The field of conventional energy conversion using radioisotopes has almost exclusively focused on solid-state materials. Herein, we demonstrate that liquids can be an excellent media for effective energy conversion from radioisotopes. We also show that free radicals in liquid, which are continuously generated by beta radiation, can be utilized for electrical energy generation. 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. This work introduces a new route for the development of next-generation power sources.
Structure and mechanism of the plasmon-assisted radiolytic water
Radio-current output at the Pt/nanoporous TiO2 electrode. Figure 3
MC simulation of the electron beam in the radiochemical cell. Figure 4
FDTD simulations of Pt/nanoporous TiO2 on glass under an e-beam irradiation of 546 keV and optical measurement.
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 wide range of applications, from energizing cardiac pacemakers in human body1 to challenging outer planet missions2. As compared to various indirect conversion methods, which collect electricity from the secondary energy forms of heat or light generated by radiation3, 4, direct conversion methods produce electric power straight from energetic particles5, 6. Beta particles can produce electron-hole pairs in semiconductors via their loss of kinetic energy and can contribute to the generation of electric power6. Although potential applicability of radioisotopes in portable power sources that do not require recharging seems very attractive7, it has been reported that only a small portion of the entire radiation energy can be converted into electrical energy5, 6, 7, 8, 9, 10. 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 particles11. Alternatively, to minimise 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 efficiencies12, 13. Little improvement has been made even after vigorous research on various improvement methods utilizing porous structures14, inverted pyramidal cavities15, and three dimensional silicon pillar structures15 to increase rectifying junction areas. Although, thus far, there is no method for completely avoiding radiation damage to semiconductors, the use of a liquid-phase semiconductor material has been introduced as a possible means to overcome the radiation damage and structural defect problem16. One major benefit of utilising a liquid-phase material is the well-known ability to efficiently absorb the kinetic energy of beta particles17.
Since the advent of nuclear power, liquids have been intensively studied for use as a radiation-shielding material. 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·) can be produced through radiolytic interactions18, 19, 20. These free radicals are known to affect the generation of molecular by-products, such as H2O2 and H221, 22, 23, 24. However, counterintuitively, the generation of electricity from a solution containing an ample amount of absorbed radiation energy has not yet been vigorously studied. Here, we demonstrate a new method for the generation of electricity using a device that separates the radiolytic current from the free radicals by splitting the water.
Design and operating principle of Pt/nanoporous TiO2 radiolytic cell
Our water splitter is composed of a nanoporous semiconductor coated with a thin Pt film to produce a specially designed metal-semiconductor junction (Fig. 1a). As a suitable semiconductor material for water decomposition, we used a very stable and common large band gap oxide material, TiO2, because the use of large band gap oxide materials as a semiconducting catalyst can improve the radiolysis yield25, 26, 27. Nanoporous TiO2 was formed by anodising and subsequently thermally oxidising a thin Ti film. The large surface area of the porous structure can provide more chemical reaction sites than a planar surface. To form a stable Schottky contact at the semiconductor/metal interface, a thin Pt film was uniformly deposited using a radio frequency (RF) sputtering system. As shown in the band diagram of the junctions in Fig. 1b, a Schottky barrier of 0.45 eV is formed because the Fermi energy (EF) of TiO2, an n-type semiconducting oxide, is 5.2 eV28, 29 while that of Pt is 5.65 eV30 with respect to the vacuum level. To experimentally confirm the Schottky barrier height between Pt and TiO2, we performed XPS analysis and measured the Schottky barrier height of 0.6 eV between Pt and Pt/TiO2 layers. 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. Since the first report31 of the generation of surface plasmons on rough metal surfaces by electron illumination in 1977, many studies have examined various metal structures32, 33, 34, 35. 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.
Figure 1: Structure and mechanism of the plasmon-assisted radiolytic water splitter.
Structure and mechanism of the plasmon-assisted radiolytic water splitter.
(a), Cross-sectional schematic illustration of nanoporous TiO2 prepared by anodising and thermally oxidising a thin Ti film deposited on a glass substrate. The thin Pt film is deposited on top of the TiO2 nanopores using an RF sputtering system. (b), Energy level diagram of a surface-plasmon-assisted radiolytic water splitter. CB, conduction band; VB, valence band; EF, Fermi energy; eaq−, aqueous electron; ·OH, hydroxyl free radical; β, beta radiation.
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In contrast to photocatalytic cells, high-energy beta radiation in our device can produce 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 molecules36, 37, 38. 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.
