Patent Application
20200220187
Particles emitted from radioisotopes can be used to convert the radiant energy into electricity. Radioisotope energy conversion for power generation has been intensively studied to develop power sources for a wide range of applications from energizing cardiac pacemakers in the human body, to challenging outer planet missions. As compared to various indirect conversion methods collecting the electricity from the secondary energy forms of heat or light generated by radiation, direct conversion methods produce electric power directly from energetic particles. Beta particles can produce electron-hole pairs in semiconductors via their loss of kinetic energy and can contribute to the generation of electric power. Although potential applicability of radioisotopes in portable power sources that do not require recharging seems very attractive, it has been reported that only a small portion of the whole radiation energy can be converted into electrical energy. Moreover, most betavoltaic cells suffer from serious radiation damage to the lattice structures of semiconductors and subsequent performance degradation due to the high kinetic energy of the beta particles. Alternatively, to minimize lattice damage in semiconductors, wide band gap materials are typically used. However, radiation-resistive materials, such as SiC and GaN, still show very low energy conversion efficiencies. Very little improvements have been made even after vigorous research on various improvement methods utilizing porous structures, inverted pyramidal cavities, and three dimensional silicon pillar structures to increase rectifying junction areas. In view of the foregoing, a need still exists for a more efficient method and device for producing electrical power from radiation.
In one embodiment, the invention is directed to a radiolytic electrochemical system comprising:
In another embodiment, the invention is directed to a radiolytic electrochemical system comprising:
In another embodiment, the invention is directed to a radiolytic electrochemical system comprising a cathode, an anode that comprises a semiconductor, an aqueous electrolyte solution disposed between the cathode and anode, and ionizing radiation, wherein the ionizing radiation splits water molecules in the solution and forms solvated free radicals that migrate to the anode or cathode, depending upon a radical's charge, and participate in redox reactions at the anode and cathode thereby producing electrical current capable of performing work when the anode and cathode are electrically connected.
In another embodiment, the invention is directed to a method of conducting a radiolytic electrochemical reaction comprising operating any of the foregoing radiolytic electrochemical systems with the anode and cathode electrically connected.
In yet another embodiment, the invention is directed to a method of generating electrical current to perform work comprising operating any of the foregoing radiolytic electrochemical systems with the anode and cathode electrically connected.
In still another embodiment, the invention is directed to a method of detecting the presence of radioisotopes comprising generating an electrical current with any of the foregoing radiolytic electrical systems with the anode and cathode electrically connected, wherein the electrical current has a magnitude that is indicative of radiation source's presence, strength, location, or a combination thereof.
Further, in one embodiment the invention is directed to a method for producing hydrogen gas comprising conducting a radiolytic electrochemical reaction comprising operating any of the foregoing radiolytic electrochemical systems with the anode and cathode electrically connected and collecting hydrogen gas produced during the redox reactions.
The field of conventional energy conversion using radioisotopes has almost exclusively focused on solid-state materials. Although, thus far, there has been no method for completely avoiding radiation damage to semiconductors, the use of a liquid-phase material has been introduced as a means to reduce or eliminate radiation damage and related structural defect problems because liquid-phase material efficiently absorbs the kinetic energy of ionizing radiation such as beta particles.
In fact, relatively large amounts of radiation energy can be absorbed by water. When radiation energy is absorbed by an aqueous solution, free radicals (e.g., eaq−, .OH, H., HO2.) may be produced through radiolytic interactions. These free radicals may result in the generation of molecular by-products, such as H2O2 and H2. The present invention utilizes the foregoing for the generation of electricity using a device that separates the radiolytic current from the free radicals by splitting the water.
Herein, it is demonstrated that liquids can be excellent media for effective energy conversion from radioisotopes. It is also shown that free radicals in liquid, which are continuously generated by ionizing radiation, can be utilized for electrical energy generation.
In one embodiment, the radiolytic electrochemical system comprises a cathode, an anode that comprises a semiconductor, an aqueous electrolyte solution disposed between the cathode and anode, and ionizing radiation, wherein the ionizing radiation splits water molecules in the solution and forms solvated free radicals that migrate to the anode or cathode, depending upon a radical's charge, and participate in redox reactions at the anode and cathode thereby producing electrical current capable of performing work when the anode and cathode are electrically connected.
In another embodiment, the radiolytic electrochemical system comprises:
Advantageously, in certain embodiments some of the ionizing radiation forms electron-hole pairs in the semiconductor component that separate with holes migrating to, and participating in, the redox reactions at the anode/solution interface and electrons migrating to, and participating in, the redox reactions at the cathode/solution interface thereby contributing to the electrical current. Generally, to facilitate such formation of electron-hole pairs and the transport electrons and holes, the semiconductor component has a thickness that is in a range of about 10 nm to about 500 μm.
