Thiolated Gold Nanoclusters for Light Energy Conversion - ACS

Thiolated Gold Nanoclusters for Light Energy Conversion Muhammad A. Abbas,† Prashant V. Kamat,*,‡ and Jin Ho Bang*,†,§,∥

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Department of Advanced Materials Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ‡ Notre Dame Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Bionano Technology, Hanyang University, Ansan, Republic of Korea ∥ Department of Chemical and Molecular Engineering, Hanyang University, Ansan, Republic of Korea ABSTRACT: Few-atom gold nanoclusters (NCs) exhibit moleculelike properties due to a discrete electronic structure driven by the quantum confinement effect. Unlike plasmonic Au particles, these nonplasmonic particles of diameter less than 2 nm, commonly referred to as nanoclusters, possess a distinct excited-state behavior that can offer a new opportunity to employ them as a photosensitizer. Their size-dependent excited-state behavior enables establishing logical designing principles to build up efficient light energy conversion systems. The photodynamics of thiolated Au NCs and efforts to exploit the Au NCs in light energy conversion applications discussed in this Review show new opportunities to utilize them as photosensitizers. Current bottlenecks in implementing thiolated Au NCs in light conversion applications and new strategies and future directions to address these limitations are also discussed.

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for this purpose; however, they usually suffer from low chemical or photochemical stability. Nonetheless, the usefulness of photophysical properties of these metal NCs have been demonstrated in sensing, biomedical imaging, cancer radiotherapy, and antimicrobial agents.24,45−48 The plasmonic Au NPs have been extensively investigated for photoactive materials over decades because of their excellent ability to trap visible light via the SPR phenomenon;49,50 however, harnessing photon energy through the plasmonic NPs was not quite successful because of the ultrafast relaxation of hot electrons (less than several picoseconds).51 This limitation can be alleviated in the Au NCs by virtue of the discrete energy states that can extend the lifetime of excited electrons up to microsecond regimes. This dramatic change in the excited-state dynamics offered a new opportunity for noble metals to serve as a light harvester, and recent advances have seen their feasibility in various solar energy conversion systems. While such achievements are inspiring and look promising, many photophysical events in the NC-based light energy conversion systems remain hidden, and the relatively low power conversion efficiency (PCE) is calling for a breakthrough to tackle current bottlenecks imposed by the NC properties.

lasmonic Au nanoparticles (NPs) have been the topic of intense research over decades and have found many applications in various fields, including sensing,1,2 biomedicine,3−5 catalysis,6−8 and photovoltaics,9−12 because of their interesting physical properties. The protocols for the synthesis of Au NPs are now well-established,6,13−15 and the size and shape of the NPs have been determined to be important controlling parameters that define their photophysical and catalytic properties. For the Au NPs whose size is greater than 3 nm, surface plasmon resonance (SPR) absorption remains a primary physical phenomenon.16 Despite the distinct behavior of Au NPs as compared to bulk counterparts, the electronic structure of NPs is continuous, much like the bulk Au. However, the quantum confinement effect becomes dominant when the size of Au is further reduced below 2 nm (i.e., nanoclusters (NCs)), and this leads to the creation of discontinuous electronic structures in Au.17,18 Because of the presence of these discrete energy levels, the ultrasmall Au NCs are capable of exhibiting unusual molecule-like behaviors,16,19−21 some of which include photoluminescence,16,22−28 electron transfer,29−34 magnetism,35−37 and molecular chirality.38−42 Recent developments in synthesis protocols have set a new milestone toward diverse applications of NCs.43,44 Employing Au NCs as a photosensitizer in light energy conversion applications is one of the rapidly emerging research areas. The NCs of other metals such as Ag, Pt, and Cu have also been explored © 2018 American Chemical Society

Received: January 16, 2018 Accepted: March 2, 2018 Published: March 2, 2018 840

DOI: 10.1021/acsenergylett.8b00070 ACS Energy Lett. 2018, 3, 840−854

Review

Cite This: ACS Energy Lett. 2018, 3, 840−854

ACS Energy Letters

Review

The structure−property relationship in the NCs, which remains unknown to a large extent, is key information to acquire for the next leap. Controlling NCs at an atomic level to allow for the delicate manipulation of their physical properties is another research area of urgency. In addition, a comprehensive understanding of the excited-state relaxation dynamics of the NCs is important to establish new design principles for light energy conversion in an effective manner. This Review presents a brief overview of the current status of thiolated Au NC research advances and summarizes various efforts to utilize Au NCs in light energy conversion systems. We also discuss new opportunities that Au NCs can offer as photosensitizers. It is important to note that the discussion will be limited to Au25(SR)18 and its smaller clusters because they offer the best opportunities to design light energy conversion systems. Key Properties of Gold Nanoclusters: Structure. Gold in its bulk and NP forms exists in a face-centered cubic (FCC) crystal structure. However, most Au NCs cannot sustain the FCC crystal structure, while some Au NCs do exist in the FCC crystal structure.52,53 Instead, they form a core−shell structure,19 which varies from homoleptic plane to spherical to cylindrical structures depending upon the size of the NCs. The core, which is made of only Au atoms, is surrounded by a shell coordinating ligand, typically thiol derivatives. The outer gold atoms are strongly complexed with the ligands, thus protecting the core Au atoms. For example, Au25(SR)18 consists of a Au13 core with one Au atom located in the center that is bonded to 12 surrounding Au atoms (Figure 1A).54−56 These 12 Au atoms in

