Removing Defects in WSe2 via Surface Oxidation and Etching to

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Removing Defects in WSe2 via Surface Oxidation and Etching to Improve Solar Conversion Performance Melinda J. Shearer, Wenjie Li, Jayson Foster, Matthew J. Stolt, Robert J Hamers, and Song Jin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01922 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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ACS Energy Letters

Removing Defects in WSe2 via Surface Oxidation and Etching to Improve Solar Conversion Performance Melinda J. Shearer,1 Wenjie Li,1 Jayson G. Foster,1,2 Matthew J. Stolt,1 Robert J. Hamers,1* and Song Jin1* 1

Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA 2

Department of Physical Science, Dixie State University, St. George, Utah, 84770, USA *Corresponding author email: [email protected] and [email protected]

Abstract Layered metal dichalcogenide materials (MX2) have great potential for solar energy conversion. However, as-grown MX2 materials often contain edge and terrace defects that degrade semiconducting properties and hinder their solar performance. Herein, we demonstrate a simple approach to removing surface defects and improving the solar performance by using UV-generated ozone to oxidize the surface of WSe2 nanoplates and single crystals, followed by a simple soak in aqueous solutions to remove the oxide. Structural characterizations reveal that defective edges and basal plane defect sites are selectively oxidized and subsequently etched, and the ratio of the non-stoichiometric WSex species is reduced. After this treatment, p-type WSe2 single crystals show increased electron accumulation on the surface and significantly enhanced photoelectrochemical solar conversion efficiency. These results and insights will be useful in the improvement and utilization of layered MX2 materials based on both Se and S for solar energy conversion and other device applications.

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TOC Graph:

UV/ozone 40-60% RH

Etch (aq)

Photocurrent density (mA/cm2)

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Potential (V)

The layered transition metal dichalcogenides (MX2) materials, such as WSe2 and MoS2, were extensively studied in the 1970s and 1980s for their promising photovoltaic performance when grown as single crystals.1-3 In particular, n-type WSe2 single crystals have shown solar conversion efficiencies as high as 14% in a photoelectrochemical (PEC) liquid junction cell.2 In order to achieve such high efficiency, some surface treatment of this material was required, achieved through a variety of methods with various levels of success and reproducibility: (photo)electrochemical etching,2,

4-5

ligand modification,4,

6-7

and surface polymerization.4,

6

However, the underlying mechanisms for these treatments and improvements have not been not well understood. It was hypothesized that these surface treatments or modifications are necessary because step edges act as recombination sites for the photogenerated carriers.7-8 Because of this problem, polycrystalline thin films of these materials were never able to achieve the high efficiencies of their single crystal counterparts,9 although improvements could be made via polymerization of the surface.10 Notably, the ligand modification and polymerization treatments are only temporary fixes for the PEC performance.7 More recent work has shown promising performance for PEC solar water splitting using both single crystals11 and exfoliated thin films of WSe212-14 as the photoabsorber, with improved performance using ligand modification,15 but more work is needed to increase efficiency. Etching or corrosion of single-crystal MX2 materials, often 2 ACS Paragon Plus Environment

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ACS Energy Letters

preferentially along the step edges or dislocation defects, were commonly observed throughout the literature.2-5 Photoelectrochemical etching has been reported to significantly increase the solar efficiency of n-type WSe2 single crystals. However, it requires the use of strong acid or base at elevated temperatures.2-3, 16 One additional oddity is that the etching process actually appears to increase the number of edges,4 which would seem counterintuitive for the increased performance. In fact, if the MX2 materials are etched in a different way, these edges will degrade the PEC performance17 and can enhance the electrocatalytic activity for the hydrogen evolution reaction (HER).18 Clearly more information is needed to understand the changes in the physical properties of these materials due to various etching and other treatments and to find a more robust and reproducible method for improving their solar performance. In recent years, the emphasis has shifted to studying these layered materials in their nanostructured forms, and much progress has been made in the synthesis and properties of these materials with varying number of layers19-20 and in a variety of different layer stackings.21-23 Through this work, as well as the study of MX2 materials in hydrogen evolution reaction (HER) catalysis,24 it has been discovered that the basal planes of these MX2 layered materials are not nearly as defect-free as was once believed, and often contain many defects, particularly chalcogenide vacancies.25-29 Such basal plane defects were shown to contribute significantly to the catalytic activity of MX2 materials for HER.30-32 In fact, spatially-resolved PEC measurements revealed that the PEC performance of the different terraces of the same p-type WSe2 single crystal could vary greatly, and these terraces could actually be a larger contributor to the PEC performance loss for the overall single crystal than the edges.33 We believe that these recent findings change the way these materials should be treated, and new surface treatment methods that could passivate or remove both edges as well as defects on the basal plane are needed. One suggested method for


