Supplementary Fig. 1.

Supplementary Fig. 1. (a,b,e,f) SEM and (c,d,g,h) TEM images of (a-d) TiO 2 mesocrystals and (e-h) NiO mesocrystals. The scale bars in the panel c, d, g, and h are 500, 2, 50, and 5 nm, respectively. SAED patterns (insets of TEM images) show the single-crystal diffraction. TiO 2 mesocrystals were prepared from a precursor solution of TiF 4 (Aldrich), H 2O, NH 4NO 3, and P123 (molar ratio = 93 : 32000 : 444 : 1). NiO mesocrystals were prepared from a precursor solution of Ni(NO 3) 2 (Wako), H 2O, NH 4NO 3, and P123 (molar ratio = 55 : 32000 : 444 : 1). Supplementary Fig. 1a-d shows the field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of as-synthesized TiO 2 mesocrystals. The ~500-nm cubic single crystals are assembled from small uniform particles (Supplementary Fig. 1a). The selectedarea electron diffraction (SAED) pattern recorded for an individual TiO 2 particle (inset of Supplementary Fig. 1c) corresponds to single crystalline anatase TiO 2 along the [001] zone axis. This result is consistent with the perfectly oriented aggregation between primary TiO 2 nanocrystals. High-resolution TEM (HRTEM) images taken from the nanocrystals revealed that single-crystal lattices exhibit atomic planes of anatase (200) or (020) crystal faces and a lattice spacing of around 0.189 nm (Supplementary Fig. 1d). The NiO mesocrystals are spherical and have a size around 100 nm (Supplementary Fig. 1e and 1f). The SAED analysis revealed the perfectly oriented aggregation between primary metal oxide nanocrystals (Supplementary Fig. 1g). HRTEM images taken from the NiO mesocrystals further revealed crystalline lattice spacings of around 0.21 nm (Supplementary Fig. 1h), which are also consistent with the perfectly oriented aggregation of NiO nanocrystals.

a Volume / cm 3 (STP) g -1 10 8 6 4 2 0 dv/dd(10-4 )/cm 3 (STP) g -1 nm -1 1.5 1.0 0.5 0.0 0 20 40 60 80 100 d p / nm b dv/dd(10-2 )/cm 3 (STP) g -1 nm -1 0.4 0.3 0.2 0.1 0.0 0 20 40 60 80 100 d p / nm 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P 0 ) Supplementary Fig. 2. N 2 adsorption desorption isotherms and pore size distributions (inset) of (a) ZnO and (b) CuO mesocrystals.

a b c d Supplementary Fig. 3. SEM images of the products at different conditions; (a) Zn(NO 3) 2 aqueous solution, (b) Zn(NO 3) 2 aqueous solution containing NH 4NO 3, (c) Zn(NO 3) 2 aqueous solution containing P123, and (d) Zn(NO 3) 2 aqueous solution containing both NH 4NO 3 and P123. The concentrations of each reagent were maintained.

a b c d Supplementary Fig. 4. SEM images of the products at different conditions; (a) Cu(NO 3) 2 aqueous solution, (b) Cu(NO 3) 2 aqueous solution containing NH 4NO 3, (c) Cu(NO 3) 2 aqueous solution containing P123, and (d) Cu(NO 3) 2 aqueous solution containing both NH 4NO 3 and P123. The concentrations of each reagent were maintained.

a b 500 o C Intensity (a.u.) 500 o C 350 o C 300 o C 250 o C ZnO Intensity (a.u.) 350 o C 300 o C 250 o C 200 o C 150 o C CuO NH 4 NO 3 Zn(NO 3 )(OH) H 2 O NH 4 NO 3 Cu 2 (NO 3 )(OH) 3 10 20 30 40 50 60 2 (degree) 10 20 30 40 50 60 2 (degree) Supplementary Fig. 5. XRD patterns of the products from (a) zinc and (b) copper precursor solutions obtained at different annealing temperatures.

a b Supplementary Fig. 6. SEM images of the products obtained by annealing of zinc precursor solutions at different temperatures; (a) 250 C and (b) 300 C.

a b c Supplementary Fig. 7. SEM images of the products obtained by annealing of copper precursor solutions annealing at different temperatures; (a) 150 o C, (b) 200 o C, and (c) 250 o C.

