Electrochemically Reduced Graphene Oxide on Well-Aligned Titanium

Aug 30, 2016 - *E-mail: [email protected]. ... A 10 mCi of 63Ni/Ni source was assembled to G-TNTAs to form the sandwich-type betavoltaic devices ...
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Electrochemically Reduced Graphene Oxide on Well-Aligned Titanium Dioxide Nanotube Arrays for Betavoltaic Enhancement Changsong Chen,† Na Wang,† Peng Zhou,† Haisheng San,*,†,‡ Kaiying Wang,§ and Xuyuan Chen§ †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials Research, Department of Physics, Xiamen University, Xiamen 361005, China § Department of Micro and Nano Systems Technology, Buskerud and Vestfold University College, Tønsberg N-3103, Norwayd ‡

S Supporting Information *

ABSTRACT: We report a novel betavoltaic device with significant conversion efficiency by using electrochemically reduced graphene oxide (ERGO) on TiO2 nanotube arrays (TNTAs) for enhancing the absorption of beta radiation as well as the transportation of carriers. ERGO on TNTAs (GTNTAs) were prepared by electrochemical anodization and subsequently cyclic voltammetry techniques. A 10 mCi of 63 Ni/Ni source was assembled to G-TNTAs to form the sandwich-type betavoltaic devices (Ni/63Ni/G-TNTAs/Ti). By I−V measurements, the optimum betavoltaic device exhibits a significant effective energy conversion efficiency of 26.55% with an open-circuit voltage of 2.38 V and a short-circuit current of 14.69 nAcm−2. The experimental results indicate that G-TNTAs are a high-potential nanocomposite for developing betavoltaic batteries. KEYWORDS: TiO2, nanotube arrays, betavoltaic, graphene, electrochemical reduction

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INTRODUCTION Autonomous wireless sensor microsystems deployed in remote and inaccessible locations result in an ever-increasing demand for independent, sustainable, and maintenance-free power sources to meet continuous operations for low-power electrical applications. A significant amount of researches have been devoted to harvest/convert energy the from environment, but most of these approaches have to overcome the challenges with respect to performance, power density, and limitation of application environments. The radioisotope batteries are getting more and more attractive owing to their high-energy density, long lifetime, and insensitivity to environment. To convert radioactive decay energy into electrical energy, one of the promising techniques is the direct energy conversion method based on betavoltaic effect, employing a semiconductor p−n junction or Schottky junction where the build-in electrical field is used to separate beta-generated electron−hole pairs (EHPs).1 Silicon p−n diodes for converting beta radiation into electrical power were first suggested in 1954,2 in which the energy-conversion efficiency (ECE) was only 0.4% under irradiation of 30 mCi 90Sr-90Y, and the device performance rapidly decayed due to material lattice damage induced by strong beta-radiation. From then on, the studies of betavoltaics focus on how to increase the energy efficiency and sustainability. There are three strategies that could be used to improve batteries: (1) increasing the beta energy and flux of radioisotope sources used;3−5 (2) using the wide band gap semiconductors;5−9 and (3) using the semiconductor materials © 2016 American Chemical Society

with one dimension (1D) or three-dimension (3D) nanostructure.4,9−11 On the first point, it is clear that the increase in radiation intensity would inevitably involve the material lattice damage and result in the material performance decay, whereas the second and third points are practical and easy to implement based on current semiconductor growth and fabrication techniques. In the latest decade, the betavoltaic devices based on wide band gap semiconductor silicon carbide (SiC) and gallium nitride (GaN) have been reported,5−9 but the devices based on two-dimensional planar diode structure were subjected to low ECE due to limitation of the active area. To increase the ECE, Sun et al. developed a betavoltaic device based on 3D diode structure using porous silicon.11 The ECE of the device was measured to be about 10 times larger than planar-diode devices, which indicates that the high aspect ratio of nanoporous structures may be capable of improving energy conversion by the increase of active area and the absorptivity of beta radiation. It is believed that the wide band gap semiconductors with nanoporous structures might be promising materials for β particle capture and thus energy conversion. TiO2 nanotube arrays (TNTAs) fabricated by direct electrochemical anodization of titanium (Ti) are considered to be the best candidate due to their wide band gap (rutile: Received: July 4, 2016 Accepted: August 30, 2016 Published: August 30, 2016 24638

