Accepted Article Title: MoS2 quantum dots@TiO2 nanotube arrays: An extended spectrum-driven photocatalyst for solar hydrogen evolution Authors: Qun Wang, Jianying Huang, Hongtao Sun, Yun Hau Ng, Keqin Zhang, and Yuekun Lai This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: ChemSusChem 10.1002/cssc.201800379 Link to VoR: http://dx.doi.org/10.1002/cssc.201800379
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FULL PAPER MoS2 quantum dots@TiO2 nanotube arrays: An extended spectrum-driven photocatalyst for solar hydrogen evolution Abstract: Molybdenum disulfide quantum dots (MoS2 QDs) decorated TiO2 nanotube arrays (TiO2 NTAs), used as a novel composite photocatalyst (MoS2 QDs/TiO2 NTAs) sythnsized by a facile electrodeposition method. MoS2 QDs/TiO2 NTAs shows enhanced photocatalytic activity when compared to pristine TiO2 NTAs for solar hydrogen without adding any sacrificial agents or cocatalysts. The photocatalytic activity was influenced by the amount of MoS2 QDs coated on TiO2 NTAs, and the optimized composition was obtained with excellent photocatalytic activity, achieving H2 evolution rates of 65.3, 53.9 and 16.3 μmol·cm-2·h-1 corresponding to ultraviolet (UV, λ < 420 nm), visible (vis, λ ≥ 420 nm) and near-infrared (NIR, λ > 760) illumination, respectively. The improved photocatalytic activity owes to the decreased band-gap and the surface plasmonic properties of MoS2 QDs/TiO2 NTAs, promoting the electron-hole pairs separation and absorption capacity to visible and NIR light. This work presents a facile approach for fabricating the MoS2 QDs/TiO2 NTAs heterostructures for efficient photocatalytic H2 evolution, which will facilitate the development of designing novle photocatalysts applied in environment and energy.
Introduction As a clean and efficient energy source, H2 has been received markedly consideration for addressing issues from global environmental pollution and energy shortage.[1] The transformation of solar to fuel dramatically relies on semiconductor materials that can capture photons across the broad solar wavelengths (from ultraviolet (UV) to near-infrared (NIR) region) and synchronously produce charge-carriers for H+ reduction, thus water splitting for H2 evolution by using effective semiconductor materials provides a promising strategy to solve energy shortage with the utilization of sunlight. [2] Currently, TiO2 has long been considered as a potential photocatalyst for solar-
[a]
[b]
[c]
[d]
Q. Wang, H. T. Sun College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123 (China) Q. Wang, J. Y. Huang, K. Q. Zhang, Y. K. Lai National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou 215123 (China) Email:
[email protected] J. Y. Huang, Y. K. Lai College of Chemical Engineering, Fuzhou University, Fuzhou 350116 (China) Y. H. Ng Particles and Catalysis Research Group, School of Chemical Engineering, University of New South Wales High Street, Kensington, New South Wales 2052 (Australia) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.
driven water splitting, because of its many advantages, including photochemical stability, non-toxic and controllability and so on.[3] However, pristine TiO2 always suffers from the poor photoabsorption over the visible and NIR light due to its wide bandgap and dull charge transfer kinetics.[4] To solve these problems, the TiO2-based heterostructures are designed by heavy doping, modifying with noble metals or co-catalysts and combining other semiconductors with narrow bandgap to minish the bandgap, thereby promoting the charge separation and expanding the spectral range of light absorption to include visible or NIR.[5] However, some TiO2-based heterostructures are limited because of the complexity of synthesis process and higher cost and toxicity of noble metal. Therefore, it is in urgent need of seeking photosensitive and earth-abundant materials compounded with TiO2 for synthesizing TiO2-based heterostructures with significantly enahnced photocatalytic activity. As a two-dimensional (2D) layered transition metal dichalcogenide (TMD), molybdenum disufide (MoS2) can be prepared into ultrathin-layered structures and even reach to monolayer regime,[6] and it has been considered for the potential applications in photocatalysis,[7] electrocatalysis,[8] solar cells[9] and lithium ion batteries.[10] Recently, cocatalysts contained MoS2 have become promising photocatalysts for photocatalytic H2 evolution because of its excellent catalytic performance.[11] Bulk MoS2 with the poor intrinsic conductivity and few active sites leads to the poor photocatalytic activity. In addition, MoS2 with singlecrystal structure composed of three molecular layers (S-Mo-S) shows very poor flexibility, and the S-edges of 2D MoS2 are usually considered as the active sites, whereas the in-plane structures are not effective for catalytic reactions.[12] Therefore, fabricating MoS2 with few layers would be an effective approach to improve the conductivity and produce more active sites due to the existence of more S-edge sites, thereby enhancing their catalytic activity in the H2 evolution reaction. The TMD nanodots with a diameter of less than 10 nm have been synthesized by facile methods, and the nanodots show stronger edge effects and quantum confinement,[13] thus showing unique optical and electrical properties beyond that of the monolayer or multilayer TMD. MoS2 has been designed with various nanostructures, such as nanoparticles,[14] nanowires,[15] nanoflowers,[16] thin films,[17] mesopores,[18] and QDs, especially for monolayer QDs, are the most attractive candidate. Therefore, an effective approch is needed for preparing monolayer MoS2 QDs. Until now, several approaches have been proposed for MoS2 QDs synthesis, including sonication and solvothermal treatment of bulk MoS 2,[19] electrochemical synthesis[20] and liquid exfoliation[21]. These approaches, however, are referred to as “top-down” synthesis that has limitations such as sensitive to the environment, time consumption, harsh conditions and toxic organic solvents. In comparison, the hydrothermal synthesis of MoS2 QDs known as
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“bottom-up” route is very favorable due to its low cost and nonpollution to the environment.[22] It is worth pointing out that MoS2 usually has two phase sructures, including trigonal phase (1T-MoS2) and hexagonal phase (2H-MoS2). Compared to metallic 1T-MoS2, 2H-MoS2 displays excellent stability and semiconductor characteristics at ambient temperature, making it to be served as a co-catalyst to decorate other wide bandgap semiconductors for more efficient photocatalytic activity.[7b] However, there are few literatures highlight MoS2 as the main photocatalyst that contributes to promote photocatalytic activity upon NIR light irradiation. The bandgap of MoS2 can be tuned by controlling its size, which has been confirmed by theoretical caculations. Also, the metalchalcogenide stoichiometric ratio has effect on the energy band structure.[8d] A recent study presented that MoS2 edge defects (Svacancies) used as a co-catalyst play a significant role in plasmonic resonance upon solar light irradiation. [7d] Therefore, the light harvesting phenomena can be described with the local surface plasmonic resonance, mainly resulting from charge resonance on the surface of metal-chalcogenide transmitted by lots of anion vacancies. Previous researchers have reported that TiO2 nanofibers, nanobelts, nanoparticles, nanorods and nanotubes modified with MoS2 nanoparticles or nanosheets could improve the photocatalytic activity.[23] However, MoS2 QDs anchored on TiO2 NTAs by a facile approach of electrodeposition to synthesize MoS2 QDs/TiO2 NTAs heterostructures for enhancing photocatalytic activity has not been previously reported. Herein, we propose a “bottom-up” route to synthsize water-soluble monolayer MoS2 QDs with a narrow size distribution followed by its subsequent deposition onto TiO2 NTAs to synthesize TiO2based heterostructures. Such heterostructure exhibits a powerfully enhanced photon harvesting ability in UV-vis-NIR range by anchoring MoS2 QDs to TiO2 NTAs. The promoted electron transfer path and opportunely adjusted the conduction band’s energetic position lead to significantly enhanced chargecarrier separation and H2 generation efficiency. From the analysis of H2 evolution rate upon monochromatic light irradiation, it is concluded that the surface plasmonic resonance, principally irradiated in the spectral range of 400-600 nm, plays a dominant role in photocatalytical water splitting. It is expected that this nonmetal plasmonic heterostructure can act as a new design approach for promoted efficient photocatalysts and other photovoltaic devices.