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modifying the basal plane has been thiol ligand modification, which was shown to heal chalcogen vacancies on the surface of MX2 materials.31, 34 Additionally, work has been done on remediating these vacancies using acid treatments35-36 or annealing in a chalcogen atmosphere at high temperature.15, 37 Among these defect-remediation methods, only the last one has proven to work with both S and Se vacancies, and the effects that these methods have on the properties vary. The development of a simple defect-remediation method for both the edges and the terraces of MX2 materials that can be operated in ambient conditions would be ideal for implementation of these materials at both the bulk and nano level. Another avenue for modifying the surface of MX2 materials is oxidation: through exposure to ozone at elevated temperature,38 using UV-ozone systems,39-40 or via a focused laser beam treatment.41 One might hypothesize that defective step edges and basal plane vacancy sites are more reactive than other types of defects or defect-free surfaces and edge planes during such oxidation reactions. In fact, step edges and dislocation defects have been known to be oxidized or corroded at a macroscopic scale in photoelectrochemical cells.2,5,42 Therefore, we might be able to selectively remove the defect states of MX2 materials using mild oxidation treatments. However, previous efforts either do not intentionally remove the oxide to produce a more defect-free basal plane surface,39-41 or they remove the oxide through a harsh argon ion bombardment;38 certainly none have examined how such selective oxidation on the MX2 basal plane could have impacted the solar performance of bulk MX2 materials. In this work, we demonstrate a simple method to remove the defect states of p-type WSe2 at both the edges and the basal plane terraces using oxidation from exposure to UV-generated ozone, followed by a simple etching process in mild neutral aqueous solutions. We first use WSe2 nanoplates with many screw dislocation spiral steps and edges21 as a model system to demonstrate that the oxidation and etching can modify the


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surface, which are readily characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). After establishing the method, we then use single crystals of p-type WSe2 to show the change in the electronic properties and solar performance. Surface photoresponse (SPR) measurements reveal an increase in electron accumulation near the surface after etching.

Finally, we demonstrate that this simple oxidation and etching method can

drastically improve the solar conversion performance of p-type WSe2 single crystals in PEC liquidjunction cells.

WSe2 nanoplates were synthesized via chemical vapor deposition (CVD) using a previously described method21 (see Materials and Methods in the SI) onto silicon wafers. The synthesis produces numerous nanoplates in various shapes and sizes that contain screw dislocations with regularly repeating step edges, as shown in the atomic force microscope (AFM) images in Figure 1a and b. These nanoplates can be synthesized quickly and reproducibly to provide an easy way to study both terraces and edges, and therefore are an ideal model system for investigating oxidation and etching of the steps and surface. These WSe2 plates were oxidized via exposure to ozone generated under UV light in ambient environments, upon which ozone breaks down into reactive O species that will attack the surface of the WSe2 and create WOx species. After 1 hour of UV-ozone oxidation, the WSe2 nanoplate displays clear signs of oxidation at the edges as well as on the terraces, as shown in Figure 1c and d. The oxidation is readily apparent in AFM as raised areas on the surface of the nanoplates about 2 nm higher than the WSe2. Figure 1d shows that the terraces contain triangular oxidation features, which have been reported previously.38 The orientation of the triangular oxide features also depends on the orientation of the underlying MX2 layer. We showed in an earlier work that the two layers of the hexagonal



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spiral nanoplates are rotated by 60°.21 Therefore, the orientation of the triangular islands change from one layer to the next, which is clearly shown in Figure 1d. This change suggests that the oxidation is nucleating from defect sites such as Se vacancies and then propagating along the selenium zigzag edge, which has been verified with ab initio studies.38 Oxidation at the edges occurs most likely as a result of dangling bonds, and oxidation passivates these reactive species.43