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation of ERGO on TNTAs. 400 °C for 4 h in nitrogen atmosphere to form the crystallized anatase phase. Next, the steps are turned to the fabrication of ERGO on TNTAs. The commercial GO powder was exfoliated in 0.1 M pH 9.32 phosphate buffer solution (PBS, Na2HPO4 & NaH2PO4) by ultrasonication for 90 min to form a brown GO colloidal dispersion with a concentration of 0.5 mg mL−1. The GO dispersion was reduced with magnetic stirring by the cyclic voltammetric method through a three-electrode system (the annealed TNTAs/Ti sheet as working electrode, Pt sheet as counter electrode, and saturated calomel electrode as reference electrode) on a CHI 660E electrochemical workstation. The scan ranged from −1.5 to 1.0 V at rates of 30, 40, and 50 mV s−1, respectively. Depositions of graphene sheets on TNTAs were controlled by the number of potential cycles. After deposition, the graphene on TNTA (G-TNTAs) samples was washed with ethylene glycol and deionized water and then dried at room temperature. Preparation and Calibration of the Radioisotope Source. 63 Nickel was chosen as the energy source due to the relatively low radiation (with an average energy of 17.1 keV and a maximum energy of 66.7 keV) and long half-life of 100.1 years.6,7 The 63Ni beta source with total activity of 10 mCi, supplied by China Institute of Atomic Energy, was prepared by electrodepositing 63Ni thin film on a 10 × 20 mm2 of Ni sheet in an electrolyte of 63NiCl2. By a four-probe measurement, the square resistance of the 63Ni beta source was 207 Ω/□. For the radiation source, there are two kinds of activities that can be used, namely, inherent activity and effective activity.6,20 The inherent activity measures the disintegration rate of the radioisotope source; the effective activity measures the actual emission rate of radiation produced in its decay. Due to the self-absorption effect, only a fraction of all the decays is used to produce the beta emission, which means the effective activity is lower than the inherent one. Therefore, the effective activity is employed in the calculation of ECE. Generally, the scintillation current method can be used to determine the effective activity.21 According to the actual contact area between the 63Ni source and TNTAs, namely, 10 × 18.5 mm2, the activity available from a 10 mCi 63Ni beta source is 9.25 mCi, and the effective activity is determined as 1.37 mCi (see Supporting Information). Characterization. Morphologies and size analysis of the TNTAs and G-TNTAs were characterized using field emission scanning electron microscopy (FESEM, ZEISS microscope). The selected area electron diffraction pattern (SEAD) of graphene layers was taken using transmission electron microscopy (JEM-2100 TEM). The crystal structures of the samples were characterized by X-ray diffraction analysis (XRD, Rigaku Ultima IV). Raman scattering was performed on a Raman spectrometer (Renishaw in via) providing valuable information about the defects and stacking of graphene layers The electrical properties of TNTAs and G-TNTAs can be characterized by I−V measurements with a sandwich-type MSM structure. Figure 2 shows schematic diagrams of G-TNTA-based betavoltaic devices (Ni/63Ni/G-TNTAs/Ti) and measurement systems. The betavoltaic devices (Ni/ 63 Ni/G-TNTAs/Ti) were

band gap 3.0 eV; anatase: 3.2 eV), high surface area, excellent chemical stability, and low-cost preparation.12,13 The vertically oriented TNTAs can provide a direct pathway for carrier transport along the long axis of nanotubes to the substrate, which enables the structures to be used as the electrode materials for solar cells,14 photocatalytics,15 supercapacitors,16 etc. However, pristine TNTAs generally suffer from poor conductive behavior, as TiO2 is a wide bandgap semiconductor with limited conductivity. Therefore, substantial improvement in the conductivity of TNTAs has been achieved by introducing impurities into the oxide, modified metal, or nonmetal conductor materials, such as introducing oxygen vacancies,16 Au nanoparticles/TNTAs,17 and polyaniline nanowires/ TNTAs.18 However, to our best knowledge, there is no report about using such nanocomposite structures to enhance the properties in betavoltaics. In our previous work,9 the TNTAs were first utilized to form a sandwich-type metal/semiconductor/metal (MSM) betavoltaic device, in which 7.3% of effective energy conversion efficiency (EECE) was achieved. To further improve TNTAbased betavoltaics, we have gained enlightenment from most recent works,14,15,19 which has demonstrated that the incorporation of graphene in nanostructured TiO2 can achieve higher photocatalytic activity and ECE of solar cell than unmodified TiO2. In this work, we prepared a novel betavoltaic device based on electrochemically reduced graphene oxide (ERGO) on TNTAs. The preparation of materials, morphology, and structure of materials and device properties were investigated in detail.