Nanoscale Morphology
In certain embodiments, the semiconductor component has a nanoscale morphology. Among other things, the nanoscale morphology tends to be advantageous simply because it provides more surface area and, thus, reaction sites for redox reactions. In one such embodiment, the nanoscale morphology comprises nanopores that have an average diameter in a range about 10 nm to about 500 nm. Additionally, the semiconductor nanopores may be spaced a distance apart that is in a range of about 10 nm to about 500 μm.
Semiconductor Component Structure
The above-described nanoscale morphology may be accomplished through any appropriate method and/or structure. In certain embodiments, the radiolytic, the semiconductor component comprises a structure selected from the group consisting of nanowires, nanorods, nanotubes, sintered nanoparticles, nanosheets, nanometer-thick films, and combinations thereof, and the semiconductor nanopores correspond to the separation between said structure. Typically, nanowires or nanotubes are selected.
An exemplary semiconductor component structure is nanowires that are formed, for example, by depositing a metal film on a substrate by RF sputtering, patterning the metal film by, for example, etching, oxidizing the metal to result in a patterned semiconductor oxide.
Semiconductor Material
When ionizing radiation passes through the semiconductor component, it produces electron-hole pairs in the semiconductor component, some of which recombine but others separate and migrate due to the built in potential at the anode/liquid interface. The holes tend to move to the anode/liquid interface and react with redox couples of water molecules. Whereas the electrons tend to move through the semiconductor component to the conduction component and through to the cathode and react with redox couples of water molecules.
Typically, it is desirable for the semiconductor component to comprise a large band gap semiconductor material. It is also typically desirable for the semiconductor component to comprise a single crystal material. That said, polycrystalline materials are acceptable. In certain embodiments, the semiconductor component consists of a single crystal large band gap semiconductor material. Exemplary large band gap semiconductor material is selected from the group consisting of TiO2, Si, SiC, GaN, GaAs, ZnO, WO3, SnO2, SrTiO3, Fe2O3, CdS, ZnS, CdSe, GaP, MoS2, ZnS, ZrO2, and Ce2O3, and combinations thereof. In still further embodiments, the single crystal large band gap semiconductor material is an oxide such as TiO2.
The semiconductor component is selected from the group consisting of intrinsic semiconductor (i), n-type semiconductor (n), n+-type semiconductor (n+), p-type semiconductor (p), p+-type semiconductor (p+), and combinations thereof.
Particularly desirable combinations include structures selected from the group consisting of n, p, n-p, p-p+, n-n+, n-i-p, and n+-n-i-p-p+.
The radiolytic electrochemical system may further comprise a Schottky component comprising a Schottky metal in contact with the semiconductor component thereby forming a rectifying metal-semiconductor junction. Exemplary Schottky metals include Pt, Au, Pd, Fe, Co, Cr, Ni, Ag, Ti, Ru, Cu, Mo, Ir, and Rh, alloys thereof, and combinations of the foregoing metallic elements and/or alloys. In certain embodiments, the Schottky metal comprises one or more noble metal elements. In another embodiment, the Schottky metal is Pt.
In certain embodiments, the Schottky component has a nanoscale morphology that creates localized surface plasmons when the Schottky component is subjected to some of the ionizing radiation, wherein some of the plasmons form electron-hole pairs in the Schottky component that separate. The excited electrons transiently occupying normally empty states in the Schottky metal conduction band above the Fermi energy level and most of the excited electrons are sufficiently energetic to enter the conduction band of the semiconductor component. Additionally, it is believed that the ionizing radiation deposits energy through the depletion region near the Schottky metal-semiconductor junction, and the electric field separates the electron-hole pairs in different directions (electrons toward the semiconductor and holes toward the Schottky metal-liquid interface, which results in a potential difference between the Schottky and ohmic).
Additionally, the Schottky component preferably has a thickness that allows for holes to migrate to, and participate in, the redox reactions at the anode/solution interface and electrons to migrate (via the semiconductor component and the conduction component) to, and participate in, the redox reactions at the cathode/solution interface thereby contributing to the electrical current. Appropriate thicknesses of the Schottky component tend to be in a range of about 1 nm to about 100 nm.
Another desirable effect associated with the formation of plasmons is that some of the plasmons assist in the release of the positively charged solvated free radical ions from their surrounding water molecules for participation in the redox reactions at the anode/solution interface thereby contributing to the electrical current. For example, the nanoscale morphology of the Schottky component may comprise nanopores that have diameters that provide optimum surface Plasmon resonance within a wavelength range of about 100 nm to about 800 nm. This is typically achieved with nanopores that have an average diameter in a range about 10 nm to about 500 nm. Also, the nanopores are typically spaced a distance apart that is in a range of about 10 nm to about 500 nm.
In certain embodiments, the ohmic metal is selected from the group consisting of Al, Ag, Fe, Cr, Ti, Ni, Au, Pt, Pb, Mo, and Cu, alloys thereof, and combinations of the foregoing metallic elements and/or alloys. In certain embodiments, the conduction component is a foil, sheet, or plate, which may be the base upon which the semiconductor component is deposited. Alternatively, the conduction component may be a film deposited on a substrate and the semiconductor component is deposited on the conduction component film.
