This work was supported by MOE under AcRF Tier 2 (ARC 10/10, No. MOE2010-T2-1-060) and AcRF Tier 1 (2012-T1-001-161), Singapore National Research Foundation under CREATE programme: Nanomaterials for Energy and Water Management,NRF under CRP (NRF-CRP2-2007-01), and NTU under the Start-Up
Grant (M4080865.070.706022) in Singapore.