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MATERIALS AND METHODS

Preparation of ERGO on TNTAs. TNTAs were fabricated using the electrochemical anodization method, and the electrochemical cyclic voltammetry provides easy access to reduction of graphene oxide (GO) on TNTAs. The preparation of ERGO on TNTAs is schematically illustrated in Figure 1. The TNTAs were fabricated by anodization of Ti foil in an electrolyte solution consisting of 0.5 wt % ammonium fluoride (NH4F), 97 vol % ethylene glycol, and 3 vol % water. Before performing electrochemical anodization, Ti foils were cut into appropriate size (10 mm × 20 mm × 0.3 mm, 99.8% purity) and then were degreased and cleaned using acetone, isopropanol, and deionized water in ultrasonic bath for 15 min, respectively. After drying, the Scotch tape was used to protect the backside of Ti foil. The anodization was performed in a double electrode cell with magnetic stirring at a moderate rate under the constant potential of 50 V for 30 min at room temperature. After anodization, the samples were rinsed in absolute alcohol with low ultrasonic power to fully remove “grasslike” debris covered on TNTAs. As-prepared TNTAs were annealed at 24639

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

Research Article

ACS Applied Materials & Interfaces

TNTAs consist of compacted, highly ordered, cylindrical nanotubes with uniform pore diameter. FESEM observations confirm that there are no obvious morphological changes for TiO2 nanotubes after thermal treatment. The diameter and wall thickness of the nanotube are around 130 and 20 nm, respectively. The tube-to-tube spacing is approximately equal to wall thickness. The surface roughness of TNTAs can be observed by a high-magnification FESEM image (see Figure 3b). It can be seen that the TNTA exhibits a maximum roughness of about 500 nm in a typical rough surface due to different nanotube length. Figure 3c shows the typical FESEM images of postannealed TNTAs in cross-sectional view. The rough top surface also can be clearly seen from the 3D image of ∼4 μm thick TNTAs with good alignment and vertical orientation. A great deal of tube-to-tube connecting bridges can be seen in the exterior surface of the nanotube wall (see Figure 3d). It has been suggested that the close-packed nanotubes could provide a 3D conductive network by these vertically oriented tube-wall pathways and horizontal connecting bridges between nanotubes.22−24 The crystallized TNTAs are able to offer better carrier mobility than the amorphous phase.22,23 Figure 3e shows the XRD spectra of TNTA samples after and before thermal treatment. It can be seen that the phase of TNTAs before thermal treatment is amorphous along with the characteristic peaks of metal Ti, but the annealed TNTAs show the anatase phase agreeing well with the diffraction pattern of TiO2 anatase. The cyclic voltammograms (CVs) for electrochemical synthesis of graphene on TNTAs (G-TNTAs) are shown in the Supporting Information (see Figure S1). Figures 4a, 4b, and

Figure 2. Schematic diagrams of the G-TNTA-based betavoltaic device (Ni/63Ni/G-TNTAs/Ti) and measurement system. assembled in the probe station through a probe compressing the planar beta source (63Ni/Ni) on the G-TNTA samples. For comparison, the TNTA sample was also assembled and measured using the same beta source. I−V measurements were performed under the dark and room temperature in a Faraday cage by using a Keithley model 4200-SCS semiconductor characterization system (see Figure S2).

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RESULTS AND DISCUSSION It is now well established that the properties of the nanotube arrays are dependent upon their specific architecture, including length, wall thickness, pore diameter, and tube-to-tube spacing.22 Figure 3a shows the typical FESEM images of postannealed TNTAs in the top view. The postannealed

Figure 4. FESEM images of top view of G-TNTAs by ERGO on TNTAs for (a) 16 cycles, (b) 26 cycles, and (c) 36 cycles. The inset in (c) is the SAED pattern of ERGO on TNTAs. (d) Raman spectra of ERGO for 36 cycles at 30, 40, and 50 mV s−1.

