Betavoltaic Enhancement Using Defect ... - ACS Publications

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Betavoltaic enhancement using defect-engineered TiO nanotube arrays through electrochemical reduction in organic electrolytes Yang Ma, Na Wang, Jiang Chen, Changsong Chen, Haisheng San, Jige Chen, and Zhengdong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05151 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Betavoltaic enhancement using defect-engineered TiO2 nanotube arrays through electrochemical reduction in organic electrolytes Yang Ma,†,‡ Na Wang,†,‡ Jiang Chen,†,‡ Changsong Chen,†,‡ Haisheng San,,†,‡,§ Jige Chen,§ Zhengdong Cheng§,// †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China

‡

Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China

§

Shenzhen Betary Energy Technologies Co., Ltd., Shenzhen 518063, China

//

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA

KEYWORDS: TiO2; nanotube arrays; electrochemical reduction; organic electrolytes; betavoltaic effect

ABSTRACT: Utilizing high-energy beta particles emitted from radioisotope for long-lifetime betavoltaic cells is a great challenge due to low energy conversion efficiency (ECE). Here we report a betavoltaic cell fabricated using TiO2 nanotube arrays (TNTAs) electrochemically reduced in ethylene glycol electrolyte (EGECR-TNTAs) for the enhancement of betavoltaic effect. The electrochemical reduction of TNTAs using high cathodic bias in organic electrolytes

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is indeed a facile and effective strategy to induce in situ self-doping of oxygen vacancy (OV) and Ti3+ defects. The black EGECR-TNTAs are highly stable with a significantly narrower bandgap and higher electrical conductivity as well as UV-Vis-NIR light absorption. A 20 mCi of 63Ni betavoltaic cell based on the reduced TNTAs exhibits maximum ECE of 3.79% with open-circuit voltage of 1.04 V, short-circuit current density of 117.5 nA cm-2, and maximum power density of 39.2 nW cm-2. The betavoltaic enhancement can be attributed to the enhanced charge carrier transport and separation as well as multiple exciton generation of electron-hole pairs due the generation of OV and Ti3+ interstitial bands below the conductive band of TiO2.

Introduction Wireless sensing networks deployed in remote and inaccessible locations have resulted in an ever-increasing demand for long-lifetime, high efficiency, and maintenance-free power sources to meet continuous operations for low-power electrical applications. A significant number of studies have been devoted to harvest/convert energy from environment energy, such as light energy, thermal energy, vibration energy, and RF energy etc., but most of these techniques have to overcome the challenges with respect to performance, reliability, and limitation on the application environment. Betavoltaic energy conversion is a promising strategy to meet the power demand of micro-systems due to their high energy-density, long lifetime, safe operation, and insensitivity to environment.1 Betavoltaic energy conversion involves to the generation of power by coupling a beta source to a semiconductor junction device. In that way, the electron-hole (e-h) pairs are generated by impact ionization of incident beta particles in semiconductor lattice and are drawn off as current by the Volta effect when the beta particles emitted form a beta source traverse a semiconductor junction device.2-3


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In 1968, Olsen’s group developed the first commercial betavoltaic cell using stacked layers of Si cells coupled to 147Pm source, which was able to achieve an energy conversion efficiency (ECE) of 4% and 400 μW of power,4 but the products were not expanded further in market due to the high cost of 147Pm and the weak γ-rays decayed from 147Pm which brought radiation risk to people. In recent decades, a great number of studies have been devoted to improve betavoltaic performance through using relative low energy of beta sources (e.g., 63Ni and 3H) and the wide band-gap semiconductors (WBGS).5-8 According to Olsen’s calculation model, the maximum theoretical ECE could achieve 30% for betavoltaics using WBGS.4 However, Liu et al. have done more detail work on the efficiency limits.9 There are two kinds of efficiencies that should be considered: (1) total efficiency based on source total activity and (2) effective efficiency based on apparent or incident particles emitted from surface of the source. However, a great number of experiments and theoretical calculations have shown that the ECE was still less than 5% for the betavoltaics based on planar diode structure fabricated using available WBGS, such as SiC and GaN.3, 7-8, 10-11 But recently, Widetronix Corp. has reported a 12% efficient of 4Hsilicon carbide betavoltaic cell based on the radioisotope tritium.12 It is due to the fact that the tritium betas have an extremely shallow absorption in 4H-silicon carbide, resulting in a low surface recombination of e-h pairs and thus high beta ECE. Furthermore, Sun’s work has demonstrated that the ECE of betavoltaic cell based on 3D porous silicon p-n diodes was 10 times larger than that based on 2D planar one.5 3D porous diodes can bring about a substantial increase of the junction area, and the porous channels can be used to store the radioisotope source, providing significant benefits for the enhancement of betavoltaic ECE. Titania nanotube arrays are considered to be the ideal candidate owing to their wide



