Correlating Changes in Electron Lifetime and ... - ACS Publications

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The Journal of Physical Chemistry

Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-modified TiO2 Aerogels Paul A. DeSario,1 Jeremy J. Pietron,1* Dereje H. Taffa,2† Ryan Compton,3 Stefan Schünemann,2 Roland Marschall,2‡ Todd H. Brintlinger,4 Rhonda M. Stroud,4 Michael Wark,2†* Jeffrey C. Owrutsky,3 Debra R. Rolison1 1

Surface Chemistry Branch (Code 6170), U. S. Naval Research Laboratory, Washington D.C. 20375, USA Laboratory for Industrial Chemistry, Ruhr-University Bochum, Bochum, Germany 3 Chemical Dynamics and Diagnostics Branch (Code 6110), U. S. Naval Research Laboratory, Washington D.C. 20375, USA 4 Materials and Sensors Branch (Code 6360), U. S. Naval Research Laboratory, Washington D.C. 20375, USA 2

†

Present address: Institute of Chemistry, Carl-von-Ossietzky University Oldenburg, Oldenburg, Germany Present address: Institute of Physical Chemistry, Justus-Liebig-University Giessen, Giessen, Germany *[email protected] *[email protected]

‡

Abstract: We use intensity-modulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) to characterize carrier dynamics in titania (TiO2) aerogels under photocatalytic conditions. By systematically increasing the weight fraction of the sol–gel precursor during TiO2 sol–gel synthesis, we are able to impart drastic changes in carrier transport/trapping and improve the photocatalytic activity of TiO2 aerogels for two mechanistically divergent photochemical reactions—reductive water splitting (H2 generation) and oxidative degradation of dichloroacetate (DCA). The lifetimes of photogenerated electrons increase in going from lowest-to-highest precursor concentrations, as measured by IMVS, indicating increasing site density for electron traps, a trend that correlates with an 8× improvement for photocatalytic H2 generation. Electron mobility in the aerogel films, as measured by IMPS, decreases with increasing trap density, further implicating the trapping sites as reactive sites. In contrast, photocatalytic DCA degradation—driven primarily by direct hole transfer to adsorbed DCA—depends only weakly on the electron dynamics in the film. Transient infrared spectroscopy shows no difference in carrier decay amongst the aerogel samples on picosecond timescales, indicating that changes in carrier dynamics within these networked nanomaterials are only observable at timescales measured by IMPV and IMPS. Correlating holemediated and electron-mediated photocatalytic activity with direct measurement of electron dynamics under photocatalytically relevant conditions and timescales comprises a powerful approach to determine how synthetic modifications to networked nanostructured photocatalysts affect the relevant physicochemical phenomena underlying their photocatalytic performance. Keywords: TiO2, aerogel, photocatalysis, hydrogen generation, intensity-modulated photovoltage spectroscopy (IMVS), intensity-modulated photocurrent spectroscopy (IMPS) 1.0 Introduction In the present study, we take initial steps towards quantifying the advantages of networked ultraporous nanoarchitectures for photocatalysis, with a particular focus on the photocatalytic 1 ACS Paragon Plus Environment


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splitting of water to generate hydrogen. The advantages of porous nanoscale semiconductor networks in photoelectrochemical devices, particularly for nanocrystalline dye-sensitized solar cells (DSSCs), have been exploited for over 20 years,1,2 and to large extent are well-understood for that application. Namely, the high surface area afforded by the porous nanostructured semiconductor enables efficient absorption of light3 and the connectivity of the networked nanoparticles provides long diffusion pathways for photogenerated electrons. These paths are macroscale in length,4,5 extending to the current collector and enabling efficient separation of electron from hole with nearly unit quantum efficiency for electron collection at wavelengths corresponding to the broad absorption maxima of state-of-the-art dyes.2,6,7,8 The possibility of exploiting networked nanostructures as suspended heterogeneous catalysts, especially for solar fuels photocatalysis, is strongly appealing. Such materials should separate the electron from the point of electron–hole pair generation and efficiently transport it via the networked semiconductor, not to a current collector, but to a catalytic site in the network where it can reduce adsorbed substrates such as water to hydrogen or carbon dioxide to fuels or fuel precursors. An open question remains: to what extent can transporting photogenerated electrons in nanoscale networks improve reactivity in suspensions of now-networked heterogeneous photocatalysts? Several examples indicate that networking nanoscale TiO2 improves photocatalytic performance: Kuznetsov et al. measured highly efficient electron–hole generation in wet TiO2 gels (η = quantum efficiency = 0.46) as compared to TiO2 colloids (η = 0.16), which they ascribed to increased degrees of freedom for charge mobility in the gel network versus the confinement characteristic of individual TiO2 colloids.9 Similarly, Panayotov et al. observed enhanced UV photochemical oxidation of adsorbed methoxy species at networked TiO2 aerogels