4c show the FESEM top-view images of TNTAs coated by ERGO using CVs of 16 cycles, 26 cycles, and 36 cycles, respectively. It can be seen that the surface coverage of graphene clearly increases with the increase of cycle numbers of ERGO. At 16 and 26 cycles, graphene islands are observed to coexist with uncovered regions of TNTAs. At 36 cycles, the TNTAs surface is thoroughly covered by a closed and smooth graphene film. The individual graphene sheets usually have size distribution in the range of hundreds of nanometers to tens of

Figure 3. Typical low- and high-magnification FESEM images of postannealed TNTAs in top view (a) and (b) and cross-sectional view (c) and (d). (e) XRD spectra of TNTAs after and before thermal treatment. 24640

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

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ACS Applied Materials & Interfaces micrometers far larger than the nanotube diameter,25,26 meaning that the graphene sheets are too large to deposit into the inwall of the TiO2 nanotube. The transparency variation of graphene films can be explained by the variation of film thickness due to the restacking of graphene sheets. By observing the SAED pattern as shown in the inset of Figure 4c, the well-defined 6-fold symmetric diffraction spots suggest a crystallized graphene structure, which is favorable for carrier transport.27,28 Raman spectroscopy is performed to measure the quality and layer number of graphene film. Raman spectra of ERGO for 36 cycles at different deposition rates are shown in Figure 4d. Three prominent graphene peaks at ∼1327, ∼1587, and ∼2650 cm−1, corresponding to the D, G, and 2D bands, are observed in the Raman spectra. The weak and broadened D band and G band under deposition rates of 40 and 50 mVs−1 show that the oxygen functional groups in the GO cannot be fully removed due to the fast deposition rate, resulting in the absence of sp2. However, as the deposition rate was reduced to 30 mV s−1, the D and G bands became more prominent and sharper, exhibiting typical features of ERGO.29,30 The high intensity G band implies that a significant number of sp2 carbon networks are generated. Meanwhile, the intensity increase in D band can be due to defects introduced into the GO during electrochemical reduction.30,31 Furthermore, the weak and broadened 2D peak indicates the generation of multilayered graphene on TNTAs.32 The multiple layers of graphene sheets that are overlapped and interconnected ensure a conduction pathway well contacted with the nanotubes. A Ni sheet was assembled, respectively, to TNTAs/Ti and GTNTAs/Ti structures for comparing their dark I−V characteristics. Figure 5a shows the forward and reverse dark I−V

Ti, and graphene as shown in Figure 5b, it can be well understood that a Schottky junction can be formed in the interface of graphene/TiO2, Ni/graphene, and TiO2/Ti. Annealed TNTAs are n-type semiconductors with doping level in the order of 1018−1020 cm−3 owing to oxygen vacancies,35 and the TiO2/Ti interface with weak Schottky barrier can be seen as an ohmic contact. An equivalent circuit is used to model the assembled devices as shown in the inset of Figure 5a. The great turn-on voltages are due to great series resistance (Rs) mainly introduced by the contact resistance between the electrode and TNTAs. The Rs of G-TNTA-based and TNTA-based devices are extracted from Figure 5c and 5d to be 4.5 × 101 Ω cm−2 and 5.9 × 103 Ω cm−2, respectively. The great Rs in the TNTA device can be explained by the rough surface of TNTAs due to the irregular length of nanotubes as shown in Figure 3b and 3c, reducing the effective contact area between the nanotube and Ni sheet. It is considered that the soft graphene film by ERGO on TNTAs enables a fully electrical contact with TNTAs as well as the Ni sheet, decreasing the interface contact resisitance as a result. The GTNTA-based device produces about 102 times more reverse leakage current density than TNTA-based devices, also indicating that the reverse leakage current is nearly proportional to the effective area of the contact. Figure 6a shows a comparison of I−V characteristics of a 36cycle G-TNTA-based device with and without the 63Ni source.

Figure 6. (a) Comparison of I−V characteristics of G-TNTA-based devices with and without 63Ni radiation, (b) comparison of I−V characteristics of betavoltaic devices with and without graphene on TNTAs, (c) comparison of I−V characteristics of betavoltaic devices with G-TNTAs prepared by CVs for 16, 26, and 36 cycles, and (d) calculated J−V and P−V characteristics according to the I−V characteristics of the optimum betavoltaic device. Figure 5. (a) I−V characteristics of TNTA-based and G-TNTA-based devices with an equivalent circuit of device in the inset. (b) Schematic energy-level diagram for TiO2, Ni, Ti, and graphene.33,34 The series resistance (R s) of G-TNTA-based and TNTA-based devices extrapolated from the linear regimes in (c) and (d) are 4.5 × 101 Ω cm−2 and 5.9 × 103 Ω cm−2, respectively.

