Structuring Pd Nanoparticles on 2H-WS2 Nanosheets Induces

Sep 27, 2017 - Structuring Pd Nanoparticles on 2H-WS2 Nanosheets Induces Excellent Photocatalytic Activity for Cross-Coupling Reactions under Visible ...
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Cite This: J. Am. Chem. Soc. 2017, 139, 14767-14774

Structuring Pd Nanoparticles on 2H-WS2 Nanosheets Induces Excellent Photocatalytic Activity for Cross-Coupling Reactions under Visible Light Faizan Raza, DaBin Yim, Jung Hyun Park, Hye-In Kim, Su-Ji Jeon, and Jong-Ho Kim* Department of Chemical Engineering, Hanyang University, Ansan 15588, Republic of Korea S Supporting Information *

ABSTRACT: Effective photocatalysts and their surface engineering are essential for the efficient conversion of solar energy into chemical energy in photocatalyzed organic transformations. Herein, we report an effective approach for structuring Pd nanoparticles (NPs) on exfoliated 2H-WS2 nanosheets (WS2/PdNPs), resulting in hybrids with extraordinary photocatalytic activity in Suzuki reactions under visible light. Pd NPs of different sizes and densities, which can modulate the photocatalytic activity of the asprepared WS2/PdNPs, were effectively structured on the basal plane of 2HWS2 nanosheets via a sonic wave-assisted nucleation method without any reductants at room temperature. As the size of Pd NPs on WS2/PdNPs increased, their photocatalytic activity in Suzuki reactions at room temperature increased substantially. In addition, it was found that protic organic solvents play a crucial role in activating WS2/PdNPs catalysts in photocatalyzed Suzuki reactions, although these solvents are generally considered much less effective than polar aprotic ones in the conventional Suzuki reactions promoted by heterogeneous Pd catalysts. A mechanistic investigation suggested that photogenerated holes are transferred to protic organic solvents, whereas photogenerated electrons are transferred to Pd NPs. This transfer makes the Pd NPs electron-rich and accelerates the rate-determining step, i.e., the oxidative addition of aryl halides under visible light. WS2/ PdNPs showed the highest turnover frequency (1244 h−1) for photocatalyzed Suzuki reactions among previously reported photocatalysts.

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INTRODUCTION C−C coupling reactions are of great importance in various organic transformations and polymer syntheses.1−3 C−C bonds can be formed via transition metal-catalyzed coupling reactions, such as the Suzuki−Miyaura, Heck, and Sonogoshira reactions.4−10 These C−C coupling reactions involve redox processes, including the oxidative addition of substances to metal catalysts and the reductive elimination of the corresponding products.11,12 The redox processes occur through electron transfer between transition metal catalysts and organic substances, which typically requires elevated temperatures or sacrificial reagents. However, electron transfer-mediated C−C coupling can also be triggered by visible light at room temperature in the presence of photocatalysts.3,13−15 Photoexcitation induces electron−hole separation in the photocatalysts, followed by electron or hole transfer to substances for initiating C−C coupling reactions. Two types of photocatalysts are widely employed in photoredox reactions: organometallic compounds13,14 and semiconductors.16−20 In particular, semiconductors are considered promising photocatalysts for organic transformations due to effective absorption of visible light, good durability, capacity for multivalent binding to substances, and cost-effectiveness. To improve the photocatalytic activity of semiconductor photocatalysts in C−C coupling reactions, proper modification is required to adjust © 2017 American Chemical Society

their redox potentials and allow effective transfers of excited electrons or holes to substances under visible light. Hence, developing facile and effective approaches for modulating the photophysicochemical properties of semiconductor photocatalysts is of great interest in visible light-driven photocatalysis. Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently received significant attention because of their intrinsic semiconducting properties and unique electronic and optical characteristics.21−26 In particular, TMDs exhibit thickness-dependent electronic structures; that is, they have direct band gaps in the visible spectrum when their thickness decreases to monolayer from a multilayered bulk with indirect band gaps.27−29 Among TMDs, exfoliated WS2 nanosheets have a band gap (1.9 eV) and large absorption in the visible spectrum and, thus, exhibit good performance in several photocatalytic reactions, including degradation of dyes,30 H2 evolution,31 reduction of nitrophenol,32 and the oxidative coupling of amines.20 To extend the photocatalytic applicability of semiconducting (2H) WS2 nanosheets to C−C coupling reactions, it is essential to modify their interfaces with organic molecules or metals. However, the proper functionalization of semiconducting WS2 nanosheets able to promote C− Received: August 18, 2017 Published: September 27, 2017 14767

