Ultrathin nanosheets of palladium in boosting its cocatalyst role and plasmonic effect towards enhanced photocatalytic hydrogen evolution

Abstract

The combination of a metal with a semiconductor is a promising route to improve the solar-to-chemical conversion efficiency of photocatalysts. In this article, ultrathin Pd nanosheets are integrated with semiconductor TiO₂ nanosheets for photocatalytic hydrogen evolution, which acts as a cocatalyst and plasmonic agent in ultraviolet and visible-near-infrared spectral regions, respectively. Owing to the unique two-dimensional (2D) nanostructure, the Pd nanosheet cocatalyst realizes the large TiO₂–Pd interfacial area for electron transfer as well as a large Pd exposed area for reduction reactions, while the plasmonic Pd nanosheets offer strong vis-NIR light absorption for “hot” electron production as well as a large interfacial area for “hot” electron injection. As a result, the Pd nanosheets achieve improved photocatalytic activity in comparison with three-dimensional Pd nanotetrahedrons under both light irradiations. This work underlines the importance in choosing a suitable shape of metal in the surface and interface design of semiconductor–metal hybrid photocatalysts as well as the advantages of 2D metal nanostructures in realizing high photocatalytic performance.

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Introduction

Sunlight-driven water splitting represents a promising approach to transform solar energy into hydrogen fuel.^1–5 When bare semiconductors were used as photocatalysts, the hydrogen evolution efficiency is mainly limited by three unfavorable factors.^6,7 The first factor is the limited light absorption range or low redox abilities of photogenerated carriers. For wide-bandgap semiconductors, the photoexcitation of which can generate charge carriers with high redox abilities, but the light absorption range of them is limited to ultraviolet (UV) light (wavelength (λ) < 400 nm), which accounts for a very small fraction (ca. 5%) of the solar spectrum. While the narrow-bandgap semiconductors can harvest visible and near-infrared (vis-NIR) light (λ > 400 nm), but in lower redox abilities. Secondly, the absence of driving force for charge separation leads to serious electron–hole recombination in bare semiconductor. Thirdly, semiconductor surface does not possess high activation ability for H₂ evolution reactions, and the accumulated charges on the surface induce degradation of the semiconductor. The combination of semiconductor (generally in n-type) with noble metal is a promising route in circumventing the undesired situation and enhancing the H₂ evolution activity.^8,9 On one hand, noble metal nanocrystals (e.g. Pt, Pd) could be used as reduction co-catalysts for H₂ production reactions. The metal cocatalyst can not only trap photo-generated electrons from the semiconductor and promote the electron–hole separation, but also serve as the highly active reaction sites to consume the trapped electrons for H₂ production, thus enhancing the surface reactive performance and suppressing the semiconductor photocorrosion (Fig. S1a†).^8–11 On the other hand, noble metal nanocrystals (e.g. Au, Ag) with surface plasmon can absorb vis-NIR light and inject energetic “hot” electrons into the conduction band (CB) of wide-bandgap semiconductor for H₂ evolution, thus extending the spectral range for light absorption, in the meanwhile, the high redox ability is well maintained (Fig. S1b†).^8,9,12,13

For both metal cocatalyst and plasmonic metal, the contribution of which to the photocatalytic performance is greatly determined by their exposed surface as well as the interface in contact with semiconductor.^14,15 The surface of metal cocatalyst is the position where the redox reactions happen, which greatly influence the photocatalytic H₂ production activity, while the interface between metal cocatalyst and semiconductor is the location where the photogenerated electrons are transferred through, which greatly impact the charge separation efficiency. As for plasmonic metal, the surface influences the light absorption ability, while the interface formed with semiconductor determines the efficiency of “hot” electron injecting. Thus a good surface and interface design is critical to boosting the contribution of metal in enhancing the photocatalytic H₂ production. Generally, increasing the surface exposed area of metal can increase the catalytic active sites of cocatalyst and enhance the light absorption ability of plasmonic metal, while enlarging the metal–semiconductor contacted area can facilitate the interfacial electron transfer between metal and semiconductor. Therefore, rational metal architectural structure choice to obtain large surface/interfacial area is a promising route to realize high-efficient photocatalysts. Moreover, considering the high cost and limited source of noble metal, architectural structure with high surface-to-volume ratio can also reduce the metal consumption and increase the H₂ economy.

