Determination of the Flat Band Potential of Nanoparticles in Porous

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Determination of the Flat Band Potential of Nanoparticles in Porous Electrodes by Blocking the Substrate-Electrolyte Contact Hendrik Naatz, Ron Hoffmann, Andreas Hartwig, Fabio La Mantia, Suman Pokhrel, and Lutz Mädler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11423 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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The Journal of Physical Chemistry

Determination of the Flat Band Potential of Nanoparticles in Porous Electrodes by Blocking the Substrate-Electrolyte Contact

Hendrik Naatz,1,2 Ron Hoffmann,3,4 Andreas Hartwig,3,4 Fabio La Mantia,5 Suman Pokhrel,1,2 Lutz Mädler1,2*

1

University of Bremen, Faculty of Production Engineering, Badgasteiner Str. 1, 28359 Bremen,

Germany; 2Leibniz Institute for Materials Engineering IWT, Badgasteiner Str. 3, 28359 Bremen, Germany; 3University of Bremen, Department 2 Biology / Chemistry, Leobener Str. 3, 28359 Bremen, Germany; 4Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), Wiener Str. 12, 28359 Bremen, Germany; 5University of Bremen, Energy Storage and Energy Conversion Systems, Bibliothekstr. 1, 28359 Bremen, Germany

Phone: +49 421 218-51200 Fax: +49 421 218-51211 E-Mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract: The determination of the flat band potential of metal oxide nanoparticles is essential to understand their electrochemical behavior in aqueous environments. The electrochemical behavior determines the possible applications and governs the environmental impact of a nanomaterial. Hence, a new electrode fabrication method is demonstrated, that allows determining the flat band potential of nanoparticles in porous nanoparticle electrodes via electrochemical impedance spectroscopy. In such electrodes, the electrolyte is in contact with the substrate material and contributes significantly to the ac response of the entire electrode. To block the substrate-electrolyte contact, the nanoparticle layers were imbibed in a liquid diacrylate monomer, followed by polymerization. To reestablish the contact between the outermost polymer-covered particles and the electrolyte an O2 plasma treatment was conducted. Based on this new electrode fabrication procedure, the flat band potential of TiO2, WO3 and Co3O4 nanoparticles in porous electrodes was determined with high precision. We believe that this new and economical method will offer an alternative to expensive ultraviolet photoelectron spectroscopy measurements at synchrotron facilities.


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INTRODUCTION A key mechanism for metal oxide nanoparticle functionality and biological response is the charge transfer between particles and liquid environment.1-2 An efficient charge separation at the particle-liquid interface depends on the magnitude of band bending, which is governed by the difference in electrochemical potentials of the electrons in the particles, i.e. Fermi-level, and the surrounding media.3-4 In biological environments, the position of the conduction band (CB) correlates with the toxicity of metal oxide nanoparticles, when it overlaps with a defined biological redox window.2,

5

In this context, the absolute energy positions of valence and

conduction band (EV and EC) and the Fermi-level (EF) are key properties that determine the particle-liquid interface.1-2, 4 The exact energy positions depend on the synthesis (e.g. high temperature synthesis, oxygen vacancies, metastable phases or phase composition) and may even be engineered via doping.6-7 Compared to ultraviolet photoelectron spectroscopy (UPS), the Mott-Schottky-Analysis is a fast and economical method to obtain information on the band structure, i.e. EC, EV and EF, of nanoparticles directly in aqueous biological environments.8 This concept allows to determine the flat band potential UFB using electrochemical impedance spectroscopy (EIS). It also offers the quantitative determination of charge transport and trapping or surface reactions in porous nanoparticle electrodes.9 However, the characterization of such electrodes via electrochemical impedance spectroscopy is different from bulk electrodes. The impedance spectrum of a bulk electrode is described by three contributions: The bulk material, the solid-liquid interface and the material-substrate contact.9 In nanoparticle electrodes, the electrolyte penetrates the porous layer having contact with the back electrode. A three-phase contact between substrate, particle layer and electrolyte is formed, which can have a large contribution to the impedance spectra.1011

The contribution of the substrate-electrolyte contact causes a shift in the flat band potential

as described for TiO2.12-13 If the porous nanoparticle layers possess a low conductivity, i.e. 3 ACS Paragon Plus Environment


