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Thiolated Gold Nanoclusters for Light Energy Conversion Muhammad A Abbas, Prashant V. Kamat, and Jin Ho Bang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00070 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Energy Letters

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Thiolated Gold Nanoclusters for Light Energy

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Conversion Muhammad A. Abbas,† Prashant V. Kamat,*,‡ and Jin Ho Bang*,†,§,ǁ

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†

Department of Advanced Materials Engineering, Hanyang University, 55 Hanyangdaehak-ro,

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Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ‡

Notre Dame Radiation Laboratory and Department of Chemistry and Biochemistry, University

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of Notre Dame, Notre Dame, Indiana 46556, United States §

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ǁ

Department of Bionano Technology, Hanyang University, Ansan, Republic of Korea

Department of Chemical and Molecular Engineering, Hanyang University, Ansan, Republic of

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Korea

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AUTHOR INFORMATION

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Corresponding Authors

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* [email protected] (J.H.B.)

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* [email protected] (P.V.K.)

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ACS Paragon Plus Environment

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ABSTRACT. A few atom gold nanoclusters (NCs) exhibit molecule-like properties due to a

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discrete electronic structure driven by the quantum confinement effect. Unlike plasmonic Au

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particles, these non-plasmonic particles of diameter less than 2 nm, commonly referred to as

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nanoclusters, possess a distinct excited state behavior that can offer a new opportunity to employ

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them as a photosensitizer. Their size-dependent excited state behavior enables establishing

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logical designing principles to build up efficient light energy conversion systems. The

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photodynamics of thiolated Au NCs and efforts to exploit the Au NCs in light energy conversion

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applications discussed in this review show new opportunities to utilize them as photosensitizers.

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Current bottlenecks in implementing the thiolated Au NCs in light conversion applications, and

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new strategies and future directions to address these limitations are also discussed.

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TOC GRAPHICS

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Plasmonic Au nanoparticles (NPs) have been the topic of intense research over decades

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and have found many applications in various fields including sensing,1,2 biomedicine,3-5

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catalysis,6-8 and photovoltaics9-12 due to their interesting physical properties. The protocols for

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the synthesis of Au NPs are now well established,6,13-15 and the size and shape of the NPs have

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been determined to be important controlling parameters that define their photophysical and

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catalytic properties. For the Au NPs whose size is greater than 3 nm, surface plasmon resonance

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(SPR) absorption remains a primary physical phenomenon.16 Despite the distinct behavior of Au

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NPs as compared to bulk counterparts, the electronic structure of NPs is continuous, much like

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the bulk Au. However, the quantum confinement effect becomes dominant when the size of Au

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is further reduced below 2 nm (i.e., NCs), and this leads to the creation of discontinuous

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electronic structures in Au.17,18 Due to the presence of these discrete energy levels, the ultrasmall

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Au NCs are capable of exhibiting unusual molecule-like behaviors,16,19-21 some of which include

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photoluminescence,16,22-28 electron transfer,29-34 magnetism,35-37 and molecular chirality.38-42

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Recent developments in synthesis protocols have set a new milestone towards diverse

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applications of NCs.43,44 Employing Au NCs as a photosensitizer in light energy conversion

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applications is one of the rapidly emerging research areas. The NCs of other metals such as Ag,

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Pt, and Cu have also been explored for this purpose; however, they usually suffer from low

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chemical or photochemical stability. Nonetheless, the usefulness of photophysical properties of

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these metal NCs have been demonstrated in sensing, biomedical imaging, cancer radiotherapy,

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and antimicrobial agents.24,45-48 The plasmonic Au NPs have been extensively investigated for

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photoactive materials over decades because of their excellent ability to trap visible light via the

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SPR phenomenon;49,50 however, harnessing photon energy through the plasmonic NPs was not

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quite successful due to the ultrafast relaxation of hot electrons (less than several picoseconds).51


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This limitation can be alleviated in the Au NCs by virtue of the discrete energy states that can

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extend the lifetime of excited electrons up to microsecond regimes. This dramatic change in the

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excited state dynamics offered a new opportunity for noble metals to serve as a light harvester,

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and recent advances have seen their feasibility in various solar energy conversion systems.

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While such achievements are inspiring and look promising, many photophysical events in

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the NC-based light energy conversion systems remain veiled, and the relatively low power

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conversion efficiency (PCE) is calling for a breakthrough to tackle current bottlenecks imposed

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by the NC properties. The structure-property relationship in the NCs, which remains unknown to

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a large extent, is key information to acquire for the next leap. Controlling NCs at an atomic level

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to allow for the delicate manipulation of their physical properties is another research area of

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urgency. In addition, a comprehensive understanding of the excited state relaxation dynamics of

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the NCs is important to establish new design principles for light energy conversion in an

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effective manner. This review presents a brief overview of the current status of thiolated Au NC

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research advances and summarizes various efforts to utilize Au NCs in light energy conversion

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systems. We also discuss new opportunities that Au NCs can offer as photosensitizers. It is

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important to note that the discussion will be limited to Au25(SR)18 and its smaller clusters since

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they offer the best opportunities to design light energy conversion systems.

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1. KEY PROPERTIES OF GOLD NANOCLUSTERS

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1.1 Structure

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Gold in its bulk and NP form exist in a face-centered cubic (FCC) crystal structure.

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However, most of Au NPs cannot sustain the FCC crystal structure while some Au NCs do exist

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in the FCC crystal structure.52,53 Instead, they form a core-shell structure,19 which varies from

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homoleptic plane to spherical to cylindrical structures depending upon the size of NCs. The core,


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which is made of only Au atoms, is surrounded by a shell coordinating ligand, typically thiol

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derivatives. The outer gold atoms are strongly complexed with the ligands thus protecting the

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core Au atoms. For example, Au25(SR)18 consists of an Au13 core with one Au atom located in

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the center that is bonded to 12 surrounding Au atoms (Figure 1A).54-56 These 12 Au atoms in the

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Au13 core form an icosahedron. The remaining 12 Au atoms form six S-Au-S-Au-S motifs by

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bonding with the thiol groups of ligands. These motifs surround the Au13 icosahedron. Each Au

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atom in the motifs is located on 12 faces of the icosahedron. However, eight faces of the Au13

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core remain uncovered. A similar symmetrical core-shell structure has been observed for larger