where an Au4 core is surrounded by the motifs made of two trimers (S−Au−S−Au−S) and one cyclic pentamer (S−Au− S−Au−S−Au−S−Au−S−Au). On the other hand, some small thiolated Au NCs (i.e., Au 10 (SR) 10 , Au 11 (SR) 11 , and Au12(SR)12) are known to possess no metal core. Instead, they form a homoleptic structure with interlocked cyclic Au−S motifs.61,62 Optoelectronic Properties. Distinctive absorption features, which result from electronic transitions among various molecular orbitals, appear in the absorption spectra of Au NCs depending upon the size and nature of the Au NCs. Akin to quantum dots (QDs), the absorption onset of Au NCs generally shifts toward shorter wavelengths with decreasing cluster size. For example, the absorption onsets of glutathione (GSH)-protected Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18 are 450, 650, 700, and 900 nm, respectively (Figure 2A),33,64 indicating an increase in the optical gap with decreasing cluster size. Interestingly, however, there are several exceptions to this universal trend. For example, Au22(SR)18 is larger than Au18(SR)14, but its absorption onset (680 nm) is 20 nm less than that of Au18(SR)14 (700 nm).23,25,65 This deviation from the general size dependency implies that the optical properties of the Au NCs are dictated not only by the number of Au atoms (i.e., the size of Au NCs) but also by the ligand/metal ratio. Besides the ratio, ligand engineering can significantly alter the optical properties by tuning the molecular orbitals positions and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the Au NCs. For example, Tlahuice-Flores et al.34 demonstrated that the introduction of functional groups to a ligand can cause more distortion in the structure of Au NCs resulting in a reduced HOMO−LUMO gap. Substituting ligands is another strategy to manipulate optical properties. Jin and coworkers demonstrated a red shift in the electronic transition peaks of Au25(SR)18 by replacing aliphatic thiol ligands with aromatic thiol ligands.66 A multiple-ligand stabilization strategy that was recently demonstrated by the Xie group has proven very effective in extending the absorption range of Au NCs while preserving the number of Au atoms.67,68 The use of multiple ligands with different charges (e.g., −COO−, −NH3+, and −OH) can give rise to the structural distortion of Au25 NCs that leads to a significant variation in their optical properties (Figure 2B). A similar change in the absorption feature via the surface charge modulation was observed even in monothiolateprotected Au25 NCs when converting the negatively charged Au25 into neutral NCs by chemical oxidation using O2 and H2O2.69−71 Replacing one or more of the Au atoms in the Au NCs with other metallic elements can have a dramatic effect on their optoelectronic properties as well.72−74 For instance, the HOMO−LUMO levels of Au25(SR)18 can be shifted to more negative values when one of the Au atoms in the outer shell is replaced by a Hg atom (Figure 2C).75 In the case of Pd or Pt doping, the electronic structure in the vicinity of HOMO− LUMO levels of PdAu24(SR)18/PtAu24(SR)18 is dramatically different as compared to their undoped parent (Figure 2D).76 The substitution of one Au atom with a single Pd or Pt atom not only causes the electrochemical gap, which echoes the HOMO−LUMO gap, to decrease from 1.67 to 0.75 V, but it also changes the redox behavior of Au25(SR)18 significantly. On the other hand, Ag doping can cause a blue shift in the absorption peak of Au NCs with the increase in Ag atoms (Figure 2E).74,77−81 It also brings an increase in absorbance

Figure 1. (A) Core−shell structure of Au25(SR)18 NCs. Reprinted from ref 63. Copyright 2015 American Chemical Society. (B) Crystal structure of Au38(SR)24. Reprinted from ref 57. Copyright 2010 American Chemical Society. (C) The lowest-energy structure of Au15(SR)13 (red atoms, Au core; orange atoms, Au atoms in the ligand shell; blue atoms, S; green atoms, C; and pink atoms, H) as determined by DFT calculations. Reprinted from ref 60. Copyright 2013 American Chemical Society.

the Au13 core form an icosahedron. The remaining 12 Au atoms form six S−Au−S−Au−S motifs by bonding with the thiol groups of ligands. These motifs surround the Au13 icosahedron. Each Au atom in the motifs is located on 12 faces of the icosahedron. However, eight faces of the Au13 core remain uncovered. A similar symmetrical core−shell structure has been observed for larger Au NCs (e.g., Au 38 (SR) 24 57 and Au102(SR)4458,59) (Figure 1B). While many thiolated Au NCs, including the ones smaller than Au25(SR)18, usually possess such core−shell structure, they are not as symmetrical as Au25(SR)18. Au15(SR)13 is the smallest known Au NC that has a core−shell structure.60 Recent theoretical calculations revealed the most probable structure of Au15(SR)13 (Figure 1C),33,60 841


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Figure 2. (A) UV−vis absorption spectra of Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18. Insets show the digital photographs of the corresponding Au NCs. Adopted from ref 84. Copyright 2014 American Chemical Society. (B) Schematic illustration of Au25(SR)18 NCs with heteroligands and UV−vis absorption spectra of monothiolate-protected Au25(MHA)18, bithiolate-protected Au25(MHA)11−17(MPA)7−1, 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 1). Adopted with permission from ref 67. Copyright 2016 Royal Chemical Society. (C) Effect of Hg doping on the structure and the HOMO−LUMO gap of Au25(SR)18. Adopted from ref 75. Copyright 2015 American Chemical Society. (D) HOMO−LUMO gap of Au25(SR)18 (black), PdAu25(SR)18 (red), and PtAu25(SR)18 (blue) as determined by cyclic voltammetry. Adopted from ref 76. Copyright 2015 American Chemical Society. (E) Absorption spectra and the corresponding digital photographs of Au18−xAgx(SR)14 NCs (the ratio of Ag doping increases from top to bottom). Adopted from ref 74. Copyright 2016 American Chemical Society.