Results and Discussion Figure 1a and b show representative transmission electron microscope (TEM) images of as-synthesized MoS2 QDs, indicating that the size distribution of QDs ranges from 1.5 to 6.0 nm, and the average diameter is about 3.4 nm, agreeing well with the test result of dynamic light scattering (DLS) histogram (Figure S1, see supporting information). The high resolution transmission
electron microscope (HRTEM) image and the fast Fourier transform (FFT) pattern of a chosen rectangular area (Figure 1c) indicates that high crystalline hexagonal structure maintains a lattice fringe spacing of 0.23 nm, corresponding to the (103) lattice plane of MoS2.[21b,24] However, there is no (002) lattice plane in crystalline MoS2 QDs, revealing that the as-synthesized MoS2 QDs contain only a few layers. This result was further verified by the selected area electron diffraction (SAED) pattern of MoS 2 QDs (Inset of Figure 1a). The typical tapping atomic force microscopy (AFM) was carried out to further confirm the thickness and morphology of the as-synthesized MoS2 QDs. The average thickness of MoS2 QDs was about 0.7 nm as shown in Figure 1d, which is similar to that of MoS2 prepared with a approach of mechanical exfoliation, indicating that most MoS2 QDs are monolayers without large amounts of re-stacked QDs.[13,25]
Figure 1. (a, b) TEM images and SAED pattern (inset), (c) HRTEM image and FFT pattern of chosen area (inset), and (d) AFM image and the corresponding height profile of MoS2 QDs.
The X-ray photoelectron spectroscopy (XPS) analysis was performed to study the elementary composition and chemical state of MoS2 QDs. The peaks of Mo and S can be obviously found from the XPS spectrum of MoS2 QDs (Figure S2a), and the atomic ratio of Mo : S is caculated about 1 : 2, which also can be verified by the energy dispersive X-ray spectroscopy (EDX) results (note that Cu element in the EDX spectrum is from the copper grid substrate) (Figure S2b), further confirming the composition of MoS2. Moreover, as can be seen from the highresolution XPS of MoS2 QDs in the Mo 3d spectrum (Figure S2c), a peak at 225.9 eV is S 2s of MoS2 QDs, two characteristic peaks, Mo 3d5/2 (228.7 eV) and Mo 3d3/2 (231.9 eV), are typical for Mo4+ in MoS2, while the characteristic peak Mo 3d (235.1 eV) shows slight oxidation of Mo from Mo4+ state in MoS2 to Mo6+, which may ascribed to the exposure of MoS2 in air.[2d,20b,21a,26] The characteristic peaks at 161.4 and 162.7 eV in the S 2p spectrum (Figure S2d) are assigned to S 2p3/2 and S 2p1/2, respectively.[20a,27] The XPS analyses indicates the predominant 2H MoS2 phase in MoS2 QDs’ crystal structure. The X-ray diffraction (XRD) and Raman spectroscopy were used to investigate the crystal structure of the as-synthesized
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MoS2 QDs. From the XRD pattern shown in Figure S3a, the diffraction peak at 2θ = 14.4° agreeing with the (002) plane of MoS2 disappears in MoS2 QDs compared to bulk MoS2, indicating the monolayer nature of the QDs.[28] The two weak peaks at 39.5° and 44.2°in MoS2 QDs’ XRD pattern are assigned to the (103) and (006) diffractions, respectively, resulting from the likely formation of some re-stacked MoS2 QDs during the sample’s drying process.[29] Two strong peaks at 376.4 and 402.6 cm-1 shown in the Raman spectrum of bulk MoS2 (Figure S3b) are attributed to the in-plane (E12g) and vertical plane (A1g) vibrations of the Mo-S bonds in 2H-MoS2, respectively.[30] The corresponding bands for MoS2 QDs located at 379.5 and 403.6 cm-1, which are much weaker than that of bulk MoS2 under the identical test conditions. Especially, the peak for the E12g mode of MoS2 QDs is red-shifted by 3.1 cm-1 compared to that of bulk MoS2, which is similar to that of monolayer boron nitride QDs with small lateral dimensions.[31] In addition, the peak spacing (Δk) between E 12g and A1g of bulk MoS2 (26.4 cm-1) is bigger than that of MoS2 QDs (24.1 cm-1), indicating the successful synthesis of MoS2 QDs with only a few layers.[32] Furthermore, the fourier transform infrared (FTIR) spectrum was used to investigate the surface functional groups of MoS2 QDs, as shown in Figure S4. The peak at 3440 cm-1 corresponds to stretching vibration of hydroxyl groups (-OH). The obvious peaks centered at 2919 and 2854, and 1502 cm -1 are assigned to stretching vibration of C-H and N-H bands, respectively, resulting from usage of dibenzyl disulfides and ammonium molybdate tetrahydrate as precursors. Moreover, the characteristic peak at 920 cm-1 is attributed to stretching vibration of Mo-S band.[23b] The MoS2 nanostructures have particular photoluminescence abilities because of the effect of quantum confinement and edge.[25,33] Figure 2a shows the UV-vis absorption spectrum of MoS2 QDs solution, an obvious broad peak located at 293 nm was observed, which is attributed to the excitonic features of MoS2 QDs.[34] It is different from the 2D MoS2 counterpart with large lateral dimensions, which shows the known peaks at around 668 nm and 609 nm attributed to A and B excitonic peaks, respectively (Figure S5), resulting from Brillouin zone’s K point.6a The strong blue shift in the optical absorption was ascribed to the quantum size effect,[35] and the broad absorption at 293 nm can be ascribed to the blue-shifted convoluted peaks for Z, C and D excitonic.[36] The optical band-gap energy was calculated by plotting the square root of absorption energy (αhν, where α and hν is the absorbance and photon energy, respectively) against hν (inset in Figure 2a), the band-gap energy is calculated to be 4.19 eV, larger than that of monolayer MoS2 (1.9 eV) and bulk MoS2 (1.2 eV),[6,36b,37] and similar to that of reported monolayer MoS2 QDs (4.96 eV).[2d] Figure 2b shows the photoluminescence excitation (PLE) spectrum recorded with the detection wavelength of 420 nm and photoluminescence (PL) spectrum excited at 302 nm of MoS2 QDs solution. A strong sharp peak located at 302 nm is observed from PLE spectrum, which corresponds to the absorption peak located at 293 nm in UV-vis absorption spectrum. Also, a emission peak located at 420 nm can be observed from