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behave as dielectrics, the contribution of the nanoparticles to the overall impedance becomes rather small. Fabregat-Santiago et al.10 showed, that the capacitance contribution of TiO2 nanoparticles to the impedance in a porous nanoparticle electrode with a conductive FTO (fluorine doped tin oxide) substrate is negligible. Their results and theoretical analyses14-15 indicate that the field lines do not penetrate the nanoparticle layer much further than the first particle layer and effectively block parts of the substrate surface. In this case, the main contribution to the impedance spectra results only from the substrate in contact with the electrolyte. Hence, to determine the flat band potential of nanoparticles is challenging. The aim of this work is to perform a Mott-Schottky analysis on porous nanoparticle electrodes by implementing our new electrode fabrication procedure, which blocks the substrateelectrolyte contact. For validation three metal-oxide nanoparticle materials (n-type TiO2 and WO3 and p-type Co3O4) with a significant difference in their flat band potentials were chosen and synthesized using flame spray pyrolysis (FSP).16-17 The nanoparticles collected on a filter unit were directly transferred to a conductive FTO glass substrate via roll-to-roll lamination.18 To avoid the substrate-electrolyte contact, the porous nanoparticle electrodes were imbibed in a liquid monomer that was subsequently polymerized. A surface etching was carried out using O2 plasma to remove the outermost polymer layer and expose the particles to the electrolyte. With the blocked substrate-electrolyte contact, a determination of the flat band potential was possible for the chosen n-type and p-type nanoparticles. The data was validated through an extensive comparison with values reported in literature.


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EXPERIMENTAL SECTION Synthesis of the TiO2, WO3 and Co3O4 nanoparticles and electrode fabrication. Flame spray pyrolysis (FSP) was used to synthesize the metal oxide nanoparticles.16-17 The metalorganic precursors titanium(IV) isopropoxide (TTIP, Sigma Aldrich, 97% pure), tungsten carbonyl (Strem Chemicals, 99 % pure) and cobalt naphthenate (Strem Chemicals, 6% Co) were used for the synthesis of TiO2, WO3 and Co3O4, respectively. For the synthesis of TiO2 and Co3O4, TTIP and cobalt naphthenate were separately dissolved in xylene (Strem Chemicals, 99.95% pure) to obtain two spray solutions of 0.5 M concentration (by metal). For the synthesis of WO3, tungsten hexacarbonyl was dissolved in tetrahydrofuran (THF, VWR Chemicals, 99.9 % pure) to obtain a 0.1 M concentration (by metal) of the precursor solution. The liquid precursors were fed into a nozzle at a constant flow rate of 5 mL/min using a syringe pump (KD Scientific, KDS-100-CF) and an O2 flow rate of 5 L/min with a constant pressure drop of 1.5 bar at the nozzle to disperse the liquid precursors.19 A support flame consisting of premixed gases CH4 (1.5 L/min) and O2 (3.2 L/min) ignites the spray. The synthesized nanoparticles were collected on a glass fiber filter unit (Pall, Type A/E, 257 mm diameter) placed 60 cm above the nozzle.17, 20-21 Following the synthesis, a direct transfer of the particles from the collecting unit to the substrates was performed via low pressure roll-to-roll lamination (Hot Roll Laminator HL-101, Cheminstruments) resulting in highly porous nanoparticle electrodes.18 The nanoparticle layers deposited on glass fiber filters were placed on top of conductive FTO glass slides (7 Ω cm-2, Sigma Aldrich) and compressed during lamination at a pressure of 3.2 MPa, followed by a heat treatment of the electrodes at 300 °C for 4 h to enhance the particle-substrate contact. After lamination the layers were mechanically stabilized and the filter was removed with pressurized air.