Figure 1. AFM images of a hexagonal WSe2 nanoplate before and after oxidation and etching. (a) AFM image of the original WSe2 nanoplate with two hexagonal spirals. xy scale bar: 2 µm and z height scale bar: 80 nm. (b) Zoom-in view of the center of the original hexagonal nanoplate. (c) AFM image of the center of the same plate after 1 h of UV-ozone exposure, causing oxidation. (d) Zoom-in view of the region marked by the black box in c. (e) AFM image of the same plate after etching in pH 7.4 phosphate buffer for 10 min. (f) Zoom-in view of the area surrounded by the black box in e. Scale bars: xy scale bar: 500 nm for (b-f). z-height scale bar: 20 nm for b, c, and e and 4.5 nm for d and f. With increasing UV light exposure time, these oxide features continue to form on the surface, as shown in Supporting Information (SI) Figure S1. Eventually they cover the entire surface of the WSe2 nanoplates, as shown in SI Figure S2a. Interestingly, we observed that the 6 ACS Paragon Plus Environment

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oxidation occurs more readily and consistently in high relative humidity (RH) conditions (4565%), as shown in SI Figure S2, and therefore we have controlled the environmental humidity during the oxidation steps (see Materials and Methods). The oxide on the WSe2 surface can be easily removed by immersing the oxidized samples in a 1.0 M pH 7.4 phosphate buffer for 10 min. The dissolution occurs following the equilibrium equation:44 WO3 + H2O

WO42- + 2H+

Therefore, the dissolution can be promoted by immersing in slightly basic conditions, but will occur in neutral and slightly acidic conditions (pH 4-5) as well, as indicated by the W Pourbaix diagram. Figure 1e and f show the morphology of the nanoplates after the etching step. Etching of the oxide leads to many triangular etch pits on the surface of the nanoplates in the middle of the terraces, as well as triangular sawtooth edges that were etched from the step edges, as shown in Figure 1f. These further confirm the selective oxidation and removal of the reactive defect sites at both the terraces and edges of WSe2. To verify that the UV light exposure was causing oxidation of the surface, we collected XPS spectra on the WSe2 nanoplates before oxidation (labeled original), after 1 hour oxidation, and after etching in phosphate buffer (Figure 2).

The corresponding AFM images of a

representative WSe2 nanoplate from this sample are shown in Figure S3. High resolution data of the W 4f and Se 3d peaks are shown in Figure 2a and 2b, respectively. The bottom spectra are from the original nanoplates, which were fit into two distinct sets of peaks: stoichiometric WSe2 (black)45 and non-stoichiometric WSex (blue),38, 46 in which the two peaks are the 4f7/2 and the 4f5/2 spin-orbit component for the W and the 3d5/2 and 3d3/2 spin-orbit component for the Se of each species. For the original plates, the stoichiometric peaks (black) are at 33.0 and 35.2 for the W



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region and 55.3 and 56.1 eV for the Se, while the non-stoichiometric peaks (blue) are at 31.9 and 34.1 eV for the W and 54.2 eV and 55.0 eV for the Se. The presence of the non-stoichiometric WSex peaks suggests that there are significant concentrations of Se vacancies at the surface of the material. The small green peak is most likely from a combination of a small amount of WOx species from exposure to air as well as the W 5p3/2 peak, which is also in that region.

Figure 2. XPS data of the WSe2 nanoplates before oxidation (original), after 1 h of oxidation, and after 10 min etch in pH 7.4 phosphate buffer. (a) W 4f and (b) Se 3d region, with peak fittings for the four species found on the surface: WSe2 (black), WSex (blue), and WOx (green, left) and SeOx (green, right). The middle XPS spectra are of the nanoplates after 1 hour oxidation via UV exposure. Both the W and the Se spectra show an increase in the higher binding energy peaks (green). These peaks, at 36.2 eV and 38.4 eV for W and 59.4 eV and 60.2 eV for Se, are oxide peaks.47 The oxide species for W is most likely WO3 but could contain some non-stoichiometric oxide (WOx, where