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Au NCs (e.g., Au38(SR)2457 and Au102(SR)4458,59) (Figure 1B). While many thiolated Au NCs,

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including the ones smaller than Au25(SR)18, usually possess such core-shell structure, they are

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not as symmetrical as Au25(SR)18. Au15(SR)13 is the smallest known Au NC that has a core-shell

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structure.60 Recent theoretical calculations revealed the most probable structure of Au15(SR)13

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(Figure 1C),33,60 where an Au4 core is surrounded by the motifs made of two trimers (S-Au-S-

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Au-S) and one cyclic pentamer (S-Au-S-Au-S-Au-S-Au-S-Au). On the other hand, some small

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thiolated Au NCs (i.e., Au10(SR)10, Au11(SR)11, and Au12(SR)12) are known to possess no metal

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core. Instead, they form a homoleptic structure with interlocked cyclic Au-S motifs.61,62


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Figure 1. (A) Core-shell structure of Au25(SR)18 NCs (Reprinted with permission from ref.63

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Copyright (2015) American Chemical Society). (B) Crystal structure of Au38(SR)24 (Reprinted

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with permission from ref.57 Copyright (2010) American Chemical Society). (C) The lowest

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energy structure of Au15(SR)13 (red atoms: Au core, orange atoms: Au atoms in the ligand shell,

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blue atoms: S, green atoms: C, and pink atoms: H) as determined by DFT calculations (Reprinted

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with permission from ref.60 Copyright (2013) American Chemical Society).

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1.2 Optoelectronic Properties

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Distinctive absorption features, which result from electronic transitions among various

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molecular orbitals, appear in the absorption spectra of Au NCs depending upon the size and

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nature of the Au NCs. Akin to quantum dots (QDs), the absorption onset of Au NCs generally

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shifts towards shorter wavelengths with decreasing cluster size. For example, the absorption

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onset of glutathione (GSH)-protected Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14, and Au25(SR)18

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are 450, 650, 700, and 900 nm, respectively (Figure 2A),33,64 indicating an increase in the optical


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gap with decreasing cluster size. Interestingly, however, there are several exceptions to this

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universal trend. For example, Au22(SR)18 is larger than Au18(SR)14, but its absorption onset (680

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nm) is 20 nm smaller than that of Au18(SR)14 (700 nm).23,25,65 This deviation from the general

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size-dependency implies that the optical properties of the Au NCs are dictated not only by the

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number of Au atoms (i.e., the size of Au NCs), but also by the ligand/metal ratio. Besides the

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ratio, ligand engineering can significantly alter the optical properties by tuning the molecular

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orbitals positions and the energy gap between the highest occupied molecular orbital (HOMO)

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and the lowest unoccupied molecular orbital (LUMO) of the Au NCs. For example, Flores et

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al.34 demonstrated that the introduction of functional groups to a ligand that can cause more

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distortion in the structure of Au NCs resulting in a reduced HOMO-LUMO gap. Substituting

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ligands is another strategy to manipulate optical properties. Jin and co-workers demonstrated a

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red shift in the electronic transition peaks of Au25(SR)18 by replacing aliphatic thiol ligands with

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aromatic thiol ligands.66 A multiple-ligand stabilization strategy that was recently demonstrated

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by the Xie group has proven very effective to extend the absorption range of Au NCs while

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preserving the number of Au atoms.67,68 The use of multiple ligands with different charges (e.g.,

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–COO-, –NH3+, and –OH) can give rise to the structural distortion of Au25 NCs that leads to a

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significant variation in their optical properties (Figure 2B). A similar change in the absorption

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feature via the surface charge modulation was observed even in mono-thiolate-protected Au25

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NCs when converting the negatively charged Au25 into neutral NCs by chemical oxidation using

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O2 and H2O2.69-71

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Replacing one or more of Au atoms in the Au NCs with other metallic elements can have a

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dramatic effect on their optoelectronic properties as well.72-74 For instance, the HOMO-LUMO

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levels of Au25(SR)18 can be shifted to more negative values when one of Au atoms in the outer


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shell is replaced by a Hg atom (Figure 2C).75 In the case of Pd or Pt doping, the electronic

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structure in the vicinity of HOMO−LUMO levels of PdAu24(SR)18/PtAu24(SR)18 is dramatically

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different as compared to their undoped parent (Figure 2D).76 The substitution of one Au atom

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with a single Pd/Pt atom not only causes the electrochemical gap, which echoes the

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HOMO−LUMO gap, to decrease from 1.67 to 0.75 V, but it also changes the redox behavior of

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Au25(SR)18 significantly. On the other hand, Ag doping can cause a blue shift in the absorption

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peak of Au NCs with the increase in Ag atoms (Figure 2E).74,77-81 It also brings an increase in

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absorbance within the absorbance range of the Au NCs. In contrast to this observation, Cu

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doping was found to induce a red shift of the absorption edge of Au NCs.82 Interestingly, in the

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case of Cu-doped Au144(SC6H13)60, introducing more than eight Cu atoms into the parent Au NC

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induces a transition from molecule-like NCs to plasmonic nanomolecules that exhibit a sharp

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SPR peak at ~520 nm.83

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Figure 2. (A) UV-vis absorption spectra of Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14, and

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Au25(SR)18, respectively. Insets show the digital photographs of the corresponding Au NCs.

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Adopted with permission from ref.84 Copyright (2014) American Chemical Society. (B)

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Schematic illustration of Au25(SR)18 NCs with hetero-ligands and UV-vis absorption spectra of

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mono-thiolate-protected

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Au25(MHA)11-13(Cystm)7-5, Au25(MHA)11-15(MetH)7-3, and Au25(MHA)18-2x(EDT)x (x =3, 2, and

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1). Adopted with permissions from ref.67 Copyright (2016) Royal Chemical Society. (C) Effect

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of Hg doping on the structure and the HOMO-LUMO gap of Au25(SR)18 (Adopted with

Au25(MHA)18,

bi-thiolate-protected

Au25(MHA)11-17(MPA)7-1,


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permission from ref.75 Copyright (2015) American Chemical Society). (D) HOMO-LUMO gap

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of Au25(SR)18 (black), PdAu25(SR)18 (red), and PtAu25(SR)18 (blue) as determined by cyclic

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voltammetry (Adopted with permission from ref.76 Copyright (2015) American Chemical

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Society). (E) Absorption spectra and the corresponding digital photographs of Au18-xAgx(SR)14

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NCs (the ratio of Ag doping increases from top to bottom). Adopted with permission from ref. 74

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Copyright (2016) American Chemical Society.