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To avoid the undesired contact of the electrolyte with the substrates (back electrodes) during electrochemical impedance measurements, the porous layers were infiltrated with a 1,6hexanediol diacrylate monomer (Laromer® HDDA, BASF) containing 1 wt. % of UV photoinitiator (Omnirad 819, IGM Resins). The infiltration without particle detachment was possible due to the compression of the layers with an appropriate lamination pressure. Subsequently, the infiltrated layers were illuminated with a UV light source (UVACUBE inert, Dr. Hönle AG) for 90 s in a carbon dioxide inert atmosphere to polymerize the monomer. Finally, the polymer coating on the outermost nanoparticles was removed via O2 plasma treatment. The plasma source was an atmospheric pressure plasma jet (RD1004 powered by a generator type FG5001 and transformator HTR12 / Plasmatreat, Steinhagen, Germany) which runs at a pulse-pause-ratio of 100%, frequency of 19 kHz and voltage of 300 V. The O2 process gas was feed through the nozzle with a flow rate of 33 L/min placed at a distance of 6 mm from the electrode surface. The process speed was 2 m/min with 30 treatments at a line distance (meandering sample treatment) of 20 mm. The plasma parameters were examined by exposing first particles on the surface from the polymer matrix. 22 Particle characterization. Structural parameters and morphology of the TiO2, WO3 and Co3O4 nanoparticles were determined from XRD, BET and HRTEM measurements/imaging. A diffractometer (D8 Advance, Bruker) equipped with Ni-filtered Cu Kα (λ = 0.154 nm) radiation was used for the XRD measurements. The diffraction patterns were recorded in the 2θ range from 20° to 80° with a step width of 0.016° and a measurement time of 30 s per step. Prior to the BET measurements with a gas sorption system (NOVA 4000e, Quantachrome Instruments), all samples were degassed at 200 °C in vacuum for 4 h to clean the surface. The BET measurements (5-point adsorption isotherm) were carried out at 77 K (liquid nitrogen) to determine the specific surface areas SA of the samples. The BET diameter, dBET, was calculated from the specific surface area SA (m²/kg) and the theoretical density ρ (kg/m3) of the samples 6 ACS Paragon Plus Environment

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as dBET = 6/(ρ·SA).23-24 High resolution TEM images were recorded using a transmission electron microscope (TEM, FEI Titan 80/300) operated with an acceleration voltage of 300 kV. For the sample preparation, a carbon-coated copper grid was immersed in a suspension containing ∼1 mg of nanoparticles mixed with 5 mL ethanol (Strem Chemicals, AR grade) followed by drying at room temperature. UV-Vis diffuse reflection spectra were recorded with a photospectrometer (UV-2600, Shimadzu) equipped with an integrating sphere using BaSO4 as background material. A Kubelka-Munk transformation was conducted with the acquired data to determine the band gap of the nanoparticles. Morphology of the electrodes. The morphology of the electrodes was investigated with scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The top views and the cross sections of the electrodes were recorded after each process step. To obtain such cross sections, the FTO glass was scratched from the back side with a glass cutter to gain a sharp breaking edge. The SEM images were recorded with a scanning electron microscope (Leo 1530, Gemini) with an acceleration voltage of 10 kV. The chemical compositions of the electrodes were evaluated using EDX (X-Flash 6/30, Bruker and a detector area of 30 mm²). To avoid charging, the electrodes were sputtered with gold using a sputter coater (K550, EMITECH) for 40 s at a current of 20 mA prior to scanning. Electrochemical characterization. A three-electrode electrochemical cell25 was used to characterize the nanoparticle layers via electrochemical impedance spectroscopy (EIS). The cell consists of the porous nanoparticle working electrode (8 cm²), a Pt-coated grid as counter electrode and an Ag/AgCl (3 M KCl, SI Analytics) reference electrode. The working electrodes were fabricated as described earlier. The impedance spectra were recorded on a potentiostat (VMP300, Biologic with the Software EC-Lab®) between 0.1 Hz and 1 MHz with an amplitude of 25 mV in a 0.5 M Na2SO4 electrolyte (pH 6.7). For each applied potential an equilibration time of 10 minutes was applied to reach a steady state current before the impedance 7 ACS Paragon Plus Environment


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measurement. The Mott-Schottky analysis was performed in a frequency range of 4 to 45 kHz at room temperature. RESULTS AND DISCUSSION Particle characterization. The TiO2, WO3 and Co3O4 nanoparticles were analyzed with BET, UV-Vis, HRTEM and XRD. Figure 1A shows HRTEM images of the nanoparticles. The BET diameters are 12.5, 12.3 and 11.3 nm for TiO2, WO3 and Co3O4, respectively and reasonably agree with the particle sizes observed in the TEM measurements. Figure 1B shows the XRD patterns for the particles. To exclude an influence arising from the O2 plasma treatment on the particle crystal structure, XRD patterns were recorded before and after the treatment without showing detectable differences. Band gaps of 3.1, 2.71 eV for TiO2 and WO3 nanoparticles were determined from their UV-Vis diffuse reflectance using Tauc plots (insets in Figure 1C) assuming an indirect band gap (n = 0.5).26 For Co3O4 two direct transitions (n = 2) of 1.43 and 1.84 eV were determined according to the DASF method. 27