Intensity (a.u.) 250 o C 200 o C NH 4 NO 3 Cu 2 (NO 3 )(OH) 3 Zn(NO 3 )(OH) H 2 O 10 20 30 40 50 60 70 2 (degree) Supplementary Fig. 8. XRD patterns of the products obtained from zinc nitrate and copper nitrate as the mixed precursor by annealing at 200 and 250 o C.

Intensity (a.u.) Zn:Cu = 1.4:1.0 1.0:1.0 0.4:1.0 CuO ZnO 20 30 40 50 60 2 (degree) Supplementary Fig. 9. XRD pattern of ZnO-CuO mesocrystals.

Weight loss (%) 0-20 -40-60 -80-100 P123 ZnO-CuO 0.0-0.1-0.2-0.3 150 200 250 300 350 100 200 300 400 500 600 700 Temperature ( o C) Supplementary Fig. 10. TGA curves of P123 and ZnO-CuO mesocrystals in air.

Transmittance (10%/div.) ZnO-CuO P123 C-H stretching C-O-C stretching 4000 3500 3000 25002000 1500 1000 500 Wavenumber (cm -1 ) Supplementary Fig. 11. FTIR spectra of P123 and ZnO-CuO mesocrystals in air.

a b c 2000 Cu 1500 Zn: 44 wt% (44 at%) Cu: 52 wt% (53 at%) Zn Counts 1000 500 Cu Zn Mo Cu Zn d e 0 0 2 4 6 8 10 Energy (kev) Supplementary Fig. 12. (a) FESEM and (b) TEM images of ZnO(1.0)-CuO(1.0) mesocrystals. The inset in panel b is the SAED pattern. The scale bar in the panel b is 500 nm. (c) A typical EDX spectrum of ZnO(1.0)-CuO(1.0) mesocrystal. (d) HAADF-STEM image and (e) EDX elemental mapping of Cu and Zn in the cross section of a ZnO(1.0)-CuO(1.0) mesocrystal.

a b c 1000 Zn: 61 wt% (61 at%) Cu: 34 wt% (35 at%) Zn Counts 500 Cu Zn Cu Mo Cu Zn d e 0 0 2 4 6 8 10 Energy (kev) Supplementary Fig. 13. (a) FESEM and (b) TEM images of ZnO(1.4)-CuO(1.0) mesocrystals. The inset in panel b is the SAED pattern. The scale bar in the panel b is 500 nm. (c) A typical EDX spectrum of ZnO(1.4)-CuO(1.0) mesocrystal. (d) HAADF-STEM image and (e) EDX elemental mapping of Cu and Zn in the cross section of a ZnO(1.4)-CuO(1.0) mesocrystal.

Volume / cm 3 (STP) g -1 50 0 dv/dd(10-2 )/cm 3 (STP) g -1 nm -1 0.1 0.0 0 20 40 60 80 100 d p / nm 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P/P 0 ) Supplementary Fig. 14. N 2 adsorption desorption isotherm and pore size distribution (inset) of ZnO- CuO mesocrystal.

Intensity (a.u.) Zn 2p 3/2 1021.3 ev Zn 2p 3/2 1021.4 ev 1026 1024 1022 1020 1018 1016 Binding Energy (ev) ZnO CuO ZnO-CuO Intensity (a.u.) ZnO CuO ZnO-CuO Cu 2p 1/2 953.0 ev Sat Cu 2p 1/2 953.1 ev Sat Cu 2p 3/2 933.0 ev Cu 2p 3/2 933.1 ev 960 950 940 930 920 Binding Energy (ev) Supplementary Fig. 15. XPS spectra of ZnO, CuO, and ZnO-CuO mesocrystals. All the BE values are calibrated by using the standard BE value of contaminant carbon (BE = 284.6 ev for C 1s) as reference. No metal impurities were detected.