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as well as higher populations of trapped electrons relative to isolated P25 TiO2 particles.10 Ismail et al. posited that networked, mesoporous TiO2 modified with gold (Au) nanoparticles strongly enhanced UV-photocatalytic oxidation of methanol because electrons traveled over multiple nanoparticle lengths to reach the Au nanoparticles.11 In

these

and

other

examples

of

photocatalysis

at

networked

nanostructured

titania,12,13,14,15,16,17,18,19 the fundamental effects of electron (and hole) lifetime as well as electron mobility on photocatalytic activity remain open. In order to exploit the promise of a wired photocatalytic system, the dynamics of the charge carriers in the semiconductor network must be more rigorously understood. In the present study, we chose to use intensity-modulated photovoltage spectroscopy (IMVS)20,21 and intensity-modulated photocurrent spectroscopy (IMPS)22,23,24 to obtain lifetime and mobility, respectively, for conduction band and/or trapped electrons for a series of TiO2 aerogels in which the weight fraction of the Ti sol–gel precursor is systematically increased. Unlike more traditional, purely spectroscopic methods, such as timeresolved spectroscopy using flash photolysis methods to obtain electron lifetime,25,26,27 and microwave28 or terahertz29,30,31,32 spectroscopy to measure electron mobility, both sets of information are acquired in the same reaction configuration merely by running the experiment at open-circuit (IMVS) or closed-circuit (IMPS) conditions. Optical excitation is employed, but with electrical detection, which obviates the need to measure transmitted or reflected radiation, especially in scattering environments. We then correlate the materials parameters derived from IMVS and IMPS to the activity obtained for two photocatalytic reactions with significantly different mechanisms: photocatalytic hydrogen (H2) generation and the photocatalytic degradation of dichloroacetate (DCA).



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2.0 Methods 2.1

Synthesis of TiO2 Aerogels

Titania (TiO2) aerogels were prepared using a sol–gel method described previously by Dagan and Tomkiewicz,12 but in which the mass of the precursor, titanium (IV) isopropoxide (Ti(iOPr)4, 97+%, Alfa Aesar) was varied while holding the total volume of solvent constant. An ethanolic (absolute ethanol, Warner-Graham) solution consisting of 2.5 g, 3.0 g, 3.4 g, or 3.9 g of Ti(iOPr)4 in 3.9 g of ethanol was added to a mixture of water (H2O, 18 mΩ cm, Barnstead Nanopure, in which the mole ratio of H2O:Ti(iOPr)4 is kept constant at 2.5:1), ethanol (4.0 g), and a catalytic amount of nitric acid (78 mg of 70% HNO3 (Fisher)). The ratios of Ti(iOPr)4 :solvent (0.25–0.39 g mL–1) cover the range under which a stable gel forms: below this ratio, a gel did not form and above this ratio the oxide precipitated before forming a gel. The solution begins to gel shortly after mixing and is cured for 24 h. The wet gels were rinsed with acetone and loaded under acetone into a supercritical dryer, then rinsed several times with liquid CO2 at 10°C to exchange the pore fluid. The dryer was then heated to 41°C (above the critical temperature of CO2) before venting to atmospheric pressure. The amorphous as-prepared aerogels were calcined in air at 425°C or 450°C (2°C min–1 ramp, 4h dwell, 2°C min–1 cooling ramp) to yield nanocrystalline anatase aerogels. The aerogel samples deriving from syntheses using 2.5, 3.0, 3.4 and 3.9 g of Ti(iOPr)4 are designated as TiO2-1, TiO22, TiO2-3, and TiO2-4, respectively. 2.2

Structural and Physical Characterization

The crystalline phase of the calcined TiO2 aerogels was characterized with X-ray diffraction (Rigaku SmartLab, 40 kV and 44 mA, 2° min–1 scan rate). The average crystallite diameter was determined via the Scherrer equation using the full width at half maximum (FWHM) of the 4 ACS Paragon Plus Environment

