Dynamics of Photocatalytic Hydrogen Production in Aqueous

Aug 21, 2018 - An intrinsic proton reduction rate constant was extracted that may be extended to any photoelectrochemical or electrochemical hydrogen ...
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Dynamics of Photocatalytic Hydrogen Production in Aqueous Dispersions of Monolayer-Rich Tungsten Disulfide Jeremy R. Dunklin,† Hanyu Zhang,† Ye Yang,† Jun Liu,‡ and Jao van de Lagemaat*,† †

Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States

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‡

S Supporting Information *

ABSTRACT: Two-dimensional tungsten disulfide (WS2) is an emerging semiconducting photocatalyst featuring high optical absorption, carrier mobility, and catalytic activity toward hydrogen evolution. While characterization of its optical and electrocatalytic properties has advanced, less is known about its ultrafast carrier dynamics and intrinsic photocatalytic activity in aqueous systems producing hydrogen. This work removed extraneous variables often found in photoelectrochemical systems, thereby allowing the intrinsic proton reduction rate for monolayer-rich WS2 nanosheets to be estimated via transient absorption lifetimes and a developed kinetic scheme. Addition of a hole scavenger, ascorbic acid (AA), resulted in a 3-fold increase in carrier lifetimes following photoexcitation. Longer electron lifetimes with AA yielded a 14-fold increase in hydrogen production. An intrinsic proton reduction rate constant was extracted that may be extended to any photoelectrochemical or electrochemical hydrogen evolution scheme involving small, monolayer-rich WS2 catalysts. This represents an important step in better understanding catalytic systems utilizing TMD catalysts.

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voltage driving force or co-catalysts undoubtedly improves catalytic efficiency, they obscure deeper understanding of the photocatalytic behavior of bare TMDs. This study aims to remove all extraneous variables found in photochemical and photoelectrochemical systems to better understand the fundamental photochemistry on monolayer-rich WS2 in water. This work represents the first study of carrier lifetimes, probed via transient absorption spectroscopy (TAS), in monolayer-rich aqueous TMD dispersions where the TMD may generate hydrogen. Spectral signatures of excitons and charge carriers in TMDs have been tuned via a gate voltage,22 chemical doping,23 substrate doping,24 temperature,25 and photoexcitation26 and probed by absorption and emission spectroscopies. TAS is a powerful tool to study TMD photophysics with time-resolved spectra across a range of TMD systems generally exhibiting photobleaching (PB) of excitonic transitions and associated red-shifted photoinduced absorption (PIA) bands. Explanation of these transient spectral features remains controversial as state filling,27,28 stimulated emission,29 or exciton−exciton interactions30 have been used

hotochemical water splitting represents a clean, sustainable method to store solar energy in a chemical fuel, i.e., hydrogen.1 In contrast to systems where photovoltaics drive electrolysis,2−5 direct photochemical water splitting on emerging nanoscale semiconductor catalysts offers the opportunity to leverage both their optoelectronic and catalytic properties.6,7 However, challenges remain in finding earth-abundant, corrosion-resistant materials capable of generating and separating photoexcited carriers while driving water oxidation and reduction. Since the 1970s,8,9 transition metal dichalcogenides (TMDs) have garnered interest in both photovoltaic and electrochemical cells. Renewed interest in monolayer or few-layer TMDs for optoelectronics10,11 and catalysis12−15 is attributable to their intriguing optical properties16,17 and surface chemistry.18 In contrast to metal oxides like TiO2, limited by large bandgaps ill-suited for solar harvesting and sluggish reaction kinetics,7 TMDs like WS2 are corrosion-resistant, visible bandgap semiconductors with potential for large-scale solar energy conversion.19 While TMDs have the highest absorption coefficient of any known material,11 most rapid progress has been made in engineering TMDs as efficient hydrogen evolution reaction (HER) electrocatalysts2−5 or as co-catalysts with other light absorbers;20,21 work highlighting their potential in photochemical systems remains sparse.15 Although addition of a bias © XXXX American Chemical Society

Received: July 20, 2018 Accepted: August 21, 2018 Published: August 21, 2018 2223