Wu et al.71 demonstrated that the PL enhancement is directly related to the electron-donating ability of ligands. Among the different ligands tested (SC6H13, SC12H25, and SCH2CH2Ph), the Au25(SR)18 stabilized with SCH2CH2Ph showed the highest PL QY because of the high charge-donating ability (chargetransfer character) of the ligand.71 Furthermore, increasing the electropositivity of the metal core can also enhance the PL of Au NCs as it stabilizes the LMCT state.16,71,88 Doping the core with other elements and changing the ratio of Au(I)/Au(0) in the NCs are two other parameters that can tune the PL of Au NCs.71,89−92 On the other hand, rigidifying the Au(I)−thiolate shell using bulky surfactants such as tetraoctylammonium bromide (Figure 3A)25,93,94 or spatially confining Au(I)−thiolate complexes in a polymer matrix95 have been demonstrated as new ways to enhance PL. The common strategy of PL enhancement found in all these examples is to

within the absorbance range of the Au NCs. In contrast to this observation, Cu doping was found to induce a red shift of the absorption edge of Au NCs.82 Interestingly, in the case of Cu-doped Au144(SC6H13)60, introducing more than eight Cu atoms into the parent Au NC induces a transition from molecule-like NCs to plasmonic nanomolecules that exhibit a sharp SPR peak at ∼520 nm.83 Photoluminescence (PL) is a convenient probe to explore the excited-state behavior of Au NCs. While initial reports of PL quantum yield (QY) of Au NCs showed only 0.1%,85 further efforts to manipulate ligand chemistry and metal ion doping have led to the achievement of emission quantum yield as high as 60% (Figures 3A).25 The origin of Au NCs’ PL remains unclear, but it is generally attributed to the relaxation of ligand-to-metal charge transfer (LMCT) or semiring states (S−Au−S−Au−S), as well as that of core metal atoms.86,87 842


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Figure 3. (A) Schematic illustration of wrapping of Au22(SR)18 NCs with bulky ligands to rigidify its shell, a digital photograph of Au22(SR)18 in water and toluene under UV irradiation, and the PL spectra of water-soluble Au22(SR)18 NCs, tetraoctylammonium (TOA+) wrapped Au22(SR)18 NCs, and rhodamine B. Adopted from ref 25. Copyright 2015 American Chemical Society. (B) Schematic mechanism of aggregation-induced emission from Au oligomeric complexes. Adopted from ref 97. Copyright 2012 American Chemical Society. (C) Theoretical and experimental UV−vis absorption spectra of Au18(SR)14 indicating specific orbital transitions for various peaks along with the HOMO and LUMO orbitals of Au18(SR)14 as calculated by DFT. The right side of panel C shows the calculated orbital energy levels of Au18(SR)14 with color-coded contributions from the constituent atoms. Adopted with permission from refs 99 and 100. Copyright 2015 WileyVCH Verlag GmbH & Co. KGaA, Weinheim, respectively.

whereas the LUMO levels have a larger contribution from Au 6sp orbitals (Figure 3C).99 This insight helped to identify the origin of light absorption in Au18(SR)14, which occurs via d → sp intraband transitions. In another theoretical investigation, Aikens and co-workers attributed the PL of Au25(SR)18 to several excited states mainly associated with core-based orbitals. Ligands were found to be a primary factor to govern the PL through the interaction with the Au core.101 Despite the benefits of new insights provided by many theoretical studies, it is noteworthy that extra care must be taken when interpreting the optoelectronic properties of the Au NCs because of the assumptions and approximations used in calculations.102 Synthesis and Characterization. The synthesis of metal NCs of defined size has improved since the early efforts to synthesize a mixture of Au NCs resulted in a low yield.85 Methods are now available to synthesize size-selective NCs in one pot with a high yield.23,43,64 Two approaches are commonly used to synthesize thiolated Au NCs (Figure 4A): (1) Metal−thiol complexes can be reduced to form Au NCs of desired size.65 (2) Large Au NPs are etched by thermal energy or excess thiol group to form desired Au NCs.103 Sodium borohydride (NaBH4) is one of the commonly used reducing agents for the synthesis of Au NCs. However, the Au NCs formed by the NaBH4 reduction are typically polydisperse in size and hence require subsequent separation processes to obtain Au NCs of a well-defined size.85 While selection of the ligand-to-metal ratio can give some control over the formation of Au NCs, it is still inadequate to achieve monodispersity. Recently, a thermal thiol-etching strategy in combination with ligand exchange has emerged as one of the methods to size-focus these crude initial products formed by the NaBH4 reduction.104,105 Jin and co-workers employed this twostep approach to synthesize monodispersed Au38(SC2H4Ph)24 NCs with 25% yield (Figure 4B).104 This size-focusing method is now extended further to prepare other Au NCs.105−107 It was found that the stability of Au NCs of a specific size plays a