a b c d Intensity (counts) 80 Intensity (counts) 10 Intensity (counts) 30 0 0 0 Supplementary Fig. 16. Optical transmission (upper) and emission (lower) images of (a) ZnO mesocrystals (bar = 1 m), (b) ZnO-CuO mesocrystals (bar = 2 m), and (d) ZnO-CuO nanocrystals immobilized on quartz glasses (excitation wavelength was 365 nm) (bar = 1 m). (c) Light scattering image of ZnO-CuO mesocrystals in (b) (i.e., without longpass filter) (bar = 2 m).

a b ZnO-CuO nanocrystals Intensity (a.u.) CuO ZnO 20 30 40 50 60 2 (degree) Supplementary Fig. 17. (a) TEM image and (b) XRD pattern of ZnO-CuO nanocrystals. The scale bar in the panel a is 200 nm. The average sizes of ZnO and CuO nanocrystals were estimated to be 36 and 20 nm, respectively, from the Scherrer equation.

30 20 40 o C Intensity distribution (%) 10 0 30 20 10 0 20 60 o C 80 o C 10 0 1 10 100 1000 10000 100000 Particle size (nm) Supplementary Fig. 18. Particle size distributions obtained for the precursor solutions of ZnO-CuO mesocrystals from in-situ DLS measurements. P123 micelles with a mean size of 20 nm were confirmed at 40 o C. When the temperature is increased to 60 o C, a new distribution appeared at 6 10 m, and the particle size increased to several tens of micrometers at 80 o C. These distributions were not observed for P123 (i.e., without metal oxide precursors) and thus might be attributable to the aggregated intermediate particles. Isolated intermediate particles were not found probably due to their small scattering crosssection and low concentrations.

Supplementary Fig. 19. (a,c,d) TEM and (b) SAED images and (e) EDX spectrum of isolated intermediate crystals of ZnO-CuO mesocrystals. The red circles indicate the diffraction spots from Cu 2(NO 3)(OH) 3 and the blue circles indicate the diffraction spots from Zn(NO 3)(OH) H 2O, NH 4NO 3, and Zn(NO 3) 2. The panel d is the enlarged image of the area marked by the red rectangle in the panel c. The scale bars in the panel a, c, and d are 500, 20, and 5 nm, respectively.

Supplementary Fig. 20. (a) FESEM image. The scale bar is 100 nm. (b) TEM image (inset is SAED pattern). The scale bar is 500 nm. (c) HRTEM image of Zn 0.2Ni 0.8O mesocrystal. The scale bar is 5 nm. (d) HAADF-STEM image of a typical Zn 0.2Ni 0.8O mesocrystal. The scale bar is 50 nm. (e) EDX spectrum of Zn 0.2Ni 0.8O mesocrystal (the K peaks of Ni (7.47 kev) and Zn (8.63 kev) are highlighted in blue and green, respectively). (f) EDX line scan profile across the single Zn 0.2Ni 0.8O mesocrystal along the red line in image (g). The Zn 0.2Ni 0.8O mesocrystals are spherical and are approximately 800 nm in diameter (Supplementary Fig. 20a,b). The SAED pattern recorded on one particle was consistent with the single-crystal structure (inset of Supplementary Fig. 20b). The HRTEM image of the edge of one representative Zn 0.2Ni 0.8O mesocrystal shows uniform lattice fringes across the edge of the mesocrystal (Supplementary Fig. 20c). This information indicates that the primary nanocrystals attached with each other while remaining oriented in the same crystallographic directions. The specific surface area of the Zn 0.2Ni 0.8O mesocrystal is 15.6 m 2 /g, and the pore diameter is 35 nm and has a broad distribution (Supplementary Table 1). Supplementary Fig. 20d shows the typical HAADF-STEM image of Zn 0.2Ni 0.8O mesocrystal. The HAADF-STEM-EDX line scan results clearly illustrate the signal intensity patterns of Ni and Zn elements are the same (Supplementary Fig. 20e,f). As shown in Supplementary Fig. 20f, the Ni and Zn elements are homogeneously distributed throughout the crystal, thus suggesting that the Zn 0.2Ni 0.8O mesocrystal is a solid solution of Ni and Zn (or Zn-doped NiO). The intensity fluctuation is mainly due to the porous structure on the surface of the Zn 0.2Ni 0.8O mesocrystal.