DOI: 10.1021/acsenergylett.8b01287 ACS Energy Lett. 2018, 3, 2223−2229

Letter

Cite This: ACS Energy Lett. 2018, 3, 2223−2229

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ACS Energy Letters

and height (section SI.3). The mean nanosheet length was 18.5 ± 8.6 and 40.4 ± 15 nm based on TEM and AFM, respectively. The measured mean height from AFM was 2.2 ± 1.5 nm. Due to residual solvent/surfactant, apparent monolayer nanosheets made via LPE typically measure ca. 2 nm for the first layer and then ca. 0.7 nm for each additional, discrete layer.41,43 The mean length of the fabricated nanosheets is similar to the charge diffusion length in TMDs,33 indicating a significant probability of photogenerated charges reaching active edges sites. Furthermore, the small nanosheets produced via LPE may be ideal for photocatalysis because the active sites are primarily at nanosheet edges.18,44 PL spectroscopy was performed on the aqueous WS2 dispersions in a confocal Raman microscope using 532 nm excitation. The PL intensity and energy in TMD systems depend on the substrate and dielectric environment.45 In aqueous media, the PL maximum is located at the Aexciton,41,43 as shown in Figure 1 (right y-axis). PL is appreciable only from monolayer TMDs, another indication of the high monolayer content herein. The addition of 1 mM ascorbic acid (AA), an effective hole capture agent, had no significant effect on PL. PL lifetimes of small LPE MoS2 were previously reported to be much shorter than that of larger, micron-sized monolayers supported on substrates.46 As such, nonradiative processes in small nanosheets in water largely outcompete radiative recombination, resulting in comparatively weak overall emission. This necessitates the use of timeresolved TAS to fully probe photoexcited carrier dynamics. This work is the first to characterize aqueous dispersions of monolayer-rich TMDs using TAS. The femtosecond (fs) to ns TAS setup (section SI.5) features a tunable pump and UV−vis broad-band probe. The 530 nm excitation was used to match PL and photocatalytic excitation; estimated excitation densities were on the order of 104 and 105 nJ/cm2. Figure 2a plots ΔA spectra at 1, 30, and 1000 ps probe delays for both the WS2 reference and WS2-AA solutions. In general, the ΔA spectra show A-exciton bleaching, within instrument response, due to ground-state depletion. A lower-energy PIA feature, associated with photoexcited carrier transitions, shows a few-hundred fs delayed rise following excitation with similar decay kinetics. The ΔA spectra for WS2-AA and WS2 reference solutions are nearly identical at 1 ps. However, at 30 and 1000 ps, while the spectral shape of each is nearly identical, the signal is larger for WS2-AA. It is believed that photoexcited holes partially oxidize AA; this reduces the likelihood of recombination, thus increasing the photoexcited electron lifetime and resulting ΔA signal. The AA has little effect on the lifetime of the initial exciton, believed to be the primary initial photoexcited species,32,33,39 because the exciton must be trapped prior to hole capture by AA. Recent work with TMDs in a photoelectrochemical cell highlighted the importance of hole transport on solar-to-hydrogen efficiency.15 A control experiment with the oxidized form of AA, dehydroascorbic acid (DA), yielded no appreciable difference in lifetime compared to the WS2 stock solution (section SI.6). pH values of the WS2 reference, WS2-AA, and WS2-DA were 6.9, 3.8, and 3.7, respectively. The absolute values of kinetic traces at 613 and 637 nm, associated with the primary PB and PIA features, respectively, are plotted in Figure 2b. It is apparent that both features in the WS2 -AA sample live significantly longer than the WS2 reference. Because the ΔA spectra shift significantly as a function of time, global analysis (GA)47 was used to extract

to describe the PB behavior, while PIA bands have been attributed to carrier-induced broadening28,30 or biexcitons.31 Similarly, short picosecond (ps)-scale kinetics have been attributed to excitons,32,33 bandgap renormalization,34,35 carrier trapping,28,36,37 and carrier cooling,38 while slower nanosecond (ns)-scale kinetic features have been ascribed to interband recombination,28 exciton−phonon scattering,25 and free carriers.39 While the aforementioned studies have thoroughly probed photophysical behavior across a range of sample preparations, excitation regimes, and local environments, dynamics of monolayer-rich dispersions in aqueous photochemical systems remains unexplored. Lack of scalable, reproducible fabrication techniques also limits implementation of 2D TMDs. Liquid-phase exfoliation (LPE) represents a low-cost, scalable method of producing TMD nanosheets.19,40 In this work, a previously developed protocol41 is utilized for production of monolayer-rich WS2 dispersions (see section SI.1). Steady-state absorption and photoluminescence (PL) spectroscopy yield insight into the physical size and optical response of 2D WS2 nanosheets. The absorbance spectra of the 0.4 mM WS2 dispersion in aqueous surfactant solution (1 mM sodium cholate), shown in Figure 1

Figure 1. Steady-state absorbance (left y-axis) and PL emission (right y-axis) spectra for WS2 reference solution (blue) and WS2 in 1 mM AA (WS2-AA; red). (Inset) Image of WS2 reference (left) and WS2-AA (right) solutions.

