lution TEM studies were carried out on a JEOL 3011 transmission electron microscope. The composition of the nanometer-sized dots was analyzed by X-ray EDS. [4] sensors, [5][6][7][8][9] and in other areas. The fabrication of ordered porous films typically involves electron (e)-beam lithography, [10] soft-lithography-related methods, [11] or the use of self-assembly techniques.[12]Recently developed two-dimensional (2D) colloidal crystal methods have proven to be flexible, low-cost, and universal alternatives for the synthesis of porous films; noble-metal, [13] TiO 2 , [1,2] and SiO 2 [14] porous films have been fabricated by these methods. Such films are generally synthesized on flat substrates, which is sufficient for certain applications. However, due to limitations of the manipulation techniques, these films can not be grown on substrates with curved surfaces. In fact, for many practical applications and for scientific research, films need to be prepared on substrates with cylindrical, spherical, or concave surfaces. The fabrication of nanostructured porous films on curved surfaces will further expand the applicability of ordered porous films, especially when constructing nanostructured devices, such as gas sensors on ceramic tubes. [15] We have recently reported the fabrication of a porous monolayer film on a curved surface by a two-step manipulation procedure. [16] However, the films are easily lifted off due to weak adhesion between the film and the curved surface. In addition, the manipulations are complicated and the method is not universal. Thus, fabrication of nanostructured porous films on curved surfaces still remains a considerable challenge.As is well known, a polystyrene (PS) sphere colloidal monolayer on a substrate can be stripped off in water and transferred onto other substrates.[17] We have used this phenome-
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Homogenous thin films are preferable for high-performance gas sensors because of their remarkable reproducibility and long-term stability. In this work, a low-temperature fabrication route is presented to prepare crack-free and homogenous metal oxide periodic porous thin films by oxygen plasma irradiation instead of high temperature annealing by using a sacrificial colloidal template. Rutile SnO2 is taken as an example to demonstrate the validity of this route. The crack-free and homogenous porous thin films are successfully synthesized on the substrates in situ with electrodes. The SnO2 porous thin film obtained by plasma irradiation is rich in surface OH groups and hence superhydrophilic. It exhibits a more homogenous structure and lower resistance than porous films generated by annealing. More importantly, such thin films display higher sensitivity, a lower detection threshold (100 ppb to acetone) and better durability than those that have been directly annealed, resulting in enhanced gas-sensing performance. The presented method could be applied to synthesize other metal oxide homogenous thin films and to fabricate gas-sensing devices with high performances.
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