Electrode characterization. The electrode fabrication procedure consists of three steps as shown in Figure 2A-C: (A) Dry printing: Particle synthesis using FSP followed by direct transfer of the particles on the conductive substrate (back electrode) using lamination. (B) Monomer imbibition and polymerization: Infiltration of the nanoparticle network with a liquid monomer followed by polymerization to avoid the substrate-electrolyte interface. (C) Surface exposure: Plasma etching to expose the upper part of the nanoparticle layer and form a nanoparticle-electrolyte contact.


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Figure 1. (A) TEM images, (B) XRD patterns and (C) UV-Vis diffuse reflectance spectra of the TiO2, WO3 and Co3O4 nanoparticles forming the electrodes. The crystal structure was preserved after the plasma etching process used for the electrode fabrication. The optical band gaps were determined from the Tauc plot shown in the inset of (C).26

The SEM images in Figure 2A-C show the cross sections (middle) and top views (bottom) of a TiO2 electrode after each process step: (A) During dry printing the nanoparticles are transferred to conductive substrates (back electrode, i.e. FTO glass) via a lamination process. After FSP synthesis, the particles are accumulated on glass fiber filter. The particle layer together with the filter is placed on top of the FTO glass and mechanically compressed at low pressures, i.e. 3.2 MPa. Subsequently the filter is removed using pressurized air. Electrodes with homogenous and highly porous (porosity of ~80 %

18

) layers are formed, which are stable against liquid

imbibition. These electrodes preserve the properties of the particles, but the electrolyte would have contact with the substrate surface through the porous layer (Figure 2A). 9 ACS Paragon Plus Environment


(A)

(B)

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(C)

Figure 2. Schematic representation (top), SEM images of the cross sections (middle) and top views (bottom) after each of the three electrode fabrication steps for a TiO2 electrode. (A) Dry printing – shows the electrode after particle transfer to the substrate. The substrate-electrolyte contact contributes to the impedance spectra due to the porous structure, (B) Monomer imbibition and polymerization – the porous structure is filled with a liquid monomer containing a UV-initiator. After polymerization the substrate-electrolyte contact is blocked, leaving also the surface particles covered. (C) Surface exposure – to enable the nanoparticleelectrolyte contact, an O2 plasma treatment was performed.

(B) After imbibition with a monomer and a subsequent polymerization, the layer is filled up with polymer. Although the substrate-electrolyte contact is avoided, the required particle surface is covered as well, hence an O2 plasma treatment was necessary. (C) After the treatment, the particles at the surface were exposed for efficient particle-electrolyte contact as required for electrochemical impedance measurements. The pores leading to the conductive substrate remain filled and block th substrate-electrolyte contact. The cross sectional SEM images (Figure 2B, middle) show that the surface and larger pores are coated/filled with polymer and that the first layer of the nanoparticles on the surface is exposed only after the O2 plasma treatment (Figure 2C, middle). To demonstrate the complete filling of 10 ACS Paragon Plus Environment

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the nanoporous structure with polymer, EDX analysis was conducted at different spots within the electrode layers. Figure 3A shows EDX spectra for the pure polymer and the polymerized TiO2 and WO3 electrodes. Figure 3B and C show the analyzed areas on the sample crosssections. The quantitative analysis (see Table S1) showed carbon content of 80 at. % for the pure polymer. The carbon concentrations are 46 at. % and 51 at. % for the TiO2 + polymer and WO3 + polymer composite layers, respectively. The carbon content in particle layers without polymer is one order of magnitude smaller. The results evidently show the presence of the polymer within the porous structure of the nanoparticle network. The cross sections and top views for WO3 are presented in Figure S1.

