ExperimentalChemicals: Vanadyl sulfate pentahydrate (purum, Fluka), TEOS (98 %, Fluka), hydrogen peroxide (30 %, Perhydrol, p.a., Merck), ammonia (25 %, p.a., Merck), isopropanol (99.5 %, J. T. Baker) were used as purchased.Synthesis: Fibrous V 3 O 7´H2 O template crystals were prepared hydrothermally according to : an aqueous solution of VOSO 4 (0.15 M) was sealed in a poly(tetrafluoroethylene)-lined autoclave (Parr bomb 4749, 23 mL capacity) and heated at 180±220 C for 1±2 days. The resulting suspension was filtered, washed several times with water and dried overnight under vacuum (~10 ±3 mbar). Coating of the as-prepared green, paper-like template as well as the subsequent core removal were performed in one pot. The fibrous solid (35 mg) was dispersed in a 250 mL glass flask containing an isopropanol/ammonia/water solution (respective volumes [mL]: 200:8.3:7.5) by means of an ultrasonic bath set at 40 C (Bandelin Sonorex DK 255 P apparatus, 35 kHz, 320 W). After addition of 0.1 mL TEOS, the ultrasound intensity was maintained at~200 W during the whole coating reaction (75 min). Then, 1 mL H 2 O 2 was added directly into the dispersion, which was further stirred for about 45 min. The solid was collected by filtration, washed extensively with isopropanol, and afterwards with water. To achieve complete core dissolution as well as elemental purity, the product was redispersed in a diluted H 2 O 2 aqueous solution (0.3 M; 30 mL), stirred for 48 h, washed several times with water, and dried under vacuum.Characterization: Samples were investigated in glass capillaries with a STOE STADI P X-ray powder diffractometer equipped with a curved Ge monochromator, a linear position sensitive detector, and using Cu Ka radiation. Scanning electron microscopy (SEM) was performed on a LEO 1530 Gemini apparatus, which was operated at low acceleration voltage (V acc = 1 kV) to minimize charging of the as-synthesized samples. For transmission electron microscopy (TEM), the samples were deposited on a holey carbon foil supported on a copper grid. TEM images were recorded on a CM30 microscope (Philips, Eindhoven, V acc = 300 kV, LaB 6 cathode). Elemental maps of vanadium were obtained at the L ionization edge applying the three-window method [33] on a Tecnai 30F apparatus (Philips, Eindhoven, V acc = 300 kV, field emission gun) equipped with a GIF (Gatan imaging filter). Laser elemental analysis was carried out on a pressed sample pellet using a Perkin Elmer/Sciex Elan 6100 DRC LA-ICP-MS machine.

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