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Photoluminescence (PL) is a convenient probe to explore the excited state behavior of Au

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NCs. While initial reports of PL quantum yield (QY) of Au NCs showed only 0.1%,85 further

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efforts to manipulate ligand chemistry and metal ion doping have led to the achievement of

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emission quantum yield as high as 60% (Figures 3A).25 The origin of Au NCs’ PL remains

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unclear, but it is generally attributed to the relaxation of ligand-to-metal charge transfer (LMCT)

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or semi-ring states (S-Au-S-Au-S), as well as that of core metal atoms.86,87 Wu et al.71

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demonstrated that the PL enhancement is directly related to the electron donating ability of

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ligands. Among the different ligands (SC6H13, SC12H25, and SCH2CH2Ph) tested, the Au25(SR)18

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stabilized with SCH2CH2Ph showed the highest PL QY because of the high charge donating

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ability (charge transfer character) of the ligand.71 Furthermore, increasing the electropositivity of

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the metal core can also enhance the PL of Au NCs as it stabilizes the LMCT state.16,71,88 Doping

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the core with other elements and changing the ratio of Au(I)/Au(0) in the NCs are two other

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parameters that can tune the PL of Au NCs.71,89-92 On the other hand, rigidifying the Au(I)–

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thiolate shell using bulky surfactants such as tetraoctylammonium bromide (Figure 3A)25,93,94 or

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spatially confining Au(I)-thiolate complexes in a polymer matrix95 has been demonstrated as new

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ways to enhance PL. The common strategy of PL enhancement found in all these examples is to

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enhance LMCT while suppressing non-radiative recombination in the core.


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Another mechanism associated with the high PL QY in the thiolated Au NCs is the

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aggregation-induced emission (AIE). Many organic, polymeric, and organometallic luminophore

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systems have been identified to show a dramatic increase in their PL upon aggregation.96 This

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AIE phenomenon has also proven to play a significant role in enhancing the PL of Au NCs.97,98

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This PL enhancement is driven by the aggregation of Au(I)−thiolate complexes onto Au core that

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can reinforce the Au(I)···Au(I) interaction (known as aurophilic interaction), which results in the

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intense PL from the Au(I) complexes. For example, the Xie group demonstrated that Au(I)-

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thiolate complexes that are non-luminescent in aqueous solution showed a dramatic increase in

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the PL when aggregation was induced by adding a weakly polar solvent (Figure 3B).97 The

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addition of such solvents (e.g., ethanol) disrupted hydration shell and neutralized surface charge,

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therefore, leading to the formation of aggregates. The mechanism and recent research about the

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PL enhancement by the AIE has been well summarized in a recent review.16

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Figure 3. (A) Schematic illustration of wrapping of Au22(SR)18 NCs with bulky ligands to

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rigidify its shell, a digital photograph of Au22(SR)18 in water and toluene under UV irradiation,

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and the PL spectra of water-soluble Au22(SR)18 NCs, tetraoctylammonium (TOA+) wrapped

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Au22(SR)18 NCs, and rhodamine B. Adopted with permission from ref.25 Copyright (2015)

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American Chemical Society. (B) Schematic mechanism of aggregation induced emission from

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Au oligomeric complexes. Adopted with permission from ref.97 Copyright (2012) American

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Chemical Society. (C) Theoretical and experimental UV-vis absorption spectra of Au18(SR)14

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indicating specific orbital transitions for various peaks along with the HOMO and LUMO

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orbitals of Au18(SR)14 as calculated by DFT. The right side of Figure 3C shows the calculated

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orbital energy levels of Au18(SR)14 with color-coded contributions from the constituent atoms.

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Adopted with permissions from ref.99 and ref.100 Copyright (2015) WILEY-VCH Verlag GmbH

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& Co. KGaA, Weinheim respectively.

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Fundamental understanding of Au NCs’ discrete energy states is an important

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prerequisite to explain their optoelectronic properties. Since the groundbreaking work by

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Kornberg and co-workers in 2007 (determination of the crystal structure of Au102(SR)44),58 the

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crystal structure of several Au NCs was revealed.55-57,100 These recent advances in the

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crystallographic studies plus the computational studies revealed the secret of the electronic

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structure of Au NCs. Chen et al., who resolved the crystal structure of Au18(SR)14, revealed that

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the NC consists of an Au9 bioctahedral inner core that is protected by three Au(SR)2 staples as

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well as Au2(SR)3 and Au4(SR)5 staples.100 This structural information enabled them to calculate

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the Kohn–Sham orbitals of Au18(SR)14, which revealed that the HOMO to HOMO-20 orbitals

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arise from Au 5d orbitals, whereas the LUMO levels have a larger contribution from Au 6sp


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orbitals (Figure 3C).99 This insight helped to identify the origin of light absorption in Au18(SR)14,

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which occurs via d→sp intra-band transitions. In another theoretical investigation, Aikens and

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co-workers attributed the PL of Au25(SR)18 to several excited states mainly associated with core-

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based orbitals. Ligands were found to be a primary factor to govern the PL through the

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interaction with the Au core.101 Despite the benefits of new insights provided by many

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theoretical studies, it is noteworthy that extra care must be taken when interpreting the

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optoelectronic properties of the Au NCs because of the assumptions and approximations used in

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calculations.102

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2. SYNTHESIS AND CHARACTERIZATION

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The synthesis of metal NCs of defined size has improved since the early efforts to

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synthesize a mixture of Au NCs resulted in a low yield.85 Methods are now available to

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synthesize size-selective NCs in one pot with a high yield.23,43,64 Two approaches are commonly

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used to synthesize thiolated Au NCs (Figure 4A); (1) Metal-thiol complexes can be reduced to

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form Au NCs of desired size.65 (2) Large Au NPs are etched by thermal energy or excess thiol

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group to form desired Au NCs.103 Sodium borohydride (NaBH4) is one of the commonly used

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reducing agents for the synthesis of Au NCs. However, the Au NCs formed by the NaBH4