In certain embodiments, the cathode comprises a cathode metal selected from the group consisting of Pt, Au, Pd, Fe, Cr, Co, Ni, Ag, Ti, Ru, Cu, Mo, and Ir, alloys thereof, and combinations of the foregoing metallic elements and/or alloys. In other embodiments, the cathode comprises noble metal elements. In still other embodiments, the cathode metal is Pt.
In certain embodiments, the cathode has a nanoscale morphology that creates localized surface plasmons when the cathode is subjected to some of the ionizing radiation, wherein some of the plasmons assist in the release of the negatively charged solvated free radical ions from their surrounding water molecules for participation in the redox reactions at the cathode/solution interface thereby contributing to the electrical current. This is typically achieved with nanopores that have an average diameter in a range about 10 nm to about 500 nm. Also, the nanopores are typically spaced a distance apart that is in a range of about 10 nm to about 500 nm.
In certain embodiments, the aqueous electrolyte solution is selected to have a pH such that a stable rectifying junction at the anode (semiconductor component)-solution interface is formed. For a basic electrolyte solution, the chemical energy of redox couple is changed by E=E0−pH*0,059 eV. For example, 2H++2e−→H2 (g) has 0 V at pH=0 but when pH=14, the chemical energy is changed from 0V to −0.826V. Which means that a p-type semiconductor cannot form the Schottky contact (rectifying junction) between the semiconductor-liquid interface but an n-type semiconductor can form a more stable Schottky contact. As such, the aqueous solution has a basic pH in the event an n-type semiconductor and/or n+-type semiconductor is in contact with the aqueous electrolyte solution. Alternatively, the aqueous electrolyte solution has an acid pH in the event a p-type semiconductor and/or p+-type semiconductor is in contact with the aqueous electrolyte solution.
In certain embodiments, the basic solution comprises an alkaline electrolyte selected from the group consisting of KOH, NaOH, and mixtures thereof, and the acidic solution comprises an acidic electrolyte selected from the group consisting of H2SO4, HNO3, LiSO4, and mixtures thereof. Also, the basic electrolyte is at a concentration in the aqueous electrolyte solution in a range of about 0.1 M to about 5 M, and the acidic electrolyte is at a concentration in the aqueous electrolyte solution in a range of about 0.1 M to about 5 M.
The ionizing radiation is from a radiation source that comprises one or more radioisotopes that emit from the group consisting of alpha particles, beta particles, neutrons, gamma rays, and combinations thereof. In certain embodiments, the radiation source is a beta particle emitting radioisotope selected from the group consisting of 63Ni, 90Sr, 35S, 204Tl, and 3H, 148Gd, and 137Cs. In other embodiments, the radiation source is an alpha particle emitting radioisotope selected from the group consisting of 210Po, 244Cm, 238Pu, and 241Am.
In certain embodiments, the system is an electrochemical cell and the ionizing radiation is from a radiation source that is encapsulated by a protective layer and at least a portion of the protective layer is in contact with the aqueous electrolyte solution, wherein the protective layer prevents chemical reactions between the radiation and the aqueous electrolyte solution. In still another embodiment, the electrochemical cell is at least substantially sealed and the cathode, the anode, the encapsulated radiation source, and the aqueous electrolyte solution are within the at least substantially sealed electrochemical cell. The phrase “at least substantially sealed” is intended to, for example, allow for the presence of vents or breathing holes that allow for generated gasses to escape and/or improving the movement of liquid or gas in, out, and/or through the system, cell, or device. Additionally, the terms “electrochemical system” and “electrochemical cell” are not intended to exclude such systems or cells that comprise multiple containers or chambers. For example, a system may comprise a multiplicity of cells. Additionally, a system or cell may comprise a multiplicity of containers or chambers. For example, a system or cell may comprise one or more containers or chambers comprising the anode component(s) and electrolyte solution and one or more containers of chambers comprising the cathode and a different electrolyte solution having, for example, different pH conditions connected by a bridge(s) and/or membrane(s).
In another embodiment, the radiation source is spaced from the anode a distance that is at least sufficient to prevent or limit lattice damage to the semiconductor component and no greater than about the distance the ionizing radiation can travel in the aqueous electrolyte solution. For example, if the radiation source is 90Sr, it is usually spaced from the anode a distance that is in a range of about 1 mm to about 4 mm. It may be closer to the cathode such as at a distance that is in a range of 0.1 mm to 10 mm.
External Radiation Source
In certain embodiments, the ionizing radiation is from a radiation source that is not in contact with the aqueous electrolyte solution. For example, the anode, and the aqueous electrolyte solution are within a sealed container through which the ionizing radiation from the radiation source passes.