Figure 3. (A) EDX analysis of the polymer infiltrated WO3 and TiO2 nanoparticle layers. The high carbon content (46 and 51 at. % for TiO2 and WO3, respectively) within the nanoparticle network indicates the presence of polymer throughout the layer. (B) and (C) cross sectional areas of the WO3 and TiO2 layers selected for the EDX analysis. Gold was used to increase the conductivity and prevent charging during SEM imaging.

The characterization of the electrodes with SEM and EDX indicates the successful blocking of the substrate-electrolyte contact with the three-step electrode fabrication. The use of these electrodes for the determination of the flat band potential UFB of nanoparticles in porous electrodes is described in the next section.



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Electrochemical characterization of porous electrodes with substrate-electrolyte contact. In agreement with Fabregat-Santiago et al.10, who used commercial TiO2 electrodes (P25, Evonik Degussa) for impedance measurements, the nanoparticle contribution to the impedance was negligible for the FSP synthesized TiO2 nanoparticles. A comparison of the impedance spectra (0.1 Hz to 1 MHz) for a planar FTO electrode with a TiO2-coated FTO electrode is shown in the Nyquist plot in Figure 4A exemplarily at an applied voltage of -200 mV vs. Ag/AgCl. The impedance spectrum for the pure FTO electrode shows a very similar behavior compared to the TiO2-coated FTO electrode in the recorded potential range. Both spectra resemble a semi-circle with a small offset from the origin, which is well described by a classic Randles circuit (see inset of Figure 4A). The circuit comprises the electrolyte resistance REl in series with a charge transfer resistance RCT,FTO at the electrode surface. The charge transfer resistance is in parallel with a capacitive contribution CFTO resulting from the electrochemical double layer at the electrolyte-FTO glass interface. This equivalent circuit is adequate to describe simple planar electrodes.28 Compared to the pure FTO glass, the charge transfer resistance RCT,FTO of the TiO2-coated FTO electrode is higher due to partial blocking of the FTO glass by the TiO2 nanoparticles.


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Figure 4. (A) Nyquist plot for a pure FTO electrode (FTO) and a TiO2-coated FTO electrode (TiO2@FTO) recorded from 0.1 Hz to 1 MHz at an exemplarily applied voltage of -200 mV vs. Ag/AgCl, (B) schematic of the porous nanoparticle electrode with two paths contributing to the impedance spectra and (C) simplified equivalent circuit representing the nanoparticle layer by a transmission line according to Bisquert et al.9. If the nanoparticle resistance rNP is much higher than the resistance of the electrolyte in the porous structure, the impedance spectra are dominated by the substrate-electrolyte contact. The particles block the substrate and the effective electrode surface area is reduced increasing the charge transfer resistance RCT,FTO.

If the internal nanoparticle resistance rNP is much higher than the electrolyte resistance (see equivalent circuit in Figure 4C), the particle contribution becomes negligible. Thus, the effective contact area of the FTO with the electrolyte is reduced,10 especially at higher frequencies, where the contribution from the nanoparticles ceases. For the polymerized TiO2 electrode (TiO2+polymer@FTO) a significantly increased charge transfer resistance is observed, which must result from the contribution at the particle-electrolyte interface rCT. The larger offset of the first intersection point with the x-axis is due likely due to the higher resistance of the particle film, compared to the electrolyte resistance in the porous structure.

Electrochemical characterization of porous electrodes with blocked substrate-electrolyte contact. To determine the flat band potential UFB of TiO2, WO3 and Co3O4 nanoparticles, a Mott-Schottky analysis was performed for the nanoparticle-polymer electrodes. According to



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the Mott-Schottky8 relation (see Equation 1), the space-charge capacitance C varies as a function of the applied voltage U: 1

C

2

=

2

εr ε0 e A²

U − UFB −

kB T , e

(1)

where ND is the majority carrier density, e is the electron charge and εr and ε0 are the dielectric constant of the material and the vacuum permittivity, respectively. The surface area of the electrode is given by A, kB is the Boltzmann constant and T the absolute temperature. The intersection point of a Mott-Schottky plot (1/C² vs. U) with the x-axis gives the flat band potential UFB, including the kB T/e term (≈ 25 mV at 25 °C). The slope contains information on the charge carrier density ND and allows calculating the difference between conduction band and Fermi-level for n-type semiconductors: ∆EF = EC − EF = kB T ln

ND , NV

with the effective density of states in the conduction band NV = 2

2π m* kB T h2

,

(2)

where m∗ is the effective mass:

/

.