(left y-axis), exhibit characteristic peaks attributed to the Aand B-excitons arising from valence band splitting.42 The absorbance spectra provide information on the nanosheet mean layer number, monolayer volume fraction, and length.41,43 The mean layer number and monolayer volume fraction are characterized by the A-exciton energy, which blue shifts with decreasing layer number, while nanosheet length is correlated with absorbance peak ratios.41 Using these empirically derived spectral metrics (section SI.2), a mean layer number of 1.2, a monolayer volume fraction of 60%, and a mean nanosheet length of ca. 30 nm were estimated for the stock WS2 dispersion. Subsequent characterization via transmission electron microscopy (TEM) and atomic force microscopy (AFM) estimated the mean nanosheet length 2224

DOI: 10.1021/acsenergylett.8b01287 ACS Energy Lett. 2018, 3, 2223−2229

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Figure 2. (a) Transient ΔA spectra at 1 ps, 30 ps, and 1 ns for the WS2 reference and WS2-AA solutions, (b) kinetic trace of the absolute value of the primary PB (613 nm) and PIA (637 nm) for each solution, (c) three spectral components derived from global analysis, and (d) population of the three components as a function of probe delay.

kinetic lifetimes. The GA protocol used herein modeled the system as a three-component sequential cascade describing the decay of photoexcited excitons to trapped excitons to trapped electrons (with AA), similar to the approach employed by previous related studies.33,39 The GA model fit the data to a sum of three exponential decays, A = ∑3i=1 αi(λ) exp(−t/τi), where αi(λ) are pre-exponential amplitudes used to calculate the decay-associated spectra, λ is probe wavelength, t is pump− probe delay, and τi are the component lifetimes. The preexponential amplitudes may vary as a function of λ, but the extracted lifetimes are wavelength-independent. This three-component sequential global model fit the experimental data exceptionally well (section SI.8) and was validated by the developed kinetic scheme below. A twocomponent sequential fit yields a second component lifetime intermediate between the second and third components of the three-component fit that is longer-lived with AA present, but overall agreement between measured and fitted spectra is poor (section SI.8). Figure 2c illustrates the GA-derived component spectra, while Figure 2d shows the GA-derived population of each component as a function of pump−probe delay. Components 1, 2, and 3 are believed to represent populations of excitons, trapped excitons, and trapped electrons (with AA) or deep-trapped excitons (without AA), respectively. Components 2 and 3 have similar spectral features as it is believed that they are of similar origin; deep-trapped excitons could arise primarily from low-energy, long-lived dark excitonic states,

while trapped excitons may represent bright, emissive excitonic states.48 The decay constant associated with exciton trapping, τ1, just after photoexcitation ranged from 0.1 to 0.6 ps for WS2 in both water and aqueous 1 mM AA; these values are in good agreement with previously reported lifetimes in similar TMD samples.32,33,49 Tabulated values for each data set are in Table 1. Critically, values of derived time constants of component 3, Table 1. GA-Derived Component Lifetimes for WS2 Reference and WS2-AA Solutions sample WS2 reference WS2-AA

excitation energy (nJ) 200 20 200 20

τ1 (ps) 0.6 0.1 0.5 0.1

± ± ± ±

0.2 0.1 0.1 0.1

τ2(ps) 25 23 37 49

± ± ± ±

8 19 12 37

τ3 (ps) 336 576 850 1920

± ± ± ±

170 340 310 710

τ3, were over 3-fold greater with AA present; the lifetime of the third component, averaged over both excitation densities, for WS2-AA was 1400 versus 410 ps for the WS2 reference. Average extracted lifetimes of the second component for each solution were within error. Component 2 and 3 lifetimes decreased with increasing excitation energy; this trend was previously attributed to a carrier cooling behavior.38,49 Overall, 2225