Such external radiation source embodiments allow for a radiolytic electrochemical system that is useful for detecting the presence of radioisotopes. For example, when in the presence of ionizing radiation from an external source, an electrical current may be generated when anode and cathode electrically connected, wherein the electrical current has a magnitude that is indicative of radiation source's presence, strength, location, or a combination thereof.
In certain embodiments, the radiolytic electrochemical system may further comprise a substrate component in contact with the conduction component, and wherein the conduction component is a layer deposited on the substrate component. The substrate component may be selected from any appropriate material. For example, in some embodiments, the substrate component comprises a substrate material selected from the group consisting of glass, Si, plastic, and metals and alloys thereof, and combinations of the foregoing.
In certain embodiments, wherein the aqueous electrolyte solution further comprises a redox compound that provides a redox couple that participates in the redox reactions are conducted at the cathode/solution interface and the anode/solution interface thereby reducing or eliminating the production of one or more gaseous products. Exemplary redox compounds and the redox couples, respectively, may be selected from the group consisting of ZnSO4 [Zn+|Zn2+], CoSO4 [Co|Co2+], CdSO4 [Cd+|Cd2+], TlSO4 [Tl0|Tl+], Pb(ClO4)2 [Pb+|Pb2+], NiSO4 [Ni+|Ni2+], and Cr(ClO4)3 [Cr+|Cr3+]. If present, the redox compound is typically at a concentration in the aqueous electrolyte solution in a range of about 1 μM to about 5 M.
In another embodiment, the radiolytic electrochemical system comprises:
In another embodiment, the invention is directed to a method of conducting a radiolytic electrochemical reaction comprising operating any embodiment of the above-described radiolytic electrochemical system with the anode and cathode electrically connected.
In yet another embodiment, the invention is directed to a method of generating electrical current to perform work comprising operating any embodiment of the above-described radiolytic electrochemical system with the anode and cathode electrically connected.
In yet another embodiment, the invention is directed to a method for producing hydrogen gas comprising conducting a radiolytic electrochemical reaction comprising operating any embodiment of the above-described electrochemical system with the anode and cathode electrically connected, wherein the electrolyte solution does not comprise the above-described Redox compounds, and collecting hydrogen gas produced during the redox reactions.
For ease of discussion, the Schottky component will be platinum, the semiconductor component will be nanoporous titania, and the ionizing radiation is beta radiation, but the described principles and operation apply for other appropriate materials selected in accordance with the foregoing.
When high-energy beta radiation passes through Pt and nanoporous TiO2, electron-hole pairs are produced inside the nanoporous TiO2. In particular, the holes generated in TiO2 move toward the Pt/liquid interface and then react with redox couples of water molecules, while the electrons are transported through the nanoporous TiO2 to the other electric contact due to the built-in potential at the Pt/electrolyte interface. In general, TiO2 is resistant to corrosion, but the additional layer of Pt can further protect the TiO2 layer under the harsh conditions of high pH values that are needed for water splitting. In addition, the porosity of the TiO2 leads to myriad nanoholes in the Pt film, which create localised surface plasmons that act as harmonic oscillators in response to an oscillating external electric field.
Surface plasmons excited on the Pt surface can produce electron-hole pairs, with the excited electrons transiently occupying normally empty states in the Pt conduction band above the Fermi energy level. Most of the excited electrons are sufficiently energetic to enter the conduction band of TiO2. The beta radiation in the electrochemical system produces free radicals in water through the loss of kinetic energy. In a meta-stable state, the free radicals are recombined into water molecules or trapped in water molecules. Thus, the free radicals produced by the radiation can be converted into electricity by a plasmon-assisted, wide band gap oxide semiconducting material using a water splitting technique at room temperature. For example, under beta radiation, surface plasmon obtained by the metallic nanoporous structures on TiO2 enhanced the radiolytic conversion via the efficient energy transfer between plasmons and free radicals.
To understand the highly enhanced output power, it is necessary to understand the details of water radiolysis. High-energy electrons passing through an aqueous solution can ionise or excite water molecules, forming a number of transition species and stable products, as shown by the equation,
High levels of these products are formed by the absorption of 100 eV in the medium. The generated radicals are powerful redox reagents (i.e., eaq− is a strong reducing agent, H. is an equally strong reducing and oxidising agent, and .OH is a powerful oxidising agent). In water at a high pH, eaq− and .OH are produced in the largest quantities by the beta radiation whereas .OH tends to be predominantly produced in an acidic electrolyte. The standard potentials of eaq− and .OH are E0 (H2O/eaq−)=−2.9 VNHE and E0 (.OH/H2O)=+2.7 VNHE, respectively. For completeness, one or more of the following reactions of ((R1)-(R50)) set forth in Table A may be generated or occur during water radiolysis. T. Palfi et al., Rad. Phys. Chem. 79, 1154 (2010).