bandgap, high specific surface area, excellent chemical stability, and low-cost preparation, which have been applied in photocatalytics, solar cells, and supercapacitors.13-16 Meanwhile, Kwon's group has designed water-based nuclear batteries utilizing platinum-plated TNTAs.17 However, pristine TNTAs have high resistivity, thereby resulting in high internal resistance of betavoltaic cell and more possibilities for recombination of e-h pairs.18 Fortunately, extensive efforts have been devoted to modify pristine TiO2 for the enhanced conductivity. A common strategy is to dope pristine TiO2 with foreign impurity species (e.g., non-metallic N, F, C, and metallic Zn, Fe, Mn etc.).19-22 However, the introduction of dopants generally serves as carrier recombination centers and could lead to a decrease of ECE of solar cells.19, 21, 23 Another strategy is to self-dope TiO2 with the oxygen vacancy (OV) and Ti3+ defects by annealing TiO2 in a reductive/inert atmosphere or vacuum.14,

21, 24-26

Recently, it has been found that

electrochemical reduction of TNTAs under cathodic bias can enhance the performance of photocatalysts by self-doping Ti3+ into TiO2.27-29 Therefore, it is considered that, with no exception, the electrochemically reduced TNTAs are able to considerably improve ECE performances of betavoltaic cells. However, some reports have demonstrated that the cathodically treated TiO2 suffers from low stability with the reduction effect fading away quickly when removing the bias (e.g., within minutes or hours).27, 30 Possible explanation has been given in relevant studies that the dynamics of ion intercalation/release into/from TNTAs could be highly dependent on the electrolyte viscosity.31 In this work, we present a betavoltaic cell based on the electrochemically reduced TNTAs and demonstrate its betavoltaic performance for the first time. A substantial improvement in conductivity has been achieved by electrochemical reduction induced self-doping of oxygen


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vacancy and Ti3+ defects. The use of organic electrolytes was proved to be a facile and effective method to enhance the betavoltaic performance and its stability. Materials and methods Preparation of TNTAs TNTAs were fabricated by electrochemical anodization method (Figure S3 shows the picture of the preparation equipment of TNTAs) in organic electrolyte system containing 0.5 wt% ammonium fluoride (NH4F), 97 vol% ethylene glycol, and 3 vol% deionized water. Before conducting anodization, Ti foils were cut into suitable size (22 mm × 30 mm × 0.2 mm, 99.8% purity) and were successively cleaned with acetone, ethanol, deionized water in an ultrasonic cleaner for 15 minutes to completely remove organic impurities. After drying, the scotch tape was used to protect the backside of Ti sheet. The anodization was performed in a two-electrode (Pt as counter electrode and Ti sheet as work electrode) electrochemical cell accompanied by magnetic stirring at an appropriate rate under constant voltage of 50 V for 1 h at room temperature. After anodization, the samples were rinsed in absolute alcohol with low ultrasonic power to fully remove “nanograss” debris covered on surface of TNTAs. Next, the sample was rinsed in ethanol and DI water, respectively, and then dried in nitrogen. The as-anodized TNTAs were annealed at 450 °C in air for 2 h to form anatase crystals (denoted as Air-TNTAs). For the sake of comparison, the same as-anodized TNTAs were annealed in argon atmosphere (denoted as Ar-TNTAs).

Electrochemical reduction of TNTAs The electrochemical reduction of TNTAs was carried out in a two-electrode (Pt as counter



electrode and TNTAs on Ti substrate as work electrode) cell at room temperature. A cathodic voltage (-60 V for 35 s) was then applied to the air-annealed TNTAs in a mixed ethylene glycol electrolyte consisting of 200 ml glycol solution and 10 ml aqueous solution with 1 mol L-1 of Na2SO4 (denoted as EGECR-TNTAs). For comparison, the air-annealed TNTAs was also performed with electrochemical reduction in complete aqueous solution with 1 mol L-1 of Na2SO4. (5 V for 35 s, denoted as AECR-TNTAs).

