2D Heterojunctions

Sep 21, 2017 - immigration of photoexcited charge.29−32 To date, some .... chromatography (GC-14C, Shimadzu, Japan, TCD, with nitrogen as a carrier ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9749-9757

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SrTiO3 Nanoparticle/SnNb2O6 Nanosheet 0D/2D Heterojunctions with Enhanced Interfacial Charge Separation and Photocatalytic Hydrogen Evolution Activity Yu Jin,† Deli Jiang,*,† Di Li,‡ Peng Xiao,† Xiaodong Ma,† and Min Chen*,† †

School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China

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‡

ABSTRACT: Exploration of high-performance and stable metal-oxide-based hybrid photocatalysts for hydrogen evolution is highly desirable. In this work, novel SrTiO3 nanoparticle/SnNb2O6 nanosheet hybrid 0D/2D heterojunctions with an interfacial interaction were constructed by a facile two-step wet chemistry strategy. Different characterization techniques were adopted to investigate the microscopic structures and physicochemical properties of the as-prepared hybrid heterojunctions. The optimal weight percent of SrTiO3 loading is 20 wt %, generating the highest H2 evolution amount of 17.16 μmol, which is 298 and 2 times higher than that of bare SrTiO3 and SnNb2O6. It can be suggested that an interfacial interaction among SrTiO3 and SnNb2O6 could result in efficient charge separation and enhanced H2 generation activity, which was confirmed by photoelectrochemical analyses. This work implies that the construction of 0D/2D metal-oxide-based hybrid heterojunctions with an interfacial interaction is an effective way to fabricate high-performance photocatalyst for solar fuel production. KEYWORDS: Photocatalysis, Hydrogen evolution, Heterojunction, Charge separation, Mechanism

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INTRODUCTION Owing to the depletion and combustion of fossil fuels, it is a worldwide priority target to develop renewable, ecofriendly, and economical energy technologies to alleviate the energy and environmental crisis.1−4 As a promising strategy, heterogeneous semiconductor photocatalytic hydrogen production driven by renewable solar energy has attracted a great deal of attention around the world, which could achieve the sustained solar-tofuel conversion and ease the above encountered problems.5−9 Therefore, various types of heterogeneous semiconductors aimed at H2 generation have been developed. The thin twodimensional (2D) crystal with exceptional electronic, optical, and mechanical properties is a significant category of nanostructured materials to which tremendous attention has been paid recently.10−14 It is noteworthy that 2D nanomaterials with outstanding photocatalytic properties benefit from high two-dimensional anisotropy and atomic-layer thickness which can provide more reactive sites and efficient charge separation.15−17 As typical 2D nanosheet materials, SnNb2O6 consists of a two-octahedron-thick sheet built by corner-sharing NbO6 octahedral flat and edge-sharing SnO8 square antiprisms flat. Besides, SnNb2O6 possesses the narrow band gap (∼2.3 eV) and proper CB position, which are beneficial for the visiblelight harvesting and photocatalytic H 2 generation.18−21 Recently, Seo et al. synthesized SnNb2O6 nanoplates by a facile template-free solvothermal method, and the synthesized nanoplates exhibited enhanced photocatalytic H2 evolution rate of 18.4 μmol/g h−1.22 Zhou et al. reported the ultrathin © 2017 American Chemical Society

SnNb2O6 nanosheets, which were synthesized via a facile hydrothermal method and showed improved visible-light photocatalytic H2 production rate of 13.2 μmol h−1.23 Yuan et al. fabricated a SnNb2O6-GR nanocomposite by an efficient electrostatic self-assembly method, and almost 100% of RhB was degraded after 60 min.24,25 Nevertheless, the photocatalytic efficiency of SnNb2O6 is limited by the high recombination rate of photogenerated electron−hole pairs, which greatly restrict its practical applications in photocatalytic H2 production. For the sake of further improving the visible-light-harvesting property of SnNb2O6 as well as suppressing the photogenerated charge recombination, extensive modification strategies have been undertaken including morphological control,22,23 noble metal deposition,26 and coupling with the others semiconductors to construct the heterojunction.24,27,28 Particularly, different semiconductor materials with matched band gaps cooperating to form a heterojunction is an effective strategy to improve the photocatalytic performance because of the synergistic effect, which could lead to enhanced visiblelight-absorption ability and improved efficient separation and immigration of photoexcited charge.29−32 To date, some semiconductors such as GR,24 g-C3N4,27 and WO328 have been employed to couple with SnNb2O6 to form heterojunctions with enhanced photocatalytic activity. However, Received: May 16, 2017 Revised: August 20, 2017 Published: September 21, 2017 9749