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1 ≤ x ≤ 3) as well.48-49 Although there is significantly more oxidation of W than of Se, some Se is shown to be oxidized to form SeOx.47 The stoichiometric WSe2 peaks are now at 32.9 and 35.1 eV for the W and 55.1 and 55.9 eV for the Se, and the non-stoichiometric WSe2 peaks are now at 32.4 and 34.6 eV for W and 54.5 and 55.4 eV for the Se region. The non-stoichiometric peaks have been shifted to slightly higher binding energies, which could be a result of the oxides changing the local chemical environment of this species. Raman spectra of the WSe2 nanoplates after oxidation showed no WO3 peaks (see SI Figure S4), suggesting that either the oxide is amorphous or the signal is below the sensitivity of the instrument. After a 10 min phosphate buffer etch, the oxide peak is reduced to its original intensity for the W and eliminated for the Se. The stoichiometric WSe2 peaks are at 32.5 and 34.6 eV for W and 54.8 and 55.7 eV for Se, and the non-stoichiometric WSe2 peaks are now at 31.2 and 33.4 eV for W, and 53.5 and 54.3 eV for Se. Upon etching of the oxide, the non-stoichiometric peaks have shifted back to lower binding energies. Interestingly, the WSex peaks have also been significantly reduced in total peak area relative to the WSe2 peaks. This change in peak areas suggests that the oxidation and etching lead to surfaces that have fewer Se vacancies. Therefore, this simple UVozone treatment followed by a mild etching step can be a method to remove the defects on the WSe2 surface, exposing less reactive edges, and yield surfaces that contain fewer Se vacancies at basal planes and fewer dangling bonds at the edges than as-grown materials. Because of their facile synthesis and regular step edges, the WSe2 nanoplates were a good model system for us to develop the oxidation and etching procedure, as well as to study the effects of such oxidation on the basal plane and the edges. Following the establishment of the best oxidation and etching procedures, we switched to WSe2 single crystals in order to understand the effects of this surface treatment on the electronic properties. Plate-like p-type WSe2 single crystals



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of 1-4 mm in size were synthesized via chemical vapor transport (CVT) (see Materials and Methods in the SI). A representative Raman spectrum and XRD pattern of the single crystals (shown in SI Figure S5) confirm that they are 2H-WSe2. Optical microscope images with the corresponding AFM data from a representative WSe2 single crystal are shown in Figure 3. Figure 3a and b show the optical images of the overall crystal and a zoom-in view of the visible step edges on the crystal surface. AFM images taken in the blue boxed area in Figure 3b reveal some step edge features not visible under optical microscopy. AFM images were collected on the same general area of the single crystal before oxidation (original, Figure 3c, d), after 2 hours of oxidation (Figure 3e, f), and after etching in 0.5 M pH 4.3 phosphate solution for 10 min (Figure 3g, h). The etching solution was chosen to be the same electrolyte used in the photoelectrochemical (PEC) measurements (discussed later). From these images, it is clear that the surface of the WSe2 crystals is oxidized under UV-ozone treatment and the generated oxide is removed after etching, similar to the behaviors observed for the WSe2 nanoplates. Additional AFM images of the single crystal after only 1 hour oxidation as well as 2 additional hours of oxidation can be found in SI Figure S6. In addition, we can carry out the etching step in deionized (DI) water and 1 M phosphate buffer solution at pH 7.4, and the etching effect on WSe2 samples is almost identical within the pH range tested (4.3 to 7.4).


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Figure 3. Optical microscopy and AFM images of a WSe2 single crystal going through the oxidation and etching process. (a, b) Optical microscope images of the WSe2 single crystal, where b shows the zoom-in region depicted by the black box in a. Blue box shows the area where the AFM images were taken. Scale bars: 100 µm for (a) and 50 µm for (b). (c, d) AFM images of the original single crystal, zoomed in subsequently from c to d. z scale bar: 3.5 nm (c) and 0.4 nm (d). (e, f) The same crystal after 2 hours of oxidation, where zoom-in view f reveals triangular oxide features decorating the surface. z scale bar: 8 nm (e) and 5.5 nm (f). (g, h) The crystal after etching in pH 4.3 phosphate buffer for 10 min. Zoom-in image in h shows etching of the oxide features. z scale bar: 15.5 nm (g) and 10 nm (h). xy scale bars in c-h: 500 nm. After confirming that the single crystals behave similarly to the nanoplates during oxidation and etching, time-resolved surface photoresponse (SPR) measurements were collected on the single crystals. SPR is a non-contact metal-insulator-semiconductor technique that allows for the measurement of charge accumulation on the surface of the sample upon illumination.50-51 Figure 4 shows the results of the SPR measurements of two samples of WSe2 single crystals under illumination at 500 nm, including both the surface photovoltage (SPV) as well as the photoinjected



