Accurate Control of Multishelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency

All reagents were analytical grade and purchased from Beijing Chemical Co. Ltd., and used without further purification. Hydrated zinc nitrates Zn(NO3)2·6H2O were used as metal precursors. Taking triple-shelled ZnO hollow microspheres with close double shells in the interior as an example, the typical synthesis process is described as follows. Carbonaceous microspheres were synthesized through the emulsion polymerization reaction of sugar under hydrothermal conditions as described elsewhere. Briefly, newly prepared carbonaceous microspheres (0.6 g) were dispersed in zinc nitrate solution (30 mL, 5 M) with the aid of ultrasonication. After ultrasonic dispersion for 15 min, the resulting suspension was aged for 6 h at room temperature, filtered, washed, and dried at 80 ℃ for 12 h. The resultant composite microspheres were heated to 500 ℃ in air at the rate of 1 ℃ min−1, with holding of the temperature at 400 ℃ for 30 min. Triple-shelled ZnO hollow microspheres were subsequently formed as a white-powder product.

Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance

CVD-Synthesis : Basic Aerographite confi guration (hollow, with closed graphitic shells) can be gained by placing ZnO templates in the maximum temperature zone of a two zone split tube furnace (quartz tube; l = 1300 mm; d = 110 mm): At a constant temperature profile of 200℃ in injection zone and 760℃ in main zone under Ar gas flow (0.02 L min⁻¹ , atmosphere pressure), injection of toluene (99% Alfa Aesar) at 5.5 mL h⁻¹ is started. At start of the injection by a syringe pump, gas fl ow rates are changed to 0.2 L min⁻¹ Ar/0.02 L min⁻¹ H_² for 120 min. A subsequent 45 min pure H_² (0.6 L min⁻¹ ) gas flow without injection is followed by a 120 min mixed atmosphere Ar/ H_² (0.2 L min⁻¹ / 0.02 L min⁻¹ ) with injection of toluene. A final pure hydrogen treatment with 0.6 L min⁻¹ was conducted for 20 min until cooling down under pure Ar purge flow of 0.6 L min⁻¹ . Synthesis depends on template surface area and time depended variations of gas flow rates or temperatures. For example, the parameters for the ultra lightweight, hollow-framework variant are given by a decreased 2 mL h⁻¹ toluene injection for 4 h with a 0.06 L min⁻¹ H^₂ and 0.2 mL min⁻¹ Ar gas flow at 760 ℃ and a 1 h post-treatment with no injection and pure H_² flow of 0.09 L min⁻¹ at 800℃.

The formation of the Aerographite can be revealed by stopping the synthesis in an intermediate state. The growth of carbon nano layers on the outside and the simultaneous removal of the inner ZnO template are shown in figure 2 e-h (Figures S7-S10). Simultaneously with the carbon deposition a controlled hydrogen gas flow enables a continuous reduction of ZnO to Zn and thus the template removal. The metallic Zn is transported downstream through the CVD exhaust system where it precipitates as metallic thin film on cold areas. This hydrogen etching exhibits a clear preference on crystallographic orientations of the ZnO crystals. Interestingly, decomposition leads to an axial and radial vanishing of template sections. Massive blocks of remaining ZnO are held in place only by the partly grown carbon nano layers, Figure 2f-h (Figures S7-S9). Furthermore, the atomic structure of these layers can be controlled by the CVD growth parameters reaching from a substantially graphitic to a predominantly amorphous to glassy state. This is in contrast to other growth processes for carbon materials which form either highly ordered graphene and graphite like materials or pyrolytic low ordered structures [18, 19], therefore Aerographite might be also described as ‘pyrolytic graphite’. Besides EELS spectra, also energy filtered TEM (EFTEM) elemental mapping ensures us of the absence of oxygen and confirms the sp²-hybridization state for both arrangements. An amorphous carbon state occurs at higher template decomposition rates, e.g., as induced by higher temperatures and higher hydrogen concentration.

 As our model proposes, initial deposition of small carbon nucleation belts on template surfaces and in-plane biaxial growth of carbon leads to the variety of possible sp²-hybridized layers: For fully enclosing, smooth graphitic layers template etching and growth has to be in equilibrium. A slightly faster template removal by changing CVD-parameters leads to a higher ratio of amorphous to graphitic carbon. Further: A quicker removal of surfaces before enclosing shells can develop (high hydrogen concentration and increased temperatures of 900 ℃ creates amorphous carbon ribbons, which assemble in a hierarchical hollow framework of high mechanical strength (Figure 1e-h, S16).