DOI: 10.1021/acssuschemeng.7b01548 ACS Sustainable Chem. Eng. 2017, 5, 9749−9757

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ACS Sustainable Chemistry & Engineering

Figure 1. TEM images of (a) SNO, (b) 10%-STO/SNO, (c) 20%-STO/SNO, (d) 30%-STO/SNO, (e) 40%-STO/SNO, (f) HRTEM image of 20%-STO/SNO, (g−m) HAADF-STEM image and the corresponding EDS elemental mapping images of the 20%-STO/SNO heterojunction.

separation between the STO and SNO heterojunction during the photocatalytic reaction.

despite this great progress in constructing heterojunction photocatalysts, their efficiency is not enough for its applications in photocatalytic environmental remediation due to their limited interfacial contact and low charge separation efficiency. Thus, further stimulating the development of novel SnNb2O6based heterojunctions with high-performance photocatalytic activities for hydrogen generation is still a huge challenge. Strontium titanate (SrTiO3), as one of the promising perovskite-type photocatalyst candidates, has been widely investigated and found to be favorable and catalytically active in water splitting, due to its outstanding corrosion resistance, heat resistance, and chemical and structural stability.33−41 In this contribution, SrTiO3 is probably an ideal candidate to construct heterojunctions with a SnNb2O6 nanosheet owing to the following reasons: the band-edge positions of SnNb2O6 match well with those of SrTiO3, which could theoretically form the type-II heterojunction and therefore improve the separation efficiency of photoinduced electron−hole pairs, consequently enhancing the photocatalytic activity. To our knowledge, there is no research on the construction of a SrTiO3 nanoparticle/SnNb2O6 nanosheet hybrid photocatalyst with high hydrogen production efficiency under visible-light irradiation. Herein, we report a facile fabrication of a SrTiO3/SnNb2O6 (denoted as STO/SNO) 0D/2D hybrid heterojunction with an interfacial interaction. The photocatalytic hydrogen evolution of as-prepared 0D/2D hybrid heterojunction was investigated. The results demonstrated that the STO/SNO hybrid heterojunction exhibits remarkably enhanced photocatalytic H2 generation efficiency compared with that of bare STO and SNO. It is postulated that the excellent photocatalytic activity for H2 production of STO/SNO can be mainly ascribed to the formed interfacial contact and enhanced interfacial charge

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EXPERIMENTAL SECTION

Materials. Potassium hydroxide (KOH), niobium oxide (Nb2O5), hydrochloric acid (HCl), stannous chloride dehydrate (SnCl2·2H2O), tetrabutyl titanate (Ti(C4H9O)4), ethylene glycol (EG), strontium nitrate (Sr(NO3)2), sodium hydroxide (NaOH), and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China, and were used as received with no purification. Deionized water was used in the experiments. Synthesis of STO/SNO Heterojunction. STO was synthesized by a simple hydrothermal method process according to a previously report.42 Typically, 10 mmol of Ti(C4H9O)4 was added into 20 mL of ethylene glycol (EG) to form a clear solution. Subsequently, 20 mL of 0.5 M Sr(NO3)2 aqueous solution and 10 mL of 5 M NaOH aqueous solution were dissolved in the obtained solution under stirring for 30 min. The obtained suspension was transferred to a 100 mL Teflonlined vessel, followed by heat-treating at 473.15 K for 24 h, then washed and collected with distilled water and absolute ethanol until the pH value was 7, and dried at 343.15 K overnight. SNO nanosheets were prepared similarly to those in the former report.18,27 In detail, certain amounts of Nb2O5 and KOH were mixed in 40 mL of distilled water. The mixed solution was stirred vigorously for 10 min and transferred into a 50 mL Teflon-lined stainless steel autoclave, and heat-treated at 453.15 K for 2 days. HCl aqueous solution (2 mol L−1) was added to the obtained clear solution until the pH value was adjusted to 7. After that, SnCl2·2H2O was quickly dispersed into the solution, and HCl (2 mol L−1) aqueous solution was dropped into the solution until the pH reached 2 by continuous stirring. The obtained suspension was transferred to a 100 mL Teflonlined vessel, hydrothermally treated at 473.15 K for 24 h. Eventually, the resultant yellow sample was centrifuged and washed thoroughly with DI water and anhydrous alcohol, and then dried at 343.15 K overnight. 9750