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anatase (101) diffraction peak at 2θ = 25.2°. The Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distributions were derived from N2 physisorption isotherms (Micromeritics ASAP2020). The isotherm data (Figure S1) were fitted with Micromeritics DFT Plus® software using a density functional model theory assuming cylindrical geometry and Halsey curve thickness. Scanning electron microscopy (SEM) images were recorded using a LEO Supra 55 field-emission microscope after sputtering the insulating samples with gold to improve image resolution. Particulates of ground titania aerogel were dispersed in ethanol, and then drop cast onto lacey carbon substrates for transmission electron microscopy (TEM; JOEL2200FS). Using bright-field imaging, titania aerogels showed similar particle composition (mostly anatase with a small fraction of brookite) and size (roughly 7–15 nm). Diffuse reflectance UV–visible data (Perkin-Elmer 750 spectrophotometer with an integrating sphere) were converted to absorption values using the Kubelka–Munk transformation. Optical bandgaps were determined from Tauc plots by extrapolating the linear portion of the plot near the absorption edge to the energy axis. The X-ray photoelectron spectra (Thermo Scientific K-Alpha, Al-Kα radiation) were recorded using a flood gun to negate charging on the insulating samples. High-resolution spectra were recorded in the valence band, O1s, and Ti2p regions. All peak positions were referenced to the C1s speak at 284.5 eV. Diffuse reflectance IR spectra (DRIFTS-IR) were measured using a Nicolet 6700 FT-IR spectrometer with a DRIFTS attachment (Thermoscientific) and a liquid nitrogen-cooled mercury cadmium telluride detector. Aerogel samples ground to a powder in a mortar and pestle and diluted into KBr (Buck Scientific) were placed in a temperature-controllable DRIFTS sample holder (Spectra Tech) before spectra were taken at 400°C under flowing nitrogen. Each spectrum comprised an



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average of 64 scans between 650–4000 cm–1 with a resolution of 0.4 cm–1. Background spectra of the KBr over the same range and at 400°C were subtracted from each spectrum. 2.3

Photocatalytic H2 generation

Photochemical hydrogen generation rates at suspensions of titania aerogels were determined in a slurry reactor in the presence of a hole scavenger.33 Aerogel photocatalyst powders were ground and then suspended at a concentration of 2 g of TiO2 L–1 in a 4:1 (v:v) 0.1 M NaOH : methanol solution containing 0.02 M ethylenediaminetetraacetic acid (EDTA, 99% Aldrich). A 50-mL Pyrex™ reactor was filled with 25 mL of the slurry and sealed; the slurry was stirred and the reactor headspace purged for several hours with argon to remove oxygen. The slurry was illuminated with broadband light from a 500-W Xe arc lamp (Newport-Oriel) equipped with an AM 1.5 filter (Newport-Oriel). At 1-h intervals, 100 µL of the headspace was sampled and injected into a gas chromatograph to quantify the H2 yield. The GC (Shimadzu GC2010) was equipped with a thermal conductivity detector (40°C, Ar carrier gas) and a micropacked ShinCarbon column (Restek ST 100/120). The reactions were repeated for duplicate batches of TiO2-1–4 aerogels. Under identical illumination conditions, no H2 was detected in the absence of photocatalysts. 2.4 Intensity-modulated photovoltage spectroscopy (IMVS) and photocurrent spectroscopy (IMPS) Briefly, these methods involve illuminating a photoelectrochemical cell with intensitymodulated light at a range of modulation frequencies while the photoelectrode is held at either open- (IMVS) or closed-circuit (IMPS). To prepare the photoanode, aerogels were ground into a paste with a water and Triton-X surfactant solution. The paste was blade-coated onto a transparent FTO glass slide (Pilkington Glass; pre-cleaned by sonication 5 min each in acetone, isopropanol, and DI water) before drying in air and heating at 450°C to remove surfactant. The 6 ACS Paragon Plus Environment

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resultant films are opaque and 10–15-µm thick, as measured on a DekTak 6M Stylus Profiler (Veeco). The films consist of mesoporous aerogel pieces on the order of 3–7 µm (Figure S2). The photoanodes were incorporated into a photoelectrochemical cell (Model PECC-1, ZahnerElektrik) with a 20-mm–diameter quartz optical window, 0.1 M NaOH (aq) electrolyte, Pt coil counter electrode, and Ag/AgCl reference electrode. The IMVS and IMPS measurements were executed using a Zahner CIMPS Photoelectrochemical Workstation (Zahner-Elektrik) controlled by CIMPS and Thales Z software packages (Zahner-Elektrik). The photoanodes were backlit with a 376-nm LED. In the IMVS experiments, DC light intensities of 42.2, 22.2 and 12.2 W m–2 were applied with an AC perturbation of 1.3 W m–2 scanned from 500 Hz→1 kHz→10 mHz, with 10 measuring steps (frequencies)/decade. At frequencies below 66 Hz, five measurement points were averaged at each measurement frequency; at the higher frequencies, ten points were averaged at each frequency. In IMPS experiments, DC light intensities of 42.2, 22.2 and 12.2 W m–2 were used as well, but the AC perturbation was 6.3 W m–2 and the AC frequency range is run from 1 kHz→10 kHz→500 mHz, again with 10 measuring steps (frequencies)/decade. Five measurement points were averaged at 6× increase in H2 generation activity for TiO2-4 relative to TiO2-1 (Fig. S7, Supporting information), demonstrating that the effects of aerogel network modification on reductive photochemical reactions persist across reaction schemes that yield different concentrations of injected electrons. 3.3 Comparison of electron lifetime and relative electron mobility in TiO2 aerogels by intensity-modulated photovoltage spectroscopy (IMVS) and photocurrent spectroscopy (IMPS) 3.3.1 IMVS/IMPS background To probe the physicochemical features that affect photocatalytic H2 generation, we employed IMVS and IMPS to measure electron lifetime (τn) and electron mobility (τD), respectively, in the four different TiO2 aerogels. To date, these techniques have been primarily