The I−V curve without beta radiation crosses the coordinate (0, 0), indicating that no betavoltaic effect occurs. On the contrary, the I−V curve with beta radiation behaves as an obvious betavoltaic effect with a high open-circuit voltage (VOC) of 2.38 V and short-circuit current (ISC) of 27.2 nA. In order to verify the important role of graphene film in betavoltaic enhancement, a comparison of I−V characteristics of betavoltaic devices based on G-TNTAs and bare TNTAs is presented. As shown in Figure 6b, the open-circuit voltage and short-circuit current of G-TNTA-based betavoltaic devices are almost 2 times and 1.3 times larger than that of the bare TNTA-based device, respectively. The results demonstrate that

characteristics of Ni/TNTAs/Ti and Ni/G-TNTAs/Ti structures. It is noted that the current on reverse bias voltage is shown in a logarithm scale and linear scale on forward. As shown in Figure 5a, both devices exhibit a diode performance with high turn-on voltage (2 V for G-TNTAs and 3.5 V for TNTAs) and ∼10−7−10−9 A cm−1 of reverse leakage current at −4 V. By the band gap energy and work functions of TiO2, Ni, 24641

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

Research Article

ACS Applied Materials & Interfaces Table 1. Electrical Parameters of TNTA-Based and G-TNTA-Based Betavoltaic Devices electrochemical deposition

open-circuit voltage

short-circuit current

max power density

filling factor

total efficiency

effective efficiency

G-TNTA samples

cycle number

VOC (V)

ISC (nA cm−2)

Wm (nW cm−2)

FF

ηa

ηeb

1 2 3 4

0 16 26 36

1.22 1.51 1.85 2.38

11.54 11.99 14.10 14.69

6.97 9.26 15.42 20.24

0.49 0.51 0.59 0.58

1.35% 1.80% 2.99% 3.93%

9.14% 12.14% 20.22% 26.55%

a

η is calculated according to the total activity of 9.25 mCi. bηe is calculated according to the effective activity of 1.37 mCi.

Figure 7. Schematic illustrations of the operation principle of the G-TNTA-based betavoltaic device: (a) the scattering and absorption of beta particles in the nanotube array accounting for the highly efficient creation of EHPs, (b) the orthogonal separation and transportation of EHPs in the nanotube wall, and energy band diagrams of (c) the graphene/TiO2 interface and (d) nanotube-wall under beta-radiation.

The energy conversion efficiency of betavoltaics depends on the creation, separation, and transport of beta-generated carriers in TNTAs, which competes with the carrier recombination.37 When compared to conventional betavoltaics based on the planar diode structure, the beta-energy conversion material based on G-TNTAs provides multiple strategies in betavoltaic enhancement by suppressing carrier recombination and improving carrier transport. First, the porous morphologies of 1D TNTAs define a very large internal surface area capable of trapping the incident beta particles and enabling strong betaabsorption by beta-scattering inside and outside of the nanotubes. Second, extreme surface curvature of the nanotube may result in enhanced surface activity;38 for example, the chemisorption of oxygen in air (production of O2−) takes place on either side of the TiO2 nanotube walls, creating an electron depletion region in the nanotube surface.13,22,39 The holes migrate to the surface of the tube-wall along the potential gradient produced by band-bending, while the electrons transport to the substrate electrode through the inner tubewall (the orthogonal EHP separation in the tube-wall is illustrated in Figure 7c). Third, the graphene sheets are covered in the top surface of TNTAs to improve the carrier collection and suppress carrier recombination. The flexible graphene as an electrode may benefit the charge collection and transport due to not only its high conductivity but also its excellent electrical contact with TNTAs. The graphene sheets making a fully electrical contact with TNTAs are capable of rapidly collecting the holes from nanotubes and thus effectivly suppress the combination of electrons in nanotubes. The operation principle of G-TNTA-based betavoltaic batteries is similar to that of photovoltaic solar cells. The beta particles emitting from the 63Ni radioisotope source penetrate through multilayer graphene and impinge upon TNTAs; in the process the kinetic energy of the β particle is deposited along passed lattices and creates EHPs. The EHPs