Recently, ultrathin metal nanosheets, important members of two-dimensional (2D) material family, have been widely developed.^16–18 The 2D nanostructures with a large surface area, low thickness, and high surface-to-volume ratio offer an ideal model in the surface and interface design of semiconductor–metal hybrid photocatalysts.^19,20 In this work, we report a design of TiO₂–Pd photocatalyst with ultrathin Pd nanosheets and TiO₂ nanosheets used as metal and semiconductor model, respectively. In the design, Pd nanosheets act as cocatalyst and plasmonic absorber when the hybrid photocatalysts were irradiated with UV and vis-NIR light, respectively. Under both light irradiations, the 2D nanostructure of Pd sheets with large surface exposure area and interface contact area with semiconductor brings about significantly higher photocatalytic performance in hydrogen evolution in comparison with three-dimensional (3D) Pd nanotetrahedrons.

Experimental

Synthesis of TiO₂–Pd nanosheets

The TiO₂–Pd nanosheets sample was synthesized with the self-assembly of presynthesized Pd nanosheets onto the TiO₂ nanosheets. The Pd nanosheets were synthesized according to previous protocol.²¹ In a typical synthesis, 16 mg of Pd(acac)₂, 30 mg of PVP, 60 mg hexadecyltrimethylammonium bromide (CTAB), and 10 mg of citric acid were dissolved in 10 mL of N,N-dimethylformamide (DMF) and stirred for 1 h. The resulting solution was then transferred to a 50 mL three-neck flask, and subsequently 100 mg of W(CO)₆ was then added into the flask under an Ar atmosphere. Then the reaction was allowed to proceed at 80 °C for 1 h. The product was collected by centrifugation, and washed with acetone and ethanol several times to remove excessive PVP. The as-obtained Pd nanosheets were re-dispersed in ethanol with the determined Pd concentration. The deposition of Pd nanosheets on TiO₂ nanosheets was achieved by sonicating the mixed aqueous suspension of Pd nanosheets and TiO₂ nanosheets, followed by a hydrothermal treatment. In a typical process, 20 mg TiO₂ nanosheets were dispersed in 20 mL water through sonication. Subsequently, 100 μL suspension of Pd nanosheets (10 mg mL⁻¹ in ethanol) was added into the dispersion. The mixture was further sonicated for 10 min, which was then transferred to a Teflon-lined stainless-steel autoclave and heated at 80 °C for 1 h. After the autoclave had cooled down to room temperature, the resultant product was separated by centrifugation, washed with acetone once and water three times, and dried at 45 °C in vacuum.

Synthesis of TiO₂–Pd nanotetrahedrons

In a typical synthesis, K₂PdCl₄ (4.9 mg), Na₂C₂O₄ (50.3 mg) and PVP (8.3 mg) were dissolved in 5 mL of aqueous suspension containing 30 mg TiO₂ nanosheets, followed by the addition of 200 μL formaldehyde solution. The pH value of the solution was adjusted to 4 by adding drops of 1 M HCl solution. Subsequently the total volume of the solution was diluted to 7.5 mL with water. The solution was then transferred to a 10 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 2 h. After the autoclave had cooled down to room temperature, the resultant product was separated by centrifugation, washed with acetone once and water three times, and dried at 45 °C in vacuum.

Photoelectrochemical measurements

3.0 mg as-synthesized products were dispersed in a mixture of 10 μL ethanol and 10 μL Nafion, which were then uniformly spin-dropped onto a 1.25 cm × 1.25 cm indium tin oxide (ITO)-coated glass by a spin coater (SC-1B, China). Subsequently, the ITO-coated glass was heated at 80 °C in a vacuum oven for 1 h. The photocurrents were measured on a CHI 660D electrochemical station (Shanghai Chenhua, China) in ambient conditions under irradiation of a 300 W Xe lamp (Solaredge 700, China). UV and vis-NIR light were used as illumination source, which was realized in the presence of short-wave-pass and long-wave-pass 400 nm cutoff filter, respectively. The power density of UV and vis-NIR light were measured to be 2.7 and 100 mW cm⁻², respectively. Standard three-electrode setup was used with the ITO coated glass as photoelectrode, a Pt foil as counter electrode, and a Ag/AgCl electrode as reference electrode. The three electrodes were inserted in a quartz cell filled with 0.5 M Na₂SO₄ electrolyte. The Na₂SO₄ electrolyte was purged with Ar for 30 min prior to the measurements. The photoresponse of the prepared photoelectrodes (i.e., I–t) was operated by measuring the photocurrent densities under chopped light irradiation (light on/off cycles: 60 s) at a bias potential of 0.4 V vs. Ag/AgCl for 400 s.