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reduction are typically polydisperse in size and hence require subsequent separation processes to

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obtain Au NCs of well-defined size.85 While selection of the ligand-to-metal ratio can give some

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control over the formation of Au NCs, it is still inadequate to achieve monodispersity. Recently,

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a thermal thiol-etching strategy in combination with ligand exchange has emerged as one of the

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methods to size-focus these crude initial products formed by the NaBH4 reduction.104,105 Jin and

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coworkers employed this two-step approach to synthesize monodispersed Au38(SC2H4Ph)24 NCs


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with 25% yield (Figure 4B).104 This size-focusing method is now extended further to prepare

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other Au NCs.105-107 It was found that the stability of Au NCs of a specific size plays a crucial

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role in the size-focusing step. A fine-tuned experimental condition renders less stable NCs to

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disintegrate while the most stable NCs remain intact. Different sized Au NCs can thus be

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obtained first by forming NCs with wide size-distribution initially, which is then followed by the

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size-focusing treatment to obtain desired size (Figure 4B).108 In another modified approach, Jin

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and co-workers reported a ligand-controlled size-focusing strategy that can be applied to a

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mixture of dramatically different sizes.109 In this work, they used para-, meta-, and ortho-

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methylbenzenethiols (MBT) to synthesize an initial product and subjected it to the size-focusing

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process. Interestingly, depending upon the ligand (p-MBT, m-MBT, or o-MBT), pure Au130(p-

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MBT)50, Au104(m-MBT)41 and Au40(o-MBT)24 were formed, respectively.

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Among the thiolated Au NCs, GSH-protected Au NCs prepared in aqueous media are of

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particular interest owing to their good stability and versatility in functionalization. In fact, these

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characteristics were instrumental for the first demonstration of Au NCs as photosensitizers in

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solar energy conversion applications.110 The GSH-protected Au NCs can be readily synthesized

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without using a reducing agent because amine functional groups provide mild reducing power to

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reduce Au(III) ions and form Au NCs.97,111 However, another method developed by the Xie

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group became popular for the synthesis of GSH-protected Au NCs as it employs slow reducing

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capability of carbon monoxide (CO) to reduce an Au precursor while fine-tuning the size of NCs.

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Four fine-tuned Au NCs (Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14, and Au25(SR)18) can be

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formed by varying pH of the reaction medium. A higher pH was found to provide a relatively

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stronger reducing environment that in turn leads to the formation of larger NCs (Figure 4C).64

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Xie and co-workers also developed a two-stage combination of acidic and basic conditions with


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the CO reduction to synthesize Au22(SR)18 NCs that show an ultrabright luminescence.23 Note

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that many other articles related to the synthesis of gold NCs that are not covered in this review

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have been presented in several recent review articles.103,108,112-114

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Figure 4. (A) Illustration of two common synthesis approaches used to synthesize thiolated Au

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NCs (Adopted with permission from ref.103 Copyright (2015) American Chemical Society), (B)

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two-step synthesis method produces mixed size clusters in the first step, followed by the second

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step that selectively keeps the most stable Au NCs in the reaction mixture (Adopted with

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permission from ref.108 Copyright (2010) American Chemical Society), and (C) CO-based

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reduction method to synthesize Au NC size by controlling pH. Adopted with permission from

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ref.64 Copyright (2013) American Chemical Society.


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Despite the recent advances in analytical techniques, the ultrasmall nature of the Au NCs

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imposes a great challenge on their full characterization. A quick indication of the formation of

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Au NCs can be readily obtained by UV-vis absorption spectroscopy. The absence of the SPR

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peak gives a sign of the molecular nature of the synthesized particles. Although comparing the

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absorption feature of Au NCs to those of well-defined, pure NCs is simple and intuitive for

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determining the size of Au NCs, it cannot guarantee the purity of products because other NCs

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might be present in very low amounts and their contribution to the absorption spectrum go

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unrevealed. Therefore, as-synthesized Au NC solutions need to be separated by a proper

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separation technique (e.g., polyacrylamide-gel electrophoresis) to determine how many types of

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NCs are present in the product.85 To determine the size and composition of Au NCs, mass

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spectrometry (MS) must be performed. It has been noted by Lu et al.115 that high-resolution

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electrospray ionization mass spectrometry (ESI-MS) is a common technique to determine the

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composition of Au NCs while matrix-assisted laser desorption/ionization time-of-flight

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(MALDI-TOF) mass spectrometry can be utilized to determine the purity of Au NCs.116-119 For

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more details, refer to a recent review on the use of MS in the characterization of metal NCs.115

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As compared to MS analysis, single crystal X-ray crystallography is the most accurate and

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informative technique that provides the comprehensive structural information of Au NCs.

19

However, implementing this analysis has been limited because of the difficulty in synthesizing

20

highly pure single crystalline NCs.55,58,112,120-127

21

spectroscopy (XAS) is often performed to extract information about the structural and electronic

22

properties of Au NCs.128 For example, XAS has been used to determine the ratio of core and

23

surface atoms in a single Au NC; it also helps identify the location of a dopant in doped Au

On the other hand, X-ray absorption


16

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NCs.77,129 Details about the characterization methods of Au NCs have recently been reviewed

2

elsewhere.112,114,115,128

3

3. EXCITED STATE PROPERTIES

4

Although the size-dependent optical properties of semiconductor QDs and metal NCs appear

5

to be similar, the origin of spectral shifts as well as the excited state deactivation processes vary.

6

Unlike the semiconductor QDs, Au NCs possess a well-defined electronic structure and do not

7

possess surface defects.130 The charge recombination induced via the surface states, which

8

dominate the excited QD relaxation dynamics, is not a factor in the deactivation of excited Au

9

NCs.131 The structure-specific characteristics of the Au NCs give rise to distinct size-dependence

10

on the carrier relaxation dynamics.

11

Several models have been proposed to explain the excited state relaxation of excited Au NCs.