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applied to measure charge-recombination dynamics in nanocrystalline TiO2 photanodes in operating dye-sensitized solar cells.20,21 In IMVS, the experiment is run using a modulated light source, and the electron lifetimes (τn) are derived from the phase relationship of the photovoltage perturbation with the light modulation, i.e., τn, can be calculated directly from the complex plane representation of the IMVS response (Eq. 2),40,41 τn = 1/ωmin,IMVS = 1/(2πfmin,IMVS)

(2)

where fmin, IMVS is the frequency of the minimum of the semicircle (the maximum negative voltage response in the imaginary plane). The photovoltage decay is primarily driven by reaction of trapped electrons in the nanocrystalline film with electron-accepting chemical species in the electrolyte.20,21,40 Similarly, the complex-plane representation of the IMPS data obtained by monitoring the photocurrent perturbation at closed-circuit conditions yields the average transit time (τD) of photogenerated electrons to the current collecting back-contact of the nanocrystalline film (and thus electron mobility) in the IMPS experiment.22,23 The electron transit time, τD, is extracted from the minimum in the IMPS complex plane plot:21,40,41 τD = 1/ωmin,IMPS = 1/(2πfmin, IMPS)

(3)

3.3.2 IMVS results The IMVS data for the four different TiO2 aerogels reveal that the shortest lifetimes occur in TiO2-1 and TiO2-2 (τn = 50 ms and 70 ms, respectively, at 42.2 W cm–2), with approximately two-fold longer lifetimes in TiO2-3 and TiO2-4, at τn = 150 ms and 100 ms, respectively (Figure 5). The trend toward longer lifetime with increasing precursor concentration is particularly clear in Figure 5B, where the imaginary component of the modulated photovoltage is plotted versus 16 ACS Paragon Plus Environment

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light modulation frequency. Increasing IMVS electron lifetimes indicate increasing residence time in electron trap sites.20,21,41 Our results suggest that we generate more trapping sites into TiO2 aerogels as we incorporate more of the Ti(iOPr)4 precursor into the networked gel during synthesis. However, the value of τn measured at TiO2-4 of 103 ms, while still longer than those measured at TiO2-1–2, is substantially shorter than that measured at TiO2-3, suggesting the onset of an additional, more facile photovoltage decay mechanism at aerogel TiO2-4. Further insight into the functional difference between aerogels TiO2-4 and TiO2-1–3 can be obtained from the open-circuit photovoltages measured during IMVS (Table 2). At films constructed from TiO2-1– 3, τn increases with higher precursor concentrations (due to higher densities of trapping sites in the resultant aerogels), however, photovoltages at a given DC light intensity remain effectively identical for all three films, indicating that the surface trap states at these aerogels are energetically similar. Unlike the other TiO2 aerogels in this study, the film constructed from TiO2-4 has open-circuit photovoltages that are 80–90 mV more negative at all intensities, indicating the existence of surface trap states at TiO2-4 with higher electron energy (i.e., residing closer to the TiO2 conduction band). Although the increase in open-circuit photovoltage observed at TiO2-4 may appear to be relatively modest, if one applies a –85-mV shift when solving the equilibrium for electrochemical water reduction (Eq. 1) via the well-known Nernst equation, the equilibrium shifts ~ 30× to the product side of the reaction. While catalysis often occurs at conditions far from equilibrium, the Nernst calculation demonstrates that the additional 80–90 mV of photovoltage in TiO2-4 is adequate to drive the ~ 8× improvement in photocatalytic H2 generation rates observed at TiO2-4 films relative to those at films constructed from TiO2-1.



