Enhanced photocatalytic activity of homoassembled ZnO nanostructures on electrospun polymeric nanofibers: A combination of atomic layer deposition and hydrothermal growth

Abstract

We report on the synthesis and photocatalytic activity (PCA) of electrospun poly(acrylonitrile) (PAN) nanofibrous mat decorated with nanoneedles of zinc oxide (ZnO). Apart from a detailed morphological and structural characterization, the PCA has been carefully monitored and the results are discussed elaborately when juxtaposed with the photoluminescence. The present hierarchal homoassembled nanostructures are a combination of two types of ZnO with diverse optical qualities, i.e. (a) controlled deposition of ZnO coating on nanofibers with dominant oxygen vacancies and significant grain boundaries by atomic layer deposition (ALD), and (b) growth of single crystalline ZnO nanoneedles with high optical quality on the ALD seeds via hydrothermal process. The needle structure (~25. nm in diameter with an aspect ratio of ~24) also supports the vectorial transport of photo-charge carriers, which is crucial for high catalytic activity. Furthermore, it is shown that enhanced PCA is because of the catalytic activity at surface defects (on ALD seed), valence band, and conduction band (of ZnO nanoneedles). PCA and durability of the PAN/ZnO nanofibrous mat have also been tested with aqueous solution of methylene blue and the results showed almost no decay in the catalytic activity of this material when reused. © 2014 Elsevier B.V

Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density

“…[1][2][3] However, to increase the efficiency of a semiconductor catalyst one needs to isolate 4 or at least delay the recombination of photogenerated electrons and holes. 1,[5][6][7][8][9][10][11][12][13] To achieve this, lattice defects are induced in the semiconductor 1,6,[8][9][10][11][12][13] in addition to the fabrication of heterojunctions (either with noble metals [5][6][7] or another semiconductors 4,14 ). In connection to lattice defects, depending on the type and energetic location of the defect, their character is determined to capture either an electron or a hole, thereby delaying the recombination process.…”

“…25,26 Note that the controllability of the defect density depends on the type of defect and the semiconductor itself. 1,6,[8][9][10][11][12][13] In the context of ZnO, numerous studies deal with V O s, 4,8,10,11,13,21 which is not the case with Zn i s. It is also notable that the balance between the relative densities of Zn i s and V O s is still challenging. 15 The presence of Zn i s and V O s in ZnO can be easily identied by simple photoluminescence (PL) experiments, [15][16][17]27 indicated by blue and green emissions, respectively.…”

“…16,17,[28][29][30][31][32] In contrast, typically synthesized ZnO is predominant with V O s. 4,8,16,17,27,31,32 Consequently, photocatalysis studies involving V O s dominate the literature. 1,4,6,[8][9][10]13,21 In this context, it is very useful and important to study the effect of Zn i s in PCA and their efficiency against V O s. In addition to the fundamental interest, the results of such a study enable an efficient and cost effective design of novel photocatalysts. As shown earlier, 33 Zn i s are very unstable when compared to V O s, which are found to be stable even at 400 C. Aer nearly two years, theoretical support for this instability was provided by Janotti et al, 34 where the authors suggest that Zn i s diffuse faster through a migration barrier as low as 0.57 eV, cf.…”

Transformation of polymer-ZnO core–shell nanofibers into ZnO hollow nanofibers: Intrinsic defect reorganization in ZnO and its influence on the photocatalysis

“…It is known that delaying the recombination of photogenerated electron-hole pair increases the efficiency of the catalysts [10,12,[15][16][17][18][19][20][21][22][23]. On the other hand, the efficiency of the catalyst is dependent on the crystal-phase [1,3].…”

“…This not only minimizes the usage of catalyst but also enhances the active sites on the surface [10][11][12][13]18]. One should note that other significant attempts of enhancing the photocatalytic activity (PCA) or delaying the recombination are lattice defects [12,16,[18][19][20][21][22][23], heterojunctions [4,10,[14][15][16][17][24][25][26] etc. It is also notable that PCA is a very complex process and it is an integral effect of surface sites, bulk of the lattice etc.…”