Supporting information

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1 Supporting information Evolution of Hollow TiO 2 Nanostrutures via the Kirkendall Effet Driven y Cation Exhange with Enhaned Photoeletrohemial Performane Yanhao Yu, 1 Xin Yin, 1 Alexander Kvit, 1,2 Xudong Wang 1, * 1. Department of Materials Siene and Engineering, University of Wisonsin-Madison 2. Materials Siene Center, University of Wisonsin-Madison * xudong@engr.wis.edu Experimental Details (1) Pulsed vapor deposition setup and hollow TiO 2 struture evolution ondition. A home-made atomi layer deposition (ALD) system was used to arry out the pulsed vapor deposition of hollow TiO 2 nanostrutures from ZnO NW templates. The target sustrate was loaded on a quartz tue and plaed at the position 5 m away from the preursor inlet nozzle. N 2 gas with a flow rate of 40 sm was introdued into the hamer to serve as the arrier gas. The system s ase pressure was kept at 3.2 Torr. The hollow TiO 2 evolution and 3D TiO 2 nanorod (NR) growth were onduted under the same growth onditions. The hamer temperature was maintained at 600 C during the entire growth proess. TiCl 4 and H 2 O vapors were pulsed into the deposition hamer separately with a pulsing time of 500ms and separated y 60s N 2 purging. Therefore, one deposition yles involves 500 ms of H 2 O pulse + 60s of N 2 purging ms of TiCl 4 pulse + 60 s of N 2 purging with a TiCl 4 pressure hange of 110 millitorr (hamer pressure differene efore and after ALD valve open). The hamer was ooled down naturally under N 2 flow after growth. To aquire different stages of nanostruture (Figure 1and 4, Figure S8 and S9), 1

2 deposition was terminated at the yles of 2, 5, 10, 25, 50, 100, 200 and 400 and the samples were removed from the hamer for haraterization. (2) Control experimental details and polyrystalline film oating ondition. All ontrol experiments and film oating were performed in the same ALD system and under idential onditions as desried aove exept preursor supply and temperature adjustments. In the ontrol experiments of surfae reation study, three preursor and temperature ompositions was applied: 10 yles of TiCl 4 pulse without H 2 O supply at 600 C, 10 yles of TiCl 4 pulse without H 2 O supply at 300 C, and 10 yles of H 2 O pulse without TiCl 4 supply at 600 C. To study the ZnO diffusion effet, a onformal amorphous TiO 2 thin film was oated on the ZnO nanowire (NW) y 200-yle ALD at 100 C. As soon as the oating was finished, without ooling down the hamer and taking out the sample, the hamer temperature was diretly raised to 600 C and annealed the sample at this temperature for 10 hours under N 2 flow. To oat the ZnO NWs with polyrystalline TiO 2 films for PEC performane omparison, the deposition temperature was set at 300 C and 400 yles and 800 yles of ALD growth were applied. (3) Hydrothermal growth and hemial vapor deposition of ZnO nanostruture. The straight ZnO NW arranges were growth y the hydrothermal method, following the previously reported proedure. 1 A thin film of ZnO seeds was deposited onto fluorine doped tin oxide (FTO) ondutive glass sustrate y dropping 5 mm zin aetate solution and aking for 20 minutes at 300 C. After that, the sustrate with seeds was immersed in the nutrient solution with the seeds surfae faing downward. The nutrient solution onsisted 25 mm zin nitrate and 25 mm hexamethylenetetramine (HMT). After 6 hour growth at 90 C, FTO sustrate was taken out from nutrient solution, washed y DI water and dried in the air. The plate-aped ZnO NWs and 3D ZnO nanosheets were synthesized y Chemial vapor deposition. Pure ZnO powder was used as the preursor and loated at the enter of the furnae. Polyrystalline alumina sustrates were plaed downstream in the furnae. The system was kept at a low pressure ( 50 Pa). 50 sm argon and 5 sm oxygen were used as the arrier gas. The system was heated to 1000 C during the first 40 minutes and reahed 1300 C after another 40 minutes. Susequently, the system was ooled down to room temperature naturally. The 3D ZnO nanosheets (Figure S6) were aquired at the hamer temperature of 1200 C. 2