-0.5 100 Hz

Aerogel TiO2-1 TiO2-2 TiO2-3 TiO2-4

tn 50 ms 70 ms 150 ms 100 ms

0.12 Hz

fmin = 1.0 Hz

-1.0

fmin = 1.5 Hz fmin = 2.2 Hz

-1.5

A

1.0

-0.2 -0.4

tn 50 ms 70 ms 150 ms 100 ms

-0.6 1.0 Hz

-0.8 -1.0

1.5 Hz 2.2 Hz

-1.2 -1.4

fmin = 3.2 Hz

0.0


0.0 Im (photovoltage, mV)

0.0 Im (photovoltage, mV)

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B

3.2 Hz

-1.6

2.0

3.0

0.1

Re (photovoltage, mV)

1

10

100

freq (Hz)

Figure 5. Intensity-modulated photovoltage spectroscopy (IMVS) data measured under open-circuit conditions for films cast from aerogels TiO2-1–TiO2-4 onto FTO-coated glass photoanodes (electrolyte: 0.1 M NaOH (aq); Ref: Ag/AgCl (3 M NaCl); Pt wire counter electrode; DC light intensity of 42.2 W m–2 at 376 nm; ~3 % AC light perturbation); the frequency of the negative photovoltage intensity maximum in the imaginary plane (fmin,IMVS) and photoelectrochemical electron lifetimes (tn) are indicated for each aerogel: (A) complex plane plots; (B) plots of photovoltage in the imaginary plane versus incident light modulation frequency.

Table 2. Electron lifetimes (τn) and measured photovoltages obtained during IMVS measurements at photoanodes constructed from TiO2 aerogels TiO2-1–TiO2-4.


12.2 W m–2 VOC (V) τn (ms) –0.233 150 –0.241 230 –0.257 520 –0.351 230

Incident Power 22.2 W m–2 VOC (V) τn (ms) –0.288 100 –0.284 100 –0.303 340 –0.370 150

42.2 W m–2 VOC (V) τn (ms) –0.330 50 –0.324 70 –0.336 150 –0.421 100

Mechanistic insight into the nature of the IMVS photovoltage decay is obtained via the light-intensity dependence of the IMVS amplitude. When extensive electron trapping occurs in semiconductor photoelectrodes, both τn and the magnitude of the IMVS amplitude decrease with increasing DC light intensity because the increased photovoltage under higher photon flux more readily drives the reaction between conduction band/trapped electrons with electron-accepting species in the electrolyte.20,40,41 We observe this predicted trend of monotonically decreasing τn at


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three illumination intensities (12.2 W m–2, 22.2 W m–2, and 42.2 W m–2) for all four aerogels (Figure 6), further implicating a surface-state–derived trapping mechanism.

1 electron lifetime (s)

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aerogel slope TiO2-1 –0.90 TiO2-2 –0.92 TiO2-3 –1.0 TiO2-4 –0.63

0.1

5 × 1015

1 × 1016

2 × 1016

Incident photon flux (s–1 cm–2)

Figure 6. Log–log plot of electron lifetimes calculated from the IMVS responses for photoanodes constructed from TiO2-1–4 aerogels at DC illumination intensities of 12.2 W m–2, 22.2 W m–2, and 42.2 W m–2.

For TiO2-1–3, the slope of the log–log plots of τn versus DC illumination intensity is ~ –1, indicating that the reactions driven by trap-state electrons are (1) on average, first order in light intensity; and (2) likely the same in all three cases. Reduction reactions between trapped electrons and one or more electrolyte components, including but not limited to reductive water splitting (Eq. 1) and reduction of dissolved oxygen, are likely the main drivers of photovoltage decay. At each illumination intensity, τn values for TiO2-1–3 increase with increasing precursor concentration (consistent with the single intensity IMVS data, Figure 5), indicating an increased density of trap states in the films derived from higher Ti(iOPr)4 concentrations. At aerogel TiO2-4, not only are the electron lifetimes lower at all intensities than at TiO2-3, but the slope of the log–log plot changes to –0.63, indicating the onset of an approximately



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second-order reaction (which would have an ideal slope of –0.5, as described for other IMVS decay pathways involving two electrons40,41), i.e., a change in the dominant mechanism by which electrons are transferred from TiO2 electron trap states to the electrolyte. Given that photocatalytic H2 generation is second order in trapped electron concentration (Eq. 1), this change to an approximately second-order photovoltage decay mechanism is consistent with the photocatalytic H2 generation data for TiO2-4 (Figure 4). It is likely that once a critical density of trap states is reached in the aerogel materials, the second-order reaction, i.e., involving two active sites, becomes the dominant mechanism and H2 yields drastically improve. It is likely that the traps that are incorporated into the aerogels are located in the interfacial region of the interparticle “necks,” which are thickened as the precursor ratio is increased. It has been previously reported that the interfacial region between TiO2 nanoparticles contains a unique population of tetrahedrally coordinated, distorted, active sites.42,43 These so-called catalytic “hot spots” control the adsorptive affinity and reactivity of TiO2 photocatalysts, and similar active/trapping sites have been reported at the solid–solid interfacial regions between TiO2 and other oxides.44,45 3.3.3 IMPS results: refining our understanding of the relationship between IMVS results and photocatalytic H2 generation The IMPS-measured electron mobility in TiO2 aerogel films should to a first approximation report on the density of trapping sites in the films: as trapping increases, mobility decreases and average transit times (τD) increase. The IMPS measurements on TiO2-1–4 films yield decreasing τD with increasing Ti(iOPr)4 concentration (Figure 7). Note that the lower frequency peaks that appear in the IMPS data, particularly evident in the data for TiO2-3, are due to charging of the electrical double-layer at the high surface area characteristic of aerogel films.22 Additionally noteworthy are the approximately equal values of τD measured at TiO2-3 and TiO2-4, which tells 20 ACS Paragon Plus Environment

