Supporting Information

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1 Supporting Information Electrochemically reduced graphene oxide on well-aligned titanium dioxide nanotube arrays for betavoltaic enhancement Changsong Chen, Na Wang, Peng Zhou, Haisheng San,,, Kaiying Wang, Xuyuan Chen Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen , China Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials Research, Department of Physics, Xiamen University, Xiamen , China. Department of Micro and Nano Systems Technology, Buskerud and Vestfold University College, Tønsberg N-3103, Norwayd Corresponding Author: *

2 1. Electrochemical synthesis of graphene films Figure S1 shows 36-cycles of cyclic voltammograms (CVs) for electrochemical synthesis of graphene on TNTAs/Ti. It is seen from CVs that the reduction of GO enable a great cathodic current peak at -1.5 V, while the TNTAs/Ti electrode exhibits an oxidation peak at -0.6V. The continuous CV scans show that the current of the peak increases with the increase of cycle number (see the inset of Figure S1), suggesting that the GO sheets in the dispersion were electrochemically reduced to conductive graphene sheets and then continuously deposited onto the surface of TNTAs. The gradual increase in oxidation peak current was attributed to the enhanced conductivity with the increase of graphene loading CVs of GO Current(μA) Current(mA) Potential(V) Potential(V) Figure S1. 36-cycles of CVs for ERGO on TNTAs/Ti. The inset is a partial enlargement for the box part in CVs. 2. Calculation of the effective activity of the 63Ni beta source Activity, measured in the unit of Bequerel (Bq), is the quantity of decays occurring per unit time. It is also often expressed in units of Curie (Ci), where 1 Ci = Bq. In our device, because of the self-absorption effect in the isotope source and the difference of the emission direction of beta particles, only a small portion of beta particles escaped and entered into the TiO2 nanotube arrays (TNTAs), the S-1

3 actual quantity of beta particles escaped is used to calculate the effective activity of the beta source. For beta particles of 63 Ni, the activity of 1 mci = Bq. According to the scintillation current method, the electrical current of 10 na(i o )caused by radioluminescence of phosphors corresponds to a radiation activity of Bq. Therefore, the effective activity of the 63 Ni beta source was calculated according to the following equation: n / S 7 I I o, (S.1) here, I n presents the current caused by the actual incident beta particles of 63 Ni, and Φ s is the effective activity (mci). In this article, according to the actual contact area between 63 Ni source and TNTAs layer, namely mm 2, the activity available from a 10 mci 63 Ni beta source is 9.25 mci, and the measured effective activity is 1.37 mci. 3. Calculation of the effective activity of the energy conversion efficiency The energy conversion efficiency is calculated by: P FF V I max OC SC 100% 7, (S.2) P E e source avg where FF is the filling factor calculated by: I V mp FF I SC V mp OC, (S.3) here, P max is the maximum output power of the betavoltaic device, P source is the radiation power of the source, is the source activity (mci), E avg is the average beta energy of the isotope (ev) and e is the electron charge (C), I mp and V mp are the current and voltage at the maximum power point, respectively. S-2

4 4. Characterizations Figure S2. (a) Photograph of Keithley Model 4200-SCS semiconductor characterization system. (b) Photographs of the probe station in Faraday cage and measuring sample. (c) Screen copy image of I-V curve of the betavoltaic devices with 26-cycles CVs of G-TNTAs. 5. Comparision of experimental results reported in similar works Table S1. Experimental results reported in similar betavoltaic devices. Semiconductor materials Radiation source Device output Isotope Activity (mcicm -2 ) I SC (nacm -2 ) V OC (V) P max (nwcm -2 ) ECE Ref Ge, Si Si a-si:h 90 Sr- 90 Y % [1] 147 Pm 6800 mci na nw 0.77% [2] 3 H % [3] SiC SiC 63 Ni % 241 Am % [4] S-3

5 GaN TNTAs Se liquid 63 Ni % [5] 63 Ni % [6] 35 S 4.49 mci na nw 1.19% [7] Liquid semiconductor Porous Si 35 S % [8] 3 H gas % [9] Carbon tube-si 63 Ni % [10] Al 0.35 Ga 0.65 As Al 0.35 Ga 0.65 As 3 H % 3 H gas % [11] SiC 33 P ± ± ± ±0.04% [12] REFERENCES (1) Rappaport, P. The Electron-Voltaic Effect in P-N Junctions Induced by Beta-Particle Bombardment. Phys. Rev. 1954, 93, (2) Flicker, H.; Loferski, J. J.; Elleman, T. S. Construction of a Promethium-147 Atomic Battery. IEEE Transactions on Electron Devices, 1964,11, 2-8. (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, (4) 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, (5) 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, (6) Zhang, Q.; Chen, R. B.; San, H. S.; Liu, G. H.; Wang, K. Y. Betavoltaic Effect in Titanium Dioxide S-4

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