PL spectrum, differing from the two well-known peaks centered at around 610 nm and 661 nm in 2D MoS2 flakes with large lateral dimensions from the direct band gap hot PL of the K point.[6a] The strongly blue-shifted hot PL from the K point is assigned to the small lateral dimensions of MoS2 QDs owing to quantum size effect.[36b] Figure 2c shows the PL spectra of MoS2 QDs solution excited by different wavelengths changing from 270 nm to 380 nm, showing that with the increase in excitation wavelength (λex), a red shift in the luminescence emission ranging from 415 nm to 455 nm was observed, and its intensity decreases promptly over emission wavelengths changing from 310 nm to 380 nm. This feature is ascribed to the existence of polydispersity of MoS2 QDs,[20b,21a] the hot PL from Brillouin zone’s K point and many trap states in MoS2 QDs as observed in graphene QDs.[6a,36b,38] The excited stated lifetime of MoS2 QDs was up to approximately 5.33 ns (Figure 2d), which is superior to its 2D MoS2 and graphene QDs counterparts.[39] The fluorescent quantum yield was calculated to be 12.8% by using Rhodamine B as reference, which also outperformed to that of graphene QDs synthesized by hydrothermal method.[40]
Figure 2. (a) UV-vis absorption spectrum of MoS2 QDs aqueous solution. Left inset: MoS2 QDs aqueous solution; right inset: the band-gap energy. (b) PLE spectrum with the detection wavelength of 420 nm and PL spectrum excited at 302 nm of MoS2 QDs. (c) PL spectrum of MoS2 QDs at different excitation wavelength. (d) The PL lifetime of MoS 2 QDs and 2D MoS2.
Scheme 1 shows the schematic diagram for synthesizing TiO2 NTAs, MoS2 QDs and MoS2 QDs/TiO2 NTAs composites. TiO2 NTAs anchored with MoS2 QDs for different electrodeposition time of 5, 10, 20 and 30 min marked as MoS2 QDs/TiO2 NTAs-5, MoS2 QDs/TiO2 NTAs-10, MoS2 QDs/TiO2 NTAs-20 and MoS2 QDs/TiO2 NTAs-30, respectively. The effect of electrodeposition time on as-synthesized MoS2 QDs/TiO2 NTAs composites’ photocatalytic activity is also discussed in this work, and the result indicates that the optimized electrodeposition time is 20 min, that is, the as-synthesized MoS2 QDs/TiO2 NTAs-20 shows the best photocatalytic activity in all samples.
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Scheme 1. Schematic diagram for preparation of TiO2 NTAs, MoS2 QDs and MoS2 QDs@TiO2 NTAs heterostructures.
Nanotubes with well-ordered arrangement and uniform nanoring can be observed from scanning electron microscope (SEM) images of pristine TiO2 NTAs (Figure 3a and b), and the length and average pore diameter of TiO2 NTAs are around 3.0 μm and 70 nm, respectively. For MoS2 QDs/TiO2 NTAs composites, the amount of MoS2 QDs deposited on TiO2 NTAs is influenced by the electrodeposition time. It is found that the amount of MoS2 QDs on TiO2 NTAs is nearly negligible when the electrodeposition time is controlled by 5 min (Figure S6a and b), and then significantly increases with increasing the electrodeposition time. An obvious amount of MoS2 QDs are uniformly deposited on TiO2 NTAs when the electrodeposition time prolongs to 10 min (Figure S6c and d), and more MoS2 QDs are well distributed on TiO2 NTAs without blocking the nanotubes’ channels when the electrodeposition time prolongs to 20 min (Figure 3c and d). However, MoS2 QDs aggregate to form the shape of stack and block TiO2 NTAs when the electrodeposition time prolongs to 30 min (Figure S6e and f). Therefore, it can be seen that the electrodeposition process has no effect on nanotubes structure of MoS2 QDs/TiO2 NTAs composites, and the amount of MoS2 QDs on TiO2 NTAs can be optimized by tuning the electrodeposition time.
QDs with sizes of 1.5-6.0 nm anchored on TiO2 NTAs appear to be isolated and well distributed. Figure 4c shows the HRTEM image of MoS2 QDs/TiO2 NTAs composites, fringes with lattice spacing of 0.35 nm and 0.23 nm belong to the (101) plane of anatase TiO2 and the (103) plane of hexagonal MoS2, respectively,[24,41] revealing MoS2 QDs uniformly and seamlessly bounded to TiO2 NTAs wall surface (marked by red circular box). Moreover, this result can be further demonstrated by SAED shown in Figure 4d. According to Bragg aquation, the corresponding plane MoS2 and TiO2 can be found (2.3 Å for MoS2 (103), 3.5 Å for TiO2 (101)), confirming the formation of MoS2@TiO2 NTAs heterostructure and the existence of MoS2 and TiO2 crystal phase in this heterostructure. The dark-field scanning transmission electron microscopy (DF-STEM) was used to investigate the quality and interface of heterostructure. Figure 4e shows the DF-STEM image of the MoS2 QDs/TiO2 NTAs heterostructure, MoS2 QDs are well bounded to TiO2 NTAs along edges, which is believed to enhance the heterojunction conductivity, thereby promoting electron-hole pairs separation. In addition, the TEM-EDX analysis of the selected region by a black rectangular box in Fig. 4e demonstrates the existence of Ti, Mo, O and S in MoS2 QDs/TiO2 NTAs composites (Figure S7). Also, the EDX analysis demonstrates that the atomic ratio of Mo : S is around 1 : 2, further verifying the formation of MoS2 QDs on TiO2 NTAs. Furthermore, EDX elemental mapping analysis of a single hybrid nanotube (Figure 4f-i) obviously reveals the MoS2 QDs/TiO2 NTAs heterostructure, indicating the adequate contact between different components, and the deep hybridization giving rise to the well-distributed active sites, which are desirable for the high catalytic efficiency.
Figure 3. Typical top-view (a) and side-view (b) images of pristine TiO2 NTAs, top-view (c) and side-view (d) images of the synthesized MoS2 QDs/TiO2 NTAs with an optimized electrochemical deposition time of 20 min.
Figure 4a and b shows the low-magnification TEM images of MoS2 QDs/TiO2 NTAs composites, it can be observed that MoS2
Figure 4. (a, b) TEM, (c) HRTEM images (MoS2 QDs are in red circles), and (c) corresponding SAED pattern of the synthesized MoS2 QDs/TiO2 NTAs-20. (e)
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High magnification DF-STEM image of synthesized MoS2 QDs/TiO2 NTAs-20, and (f-i) EDX elemental mappings of the selected area by red rectangular box in (e).