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charge. The SPR collected on an ensemble of several smaller WSe2 crystals from the same CVT batch was compared before oxidation, after oxidation, and after etching in DI water for 10 min (Figure 4a), and compared to a WSe2 crystal that was not oxidized but still etched in DI water for 10 min (Figure 4b). The original SPR measurements conducted on both sets of single crystals, the dark red traces, show a negative photoinjected charge, which can be attributed to electrons accumulating in the near-surface region. The accumulation of electrons is consistent with these WSe2 single crystals being p-type, as p-type materials tend to have downward band bending due to mid-gap surface states.52

Figure 4. SPR measurements of p-type WSe2 single crystals from the same CVT batch, after various treatments. (a) Surface photovoltage (SPV, top) and photoinjected charge (bottom) over time of the original sample (dark red trace), after 1 h oxidation (blue trace), and after etching in DI water (orange trace). (b) SPV (top) and photoinjected charge (bottom) of a control sample with an original spectrum (dark red trace), a spectrum taken after 1 h but with no change to the sample (blue trace) and finally after a 10 min etch in DI water (orange trace).


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After the sample has been oxidized, the SPV and the charge spectra (blue traces, Figure 4a) show a slight decrease in overall signal but no significant change. After removal of the oxide via etching, the SPV and charge signals significantly increased (orange trace), indicating that more electrons are accumulating near the surface. This large increase suggests that after the oxidation and etching, the WSe2 crystals have decreased charge recombination at the surface. It has been theoretically predicted that chalcogen vacancies can create deep acceptor states close to the conduction band in MX2 materials,26, 53 specifically, about 0.6 eV below the conduction band for WSe2.41 These acceptor states can act as traps for electrons, limiting electron mobility and leading to recombination with holes or repulsion of subsequent illumination-induced electrons away from the surface.41 The decrease in the number of Se vacancies on the surface of the WSe2 would reduce these defect states within the band gap, and lead to less recombination and enhanced p-type conductivity. Note that such Se vacancies are different from the “mid-gap states” mentioned above that cause downward band bending, thus such surface band bending would not be significantly affected by this oxidation and etching treatment. The p-type doping we observe, which is commonly seen for WSe2, is most likely the result of tungsten vacancies,54 which in the bulk would also not be affected by our surface treatment. Interestingly, in contrast, for the control sample that was not oxidized (Figure 4b), there is only a small increase in signal after the etching step. This control experiment confirms that the changes in SPV and photoinjected charge are a direct result of the selective oxidation process, and not just due to changes at the surface from the etching step. We further tested the PEC solar performance of individual p-type WSe2 single crystals with exposed surface area of approximately 0.02-0.05 cm2 using a 50 mM solution of Ru(NH3)3+/2+ as the redox shuttle in 0.5 M pH 4.3 phosphate solution (see Materials and Methods for details on electrode fabrication and measurement conditions). Optical images of one photoelectrode before



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and after the PEC measurements are shown in Figure 5a and b. The current density (J) vs. potential (V) measurement of this WSe2 photoelectrode is shown in Figure 5c, where the crystal was measured under 1 sun (100 mW/cm2) of AM 1.5 G simulated solar illumination and compared to a dark curve taken with no illumination (dark blue curve). The performance of the photoelectrode was evaluated using the ideal regenerative cell method, where short-circuit current density (Jsc), open-circuit potential (Voc) and fill factor (FF) are all referenced to the equilibrium potential of Ru(NH3)3+/2+ redox couple.55 Compared to the original J-V curve (dark red trace), the WSe2 single crystal after 2 hours of oxidation showed a large increase in the Jsc from -1.96 to -10.6 mA/cm2 (Oxid. 2 h, teal curve). The measured Voc increased from 0.174 V to 0.289 V (onset potentials were all taken at a J of -0.2 mA/cm2), and the calculated FF also increased from 18.3% to 32.4%. We noticed that the oxide can be readily removed by the phosphate solution used during the PEC measurement, as confirmed with the AFM data shown in Figure 3. Therefore, an intentional etching step is not necessary before the PEC measurement, and can even decrease the performance of the single crystal photoelectrode (see SI Figure S7).