DOI: 10.1021/acssuschemeng.7b01548 ACS Sustainable Chem. Eng. 2017, 5, 9749−9757

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Figure 2. XPS spectra of the STO, SNO, and 20%-STO/SNO heterojunction: (a) survey spectrum, (b) Sr 3d, (c) Ti 2p, (d) Sn 3d, (e) Nb 3d, and (f) O 1s. A facile two-step wet chemistry strategy is proposed to synthesize the STO/SNO heterostructures. The different weight ratios of the STO to SNO samples (1/10, 1/5, 1/3, 2/5) were separately dispersed into 80 mL of absolute ethanol. The mixed solution was ultrasonically treated for 30 min, and it was then stirred continuously for 4 h to form a homogeneous solution. The solution was moved to a 100 mL Teflon-lined vessel, followed by heat-treating at 433.15 K for 12 h. The as-formed samples were rinsed with DI water, and then dried at 343.15 K overnight. The resultant STO/SNO composites were labeled as 10%-STO/SNO, 20%-STO/SNO, 30%-STO/SNO, and 40%-STO/ SNO, respectively. Characterization. X-ray diffraction data were collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation. The morphology of the products was also examined by transmission electron microscopy (TEM) which was recorded on a Tecnai 12 (The Dutch Philips company). The X-ray photoelectron spectroscopic technique (XPS, ESCA PHI500) was used to determine the metal oxidation states and surface element compositions of the samples. The nitrogen adsorption−desorption isotherms at 77 K were researched using a TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation). Furthermore, the ultraviolet−visible (UV−vis) diffuse absorbance and reflectance spectra of the samples in the range 200−800 nm were determined with a Shimadzu UV-2450 spectrophotometer using BaSO4 as a reference. The photoluminescence (PL) spectra of the photocatalysts were obtained by a Varian Cary Eclipse spectrometer. The transient photocurrent responses (I−t curves) were performed on an electrochemical analyzer (CHI 660B Chenhua Instrument Company) with a 420 nm optical filter. Photocatalytic Hydrogen Production. The photocatalytic H2 evolution experiments were performed in a Pyrex flask, and a closed gas circulation and evacuation system is connected to it for sampling. A 300 W Xe lamp with a UV-light cutoff filter (420 nm) was used as the visible-light source. In a typical reaction, 50 mg of as-prepared catalysts was suspended in 50 mL of aqueous solution containing 20 vol % methanol under stirring conditions. The 1.0 wt % Pt-loaded photocatalyst was conducted by an in situ photodeposition method using H2PtCl6 as a precursor before H2 evolution. Prior to irradiation, the suspension of the photocatalyst was dispersed under ultrasonic treatment for 5 min and then completely degassed to remove oxygen

by bubbling N2 for 20 min to ensure an anerobic condition. The produced hydrogen gas was sampled and measured by using gas chromatography (GC-14C, Shimadzu, Japan, TCD, with nitrogen as a carrier gas and a 5 Å molecular sieve column). The apparent quantum efficiency (QE) was estimated by eq 1: QE =

number of evolved hydrogen molecules × 2 × 100% number of incident photons (1)

Photoelectrochemical Measurement. Transient photocurrent measurements of STO/SNO heterojunctions were measured on an electrochemical analyzer in a standard three-electrode configuration. Irradiated with a 500 W Xe arc lamp with a 420 nm light filter, the prepared FTO/samples, the Ag/AgCl electrode, and Pt wire were used as the working, reference, and counter electrodes, respectively. The modified electrodes were prepared by a dip-coating method: 4 mg of STO, SNO, 20%-STO/SNO heterojunction, and 20 μL of 5 wt % Nafion solution dispersed in 1 mL of ethanol by ultrasonication, which then is dip-coated directly onto a 1 cm × 2 cm FTO slice. A 0.2 M Na2SO4 aqueous solution was used as the electrolyte.