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us that that despite the significantly higher electron lifetimes measured for TiO2-3 by IMVS, the electrons encounter similar trap densities (resulting from the similar structures of the TiO2 nanoparticle interconnects) when allowed to diffuse through the two films under the closedcircuit conditions of the IMPS measurement. Taking the IMVS and IMPS results in concert, we can conclude that it is unlikely that the shorter τn measured at TiO2-4 relative to TiO2-3 by IMVS is due to decreased trap-site density in TiO2-4, but rather due to a population of highly reactive electron trap sites in TiO2-4 that lie energetically more negative than the majority of the trap populations in TiO2-1–3 and which drive facile electron transfer to the electrolyte. 0.0

0.0

fmin,b = 4.2 Hz

-2.0e-7 -3.0e-7 -4.0e-7

fmin = 33.6 Hz Aerogel TiO2-1 TiO2-2 TiO2-3 TiO2-4

0.0

tD 2.9 ms 3.7 ms 4.7 ms 4.7 ms

fmin = 43.0 Hz

2.0e-7

33.6 Hz

-1.0e-7

fmin = 33.6 Hz

-1.0e-7

Im, IPCE

Im, IPCE

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33.6 Hz Aerogel TiO2-1 TiO2-2 TiO2-3 TiO2-4

-3.0e-7

fmin = 55.0 Hz

4.0e-7

-2.0e-7

6.0e-7

A -4.0e-7

8.0e-7

0.1

Re, IPCE

tD 2.9 ms 3.7 ms 4.7 ms 4.7 ms

1

43.0 Hz

B

55.0 Hz

10

100

1000

10000

f(Hz)

Figure 7. Intensity-modulated photocurrent spectroscopy (IMPS) data from films cast from aerogels TiO2-1–TiO2-4 onto FTO-coated glass photoanodes under closed-circuit conditions (electrolyte: 0.1 M NaOH (aq); Ref: Ag/AgCl (3 M NaCl); Pt wire counter electrode; DC light intensity of 42.2 W m–2 at 376 nm; ~ 20 % AC light perturbation) with the frequency of the negative photovoltage intensity maximum in the imaginary plane (fmin,IMPS) and photoelectrochemical electron lifetimes (tD) indicated for each aerogel: (A) complex plane plots; (B) plots of photovoltage in the imaginary plane versus incident light modulation frequency.

3.4 Photocatalytic degradation of dichloroacetate (DCA) at TiO2 aerogels: a direct holetransfer reaction with oxidation as the limiting step We turned to photocatalytic degradation of dichloroacetic acid (DCA), Eq. 4—a wellcharacterized reaction that reports directly on hole trapping and surface –OH site density27,46—to determine whether hole reactivity correlates to electron trapping in aerogel networks. UV hν CHCl2COO–ads + O2 → 2 CO2 + H+ + 2 Cl– TiO 2


(4)


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The photocatalytic DCA degradation mechanism has been extensively described elsewhere.27,46 Critical to the present study are the rate-determining steps in the mechanism and how modification of the TiO2 aerogels affects those steps. The most rate-critical elements in the mechanism are the electron-trapping step and the adsorption of DCA to hole-trapping surface -OH sites (Eq. 5),27,46 enabling its degradation via hole transfer, which is then followed by the terminal degradation steps of the reaction (Eqs. 6 and 7):46 hvb+/•OH + CHCl2COO–ads → CHCl2COO• + –OH

(5)

CHCl2COO• → CHCl2• + CO2

(6)

CHCl2• + O2–• → CO2 + H+ + 2Cl–

(7)

One would expect a positive correlation between electron trapping and DCA degradation; although we do not directly measure hole lifetime, increasing electron lifetime means that the probability of electron–hole recombination decreases, thus increasing the probability of successful hole transfer to adsorbed DCA (Eq. 5). We observe a modest increase in DCA degradation rates in the aerogels with longer lifetimes (Figure 8). The photocatalytic DCA degradation rates are enhanced at TiO2-3 and TiO24 by over 40% compared to TiO2-1 (Table 3). The slope of DCA degradation rate measured for TiO2-2 is close to that for TiO2-3 and TiO2-4 in the first 60 s (Figure 8), but the slope over 5 min is closer to that measured for TiO2-1, yielding a surface area-normalized DCA degradation rate at TiO2-2 equivalent to that at TiO2-1. The apparent intermediate position of the TiO2-2 DCA degradation rates may constitute the beginning of the generally upward trend in DCA degradation time with electron lifetime (Figure 8), however, the comparatively small effect of electron lifetimes on the hole-transfer reaction limit the ability to clearly place the activity of this