H2 + 2OH−
H2 + OH−
OH−
2OH−
•OH + OH−
O−• + OH−
O2−•
O22−
HO2−
H• + HO−
H• + H2O
H2
H2O
•OH + H2O
•OH + HO−
HO2•
H2O2
HO2−
eaq− + H2O
H2O2
HO2−
H2O + HO2•
H2O + HO2•
H2O + O2
OH− + O2
H• + H2O
O22−
H2O + O2−•
OH− + O2−•
O3−•
2OH− + O2
H• + HO−
H2O + O•
O2
H2O2 + O2
HO2− + O2
O22− + O2
HO2− + OH−
2H2O
OH− + H3O+
O−• + H2O
•OH + OH−
HO2− + H2O
H2O2 + OH−
HO2• + H2O
O2−• + H3O+
•OH + O2
O−• + O2
HO3• + H2O
O3−• + H3O+
These two species (eaq− and .OH) react with water molecules during water radiolysis and then remain in the aqueous solution. They can then no longer react with each other or with surrounding water molecules. Once these two species (eaq− and .OH) are solvated in an aqueous solution, they can be surrounded by water molecules in a meta-stable state for a few μs. It is well known that water has very large dielectric constant and consists of polar molecules, which prevent the electrostatic attraction between positive and negative charges and maintain the separated state of charges surrounded by oppositely charged ends of water dipoles. During the long lifetime of solvated electrons, beta radiation will continuously increase the number of solvated electrons in the water until they finally move to the surface of water as they are emitted as thermal energy. The floated electrons form the negatively charged surface of water.
Theoretically, water has lower standard potential (E0 (H2O/O2)=+0.82 VNHE and E0 (H2O/H2)=−0.41 VNHE) than free radicals, indicating that electrochemical energy of water is lower than that of free radicals. Additionally, the high electrochemical energy of solvated electrons (eaq−) can enable difficult chemical reactions to happen. Moreover, the presence of an external electric field or electromagnetic field can release the solvated electrons from their confined environment of surrounding molecules. This is because eaq− in water has very small diffusion efficiency (4.8×10−5 cm2/s) and electron mobility (1.84×10−3 cm2/V·s). Solvated electrons in water also need to get over the potential barrier of 0.2˜0.5 eV set by the surrounding rigid water molecule network. Moreover, because water is located between both Pt electrodes (plasmonic layer and counter electrode), if same electrochemical reactions occur on both Pt surfaces, simply no current flow is expected.
A. Anode Preparation
Referring to
As shown in the band diagram of the junctions in
B. Structural and Optical Properties
Scanning electron microscopy (SEM) was performed using an FEI Quanta 600 FEG extended vacuum scanning electron microscope with an accelerating voltage of 10 kV and an energy dispersive spectrometer (Noran System Six). X-ray diffraction (XRD) analysis was conducted using a Rigaku Miniflex 600 X-ray diffractometer with Cu Kα radiation. The reflection spectrum was measured using a Perkin-Elmer lambda 25 UV-VIS spectrometer and variable-angle reflectance accessory.
Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) data for the nanoporous TiO2 are shown in
C. Electrical Properties
Illustrations of the test setup and of the Pt/nanoporous TiO2 cathode are shown in
The current density-voltage characteristics were measured at open circuit voltage and the current density at 0 V, −0.1 V, −0.4 V, −0.7 V, and −0.9 V for 1200 seconds each under continuous irradiation.
D. Energy Absorption of Electron Beam Using Numerical Simulation
To study the mechanism of the enhanced power conversion of the radiation, the energy absorbed in an aqueous solution was calculated using Monte Carlo (MC) simulations. The number of electrons utilised in this simulation was 10,000, and the kinetic energy of the electron beam was 546 keV. A beta particle is an electron (e−) or a positron (e+) that is generated when the neutron to proton ratio in the nucleus is too large, making the nucleus unstable. In this simulation, the presence of secondary electrons generated by primary electrons was ignored, and the electron beam was defined by a Gaussian distribution. The energy absorbed by the TiO2 film was calculated to be approximately 0.25% of the total energy of the electron beam (
Beta radiation can create vacancies in a solid by primary knock-on atoms (PKA). For rutile TiO2, the threshold displacement energy is about 47 eV. A required incident kinetic energy level of beta radiation for the vacancy production can be estimated from the displacement energy equation, Tm=2(E+2mc2)E/Mc2, where E is kinetic energy of beta radiation and c is speed of light, and m and M are masses of electron and target atom, respectively. Under beta radiation, the threshold displacement energy of 47 eV is equivalent to the incident kinetic energy of 271 keV for oxygen and 633.5 keV for titanium. It was also found from the MC simulations that the water can absorb beta radiation of 356 keV. Because the PKA damage in TiO2 occurs from the kinetic energy over 271 keV, the performance degradation of our radiolytic cell might begin with the kinetic energy over 627 keV. Although beta radiation from 90Sr/90Y includes higher energy beta particles than 627 keV, the radiolytic cell didn't show any performance degradation for about 6 hours. Moreover, water can absorb 72.02% of the total kinetic energy in emission spectrum of 90Sr/90Y.