(3)

A calculation of the electron affinity gives the position of the conduction band EC with respect to the vacuum level: = UFB e + 4.64 eV + ∆EF + UH e.

(4)

The term 4.64 eV is the conversion factor from Ag/AgCl to the vacuum scale and UH = 0.059

V pH

· (pHPZZP – pH) is the potential drop across the Helmholtz layer.8

To obtain the space-charge capacitance C in dependence on the applied voltage U, electrochemical impedance spectroscopy is used. The capacitance is either determined by a fit of an appropriate equivalent circuit to the entire impedance spectrum or by extracting it from the imaginary part Im(Z) of the impedance at a certain frequency ω, according to Equation 5: 14 ACS Paragon Plus Environment

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Im Z =

-1 . jωC

(5)

The electrochemical impedance spectroscopy (EIS) measurements performed for the porous TiO2, WO3 and Co3O4 nanoparticle layers are presented in Figure 5A-C, respectively. In contrast to the spectrum of the TiO2@FTO electrode in Figure 4A, they show completely different impedance characteristic. All spectra consist of at least two to three distinct contributions resulting from the particle-electrolyte interface, the particle-polymer composite or the particle-substrate junction. Independent of assigning the different contributions to an appropriate equivalent circuit, a MottSchottky analysis allows determining the flat band potential of the particles through extraction of the capacitance from the imaginary part Im(Z) (see Equation 5). For the analysis of each material, three frequencies were chosen such that the high frequency region (Figure 5A-C) is primarily reflected. The Mott-Schottky plots in Figure 5D-F show that the intersection points with the x-axes are independent of the frequency giving reproducible flat band potentials of -0.56, -0.15 and 0.72 V vs. Ag/AgCl (pH 6.7) for TiO2, WO3 and Co3O4, respectively. The charge carrier density ND can be derived from the slope of this plot. However, additional information of ND is omitted since it would bear an unknown error related to the exposed surface in Equation 1. For comparison, Figure S2 shows the Mott-Schottky plots of TiO2 electrodes without polymer. At low frequencies (0.1 – 1 kHz, Figure S2 A) a Mott-Schottky behavior is observed, where UFB shifts towards more negative potentials with increasing frequencies. At higher frequencies (20-45 kHz) used for the polymerized electrodes (Figure S2 B) an intersection point giving the flat band potential is missing in the recorded potential range.



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Figure 5. (A) to (C) Nyquist plots of polymer infiltrated and plasma-treated TiO2, WO3 and Co3O4 polymer composite electrodes at different applied potentials. The impedance spectra allow to calculate the capacitance at different frequencies according to Equation 5. (D) to (F) Mott-Schottky plots obtained at frequencies between 4 and 45 kHz, from which reproducible flat band potentials are deduced.

Theory of amorphous semiconductor junctions to describe the ac voltage response of the nanoparticle electrodes. From Figure 5D-F it is obvious that the Mott-Schottky plot deviates from linearity with increasing distance of the applied potential to the flat band potential, i.e. increasing space charge layer (band bending). At the same time, a frequency dispersion of the capacitance is observed. By describing the density of states (DOS) distribution of the nanoparticle electrodes with the theory of amorphous semiconductor junctions (a-SC) and simulating the ac voltage response of the system both effects are explained. The approach is based on the original work of Losee

29

and the equations used for the numerical solution are

described in detail elsewhere. 30-32 While the Mott-Schottky theory enables a fast determination of the flat band potential, the theory of amorphous semiconductor junctions presents a more realistic view of the nanoparticle 16 ACS Paragon Plus Environment