12

Kwak and co-workers recently demonstrated that the exciton lifetime of Au NCs increases with

13

the decrease in size (i.e., the increase in HOMO-LUMO gap) (Figure 5A).130 This observation

14

resulted in the conclusion that the exciton dynamics in the Au NCs is governed by the energy

15

gap law (i.e., an inverse relationship between non-radiative decay kinetics and the energy

16

gap).132 On the other hand, spectroscopic analysis has revealed that Au NCs show much less

17

prominent relaxation mechanism associated with electron-phonon interactions due to their

18

discontinuous energy states.84,133

19 20


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1 2

Figure 5. (A) HOMO-LUMO gap of Au NCs as a function of the number of Au atoms in Au

3

NCs and the plot of the logarithm of nonradiative recombination ln(knr) vs the HOMO-LUMO

4

gap of Au NCs demonstrating the energy gap law (Reprinted with permission from ref.130

5

Copyright (2017) American Chemical Society). (B) Schematic diagram of relaxation pathways in

6

Au25(SR)18 NCs (Reprinted with permission from ref.86 Copyright (2010) American Chemical

7

Society).

8 9

While there are still debates over the exact nature of the excited state behavior, a common

10

consensus is that, upon excitation, ultrafast charge relaxation (less than 200 fs) occurs in the Au

11

core and the subsequent internal relaxation from the core states to semi-ring states (i.e., core-

12

shell charge transfer) takes place in the timeframe of ~1 ps (Figure 5B).86,134-138 The excited

13

electrons that reside in this semi-ring state with several orders of magnitude longer lifetimes are


18

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1

primarily responsible for the PL of Au NCs. Using transient absorption spectroscopy techniques,

2

the Goodson and Kamat groups have independently identified the semi-ring state as a major

3

contributor dictating the excited state behavior of Au NCs.86,133,139 Interestingly, the excited

4

electron relaxation dynamics of Au NCs was also found to be sensitive to factors such as charge

5

states,69 dopants,140,141 ligands,142 and solvent medium.143

6

Akin to other light harvesters, excited Au NCs are capable of transferring electrons to an

7

electron acceptor when the required conditions dictated by the Marcus theory (e.g., energy

8

difference between donor and acceptor, degree of electronic coupling, and reorganization

9

energy)144-148 are satisfactorily met. The photoinduced electron transfer kinetics in the Au NCs

10

has been investigated using methyl viologen (MV2+) as an electron acceptor.139 The semi-ring

11

state (or LMCT state) with a long lifetime was turned out to be solely responsible for the electron

12

transfer. On the other hand, the metal core transition was too short-lived to participate in the

13

photoinduced electron transfer event effectively.

14

Since the physical properties of Au NCs are highly size-dependent, it is essential to elucidate

15

how size variation affects the excited state lifetime and the efficiency of charge transfer. The

16

size-dependent excited state kinetics of four GSH-protected Au NCs (Au10-12(SR)10-12,

17

Au15(SR)13, Au18(SR)14 and Au25(SR)18) have been evaluated using transient absorption

18

spectroscopy.84 This study revealed that the fate of excited electrons in the Au NCs varied

19

depending on the size of Au NCs. While there was no noticeable decay in Au10-12(SR)10-12 and

20

Au15(SR)13 even after 1 ns, a short-lived component in the decay dynamics appeared in

21

Au18(SR)14 and Au25(SR)18 because of the presence of metal cores (Figures 6A). In addition,

22

nanosecond transient absorption spectroscopy disclosed that all four Au NCs have a long-lived

23

excited state component with the lifetime of 164, 241, 255, and 203 ns for Au10-12(SR)10-12,


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1

Au15(SR)13, Au18(SR)14 and Au25(SR)18, respectively, and among the four Au NCs, Au18(SR)14

2

has the longest excited states (Figure 6B). It is noteworthy that the observed nonradiative decay

3

trend deviates from what one would expect from the energy gap law, as demonstrated for larger

4

size Au NCs ranging from Au25 to Au333.130 This deviation seen in smaller size NCs is attributed

5

to the dominance of LMCT state and the absence of well-defined metal core transitions.

6

In order to examine the ability of Au NCs to undertake photoinduced electron transfer

7

process, a well-known probe molecule, methyl viologen, was employed as an electron acceptor.

8

The reduced methyl viologen with its characteristic absorption at 600 nm allowed the

9

spectroscopic determination of electron transfer yield and associated kinetics of charge transfer.

10

By using the long-lived absorption at 600 nm the QY of electron transfer was determined.

11

Among the four different Au NCs studied, Au18(SR)14 exhibited the highest electron transfer

12

yield of ~4% (Figure 6C). Given the light absorption capability along with the relatively high

13

electron transfer kinetics, Au18(SR)14 seems to be an optimal choice for light harvesting

14

applications. This observation was further confirmed in another recent study.33

15

Another intriguing observation regarding the excited state behavior of Au NCs has recently

16

been reported. Jin and co-workers explored the size-dependent excited state dynamics of FCC-

17

structured Au NCs (Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32) and revealed that the

18

excited state behavior of these NCs is very different from all the other NCs studied so far.52 In

19

general, the PL spectrum of Au NCs exhibits two peaks: one peak in the visible light region

20

normally associated with the core-core transitions, and the other in the near-infrared region

21

attributed to the LMCT state. However, the FCC Au NCs showed only a single peak in the

22

visible region with the near-infrared peak being absent from their PL spectra. This behavior was

23

attributed to the absence of core-shell relaxation in these NCs. Unlike the smaller size Au NCs


20

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ACS Energy Letters

1

discussed earlier, the relaxation of the excited state of FCC Au NCs slowed down with an

2

increase in size (Figure 6D). This opposite trend in the lifetime of excited states of the FCC-

3

structured Au NCs may bring very interesting prospects to the future applications of Au NCs.


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1


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Figure 6. (A) Difference absorbance spectra of Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14, and

2

Au25(SR)18 immediately and 1000 ps after excitation. The insets show the signal decays at 500

3

nm. (B) Difference absorption spectra of the Au NCs in 100 ns delay after excitation with 355

4

nm laser and the corresponding decays of the absorbance signals at 500 nm. Adopted with

5

permission from ref.84 Copyright (2014) American Chemical Society. (C) The absorbance-time

6

profiles of Au18(SR)14 NCs in the presence of different amounts of electron accepter (methyl

7

viologen) and the quantum efficiency of Au NCs for electron transfer. Adopted with permission

8

from ref.84 Copyright (2014) American Chemical Society. (D) Schematic representations of the

9

excited state lifetimes of a series of FCC Au NCs. Adopted with permission from ref.52

10

Copyright (2017) American Chemical Society.

