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Since the advent of dye-sensitized solar cells (DSCs), which have achieved $11% of power conversion efficiency (PCE) in TiO 2 -based photoelectrodes, a lot of efforts have been devoted to make low-cost, light-weight, high-performance photovoltaic devices. [1][2][3] Nanostructured metal oxides are one of key factors in determining the PCE of DSCs, because the nanostructured networks provide a huge surface area to accommodate a large quantity of dye molecules that relate to the light harvesting of a photoelectrode in DSCs.ZnO is a good alternative of TiO 2 because it has a similar band gap but higher electron mobility than TiO 2 . [4][5][6][7] The mobility of ZnO is about 115-155 cm 2 V À1 s À1, much higher than that of TiO 2 , $10 À5 cm 2 V À1 s À1. Recently, DSCs with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites demonstrated a PCE of 5.4% due to much enhanced light scattering without compromising the surface area for dye molecule adsorption. [8][9][10] A porous structured ZnO aggregates of nanocrystallites were thought to be helpful to retain their high surface area. Although this PCE is still lower than that of TiO 2 DSCs, it doubled the PCE of ZnO nanocrystallite DSCs.Atomic layer deposition (ALD) has been used to introduce extremely thin and conformal coating due to its unique self-limiting nature and low growth temperature; lots of semiconductor materials like TiO 2 , ZnO, SnO, and Al 2 O 3 can be grown by ALD. [11][12][13] In this study, we utilized ALD to deposit ultrathin TiO 2 layer on the porous structure of ZnO aggregates and demonstrated much enhanced PCE of ZnO DSC with photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites.As illustrated schematically in Figure 1a-c, TiO 2 ultrathin layer deposited by ALD would form a complete and conformal coverage on the surface and even inside pores of ZnO that would otherwise be exposed to dye electrolyte during the dye loading. Consequently, all the dye molecules would adsorb onto the surface of TiO 2 coating. Such an ultrathin and conformal ALD coating would not change the morphology the underline ZnO structures as shown in Figure 1e and 1f. The coating of TiO 2 layer on the surface of ZnO by ALD is presumably so thin that would not affect any detectable change in the morphology by means of scanning electron microscopy (SEM). Brunauer Emmett Teller (BET) results demonstrate that micropores inside each aggregate still remain after ALD, indicating that the porous structure of ZnO is preserved. As shown in Table 1, the slight decrease in the size and volume of the micropore was observed due to the introduction of ALD-TiO 2 layer. In addition, the connections between adjacent ZnO nanocrystallites would retain to ensure a favorable electron motion through ZnO (as suggested in Fig. 1d). Such structure would improve the surface stability with enhanced dye loading on the ZnO surface, while retains the advantage of high electron mobility in ZnO.It is reported that the growth rate of TiO 2 at the substrate tempera...

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Novel nanowall arrays of hydrous manganese dioxide MnO2 · 0.5H2O are deposited onto cathodic substrates by the potentiostatic method from a mixed aqueous solution of manganese acetate and sodium sulfate. The deposition is induced by a change of local pH resulting from electrolysis of H2O, and hierarchical mesoporous nanowall arrays are formed as a result of simultaneous precipitation of manganese hydroxide and release of hydrogen gas bubbles from the cathode. The morphology and lithium ion intercalation properties are found to change appreciably with the concentration of the precursor electrolyte, with a significant reduction in specific surface area with an increased precursor concentration. For example, mesoporous nanowall arrays deposited from 0.1 M solution possess a surface area of ∼96 m2 g−1 and exhibit a stable high intercalation capacity of 256 mA hg−1 with a film of 0.5 µm in thickness, far exceeding the theoretical limit of 150 mA hg−1 for manganese dioxide bulk film. Such mesoporous nanowall arrays offer much greater energy storage capacity (e.g., ∼230 mA hg−1 for films of ∼2.5 µm) than that of anodic deposited films of the same thickness (∼80 mA hg−1). Such high lithium ion intercalation capacity and excellent cyclic stability of the mesoporous nanowall arrays, especially for thicker films, are ascribed to the hierarchically structured macro‐ and mesoporosity of the MnO2 · 0.5H2O nanowall arrays, which offer large surface to volume ratio favoring interface Faradaic reactions, short solid‐state diffusion paths, and freedom to permit volume change during lithium ion intercalation and de‐intercalation.

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Nanowall arrays of hydrous manganese dioxide MnO 2 · 0.5H 2 O were deposited on cathodic substrates by the potentiostatic method from a mixed aqueous solution of manganese acetate and sodium sulfate, and the Li + ions intercalation properties of such nanowall arrays were studied. The deposition was induced by a change of local pH resulting from electrolysis of H 2 O. Composition of this new nanowall structure was investigated by means of combined XRD, XPS, and TGA and determined to be hydrous manganese dioxide. SEM study revealed that the MnO 2 · 0.5H 2 O nanowall arrays were homogeneous across the entire substrate of top thicknesses that varied from 50 to 100 nm with identical depth. The nanowall arrays of hydrous manganese dioxide exhibited an initial capacity of 270 mAh/g with a reversible capacity maintained at 220 mAh/g at the 50th charge/discharge cycle in the Li + ions intercalation capacity measurement at a high charge/discharge rate of 0.1 mA/cm 2 (C/2, 76 mAh/g). This greatly enhanced Li + ions intercalation capacity is ascribed to the large active surface area of the nanowall arrays and a short facile diffusion path for Li + ions. The nanowall arrays of hydrous manganese dioxide also displayed an improved cyclic stability attributed to the reduced strain accumulated in these nanostructures during Li + ions intercalation.

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