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electrode, since it includes the description of trap levels and defects within the band gap. In principle, this approach allows to determine the DOS distribution in the vicinity of CB for ntype and VB for p-type semiconductors, which dominate the electronic properties of the particles. A fit of this theory to the experimental data is presented in Figure S3. Qualitatively, the shape of the Mott-Schottky plots for all three materials is described by a DOS composed of an exponential tail function reaching into the band gap and a deep level state, described with a Gaussian distribution (see Figure S4), similar to the description of an amorphous semiconductor. While the states described with the exponential tail function are responsible for the frequency dispersion of the capacitance, the deep level states cause the flattening of the Mott-Schottky curves. The flat band potentials of -0.54, -0.05 and 0.63 V vs. Ag/AgCl are in good agreement with the potentials obtained according to Equation 1. Thus, the theory reasonably describes the impedance behavior of the nanoparticle electrodes and provides valuable information about the DOS distribution, including oxygen vacancies, in nanoparticle electrodes. However, the simple Mott-Schottky approach (Equation 1) enables a fast and precise determination of the flat band potential of nanoparticles using the novel electrode fabrication procedure. These measurements were compared with values reported in literature, converting the potentials from Ag/AgCl to the reversible hydrogen electrode (RHE) using: URHE = UAg/AgCl + U 0Ag/AgCl + 0.059

V · pH , pH

(6)

with U 0Ag/AgCl = 0.197 V at 25 °C.33 A comparison with literature values is provided in Table S2 and shown (sorted by value) in Figure 6. For both n-type nanomaterials (TiO2 and WO3), the UFB calculated in the present work fall within the range of the data reported in literature. While the flat band potential of n-type materials such as TiO2 and WO3 is expected to be close to the CB34-35, the CB position of TiO2 is close to the H2/H+ redox potential at -4.5 eV on the vacuum scale. For WO3, the position of the CB is known to be below the one of TiO2. However, 17 ACS Paragon Plus Environment


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Figure 6 shows a large variation in the flat band potentials for each material emphasizing the importance of a simple measurement system for nanomaterials. For TiO2 there are two crystal phases, anatase and rutile, which need to be considered at room temperature, but are rarely reported together with the flat band potential. The theoretical band positions of the VB and CB of anatase and rutile are shown in the inset in Figure 6, with band gap energies 3.23 and 3.02 eV, respectively.36-37 From the difference of the band gap energies, the CB of anatase lies 0.21 eV above the CB of rutile, provided that the O2p orbital forming the VB is considered the same for both phases.38 Kalyanasundaram and Grätzel39 reported a comparable difference of 0.2 V in the flat band potentials of anatase (-0.4 V vs. SCE, pH = 0) and rutile (-0.2 V vs. SCE, pH = 0). This difference is observed in the literature review for TiO2 in Figure 6 as well. We would like to note, that the band alignment between anatase and rutile corresponding to the direction of charge transfer is controversially discussed in literature

40

. X-ray photoelectron

spectroscopy (XPS) measurements indicate a higher position of the CB for rutile and therefore an electron transfer to anatase. 41-43 However, through XPS the VB maximum (EV) is obtained at the isoelectric point of the material, while the Mott-Schottky analysis determines EF and the distance to EC may be different from anatase to rutile. While the flat band potential of the FSP synthesized TiO2 nanoparticles (75 wt. % anatase and 25 wt. % rutile) lies well within this range, a few flat band potential values in literature are at lower energies. Such a shift occurs if the material possesses less defects, is doped or the charge carrier concentration is altered, e.g. through light/UV illumination. In fact, the most negative flat band potentials for TiO2 in Figure 6 were reported for electrodes under illumination. The characteristics of WO3 are quite similar to TiO2. The upper limit of the flat band potential is close to the CB. Since the flame spray pyrolysis is a high temperature synthesis method, there is an excess of oxygen vacancies which moves the Fermi-level of n-type materials closer to the


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CB compared to the intrinsic case.34-35 In both cases, for TiO2 and WO3, the flat band potentials for the FSP synthesized materials are close to the expected upper limit. For Co3O4 only a few flat band potentials have been reported varying in a range of almost 1 eV. To validate our flat band potential data, energy positions obtained from ultraviolet photoelectron spectroscopy (UPS) are considered besides the values obtained from electrochemical methods. The UPS measurements (see Table S2) are in good agreement to the electrochemically determined flat band potential in this work.