11 12

4. METAL-CLUSTER-SENSITIZED SOLAR CELLS (MCSSCs)

13

Despite the fact that thiolated Au NCs are relatively new to light energy conversion

14

systems, several recent findings have demonstrated their potential as a new class of light

15

harvester. The first utilization of the Au NCs in the light energy conversion application was

16

demonstrated by Tatsuma and co-workers in 2010.110 This pioneering work showed that TiO2

17

films sensitized with GSH-protected Aun (n = 15, 18, 22, 25, 29, 33, 39) could generate

18

photocurrent when exposed to light, and it also found out that Au25(SR)18 performed best among

19

the Au NCs examined (Figure 7A). While the achieved performance in this work (a short-circuit

20

current density (JSC) of 290 µA/cm2 and an open-circuit voltage (VOC) of 460 mV) looks trivial

21

when compared with the recent advances, this work laid a cornerstone for the application of Au

22

NCs in light energy conversion systems. Later, they explored the electronic structure of Au NCs

23

by investigating the photocurrent dependency on the wavelength of light and on the standard


23


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1

electrode potentials of electron donors.32 Along with the estimation of HOMO and LUMO levels

2

of the Au NCs, the knowledge garnered from this study highlighted the choice of an appropriate

3

redox couple that could play a critical role in designing an efficient MCSSC.

4

A breakthrough in designing efficient MCSSCs was made possible through the careful

5

selection of a suitable Au NC and a suitable redox couple for their MCSSC assembly.31 An

6

indirect evidence of a strong interaction between the NC and TiO2 was obtained through the PL

7

quenching of Aux NCs, and hence the presence of an efficient pathway for the electron transfer

8

from the Aux NCs to TiO2 was established. An MCSSC assembled with this Aux NC and

9

Co(bpy)3(PF6)2/Co(bpy)3(PF6)3 as a redox couple in the electrolyte achieved the record PCE of

10

2.3% for MCSSCs with a JSC of 3.96 mA/cm2 and a VOC of 832 mV (Figure 7B). This high VOC

11

and a stable photocurrent, combined with the fact that this Aux NC was only active below 525

12

nm, led to the conclusion that room for further improvement in the efficiency is possible by

13

tuning the electronic structure of Au NCs.

14

Inspired by the early work, our group systematically investigated the effect of the Au

15

NCs’ size and of redox couples on the efficiency of MCSSCs and identified the performance

16

limiting factors by electrochemical impedance spectroscopy (EIS) in combination with other

17

physical characterization techniques.33 When MCSSCs were assembled using four Au NCs

18

(Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14, and Au25(SR)18) as photosensitizers, Au18(SR)14

19

outperformed all other NCs. When analyzed by the EIS analysis, Au15(SR)13 showed the best

20

recombination kinetics, but suffered from limited light absorption. On the other hand, Au25(SR)18

21

could absorb light over a much broader spectrum, but suffered from limitations arising from

22

charge recombination. Due to the better balance between the recombination and the light

23

absorption, Au18(SR)14 showed the highest JSC of 8.18 mA/cm2, VOC of 672 mV, and the PCE of


24

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ACS Energy Letters

1

3.8%, which was a new record for MCSSCs (Figure 7C-D). The use of I-/I3- redox couple in the

2

electrolyte played a pivotal role in achieving this high PCE. The problem associated with

3

corrosion by the iodine couple was not encountered for the Au NCs in these experiments.

4

Sustained photocurrent was recorded in the presence of the iodine couple without significant

5

deterioration (photocurrent degraded to ~90% even after 1 h of continuous illumination). Despite

6

this initial promise, the long-term stability of Au NC-based energy system have not been fully

7

investigated yet, hence an urgent attention should be given to ensure sufficient stability for any

8

practical applications.

9

Besides the charge transfer capability for light harvesting, the charging properties of Au

10

NCs attracted an interest in their use in photoelectrodes.149,150 For the plasmonic Au NPs, it has

11

been shown that they have the ability to store charge with increased capacitance per electron

12

with decreasing particle size.151 When the plasmonic Au NPs were embedded in TiO2 for the

13

photoanode of dye-sensitized solar cells (DSSCs), it shifted the quasi Fermi level to more

14

negative potentials due to the stored charge in the NPs, resulting in an increase in the

15

photovoltage.11 Given the presence of a metal core in Au NCs, it would have been interesting to

16

see how the non-plasmonic Au NCs would behave when coupled with TiO2. The influence of Au

17

NCs embedded in TiO2 was explored using a photoanode composed of squaraine dye and Aux

18

NCs on TiO2 (Figure 7E).30 As the Aux NCs can also work as a light absorber, it resulted in

19

photocurrent enhancement in this co-sensitized DSSC. An interesting observation was a 240 mV

20

increase in the VOC. This increase in VOC was attributed to the charge storage ability of the Aux

21

NCs in their metal core that can induce a negative shift in the quasi Fermi level, akin to the Au

22

NPs. Therefore, the charge injection ability of Au NCs combined with the charge storage

23

capability could give them a unique perspective in the future development of MCSSCs.


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1

Another interesting feature of the thiolated Au NCs is a synergistic effect when combined

2

with the plasmonic NPs.152,153 When Au NCs were coupled with Ag NPs, the absorption was

3

enhanced significantly beyond the absorption of the individual components. The Ag NPs have

4

the ability to capture two photons using wavelengths greater than their absorption edge.154 A

5

biphotonic off-resonance excitation was used to probe the interaction between excited Ag NP

6

and Au NCs using transient absorption spectroscopy (Figure 7F). The results showed that

7

coupling with metal NCs can further enhance the plasmonic effect of metal NPs. Demonstration

8

of the plasmonic enhancement of Au photosensitizers has also been reported recently.155,156

9

The NCs of Ag, Cu, Pt, Pd, and alloyed NCs of Ag-Au and Ag-Cu have been shown to

10

work as a sensitizer in MCSSCs as well,29,157-160 and among these noble metal NCs, Ag NCs

11

stand out with a superior performance in MCSSCs.29,160 So far, the best PCE of MCSSCs other

12

than Au NCs is 1.1% (obtained using AgCu alloyed NCs).159 The availability of other metal NCs

13

as photosensitizers could offer a new possibility for MCSSCs; however, almost all non-Au NCs

14

suffer from inherent instability during light irradiation.161 Therefore, developing a suitable design

15

strategy to ensure the photostability of these non-Au NCs is needed to employ these metal NCs

16

as photosensitizers.