To determine the total kinetic energy passing through Pt/nanoporous TiO2, the directional loss of radiation source was considered. The directional loss is the geometrical effect between source and device as a function of distance and source shape. When two parallel discs with each radius of Rs and Rd are placed at a distance , solid angle (Ωs) is calculated by
Therefore, the directional loss (η) can be estimated by η=(1−Ωs)·100%. The directional loss of the present radiolytic cell was estimated to be approximately 54.88% and the amount of higher kinetic energy than 627 keV was only 12.62% of the total kinetic energy of beta radiation, indicating that performance degradation of the radiolytic cell may not be serious.
In this situation, incident high-energy beta particles can excite electrons in the semiconductor via their energy loss, which is defined as the electron-hole pair (EHP) ionisation energy given by W±≈2.8Eg+Eph, where Eg and Eph are the band gap and phonon energies (0.5≤Eph≤1 eV), respectively. For TiO2, W± is approximately 9.46 eV. Because the high EHP ionisation energy is sufficient for tunnelling through a low Schottky barrier at the TiO2/liquid interface, the radio current of nanoporous TiO2 is lower than the radio current of Pt/nanoporous TiO2, as shown in
E. Finite-Difference Time-Domain (FDTD) Simulation
One available external electric field in our device is the localised electric fields from the surface plasmons of the Pt nanoholes. To generate surface plasmons on the Pt/nanoporous TiO2 radiolytic electrode, 3D-FDTD (finite-difference time-domain) numerical simulations were conducted using a commercially available FDTD code for simulating electron irradiation (e.g., available at www.lumerical.com). The FDTD simulation was based on Pt (50 nm)/nanoporous TiO2 (1 μm) composed of a periodic array of nanoholes with a diameter of 10 nm and a depth of 505 nm on a glass substrate. The electron beam was modeled as a series of closely spaced dipoles with a temporal phase delay defined by the velocity of the electron beam. In the absence of a structure, an electron beam moving at a constant velocity does not generate any radiation. The material properties used in the calculation are based on the dispersion data set forth in Devore, J. R. Refractive indices of rutile and sphalerite, J. Opt. Soc. Am. 41, 416-419 (1951) and Palik, E. D., Handbook of optical constants of solids, Academic Press (1997). The detailed electron irradiation profiles at the different positions at the bottom level of the nanoholes in
In FDTD, the macroscopic Maxwell equations are solved in discretised space and time to follow the response of a material to an applied electric field. For a numerical investigation of the beta radiation-excited photon emission in Pt/nanoporous TiO2, the electron beam can be modelled as a line current density source represented by {right arrow over (J)}(t, {right arrow over (r)})=−evûzδ(x−x0)δ(y−y0), where e is the electronic charge, v is the electron velocity, (x0, y0) represents the position of the focused electron beam, z is the direction of the electron velocity, and ûz is the unit vector along the z direction. In the simulation, the current density was modelled as a series of dipoles with a temporal phase delay (z/v) related to the electron velocity, v (in this simulation, v=0.875c, corresponding to the beta particle's kinetic energy of 546 keV used in the experiment, and c is the velocity of light in free space). The simulations were conducted at three different points: in the middle of a pore, at the side wall of a pore, and between two pores (denoted as 1, 2, and 3 in
To simulate the near-field electric intensities and emission spectra at the different positions, the induced electric field distributions were calculated by [E]2/[E0]2, where E and E0 are the generated and minimum electric field intensities, respectively, emitted normal and parallel to the Pt surface in the wavelength range of 200 to 700 nm. The near-field intensity distributions on Pt/nanoporous TiO2 at the different positions under focused electron irradiation were investigated using FDTD simulations. As shown in
To understand the surface plasmon phenomena between two materials, Maxwell's equation was solved for the electromagnetic wave at the interface between two materials, which has relative dielectric functions ε1 (=ε′1+iε″1=(n12−κ12)+i(2n1κ1)) and ε2 (=ε′2+iε″2=(n22−κ22)+i(2n2κ2)) with the appropriated continuity relationship. Real and imaginary parts of the dielectric function are presented. The nj and κj are refractive index and extinction coefficient, respectively, and j is 1 or 2. By boundary conditions, the dispersion relation for a wave propagating on the surface is
where k (=k′+ik″), ω and c are the wave number and the frequency of the wave, the speed of light, respectively. For k′ not to be purely imaginary, ε′1=(n12−κ12)<0 and |ε′1|>ε′2=(n22−κ22). Based on this relationship, a graph shown in