enhance LMCT while suppressing nonradiative recombination in the core. Another mechanism associated with the high PL QY in the thiolated Au NCs is the aggregation-induced emission (AIE). Many organic, polymeric, and organometallic luminophore systems have been identified to show a dramatic increase in their PL upon aggregation.96 This AIE phenomenon has also proven to play a significant role in enhancing the PL of Au NCs.97,98 This PL enhancement is driven by the aggregation of Au(I)−thiolate complexes onto a Au core that can reinforce the Au(I)···Au(I) interaction (known as aurophilic interaction), which results in the intense PL from the Au(I) complexes. For example, the Xie group demonstrated that Au(I)−thiolate complexes that are nonluminescent in aqueous solution showed a dramatic increase in the PL when aggregation was induced by adding a weakly polar solvent (Figure 3B).97 The addition of such solvents (e.g., ethanol) disrupted hydration shell and neutralized surface charge, therefore leading to the formation of aggregates. The mechanism and recent research about the PL enhancement by the AIE has been well summarized in a recent review.16 Fundamental understanding of Au NCs’ discrete energy states is an important prerequisite to explain their optoelectronic properties. Since the groundbreaking work by Kornberg and co-workers in 2007 (determination of the crystal structure of Au102(SR)44),58 the crystal structure of several Au NCs has been revealed.55−57,100 These recent advances in the crystallographic studies plus the computational studies revealed the secret of the electronic structure of Au NCs. Chen et al., who resolved the crystal structure of Au18(SR)14, revealed that the NC consists of an Au9 bioctahedral inner core that is protected by three Au(SR)2 staples as well as Au2(SR)3 and Au4(SR)5 staples.100 This structural information enabled them to calculate the Kohn−Sham orbitals of Au18(SR)14, which revealed that the HOMO to HOMO−20 orbitals arise from Au 5d orbitals, 843


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Figure 4. (A) Illustration of two common synthesis approaches used to synthesize thiolated Au NCs. Adopted from ref 103. Copyright 2015 American Chemical Society. (B) Two-step synthesis method produces mixed size clusters in the first step, followed by the second step that selectively keeps the most stable Au NCs in the reaction mixture. Adopted from ref 108. Copyright 2010 American Chemical Society. (C) CO-based reduction method to synthesize Au NC size by controlling pH. Adopted from ref 64. Copyright 2013 American Chemical Society.

these characteristics were instrumental for the first demonstration of Au NCs as photosensitizers in solar energy conversion applications.110 The GSH-protected Au NCs can be readily synthesized without using a reducing agent because amine functional groups provide mild reducing power to reduce Au(III) ions and form Au NCs.97,111 However, another method developed by the Xie group became popular for the synthesis of GSH-protected Au NCs as it employs slow reducing capability of carbon monoxide (CO) to reduce a Au precursor while fine-tuning the size of NCs. Four fine-tuned Au NCs (Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18) can be formed by varying the pH of the reaction medium. A higher pH was found to provide a relatively stronger reducing environment that in turn leads to the formation of larger NCs (Figure 4C).64 Xie and co-workers also developed a two-stage combination of acidic and basic conditions with the CO reduction to synthesize Au22(SR)18 NCs that show an ultrabright

crucial role in the size-focusing step. A fine-tuned experimental condition induces less stable NCs to disintegrate while the most stable NCs remain intact. Differently sized Au NCs can thus be obtained first by forming NCs with wide size distribution initially, which is then followed by the size-focusing treatment to obtain the desired size (Figure 4B).108 In another modified approach, Jin and co-workers reported a ligand-controlled size-focusing strategy that can be applied to a mixture of dramatically different sizes.109 In this work, they used para-, meta-, and ortho-methylbenzenethiols (MBT) to synthesize an initial product and subjected it to the size-focusing process. Interestingly, depending upon the ligand (p-MBT, m-MBT, or o-MBT), pure Au130(p-MBT)50, Au104(m-MBT)41, and Au40(o-MBT)24 were formed, respectively. Among the thiolated Au NCs, GSH-protected Au NCs prepared in aqueous media are of particular interest owing to their good stability and versatility in functionalization. In fact, 844


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luminescence.23 Note that many other articles related to the synthesis of gold NCs that are not covered in this Review have been presented in several recent review articles.103,108,112−114 Despite the recent advances in analytical techniques, the ultrasmall nature of the Au NCs imposes a great challenge on their full characterization. A quick indication of the formation of Au NCs can be readily obtained by ultraviolet−visible (UV−vis) absorption spectroscopy. The absence of the SPR peak gives a sign of the molecular nature of the synthesized particles. Although comparing the absorption feature of Au NCs to those of well-defined, pure NCs is simple and intuitive for determining the size of Au NCs, it cannot guarantee the purity of products because other NCs might be present in very low amounts and their contribution to the absorption spectrum might go unrevealed. Therefore, as-synthesized Au NC solutions need to be separated by a proper separation technique (e.g., polyacrylamide-gel electrophoresis) to determine how many types of NCs are present in the product.85 To determine the size and composition of Au NCs, mass spectrometry (MS) must be performed. It has been noted by Lu et al.115 that highresolution electrospray ionization mass spectrometry (ESI-MS) is a common technique to determine the composition of Au NCs while matrix-assisted laser desorption/ionization timeof-flight (MALDI-TOF) mass spectrometry can be utilized to determine the purity of Au NCs.116−119 For more details, refer to a recent review on the use of MS in the characterization of metal NCs.115 As compared to MS analysis, single-crystal X-ray crystallography is the most accurate and informative technique that provides the comprehensive structural information on Au NCs. However, implementing this analysis has been limited because of the difficulty in synthesizing highly pure singlecrystalline NCs.55,58,112,120−127 On the other hand, X-ray absorption spectroscopy (XAS) is often performed to extract information about the structural and electronic properties of Au NCs.128 For example, XAS has been used to determine the ratio of core and surface atoms in a single Au NC; it also helps identify the location of a dopant in doped Au NCs.77,129 Details about the characterization methods of Au NCs have recently been reviewed elsewhere.112,114,115,128 Excited-State Properties. Although the size-dependent optical properties of semiconductor QDs and metal NCs appear to be similar, the origin of spectral shifts as well as the excited-state deactivation processes vary. Unlike the semiconductor QDs,