Photocatalytic water splitting measurements

To investigate the photocatalytic activities of TiO₂-based catalysts in water splitting, 15 mg photocatalysts were dispersed in 50 mL of methanol/H₂O mixture (20 vol% methanol). The samples were sonicated to form uniform suspension, followed by saturation with Ar to eliminate air. The light-irradiation experiment was performed by using a 300 W Xe lamp. UV and vis-NIR light were used as illumination source, which was realized in the presence of short-wave-pass and long-wave-pass 400 nm cutoff filter, respectively. The power density of UV and vis-NIR light was measured to be 2.7 and 100 mW cm⁻², respectively. The photocatalytic reaction was typically performed for 4 h. The amount of H₂ evolved was measured by gas chromatography (GC-2014, thermal conductivity detector, Ar carrier, Shimadzu). Three replicates were collected for each sample with relative error <10%. The during test was performed in four cycles with a 4 h-photocatalytic reaction in a cycle.

Results and discussion

The design begins with the choice of single-crystalline TiO₂ nanosheets as a semiconductor model. TiO₂ is the most widely used UV-excitable wide-bandgap n-type semiconductor in photocatalysis. The TiO₂ nanosheets were synthesized via a hydrothermal procedure, which is in anatase phase (JCPDS 21-1272) according to the X-ray diffraction (XRD) pattern (Fig. S2†).²² The edge length of the TiO₂ nanosheets is above 200 nm and the thickness is below 5 nm (Fig. S3a and b†). According to the size, the area percentage of the top and bottom flat faces is over 95% on the total surface of the TiO₂ nanosheets. As indicated by the high-resolution TEM (HRTEM) images, the flat surface is covered by (001) facets (Fig. S3c and d†). The large flat surface of the TiO₂ nanosheets facilitate the deposition of metal in various dimensions, while the low thickness not only shortens the distance of electrons transfer in bulk semiconductor and reduces the electron loss, but also increases light transmission and attenuate the shielding effect on the plasmonic metal.

In the second step of the design, presynthesized Pd nanosheets were assembled on the TiO₂ nanosheets and their interfaces were further annealed through a hydrothermal process. The work function of Pd (≈5.1 eV) is higher than that of anatase TiO₂ (≈4.9 eV), ensuring the electron trapping of Pd cocatalyst from TiO₂ as well as the electron injection of plasmonic Pd into TiO₂ as shown in Fig. S1.† As indicated by TEM images (Fig. 1), hexagon Pd nanosheets with an edge length of approximately 19 nm have been sparsely distributed over the TiO₂ nanosheets to form the hybrid structure (namely, TiO₂–Pd NSs). The HRTEM image confirmed that the Pd nanosheets are single crystals covered by (111) facets on the top and bottom surface (Fig. 1d). From the TEM image of TiO₂–Pd NSs with a standing-up TiO₂ nanosheet (Fig. 1e and f), it could be clearly seen that the TiO₂ and Pd nanosheets were combined through face-to-face contact, thus forming the TiO₂(001)–Pd(111) interface. As shown in Fig. 1f, the thickness of the Pd nanosheets in TiO₂–Pd NSs was measured to be ca. 1.1 nm. The side faces of Pd nanosheets were covered by a mix of (100) and (111) plane as illustrated by the HRTEM image (Fig. 1g). Considering the large flat surface and low thickness, the Pd nanosheets could be considered as nanocrystals dominated by (111) facet.

Fig. 1 (a) Schematic illustration, (b and c) TEM, and (d) HRTEM images of TiO₂–Pd NSs; (e) schematic illustration, (f) TEM, and (g) HRTEM images of TiO₂–Pd NSs with a standing-up TiO₂ nanosheet.