From the XPS analysis of MoS2 QDs/TiO2 NTAs composites (Figure S8), it can be seen that the as-synthesized sample contains Ti, O, Mo, S, and adventitious C. Figure 5a shows XPS spectra of Ti 2p in both pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs, it is indicated that Ti4+ presences in the two samples, resulting from a comparison of the binding energy splitting of approx. 5.8 eV between Ti 2p1/2 and Ti 2p3/2 peaks.[42] The fitted peak of Ti 2p for MoS2 QDs/TiO2 NTAs shifted negative higher energies, arising from the presence of Ti3+ states that show higher reaction activity by capturing electrons, thus leading to radicals formed on TiO2 NTAs by covalent bonding effect. These shallow defects can release captured charge carriers to the neighbouring valence or conduction bands by thermal excitation, which contributes to enhance photocatalytic activity.[43] The highresolution XPS spectra of O 1s in both pristine TiO 2 and MoS2 QDs/TiO2 NTAs are shown in Figure 5b, the major peaks at 530.0 eV and 530.6 eV ascribed to lattice oxygen, and the weak peaks at 531.6 eV and 532.4 eV are observed because of the existence of a surface hydroxyl group.[42] Figure 5c shows the highresolution XPS spectrum of Mo 3d, two peaks at 232.5 and 229.4 eV are attributed to Mo4+ 3d3/2 and Mo4+ 3d5/2, respectively, and the peak at 226.4 eV is ascribed to S 2s. The separation energy about 3.4 eV can be attributed to the typical features of Mo, which similarly exists in MoS2 QDs with corresponding binding energies of about 231.9 and 228.7 eV, respectively. For the high-resolution XPS spectrum of S 2p region, the fitted peaks at 161.9 and 162.8 eV are attributed to S 2p3/2 and S 2p1/2, respectively (Figure 5d), which are similar to MoS2 QDs with corresponding binding energies of about 161.4 and 162.7 eV, respectively. The fitted peaks of MoS2 QDs/TiO2 NTAs shifted to negative higher energies, indicating that electronic interaction exists in MoS2 and TiO2.[23a] The XPS analysis further confirms the successful introduction of MoS2 QDs to TiO2 NTAs.
Figure 5. XPS spectra of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites: (a) Ti 2p, (b) O 1s, (c) Mo 3d and (d) S 2p XPS spectrum.
The XRD was employed to investigate the crystal structure of MoS2 QDs/TiO2 NTAs. As shown in Figure S9a, the diffraction peaks of pristine TiO2 NTAs agree with the standard peaks of anatase TiO2 phase, and the diffraction patterns of MoS2 QDs/TiO2 NTAs composites contain all diffraction peaks of anatase TiO2, confirming that the crystal structure of TiO2 NTAs was not influenced during the electrodeposition process. From the XRD patterns of MoS2 QDs/TiO2 NTAs, two new peaks at 2θ of 39.8° and 44.4° can be observed, which correspond to the assynthesized MoS2 QDs’ characteristic peaks located at 39.5° and 44.2°assigned to the (103) and (006) planes of MoS2. Moreover, the intensity of MoS2 QDs’ characteristic peaks increases with prolonging the electrodeposition time, and the most intense peak is not obvious as it can be masked by the strong peak of Ti substrate. It is worth noting that some MoS2 QDs likely formed restacked shape during the electrodeposition and drying process, especially for prolonging the electrodeposition time, resulting in increasing multilayered MoS2 QDs anchored on TiO2 NTAs. Also, the peak assigned to (002) plane of the MoS2 (14.4°) is absent from the XRD patterns of MoS2 QDs/TiO2 NTAs, indicating that most MoS2 QDs deposited on TiO2 NTAs still retain the monolayer nature of the QDs without large amounts aggregating together. In addition, Figure S9b shows a comparison of Raman spectra of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs with different electrodeposition time. An intense peak at 143.2 cm-1 is attributed to the vibration mode of anatase TiO2, and other three peaks at 392.8, 513.3 and 634.2 cm-1 ascribed to the Raman active modes B1g, A1g and Eg of anatase TiO2, respectively. For MoS2 QDs/TiO2 NTAs, two new scattering peaks at 377.8 and 403.4 cm-1 can be observed, corresponding to the E12g and A1g modes of MoS2 QDs, respectively. Moreover, the intensity of the two peaks for MoS2 QDs/TiO2 NTAs increases with prolonging electrodeposition time. The result confirms the successful introduction of MoS2 QDs onto the TiO2 NTAs, and the amount of MoS2 QDs increases with extending electrodeposition time. Furthermore, compared to pristine TiO2 NTAs, the mode of E1g in MoS2 QDs/TiO2 NTAs occurs a slight blue shift, which may result from the surface strain caused by the introduction of MoS2.[44] Figure 6a shows UV-vis-NIR absorbance spectra of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites. Compared to pristine TiO2 NTAs, the MoS2 QDs/TiO2 NTAs composites exhibit a broader and stronger absorption between the wavelength range of 200 nm and 2500 nm, contributing to the significantly enhanced photocatalytic activity. Moreover, The as-synthesized MoS2 QDs/TiO2 NTAs-20 shows the strongest absorption in all samples, thereby expecting the most excellent photocatalytic activity. Also, the light absorption of MoS2 QDs/TiO2 NTAs composites broadens to NIR region without extinction, indicating a potential NIR-response photocatalyst. PL emission spectrum can be employed to study the efficiency of charge capture and photogenerated electron-hole pairs recombination in semiconductors. As shown in Figure 6b, the emission spectra of MoS2 QDs/TiO2 NTAs composites appear to be similar to pristine TiO2 NTAs, indicating that the introduction of MoS2 QDs has no
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effect on peak patterns of PL. Peaks located at about 451 nm, 468 nm, 483 nm and 599 nm in PL spectra may result from the existence of oxygen vacancies in TiO2 NTAs.[45] The lower PL intensity indicates more efficient separation of photoinduced electrons and holes, thereby expecting a higher photocatalytic activity. Compared to TiO2 NTAs, the MoS2 QDs/TiO2 NTAs composites with different electrodeposition time show the lower PL signal intensity, and the PL intensity increased in the order of MoS2 QDs/TiO2 NTAs-20 < MoS2 QDs/TiO2 NTAs-10 < MoS2 QDs/TiO2 NTAs-5 < MoS2 QDs/TiO2 NTAs-30 < TiO2 NTAs. When the electerodeposition time goes beyond a certain range, the PL intensity increases with prolonging electerodeposition time, indicating that re-stacked MoS2 QDs serve as the recombination centre, rather than promote electron-hole pairs separation, and partial re-stacked MoS2 QDs can block TiO2 NTAs’ channels. From these results, it is indicated that the MoS2 QDs with optimized size uniformly distribute on TiO2 NTAs can significantly fasten the separation of photoinduced charge and extend the lifetime of electro-hole pairs. The as-synthesized MoS2 QDs/TiO2 NTAs-20 exhibits the lowest PL intensity in all samples, thus expecting the most excellent photocatalytic activity. Time-resolved photoluminescene (TRPL) spectroscopy was carried out on pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs to investigate the photoinduced charge carriers kinetics. The biexponential function exactly as equation (1) was used to fit TRPL decay profiles (Figure 6c).[46] I (t) = A1 exp (-t/τ1) + A2 exp (-t/τ2)
where α, h, ν, A and Eg are the absorption coefficient, plank constant, light frequency, a constant and band-gap energy, respectively. The value of n is 1 for the as-synthesized MoS2 QDs, arising from that the monolayer MoS2 is a direct-gap material (n = 1).[43] Figure 6d shows the corresponding Kubelka-Munk transformed reflectance spectra, the calculated Eg of pristine TiO2 NTAs is 3.10 eV, and for MoS2 QDs/TiO2 NTAs composites with different electrodeposition time of 5, 10, 20 and 30 min, the calculated Eg are 2.75, 2.51, 1.96 and 2.81 eV, respectively. The band gap energy of MoS2 decreases with increasing crystal thickness below 100 nm because of quantum confinement and a single monolayer is predictably calculated by 1.9 eV. [27a] Therefore, the calculated Eg of MoS2 QDs/TiO2 NTAs composites reveals the formation of re-stacked MoS2 QDs during the electrodeposition and drying process. Furthermore, the analysis of band-gap energy indicates that MoS2 QDs anchored on TiO2 NTAs can improve the optical absorption properties, thus expecting the enhanced photocatalytic activity of MoS2 QDs/TiO2 NTAs composites.