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

Figure 5. (A) HOMO−LUMO gap of Au NCs as a function of the number of Au atoms in Au NCs and the plot of the logarithm of nonradiative recombination ln(knr) vs the HOMO−LUMO gap of Au NCs demonstrating the energy gap law. Reprinted from ref 130. Copyright 2017 American Chemical Society. (B) Schematic diagram of relaxation pathways in Au25(SR)18 NCs. Reprinted from ref 86. Copyright 2010 American Chemical Society.

conclusion that the exciton dynamics in the Au NCs is governed by the energy gap law (i.e., an inverse relationship between nonradiative decay kinetics and the energy gap).132 On the other hand, spectroscopic analysis has revealed that Au NCs show a much less prominent relaxation mechanism associated with electron−phonon interactions due to their discontinuous energy states.84,133 While there are still debates over the exact nature of the excitedstate behavior, a common consensus is that, upon excitation, ultrafast charge relaxation (less than 200 fs) occurs in the Au core and the subsequent internal relaxation from the core states to semiring states (i.e., core−shell charge transfer) takes place in the time frame of ∼1 ps (Figure 5B).86,134−138 The excited electrons that reside in this semiring state with several orders of magnitude longer lifetimes are primarily responsible for the PL of Au NCs. Using transient absorption spectroscopy techniques, the Goodson and Kamat groups have independently identified the semiring state as a major contributor dictating the excited-state behavior of Au NCs.86,133,139 Interestingly, the excited electron relaxation dynamics of Au NCs was also found to be sensitive to factors such as charge states,69 dopants,140,141 ligands,142 and solvent medium.143 Akin to other light harvesters, excited Au NCs are capable of transferring electrons to an electron acceptor when the required conditions dictated by the Marcus theory (e.g., energy difference between donor and acceptor, degree of electronic coupling, and reorganization energy)144−148 are satisfactorily met. The photoinduced electron-transfer kinetics in the Au NCs has been investigated using methyl viologen (MV2+) as an

Although the size-dependent optical properties of semiconductor QDs and metal NCs appear to be similar, the origin of spectral shifts as well as the excited-state deactivation processes vary. Au NCs possess a well-defined electronic structure and do not possess surface defects.130 The charge recombination induced via the surface states, which dominate the excited QD relaxation dynamics, is not a factor in the deactivation of excited Au NCs.131 The structure-specific characteristics of the Au NCs give rise to distinct size-dependence on the carrier relaxation dynamics. Several models have been proposed to explain the excitedstate relaxation of excited Au NCs. Kwak and co-workers recently demonstrated that the exciton lifetime of Au NCs 845


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systems, several recent findings have demonstrated their potential as a new class of light harvester. The first utilization of the Au NCs in the light energy conversion application was demonstrated by Tatsuma and Sakai in 2010.110 This pioneering work showed that TiO2 films sensitized with GSH-protected Aun (n = 15, 18, 22, 25, 29, 33, 39) could generate photocurrent when exposed to light, and it also determined that Au25(SR)18 performed best among the Au NCs examined (Figure 7A). While the achieved performance in this work (a short-circuit current density (JSC) of 290 μA/cm2 and an open-circuit voltage (VOC) of 460 mV) looks trivial when compared with the recent advances, this work laid a cornerstone for the application of Au NCs in light energy conversion systems. Later, they explored the electronic structure of Au NCs by investigating the photocurrent dependency on the wavelength of light and on the standard electrode potentials of electron donors.32 Along with the estimation of HOMO and LUMO levels of the Au NCs, the knowledge garnered from this study highlighted the choice of an appropriate redox couple that could play a critical role in designing an efficient metal-cluster-sensitized solar cell (MCSSC). A breakthrough in designing efficient MCSSCs was made possible through the careful selection of a suitable Au NC and a suitable redox couple.31 Indirect evidence of a strong interaction between the NC and TiO2 was obtained through the PL quenching of Aux NCs; hence, the presence of an efficient pathway for the electron transfer from the Aux NCs to TiO2 was established. An MCSSC assembled with this Aux NC and Co(bpy)3(PF6)2/Co(bpy)3(PF6)3 as a redox couple in the electrolyte achieved the record PCE of 2.3% for MCSSCs with a JSC of 3.96 mA/cm2 and a VOC of 832 mV (Figure 7B). This high VOC and a stable photocurrent, combined with the fact that this Aux NC was active only below 525 nm, led to the conclusion that room for further improvement in the efficiency is possible by tuning the electronic structure of Au NCs. Inspired by the early work, our group systematically investigated the effect of the Au NCs’ size and of redox couples on the efficiency of MCSSCs and identified the performancelimiting factors by electrochemical impedance spectroscopy (EIS) in combination with other physical characterization techniques.33 When MCSSCs were assembled using four Au NCs (Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18) as photosensitizers, Au18(SR)14 outperformed all other NCs. When analyzed by the EIS analysis, Au15(SR)13 showed the best recombination kinetics but suffered from limited light absorption. On the other hand, Au25(SR)18 could absorb light over a much broader spectrum but suffered from limitations arising from charge recombination. Because of the better balance between the recombination and the light absorption, Au18(SR)14 showed the highest JSC of 8.18 mA/cm2, VOC of 672 mV, and the PCE of 3.8%, which was a new record for MCSSCs (Figure 7C,D). The use of I−/I3− redox couple in the electrolyte played a pivotal role in achieving this high PCE. The problem associated with corrosion by the iodine couple was not encountered for the Au NCs in these experiments. Sustained photocurrent was recorded in the presence of the iodine couple without significant deterioration (photocurrent degraded to ∼90% even after 1 h of continuous illumination). Despite this initial promise, the long-term stability of the Au NC-based energy system has not been fully investigated yet; hence, urgent attention should be given to ensure sufficient stability for any practical applications.