Intensity (a.u.) 36 38 40 42 44 46 NiO Zn 0.2 Ni 0.8 O 40 50 60 70 80 2 (degree) Supplementary Fig. 21. XRD patterns of Zn 0.2Ni 0.8O and NiO mesocrystals.

E (V vs. NHE) 1 0 1 2 3 0.2 +1.5 CB VB hv e - h + CB 0.6 hv +2.6 VB CuO ZnO Supplementary Fig. 22. Mechanistic scheme for charge transfer from photoexcited ZnO to CuO. Valence band (VB) and conduction band (CB) edges of ZnO and CuO are reported elsewhere. 52,53 hv and hv mean irradiation light and PL, respectively.

Supplementary Table 1. Structural Parameters of Mesocrystals Sample SBET (m 2 /g) a Pore size (nm) b Pore volume (cm 3 /g) b Particle size (nm) c TiO2 92 10.9 0.29 24 ZnO 0.8 26.3 0.004 45 NiO 15 2.7 0.08 14 CuO 2.2 35.2 0.04 23 Zn0.2Ni0.8O 15.6 35.2 0.1 20 ZnO-CuO d 3.6 54.4 0.03 35 (ZnO), 20 (CuO) a The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area. b The pore volume and pore diameter distribution were derived from the adsorption isotherms by the Barrett-Joyner-Halenda (BJH) model. c The average size of nanocrystals was estimated from the Scherrer equation. d The molar ratio of Zn:Cu is 0.4:1.0, which was determined by ICP-AES analysis. Supplementary Table 2. Compositions of ZnO-CuO Mesocrystals Sample [Zn] (wt%) [Cu] (wt%) calc. obs. a calc. obs. a ZnO(0.4)-CuO(1.0) 26 23 54 55 ZnO(1.0)-CuO(1.0) 41 40 39 39 ZnO(1.4)-CuO(1.0) 51 47 29 32 a The weight percentages of Zn and Cu ions were determined by ICP-AES analysis.

Supplementary Table 3. Kinetic Parameters of the Emission Decay of Mesocrystals Sample 1 2 3 < PL> a ZnO mesocrystals 0.6 ns (64%) 4.8 ns (24%) 35 ns (12%) 27 ns ZnO-CuO mesocrystals 0.4 ns (78%) 3.6 ns (22%) 2.7 ns ZnO-CuO nanocrystals 0.5 ns (75%) 1.5 ns (20%) 24 ns (5%) 16 ns a The multiexponential decay curves were fitted using a nonlinear least-squares method with a multi-component decay law given by I(t) = a1exp( t/ 1) + a2exp( t/ 2) + + anexp( t/ n). The average lifetime < PL> was then determined using the equation: PL i n i 1 2 i i n a i a i 1 i i Supplementary Table 4. Kinetic Parameters of the Transient Absorption Decay Sample 1 a 2 a ZnO mesocrystals 3.4 ps (53%) 37 ps (47%) CuO mesocrystals 0.43 ps (97%) 2.7 ps (3%) ZnO-CuO mesocrystals 0.43 ps (90%) 11 ps (10%) ZnO-CuO nanocrystals 1.1 ps (81%) 16 ps (19%) a The multiexponential decay curves measured at 1100 nm were fitted using a nonlinear leastsquares method.

Supplementary References (52) Fessenden, R. W., Kamat, P. V. Rate constants for charge injection from excited sensitizer into SnO2, ZnO, and TiO2 semiconductor nanocrystallites. J. Phys. Chem. 99, 12902-12906 (1995). (53) Leung, D. Y. C.; Fu, X.; Wang, C.; Ni, M.; Leung, M. K. H.; Wang, X.; Fu, X. Hydrogen production over titania-based photocatalysts. ChemSusChem 3, 681-694 (2010).