the graphene on TNTAs can make an enhanced betavoltaic energy conversion. Figure 6c shows a comparison of I−V characteristics of betavoltaic devices based on G-TNTAs prepared by CVs of 16, 26, and 36 cycles. It can be seen that the open-circuit voltage increases with the increase of cycle number, while the shortcircuit current increases to a saturation value of around 28 nA. It can be well understood that the continuity, integrity, and coverage of graphene on TNTAs can be improved by increasing the cycle number of CVs. The increase of coverage and thickness of the graphene layer would improve carrier transportation and result in the enhancement of a betavoltaic effect. As the cycle number is further increased to form a thicker graphene film, it is considered that the graphene films are supposed to form graphite-like structure through strong π−π stacking interactions, which would increase the defects and decrease carrier mobility.36 As a result, the short-circuit current tends to saturation after sufficient CV cycles. The maximum power density and filling factor can be calculated from current density via voltage (J−V) curve and power density via voltage (P−V) as shown in Figure 6d, in which the voltage and current at the peak power are 2.04 V and 18.36 nA for a 36-cycle G-TNTA-based betavoltaic device, suggesting a filling factor of 0.58 and the maximum power density of 20.24 nW cm−2 (the calculation method can be seen in the Supporting Information). Table 1 presents the electrical parameters of measurement and calculation for betavoltaic devices based on TNTAs and G-TNTAs. The 36 cycles of GTNTAs make a maximum ECE of 26.55% with an open-circuit voltage of 2.38 V and a short-circuit current of 14.69 nA cm−2. In comparison with the experimental results of other similar works as listed in Table S1 in the Supporting Information, we can deduce that the G-TNTA-based betavoltaic devices have record values in open-circuit voltage and ECE for reported betavoltaic devices using a 63Ni source. 24642

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

Research Article

ACS Applied Materials & Interfaces are separated by build-in potential in TNTAs and collected in electrodes. As a result, the kinetic energy of the beta particles is harvested and converted into electrical energy. In comparison with a photon exciting an EHP, a β particle of 63Ni can create thousands of EHPs along its moving trajectory. Figure 7 displays a schematic illustration of operation principle of GTNTA-based betavoltaics. As a Ni sheet coated 63Ni film is compressed into the soft multilayered graphene sheet film, the beta particles emitting from 63Ni excite EHPs in TNTAs. With the action of build-in potential in graphene/TNTA Schottky junction located in the top surface of TNTAs as well as O2−/ TNTA heterojunction distributed in the nanotube-wall surface, the EHPs in the tube-wall are effectively separated (see Figure 7a). The electrons transport from the conductive band of TiO2 to the Ti substrate through the interior of the nanotube-wall, while the hole transports from the valence band of TiO2 to the graphene film through the nanotube-wall surface. The separated transport pathways for the electrons and holes significantly decrease the recombination of electrons and holes (see Figure 7b and 7c). Furthermore, the beta particles emitting from 63Ni also excite EHPs in graphene film, and the excited electrons with kinetic energy larger than the height of the Schottky barrier could pass across the graphene/TiO2 interface into the depletion region; thereby, these electrons are swept through the depletion region by build-in potential and eventually transferred to the Ti electrode (see Figure 7d). In this process the graphene film rapidly collects beta-generated holes from not only TiO2 but also itself, increasing the work function (ΦG) and thus the Schottky barrier height (Φb). As a great deal of betagenerated electrons are excited to the conductive band of TiO2, resulting in the decrease of work function of TiO2 (ΦTiO2), the difference in Fermi level between graphene and TiO2 (qVOC = EFS − EFM) is increased accounting for the high open-circuit voltage in G-TNTA-based betavoltaics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 61574117 and 61274120). The Research Council of Norway is acknowledged for the support to the Nano-Network (221860/F40), EEA-Poland (237761), and FRINATEK programme (231416/F20).