The combination of Pd nanosheets with TiO₂ is further confirmed by X-ray diffraction (XRD) pattern and the X-ray photoelectron spectroscopy (XPS). In the XRD pattern of TiO₂–Pd NSs hybrid structure (Fig. S2†), besides of the peaks of TiO₂, the additional peaks are assigned to face-centered cubic (fcc) Pd (JCPDS 65-2867). The survey XPS spectrum (Fig. S4a†) indicates the Ti, O and Pd peaks in the TiO₂–Pd NSs hybrid structure. In the high-resolution spectrum of Ti 2p (Fig. S4b†), the peaks with binding energies of 464.7 eV and 459.0 eV are attributed to Ti 2p_3/2 and Ti 2p_1/2 for Ti(IV) of the surface titania, respectively. The peaks located at 530.0 eV and 531.6 eV in the high-resolution spectrum of O 1s (Fig. S4c†) are assigned to O–Ti of TiO₂ and O–H of adsorbed OH groups on the TiO₂ surface. In the high-resolution spectrum of Pd 3d (Fig. S4d†), the binding energies of 340.3 eV (Pd 3d_3/2) and 335.0 eV (Pd 3d_5/2) are in good agreement with the zero valence of Pd.

As a reference sample of TiO₂–Pd NSs, Pd nanotetrahedrons were also in situ grown on the TiO₂ nanosheets to form TiO₂–Pd NTs hybrid structure. As shown in Fig. 2, the Pd nanocrystals with tetrahedral profiles and edge length of approximately 16 nm are uniformly dispersed on the surface of TiO₂ nanosheets. Similar to Pd nanosheets, the Pd nanotetrahedrons were enclosed by (111) planes (Fig. 2d). Moreover, the TiO₂(001)–Pd(111) interface was also formed through the face-to-face contact between TiO₂ and Pd in the TiO₂–Pd NTs structure (Fig. 2e and f). The same Pd(111) surface and TiO₂(001)–Pd(111) interface between TiO₂–Pd NSs and TiO₂–Pd NTs preclude the facet factor in comparing their photocatalytic performance. In our previous works, it has been demonstrated that different exposed facets of metal cocatalyst could result in different adsorption and activation abilities for the reactant molecules, while different semiconductor–metal facet contacts could also lead to different interfacial charge transfer abilities owing to the different electronic couplings.^23,24 With the same surface and interfacial facet parameters, the only difference between the TiO₂–Pd NSs and TiO₂–Pd NTs could be the exposed area of Pd surface and contact area of TiO₂–Pd interface.

Fig. 2 (a) Schematic illustration, (b and c) TEM, and (d) HRTEM images of TiO₂–Pd NTs; (e) schematic illustration and (f) TEM image of TiO₂–Pd NTs with a standing-up TiO₂ nanosheet.

To investigate the surface/interfacial area dependent photocatalytic performance of TiO₂–Pd, the loading amount of Pd nanosheets and nanotetrahedrons were kept the same as detected by inductively coupled plasma-mass spectrometry (ICP-MS, Table S1†). UV-vis-NIR diffuse reflectance spectra show that TiO₂–Pd NSs and TiO₂–Pd NTs exhibit comparable light absorption in UV light range as the bandgap of TiO₂ does not change when the hybrid structures are formed, while in vis-NIR light range, the TiO₂–Pd NSs exhibits apparently stronger light absorption in comparison with TiO₂–Pd NTs, resulted from the unique plasmonic property of metal nanosheets (Fig. 3a). In order to reflect the plasmonics more clearly, bare Pd nanotetrahedrons and nanosheets were synthesized and their UV-vis-NIR absorption spectra were shown (Fig. S5 and S6†). It could be clearly seen that Pd nanosheets possess distinctive plasmonic absorption in vis-NIR spectral region and bring about a blue color suspension, which can be assigned to the in-plane dipole resonance according to the previous report.^16,21 Similar cases include blue Au and Ag nanoplates.^25,26 While for Pd nanotetrahedrons, the smaller anisotropy leads to weaker plasmonic absorption (Fig. S6†). According to the light absorption characteristics of TiO₂–Pd NSs, Pd may act as a cocatalyst in trapping electrons from UV light excited TiO₂, while play a role in injecting plasmonic “hot” electron into TiO₂ under vis-NIR light irradiation.

Fig. 3 (a) UV-vis-NIR diffuse reflectance spectra of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs; (b) photocurrent vs. time (I–t) curves of photoelectrodes made of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs under UV light (λ < 400 nm); (c) PL spectra of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs excited at 315 nm; (d) photocatalytic H₂ evolution rates from water with the samples of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs under UV light (λ < 400 nm).