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

Article

Journal of the American Chemical Society

with a spacing of 0.22 nm and WS2 nanosheets with a spacing of 0.27 nm. In addition, W, S, and Pd atoms were distinctly observed in the atomic mapping images (Figure 2d−f), showing that the desired 2H-WS2/PdNPs hybrids were synthesized successfully. We then examined if the coverage of Pd NPs on the WS2 nanosheets could be controlled by varying the molar ratio of Pd(OAc)2 to WS2 at a fixed concentration of PVP. As shown in Figure 3a, the fractional coverage of Pd NPs in the assynthesized 2H-WS2/PdNPs hybrids was very low (15%) when the molar ratio was 0.5. However, the fractional coverage of Pd NPs gradually increased as the molar ratio of the Pd precursor increased (Figure 3b,c), reaching 78% at a molar ratio of 1.25. These results indicated that the density of Pd NPs on 2H-WS2 nanosheets was controllable. Next, the effect of the ratio of PVP to Pd(OAc)2 on the size of the Pd NPs on the WS2 nanosheets was investigated. The concentration of PVP is known to influence the size of NPs in typical polyol methods. As shown in Figure S1a, in the absence of PVP, WS2/PdNPs hybrids were produced, but the Pd NPs on the surface of the hybrids were severely aggregated. Therefore, PVP is required to introduce Pd NPs uniformly without aggregation on the surface of 2H-WS2 nanosheets. Subsequently, the weight ratio of PVP to Pd(OAc)2 was varied from 1, 10, 20, and to 30 during a course of sonic wave-assisted reduction and nucleation. As shown in Figure 4, the average size of the Pd NPs on the WS2 nanosheets was 2.9 nm at the ratio of 1. When the ratio of PVP increased, the size of the Pd NPs also increased. The size of the Pd NPs reached 5.5 nm when the ratio was 30. The dependence of the size of the Pd NPs in the hybrids on the ratio of PVP is opposite to the behavior observed in typical polyol methods in which the size of the NPs normally decreases as the amount of PVP is increased.40 We speculate that increasing the amount of PVP could raise the local concentration of Pd(OAc)2 on the surface of WS2 nanosheets and, thereby, accelerate the growth of Pd NPs. The obtained results suggest that the size of the Pd NPs on the surface of the 2H-WS2 nanosheets was also controllable. The chemical composition of the 2H-WS2/PdNPs was then investigated using X-ray photoelectron spectroscopy (XPS). As shown in Figure 5a, the peaks appearing at 32.4 and 34.6 eV originate from W4+, indicating that the WS2 nanosheets have a semiconducting 2H phase.20 The peak corresponding to oxidized W6+ at 37.9 eV did not increase in the XPS spectrum of 2H-WS2/PdNPs, suggesting that the 2H-WS2 nanosheets remained stable during the reduction of the Pd nucleation reaction. The XPS spectra of S 2p3/2 and S 2p1/2 also show the characteristic peaks at 162.5 and 163.8 eV for 2H-WS2 nanosheets (Figure 5b). The slight shift observed in the binding energy of S 2p can be attribute to the growth of Pd NPs on the surface of WS2 nanosheets. The deposition of Pd NPs on the WS2 nanosheets may change the electron density of W−S bonds, causing the peak shift.41 In the XPS spectrum of Pd 3d (Figure 5c), two distinct and intense peaks corresponding to Pd0 appeared at 335.25 and 340.5 eV.42 In addition, two more peaks with very low intensity were observed at 337.0 and 342.5 eV for divalent Pd (Pd2+).43 This XPS analysis revealed that most Pd NPs on the WS2/PdNPs hybrids consisted of Pd0, which is known to be an active species able to promote Suzuki coupling reactions effectively. The optical properties of the 2H-WS2/PdNPs hybrids were then measured. No newly generated peaks were detected in the absorption spectra of the 2H-WS2/PdNPs, but the absorption