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synthetic variant in the synthetic series. Thus, the increased electron trapping measured in aerogels TiO2-3 and TiO2-4 via IMVS (Table 2) correlates with a just-measureable increase in DCA degradation activity. 0.10 TiO2-1

0.08 mL 0.1M NaOH

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TiO2-2 TiO2-3

0.06

TiO2-4

0.04 0.02 0.00 0

50

100

150

200

250

300

t (s)

Figure 8. Rate of generation of protons in the photocatalytic degradation of DCA as measured by rate of dispensation of [OH–] in an auto-titrator reactor in pH-stat mode (10 mM KNO3, pH 3).

Table 3. DCA degradation rates measured at TiO2 aerogels (two trials each) derived from differing TiO2 precursor concentrations in the sol–gel syntheses. Quantum efficiency (ξ) determined as the rate of DCA degradation production per incident photon.

TiO2 Aerogel

TiO2-1 TiO2-2 TiO2-3 TiO2-4

ξ%

DCA degradation rate (10–8 mol s–1) 2.0 2.0 1.7

Rate/SA (10–9 mol s–1 m–2)

0.86 0.86 0.73

1.6 1.6 1.5

2.5 3.1 2.6 3.0

1.1 1.3 1.1 1.3

2.1 2.6 2.0 2.3

The quantitative relationship between DCA activity and electron lifetime (τn) is unclear. The relative increase in DCA degradation reactivity (~ 1.4×) realized for TiO2-3 and TiO2-4 vs. TiO21 and TiO2-2 is of the same order of magnitude as the relative increase in τn (2–3×); but this 23 ACS Paragon Plus Environment


apparent correlation does not dictate reactivity. The rate for DCA degradation is the same for TiO2-3 and TiO2-4, yet their IMVS-determined τn values significantly differ (150 ms vs. 100 ms, respectively). Although TiO2-4 can access different electron-driven photovoltage decay mechanisms, as seen by IMVS and the enhanced photogeneration of H2, these electron pathways do not affect DCA degradation, indicating that hole reactivity in the different TiO2 aerogels only weakly depends on electron reactivity. One possible explanation is that the highly reactive electron traps incorporated into TiO2-4 likely comprise a very small percentage of the total number of electron traps; they substantially enhance electron activity, but only marginally change the probability of electron–hole recombination.

3.5 Effect of increased electron trap density on the dynamics of electron trapping 1000

Absorbance Change (mOD)

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2

1

100

TiO2-4 0

5

10

15

10

TiO2-3 1

TiO2-1

0.1 0

100

200

300

400

500

600

700

Time (ps) Figure 9. Normalized transient absorption decay curves obtained for TiO2 aerogels at various precursor loading; TiO2-1, TiO2-3, and TiO2-4. All samples were pumped at 267 nm (2 µJ) with the probe maintained at 4500 nm. Inset: zoomed-in view of the early time-scale dynamics. Decay curves are offset for clarity.

Ultrafast transient absorption spectroscopy of charge trap states in TiO2-1, TiO2-3 and TiO24 was performed to determine whether the increased electron trap density from TiO2-1 to 3, and the incorporation of more electrochemically negative traps in TiO2-4, have an observable short24 ACS Paragon Plus Environment

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time effect on the dynamics in the aerogels. Conduction band electrons in TiO2 have been shown to relax into traps within the first 300 fs47 following above-bandgap excitation (267 nm), timescales not accessed by IMVS or IMPS. The probe wavelength was maintained at 4500 nm in order to measure the presence of conduction band48 and shallow trapped electrons49 without interference from absorption corresponding to deeper trapped electrons and holes. The absorptions exhibit biexponential decay for aerogels TiO2-1, 3 and 4 (Figure 9), with lifetimes on the order of 15 and 150 ps. This behavior is consistent in all three samples, with the longer decay attributed to the loss of electrons from the chemically reactive shallow traps to unreactive deeper traps, which have been observed to occur over 100s of picoseconds.50 None of the aerogels—even the most reactive aerogel, TiO2-4—exhibits any changes in dynamics on picosecond timescales (Figure 9). Thus, the incorporation of additional trapping sites into the aerogels does not change the rate of electron decay from shallow traps (reactive) to deep traps (unreactive) occurring on this ultrafast timescale in any of the materials. The observed changes in photocatalytic activity derive from trapping/transport processes observable on the IMVS and IMPS timescales. 4.0 Conclusions and outlook Subtle modification of TiO2 aerogel networks via systematic variation of synthetic conditions significantly alters both the lifetime and mobility of conduction band/shallow-trapped electrons in the aerogels on timescales observable by IMVS and IMPS, but not on shorter timescales measured by transient IR. These changes in electron lifetime and mobility have widely varying impact on different photocatalytic reactions, depending on the critical mechanistic steps in the reaction. In the case of photocatalytic H2 generation, where reductive water splitting at an electron-trapping site is the rate-determining step, both the number and the 25 ACS Paragon Plus Environment