Characterization The morphologies were examined by field emission scanning electron microscope (FESEM，ZEISS Sigma HD microscope) at 10 kV. The crystal structures of TNTAs were characterized by X-ray diffraction analysis (XRD, Rigaku Ultima IV) using Cu Kα radiation at 50 kV in a 2 range from 20ºto 80º. The electron paramagnetic resonance (EPR) signals of TNTAs were recorded to confirm the presences of oxygen vacancy and Ti3+ defects by an electron spin resonance spectrometer (BRUKER EMX-10/12). UV-Vis-NIR light absorbance spectra of TNTAs were recorded by UV-Vis-NIR spectrophotometer (Varian, Cary 5000 with Integrating Sphere Attachment) with fine BaSO4 powder as reference in the wavelength range of 150  850 nm. I-V measurements of the betavoltaic cells were performed in a Faraday cage by a Keithley Model 2450 source meter.

Results and Discussion Figure 1a shows typical FESEM image of top view of TNTAs, and its partial enlarged image is shown in Figure 1b. The TNTAs consist of compacted and highly ordered cylindrical nanotubes with uniform pore diameter of around 100 nm and tube-wall thickness of around 10


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nm (see TEM image of single TiO2 nanotube as shown in Figure S1). Comparing the partial enlarged FESEM image of TNTAs before thermal treatment and after electrochemical reduction (see Figure S2), it is confirmed that there are no obvious changes in morphology and size.

Figure 1. (a) Typical top-view of TNTAs and (b) partial enlarged top-view of TNTAs with size marking, (c) cross-sectional view of TNTAs with thickness marking and (d) partial enlarged cross-sectional view of TNTAs.

Figure 1c exhibits the typical FESEM images of TNTAs in cross-sectional view. It can be seen that the TNTAs film features self-aligned upright nanotubes standing vertically to the Ti substrate, with a grown nanotube-length of around 10 m. Great amounts of tube-to-tube connecting lines can be seen in the exterior surface of the nanotube wall, as shown in Figure 1d. The nanotube arrays could provide a 3D conductive matrix through these vertically oriented thinner walls as well as the horizontal connecting lines between nanotubes. 32 Furthermore, the close-packed nanotubes create the junction barriers between neighboring nanotubes beneficial



to the separation of e-h paris.33

Figure 2. XRD spectra of EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, Air-TNTAs, and pristine-TNTAs.

Figure 2 shows the X-ray diffraction (XRD) spectra of EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, Air-TNTAs, and pristine TNTAs. It can be seen that pristine TNTAs are amorphous along with the characteristic peaks of metal Ti, whereas the other four TNTAs exhibit crystallization characteristics with strong diffraction peaks of anatase phase (101) at 25.2º. Especially, the TNTAs after electrochemical reduction still maintain anatase phase without lattice damage.


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Figure 3. EPR spectra for EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs

The RT EPR spectra are recorded to identify the presence of OV and Ti3+ defects in TNTAs, as shown in Figure 3. The Air-TNTAs show no EPR signal, and the Ar-TNTAs show only an EPR signal peak at g = 2.003, which generally is assigned to surface electron trapping OV defects.34 These results are in line with other reports.14 In contrast, both EGECR- and ECRTNTAs exhibit two peaks, respectively at g = 1.980 and g = 2.003. The strong signal peaks at g = 1.980 generally are assigned to the Ti3+ defects,35 which are attributed to the reduction of Ti4+ to Ti3+ by electrochemical reduction. Moreover, it is found that the EPR signal intensity of OV defects is almost same for reduced TNTAs except for Air-TNTAs, but the EPR signal intensity of Ti3+ defects in EGECR-TNTAs is stronger than that in AECR-TNTAs, which means the presence of a higher density of Ti3+ defects in EGECR-TNTAs. This can be attributed to the introduction of more Ti3+ defects by using organic solvents. The application of organic solvents allows high voltage to be applied without solvents breakdown, offering a strong driving force for H+ to diffuse deeper into TiO2.29 Meanwhile, the organic solvents possibly provide the



proper viscosity than aqueous electrolyte for H+ ions to migrate and insert into TiO2 to reduce Ti4+ ions into Ti3+ ions. It has been reported that the electrolyte viscosity has a significant impact on the dynamics of the intercalation/release of the small ions into/from TiO2.31 High density of self-doping Ti3+ ions accounts for the fact that the EGECR-TNTAs have maximum conductivity in all of samples. Meanwhile, it also reveals that the Ti3+ defect states are the critical role in improving the conductivity of TNTAs.