Figure 6. Comparison of the measured flat band potentials UFB for TiO2, WO3 and Co3O4 with values reported in literature7, 44-73 (sorted by value). The inset shows the theoretical band positions of anatase and rutile. The conduction band position of anatase is 0.21 eV above the one of rutile. This agrees well with the difference of 0.2 V for the flat band potential reported for anatase and rutile in literature.39

Estimation of energy positions. The conduction bands of TiO2 and WO3 are composed of Ti3d and W5d orbitals, respectively. For transition metal oxide semiconductors with empty d orbitals, the valence band position EV is primarily determined by the O2p orbital at about 3 V vs. RHE.38 The measured band gap allows to determine the position of the conduction band EC 19 ACS Paragon Plus Environment


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for both materials. Equation 4 relates the Fermi-level EF to the flat band potential UFB. The calculated energy positions of TiO2 and WO3 are displayed in Figure 7. As expected for both n-type materials, the Fermi-level is close to their CB with ∆EF estimated to be 100 mV and 200 mV for TiO2 and WO3, respectively.

Figure 7. Estimated Fermi-level EF, conduction band EC and valence band EV for TiO2 and WO3. The Fermi-levels of the FSP synthesized nanoparticles were calculated from the measured flat band potentials. As expected for n-type materials with oxygen vacancies, the Fermi-levels are close to the conduction band.

The band gap of Co3O4 originates from the d6 and d7 orbitals of the tetrahedrally coordinated Co2+ and octahedrally coordinated Co3+ ions in the spinel structure.71 The two transitions observed in the UV-Vis spectrum (Figure 1A) result from the d-orbital crystal field splitting and are assigned to transitions between Co3+ t2g → Co2+ t2g (1.43 eV) and Co3+ t2g → Co2+ eg (1.84 eV). 74-76 To assign an absolute energy value, we chose the EV determined through UPS in our previous work.7 A distance of 400 mV is obtained for the Fermi-level from the Co3+ t2g state. This is in very good agreement with a fitting of ∆EF to the Mott-Schottky plot according to the a-SC theory described in the previous section (see Figure S3). 20 ACS Paragon Plus Environment

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CONCLUSIONS The energy positions of metal oxide nanoparticles are key to various applications including their biological response assessment. A common way to derive energy positions is through UPS measurements under ultra-high vacuum conditions. A simple and more economical alternative to extract these properties directly in aqueous biological environment is through electrochemical impedance spectroscopy. A new electrode fabrication method was established, which enables the characterization of nanoparticles in porous electrodes via electrochemical impedance spectroscopy. The method consists of three steps that avoid the substrate-electrolyte contact, which would otherwise dominate the impedance measurements. With this novel electrode fabrication, we were able to determine the flat band potentials UFB of the exemplarily chosen TiO2, WO3 and Co3O4 nanoparticles with high precision. An intensive comparison to literature shows that the method is suitable to determine the flat band potentials of nanoparticles in porous electrodes and may even enable a detailed investigation of porous structures with a low conductivity via electrochemical impedance spectroscopy. It was demonstrated that the aSC theory for amorphous semiconductors qualitatively describes the ac impedance response of the porous metal oxide nanoparticle layers. With this theory applied to nanoparticles, the presence of defects and trap levels within the band gap is described. A comparison with values for the Fermi-level from UPS measurements makes this technique an economically viable alternative to determine energy positions of nanoparticles.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXXX/acs.jpcc. . Elemental composition of the cross sections of the TiO2 and WO3 polymerized electrodes from EDX analysis, SEM images showing the cross sections and top views of a WO3 electrode, MottSchottky plots of a TiO2 electrode without polymer, summary of the literature review on the flat band potentials of TiO2, WO3 and Co3O4 and fittings of the a-SC theory to experimental data (PDF).

ACKNOWLEDGEMENTS We would like to thank the Deutsche Forschungsgemeinschaft (DFG) for funding this work under grants of MA3333/16-1 and HA2420/16-1. SP and LM would like to thank for the support provided by the US Public Health Service Grant, RO1 ES016746. The work also leveraged the infrastructure that is supported by the National Science Foundation and the Environmental Protection Agency under Cooperative Agreement Number DBI 0830117 and 1266377. Finally, the authors would like to thank Dr. Thomas Lukasczyk, Dr. Christoph Regula and Annika Stalling, Fraunhofer-Institute for Manufacturing Technology and Advanced Materials (IFAM), Bremen, Germany for the plasma treatment.

CONFLICT OF INTERESTS There are no conflicts of interest to declare.


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TOC Graphic


Determination of the Flat Band Potential of Nanoparticles in Porous

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