Given the comparable capability of TiO₂–Pd NSs and TiO₂–Pd NTs in absorbing UV light and generating photo-induced electrons, the efficiency of electron–hole separation can be reflected by photocurrent. As shown in Fig. 3b, the photocurrents turn out to be in the order of bare TiO₂ < TiO₂–Pd NTs < TiO₂–Pd NSs under UV light irradiation. This result suggests that Pd nanosheets can trap electron from TiO₂ more effectively in comparison with Pd nanotetrahedrons. As the radiative charge recombination in semiconductor generally induces luminescence, this argument is also supported by photoluminescence (PL) emission spectroscopy (Fig. 3c), which indicate that the PL of TiO₂ was quenched by Pd nanosheets in larger degree, suggesting the electron–hole recombination in TiO₂–Pd NSs was suppressed more efficiently. Considering the same TiO₂(001)–Pd(111) interface in transferring the electrons from TiO₂ to Pd, the different electron trapping abilities can only be attributed to the different TiO₂–Pd interfacial area. According to the average edge length and thickness of Pd nanosheets, the ratio of interfacial area to Pd volume (S_interface/V_Pd) in TiO₂–Pd NSs was calculated to be 0.909 nm⁻¹, much higher than that (0.227 nm⁻¹) in TiO₂–Pd NTs (calculated based the average edge length of Pd nanotetrahedrons) (Table 1 and Fig. S7†).

Table 1 Calculated TiO₂–Pd interfacial area and exposed Pd surface area as well as the corresponding interface-to-volume ratio (S_interface/V_Pd) and surface-to-volume ratio (S_surface/V_Pd) of Pd in TiO₂–Pd NSs and TiO₂–Pd NTs, respectively

Samples	Average L and H^a (nm)	Average VPd^b (nm^3)	Average Sinterface^c (nm^2)	Sinterface/VPd (nm^−1)	Average Ssurface^d (nm^2)	Ssurface/VPd (nm^−1)
a L is the edge length of a Pd hexagon nanosheet or nanotetrahedron, H is the height (thickness) of a Pd nanosheet.b VPd is the volume of a Pd nanocrystal.c Sinterface is the interfacial area.d Ssurface is the surface area. The VPd, Sinterface and Ssurface are calculated according to the equations in Fig. S7.
TiO2–Pd NSs	L = 19.1, H = 1.1	1042.6	947.8	0.909	1085.3	1.041
TiO2–Pd NTs	L = 16.2	501.0	113.6	0.227	340.9	0.680

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Having confirmed the superior interfacial electron transfer ability in TiO₂–Pd NSs, we further investigated its performance in photocatalytic water splitting under UV light irradiation with methanol as sacrificial agent. Environmentally sensible methanol was frequently used to consume the holes and reduce the charge recombination in the TiO₂-based photocatalysts.^27,28 The hydrogen production rates of bare TiO₂ and TiO₂–Pd samples were summarized in Fig. 3d. Following the same order as the photocurrents, the TiO₂–Pd NSs achieves the highest value of 2.80 mmol g_cat⁻¹ h⁻¹, reflecting the importance of interfacial electron transfer to the entire photocatalytic performance. However, in comparison with photocurrent, the difference between TiO₂–Pd NSs and TiO₂–Pd NTs appears to be significantly larger in photocatalytic hydrogen production. Therefore, Pd nanosheets as catalytic active site also contribute to the higher H₂O molecular adsorption and activation properties in addition to the electron trapping ability, further confirming the cocatalyst role of Pd. Though both Pd nanosheets and nanotetrahedrons expose Pd(111) facet for the surface reduction reaction, more surface atoms on Pd nanosheets participate in the reduction of H₂O molecule. The exposed surface-to-volume ratio of Pd (S_surface/V_Pd) in TiO₂–Pd NSs was calculated to be 1.041 nm⁻¹ based on the average edge length and thickness of Pd, higher than that in TiO₂–Pd NTs (0.680 nm⁻¹, calculated based the average edge length of Pd nanotetrahedrons) (Table 1 and Fig. S7†).