electron acceptor.139 The semiring state (or LMCT state) with a long lifetime was turned out to be solely responsible for the electron transfer. On the other hand, the metal core transition was too short-lived to participate in the photoinduced electrontransfer event effectively. Because the physical properties of Au NCs are highly sizedependent, it is essential to elucidate how size variation affects the excited-state lifetime and the efficiency of charge transfer. The size-dependent excited-state kinetics of four GSH-protected Au NCs (Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14 and Au25(SR)18) have been evaluated using transient absorption spectroscopy.84 This study revealed that the fate of excited electrons in the Au NCs varied depending on the size of Au NCs. While there was no noticeable decay in Au10−12(SR)10−12 and Au15(SR)13 even after 1 ns, a short-lived component in the decay dynamics appeared in Au18(SR)14 and Au25(SR)18 because of the presence of metal cores (Figures 6A). In addition, nanosecond transient absorption spectroscopy disclosed that all four Au NCs have a long-lived excited-state component with the lifetime of 164, 241, 255, and 203 ns for Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18, respectively, and among the four Au NCs, Au18(SR)14 has the longest excited states (Figure 6B). It is noteworthy that the observed nonradiative decay trend deviates from what one would expect from the energy gap law, as demonstrated for larger size Au NCs ranging from Au25 to Au333.130 This deviation seen in smaller size NCs is attributed to the dominance of the LMCT state and the absence of well-defined metal core transitions. In order to examine the ability of Au NCs to undertake photoinduced electron-transfer process, a well-known probe molecule, methyl viologen, was employed as an electron acceptor. The reduced methyl viologen with its characteristic absorption at 600 nm allowed the spectroscopic determination of electrontransfer yield and associated kinetics of charge transfer. By using the long-lived absorption at 600 nm, the QY of electron transfer was determined. Among the four different Au NCs studied, Au18(SR)14 exhibited the highest electron-transfer yield of ∼4% (Figure 6C). Given the light absorption capability along with the relatively high electron-transfer kinetics, Au18(SR)14 seems to be an optimal choice for light-harvesting applications. This observation was further confirmed in another recent study.33 Another intriguing observation regarding the excited-state behavior of Au NCs has recently been reported. Jin and co-workers explored the size-dependent excited-state dynamics of FCC-structured Au NCs (Au 28 (SR) 20 , Au 36 (SR) 24 , Au44(SR)28, and Au52(SR)32) and revealed that the excitedstate behavior of these NCs is very different from all the other NCs studied so far.52 In general, the PL spectrum of Au NCs exhibits two peaks: one peak in the visible light region normally associated with the core−core transitions, and the other in the near-infrared region attributed to the LMCT state. However, the FCC Au NCs showed only a single peak in the visible region, with the near-infrared peak being absent from their PL spectra. This behavior was attributed to the absence of core−shell relaxation in these NCs. Unlike the smaller size Au NCs discussed earlier, the relaxation of the excited state of FCC Au NCs slowed with an increase in size (Figure 6D). This opposite trend in the lifetime of excited states of the FCC-structured Au NCs may bring about very interesting prospects in the future applications of Au NCs. Metal-Cluster-Sensitized Solar Cells. Despite the fact that thiolated Au NCs are relatively new to light energy conversion 846


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Figure 6. (A) Difference absorbance spectra of Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18 immediately and 1000 ps after excitation. The insets show the signal decays at 500 nm. (B) Difference absorption spectra of the Au NCs in 100 ns delay after excitation with 355 nm laser and the corresponding decays of the absorbance signals at 500 nm. Adopted from ref 84. Copyright 2014 American Chemical Society. (C) Absorbance−time profiles of Au18(SR)14 NCs in the presence of different amounts of electron acceptor (methyl viologen) and the quantum efficiency of Au NCs for electron transfer. Adopted from ref 84. Copyright 2014 American Chemical Society. (D) Schematic representations of the excited-state lifetimes of a series of FCC Au NCs. Adopted from ref 52. Copyright 2017 American Chemical Society.

Besides the charge-transfer capability for light harvesting, the charging properties of Au NCs attracted an interest in their use in photoelectrodes.149,150 For the plasmonic Au NPs, it has been shown that they have the ability to store charge with increased capacitance per electron with decreasing particle size.151 When the plasmonic Au NPs were embedded in TiO2

for the photoanode of dye-sensitized solar cells (DSSCs), it shifted the quasi Fermi level to more negative potentials because of the stored charge in the NPs, resulting in an increase in the photovoltage.11 Given the presence of a metal core in Au NCs, it would have been interesting to see how the nonplasmonic Au NCs would behave when coupled with TiO2. 847


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Figure 7. (A) Photocurrent action spectra of Aun(GSH)m-sensitized TiO2 films. Adopted with permission from ref 110. Copyright 2010 WileyVCH Verlag GmbH & Co. KGaA, Weinheim. (B) Current−voltage (J−V) characteristics of Aux(GSH)- and CdS-sensitized TiO2 films along with bare TiO2 under 1 sun illumination. Adopted from ref 31. Copyright 2013 American Chemical Society. (C) Energy band diagram and (D) J−V characteristics of Au10−12(SR)10−12, Au15(SR)13, Au18(SR)14, and Au25(SR)18 NC-sensitized MCSSCs. Adopted from ref 33. Copyright 2016 American Chemical Society. (E) J−V curves showing the boost in photovoltage in Aux(GSH) NCs and squaraine dye cosensitized DSSCs. Reprinted from ref 30. Copyright 2015 American Chemical Society. (F) Scheme of the synergistic effect between Ag NPs and Au NCs. Reprinted from ref 152. Copyright 2015 American Chemical Society.