Since the surface plasmon energy occurs around |ε′1|≈ε′2, the surface plasmon wavelengths can be found from crossing points on the Pt line (the substantially linear relationship line). Compared to the water/Pt interface (dot at the intersection of the Pt line and the water line, which is horizontal intersecting the y-axis at about 2), the air/Pt interface (the dot at the intersection of the Pt line and the air line, which is horizontal intersecting the y-axis at about 1) shows the surface plasmon phenomena at a slightly shorter wavelength. These wavelengths at the crossing points of water/Pt and air/Pt in
In addition, the electron profiles (
F. Laser Excitation
To understand the surface plasmon effect better on Pt/TiO2, excitation using three different lasers on Pt/TiO2 structure was conducted. Lasers with various colors such as red (650 nm, 1.91 eV), green (532 nm, 2.33 eV), and blue (405 nm, 3.05 eV) were used and the excitation power of all the lasers was 5 mW. The surface plasmon wavelength for the air/Pt interface is found near 250 nm as shown in
G. Discussion
Electrons and holes that migrate to the surface of metal electrodes can reduce and oxidize the reactants adsorbed on the metal surfaces, respectively. The reduction (Red) and oxidation (Ox) reactions can produce the reduction current and oxidation current as shown in the J-V measurement data in
When the strongly localised electric field oscillates across the Pt surface, as illustrated in
In addition, the presence of excited electrons indicates that excited holes with the same amount of energy (10.64 eV with respect to the vacuum level) exist. These excited holes can interact with .OH on the surface of Pt. In fact, the Schottky junction of Pt/TiO2 creates a space-charge region in the n-type TiO2 layer because the electrons diffuse from the TiO2 side to the Pt side whereas the holes move reversely. In the equilibrium state, an equal amount of electrons in Pt is trapped at the Pt/TiO2 interface and builds up an internal electric field (0.45 eV) from the TiO2 side toward the Pt side, preventing further movement of carriers. When surface plasmons are generated on Pt surface by beta radiation, electrons in Pt are pushed towards TiO2 by the electric field of surface plasmons, which is higher than the internal electric field, and holes move simultaneously towards the Pt/water surface interface. Accumulated holes can attract donors like negatively charged ions or radicals. Thus, the strong electric field enhancement can significantly increase the number of accumulated holes at the Pt/water interface and create a large internal potential difference.
With the energy higher than the potential barrier (0.2˜0.5 eV) of surrounding water molecules, solvated electrons (eaq−) can be attracted toward the Pt surface rapidly in water. Moreover, surface plasmons have a resonance frequency of a few hundred of GHz in visible wavelength. It means that strong Coulombic attraction force oscillates very fast to attract eaq − in water. When the distance between the TiO2 and Pt counter electrode is 5 cm, the electric field is approximately 0.09 V/cm and the speed of electrons in water reaches 1.6256×10−4 cm/s. If eaq − in water is 1 mm far away from TiO2, it can move to the TiO2 surface in 603s and most solvated electrons will not be quickly arrived at the TiO2 electrode. Thus, a slowly saturated curve of open circuit voltage is observed on the TiO2 electrode under beta radiation, while the Pt/TiO2 electrode has very fast saturated curve. The electric field of the TiO2 electrode is much weaker than that of Pt/TiO2 electrode with the surface plasmon effect and the difference in electric field intensity may affect the collection of eaq in water. Once eaq is arrived at the electrode surface, eaq is injected without any energy loss because electrochemical energy is higher than the conduction band edge (−0.1˜−0.2 VNHE) of TiO2.
In view of the foregoing, the beta radiation produces many free radicals through the loss of kinetic energy in water. Then, the generation of localised surface plasmons on the Pt surface by beta radiation enhances the chemical reactions involving free radicals produced by beta radiation in water. The reflection peak of Pt/nanoporous TiO2 at 5.04 eV and the similarity between the simulated emission spectra and measured reflection spectrum show that the surface plasmon resonance occurs at higher energy than the band gap energy of TiO2, indicating that resonant energy transfer between TiO2 and surface plasmons.
Because the increased population of hot electrons generated by surface plasmons decreases the thickness of the Pt/TiO2 junction and increases its junction electric field, the narrow junction enables carriers obtained from free radicals to efficiently move toward the contact metals without carrier relaxation in the thin film. It was confirmed that much higher power from the device with the plasmonic Pt layer was produced than that from the device without the plasmonic Pt layer. As shown by the laser test, increased surface plasmon energy can effectively transport hot carriers generated on Pt surface. Therefore, it can be concluded that the high power density obtained from the radiolytic cell may be attributed to the conversion of continuously stored free radicals with large electrochemical energy into electricity by the surface plasmon energy generated at the Pt surface.