DOI: 10.1021/acsenergylett.8b01287 ACS Energy Lett. 2018, 3, 2223−2229

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separation and served as the reduction catalyst. Photocatalytic HER from TMD-based catalysts can be similarly improved by addition of metal co-catalysts.20 Au decoration of WS2 allows facile access to high concentrations of monolayer sheets that exhibit a 5-fold enhancement in electrocatalytic HER performance.5 The low observed QY herein is due in part to needing two electrons for H2 production; it has been reported previously that similar-sized nanostructures would absorb a single photon only every μs.52 Furthermore, HER active sites and the primary photogenerated electron trapping states are both believed to reside primarily at edges, possibly limiting photocatalytic efficiency. Passivation of defects in few-layer solution-exfoliated TMDs is crucial for efficient solar-tohydrogen production.53 In thicker WSe2 nanosheets up to 1 μm in length, edge recombination limited performance in bare TMD nanosheets. However, once edge defects were passivated by surfactant treatment, preannealing in Se gas to reduce basal defects became important.53 Defects in smaller, monolayer-rich solution-exfoliated WS2 are believed to be present primarily at edges or terraces between layers, not the basal plane, as previously visualized by the location of in situ reduced AuNPs.5 Next, a kinetic scheme, illustrated in Scheme 1, was developed to rationalize the increased kinetic lifetimes and

these results suggest that addition of hole-scavenging AA increases WS2 electron lifetimes. Subsequent electrode-free photocatalysis experiments indicated increased electron lifetimes with AA present yielded a 14-fold increase in hydrogen production from monolayer-rich WS2. Several batches of WS2 with an identical UV−vis estimated nanosheet length and layer number were combined with those used for optical characterization to yield the significant mass of nanosheets needed to perform the photochemical experiments. A 40 mW 530 nm LED was used to irradiate a 4.5 mL volume of the WS2-AA, WS2-DA, and WS2 reference solutions in a septum-sealed 9.5 mL vial (see Figure 3). Approximately 50 μL of headspace (5 mL total

Scheme 1. Proposed Kinetic Scheme for Relaxation Dynamics of an Initial Exciton Population to the Ground State (G.S.) in the WS2-AA Photochemical System

Figure 3. Photochemical hydrogen production from monolayerrich WS2-AA (red), WS2 reference (blue), and WS2-DA (gray). The light source, 40 mW of 530 nm excitation, was turned on after 1 h. (Inset) Image of the experimental setup.

headspace volume) was manually injected into a gas chromatograph with a barrier-ion discharge detector (section SI.8). An estimated average hydrogen production rate of 1 nmol/h across the entire 20 h irradiation time was produced from WS2-AA (pH = 3.8) absent an external bias. This represented a 14-fold increase in hydrogen production versus aqueous WS2 (pH = 6.9) without hole-scavenging AA, which yielded only 0.07 nmol/h. The WS2-DA control solution (pH = 3.7) yielded only 0.1 nmol/h of hydrogen, suggesting that a reduction of pH alone will not enhance hydrogen production. Photooxidation of WS2, another possible decay mechanism for photogenerated holes,8 could explain the small amount of hydrogen produced absent AA. Hole scavenging is necessary to suppress recombination because photoexcited holes in WS2 are not energetic enough to perform water oxidation.50 The production rate did decline over time, presumably due to consumption of AA or WS2 oxidation. The focus of this work was not to maximize the HER quantum yield (QY) by using co-catalysts but rather to characterize the intrinsic HER rate from monolayer-rich WS2 in aqueous media. The apparent QY for the monolayer-rich WS2 dispersion was 0.001%. The estimated rate constant for proton reduction, derived from the TA data in the following section, was comparable to values estimated from Au-CdS nanorods.51 The QY for this previous study reached 10% by utilizing a metal co-catalyst, which permitted efficient charge

hydrogen production in aqueous WS2 dispersions. First, photogenerated excitons are formed. The exciton population, [A], decays as a function of time, as expressed mathematically in eq 1 dA = −(k T + k rad)[A] + kdt[B] dt

(1)

where kT, krad, and kdt are the rate constant for exciton trapping, radiative recombination, and detrapping, respectively. The rate constant ki is related to the time constant τi by ki = 1/ τi. At steady state, assuming the detrapping rate is negligible, eq 1 suggests GA-extracted τ1 = 1/(kT + krad). Because krad is comparatively small, ∼109 s−1,48 it is estimated that kT ≈ 1/τ1 ≈ 3 × 1012 s−1 based on an average of all four τ1 values. The population of the resulting trapped excitons [B] may decay nonradiatively, via hole capture, or may detrap into [A]. The decay of trapped excitons, analogous to that of component 2 in the GA fitting, is given in eq 2 dB = −(k nr + k hc + kdt)[B] + k t[A] dt 2226

(2)