can induce a negative shift in the quasi Fermi level, akin to the Au NPs. Therefore, the charge injection ability of Au NCs combined with the charge storage capability could give them a unique role in the future development of MCSSCs. Another interesting feature of the thiolated Au NCs is a synergistic effect when combined with plasmonic NPs.152,153 When Au NCs were coupled with Ag NPs, the absorption was enhanced significantly beyond the absorption of the individual components. The Ag NPs have the ability to capture two photons using wavelengths greater than their absorption edge.154 A biphotonic off-resonance excitation was used to probe the interaction between excited Ag NP and Au NCs using transient absorption spectroscopy (Figure 7F). The results showed that coupling with metal NCs can further enhance the plasmonic effect of metal NPs. Demonstration of the plasmonic

The influence of Au NCs embedded in TiO2 was explored using a photoanode composed of squaraine dye and Aux NCs on TiO2 (Figure 7E).30 As the Aux NCs can also work as a light absorber, it resulted in photocurrent enhancement in this cosensitized DSSC. An interesting observation was a 240 mV increase in the VOC. This increase in VOC was attributed to the charge storage ability of the Aux NCs in their metal core that

The charge injection ability of Au NCs combined with the charge storage capability could give them a unique role in the future development of MCSSCs. 848


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Figure 8. (A) Schematic illustration of water oxidation by Au NC-sensitized TiO2 film under illumination, (B) digital photograph of the experimental setup showing the evolution of H2 bubbles at the Pt counter electrode, and (C) time course of hydrogen evolution following the visible light illumination of an aqueous suspension containing (a) Pt/TiO2 nanoparticles, (b) Pt/TiO2 nanoparticles in the presence of 0.01 M EDTA, (c) Aux-GSH-sensitized Pt/TiO2 nanoparticles, and (d) Aux-GSH-sensitized Pt/TiO2 nanoparticles in the presence of 0.01 M EDTA. (Inset shows the photographs (left to right) of TiO2, Pt/TiO2, and Aux(GSH)-sensitized Pt/TiO2 powders.) The experiments were conducted in aqueous solutions at pH 7. Adopted from ref 162. Copyright 2014 American Chemical Society.

aggregation of Au NCs and gave rise to a gradual transformation into plasmonic NPs.167 Interestingly, this work demonstrated an enhanced photocatalytic activity driven by the synergistic interaction between Au NCs and adjacent Au NPs formed by this photoinduced aggregation. While ensuring the photostability of Au NCs remains a primary task for this application, a judicious use of this transformation for designing a Au NC−NP coupling system is worth trying for boosting light conversion efficiency. Summary and Future Outlook. Over several decades, organic and organometallic molecules have served as the most popular light harvesters for light energy conversion applications.168 Despite the enormous advances achieved in the dye-based light conversion systems, their stagnant conversion efficiency is now calling for a new material that can provide a breakthrough. The thiolated Au NCs whose properties resemble those of molecules have emerged as a new class of materials to challenge the dominance of traditional dyes. Offering facile tunability of optoelectronic properties by ligand engineering, changing size, or doping with heteroatoms is a merit of the Au NCs over the molecules. In addition, unlike traditional dyes for which aggregation causes the deterioration of performance,169,170 the aggregation of Au NCs would rather be advantageous as it suppresses the nonradiative recombination.16,97 The nontoxic, environmentally benign nature of Au NCs as compared to inorganic semiconductors is another attractive feature. However, there are many hurdles to utilize Au NCs for practical applications. Along with the current low conversion efficiency, their chemical and photochemical stability remains a critical issue. In addition, cost of materials is likely to be another obstacle that could hinder the wider deployment of Au NC-based energy conversion systems. While the initial proof-of-concept studies have shown the feasibility of the thiolated Au NCs in light energy conversion applications, many aspects of Au NCs as a photoactive material have yet to be explored fully. Establishing a design principlebased synthesis, gaining a better understanding of the optoelectronic properties of Au NCs, and elucidating factors that govern the charge transfer and recombination kinetics would be the keys to realizing Au NCs’ full potential for light energy conversion. The knowledge of the physical properties of Au NCs gathered from a number of recent studies points out areas that are likely to see breakthroughs. The smaller Au NCs are more photoactive with longer lifetimes than their larger counterparts. Therefore, strategies to suppress the faster charge recombination in Au NCs larger than Au18(SR)14 should be developed to improve the light conversion efficiency. The excited-state