17 18


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ACS Energy Letters

1

2 3

Figure 7. (A) Photocurrent action spectra of Aun(GSH)m-sensitized TiO2 films (Adopted with

4

permission from ref.110 Copyright (2010) WILEY-VCH Verlag GmbH & Co. KGaA,

5

Weinheim). (B) Current-voltage (J-V) characteristics of Aux(GSH)- and CdS-sensitized TiO2

6

films along with bare TiO2 under 1 sun illumination (Adopted with permission from ref.31


27


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1

Copyright (2013) American Chemical Society). (C) Energy band diagram and (D) J-V

2

characteristics of Au10-12(SR)10-12, Au15(SR)13, Au18(SR)14 and Au25(SR)18 NC-sensitized

3

MCSSCs. Adopted with permissions from ref.33 Copyright (2016) American Chemical Society.

4

(E) J-V curves showing the boost in photovoltage in Aux(GSH) NCs and squaraine dye co-

5

sensitized DSSCs (Reprinted with permission from ref.30 Copyright (2015) American Chemical

6

Society). (F) Scheme of the synergistic effect between Ag NPs and Au NCs (Reprinted with

7

permission from ref.152 Copyright (2015) American Chemical Society).

8 9

5. METAL NANOCLUSTERS FOR PHOTOCATALYTIC HYDROGEN PRODUCTION

10

Another opportunity for the thiolated Au NCs in the light energy conversion applications can

11

be found in photoelectrolytic (or photocatalytic) water splitting for H2 generation. The Au NCs

12

possess a reduction peak at -0.63 V and two oxidation peaks at +0.97 and +1.51 V versus

13

reversible hydrogen electrode (RHE), thus exhibiting their ability to induce water splitting under

14

light irradiation. A photoelectrolysis cell was assembled to evaluate thiolated Aux NCs as a

15

visible light harvester for H2 production (Figure 8A).162 For this task, the Aux NCs were

16

adsorbed onto mesoporous TiO2 surface to form the photoanode in conjunction with Pt as the

17

cathode and 0.1 M phosphate buffer solution as the electrolyte (Figure 8B). The photoaction

18

spectra showed the extended photocurrent response from 400 to 520 nm and an incident photon-

19

to-current efficiency (IPCE) of 2.9% at 420 nm, which further increased to 4.8% when a

20

sacrificial hole scavenger, ethylenediaminetetraacetic acid disodium salt (EDTA), was

21

introduced. This enhanced conversion efficiency observed in the presence of EDTA was

22

attributed to the faster removal of holes from the oxidized Aux NCs. Similar demonstrations of


28

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ACS Energy Letters

1

the H2 evolution were made using Aux NC-sensitized TiO2 nanotubes by Chen and co-workers163

2

and Liu and coworkers164 as well.

3

The Au NCs were applicable not only for the photoelectrochemical system, but also worked

4

for photocatalytic H2 generation in neutral water even without the aid of applied bias (Figure

5

8C).162 This opened up a new opportunity for Au NCs to employ them as photocatalysts for

6

solar-to-fuel conversion systems. One of the critical problems is the photoinduced transformation

7

of Au NCs into larger size Au NPs, which is primarily responsible for lower conversion

8

efficiency during prolonged photoirradiation.162,165,166 A recent investigation on the photoinduced

9

transformation revealed that the photocatalytic oxidation of organic ligands protecting Au NCs

10

triggered the aggregation of Au NCs and gave rise to a gradual transformation into plasmonic

11

NPs.167 Interestingly, this work demonstrated an enhanced photocatalytic activity driven by the

12

synergistic interaction between Au NCs and adjacent Au NPs formed by this photoinduced

13

aggregation. While ensuring the photostability of Au NCs remains a primary task for this

14

application, a judicious use of this transformation for designing an Au NC-NP coupling system is

15

worth trying for boosting light conversion efficiency.

16

17


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1

Figure 8. (A) Schematic illustration of water oxidation by Au NC-sensitized TiO2 film under

2

illumination, (B) digital photograph of the experimental setup showing the evolution of H2

3

bubbles at the Pt counter electrode, and (C) time course of hydrogen evolution following the

4

visible light illumination of an aqueous suspension containing (a) Pt/TiO2 nanoparticles, (b)

5

Pt/TiO2 nanoparticles in the presence of 0.01 M EDTA, (c) Aux-GSH-sensitized Pt/TiO2

6

nanoparticles, and (d) Aux-GSH-sensitized Pt/TiO2 nanoparticles in the presence of 0.01 M

7

EDTA (Inset shows the photographs (left to right) of TiO2, Pt/TiO2, and Aux(GSH)-sensitized

8

Pt/TiO2 powders). The experiments were conducted in aqueous solutions at pH 7 (Adopted with

9

permission from ref.162 Copyright (2014) American Chemical Society).

10 11

6. SUMMARY AND FUTURE OUTLOOK

12

Over several decades, organic and organometallic molecules have served as the most

13

popular light harvesters for light energy conversion applications.168 Despite the enormous

14

advances achieved in the dye-based light conversion systems, their stagnant conversion

15

efficiency is now calling a new material that can provide a breakthrough. The thiolated Au NCs

16

whose properties resemble those of molecules has emerged as a new class of materials to

17

challenge the dominance of traditional dyes. Offering facile tunability of optoelectronic

18

properties by ligand engineering, changing size, or doping with heteroatoms is a merit of the Au

19

NCs over the molecules. In addition, unlike traditional dyes of which aggregation causes the

20

deterioration of performance,169,170 the aggregation of Au NCs would rather be advantageous as

21

it suppresses the non-radiative recombination.16,97 The nontoxic, environmentally benign nature

22

of Au NCs as compared to inorganic semiconductors is another attractive feature. However,

23

there are many hurdles to utilize Au NCs for practical applications. Along with the current low


30

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1

conversion efficiency, their chemical and photochemical stability remains a critical issue. In

2

addition, cost of materials is likely to be another obstacle that could hinder the wider deployment

3

of Au NC-based energy conversion systems.