DOI: 10.1021/acsenergylett.8b01287 ACS Energy Lett. 2018, 3, 2223−2229

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ACS Energy Letters where knr and khc are rate constants for nonradiative recombination and hole capture via AA, respectively. From eq 2, it is apparent that GA-derived τ2 with AA = 1/(knr + khc + kdt). If it is assumed that that knr ≫ khc + kdt, as nonradiative recombination is still the dominant pathway,54 knr ≈ 1/τ2 ≈ 3 × 1010 s−1 (averaging all Table 1 τ2 values). The decay of the population of the final trapped carrier populations [C] and [C′] for samples with and without AA, respectively, equivalent to the GA-derived component 3, can be expressed as dC = −k pr[C] + k hc[B] dt

(3a)

dC′ = −k nr,2[C′] + kDT[B] dt

(3b)

in hydrogen production from long-lived electrons that otherwise recombine. By removing extraneous variables often present in photoelectrochemical systems, several rate constants, including the intrinsic proton reduction rate for monolayer-rich WS2, were estimated. This value may be extended to photoelectrochemical and electrochemical systems using WS2 as a hydrogen evolution catalyst and thus represents an important advance toward design and implementation of TMD-driven catalysis.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01287. Size and optical characterization of monolayer-rich WS2, a comparison of transient absorption data and photochemical hydrogen production from few-layer WS2 dispersions (PDF)

where kpr is the rate constant of proton reduction, i.e., electron transfer to a proton with AA present, and knr,2 is nonradiative recombination from the noncharge transfer pathway of deeptrap excitons (DT) absent AA. Because WS2 is a known HER catalyst and there are no other obvious reductive pathways, the decay of this energetic third component in the presence of AA is likely associated with proton reduction. Trapped carriers are still expected to have a TA signal; therefore, the third component lifetime must correspond to the system returning to its ground state. Component 3 is not significantly redshifted, which would be an indication of low-energy midgap states. Hence, these energetic electrons, believed to be trapped at catalytic edge sites, should be fully capable of performing HER. Comparable kinetics between the WS2 reference (pH = 6.9) and WS2-DA (pH = 3.7) solutions (section SI.6) appear to exclude contributions from pH absent AA. pH-dependent kinetics in the presence of AA is the subject of future work. Estimated rate constants for each carrier relaxation pathway are summarized in Table 2. Values are based on an average of time

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kt (s−1)

knr (s−1)

kpr (s−1)

knr,2 (s−1)

109,48

3 × 1012

3 × 1010

7 × 108

2 × 109

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeremy R. Dunklin: 0000-0003-0070-8167 Hanyu Zhang: 0000-0001-9942-8186 Jun Liu: 0000-0003-0159-9767 Jao van de Lagemaat: 0000-0001-5851-6163 Author Contributions

J.R.D. fabricated samples, performed optical characterization, global analysis, and photochemical experiments, and prepared manuscript text and figures. H.Z. performed AFM and assisted with development of the work. Y.Y. assisted with initial TA experiments. J.L. performed TEM and subsequent nanosheet length analysis. J.v.d.L. directed the work and refined compilation and finalization of the manuscript.

Table 2. Estimated Rate Constants for Various Carrier Decay Pathways Based on Scheme 1 and Average of TA Lifetimes for Both 20 and 200 nJ Excitation krad (s−1)

ASSOCIATED CONTENT

S Supporting Information *

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Xiaochuan Xiao, Elisa M. Miller, and Nadezhda V. Korovina for assistance with AFM, gas chromatography, and global analysis, respectively. This work was authored by the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding was provided by Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

constants for both 20 and 200 nJ excitation. On the basis of this scheme, kpr equals GA-derived k3 with AA ≈ 7 × 108 s−1, while knr,2 equals GA-derived k3 without AA′ ≈ 2 × 109 s−1. This rate constant for HER, the first such reported for a TMD, is expected to be valid for photoelectrochemical and electrochemical systems using small monolayer-rich WS2 and is similar to reported rates (106−108 s−1) for Au-CdS nanorods.51 kpr represents an upper limit for an overall HER photocatalytic rate on WS2 because it represents only the first electron transfer step. Overall, this fundamental approach represents an essential first step in better understanding photochemical, photoelectrochemical, and electrochemical systems involving TMD catalysts. In summary, the work herein utilized a combination of optical spectroscopies, including TA, and photocatalytic measurements to study the relaxation dynamics of monolayer-rich aqueous dispersions of ca. 30 nm WS2 nanosheets. An over 3-fold increase in carrier lifetimes following light excitation was observed upon addition of hole-scavenging AA. Scavenging of photoexcited holes permitted a 14-fold increase

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