C coupling reactions under visible light has not been reported to date. Several studies have demonstrated metallic (1T) WS2 nanosheets that were modified with metal nanoparticles (NPs),33−37 but these materials are not suitable for visible light-driven photocatalysis because they are metallic. Herein, we report an effective approach for structuring Pd NPs on semiconducting (2H) WS2 nanosheets, which induced outstanding photocatalytic activity in Suzuki cross-coupling reactions at room temperature under visible light. A sonic waveassisted reduction and nucleation method without use of any reducing agents enabled to introduce Pd NPs with control over their density and size on the basal plane of 2H-WS2 nanosheets under very mild conditions (25 °C, 1.5 h). The as-prepared 2HWS2/PdNPs hybrids were then applied to photocatalyzed Suzuki cross-coupling reactions under visible light, and the effects of the density and size of the Pd NPs in the hybrids on their photocatalytic activity were investigated. In addition, the reaction mechanism responsible for the photocatalyzed Suzuki reaction, in particular, the effect of using a protic solvent on the photocatalytic activity of 2H-WS2/PdNPs was also fully investigated.

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RESULTS AND DISCUSSION To prepare 2H-WS2/PdNPs hybrids, a thin layer of 2H-WS2 nanosheets was exfoliated from bulk WS2 in N-methylpyrrolidone (NMP) using a modified liquid exfoliation method (see the Supporting Information). Then, as-prepared 2H-WS2 nanosheets were mixed with Pd(OAc)2 in ethylene glycol (EG) including poly(N-vinyl-2-pyrrolidone) (PVP), followed by sonication at 25 °C for 1.5 h (Figure 1). The mixture

Figure 1. Schematic representation of the synthesis of 2H-WS2/ PdNPs hybrids.

solution gradually turned black during sonication, indicating the formation of Pd NPs. In conventional polyol methods, the reduction and nucleation of metal ions for the formation of metal NPs require high temperatures (>100 °C) at which EG was oxidized to either acetaldehyde38 or glycolaldehyde39 to provide electrons. The sonic wave-assisted polyol method, however, allowed the rapid and effective formation of Pd NPs on the basal plane of 2H-WS2 nanosheets at a low temperature (25 °C) without the addition of any reductant. Mild bathsonication provided enough energy for EG to be rapidly oxidized at a low temperature, leading to the effective formation of Pd NPs on the basal plane of the 2H-WS2 nanosheets. 2H-WS2/PdNPs hybrids were then analyzed by transmission electron microscopy (TEM). As shown in Figure 2a, the WS2 nanosheets exfoliated in NMP had clear surface without noticeable particles, and their fast Fourier transformation (FFT) pattern revealed a semiconducting (2H) phase (inset). After sonication for 1.5 h at 25 °C, the WS2 nanosheets were densely and uniformly covered with Pd NPs of ca. 5.5 nm (Figure 2b). The Pd NPs formed on the basal plane as well as at the edge. Figure 2c clearly shows the lattice fringes of Pd NPs 14768

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

Article

Journal of the American Chemical Society

Figure 2. Structural analysis of as-prepared 2H-WS2/PdNPs hybrids. TEM images of (a) exfoliated 2H-WS2 nanosheets (inset: FFT pattern), (b) 2H-WS2/PdNPs, and (c) the lattice fringe of W and Pd. (d−f) W, S, and Pd elemental mapping images of 2H-WS2/PdNPs.