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RESULTS AND DISCUSSION The TEM and HRTEM characterization provides insights into the morphology and microstructure of the obtained photocatalysts. The typical TEM images of SNO and STO/SNO heterojunctions were displayed in Figure 1. As can be seen, SNO possessed a typical 2D nanosheet structure with a smooth surface (Figure 1a). As for STO/SNO heterojunctions, it could be clearly seen that the STO irregular nanoparticles were dispersed on the surface of SNO nanosheets. An unexpected aggregation of these STO nanoparticles was also observed (Figure 1b−e). The more detailed information about the interfacial structure of the STO/SNO heterojunctions was further confirmed by the HRTEM image. As shown in Figure 1f, the distinct fringe with an interval of 0.27 nm can be perfectly indexed to (110) lattice plane of STO, and those of 0.30 and 0.36 nm coincide with the (311) and (1̅11) lattice planes of SNO, respectively. Furthermore, there was a large and 9751

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Figure 3. (a) XRD patterns of the as-prepared samples. (b) N2 adsorption isotherms and the corresponding pore-size distribution (inset) for SNO, STO, and the 20%-STO/SNO heterojunction.

delocalization of firmly contacted STO and the SNO nanosheets.51 Pristine STO, SNO, and STO/SNO heterojunctions were determined by XRD patterns, as depicted in Figure 3a. The diffraction peaks of pristine STO located at 32.4°, 39.9°, 46.3°, 57.6°, 67.7°, and 77.0° were assigned to the characteristic diffraction peaks of (110), (111), (200), (211), (220), and (013) planes, which were consistent with the previously reported values for the perovskite SrTiO3 (JCPDS 79-0176). This suggests that pure phase STO was formed and wellcrystallized with the cubic structure.52 For the pure SNO, all peaks are in good agreement with monoclinic SNO (JCPDS 84-1810). The main diffraction peaks at 24.9°, 29.0°, 31.3°, 33.8°, 36.9°, 49.5°, 51.0°, 52.9°, 54.3°, 60.0°, 61.7°, and 65.6° can be well-indexed to the (1̅11), (311), (600), (202), (020), (022), (222), (1̅13), (911), (8̅21), (331), and (7̅13) planes, respectively.27 In addition, after the deposition of STO on the surface of SNO, no trace of any impurity phase can be discovered from the XRD patterns of STO/SNO heterojunctions, indicating the successful combination of STO/SNO heterojunctions. Moreover, the intensity of the diffraction peak of STO gradually increased with the increase of STO content, while the peak intensity of SNO gradually weakened. N2 adsorption−desorption isotherms and the corresponding pore-size distribution curves of as-obtained samples were shown in Figure 3b. The specific surface area, pore volume, and pore-size data of as-prepared samples were calculated by BET and BJH methods, as summarized in Table 1. All of the