(1)
The average excited-state lifetime is determined using the following equation (2):[46] τaverage = ∑Aiτ2i /∑Aiτi
(2)
where I is the PL intensity, t is the time after excitation, A1 and A2 are the pre-exponential factors, and τ1 and τ2 are the corresponding lifetimes, τaverage is the average lifetime for the excited state electron before transfer or recombination, Ai and τi corresponds to the parameters in equation (1). As calculated in Figure 6c (inset), the average excited electron lifetimes of MoS2 QDs/TiO2 NTAs with different electrodeposition time of 5, 10, 20 and 30 min are 4.39, 2.89, 2.23 and 4.66 ns, respectively, all of which are lower than that of pristine TiO2 NTAs (τaverage = 5.60 ns). A shorter decay lifetime means a more efficient separation of electron-hole pairs,[45] and the more efficient separation of electron-hole pairs indicates higher photocatalytic activity and weaker photoluminescence, which agrees with the trend of PL spectra.[47] The as-synthesized MoS2 QDs/TiO2 NTAs-20 shows the shortest decay lifetime in all samples, thus indicating the most excellent photocatalytic activity. Also, the result reveals that MoS2 QDs can be used as an electron filter to significantly facilitate excitons separation and suppress charge recombination. For a typical crystalline semiconductor, the band-gap can be caculated by the equation (3) as follows: [43] αhν = A (hν-Eg)n/2 (3)
Figure 6. (a) UV-vis-NIR absorption spectra, (b) PL spectra and (c) TRPL decay profiles for TiO2 NTAs and MoS2 QDs/TiO2 NTAs with different electrodeposition time for 5, 10, 20 and 30 min. (d) The plots of optical band gap of pristine TiO 2 NTAs and MoS2 QDs/TiO2 NTAs composite with electrodeposition time for 20 min.
The heterojunction created between MoS2 QDs and TiO2 NTAs also has effect on the photocurrent response. Figure 7a shows the photocurrent densities of all samples, the maximum saturation photocurrent densities of MoS2 QDs/TiO2 NTAs composites are higher than that of pristine TiO2 NTAs under full spectral wavelength light illumination, and the as-synthesized MoS2 QDs/TiO2 NTAs-20 shows the highest maximum saturation photocurrent density, revealing that TiO2 NTAs decorated with MoS2 QDs can produce more photo-induced charge carriers and enhance the separation effiency of electron-hole pairs. Movermore, compared to pristine TiO2 NTAs, the onset potential of MoS2 QDs/TiO2 NTAs composites shows negative shift, indicating that the heterojunction can liberate and accumulate
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where ND is the electron carrier density, e is the elemental charge (e = 1.6×10-19 C), C is the space charge capacitance, ε is the relative permittivity of the semiconductor (ε = 48 for anatase TiO2), and ε0 is the permittivity of a vacuum (ε0 = 8.86×10-12 F·m-1). A higher ND signifies a faster carrier transfer.[43] Clearly, the calculated ND value of MoS2 QDs/TiO2 NTAs is higher than that of pristine TiO2 NTAs, demonstrating that MoS2 QDs anchored on TiO2 NTAs can significantly promote the transfer of photogenerated charge, thus effectively inhibiting electron-hole pairs recombination. In addition, the incident photon-to-current conversion efficiencies (IPCE) was employed to further investigate the photoresponse of TiO2 NTAs and MoS2 QDs/TiO2 NTAs, as comparatively studied in Figure S10. The IPCE of pristine TiO2 NTAs shows slightly lower than that of the as-
synthesized MoS2 QDs/TiO2 NTAs-20 in the UV region (300-420 nm), but almost negligible in visible and NIR light (420-2500 nm). The as-synthesized MoS2 QDs/TiO2 NTAs-20 shows highly enhanced IPCE in visible and NIR region, indicating the much enhanced utilization of visible and NIR light for MoS 2 QDs/TiO2 NTAs composites.
Figure 7. (a) Photocurrent densities versus voltage curves and (b) photocurrent densities versus time curves of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs with different electrodeposition time of 5, 10, 20 and 30 min in 0.1 M Na2SO4 solution under xenon lamp irradiation (100 mW·cm-2) at 0.3 V vs Ag/AgCl. (c) EIS Nyquist plots of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs with electrodeposition time of 20 min in 0.1 M Na2SO4 solution under dark and xenon lamp irradiation (100 mW·cm-2) at 0 V vs Ag/AgCl. (d) MS plots of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs with electrodeposition time of 20 min in 0.1 M Na2SO4 solution.