then stirred for 3 h under visible light irradiation using a white LED lamp (60 W). 4-BT was converted into the corresponding product 4-methylbiphenyl at 92% yield (entry 1). In the absence of the WS2/PdNPs catalyst, however, no desired product was obtained (entry 2). In addition, the reaction hardly proceeded without light irradiation, even when the WS2/PdNPs catalyst was present (entry 3). These results clearly indicate that the Suzuki cross-coupling reaction of 4-BT is photocatalyzed by WS2/PdNPs hybrids under visible light. For further control experiments, either 2H-WS2 nanosheets or Pd NPs were added alone to the reaction mixture, but no desired product was obtained (entries 5 and 6). Similarly, when the 2H-WS2 nanosheets were physically mixed with Pd NPs that had been separately synthesized, as shown in Figure S3, no product was produced by the reaction (entry 7). This result indicates that the proper interfacial interaction of WS2 nanosheets with Pd NPs is crucial for the photocatalytic activity of the WS2/PdNPs hybrids in Suzuki reactions. To elucidate the mechanism responsible for photocatalyzed Suzuki cross-coupling reactions, p-benzoquinone (BQ) was added to the reaction of 4-BT with PBA as an electron scavenger to trap the electrons transferred from Pd NPs to 4BT.47−49 As shown in Table 1 (entry 8), no desired product was produced in the presence of BQ. Furthermore, diisopropylethylamine (DIPEA) was added as a hole scavenger to the reaction solution to block hole transfer to the borate salt, which generally led to completely quenching the reaction in other papers.50 The WS2/PdNPs hybrids, however, were still active to promote the photocatalyzed Suzuki reaction in the presence of DIPEA although the reaction yield slightly decreased to 79% (entry 9). We speculate that the decreased yield might be attributed to strong chemisorption of basic DIPEA on the active sites of Pd NPs, preventing the oxidative addition of 4-BT during the reaction. This result of the hole scavenger indicates that the photogenerated holes on the WS2/ PdNPs hybrids would not transfer to the borate salt, and the different reaction mechanism should exist in this photocatalyzed Suzuki reaction.

Figure 3. Effect of different Pd(OAc)2/WS2 ratios on the fractional coverages of Pd NPs on 2H-WS2/PdNPs. TEM images of 2H-WS2/ PdNPs prepared at W/Pd ratios of (a) 4:1 and (b) 1:2. (c) Plot of the fractional coverage of Pd NPs on WS2 nanosheets against the Pd(OAc)2/WS2 ratios. The ratio of PVP to Pd(OAc)2 was fixed at 3.0.

of visible light from 300 to 700 nm increased significantly after the deposition of Pd NPs on the WS2 surface (Figure S2a).35 The Raman spectrum of the WS2/PdNPs hybrids shows an increase in the distance between the two peaks of the E12g and A1g vibration modes as compared with that of WS2 nanosheets (Figure S2b). The E12g peak was red-shifted by 1.83 cm−1, whereas the A1g peak was blue-shifted by 1.46 cm−1; these changes are attributed to the doping of the WS2 nanosheets with Pd.30 Then, the fluorescence of the WS2/PdNPs hybrids was measured. The fluorescence of the WS2/PdNPs hybrids was significantly quenched by the deposition of Pd NPs, indicating that the oxidative quenching of the fluorescence occurs under light irradiation (Figure S2c).44−46 The excited electrons of the WS2/PdNPs hybrids could be transferred to metallic Pd NPs, preventing the electron−hole pairs from recombining for fluorescence emission. We expect that this photoinduced electron transfer would make the Pd NPs more electron-rich, accelerating the oxidative addition of Pd catalysts to aryl halides. Next, the photocatalytic activity of the 2H-WS2/PdNPs hybrids was examined in Suzuki cross-coupling reactions under visible light irradiation at room temperature, as shown in Table 1. The WS2/PdNPs (45% Pd coverage, 2.85 μg) were added to a EtOH/H2O solution of 4-bromotoluene (4-BT), phenylboronic acid (PBA), and K2CO3 as a base. This mixture was 14769

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

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Journal of the American Chemical Society

Figure 4. Effect of PVP/Pd(OAc)2 ratios on the size of the Pd NPs on 2H-WS2 nanosheets. TEM images and size distributions of 2H-WS2/ PdNPs prepared at ratios of (a,b) 1, (c,d) 10, and (e,f) 30. (g) Plot of the size of the Pd NPs on 2H-WS2/PdNPs against the PVP/Pd(OAc)2 ratios. The ratio of Pd(OAc)2 to WS2 nanosheets was fixed at 0.75.

