Research

Photoinduced Electron Transfer in Quantum Dot Heterostructures and Dye-Semiconductor Interfaces

We use coordination chemistry, organic chemistry, and linker-assisted assembly to organize nanoparticles into composite materials and heterostructures and to attach light-harvesting molecular dyes to surfaces. We use spectroscopy to characterize excited-state interfacial charge-transfer processes of the resulting materials interfaces. A central theme of our research is to understand the influence of interfacial composition and electronic structure on the dynamics and yield of excited-state charge-transfer processes.

Linker-Assisted Attachment of Inorganic Nanoparticles to Metal Oxide Thin Films

We tether semiconductor quantum dots (QDs) to metal oxide surfaces via bifunctional molecular linkers (Fig. 1). The specificity of surface-attachment interactions enables precise control over materials-assembly processes. The color and optical density of materials can be controlled by varying the size and fractional surface coverage of adsorbed nanoparticles. Similarly, the distance and electronic coupling between QDs and substrates are tunable by varying the properties of the molecular linker. Much of our research  involves studying the kinetics and thermodynamics of the linker-assisted assembly of various materials interfaces and heterostructures. We typically use electron microscopy, UV/visible (electronic) spectroscopy, and infrared (vibrational) spectroscopy to characterize the composite materials.

Figure 1:  Mechanisms for the linker-assisted attachment of CdS quantum dots to TiO2 surfaces to yield a materials interface that promotes photoinduced CdS-to-TiO2 electron transfer.  (From Watson, D.F. J. Phys. Chem. Lett. 2010, 1, 2299-2309,DOI:10.1021/jz100571u.)

Our ongoing research is focused on elucidating kinetics and mechanisms of materials-assembly processes and developing new materials interfaces with interesting physical properties and chemical and photochemical reactivities.

Fabrication of Quantum Dot Heterostructures via Covalent Bonding between Capping Ligands

We tether QDs to each other by forming covalent bonds between terminal functional groups of adsorbed capping ligands. To date, we have explored the N,N’-dicyclohexylcarbodiimide (DCC)-mediated formation of amide bonds between capping groups of CdS and CdSe QDs to yield QD heterostructures with programmable optical and electronic properties (Fig. 2). Our approach offers two advantages over the use of bifunctional ligands to tether QDs to one another. First, DCC-mediated coupling enables the selective attachment of two different QDs to each other to yield exclusively heterostructures without the undesired formation of homojunctions between identical QDs. Second, QDs primed for incorporation into heterostructures have no affinity for each other in the absence of DCC, enabling us to trigger the assembly of materials and to control the rate and extent of coupling reactions. Importantly, we have discovered that our first-generation amide-bridged CdS/CdSe QD heterostructures undergo efficient photoinduced inter-QD charge transfer.

Ongoing research involves the synthesis of new QD heterostructures in dispersions and on thin films, the characterization of interfacial charge-transfer reactivity of such heterostructures, and the exploration of alternative covalent coupling methodologies.

Figure 2:  DCC-mediated formation of QD heterostructures. (From Sellers, D.G. et al. J. Phys. Chem. C 2015, 119, 27737-27748. DOI: 10.1021/acs.jpccc.5b07504.)