4

While the initial proof-of-concept studies have shown the feasibility of the thiolated Au

5

NCs in light energy conversion applications, many aspects of Au NCs as a photoactive material

6

have yet to be explored fully. Establishing a design principle-based synthesis, gaining a better

7

understanding of the optoelectronic properties of Au NCs, and elucidating factors that govern the

8

charge transfer and recombination kinetics, would be the key to realizing their full potential for

9

light energy conversion.

10

The knowledge of the physical properties of Au NCs gathered from a number of recent

11

studies points out areas that are likely to see breakthroughs. The smaller size Au NCs are more

12

photoactive with longer lifetimes than their larger counterparts. Therefore, strategies to suppress

13

the faster charge recombination in Au NCs larger than Au18(SR)14 should be developed to

14

improve the light conversion efficiency. The excited state lifetime of Au NCs is governed by the

15

energy gap law; therefore, any advantage gained by the increase in absorption range in the

16

visible from larger Au NCs is likely to be counterbalanced by the decrease in the excited state

17

lifetime. Utilization of the Au NCs featured with the FCC structure could be an option to

18

overcome this issue since the FCC Au NCs showed an increase in the excited state lifetime with

19

increase in size from Au28(SR)20 to Au52(SR)32.52 A synthesis approach for two-way structural

20

transformation between FCC and non-FCC Au NCs has recently been devised as well.171 These

21

results may open up another opportunity to expand the potential of Au NCs to harvest solar

22

energy.


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1

Most of the research regarding the application of NCs for the light energy conversion has

2

exclusively used the GSH-stabilized Au NCs due to the ease of synthesis, relatively high

3

stability, and long lifetimes of their excited states.31,33,110,159,160,162 However, as GSH consists of

4

long and bulky ligands, it could reduce the Au NCs’ ability of electron transfer. Therefore, it is

5

important to investigate how the alkyl chain length or the presence of certain functional groups

6

in the ligand affects the excited states and the charge transfer kinetics of the GSH-stabilized Au

7

NCs. Future research on the effect of ligands could shed light on these points and aid in the

8

design of more efficient Au NC-based photoelectrodes. Using shorter alkyl chains for the ligand

9

will be a good starting point to achieve this goal, but may not fully suppress the internal non-

10

radiative recombination. Thus, it remains an open question whether the short ligands could

11

indeed be beneficial to the solar light harvesting applications.

12

Doping of Au NCs with other metal ions is a convenient approach to tune the

13

photophysical properties. However, such doping inevitably lowers stability. Therefore, it would

14

be useful to explore ways to stabilize doped Au NCs so that it can withstand the reactivity of the

15

redox couples employed in MCSSCs and water splitting reaction. In addition, it would be equally

16

important to gain insight into the following questions: Will the charge transfer be more efficient

17

in the doped Au NCs than native Au NCs? To what extent can the light energy conversion

18

process be improved with controlled doping? The scientific issues raised in this review lay the

19

foundation for future research efforts. Given the rapid rise of the metal NCs as photoactive

20

materials, one can expect their utilization in light harvesting assemblies, display devices, and

21

sensors.

22 23

AUTHOR INFORMATION


32

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ACS Energy Letters

1

Corresponding Authors

2

*E-mail: [email protected] (J.H.B).

3

*E-mail: [email protected] (P.V.K.).

4

Notes

5

The authors declare no competing financial interest.

6

Biographies

7

Muhammad Awais Abbas received his PhD in Advanced Materials Engineering from Hanyang

8

University in 2017. Currently, he is working as a postdoctoral researcher in the Bang group. His

9

current research is focused on the development of noble metal nanoclusters for light energy

10

conversion devices. He is also interested in Li-ion batteries and specializes in the application of

11

impedance spectroscopy to analyze the various mechanisms in energy conversion and storage

12

systems.

13

Prashant V. Kamat is Rev. John A. Zahm, C.S.C., Professor of Science in the Department of

14

Chemistry and Biochemistry and Radiation Laboratory at the University of Notre Dame and is

15

also a Concurrent Professor in the Department of Chemical and Biomolecular Engineering. For

16

nearly four decades, Prof. Kamat has made a significant contribution in developing advanced

17

nanomaterials for light energy conversion systems. He is currently serving as the Editor-in-Chief

18

of ACS Energy Letters and is a member of the advisory board of several scientific journals.

19

Jin Ho Bang is an associate professor of Chemical and Molecular Engineering, Bionano

20

Technology, and Advanced Materials Engineering at Hanyang University in South Korea. He

21

received his Ph.D. from the University of Illinois at Urbana-Champaign in 2008 and started his


33


Page 34 of 55

1

independent research career in 2010. His research interest is electrode materials for energy

2

conversion and storage devices. Prof. Bang was the recipient of the T. S. Piper Award in 2008

3

and the HYU Excellent Researcher Award in 2016.

4 5

ACKNOWLEDGMENT

6

This research was supported by grants from the Basic Science Research Program through the

7

National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and

8

Future Planning (NRF-2016R1A1A1A05005038 and NRF-2017R1E1A2A01077275). This is

9

contribution number NDRL No. 5202 from the Notre Dame Radiation Laboratory that is

10

supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic

11

Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. We

12

gratefully acknowledge Mr. J. Lee for graphic work.

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QUOTES

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1. Page 17

15

Although the size-dependent optical properties of semiconductor QDs and metal NCs appear to

16

be similar, the origin of spectral shifts as well as the excited state deactivation processes vary.

17

Unlike the semiconductor QDs, Au NCs possess a well-defined electronic structure and do not

18

possess surface defects. The structure-specific characteristics of the Au NCs give rise to distinct

19

size-dependence on the carrier relaxation dynamics.

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2. Page 25

21

The charge injection ability of Au NCs combined with the charge storage capability could give

22

them a unique perspective in the future development of MCSSCs

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3. Page 31


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ACS Energy Letters

1

Establishing a design principle-based synthesis, gaining a better understanding of the

2

optoelectronic properties of Au NCs, and elucidating factors that govern the charge transfer and

3

recombination kinetics, would be the key to realizing their full potential for light energy

4

conversion.

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Thiolated Gold Nanoclusters for Light Energy Conversion - American

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