To better understand the reaction mechanism of photocatalyzed Suzuki reactions (Figure 6a), the effects of different solvents were investigated, as shown in Figure 6b. The photocatalyzed Suzuki reaction of 4-BT with PBA over WS2/ PdNPs hybrids (45% Pd coverage) hardly proceeded in H2O because of the poor solubility of 4-BT at room temperature. Hence, mixture of polar protic or aprotic solvents with H2O were examined. A mixture of aprotic N,N-dimethylformamide (DMF) and H2O (2:1) rarely produced the desired product in this photocatalyzed Suzuki reaction, although all of the reagents were completely dissolved in the solution. In addition, a mixture containing a polar aprotic solvent (dimethyl sulfoxide [DMSO]) gave almost no product when the reaction was allowed to proceed at room temperature. This result is very different from that observed in conventional Suzuki reactions catalyzed by heterogeneous catalysts. Generally, polar DMF and DMSO are considered to be good solvents for Suzuki reactions, leading to the generation of the desired products at high yields. However, this is not the case for photocatalyzed Suzuki reactions. A mixture of protic EtOH and H2O, however, gave the desired product at very high yield (92%) in a short period of time at room temperature. In addition, other protic organic solvent mixtures involving MeOH or isopropyl alcohol

Figure 5. XPS analysis of 2H-WS2/PdNPs. High resolution spectra of (a) W 4f, (b) S 2p, and (c) Pd 3d for the 2H-WS2/PdNPs synthesized under the following conditions: PVP/Pd(OAc)2 = 30.0, and Pd(OAc)2/WS2 = 0.75.

(IPA) gave moderate yields of the product in this photocatalyzed Suzuki reaction. These results imply that protic organic solvents play an important role in photocatalyzed Suzuki reactions over WS2/PdNP hybrids. We speculate that protic organic solvents may be oxidized more readily by photogenerated hole transfer from the WS2 nanosheets of the hybrid catalysts. In contrast, aprotic organic solvents, such as DMF and DMSO could not be oxidized by this hole transfer because they have higher oxidation potentials.51 Subsequently, we investigated the effects of bases on the photocatalyzed Suzuki reaction catalyzed by WS2/PdNPs hybrids under visible light. Among the bases tested, K2CO3 was found to be most effective in the photocatalyzed Suzuki reactions over WS2/PdNPs hybrids (Figure 6c), whereas NaOH was found to be the least effective. This base effect 14770

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

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Journal of the American Chemical Society Table 1. Photocatalytic Suzuki Cross-Coupling Reaction over 2H-WS2/PdNPsa

a

Reaction conditions: 4-bromotoluene (0.1 mmol), phenylboronic acid (1.2 equiv), 2H-WS2/PdNPs (Pd 2.85 μg), K2CO3 (5 equiv), EtOH/H2O (2:1), a 60 W white LED lamp, r.t. bDetermined by gas chromatography (GC) using toluene as an internal standard. cWS2 nanosheets were mixed with Pd NPs in the reaction. d0.1 mmol of pbenzoquinone (electron scavenger) was added. e0.5 mmol of diisopropylethylamine (hole scavenger) was added. (n.r = no reaction)

Figure 7. Proposed mechanism responsible for photocatalytic Suzuki cross- coupling reactions over 2H-WS2/PdNPs.