Figure 4. Nyquist plots for EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs. The inset is the impedance spectra at high-frequency region.

Electrical properties of TNTAs are characterized by electrochemical impedance spectroscopy (EIS) obtained at 0.1 V DC bias potential with 5 mV oscillation amplitude in the frequency range from 0.01 Hz to 100 kHz under dark environment. Figure 4 shows Nyquist plots for EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs. All of samples exhibit typical semicircular arcs at high-frequency region (see inset of Figure 4), whereas no apparent


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feature of semicircular arcs is found at low frequency region. The impedance spectra at high frequency region generally represent the contribution of charge transfer resistance at counter electrode/electrolyte interface, with little dependency on the working electrode (TNTAs). On the other hand, the impedance spectra at low frequency region reflect the diffusion impedance in the electrolyte and highly depend on the resistance of the TNTAs/electrolyte interfaces.36 The slope of impedance curve at low frequency region can be used to estimate the conductive properties of TNTAs. It can be seen from Figure 4 that the EGECR-TNTAs have maximum slope in all of samples, meaning a fastest charge transfer and a smallest interfacial resistance in comparison with other three samples. The enhanced conductivity of EGECR-TNTAs can be attributed to the oxygen vacancy and Ti3+ defects. OV and Ti3+ defects can form a shallow donor level (electron traps) just below the conduction band, which are considered to not only contribute to the enhanced carrier density beneficial to the charge carrier transport, but also largely promote the charge separation process by the trapping of electrons.37-38



Figure 5. (a) UV-Vis-NIR light absorbance spectra and corresponding sample pictures for EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs and (b) their bandgap values estimated from the main absorption edge of the profile.

Beside of the significant enhancement of charge transport and separation, the presence of OV and Ti3+ defects can effectively expand the visible-light absorption range of TNTAs. As shown in Figure 5a. The Air-TNTAs are only active under UV irradiation, while the photoresponses of other reduced TNTAs are effectively extended into the visible and near infrared regions. Both Ar-TNTAs and EGECR-TNTAs samples exhibit same black color, which stands in a sharp contrast with the light-brown color of Air-TNTAs and the deep-gray color of AECR-TNTAs. The color change from light-brown to black color indicates a different extent of visible-light absorption in TNTAs, which could be ascribed to the increased defects density of OVs or Ti3+ in TNTAs. Ti3+ and OV defect states are formed in band gap of TiO2, respectively with shallow energies of 0.27  0.87 eV and deep energies of 0.75  1.18 eV below the conduction band minimum of TiO2.14, 39-40 Therefore, electronic transitions from both OVs and


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Ti3+ localized states to conduction band (CB), and from valence band (VB) to both OVs and Ti3+ localized states are responsible for Vis-NIR light absorption of reduced TNTAs. Figure 5b shows the band gap calculated by Kubelka Munk function from the absorption spectra.41 It can be seen that the bandgap of Air-TNTAs is approximately 3.10 eV, which is close to the 3.09 eV of pristine TNTAs reported by Kang et al.37 In contrast, the bandgaps of Ar-TNTAs, AECRTNTAs, and EGECR-TNTAs are 3.01 eV, 3.07 eV, and 2.95 eV, respectively. A high OV or Ti3+ concentrations can induce an OV interstitial bands or Ti3+ interstitial bands just below the conduction band, thereby resulting in the bandgap narrowing.

Figure 6. (a) Schematic diagrams of betavoltaic devices (Ni/63Ni/TNTAs/Ti) and (b) actual device photograph.

In order to investigate the performance of betavoltaics based on reduced TNTAs, the radioisotope Nickel-63 (63Ni) source with an average energy of 17.6 keV and half-life of 100.1 years is used in tests as the pure beta-emitting source. The

63

Ni beta source is a solid planar

source with a total activity of 20 mCi, which is prepared by electrodepositing 63Ni thin film on a 10 × 20 mm2 of Ni sheet in an electrolyte of 63NiCl2. The betavoltaic cells are assembled as a sandwich configuration (Ni/63Ni/TNTAs/Ti) (see Figure 6a) through a mechanical fixture to



compress the 63Ni/Ni source plate on the top surface of TNTAs on Ti substrate (see Figure 6b).