For further studying the enhanced photoresponse of MoS2 QDs/TiO2 NTAs, both transient photocurrent response and Nyquist plots of EIS for TiO2 NTAs and MoS2 QDs/TiO2 NTAs were performed under UV, visible and NIR light irradiation, respectively, as comparatively studied in Figure S11. The pristine TiO2 NTAs electrode illustrates 0.30 mA·cm-2 under UV light irradiation but nearly no response under visible and NIR light irradiation, while the as-synthesized MoS2 QDs/TiO2 NTAs-20 electrode illustrates 0.98, 0.079 and 0.032 mA·cm-2 under UV, visible and NIR light irradiation, respectively (Figure S11a). The MoS2 QDs/TiO2 NTAs electrode shows similar photocurrent under simulated solar light and UV light irradiation, and displays obvious photocurrent under visible even NIR light irradiation when compared to pristine TiO2 NTAs. Also, the arc radius on EIS Nyquist plots corresponds to photocurrent for different electrodes (Figure S11b). The result indicates that TiO2 NTAs decorated with MoS2 QDs can produce photoinduced electron-hole pairs under visible even NIR light irradiation, resulting in extending the photoresponse range from UV to NIR, thus expecting a efficiently promoted photocatalytic activity of MoS2 QDs/TiO2 NTAs under the simulated solar light irradiation. The photocatalytic activities of TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites were tested with the help of H2 and O2 evolution from pure water without adding any sacrificial agents or
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more electrons and thus resulting in more electronegatively of the anode potential.[23f] Figure 7b shows photocurrent-time (I-t) characteristics in different electrodes of pristine TiO 2 NTAs and MoS2 QDs/TiO2 NTAs composites with different electrodeposition time upon several 30 s light on/off cycles under simulated solar light irradiation at 0.3 V vs Ag/AgCl. It can be seen that the photocurrent densities of MoS2 QDs/TiO2 NTAs composites with different deposition duration of 5, 10, 20 and 30 min are 0.56, 0.76, 1.08 and 0.49 mA·cm-2 respectively, all of them are higher than that observed with blank TiO2 NTAs (0.34 mA·cm-2), which may result from the enhanced photoresponse of MoS2 QDs/TiO2 NTAs heterostructure to visible even NIR light and then promoting the separation of photoinduced electron-hole pairs. Electrochemical impendance spectroscopy (EIS) tests were used to study the characteristics of electron-hole pairs separation and delivery. The arc radius of EIS (Nyquist plots, corresponding to the real part Z' vs the imaginary part -Z" of the complex impedance Z) plot reveals the reaction rate on electrode surface, and a smaller semicircle means a lower resistance of charge transfer.[45] As shown in Figure 7c, compared to TiO2 NTAs, the as-prepared MoS2 QDs/TiO2 NTAs-20 shows smaller arc radius on both in dark and under simulated solar light irradiation, indicating a lower charge transfer resistance of MoS2 QDs/TiO2 NTAs-20. The result indicates that MoS2 QDs anchored on TiO2 NTAs can efficiently facilitate electrons transport and reduce charge-transfer resistance based on the interfacial contact between MoS2 QDs and TiO2 NTAs, leading to a significantly enhanced photoelectrical activity of MoS2 QDs/TiO2 NTAs composites when compared to pristine TiO2 NTAs. Figure 7d shows Mott-Schottky (MS) plots carried out by employing the impedance technique at 10 Hz for pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs with electrodeposition time of 20 min. The MS plots are linear in the potential range from -0.1 to +0.3 V vs Ag/AgCl. It can be seen that the as-synthesized MoS2 QDs/TiO2 NTAs-20 electrode shows smaller slope than that of pristine TiO2 NTAs in MS plots under simulated solar light illumination near the flat band potential. The following equation (4) was employed to caculate the carrier density ND.[48] 2 dΕ (4) ND eεε0 d (C 2 )
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cocatalysts under the simulated solar light illumination, as comparatively investigated in Figure 8a. As expected, the H2 and O2 generation rates of MoS2 QDs/TiO2 NTAs composites are higher than that of pristine TiO2 NTAs (7.21 and 3.42 μmol·cm-2·h1 for H2 and O2, respectively), and the MoS2 QDs/TiO2 NTAs with electrodeposition time of 20 min shows the highest H 2 evolution rate (32.64 and 16.16 μmol·cm-2·h-1 for H2 and O2, respectively) among all of the samples, which is consistent with photocurrent test under the simulated solar light illumination. The result indicates that MoS2 QDs anchored on TiO2 NTAs shows efficiently enhanced the photocatalytic water splitting when compared to pristine TiO2 NTAs. Moreover, the as-prepared MoS2 QDs/TiO2 NTAs shows outstanding stability in the durability test of photocatalytic water splitting. Figure 8b shows the production of H2 and O2 from 20 mL of pure water containing 4.5 cm2 of MoS2 QDs/TiO2 NTAs with electrodeposition time of 20 min under the simulated solar light irradiation. H2 and O2 were both quantified by typical GC signal shown in Figure S12. The molar ratio of H2 and O2 evolution is about 2 during the continuous process of gases evolution, effectively identical to the theoretical value for overall water splitting, but stopped immediately when the light was turned off. After 5 repeated cycles, the H2 and O2 evolution rates remain 31.14 and 14.27 μmol·cm-2·h-1, respectively, and a slight decrease occured when compared to the first cycle, indicating the outstanding photostability of MoS2 QDs/TiO2 NTAs. Figure 8c shows that the H2 generation rates from water photo-catalyzed by pristine TiO2 NTAs and the as-synthesized MoS2 QDs/TiO2 NTAs20 under 300-W Xe lamp irradiation with a long-pass cutoff filter (λ > 420 nm). Compared to pristine TiO2 NTAs, the H2 generation rate increased by a factor of 3.8 for MoS2 QDs/TiO2 NTAs composites with electrodeposition time of 20 min. For quantum efficiency (QE) calculations, we employed the QE determination at λ0 = 420 nm for pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites as an example. The amount of H2 molecules generated in 12 hours was 18.24 and 65.52 μmol·cm-2 for pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites, respectively, under 300-W Xe lamp irradiation applying a λ0 ±20 nm band-pass filter (Figure S13). The QE is calculated from the following equation (5) and (6):[49] Eλ (5) Np hc (6) 2N H QE
Np
where Np is the number of incident photons, E is the average intensity of light irradiation, λ is irradiation wavelength, h is Planck constant, c is the speed of light, NH is the number of evolved H2 molecules. The QE of the MoS2 QDs/TiO2 NTAs composites (electrodeposition time of 20 min) remarkably increased relative to the QE of pristine TiO2 NTAs, as comparatively shown in Figure 8d. It reached 7.11% at λ = 420 ± 20 nm (3.6 times the QE for pristine TiO2 NTAs) and 0.12% at λ = 800 ± 14 nm. The QE of the MoS2 QDs/TiO2 NTAs composites trended to 0 when irradiation wavelengths λ > 900 nm, while the catalyst system of pristine TiO 2 NTAs showed zero QE when irradiation wavelengths λ > 620 nm.
Figure 8. (a) Comparison of photocatalytic H2 and O2 evolution from water activities of pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites with different electrodeposition time of 5, 10, 20 and 30 min under the simulated solar light irradiation. (b) Typical time course of H2 and O2 evolution from water under the simulated solar light irradiation catalyzed by MoS2 QDs/TiO2 NTAs composites with electrodeposition time of 20 min. (c) The 12-hours time course of H2 generation from water photo-catalyzed by pristine TiO2 NTAs and MoS 2 QDs/TiO2 NTAs composites with electrodeposition time of 20 min under visible light irradiation (300-W Xe lamp, λ > 420 nm). (d) Wavelength-dependent QE of water splitting by catalysts with pristine TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites with electrodeposition time of 20 min applying several bandpass filter (for λ < 680 nm). A long-pass cutoff filter was used to attain λ > 700 nm light from a 300-W Xe lamp. For (a), (b) and (c), the vertical error bars indicate the maximum and minimum values obtained; for (d), the horizontal bars indicate the width of the wavelength band of the filters used.