When the TiO₂–Pd was further irradiated by vis-NIR light, the plasmonic Pd may inject energetic “hot” electrons into TiO₂. As TiO₂ nanosheets are not excited by the vis-NIR light, the two other possible mechanisms for plasmonic effect (local electromagnetic field enhancement and resonant photon scattering) cannot work in the TiO₂–Pd samples.^6,29 As shown in Fig. 4a, under vis-NIR light irradiation, the photocurrent of TiO₂–Pd NSs is much larger than that of bare TiO₂, confirming the plasmonic “hot” electron injection. However, there is not too much difference between TiO₂ and TiO₂–Pd NTs in the photocurrent, indicating the poor “hot” electron injection ability of Pd nanotetrahedrons in comparison with Pd nanosheets. The reason is that the Pd nanosheets with stronger plasmonic absorption may generate more “hot” electrons, while the larger TiO₂–Pd interfacial area also lead to smoother electron transfer from Pd nanosheets to TiO₂. The injected electrons can also drive the reduction of H₂O on the surface of TiO₂ with methanol as scavenger. The hydrogen production rates under vis-NIR light irradiation were shown in Fig. 4b. It can be seen that the TiO₂–Pd NSs exhibit apparent photocatalytic hydrogen evolution activity whereas bare TiO₂ and TiO₂–Pd NTs cannot. As the H₂O adsorption and activation happened on the same surface of TiO₂ in all the samples, the difference in hydrogen evolution can only attribute to the superior ability of Pd nanosheets in “hot” electron generation and injection.

Fig. 4 (a) Photocurrent vs. time (I–t) curves of photoelectrodes made of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs under vis-NIR light (λ > 400 nm); (b) photocatalytic H₂ evolution rates from water with the samples of bare TiO₂, TiO₂–Pd NTs and TiO₂–Pd NSs under vis-NIR light (λ > 400 nm).

Generally, TiO₂–Pd NSs exhibits significantly higher photocatalytic hydrogen evolution activity in comparison with TiO₂–Pd NTs under both UV and vis-NIR light irradiations, which highlights the superiority of 2D metal nanostructures used as cocatalyst and plasmonic agent in photocatalysis. On one hand, the large interfacial area for electron transfer as well as large surface area for catalytic reduction reaction turns Pd nanosheets into ideal nanostructures for cocatalysts (Fig. 5a). On the other hand, the strong plasmonic absorption for “hot” electron production as well as large interfacial area for “hot” electron injection makes Pd nanosheets good plasmonic nanostructures (Fig. 5b). Furthermore, the large interfacial contact ensures the strong binding of Pd nanosheets to TiO₂ nanosheets. As shown in Fig. S8a and b,† the Pd nanosheets are well retained on the TiO₂ nanosheets after the photocatalytic reaction. In contrast, after the reaction, the Pd nanotetrahedrons were detached from the TiO₂ nanosheets and agglomerated with each other (Fig. S8c and d†). As a result, the TiO₂–Pd NSs sample maintains excellent photocatalytic stability during the successive cycles, while the catalytic stability of TiO₂–Pd NTs is much poorer under both UV and vis-NIR irradiation (Fig. 6). The superior photocatalytic performance of the TiO₂–Pd NSs sample can be further confirmed by comparing its performance with other TiO₂–Pd system under similar test conditions.³⁰

Fig. 5 Schematic illustrating the photocatalytic H₂ evolution reaction on the samples of TiO₂–Pd NSs and TiO₂–Pd NTs under (a) UV (λ < 400 nm) and (b) vis-NIR (λ > 400 nm) light irradiation, respectively.

Fig. 6 Photocatalytic average hydrogen production rates for TiO₂–Pd NSs and TiO₂–Pd NTs samples under (a) UV (λ < 400 nm) and (b) vis-NIR (λ > 400 nm) light irradiation in four successive cycles, respectively.

Conclusions

In summary, we have designed a TiO₂–Pd NSs photocatalyst, in which ultrathin Pd nanosheets were used as cocatalyst and plasmonic agent under UV and vis-NIR light irradiation, respectively. The Pd nanosheets not only realize the large interfacial contact area with UV light excited TiO₂ for electron transfer as well as large exposed surface for reduction reaction when used as cocatalyst, but also offer the strong vis-NIR plasmonic absorption for “hot” electron production as well as large interfacial area for “hot” electron injection when used as plasmonic agent. As a result, though with the same Pd(111) surface and TiO₂(100)–Pd(111) interface, the TiO₂–Pd NSs exhibit superior photocatalytic performance in comparison with TiO₂–Pd NTs under both light irradiations. This work highlights the advantages in choosing ultrathin 2D nanostructures of metal cocatalyst and plasmonic metal for enhanced charge kinetics in photocatalytic reactions.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 21273203), Zhejiang Provincial Natural Science Foundation of China (No. LQ16B010001, LR15B010001, LR12B040001), and Open Research Fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges and Key Laboratory of the Ministry of Education for Advanced Catalysis Materials (Zhejiang Normal University) (ZJHX201507).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09647f

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