Figure 7. (a) I−V characteristics of Ar-TNTAs-based device with and without 63Ni radiation, comparison of (b) I−V and (c) P-V characteristics of betavoltaic cell based on EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs, (d) comparison of I−V characteristics of betavoltaic cells based on EGECR-TNTAs treated by electrochemical reduction for 5, 15, 25, and 35 seconds. Figure 7a displays a comparison of I−V curves of an Ar-TNTAs-based betavoltaic cell with and without the

63

Ni source assembled in device. The I-V curve passes through the

coordinates (0, 0) when using a Ni plate instead of 63Ni source plate, implying no occurrence of betavoltaic effect. In contrast, the I−V characteristic with 63Ni radiation exhibits a significant betavoltaic effect with open-circuit voltage (Voc) of  1.19 V and short-circuit current (Isc) of 


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156 nA. Figure 7b shows a comparison of I-V characteristics of betavoltaic cells based on EGECR-TNTAs, AECR-TNTAs, Ar-TNTAs, and Air-TNTAs. All of the betavoltaic cells exhibit betavoltaic effect with stable Isc and Voc in comparison with devices without beta source. The P-V characteristics, as shown in Figure 7c, are calculated from the results of I-V, and the electrical parameters of betavoltaic cells are extracted and presented in Table 1. Table 1. Electrical parameters of betavoltaic cells based on TNTAs treated in different conditions Betavoltaic cells (TNTAs)

Electrochemical Open-circuit Short-circuit Short-circuit reduction time voltage current current density

Max power density

Filling factor

Total efficiency

t (s)

VOC (V)

ISC (nA)

JSC (nA cm-2)

Pmax (nW cm-2)

FF

η

Air-TNTAs

0

1.18

68.10

34.05

22.13

0.55

2.10%

Ar-TNTAs

0

1.19

156.98

78.49

37.86

0.41

3.59%

AECR-TNTAs

35

1.13

195.96

97.98

35.70

0.32

3.41%

EGECR-TNTAs

5

0.91

121.98

60.99

19.11

0.35

1.85%

EGECR- TNTAs

15

0.94

149.94

74.97

23.80

0.34

2.30%

EGECR- TNTAs

25

1.13

219.02

109.51

39.06

0.31

3.78%

EGECR- TNTAs

35

1.04

235.00

117.50

39.15

0.32

3.79%

The conversion efficiency is calculated as follows:

=

Pmax FF × Voc × I sc = × 100% Psource 3.7 × 107 ×  × Eavg × e

(1)

Where Pmax is the maximum output power of the betavoltaic device, Psource is the radiation power of

63

Ni source,  is the source activity (mCi), Eavg is the average beta energy of the

isotope (eV), and e is the electron charge (C). FF is the filling factor, which can be expressed as FF = Pmax/IscVoc. It can be found from Table 1 that the betavoltaic cell based on EGECR-TNTAs has maximum ECE of 3.79% with Voc = 1.04 V, Jsc = 117.5 nA cm-2, FF = 0.32, and Pmax = 39.2 nW cm-2. Pmax of betavoltaic cells based on reduced TNTAs is about two times as high as that of the



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air-annealed TNTAs, which indicates the critical role of electrochemical reduction induced selfdoping of Ti3+ for the enhancement of betavoltaic effect. Figure 7d shows the effect of electrochemical reduction time on betavoltaic performance of betavoltaic cells based on EGECR-TNTAs. The measured results are also presented in Table 1. It is obvious that the Voc and Isc increase with the increase of reduction time until a saturation current of around 235 nA is reached for 35s of reduced time. The EGECR-TNTAs reported in this study show a high stability with black surface color and the betavoltaic effect in air for several months. The stability tests of betavoltaic effect show that the efficiency of betavoltaic cell based on EGECR-TNTAs stored in air environment for about five months (EGECR-TNTAs@5M) is only decreased 0.23% (see Figure S4). The slight degradation in efficiency is due to the oxidation of Ti3+ and OVs defects. In practical application, the issue can be resolved for betavoltaic cell by sealing packaging cell in an Ar filled glove-box for air isolation and radiation shield. In contrast, whereas the AECR-TNTAs only maintain the stability for several hours with dark colored state quickly vanishing. As previously reported, the surface OV and Ti3+ are not stable and prone to oxidation in air,34,

42

but the unique

electrochemical reduction of TNTAs using high cathodic bias in organic electrolyte enables an highly stable self-doping mechanism. The high driving force for the H+ to intercalate into the lattice of TiO2 leads to the formation of TiIII-H, TiIII-OH, and TiIII-OH2 bonds.21, 26, 29 This partially compensates for the loss of charge, which is beneficial to the structural stability. Furthermore, the high density of Ti3+ is significantly retained in the bulk phase of the EGECRTNTAs, which is expected to be more stable than those located on and near the surface.43 In order to gain more insight into the critical role of electrochemically reduced TNTAs in