Recent publications on materials assembly

  1. Rivera-González, N.; Chauhan, S.; Watson, D.F. “Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2.” Langmuir 2016, 32, 9206-9215. DOI: 10.1021/acs.langmuir.6b02704
  2. Sellers, D.G.; Button, A.A.; Nasca, J.N.; Wolfe II, Guy E.; Chauhan, S.; Watson, D.F. “Excited-State Charge Transfer within Covalently-Linked Quantum Dot Heterostructures.” J. Phys. Chem. C 2015, 119, 27737-27748. DOI:  10.1021/acs.jpccc.5b07504
  3. Kern, M.E.; Watson, D.F. “Linker-Assisted Attachment of CdSe Quantum Dots to TiO2: Time-and Concentration-Dependent Adsorption, Agglomeration, and Sensitized Photocurrent.” Langmuir 2014, 30, 13293-13300. DOI: 10.1021/la503211k
  4. Coughlin, K.M.; Nevins J.S.; Watson, D.F. “Aqueous-Phase Linker-Assisted Attachment of Cysteinate(2-)-capped CdSe Quantum Dots to TiO2 for Quantum Dot-Sensitized Solar Cells.” ACS Appl. Mater. Interfaces 2013, 5, 8649-8654. DOI: 10.1021/am402219e
  5. Kern, M.E.; Watson, D.F. “Influence of Solvation and the Persistence of Adsorbed Linkers on the Attachment of CdSe Quantum Dots to TiO2 via Linker-Assisted Assembly.” Langmuir 2012, 28, 15598-15605. DOI: 10.1021/la303504u
  6. Nevins, J.S.; Coughlin, K.C.; Watson, D.F. “Attachment of CdSe Nanoparticles to TiO2 via Aqueous Linker-Assisted Assembly: Influence of Molecular Linkers on Electronic Properties and Interfacial Electron Transfer.” ACS Appl. Mater. Interfaces 2011, 3, 4242-4253. DOI: 10.1021/am200900c
  7. Watson, D.F. “Linker-Assisted Assembly and Interfacial Electron-Transfer Reactivity of Quantum Dot-Substrate Architectures.” J. Phys. Chem. Lett. 2010, 1, 2299-2309. DOI: 10.1021/jz100571u
  8. Mann, J.R.;  Watson, D.F. “Adsorption of CdSe Nanoparticles to ThiolatedTiO2 Surfaces:  Influence of Intralayer Disulfide Formation on CdSe Surface Coverage.” Langmuir 2007, 23, 10924-10928. DOI: 10.1021/la702127t
  9. Dibbell, R.S.; Soja, G.R.; Hoth, R.M.; Watson, D.F. “Photocatalytic Patterning of Monolayers for the Site-Selective Deposition of Quantum Dots onto TiO2Surfaces.” Langmuir 2007, 23, 3432-3439. DOI: 10.1021/la063161a

 

Spectroscopic and Photoelectrochemical Characterization of Electron Transfer Reactions at Interfaces

We use time-resolved spectroscopy to probe excited-state electron transfer reactions at quantum dot(QD)-molecule-semiconductor and QD-molecule-QD interfaces. We have discovered that the efficiency of excited-state electron transfer, or electron injection, from QDs to TiO2 varies dramatically with interparticle distance, through-bridge electronic coupling, and the nature of surface-anchoring groups. We have also found that electrons are transferred from both band-edge and trap states of QDs (Fig. 3). Our findings may have implications for the design of QD-based solar cells and photocatalysts. More generally, they lend fundamental insight into the factors governing the efficiency of excited-state electron transfer between nanoparticles.

Figure 3: Transfer of electrons from band-edge and trap states of CdSe QDs, via molecular linkers, to TiO2.  (From Rivera-González, N.; Chauhan, S.; Watson, D.F. Langmuir 2016, 32, 9206-9215 (top). DOI:10.1021/acs.langmuir.6b02704 and Chauhan, S.; Watson, D.F. Phys. Chem. Chem. Phys. 2016, 18, 20466-20475 (bottom) DOI:10.1039/c6cp03813a.)

Our ongoing research is focused on fundamental studies of excited-state electron transfer reactions between nanoparticles. We are particularly interested in controlling the mechanisms and dynamics of interfacial charge-transfer processes by tuning the properties of quantum dots, molecular linkers, and semiconductor substrates.

Recent publications on electron transfer:

  1. Chauhan, S.; Watson, D.F. “Photoinduced Electron Transfer from Quantum Dots to TiO2: Elucidating the Involvement of Excitonic and Surface States.” Phys. Chem. Chem. Phys. 2016, 18, 20466-20475. DOI:10.1039/c6cp03813a
  2. Rivera-González, N.; Chauhan, S.; Watson, D.F. “Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2.” Langmuir 2016, 32, 9206-9215. DOI:10.1021/acs.langmuir.6b02704
  3. Sellers, D.G.; Button, A.A.; Nasca, J.N.; Wolfe II, Guy E.; Chauhan, S.; Watson, D.F. “Excited-State Charge Transfer within Covalently-Linked Quantum Dot Heterostructures.” J. Phys. Chem. C 2015, 119, 27737-27748. DOI: 10.1021/acs.jpccc.5b07504.
  4. Kern, M.E.; Watson, D.F. “Linker-Assisted Attachment of CdSe Quantum Dots to TiO2: Time-and Concentration-Dependent Adsorption, Agglomeration, and Sensitized Photocurrent.” Langmuir 2014, 30, 13293-13300. DOI: 10.1021/la503211k
  5. Coughlin, K.M.; Nevins J.S.; Watson, D.F. “Aqueous-Phase Linker-Assisted Attachment of Cysteinate(2-)-capped CdSe Quantum Dots to TiO2 for Quantum Dot-Sensitized Solar Cells.” ACS Appl. Mater. Interfaces 2013, 5, 8649-8654. DOI: 10.1021/am402219e
  6. Sellers, D.G.; Watson, D.F. “Probing the Energetic Distribution of Injected Electrons at Quantum Dot-Linker-TiO2 Interfaces.” J. Phys. Chem. C. 2012, 116, 19215-19224. DOI: 10.1021/jp307196z
  7. Nevins, J.S.; Coughlin, K.C.; Watson, D.F. “Attachment of CdSe Nanoparticles to TiO2 via Aqueous Linker-Assisted Assembly: Influence of Molecular Linkers on Electronic Properties and Interfacial Electron Transfer.” ACS Appl. Mater. Interfaces 2011, 3, 4242-4253. DOI: 10.1021/am200900c
  8. Dibbell, R.S.; Youker, D. G.; Watson, D.F. “Excited-State Electron Transfer from CdS Quantum Dots to TiO2 Nanoparticles via Molecular Linkers with Phenylene Bridges.” J.  Phys. Chem. C 2009, 113, 18643-18651. DOI: 10.1021/jp9079469
  9. Dibbell, R.S.; Watson, D.F. “Distance-Dependent Electron Transfer in Tethered Assemblies of CdS Quantum Dots and TiO2  Nanoparticles.” J. Phys. Chem. C 2009, 113, 3139-3149. DOI: 10.1021/jp809269m

 

Quantum Dot-Nanowire Heterostructures: Exploiting Quantum Confinement and Intercalative Mid-Gap States in Interfacial Charge Transfer

In collaboration with Prof. Sarbajit Banerjee’s research group at Texas A&M University, Prof. Louis Piper’s research group at Binghamton University, Prof. Peihong Zhang’s research group at UB, and Prof. Shengbai Zhang’s research group at RPI, we are fabricating quantum dot (QD)-nanowire (NW) heterostructures with intriguing electronic and optical properties for light-harvesting and charge transfer. To date, we have reported on the synthesis, characterization, and excited-state charge-transfer reactivity of CdSe-Pb0.33V2O5 heterostructures. The Pb0.33V2O5 NWs, which exhibit intercalative mid-gap electronic states derived from the stereoactive lone pairs of Pb2+cations, can be functionalized with CdSe QDs through linker-assisted assembly or successive ionic layer adsorption and reaction (SILAR) to yield heterostructures (Fig. 4). Transient absorption spectroscopy has revealed that photoexcitation of QDs results in the transfer of photogenerated holes from the QDs to the NWs (Fig. 5). Thus, the QD-NW heterostructures are intriguing as light harvesters and charge concentrators for photocatalysis.

Figure 4:  TEM images of an uncoated Pb0.33V2O5 NW (A) and CdSe/Pb0.33V2O5 QD/NW heterostructures prepared by linker-assisted assembly (B) and SILAR (C, D). (From Pelcher, K.E.; Milleville, C.C. et al. Chem. Mater. 2015, 27, 2468-2479. DOI: 10.1021/cm54574h)

Figure 5:  Transient absorbance spectrum of CdSe/Pb0.33V2O5 QD/NW heterostructure. (From Milleville, C.C.; Pelcher, K.E. et al. J. Phys. Chem. C 2016, 120, 5221-5232. DOI: 10.1021/acs.jpcc.6b00231)

Recent publications on the QD/NW heterostructures:

  1. Pelcher, K.E.; Milleville, C.C.; Wangoh, L.; Cho, J.; Sheng, A.; Chauhan, S.; Sfeir, M.Y.; Piper, L.F.J.; Watson, D.F.; Banerjee, S. “Programming Interfacial Energetic Offsets and Charge Transfer in β-Pb0.33V2O5/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States.” J. Phys. Chem. C 2016, 120, 28992-29001. DOI:10.1021/acs.jpcc.6b10863
  2. Milleville, C.C; Pelcher, K.E.; Sfeir, M.Y.; Banerjee, S.; Watson, D.F. “Directional Charge Transfer Mediated by Mid-Gap States: A Transient Absorption Spectroscopy Study of CdSe Quantum Dot/β-Pb0.33V2O5Heterostructures.” J. Phys. Chem. C 2016, 120, 5221-5232. DOI: 10.1021/acs.jpcc.6b00231
  3. Pelcher, K.E.; Milleville, C.C.; Wangoh, L.; Chauhan, S.; Crawley, M.R.; Marley, P.M.; Piper, L.F.J.; Watson, D.F.; Banerjee, S. “Integrating PbxV2O5 Nanowires with CdSe Quantum Dots: Towards Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer.” Chem. Mater. 2015, 27, 2468-1479. DOI: 10.1021/cm54574h