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nature of the trapping site are critical to photoactivity. In the case of DCA degradation, which employs a direct hole-transfer mechanism (i.e., the electron to a first approximation is not involved), the effect of the electron dynamics on photocatalytic activity is modest, existing only to the extent that electron trapping helps to slow recombination losses of holes. More importantly, we have demonstrated that the dynamics of electrons within the aerogel nanoscale network can be analyzed both qualitatively (in terms of general trends) and semiquantitatively (relative values of lifetime, mobility, and photovoltage) by dynamic photoelectrochemical methods in a way that lends meaningful insight into the performance of the networked nanostructures as freely suspended heterogeneous photocatalysts. The fractal nature of the aerogels makes them structurally identical on the nanometer-to-micrometer length scales relevant to their photoactivity, making the results obtained from the two methods compatible. The combined information obtainable from mechanistically divergent photocatalytic reactions and the IMPS and IMVS methods will be extremely helpful in the design of more hierarchically integrated photocatalysts based on networked nanostructures. Incorporation of elements such as plasmonic sensitizers and reduction co-catalysts, as well as changing other fundamental aspects of the network semiconductors such as nanoparticle size and doping, will impact electron lifetime and mobility in ways that will profoundly impact performance, and thus will be critical to understand.

Acknowledgments This work was partly supported by the U. S. Office of Naval Research (P. A. D., J. J. P., R. C., T.H.B., R.M.S, J. C. O., and D. R. R.) and by the Deutsche Forschungsgeminschaft (S.S., R.M., D. H. T., and M.W., WA 1116/23). J. J. P. acknowledges an Advanced Graduate Research 26 ACS Paragon Plus Environment

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(sabbatical) fellowship at Ruhr-University Bochum supported by the U. S. Office of Naval Research, and would like to especially thank Professor Michael Wark and the Lehrstuhl für Technische Chemie (Laboratory for Industrial Chemistry) at Ruhr-University Bochum for hosting him during his sabbatical study. P. A. D. (2011–2014) and R. C. (2012–2015) were Naval Research Laboratory–National Research Council postdoctoral associates. Conflict of interest disclosure: The authors declare no competing financial interest. Supporting Information Figures containing adsorption/desorption isotherms, additional SEM micrographs, diffuse reflectance absorption spectroscopy, XPS, and IR spectroscopy, and reactivity data for an alternate photocatalytic H2-generation reaction are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References 1. O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737–740. 2. Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of Light to Electricity by cis-X2Bis(2,2’-bipyridyl-4,4’dicarboxylate)ruthenium(II) Charge-Transfer Sensitizers (X = Cl–, Br–, I–, CN–, and SCN–) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390. 3. Anderson, M. L.; Stroud, R. M.; Morris, C. A.; Merzbacher, C. I.; Rolison, D. R. Tailoring Advanced Nanoscale Materials Through Synthesis of Composite Aerogel Architectures. Adv. Eng. Mater. 2000, 2, 481–488. 4. Grätzel, M. Mesoporous Oxide Junctions and Nanostructured Solar Cells. Curr. Opin. Coll. Interfac. Sci. 1999, 4, 314–321. 5. Barnes, P. R. F.; Miettunen, K.; Li, Z.; Anderson, A. Y.; Bessho, T.; Grätzel, M.; O’Regan, B. C. Interpretation of Optoelectronic Transient and Charge Extraction Measurements in Dye-Sensitized Solar Cells. Adv. Mater. 2013, 25, 1881–1922. 6. Kalyanasundaram, K.; Grätzel, M. Application of Functionalized Transition Metal Complexes in Photonic and Optoelectronic Devices. Coord. Chem. Rev. 1998, 177, 347–414. 7. Hagfeldt, A.; Grätzel, M. Molecular Photovoltaics. Acc. Chem. Res. 2000, 33, 269–277.



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Table of Contents Graphic (TOC)

–

–

TiO2(ecb ) + H2O → OH + ½ H2 Thickened via increased precursor concentration TiO2-4 Inter-particle “neck regions” 12–15 nm particles

TiO2-1

TiO2


Correlating Changes in Electron Lifetime and ... - ACS Publications

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