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enhancement of betavoltaic effect, a schematic illustration on the betavoltaic enhancement mechanisms is shown in Figure 8. Schottky contact will be fabricated in Ni/TNTAs interface because the work function of Ni (5.15 eV) is higher than that of TiO2 (4.2 eV). In contrast, TNTAs/Ti interface can be approximately seen as the ohmic contact because the work function of TiO2 is close to that of Ti (4.33 eV). The enhancement in betavoltaic ECE depends on the creation, separation, and transport of beta-generated carriers in TNTAs, which competes with the carrier recombination. The reduced TNTAs provide several strategies to increase the ECE of betavoltaic cells.

Figure 8. Schematic illustration of betavoltaic enhancement mechanisms. (a) Separation and transportation of charge carriers in the nanotube wall under beta-radiation and energy band diagram of Ni/TNTAs/Ti device, (b) interband transition induced multiple exciton generation of e-h pairs in self-doped TNTAs under beta-radiation.

Firstly, the multi-porous array structures of TNTAs provide a great specific surface area,



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thereby increasing the active area of beta radiation absorption and energy conversion. Secondly, the extreme surface curvature of the nanotube can lead to enhanced surface activity,

33

which

results in the formation of junction barriers between neighboring tubes (NT-NT junction barriers), as shown in Figure 8a. The junction barriers enable orthogonal carrier separation (hole to the surface of tube-wall, electron to the inner of tube-wall) beneficial to the suppression of carrier recombination. Thirdly, electrochemical reduction induces the self-doping of OVs and Ti3+ in TNTAs, which are known to be electron donors/traps below the conduction band minimum of TiO2 (shallow for Ti3+ and deep for OV), not only improve the charge carrier transport in TiO2 due to the enhanced donor density, but also promote the charge separation due to acting as trap centers, thereby resulting in the reduction of carrier recombination. Finally, the mid-band gap electronic states created by OVs and Ti3+ are mostly responsible for the ECE enhancement by the multiple exciton generation (MEG). High-energy beta particles excite great amounts of e-h pairs in nanotube-walls by the impact ionization process of beta particles. These beta-generated electrons and holes with excess kinetic energy above the threshold energy for absorption (e.g., the bandgap in TiO2 and the different energy gaps between VB/CB and OVs/Ti3+) would create additional e-h pairs (see the Figure 8b), which has been well known as MEG process and reported on the studies of quantum dots solar cells.44-45 These extra-generated electrons and holes can be separated, transported, and collected to electrodes, contributing to the enhanced current in the betavoltaic cells.

Conclusion In summary, we have prepared well-aligned TNTAs using electrochemical anodization. By thermal treatment for TNTAs at 450 °C in air and subsequent electrochemical reduction


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using 50 V of cathodic bias in ethylene glycol electrolyte, the highly stable EGECR-TNTAs are obtained and exhibit enhanced electrical conductivity and UV-Vis-NIR light absorption as well as bandgap narrowing. The presence of OV and Ti3+ defects in EGECR-TNTAs is verified by ESR measurements. It is suggested that OV and Ti3+ defects serve as electron donors as well as electron traps, not only contributing to the enhanced carrier density beneficial to the charge carrier transport, but also facilitating the charge carrier separation. As a result, the carrier lifetime is prolonged and thus e-h recombination are suppressed. A 20 mCi

63

Ni source was

assembled to EGECR-TNTAs to form the sandwich-type betavoltaic cell (Ni/63Ni/ EGECRTNTAs/Ti). By I−V measurements, the betavoltaic cell exhibites maximum ECE of 3.79% with Voc = 1.04 V, Jsc = 117.5 nA cm-2, and Pmax = 39.2 nW cm-2. The betavoltaic enhancement can be attributed to the enhanced charge carrier transport and separation as well as multiple exciton generation of e-h pairs due the generation of OV and Ti3+ interstitial bands below the conductive band of TiO2. ASSOCIATED CONTENT Supporting Information. TEM image of single TiO2 nanotube; partial enlarged top-view of TNTAs before thermal treatment and after electrochemical reduction; the picture of the preparation equipment of TNTAs; and the stability tests of I−V characteristics of betavoltaic cells based on EGECR-TNTAs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].



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