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Semiconductor Nanomaterials in Photocatalytic Bipyridine Reduction and H2 Generation

Yu, Li (2017)
Master's Thesis (43 pages)
Committee Chair / Thesis Adviser: Lian, Tim
Committee Members: Dyer, Brian ; Heaven, Michael
Research Fields: Physical chemistry; Nanoscience; Materials science
Keywords: nano materials; proton coupled electron transfer; photocatalysis; ultrafast spectroscopy
Program: Laney Graduate School, Chemistry
Permanent url: http://pid.emory.edu/ark:/25593/s3t8k

Abstract

Semiconductor nanoparticles have been used in the study of proton coupled electron transfer from CdS quantum dots (QDs) to N-heptyl-4,4'-bipyridinium (bPYD) and photocatalytic H2 generation with Pt tipped CdS nanorods (NRs) under alkaline solutions. For the QDs/bPYD system, transient absorption (TA) spectroscopy was carried out to examine the pH dependence and bPYD concentration dependence of the interfacial electron and proton transfer. pH dependence experiments revealed that electron transfer happened prior to proton transfer, with the apparent electron transfer rates stayed unchanged at various pH ranging from 4 to 8. Global fitting of the concentration dependence data showed an intrinsic ET/PT rate of 7.51 ± 0.15 x 10-9 s-1 at pH 8 and 4.39 ± 0.07 x 10-9 s-1 at pH 5, confirming the pH independent electron transfer process. For the Pt tipped CdS NRs in photocatalytic H2 generation, hydroxyl anions were utilized as an electron donor for fast and efficient hole removal. An external quantum efficiency of 29.3% and H2 generation rate of 34.9 umol/h was achieved under LED light of 455 nm.

Table of Contents

Figure 2.1 (A) Schematic depiction of the working principles of the QDs/bPYD system. UV-Vis absorption of CdS QDs and bPYD (B) absorption of CdS QDs and with 1 mM bPYD at pH 8. (C) absorption of CdS QDs with 5 mM bPYD, at different pH. ...........................................................5

Figure 2.2 TA spectra and trace kinetics. (A) and (C) TA spectra of CdS QDs and CdS QDs with 5 mM bPYD, at pH 8. (B) and (D) are the broadened view of (A) and (C). (E) and (F) The comparison of XB and bPYD radical signal kinetics at pH 8 and 7, separately. .....................................6

Figure 2.3 Exciton bleach recovery of CdS QDs with 5 mM bPYD (A) and only QDs (B), at different pH. ..............................7

Figure 2.4 Panel on the left: energy diagram of the apparent equilibrium redox potential of bPYD. Panel on the right: the table of pH dependent equilibrium redox potential from molecule 1 to 4. ....8

Figure 2.5 UV-Vis absorption of CdS QDs with increasing bPYD concentration at pH 8 (A) and pH 5 (B). (C) and (D) are the corresponding bleach recovery kinetics of CdS QDs. ...........................9

Figure 2.6 Global fitting curves of XB kinetics of CdS QDs with different bPYD concentrations, at pH 8 (A) and pH 5 (B), separately. ...................12

Table 2.1 Fitting parameters of 1S XB kinetics of free QDsa and m value at different bPYD concentrations, at pH 8 and 5. .................................12

Figure 2.7 The average numbers of adsorbed bPYDs (m) as a function of [bPYD]. ............................13

Figure 3.1 a, b) Transmission electronic microscopy (TEM) images of CdS-Pt NRs and CdSe/CdS-Pt NRs, respectively. c) Absorption spectra of the as-synthesized nanorods and Pt tipped nanorods. Inserted is the enlarged graph at a wavelength range from 500 to 650 nm. d) Steady state PL intensities of CdS NRs and CdSe/CdS NRs. e,f) Schematic illustration of existed distinct exciton states (X1, X2, X3) in CdS NRs and CdSe/CdS NRs. Also shown below is the simplified schematic energy levels and possible band edge transitions. ....................................................................21

Figure 3.2 a) Schematic depicture of the photocatalytic H2 generation process b) H2 generation efficiency under different pH and second electron donors. .......................23

Table 3.1 H2 generation efficiencies under different conditions. ............................................23

Figure 3.3 a) UV-Vis absorption of the CdS NRs after ligand exchange to aqueous solution. b) Steady state PL intensity of CdS NRs under different pH. .......................25

Figure 3.4 a) PL lifetime of CdS trap emission. b) Exciton bleach (XB) feature of CdS NRs probed at 466 nm, in different pH solutions. c) Merged graph of a) & b) for better comparison. ...............26

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