Furthermore, the photocatalytic activity of TiO2 NTAs and MoS2 QDs/TiO2 NTAs composites (electrodeposition time of 20 min) were tested by H2 and O2 evolution from pure water under UV, visible and NIR light illumination, respectively, as comparatively shown in Figure S14a. For TiO2 NTAs, the H2 evolution rate is 1.41 μmol·cm-2·h-1 under UV light illumination, which is quite analogous to that under the simulated solar light irradiation, and nearly no H2 evolution under visible and NIR light illumination. However, the MoS2 QDs/TiO2 NTAs composites achieves H2 evolution rates of 31.36, 5.29 and 1.67 μmol·cm-2·h1 under UV, visible and NIR light illumination, respectively. Obviously, MoS2 QDs act as an important role for TiO2 NTAs used as catalyst to photocatalytic H2 evolution under visible and NIR light irradiation. The molar ratio of H2 to O2 is 2:1, indicating that MoS2 QDs/TiO2 NTAs can efficiently drive water splitting under UV, visible and NIR light irradiation. Moreover, the MoS2 QDs/TiO2 NTAs composite also exhibits excellent stability in durability test of photocatalytic water splitting. The H2 and O2 generation rates decreased a little after 5 successive 2-h cycles under UV, visible and NIR light irradiation, respectively, As shown in Figure S12b, indicating steady water splitting with time of MoS2 QDs/TiO2 NTAs. The optimum amount of MoS2 QDs anchored on TiO2 NTAs can effectively promote the heterostructure’s photocatalytic activity in water splitting upon the simulated solar light irradiation,
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indicating that such heterostructure has a strong photon acquisition ability in full solar spectrum by confining a plasmonic MoS2 coating on the surface of TiO2 substrate.[7d] The surface plasmon can facilitate the electron-hole pairs generation and separation by resonant energy transfer (RET) from plasmonic dipoles to electron-hole pairs in semiconductor by a near-field electromagnetic interaction.[50] The RET facilitate charge carrier generation and separation at the energies both below and above the bandgap, without being restricted by the electronic band structure of semiconductor.[50a] In this work, considering both the photocurrent spectra and the high photocatalytic H2 evolution activity, we can briefly propose a tentative mechanism referred in the designed system in full spectral wavelength, as illustrated in Scheme 2. UV light (and partial visible light) can photoexcite TiO2 to generate the electron-hole pairs, the VB electrons of TiO2 transfer to the CB, and then instantly transfer to MoS2 through the intimate interface contacts for H2 evolution. Meanwhile, visible light illumination activates the surface plasmon of MoS 2 QDs, whose RET process can significantly enhance the electron-hole pairs generation and separation. Also, the RET mechanism depends on the simultaneous photoexcitation of TiO2 and MoS2 QDs, thereby resulting in its dramatically promoted photocatalytic activity under the simulated solar light illumination. Additionally, MoS2 QDs itself is highly active for H2 evolution because of the quantum-confinement effect and the presence of surface defects and edge sites in MoS2 nanostrctures,[19a,51] which is highlighted by the H2 generation on MoS2 QDs/TiO2 NTAs heterostructure upon visible and NIR light irradiation.
Scheme 2. Schematic illustration of the energy band structure and the proposed charge transfer mechanism of MoS2 QDs@TiO2 NTAs heterostructures under full-spectrum irradiation.
Conclusions A novel type of hierarchical MoS2 QDs/TiO2 NTAs composites by decorating MoS2 QDs onto self-ordered TiO2 NTAs is successfully synthesized by a facial electrodeposition method. An appropriate amount of MoS2 QDs uniformly coated on anatase TiO2 NTAs with clean top surface is the key to grow perfect MoS2 QDs/TiO2 NTAs composites with dramatically promoted photocurrent response and enhanced photocatalytic activity due to expanded spectrum range of energy utilization and enhanced charge separation. In
detail, the as-synthesized MoS2 QDs/TiO2 NTAs with UV-vis-NIR broad spectrum absorption can be used as an expanded spectrum-driven photocatalyst for water splitting into H 2 and O2. As expected, the H2 and O2 evolution behavior from water over MoS2 QDs/TiO2 NTAs composites was realized under UV (λ < 420 nm), visible (λ ≥ 420 nm) and NIR (λ > 760 nm) light irradiation without the assist of any sacrificed agents or cocatalysts, and the H2 evolution rates corresponding to UV, visible and NIR light irradiation consist with the trend of photocurrent densities under the identical light irradiation. Moreover, the excellent photocatalytic stability of as-synthesized MoS2 QDs/TiO2 NTAs can make it a promising candidate in the future for practical applications in environment and energy. Significantly, the facial and low-coast method provides a new insight into designing novel dual-semiconductor material system for efficiently photocatalytic hydrogen evolution reaction and photovoltaic devices.
Experimental Section Synthesis of MoS2 QDs/TiO2 NTAs MoS2 QDs were prepared by a hydrothermal process using ammonium molybdate tetrahydrate (H24N6Mo7O24·4H2O) and dibenzyl disulfides (C14H14S2) as molybdenum and sulfur sources, respectively. In brief, 0.7 g of ammonium molybdate tetrahydrate was dissolved in 40 mL of deionized water with magnetic stirring for 20 min, and then 0.48 g of dibenzyl disulfide and 40 mL of ethanol were dissolved in the above solution by the assitance of ultrasonication for 20 min. The mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 220℃for 20 h. Upon cooling naturally to room temperature, drain the mother liquor and add new deionized water into the autoclave. The obtained solution was filtrated with slow-speed quantitative filter paper via vacuum pumping, and then was centrifuged at 12000 rpm for 30 min to eliminate the precipitate. Finally, the synthesized brown supernatant was concentrated by rotary evaporation at 80 ℃ to obtain MoS2 QDs solution with a concentration of 0.1 mg·mL-1, and then saved as standby in ambient temperature. The titanium foils purchased from Sigma-Aldrich (0.25 mm thick, 99.7% purity) were ultrasonically cleaned in acetone, ethanol, and deionized water, respectively, and then dried in a nitrogen stream before electrochemcial anodization. The anodization process was performed in a conventional twoelectrode system with Ti foil as the working anode and Pt foil as the counter electrode. The Ti working anode was pressed together with an Al foil against an O-ring, defining a working area of 4.5 cm2 (1.5×3.0 cm2). A mixed solution consists of ethylene glycol (99.8%, Sigma-Aldrich) solution with 0.5 wt% NH4F (98.0%, Sigma-Aldrich) and 2.0 vol% H2O was used as the electrolyte. The Ti foils were pre-anodized at 50 V for 2 h, and then removed the as-anodized oxide layer by ultrasonicating in ethanol. Then the second anodization was peformed in fresh electrolyte with appling the voltage of 50 V for 10 min to grow the self-ordered
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nanotube structures with clean top surface. To obtain TiO 2 NTAs with anatase phase, the anodized samples were annealled at 450℃ in air for 3 h with a heating rate of 2℃ per min. The MoS2 QDs/TiO2 NTAs heterostructure was synthesized by a facile electrodeposition method. The electrodeposition process was performed on an electrochemical workstation (PGSTAT302N, Autolab, Switzerland) with a three-electrode configuration, TiO2 NTAs/Ti foil, Pt foil and saturated Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. MoS2 QDs solution (0.1 mg·mL-1) was used as electrolyte that contains citric acid-sodium citrate buffer solution (0.1 M, pH 4.5) with a volume ratio 1:1. MoS2 QDs were deposited onto TiO2 NTAs photoelectrodes by electrodeposition process, and the obtained samples were rinsed with deionized water and dried at ambient temperature after different electrodeposition time for 5, 10, 20 and 30 min, respectively.