3 (4) Charaterization. SEM measurements were performed on Zeiss Leo 1530 field- emission mirosopes and TEM measurements were done on FEI TF30 and Titan mirosopes. Sanning transmission eletron mirosopy (STEM) experiments were performed on a FEI Titan mirosope with a CEOS proe aerration-orretor operated at 200 kev. STEM images were olleted with a 24.5 mrad proe semi-angle, 25 pa proe urrent, and STEM resolution of 0.8 ÅA. STEM eletron energy loss spetrosopy (EELS) spetrum images (SIs) were aquired using a 24.5 mrad proe semi-angle, spetrometer olletion angle of 102 mrad, pa proe urrent, and STEM resolution of 2.1A1.55 Å. Energy dispersive spetrosopy (EDS) measurements were done at proe urrent pa. Short exposure time (500 ms) used to avoid sample damaging artifats during EDS SI. The samples were tested for damaging after SI. EDS results on Figure S6d and S7e were performed on SEM LEO X-ray diffration pattern were aquired from the Bruker D8 Disovery with Cu K radiation. (5) PEC measurements. Prior to integrated to PEC system, the as-synthesized TiO 2 nanostrutures were overed y an additional layer of polyrystalline anatase TiO 2 film deposited y 400 yles of ALD to prevent the ontat etween FTO surfae and eletrolyte. A typial three-eletrode eletrohemial ell setup was applied to arry out the PEC haraterization with 3D hollow TiO 2 nanoforest as the working eletrode, Pt wire as the ounter eletrode and a saturated alomel eletrode (SCE) as the referene eletrode. All eletrodes were emerged in a 1 M KOH eletrolyte and working eletrode was illuminated y a light soure with intensity of 100 mw/m 2 provided y a 150 W Xenon lamp. Referene: (1) Wang, F.; Seo, J. H.; Bayerl, D.; Shi, J.; Mi, H.; Ma, Z.; Zhao, D.; Shuai, Y.; Zhou, W.; Wang, X. D. Nanotehnology 2011, 22,

4 Intensity (a.u.) Intensity (a.u.) a 500 nm 1 µm Figure S1. SEM images of (a) ZnO NWs after 2 yles of deposition; and () the ross setion of ZnO NW template with a length ~2 µm. a Anatase TiO ZnO Anatase TiO ZnO 600 C H 2 O ZnTiO C film + 10 h 600 C annealing 300 C TiCl C film 600 C TiCl 4 FTO FTO Theta (degree) 2 Theta (degree) Figure S2. (a) X-ray diffration (XRD) pattern of ZnO NW samples after 10 yles of TiCl 4 pulses without H 2 O supply at 600 C (red); 10 yles of TiCl 4 pulse without H 2 O supply at 300 C (lue); and 10 yles of H 2 O pulse without TiCl 4 supply at 600 C (purple). () XRD of ZnTiO 3 /ZnO ore/shell struture fariated via 200 yles of ALD TiO 2 oating on ZnO NWs at 100 C followed y 10 hours annealing at 600 C. The spetrum of a 300-yle ALD TiO 2 film deposited at 300 C on ZnO NWs without annealing is inluded for referene (lue). 4

5 pores 20 nm 2 nm d d 50 nm 2 nm Figure S3. (a,) TEM images of a ZnO NW after 10 yles of H 2 O pulse without TiCl 4 supply at 600 C, exhiiting a lear ZnO lattie with defet pores loated on the NW surfaes. (,d) TEM images of a ZnO NW after 10 yles of TiCl 4 pulse without H 2 O supply at 300 C, showing the ZnO lattie in the enter region with an amorphous film TiO 2 film on the surfae. 5

6 20 nm 20 nm 5 nm Figure S4. TiO 2 /ZnO ore-shell struture formed y 5 seond ontinuous TiCl 4 exposure at 600 C. 6

7 1 µm 200 nm d 1 µm 1 µm Figure S5. (a,) Plate-aped ZnO NW undles produed y hemial vapor deposition at 1300 C. (,d) Bright field STEM images of the plate-aped NW struture after 300 yles () and 600 yles (d) of TiCl 4 deposition, demonstrating the preisely preserved morphology and hollow feature. 7

8 2 µm 2 µm d 1 µm kev Figure S6. Demonstration of the universal apaility of morphology dupliation. (a) SEM image of large-area 3D-interonneted ZnO nanosheet struture. () TiO 2 hollow struture reated y 400 yles of deposition. () Higher magnifiation SEM image showing the surfae feature of a TiO 2 sheet replia. (d) EDS aquired from the repliated struture reveals the pure TiO 2 omposition. 8

9 200 nm 200 nm 200 nm 200 nm d e 5 nm kev Figure S7. SEM images of (a) ross-setional view and (, ) top view of 3D TiO 2 nanoforest. Inset of (a) is the TEM image of a single ranhed TiO 2 NW. (d) High resolution TEM image of the TiO 2 ranh revealing its single rystalline feature. (e) EDS otained from the TiO 2 nanoforest onfirming the pure TiO 2 omposition. 9

10 25 yles 50 yles 100 nm 100 nm 100 yles d 200 yles 100 nm 100 nm Figure S8. SEM oservation of TiO 2 NRs growth proess via SPCVD. Aspet ratio of the ranh inreases following the deposition yles: 25 yles (a), 50 yles (), 100 yles (), 200 yles (d). 10

11 800 yles Intensity (a.u.) 400 yles a i ii Anatase TiO ZnO ii 800 yles 100 nm 20 nm 400 yles i ii ii 51 nm ZnO FTO 200 nm 50 nm Theta (degree) Figure S9. TEM images of TiO 2 /ZnO ore-shell NW struture, where the TiO 2 shells were deposited y 400 yles (a) and 800 yles () of ALD. () XRD results of orresponding samples onfirming the ZnO and anatase TiO 2 phases. 11