 

New Organic Dyes for Photoelectrochemical Cells and Photocatalysis

In collaboration with Prof. Michael Detty’s research group in our department, we are developing novel classes of organic sensitizers for light harvesting and excited-state charge transfer. Our goals are (1) to increase the light-harvesting efficiency of the dyes by controlling dye-surface orientation and the aggregation state of adsorbed dyes and (2) to understand the influence of dye structure, dye-surface orientation and anchoring mode, and the nature and extent of aggregation of dyes on interfacial electron-transfer reactivity. We have demonstrated that controlled H-aggregation of chalcogenorhodamine and related dyes leads to enhanced light-harvesting, increased quantum yields of electron injection into TiO2 (Fig. 6), and increased photocurrent efficiencies in dye-sensitized solar cells. We have also shown that the persistence and stability of dyes on TiO2, as well as the excited-state electron injection yields, vary greatly with the dye-surface anchoring chemistry.

Figure 6:  Transient absorption spectra reveal that the photoexcited H-aggregated dyes (a) inject electrons more efficiently than non-aggregated dyes (b) into TiO2.  (From Mulhern, K.R.; Detty, M.R.; Watson, D.F. J. Phys. Chem. C 2011, 115, 6010-6018.  DOI: 10.1021/jp111438x.)

Our ongoing research is focused on further enhancing the light-harvesting efficiencies of the dyes and understanding and controlling the electron-transfer reactivity of these and related organic sensitizers.

Recent publications on the organic dyes:

  1. Kryman, M.W.; Nasca, J.N.; Watson, D.F.; Detty, M.R. “Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance”Langmuir 2016, 32I 1521-1532. DOI: 10.1021/acs.langmuir.5b04275
  2. Sabatini, R.P.; Eckenhoff, W.T.; Orchard, A.; Liwosz, K.R.; Detty, M.R.; Watson, D.F.; McCamant, D.W.; Eisenberg, R. “From Seconds to Femtoseconds:  Solar Hydrogen Production and Transient Absorption of Chalcogenorhodamine Dyes.” J. Am. Chem. Soc. 2014, 136, 7740-7750. DOI: 10.1021/ja503053s
  3. Bedics, M.A.; Mulhern, K.R.; Watson, D.F.; Detty, M.R. “Synthesis and Photoelectrochemical Performance of Chalcogenopyrylium Monomethine Dyes Bearing Phosphonate/Phosphonic Acid Substituents.” J. Org. Chem. 2013, 78, 8885-8891. DOI: 10.1021/jo401280s
  4. Mulhern, K.M.; Detty, M.D.; Watson, D.F. “Effects of Surface-anchoring Mode and Aggregation State on Electron Injection from Chalcogenorhodamine Dyes to Titanium Dioxide.” J. Photochem. Photobiol. A: Chem. 2013, 264, 18-25. DOI: 10.1016/j.jphotochem.2013.04.028
  5. Mulhern, K.R.;  Orchard, A.; Watson, D.F.; Detty, M.R. “Influence of Surface-Attachment on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO2.” Langmuir 2012, 28, 7071-7082. DOI: 10.1021/la3000668k
  6. Mulhern, K.R.;  Detty, M.R.; Watson, D.F. “Aggregation-induced Increase of the Quantum Yield of Electron Injection from Chalcogenorhodamine Dyes to TiO2.” J. Phys. Chem. C 2011, 115, 6010-6018. DOI: 10.1021/jp111438x
  7. Mann, J.R.;  Gannon, M.K.; Fitzgibbons, T.C.; Detty, M.R.; Watson, D.F. “Optimizing the Photocurrent Efficiency of Dye-Sensitized Solar Cells through the Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline Titania Films.” J. Phys. Chem. C 2008, 112, 13057-13061. DOI: 10.1021/jp803990b