and counter electrode, respectively. A 0.1 M Na2SO4 (pH = 6.0) solution was used as the electrolyte, and the Na2SO4 electrolyte was purged with Ar air for 30 min prior to the measurement. A 300-W Xe lamp (Beijing Trusttech Co. Ltd, China) was acted as the irradiation source to provide full spectrum light, UV light, visible light and NIR light, under the absence of cutoff filter to ensure the light as full spectrum light source, the presence of 420 nm cutoff filter to ensure the light as UV and visible light source, and the assistance of 760 nm cutoff filter to ensure the light as NIR light source. The distance between the light source and the working electrode was 15 cm. Linear sweep voltammetry curves were tested at a scanning rate of 20 mV·s -1, ranging from -0.5 V to 1.0 V vs Ag/AgCl electrode. The photocurrent were performed by measuring the photocurrent densities at a bias potential of 0.3 V vs Ag/AgCl. Electrochemical impedance spectroscopy was performed over a frequency range of 0.1-105 Hz with an AC voltage amplitude of 5.0 mV at a bias of 0 V vs Ag/AgCl in 0.1 M Na2SO4 solution.
Characterization The field-emission scanning electron microscope (FESEM) images were obtained using a S4800 SEM (Hitachi, Japan). The microstructure and composition were characterized by Tecnai F20 high resolution transmission electron microscope (FEI, USA), combining with energy dispersive X-ray spectroscopy. Atomic force microscope (Agilent 5500, USA) was applied to charaterize the thickness profile of MoS2 QDs. X’pert-Pro MRD X-ray diffractometer with Cu-Kα radiation (PANalytical, Holland) was employed to study crystal phases. Axis Ultra HAS X-ray photoelectron spectroscopy (KRATOS, Japan) equipped with an Al monochromatic X-ray source was empolyed to analyze the chemical composition. The binding energies were normalized to the signal for adventitious C 1s at 284.8 eV. The Raman spectra were obtained from a FM4P-TCSPC Raman spectrometer (HOKIBA Jobin Yvon, UK) with an air-argon ion laser as the excitation source (λex = 532 nm). The Nicolet 6700 fourier transform infrared spectrometer (Thermo Inc., USA) was used to investigate the maiterials surface functional groups. UV-vis absorption spectra were measured by a Cary 5000 UV-vis spectrophotometer (Agilent, USA) equipped with an integrating sphere attachment. The PLE and PL spectra were recorded using an FLS 980 spectrofluorometer (Edinburgh Instruments, UK) with a xenon lamp as the excitation source (λ ex = 375 nm). Timeresolved PL measurements were measured under ambient conditions by detecting the modulated luminescence signal with a PMT (Hamamatsu, H10330-75, Japan), and then analyzing the signal with a photon-counting multichannel scaler.
Photoelectrochemical measurements Photoelectrochemical characterizations were performed on a PGSTAT302N electrochemical workstation (Autolab, Switzerland) with a standard three-electrode configuration, a saturated Ag/AgCl electrode and a Pt foil served as the reference
Photocatalytic water splitting measurements The Pyrex glass photoreactor was applied to test the photocatalytic H2 production, which connected with a closed gascirculation system. The samples (4.5 cm2) as photocatalysts were put into 20 mL deionized water contained in the above photoreactor, followed by Ar air bubbling for 30 min. A 300-W xenon lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd, China) was used as the simulated solar light source. The amount of H 2 was obtained on a GC-7900 gas chromatography set up with a 5 Å molecular sieve column and a thermal conductivity detector (TCD). The temperatures of GC oven, injection and detector were 80, 100 and 120℃, respectively. Ar air with the flow rate of 30 mL·min-1 was used as carrier gas.
Acknowledgements We thank the funding from National Natural Science Foundation of China (21501127 and 51502185), Nantong Science and Technology Project (GY12016030), and Jiangsu Advanced Textile Engineering Center Project (Project No. SPPGO[2014] 22). Keywords: MoS2 quantum dots • TiO2 nanotube arrays • photocatalytic activity • water splitting • hydrogen evolution [1] a) J. A. Turner, Science 2004, 305, 972974; b)X. Yu, L. Yu, H. Wu, X. W. Lou, Angew. Chem., Int. Ed. 2015, 54, 1-6; c) X. N. Wang, R. Long, D. Liu, D. Yang, C. M. Wang, Y. J. Xiong, Nano Energy 2016, 24, 87-93; d) M. Z. Ge, Q. S. Li, C. Y. Cao, J. Y. Huang, S. H. Li, S. N. Zhang, Z. Chen, K. Q. Zhang, S. S. Al-Deyab, Y. K. Lai, Adv. Sci. 2017, 4, 1600152. [2] a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 2001, 293, 269-271; b) K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 2006, 440, 295; c) X. M. Zhou, N. Liu, P. Schmuki, ACS Catal. 2017, 7, 32103235; d) X. P. Ren, L. Q. Pang, Y. X. Zhang, X. D. Ren, H. B. Fan, S. Z. Liu, J. Mater. Chem. A 2015, 3, 10693-10697.
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FULL PAPER Qun Wang, Jianying Huang, Hongtao Sun, Yun Hau Ng, Ke-Qin Zhang, and Yuekun Lai * Page 1 – Page 12 MoS2 quantum dots@TiO2 nanotube arrays: An extended spectrum-driven photocatalyst for solar hydrogen evolution
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This work presents a facile approach for fabricating the MoS2 QDs/TiO2 NTAs heterostructures for efficient photocatalytic H2 evolution without adding any sacrificial agents or cocatalysts. The improved photocatalytic activity owes to the decreased band-gap and the surface plasmonic properties of MoS2 QDs/TiO2 NTAs, promoting the electron-hole pairs separation and absorption capacity to visible and NIR light.
