Synthesis and characterization of titania-coated silver nanoparticles. Journal of NanoScience and NanoTechnology 2004;4(3):299-303.
- Kumbhar A, Chumanov G. Synthesis of iron(III)-doped titania nanoparticles and its application for photodegradation of sulforhodamine-B pollutant. Journal of Nanoparticle Research 2005;7(4-5):489-498.
- Daniels JK, Chumanov G. Spectroelectrochemical studies of plasmon coupled silver nanoparticles. Journal of Electroanalytical Chemistry 2005;575(2):203-209.
- Chumanov G, Malynych SZ. Coherent plasmon coupling and cooperative interactions in two-dimensional array of silver nanoparticles. In: Kotov N, ed. Nanoparticle Assemblies and Superstructures. Boca Raton, FL: CRC Press, 2005.
- Malynych S, Chumanov G. Coupled planar silver nanoparticle arrays as refractive index sensors. Journal of Optics A: Pure and Applied Optics 2006;8(4):S144-S147.
Journal Articles on this Report : 10 Displayed | Download in RIS Format
Other project views: | All 25 publications | 11 publications in selected types | All 10 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Bao H, Chumanov G, Czerw R, Caroll DL, Foulger SH. Synthesis of core-shell silver colloidal particles by surface immobilization of an azo-initiator. Colloid and Polymer Science 2005;283(6):653-661. |
R829603 (Final) |
not available |
|
Daniels JK, Chumanov G. Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering. Journal of Physical Chemistry B 2005;109(38):17936-17942. |
R829603 (Final) |
not available |
|
Daniels JK, Chumanov G. Spectroelectrochemical studies of plasmon coupled silver nanoparticles. Journal of Electroanalytical Chemistry 2005;575(2):203-209. |
R829603 (Final) |
not available |
|
Evanoff DD, Chumanov G. Size-controlled synthesis of nanoparticles: 1. “Silver-only” aqueous suspensions via hydrogen reduction. Journal of Physical Chemistry B 2004;108(37):13948-13956. |
R829603 (Final) |
not available |
|
Evanoff DD, Chumanov G. Size-controlled synthesis of nanoparticles: 2. Measurement of extinction, scattering, and absorption cross sections. Journal of Physical Chemistry B 2004;108(37):13957-13962. |
R829603 (Final) |
not available |
|
Evanoff DD, Chumanov G. Synthesis and optical properties of silver nanoparticles and arrays. ChemPhysChem 2005;6(7):1221-1231. |
R829603 (Final) |
not available |
|
Kumbhar AS, Kinnan MK, Chumanov G. Multipole plasmon resonances of submicron silver particles. Journal of the American Chemical Society 2005;127(36):12444-12445. |
R829603 (Final) |
not available |
|
Kumbhar A, Chumanov G. Synthesis and characterization of titania-coated silver nanoparticles. Journal of NanoScience and NanoTechnology 2004;4(3):299-303. |
R829603 (Final) |
not available |
|
Kumbhar A, Chumanov G. Synthesis of iron(III)-doped titania nanoparticles and its application for photodegradation of sulforhodamine-B pollutant. Journal of Nanoparticle Research 2005;7(4-5):489-498. |
R829603 (Final) |
not available |
|
Malynych S, Chumanov G. Coupled planar silver nanoparticle arrays as refractive index sensors. Journal of Optics A: Pure and Applied Optics 2006;8(4):S144-S147. |
R829603 (Final) |
not available |
Supplemental Keywords:
nanotechnology, EPA, remediation, photocatalysis, photocatalytic activity, titania, silver nanoparticles, hybrid nanoparticles, synthesis of silver nanoparticles, sol-gel, core-shell structure, plasmon, enhanced local field, photoinduced electron transfer, photooxidation plasmon coupling,, RFA, Scientific Discipline, Waste, Sustainable Industry/Business, Sustainable Environment, Physics, Environmental Chemistry, Remediation, Technology for Sustainable Environment, Biochemistry, New/Innovative technologies, Chemistry and Materials Science, Environmental Engineering, nanoparticle remediation, titanium dioxide, bioengineering, remediation technologies, nanotechnology, environmental sustainability, bio-engineering, plasmon sensitized titanium dioxide nanoparticles, solar energy, photoremediation, environmentally applicable nanoparticles, sustainability, plasmon sensitized nanoparticles, innovative technologies, photocatalystProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.