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C: Energy Conversion and Storage; Energy and Charge Transport
Revealing the Relative Electronic Landscape of Colloidal ZnO and TiO Nanoparticles via Equilibration Studies 2
Janelle Castillo-Lora, Ryoji Mitsuhashi, and James M. Mayer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00829 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019
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Revealing the Relative Electronic Landscape of Colloidal ZnO and TiO2 Nanoparticles via Equilibration Studies Janelle Castillo-Lora,à Ryoji Mitsuhashi,à, and James M. Mayer*,à à
Department of Chemistry, Yale University, 255 Prospect St, New Haven CT 06520-8107 USA
[email protected] Current address; Department of Applied Chemistry for Environment, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337 Japan
ABSTRACT: Interest in metal oxide semiconductors for energy processes has increased due to their prominent roles in photocatalysis, electrical energy storage, and conversion. However, an understanding of the thermochemistry of electron transfer (ET) reactions of these systems has lagged behind photophysical studies. This report investigates ET equilibria between reduced forms of well-characterized, ligated ZnO and TiO2 nanoparticles (NPs) suspended in toluene. Multiple electrons were added to each type of NP, either photochemically or with a chemical reductant. Equilibration experiments monitoring these added electrons are used to construct a qualitative band diagram. Surprisingly, the difference between the ÔreducibleÕ oxide TiO2 and the formally Ônon-reducibleÕ ZnO are reflected not in the relative band energies but rather in the relative width of the bands (the density of trap and/or band states). Moreover, the position of the electron equilibrium shifts upon addition of excess dodecylamine or oleic acid capping ligands. The directions of the equilibrium shifts suggest that they are due to the acid/base or hydrogen bond donor/acceptor properties of capping ligands. This suggests a coupling of protons with the electron transfers in these systems. These findings provide a more nuanced and detailed picture of ET thermodynamic landscapes at nanoparticles than what is provided in a typical nanoparticle band energy scheme. Aspects of this understanding could be valuable for the use of nanoscale oxides in energy technologies.
Introduction The study and use of metal oxide nanomaterials, such as TiO2 and ZnO, have steadily grown over the last several decades. Electron transfer (ET) across the surfacesolution interface is central to many of their applications. These include energy storage, solar energy conversion to electricity and fuels, self-cleaning surfaces, and catalysis.1-4 While the spectroscopic properties of charge carriers in these materials have been studied in detail, much less is known about the thermochemistry of their ET processes. Greater understanding of the energetics of interfacial redox processes would be broadly valuable as electron transfer is central to many semiconductor applications. A number of studies have examined the energies of the valence and conduction band edges of oxide semiconductors, EVB and ECB.2-16 These typically examined ET across an oxide/aqueous solution interface, and usually found a pH dependence of EVB and ECB of ca. 60 mV per pH unit. However, while the band gap energies are well known, there is significant variation in the reported EVB and ECB between these studies. A compendium of band energies for TiO2 by Finklea shows a spread
of ±0.5 V in the reported ECB at each pH.6 For example, the detailed study of Enright and Fitzmaurice on both ZnO and TiO2 electrodes gave a ~0.2 V lower potential for TiO2,10 but the classic report by GrŠtzel shows them as being at the same energies.4 Studies of colloidal oxide nanoparticles (NPs) provide a direct way to look at the energetics of nanocrystal redox reactivity. Colloidal NPs are in some ways simpler than porous or dense thin films, for which diffusion of electrons, intercalants and electrolyte can be complicating factors.10, 17-19 While direct measurements of reduction potentials for colloidal NPs are rare, a notable example is the direct electrochemical study of aqueous IrO2 by Royce Murray and coworkers.20 Gamelin et al. have recently developed an elegant potentiometric approach to examine the thermochemistry of non-aqueous ZnO and other colloidal NPs.12-13 Equilibration between NPs has been shown to be a sensitive probe of relative energetics, between different materials, different sizes of the same NPs, different doping levels and other aspects of band engineering.21-23 The electronic energy landscape of colloidal NPs is often depicted with simple, yet peculiar schematics such