intimate contact interface between STO and SNO, which is beneficial for photogenerated charge separation in the photocatalytic process. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental scanning are used to reveal the composition and microstructure of the binary 20%STO/SNO heterojunction, which are shown in Figure 1g−m, confirming the existence of Sr, Ti, O, Sn, and Nb elements in the 20%-STO/SNO heterojunction, and the STO deposited on the surface of SNO nanosheets can also be confirmed by the signal intensity of different elements. The surface elemental compositions and bonding configuration of the STO, 20%-STO/SNO heterojunction, and SNO can be evidenced by X-ray photoelectron spectra (XPS), as displayed in Figure 2. The XPS survey spectra (Figure 2a) confirmed the coexistence of the Sr, Ti, O, Sn, and Nb elements in the STO/SNO heterojunction and ruled out the possibility of the existence of other impurity elements. The small C peak at 284.6 eV was ascribed to surface adventitious reference carbon during the XPS measurement, which is unavoidable.43 The high-resolution spectrum of Sr 3d (Figure 2b) presented two characteristic peaks centered at approximately 132.5 and 134.2 eV, which could be assigned to Sr 3d5/2 and Sr 3d3/2, respectively. The peak located at around 132.5 eV was in accordance with reports for STO perovskite materials, and the higher energy peak can be attributed to SrO complexes.44,45 Titanium existing as the Ti4+ state was verified by two separate peaks at 457.8 eV (Ti 2p3/2) and 463.7 eV (Ti 2p1/2) due to the spin−orbit splitting (Figure 2c). The curves of Sn 3d show that the binding energies of Sn 3d5/2 and Sn 3d3/2 appeared at 485.7 and 494.1 eV (Figure 2d), revealing the oxidation state of Sn present as +2 in the sample.46 The spin−orbit separation of Nb 3d is 2.8 eV, and the distinct peaks at 206.4 eV in Nb 3d5/2 are features of the Nb5+ in the SNO (Figure 2e).47 Additionally, in the spectrum of O 1s (Figure 2f), the characteristic peaks were centered at 529.8 and 530.8 eV. The binding energy peak of 529.8 eV was associated with the O 1s core level of the O2− anions in the SNO crystal lattice, and the binding energy peak of 530.87 eV could be ascribed to titanium bonded oxygen (Ti−O) and hydroxides present on the surface.48−50 Notably, the peaks of Sr 3d, Ti 2p, Sn 3d, Nb 3d, and O 1s in 20%-STO/ SNO shift toward the higher binding energies. This result indicates that there was an interfacial interaction between STO nanoparticles and SNO nanosheets in the heterojunction, which was driven from the probable electron transfer and

Table 1. BET Surface Areas and Pore Parameters of Different Samples sample

surface area (m2/g)

pore volume (cm3/g)

av pore size (nm)

SNO STO 20%-STO/SNO

22.84 30.31 25.35

0.028 0.048 0.032

4.74 6.43 5.39

samples possess a classical type-IV isotherm with a H3 adsorption hysteresis loops in terms of the International Union of Pure and Applied Chemistry (IUPAC) classification, indicating the existence of mesoporous (2−50 nm) structures.53−55 As displayed in Table 1, SNO, STO, and 20%-STO/ SNO exhibit the pore-size distributions with mean pore diameters of 4.74, 6.43, and 5.39 nm, respectively, further suggesting their mesoporous structures. The Brunauer− 9752

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Figure 4. (a) UV−vis diffused reflection spectra of pristine STO, SNO nanosheet, and STO/SNO heterojunctions. (b) Plot of (αhν)1/2 vs hν for the band gap energy of STO and plot of (αhν)2 vs hν for the band gap energy of SNO.

Emmett−Teller (BET) specific surface areas were calculated to be 22.84, 30.31, and 25.35 m2 g−1 for pure SNO, STO, and 20%-STO/SNO, respectively, following the order STO > 20%STO/SNO > SNO. This result is inconsistent with the photoactivity order of these samples. Therefore, it is reasonable to deduce that the surface area was not the primary factor to affect the photocatalytic performance in this study. The optical characterizations of all samples were recorded by the UV−vis diffuse reflectance spectra (UV−vis DRS), as depicted in Figure 4a. The majority of pristine STO harvested light below 400 nm can be clearly observed, and SNO has a strong absorption edge located at 550 nm. When SNO was combined with STO, the absorption edges of STO/SNO heterojunctions are gradually expanded toward a high wavelength with an increasing SNO amount, indicating that there is optical property enhancement via hybridization with SNO compared to that of bare STO. The enhanced visible-light absorption indicated that the composite photocatalysts could utilize more visible light, which will be favorable for a photocatalytic reaction. The band gap energies of both STO and SNO were calculated by the Tauc plot.56 As shown in Figure 4b, where the intercept of the tangents on horizontal axis gives the value of the band gap energies, the band gap energies for the STO were measured to be 3.2 eV (n is equal to 2 for indirect transition), and that of the SNO was 2.59 eV (n is equal to 1/2 for direct transition), similar to the previous reports.27,37 To give more information about the charge-transfer properties of the as-obtained samples, the time-resolved photocurrent behaviors were performed under intermittent visible-light irradiation (λ > 420 nm). As displayed in Figure 5, the photocurrent densities climbed up to a high current level when the light was activated and rapidly decreased as soon as the light was turned off. It is observed that the photoresponse of the 20%-STO/SNO heterojunction is much higher compared to that of pristine STO and SNO, indicating the heterostructure formed between STO and SNO significantly facilitates the photoinduced charge transfer, which should contribute to the obvious enhanced photocatalytic hydrogen production. Figure 6 presented the photocatalytic H2 generation performances of as-prepared photocatalysts deposited with 1 wt % Pt using methanol as a scavenger, and irradiated by visible-light irradiation (λ > 420 nm). Before the practical photocatalytic experiment, a control experiment was carried out to demonstrate that H2 was generated by photocatalytic reaction as there was no appreciable H2 production under the