We then investigated the effects of the coverage and size of Pd NPs on the photocatalytic activity of WS2/PdNPs hybrids in Suzuki reactions under visible light (Figure 6d,e). The fractional coverage of Pd NPs on WS2 nanosheets was controlled from 15% to 78% by varying the precursor ratio of Pd and WS2. As shown in Figure 6d, the WS2/PdNPs with 45% fractional coverage gave the highest yield of the desired product, 4-methyl-1,1′-biphenyl. When the coverage of Pd NPs exceeded 45%, the product yields decreased because Pd NPs shield the WS2 nanosheets from visible light by absorbing it. This shielding effect prevents the WS2 nanosheets from being excited by visible light, which is a crucial step in photocatalyzed Suzuki reactions. The effect of the size of the Pd NPs on WS2/ PdNPs hybrids on their photocatalytic activity was also explored. As shown in Figures 4 and S1, the size of the Pd NPs on the WS2/PdNPs hybrids was effectively varied from 2.9 to 5.5 nm. Interestingly, the photocatalytic activity of the WS2/ PdNPs hybrids increased as the size of the Pd NPs increased (Figure 6e). Generally, photocatalytic activity tends to increase as the particle size of the catalyst decreases because of the increased number of active sites available on smaller catalysts. In order to understand this discrepancy in the size effect, the oxidation states of the smaller size (2.9 nm) and larger (5.5 nm) Pd NPs on the WS2/PdNPs hybrids were analyzed by XPS (Figure S4). As the Pd NP size on the WS2/PdNPs increased, the proportion of active Pd0 species also increased from 54% (2.9 nm) to 69% (5.5 nm). This higher proportion of active Pd0 species in larger Pd NPs led to the enhanced photocatalytic activity of the resulting hybrids in Suzuki reactions. Finally, 2H-WS2/PdNPs hybrids were applied to the photocatalyzed Suzuki reactions of various aryl halides and boronic acids at room temperature (Table 2). All aryl iodides were converted to the corresponding products in high yields, regardless of whether electron-donating or electron-withdrawing substituents were present (entries 1−5). However, the substituents on the aromatic rings of aryl bromides were observed to exert an effect. The reaction yields for bromoanisole (entries 6 and 7) and 4-bromophenol (entry 8) with electron-donating substituents decreased slightly relative to those for aryl bromides bearing electron-withdrawing substituents, such as 2- or 4-bromobenzonitrile and 4bromobenzaldehyde. According to the experimental data shown in entries 6 and 9 of Table 2, the steric hindrance of aryl bromides rarely affects the yields of the photocatalyzed Suzuki reactions. On the basis of the photocatalyzed Suzuki reaction of iodotoluene with PBA, we calculated the turnover

Figure 6. Photocatalytic activity of 2H-WS2/PdNPs under various conditions. (a) Reaction scheme for the photocatalyzed Suzuki crosscoupling reaction of 4-bromotoluene. Effects of (b) solvents and (c) bases on the photocatalytic activity of 2H-WS2/PdNPs. (d) Effect of the fractional coverage of the Pd NPs on the photocatalytic activity of 2H-WS2/PdNPs. (e) Effect of the size of the Pd NPs on the photocatalytic activity of 2H-WS2/PdNPs.

on photocatalysis is very similar to that observed in conventional Suzuki reactions. On the basis of the above experimental results, we propose a mechanism responsible for photocatalyzed Suzuki crosscoupling reactions catalyzed by WS2/PdNPs hybrids under visible light, as shown in Figure 7. Upon visible light irradiation, the WS2/PdNPs hybrids generate electron−hole pairs within the 2H-WS2 nanosheets. Then, the photoexcited electrons are transferred to Pd NPs, and the photogenerated holes are transferred to a protic solvent, such as EtOH. These electronrich Pd NPs then undergo the oxidative addition of an aryl halide, followed by transarylation of the borate salt. Finally, reductive elimination gives the desired product, and the WS2/ PdNPs are regenerated for further reactions. 14771

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

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Journal of the American Chemical Society Table 2. Photocatalyzed Suzuki Cross-Coupling Reactions of Various Aryl Halides by 2H-WS2/PdNPsa

Figure 8. Comparison of the TOF value of 2H-WS2/PdNPs with those of previously reported photocatalysts for photocatalyzed Suzuki reactions.