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slopes of the titration data (i.e. !600nm = 700 ± 70 for eÐ/TiO2 and !850nm =1000 ± 100 for eÐ/ZnO) are independent of the concentration of NPs and agree with previously published results.40 The availability of molar extinction coefficients for each of the NPs at different wavelengths allowed determination of the concentration or number of electrons on both NPs, for both chemically and photochemically reduced samples (SI Equation 1). Dividing this by the NP concentration afforded the number of electrons per NP. Photochemical reduction yielded 150 eÐ per TiO2 NP, 5 eÐ per large 5.6 nm ZnO NP, and 3 eÐ per small 3.7 nm ZnO NP. The ±10% uncertainty in the molar extinction coefficient values is the primary uncertainty in these studies and applies to all values reported here. Chemical Reduction Experiments. Chemical reduction studies were typically performed by titrating the outer-sphere chemical reductant, decamethylcobaltocene (CoCp*2, E1/2 = Ð1.9 V vs. Fc/Fc+ in THF) into a solution of diluted uncharged NPs.35 Chemical reduction involved the formation of eÐ/MOx with a tightly ion-paired CoCp*2 since reactions were performed in low-polarity toluene. NP reduction resulted in the growth of the same optical features as photochemical reduction, and maximum reduction was indicated by both peak formation at 350-400 nm attributable to CoCp*2 and a lack of optical change in the visible region (Figure S2). Typically, 0.01 mM ZnO was treated with excess reductant resulting in 1-3 eÑ per ZnO, depending on size. A solution with 0.01 mM TiO2 was also treated with excess CoCp*2 and was reduced to 90 eÑ/TiO2. Interparticle Electron Equilibration Experiments. Studies of ET between ZnO and TiO2 nanoparticles were performed and monitored primarily by optical and EPR spectroscopies. In a typical UV-Vis study, a solution containing 0.2 mM eÐ on 0.002 mM of TiO2 NPs (100 eÐ /TiO2) was titrated with 0.02 mM of uncharged 5.6 nm ZnO NPs, corresponding to a 10:1 ratio of ZnO : TiO2 NP (excess ZnO stoichiometry explained below), and monitored in a quartz cuvette at room temperature. Changes in the optical spectrum were monitored and quantified to determine the number of electrons on either ZnO or TiO2 at equilibrium. This protocol is similarly performed when analyzing how capping ligands perturb equilibrium. ET reactions were also monitored by EPR spectroscopy and performed similarly. Typically, a solution composing 0.5 mM eÐ on 0.005 mM of TiO2 NPs (100 eÐ /TiO2) were treated with 0.1 mM ZnO NPs. Samples were flash frozen at various time points and measured at 7K (Figure S3). Analysis to determine whether electrons had completely or partially transferred from TiO2 re-
quired fitting the EPR spectrum as either a single (electrons on ZnO only) or a multi-component system (electrons on TiO2 and ZnO) (Figure S4). Table 1. Differences in reduction level based on charging method and NP profilea
NP
eÐ/NP Carrier (h!) densityb
eÐ/M+
eÐ/NP eÐ/NP * from CoCp 2 from TiO2
TiO2
150
1021
8%
90
N/A
ZnO 5.6 nm
5
1019
0.08%
3
7
ZnO 3.7 nm
3
1019
0.07%
1
4-5
a
Charging by UV irradiation in the presence of ethanol (h!), or by electron transfer from CoCp*2 or eÐ/NP. b Carrier density in eÐ/cm3.
Results I.!Approach to quantifying electrons in nanoparticle containing reaction mixtures. Different reduction methods of the two types of nanoparticles led to a range of maximum number of electrons per nanoparticle (Table 1). These experiments involved colloidal ZnO and TiO2 NPs that are roughly the same size (d = 3.7 Ð 4.5 nm) with similar extinction coefficients per electron from 400-900 nm. In contrast to these similarities, a TiO2 NP is able to hold 30-50 times more electrons than a ZnO NP of equivalent size. Responding to this large difference in eÑ/NP, the equilibration experiments described below were done with much higher concentrations of ZnO NPs than TiO2 NPs (concentration = moles of NPs/L). This allowed the concentration of electrons on TiO2 NPs (moles of eÐ/L) to be similar to the electron concentrations on ZnO. The interparticle experiments below are typically described with the metric of percent electrons on TiO2 or ZnO (%eÐTiO2/ZnO). This is an accessible way to describe where electrons have transferred and equilibrated. The electron per NP (eÐ/NP) notation is also used and provides a more nuanced view of the reduction of each nanoparticle. In a hypothetical example, 150 eÐ can be held in a single TiO2 NP, whereas dozens of ZnO NPs would be needed. Ultimately, the experiments below reveal the reasons why the maximum number of electrons is different between ZnO and TiO2 and how the respective NP reactivity is impacted.
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all on ZnO, all on TiO2, or distributed between both NPs (Figure 1C). Monitoring the equilibration of electrons upon addition of uncharged ZnO to eÐ/TiO2 showed substantial ET from TiO2 to ZnO, similar to Figure 1B. The reverse direction (uncharged TiO2 to eÐ/ZnO) did not result in a dramatic change. Mixing eÐ/ZnO and eÐ/TiO2 with the same total electron concentration gave the same equilibrium distribution: 75% of the electrons were on ZnO (3.5 eÐ/ZnO) and 25% were on TiO2 (~21 eÐ/TiO2) (