A. Anode Preparation
Referring to
B. Structural and Optical Properties
Scanning electron microscopy (SEM) was performed using an FEI Quanta 600 FEG extended vacuum scanning electron microscope with an accelerating voltage of 10 kV and an energy dispersive spectrometer (Noran System Six). X-ray diffraction (XRD) analysis was conducted using a Rigaku Miniflex 600 X-ray diffractometer with Cu Kα radiation. The reflection spectrum was measured using a Perkin-Elmer lambda 25 UV-VIS spectrometer and variable-angle reflectance accessory. Spectrum of a fluorescent lamp was measured by Ocean optics HR2000+ spectrometer. As illustrated in
We have studied x-ray diffraction (XRD) analysis because the component ratio and size of nanoparticles significantly affect the enhancement of water splitting at TiO2/electrolyte interface.
where IA and IR are the x-ray intensities of the anatase and rutile peaks, respectively. Estimated weight percentages of anatase and rutile are 82% and 18%. Average crystalline sizes were calculated by Scherrer's formula given by Lc=kl/bcosq, where k (0.94) and/(Cu kα1=0.1540593 nm) are related to the shape of polycrystals and the x-ray wavelength, b and q are the full-width at half maximum (FWHM) of the respective diffraction peak and Bragg angle. To obtain the value of the FWHM of a peak from the XRD data, the observed diffraction patterns were fitted by a Gaussian function. Hence, the particle sizes of anatase and rutile TiO2 were about 20.3 nm and 28.4 nm, respectively.
C. Energy Absorption of Electron Beam Using Numerical Simulation
In order to understand the energy loss of the beta radiation in TiO2 (band gap: 3.2 eV), the absorbed energy in the aqueous solution was calculated using Monte Carlo (MC) simulations. The number of electrons utilized in this simulation was 100,000 and the kinetic energy of electron beam was 1.176 MeV. A beta particle is an electron (e−) or a positron (e+), and is generated when the neutron to proton ratio is unstable. Although radioisotope 137Cs emits beta particles (electrons, 1.176 MeV) and gamma rays (661.6 keV), it was assumed that it emits only beta particles for this simulation. Since gamma rays are high energy photons, gamma rays can excite high energy electrons due to Compton scattering and photoelectric absorption in semiconductor and then these energetic electrons lose their energy in the material via electron-electron collision and the creation of various excitations. However, the mass attenuation coefficients of gamma rays (661.6 keV) are about 0.0778 cm2/g for Si and about 0.028 cm2/g for TiO2, while the mass absorption coefficient for beta particles (1.176 MeV) is about 14.13 cm2/g. The mass attenuation and absorption coefficients (μ) are related to
wherein N0 and N(t) are the initial number of beta particles or photons and their numbers at thickness t of semiconductor, respectively. Therefore, the effect of gamma ray in this simulation may be excluded because the interaction of gamma ray with semiconductor is much less than that of beta particles with semiconductor.
For mimicking our real testing setup, the diameter of the electron beam was 7.5 mm and the beam source was set to be 1 mm away from electrode in water. The electrode consisted of TiO2 (10 μm), silicon (300 μm), and epoxy (2 mm). Note that this simulation ignored the presence of secondary electrons generated by primary electrons.
D. Electrical Properties
For measuring radio- and photo-current, the testing setup and electrode configuration is schematically shown in
E. Generation of Free Radicals
Furthermore, the generation of free radicals by water radiolysis was evaluated. High energy electrons passing through an aqueous solution can ionize or excite water molecules forming a number of transition species and stable products, as given by
A lot of molecules of the given products are formed by the absorption of 100 eV in the medium. The generated radicals are powerful redox reagents; i.e., eaq − is a strong reducing agent, H. is an equally strong reducing and oxidizing agent and .OH is a powerful oxidizing agent. In order to understand radiolytic activity of aqueous solutions, the radiolytic decolorization of 5 mg/L Methylene Blue (MB, C16H18ClN3S) aqueous solution having 0.1 M Li2SO4 electrolyte was evaluated by UV-VIS spectrometer. A 20 ml sample of this dye solution was placed in a bottle under 137Cs radiation and the absorption spectra were measured to determine the concentration of MB as a function of time with a reference. There were unique peaks of 292 nm, 613 nm, and 663 nm in MB solution. Those peak intensities of both solutions were gradually dropped with increasing time. The rate of decolorization was estimated in terms of change in intensity at Δmax=663 nm, as shown in
The radiocatalytic effect of the TiO2/Si electrode was demonstrated. The radiocatalytic chemical cell has shown larger generation of current than a photocatalytic chemical cell. Due to enhanced energy absorption of the TiO2 nanoparticle film using high electron-hole pair ionization energy, and multiplication of electron-hole pairs and free radicals generated by kinetic energy loss of beta particles, the performance of the radiocatalytic cell was significantly enhanced.
Having illustrated and described the principles of the present invention, it should be apparent to persons skilled in the art that the invention can be modified in arrangement and detail without departing from such principles.
Although the materials and methods of this invention have been described in terms of various embodiments and illustrative examples, it will be apparent to those of skill in the art that variations can be applied to the materials and methods described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.
This application is a continuation application of U.S. Ser. No. 14/762,869, filed on Jul. 23, 2015, which is a national stage application of PCT/US2014/014262, filed on Jan. 31, 2014, which claims the benefit of provisional application U.S. Ser. No. 61/849,660, filed Jan. 31, 2013, each of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61849660 | Jan 2013 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14762869 | Jul 2015 | US |
| Child | 16723611 | US |