Structural properties of radiolytic electrode
Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) data for the nanoporous TiO2 are shown in Fig. 2. The cross-sectional SEM image shows the existence of nanopores 100 nm in diameter and 1 μm deep with a spacing of 100 nm (Fig. 2a). Figure 2b shows a top-viewed SEM image of a 50-nm-thick Pt film coated surface of nanoporous TiO2. After Pt deposition on nanoporous TiO2 film, the size of Pt nanoholes turns out to be approximately 10 ~ 20 nm (Fig. 2b, inset). The density of holes is 3 ~ 5 × 109 cm−2. As shown by the XRD data in Fig. 2c, after the as-deposited Ti was anodised for 5 min, the intensity of the Ti (002) peak decreased compared to that of the Ti (001) peak, indicating the presence of vertically arranged uniform nanopores along the direction of the Ti. After the anodised Ti was thermally oxidised at 450°C for 2 hours, the (001) and (002) peaks of Ti disappeared, and a new peak (37.24°) corresponding to the rutile crystalline structure appeared, indicating that TiO2 has a band gap of 3.2 eV39.
Figure 2: Radiolytic electrode.
(a), Cross-sectional SEM image of nanoporous TiO2 on glass. (b), SEM image of Pt-coated nanoporous TiO2 viewed from the top and the inset is a SEM image of a nanohole. (c), XRD data of as-deposited Ti (black line), anodised Ti (blue line), and rutile TiO2 (red line).
Electrical properties of radiolytic cell
Illustrations and photographs of the test setup and of the Pt/nanoporous TiO2 cathode are shown in Figs. 3a and b, respectively. During our experiment, gas bubbles on the PET plastic shielding film of the Sr-90/Y-90 source (Fig. 3a) clearly demonstrated the occurrence of water splitting. To evaluate the radiolytic performance of the Pt/nanoporous TiO2 electrode in a 1 M KOH aqueous solution, we employed a potentiostat. We measured the current density-voltage characteristics (Fig. 3c), 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 (Fig. S2). Figure 3c shows the current density - voltage (J-V) characteristics for the Pt/nanoporous TiO2 radiolytic electrode (red line) under irradiation. For comparison, we measured the radio current of a nanoporous TiO2 electrode (blue line) under irradiation and the dark current of Pt/nanoporous TiO2 (black line) under no irradiation. While the radio current of nanoporous TiO2 is slightly larger than the dark current of Pt/nanoporous TiO2, the radio current of Pt/nanoporous TiO2 is significantly larger than both the radio current of nanoporous TiO2 and the dark current of Pt/nanoporous TiO2. The radio current density of Pt/nanoporous TiO2 is saturated at −175.4 μA/cm2 for 0 V, while the dark current density of Pt/nanoporous TiO2 and the radio current density of TiO2 are approximately −1.051 μA/cm2 and −0.0719 μA/cm2, respectively. At −0.9 V, the radio current density of Pt/nanoporous TiO2, the dark-current density of Pt/nanoporous TiO2, and the radio current density of TiO2 are −83.336 μA/cm2, 70.31 μA/cm2, and 2.85 μA/cm2, respectively. These measurement data are summarized and reshaped into Table S1 to compare the performance of the irradiated devices with Pt/nanoporous TiO2 and nanoporous TiO2. Figure 3d manifests the clear difference in output power from devices with and without the plasmonic Pt layer. The output power densities (11.59 μW/cm2 at −0.1 V and 75.02 μW/cm2 at −0.9 V) of the Pt/nanoporous TiO2 electrode are higher than the power densities (−0.0027 μW/cm2 at −0.1 V and −2.565 μW/cm2 at −0.9 V) of nanoporous TiO2. For a radioactive material activity of 15 mCi (±10%), the total number of beta particles per unit time is estimated to be 5.55 × 108 s−1, and the total input power density of the beta particles is 139.238 μW/cm2, when the average kinetic energy of Sr-90/Y-90 is 490.96 keV. From this data, maximum energy conversion efficiency (η) of our device was approximately estimated to be 53.88% at −0.9 V using , where Prad, Pchem, and Pout are radiation power density of source (139.238 μW/cm2), chemical power density in water, and, output power density of device (75.02 μW/cm2), respectively. One possible reason for the high output power density under irradiation is that a certain level of the EHP ionisation energy of beta particles can easily excite electrons because the EHP ionisation energy is much higher than the band gap of each material while a large portion of the spectrum of solar light is below the TiO2 band gap, indicating that the TiO2 layer does not absorb sunlight well. Therefore, beta particles are a reliable energy source for electricity generation via water splitting.