Furthermore, this single crystal

photoelectrode was oxidized for an additional 2 hours, but while the overall Jsc performance did increase slightly to -11.2 mA/cm2 (Oxid. 4 h, orange trace), the enhancement is much smaller than that observed after the first oxidation step, and the Voc and FF decreased slightly to 0.259 V and 27.6%, respectively. This suggests that the removal of defects from the surface is self-limiting, and continuous exposure to UV-ozone and more oxidation will not lead to continuous significant improvement in solar performance. This is also supported by Figure 1 and Figure S1 showing that the most significant oxidation occurred during the first UV treatment period, and subsequent treatments continued to oxidize more materials but only slightly increased the oxidized area. Moreover, as shown in SI Figure S8, when the same fabricated photoelectrode was re-measured


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after storage in ambient conditions for 24 days, the improved performance from the oxidation and etching was still apparent. Additional PEC measurements on other WSe2 single crystals and with other treatment conditions are shown in SI Figures S7 and S9. Of the 5 different single crystal electrodes measured, all showed improvements in the photocurrent density and fill factor after the oxidation treatment, although the overall best PEC performance of the single crystal photoelectrodes varied greatly depending on the quality of both the crystals and the fabricated photoelectrodes. No significant performance increase was observed for a control sample that was not oxidized (see SI Figure S7b).

Optical and AFM images taken before and after PEC

measurements show little evidence of photocorrosion (see Figure 5a,b and S10), which is different from the observations of significant photocorrosion along step edges and dislocations previously reported.2,5,42



a Original

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b After PEC

c

Figure 5. Optical microscope images and PEC performance of a WSe2 single crystal photoelectrode with an exposed surface area of approximately 0.026 cm2. (a) Optical image of the original WSe2 crystal after mounting in epoxy to make the PEC electrode. (b) Optical image after the oxidation and PEC measurements. Scale bars: 100 µm. (c) Photocurrent density vs. potential graph of the single crystal WSe2 photoelectrode shown in a and b. The potential was recorded relative to the reference standard calomel electrode (SCE). Scans were taken under 1 Sun illumination before oxidation (dark red trace), after oxidation for 2 h (Oxid. 2 h, teal trace), and after oxidation for an additional 2 h, for a total of 4 h oxidation time (Oxid. 4 h, orange trace). A dark scan (dark blue trace) is also shown as reference. The vertical dashed line indicates the equilibrium potential for the redox couple, measured with a Pt mesh working electrode. The variations in the noisy level of different curves were caused by the variations in the disturbance of measurements caused by stirring. The source of this PEC solar performance improvement is likely from the oxidation and etching of the WSe2 surface that removes the more reactive edges and surface defects, particularly the Se vacancies. The observation of the lack of further improvement in PEC performance after additional oxidation suggests that the defective sites are selectively oxidized and removed during 16 ACS Paragon Plus Environment

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this oxidation and etching treatment. The removal of these reactive and defective sites leads to enhanced accumulation of electrons in the near-surface region, as shown by the SPR measurements discussed above. Additionally, the modified surface states could contribute to a change in the semiconductor band positions, which may promote charge transfer at the semiconductorelectrolyte interface.

While previous study showed that annealing MX2 monolayers in a

chalcogen-rich atmosphere at high temperature can increase the conductivity of the material as the chalcogen vacancies are healed,37 the new approach we describe here perhaps yields similar results, but under ambient conditions with a common laboratory instrument (a UV lamp). Prior studies of photoetching and photoelectrochemical etching on WSe2 single crystals have used treatments that likely remove a significant amount of material especially along step edges and screw dislocation defects,2, 4, 5, 42 which would likely be problematic when using nanomaterials. In contrast, because only some of the surface material is removed by our mild oxidation and etching treatment, we anticipate that it is better suited for thin films14 and the very thin, 2-dimensional MX2 materials that are currently of intense interest.34-41 We believe that this oxidation and etching treatment method could be a general approach to improve the solar performance of p-type MX2 materials that are either S or Se based, and have preliminarily demonstrated that this treatment also leads to improved PEC performance for p-type MoS2 single crystals found naturally, as shown in SI Figure S11. We have demonstrated that a simple and mild oxidation treatment using UV-generated ozone with subsequent etching in aqueous solutions can remove the defects on the surface of WSe2 nanoplates and single crystals and improve their solar performance. Using spiral WSe2 nanoplates, we showed with AFM that the oxidation occurs at the edges as well as on the terraces of these materials, and verified that this treatment created surfaces with fewer Se vacancies using XPS.