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REFERENCES

(1) Kavetsky, A. G.; Meleshkov, S. P.; Sychov, M. M.; Bower, K. E. Polymers, Phosphors, and Voltaics for Radioisotope Microbatteries; CRC Press: Boca Raton, FL, 2002. (2) Rappaport, P. The Electron-Voltaic Effect in P-N Junctions Induced by Beta-Particle Bombardment. Phys. Rev. 1954, 93, 246−247. (3) Liu, B.; Chen, K. P.; Kherani, N. P.; Zukotynski, S.; Antoniazzi, A. B. Betavoltaics Using Scandium Tritide and Contact Potential Difference. Appl. Phys. Lett. 2008, 92, 083511−083513. (4) Kim, B. H.; Kwon, J. W. Plasmon-Assisted Radiolytic Energy Conversion in Aqueous Solutions. Sci. Rep. 2014, 4, 5249−5249. (5) Eiting, C. J.; Krishnamoorthy, V.; Rodgers, S.; George, T.; Robertson, J. D.; Brockman, J. Demonstration of a Radiation Resistant, High Efficiency SiC Betavoltaic. Appl. Phys. Lett. 2006, 88, 064101− 064103. (6) Qiao, D. Y.; Chen, X. J.; Ren, Y.; Yuan, W. Z. A Micro Nuclear Battery Based on SiC Schottky Barrier Diode. J. Microelectromech. Syst. 2011, 20, 685−690. (7) Cheng, Z. J.; Chen, X. Y.; San, H. S.; Feng, Z. H.; Liu, B. A High Open-Circuit Voltage Gallium Nitride Betavoltaic Microbattery. J. Micromech. Microeng. 2012, 22, 074011−074016. (8) Chandrashekhar, M. V. S.; Thomas, C. I.; Li, H.; Spencer, M. G.; Lal, A. Demonstration of a 4H SiC Betavoltaic Cell. Appl. Phys. Lett. 2006, 88, 033506−033508. (9) Zhang, Q.; Chen, R. B.; San, H. S.; Liu, G. H.; Wang, K. Y. Betavoltaic Effect in Titanium Dioxide Nanotube Arrays under Buildin Potential Difference. J. Power Sources 2015, 282, 529−533. (10) Wacharasindhu, T.; Kwon, J. W.; Meier, D. E.; Robertson, J. D. Radioisotope Microbattery Based on Liquid Semiconductor. Appl. Phys. Lett. 2009, 95, 014103−014105. (11) Sun, W.; Kherani, N. P.; Hirschman, K. D.; Gadeken, L. L.; Fauchet, P. M. A Three-Dimensional Porous Silicon P-N Diode for Betavoltaics and Photovoltaics. Adv. Mater. 2005, 17, 1230−1233. (12) Wang, K.; Liu, G.; Hoivik, N.; Johannessen, E.; Jakobsen, H. Cheminform Abstract: Electrochemical Engineering of Hollow Nanoarchitectures: Pulse/Step Anodization (Si, Al, Ti) and Their Applications. Chem. Soc. Rev. 2014, 43, 1476−1500. (13) Lee, K.; Mazare, A.; Schmuki, P. One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes. Chem. Rev. 2014, 114, 9385− 9454. (14) Wang, T. W.; Ball, J. M.; Barea, E. M.; Abate, A.; AlexanderWebber, J. A.; Huang, J.; Saliba, M.; Mora-Sero, I.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Low-Temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano Lett. 2014, 14, 724−730.

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CONCLUSION In summary, we have prepared the well-aligned TNTAs using electrochemical anodization, and the electrochemical cyclic voltammetry method was used to reduce and deposite GO sheets on TNTAs. The FESEM and Raman scattering analyses indicate that the continuous, multilayered, reduced GO sheets were deposited in the top surface of TNTAs rather than in the surface of the tube-wall. A 10 mCi 63Ni/Ni source was assembled to TNTAs and G-TNTAs, respectively, to form the sandwich-type betavoltaic devices (Ni/63Ni/G-TNTAs or TNTAs/Ti). By I−V measurements, the betavoltaic device with 36-cycle CVs of G-TNTA structure exhibits a significant EECE of 26.55% with an open-circuit voltage of 2.38 V and a short-circuit current of 14.69 nAcm−2, which indicates that the G-TNTAs are able to generate an enhanced betavoltaic effect in comparison with bare TNTAs. The betavoltaic enhancement induced by G-TNTAs is attributed to the large specific surface area of 1D TNTAs and the effective electrical contact between graphene and TNTAs, making highly efficient creation and transportation of carriers and suppression of carrier combination.

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Electrochemical synthesis of graphene films; calculation of the effective activity of the 63Ni beta source and energy conversion efficiency; photographs of measurement system, device, and typical measurement results of I− V; experimental result list of similar works for comparison (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08112. 24643

DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644

Research Article

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DOI: 10.1021/acsami.6b08112 ACS Appl. Mater. Interfaces 2016, 8, 24638−24644