Table 3. Photocatalyzed Suzuki Cross-Coupling Reactions of Several Arylboronic Acids with 4-Bromotoluene by 2H-WS2/ PdNPsa

a

Reaction conditions: Aryl halide (0.1 mmol), phenylboronic acid (1.2 equiv), 2H-WS2/PdNPs (Pd 2.85 μg), K2CO3 (5 equiv), EtOH/H2O (2:1), a 60W white LED lamp, r.t. bDetermined by GC using toluene as an internal standard.

frequency (TOF) of the WS2/PdNPs hybrids, which was 1244 h−1. This TOF is higher than the values determined for previously reported photocatalysts, as shown in Figure 8 and Table S1.46,50,52−63 Less-reactive aryl chlorides were also subjected to photocatalyzed Suzuki reactions over WS2/ PdNPs hybrids, but no desired product was obtained. As shown in Table 3, three different types of arylboronic acids bearing electron-donating or electron-withdrawing substituents were exploited as starting compounds. pTolylboronic acid containing an electron-donating group was converted to the corresponding biphenyl product quantitatively (entry 1). However, the arylboronic acids with electronwithdrawing groups showed decreased product yields and required longer reaction times (entries 2 and 3). According to the experimental results, the substituent effect is more significant in arylboronic acids than in aryl halides in photocatalyzed Suzuki reactions. This synthetic approach for structuring NPs on the surface of TMDs can be extended to various two-dimensional nanomaterials including transition metal oxides to induce outstanding photocatalytic activity under visible light in various energy applications.64 In addition, 2H-WS2/PdNPs can be applied as an effective catalyst for other cross-coupling reactions such as Heck and Sonogashira reactions.

a

Reaction conditions: 4-bromotoluene (0.1 mmol), arylboronic acid (1.2 equiv), 2H-WS2/PdNPs (Pd 2.85 μg), K2CO3 (5 equiv), EtOH/ H2O (2:1), a 60 W white LED lamp, r.t. bDetermined by GC using toluene as an internal standard.

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CONCLUSION We developed an effective approach to modulate the photocatalytic activity of 2H-WS2 nanosheets via hybridization with Pd NPs under very mild conditions. The size and density of the Pd NPs on as-prepared 2H-WS2/PdNPs hybrids were readily controlled during the synthetic process and were found to be crucial factors for modulating the photocatalytic activity of the 2H-WS2/PdNPs in Suzuki reactions under visible light. Moreover, the photocatalytic activity of the 2H-WS2/PdNPs hybrids for Suzuki reactions was extraordinarily enhanced in polar protic solvents, whereas no photocatalytic activity was observed in polar aprotic solvents. 2H-WS2/PdNPs hybrids were successfully applied to the photocatalyzed Suzuki reactions of various aryl halides with several aryl boronic acids at room temperature. The TOF value of 2H-WS2/PdNPs hybrids for photocatalyzed Suzuki reactions (1244 h−1 for) is the highest reported for any previously investigated photocatalyst. We expect that this approach for modulating the 14772

DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

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photocatalytic activity of catalysts will be applicable for diverse heterogeneous photocatalysts. In addition, 2H-WS2/PdNPs hybrids could potentially be employed as photocatalysts in other C−C forming reactions, such as Sonogashira, Heck, and Stille reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08619. Materials; experimental produces for the synthesis of WS2 nanosheets and 2H-WS2/PdNPs hybrids, and Suzuki reactions under visible light; instrumentation; XPS, PL, and 1NMR data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jong-Ho Kim: 0000-0001-6256-7713 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2008-0061891, NRF-2014R1A2A1A11051877, and NRF-2017R1A2B2008455) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. Faizan Raza gratefully acknowledges the scholarship support provided by the Higher Education Commission of Pakistan (H.E.C.) through its HRDI-UESTPS/UETS Faculty Development Program.

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DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774

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DOI: 10.1021/jacs.7b08619 J. Am. Chem. Soc. 2017, 139, 14767−14774