enhancement of Au photosensitizers has also been reported recently.155,156 The NCs of Ag, Cu, Pt, and Pd and alloyed NCs of Ag−Au and Ag−Cu have been shown to work as a sensitizer in MCSSCs as well,29,157−160 and among these noble metal NCs, Ag NCs stand out with a superior performance in MCSSCs.29,160 To date, the best PCE of MCSSCs other than Au NCs is 1.1% (obtained using AgCu alloyed NCs).159 The availability of other metal NCs as photosensitizers could offer a new possibility for MCSSCs; however, almost all non-Au NCs suffer from inherent instability during light irradiation.161 Therefore, developing a suitable design strategy to ensure the photostability of these non-Au NCs is needed to employ these metal NCs as photosensitizers. Metal Nanoclusters for Photocatalytic Hydrogen Production. Another opportunity for the thiolated Au NCs in light energy conversion applications can be found in photoelectrolytic (or photocatalytic) water splitting for H2 generation. The Au NCs possess a reduction peak at −0.63 V and two oxidation peaks at +0.97 and +1.51 V versus reversible hydrogen electrode (RHE), thus exhibiting their ability to induce water splitting under light irradiation. A photoelectrolysis cell was assembled to evaluate thiolated Aux NCs as a visible light harvester for H2 production (Figure 8A).162 For this task, the Aux NCs were adsorbed onto mesoporous TiO2 surface to form the photoanode in conjunction with Pt as the cathode and 0.1 M phosphate buffer solution as the electrolyte (Figure 8B). The photoaction spectra showed the extended photocurrent response from 400 to 520 nm and an incident photon-to-current efficiency (IPCE) of 2.9% at 420 nm, which further increased to 4.8% when a sacrificial hole scavenger, ethylenediaminetetraacetic acid disodium salt (EDTA), was introduced. This enhanced conversion efficiency observed in the presence of EDTA was attributed to the faster removal of holes from the oxidized Aux NCs. Similar demonstrations of the H2 evolution were made using Aux NC-sensitized TiO2 nanotubes by Chen and co-workers163 and Liu and co-workers164 as well. The Au NCs not only were applicable for the photoelectrochemical system but also worked for photocatalytic H2 generation in neutral water even without the aid of applied bias (Figure 8C).162 This opened up a new opportunity for Au NCs to employ them as photocatalysts for solar-to-fuel conversion systems. One of the critical problems is the photoinduced transformation of Au NCs into larger Au NPs, which is primarily responsible for lower conversion efficiency during prolonged photoirradiation.162,165,166 A recent investigation on the photoinduced transformation revealed that the photocatalytic oxidation of organic ligands protecting Au NCs triggered the 849


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Notes

Establishing a design principle-based synthesis, gaining a better understanding of the optoelectronic properties of Au NCs, and elucidating factors that govern the charge transfer and recombination kinetics would be the keys to realizing Au NCs’ full potential for light energy conversion.

The authors declare no competing financial interest. Biographies Muhammad Awais Abbas received his Ph.D. in Advanced Materials Engineering from Hanyang University in 2017. Currently, he is working as a postdoctoral researcher in the Bang group. His current research is focused on the development of noble metal nanoclusters for light energy conversion devices. He is also interested in Li-ion batteries and specializes in the application of impedance spectroscopy to analyze the various mechanisms in energy conversion and storage systems. Prashant V. Kamat is Rev. John A. Zahm, C.S.C., Professor of Science in the Department of Chemistry and Biochemistry and Radiation Laboratory at the University of Notre Dame and is also a Concurrent Professor in the Department of Chemical and Biomolecular Engineering. For nearly four decades, Prof. Kamat has made a significant contribution in developing advanced nanomaterials for light energy conversion systems. He is currently serving as the Editor-in-Chief of ACS Energy Letters and is a member of the advisory board of several scientific journals.

lifetime of Au NCs is governed by the energy gap law; therefore, any advantage gained by the increase in absorption range in the visible from larger Au NCs is likely to be counterbalanced by the decrease in the excited-state lifetime. Utilization of the Au NCs featuring the FCC structure could be an option to overcome this issue because the FCC Au NCs showed an increase in the excited-state lifetime with increase in size from Au28(SR)20 to Au52(SR)32.52 A synthesis approach for two-way structural transformation between FCC and non-FCC Au NCs has recently been devised as well.171 These results may open up another opportunity to expand the potential of Au NCs to harvest solar energy. Most of the research regarding the application of NCs for light energy conversion has exclusively used the GSH-stabilized Au NCs because of the ease of synthesis, relatively high stability, and long lifetimes of their excited states.31,33,110,159,160,162 However, as GSH consists of long and bulky ligands, it could reduce the Au NCs’ electron-transfer ability. Therefore, it is important to investigate how the alkyl chain length or the presence of certain functional groups in the ligand affects the excited states and the charge-transfer kinetics of the GSH-stabilized Au NCs. Future research on the effect of ligands could shed light on these points and aid in the design of more efficient Au NC-based photoelectrodes. Using shorter alkyl chains for the ligand will be a good starting point to achieve this goal but may not fully suppress the internal nonradiative recombination. Thus, it remains an open question whether the short ligands could indeed be beneficial to solar light-harvesting applications. Doping of Au NCs with other metal ions is a convenient approach to tune the photophysical properties. However, such doping inevitably lowers stability. Therefore, it would be useful to explore ways to stabilize doped Au NCs so that they can withstand the reactivity of the redox couples employed in MCSSCs and the water-splitting reaction. In addition, it would be equally important to gain insight into the following questions: Will the charge transfer be more efficient in the doped Au NCs than in native Au NCs? To what extent can the light energy conversion process be improved with controlled doping? The scientific issues raised in this Review lay the foundation for future research efforts. Given the rapid rise of the metal NCs as photoactive materials, one can expect their utilization in lightharvesting assemblies, display devices, and sensors.

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Jin Ho Bang is an associate professor of Chemical and Molecular Engineering, Bionano Technology, and Advanced Materials Engineering at Hanyang University in South Korea. He received his Ph.D. from the University of Illinois at Urbana−Champaign in 2008 and started his independent research career in 2010. His research interest is electrode materials for energy conversion and storage devices. Prof. Bang was the recipient of the T. S. Piper Award in 2008 and the HYU Excellent Researcher Award in 2016.

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ACKNOWLEDGMENTS This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016R1A1A1A05005038 and NRF2017R1E1A2A01077275). This is contribution number NDRL No. 5202 from the Notre Dame Radiation Laboratory that is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Award DE-FC02-04ER15533. We gratefully acknowledge Mr. J. Lee for graphic work.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Muhammad A. Abbas: 0000-0002-8338-9802 Prashant V. Kamat: 0000-0002-2465-6819 Jin Ho Bang: 0000-0002-6717-3454 850


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Thiolated Gold Nanoclusters for Light Energy Conversion - ACS

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