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The same surface treatment on p-type WSe2 single crystals increases the amount of electron accumulation near the surface of the single crystals, most likely due to decreased recombination at the surface as a result of the removal of these vacancies. Finally, this surface treatment can significantly improve the solar conversion performance (photocurrent density, photovoltage and fill factor) of p-type WSe2 single crystals in PEC liquid-junction solar cells. The surface treatment developed here selectively removes reactive edges as well as defect centers within the basal plane in a self-limiting process under ambient conditions, and can be generally applied to both S and Se based MX2 materials. These results and new insights will be useful in the improvement and utilization of these layered MX2 semiconductor materials for solar energy conversion, such as PEC hydrogen generation and CO2 reduction, and other device applications.

Acknowledgements This research is supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under Award DE-FG02-09ER46664. M.J.S. and M.J.S. also thank the NSF Graduate Research Fellowship Program for support. Support was also provided by the Graduate School and the Office of the Vice Chancellor for Research and Graduate Education at the University of Wisconsin—Madison with funding from the Wisconsin Alumni Research Foundation. J.G.F. also thanks the REU support from the University of WisconsinMadison Graduate School and National Science Foundation through the University of WisconsinMadison Materials Research Science and Engineering Center (DMR-0520527) and CHE1659223. Additional Information


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ACS Energy Letters

The Supporting Information is available free of charge on the ACS Publications website. Additional AFM images and Raman spectra of the WSe2 spiral plates before and after the oxidation and etching treatment, as well as additional AFM images, XRD and Raman data of the WSe2 single crystals. Additional results of WSe2 single crystal photoelectrochemical performance, along with XRD and Raman characterization of a p-type MoS2 single crystal with photoelectrochemical performance before and after oxidation and etching treatment. Corresponding Authors: Correspondence

should

be

addressed

to

R.J.H.

([email protected])

and

S.J.

([email protected]). References (1) Tributsch, H. Layer-Type Transition-Metal Dichalcogenides - New Class of Electrodes for Electrochemical Solar-Cells. Ber. Bunsen-Ges. Phys. Chem 1977, 81, 361-369. (2) Tenne, R.; Wold, A. Passivation of Recombination Centers in n-WSe2 Yields High-Efficiency (Greater-Than-14-Percent) Photoelectrochemical Cell. Appl. Phys. Lett. 1985, 47, 707-709. (3) Prasad, G.; Srivastava, O. N. The High-efficiency (17.1%) WSe2 Photo-electrochemical Solar Cell. J. Phys. D: Appl. Phys. 1988, 21, 1028. (4) Lévy-Clément, C.; Tenne, R., Modification of Surface Properties of Layered Compounds by Chemical and (Photo)Electrochemical Processes. In Photoelectrochemistry and Photovoltaics of Layered Semiconductors, 1992; pp 155-194. (5) Jakubowicz, A.; Mahalu, D.; Wolf, M.; Wold, A.; Tenne, R. WSe2 - Optical and ElectricalProperties as Related to Surface Passivation of Recombination Centers. Phys. Rev. B 1989, 40, 2992-3000. (6) Canfield, D.; Parkinson, B. Improvement of Energy-Conversion Efficiency by Specific Chemical Treatments of MoSe2-Photoanode and WSe2-Photoanodes. J. Electrochem. Soc. 1981, 128, C114-C115. (7) Parkinson, B. A.; Furtak, T. E.; Canfield, D.; Kam, K. K.; Kline, G. Evaluation and Reduction of Efficiency Losses at Tungsten Diselenide Photoanodes. Faraday. Discuss. 1980, 70, 233-245. (8) Lewerenz, H. J.; Ferris, S. D.; Doherty, C. J.; Leamy, H. J. Charge Collection Microscopy on p-WSe2 - Recombination Sites and Minority-Carrier Diffusion Length. J. Electrochem. Soc. 1982, 129, 418-423. (9) Cabrera, C. R.; Abruna, H. D. Synthesis and Photoelectrochemistry of Polycrystalline ThinFilms of p-WSe2, p-WeS2, and p-MoSe2. J. Electrochem. Soc. 1988, 135, 1436-1442. (10) Cabrera, C. R.; Abruna, H. D. Blocking of Recombination Sites and Photoassisted Hydrogen Evolution at Surface-modified Polycrystalline Thin Films of p-Tungsten Diselenide. J. Phys. Chem. 1985, 89, 1279-1285. 19 ACS Paragon Plus Environment


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ACS Energy Letters

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Removing Defects in WSe2 via Surface Oxidation and Etching to

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