Figure 5. Transient photocurrent response for pristine STO, SNO, and 20%-STO/SNO heterojunction.

nonirradiation or without photocatalyst. As observed in Figure 6a, the photoactivity trends over all the samples are the same during the whole photocatalytic H2 evolution reaction, implying that photocatalysts are rather stable under light irradiation. Figure 6b showed the comparison of the H2 evolution amount of all the photocatalysts (50 mg) during 3 h of reaction time. Apparently, bare STO showed a negligible H2 evolution amount, ca. 0.06 μmol, and bare SNO showed a clearly lower H2 evolution amount (8.99 μmol). When a certain amount of STO was attached on the surface of SNO nanosheet, there was a dramatic increase in H2 production as compared to that of the pure STO and SNO. Especially, it was found that among a series of STO/SNO heterojunctions, the 20%-STO/SNO possesses the maximum H2 evolution amount of ca. 17.16 μmol with apparent quantum efficiency of about 3.2%, which represented an increase of up to 298 and 2 times higher H2 generation amount with respect to bare STO and SNO, respectively. With a further increase of the content of STO, the H2 generation activity of the heterojunction samples decreased, possibly due to the fact that the overloading of STO could restrain the separation of photoinduced charges. It is worth noting that the present STO/SNO heterojunction exhibits the higher H2 evolution activity as compared with the other STObased photocatalysts reported previously (Table 2).40,41,57 In addition, we also examined the stability and recyclability of the 20%-STO/SNO heterojunction. As displayed in Figure 7, no clear inactivation can be found after 12 h, suggesting that the as-prepared STO/SNO heterojunction is very stable in the photocatalytic reaction. The processes of separation, transfer, and recombination of photoinduced e−h pairs in photocatalysts could be detected by 9753

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Figure 6. (a) Time courses of photocatalytic H2 evolution over different photocatalysts under visible light (λ > 420 nm). (b) Comparison of photocatalytic H2 evolution activity over different photocatalytsts in 3 h.

Table 2. Comparison of Photocatalytic H2 Evolution Behavior over Our Work and the Other STO-Based Photocatalysts photocatalyst Cr-SNO N-STO Cr/N-STO Rh-STO STO/SNO

light source 300 300 300 300 300

W W W W W

Xe Xe Xe Xe Xe

lamp lamp lamp lamp lamp

(λ (λ (λ (λ (λ

> > > > >

reactant solution 420 420 420 420 420

nm) nm) nm) nm) nm)

methanol methanol methanol methanol methanol

cocatalyst 0.6 0.5 0.5 0.5 1 wt % pt

wt wt wt wt

% % % %

pt pt pt pt

activity (μmol h−1)

QE

ref

82.6 1.2 106.7 47 114.4

2.95%

39 40 40 56 Our sample

3.1% 3.2%

nm. Since PL emission primarily arises from the recombination of charge carriers, the low PL intensity indicates inhibited recombination of charge carriers.60 All of the samples showed a strong emission peak around 533 nm. In particular, STO/SNO nanocomposites displayed an obviously diminished PL intensity, which indicated that the recombination rate of e−h pairs was efficiently inhibited and the photocatalytic hydrogen production activities were improved. These results are in good agreement with the corresponding photocatalytic H2 evolution activities, further suggesting that the interfacial electron transfer is beneficial for the improvement of the H2 generation activity over the STO/SNO heterojunctions. The enhanced efficiency in prolonging the lifetime of photogenerated charge carriers caused by the interface interaction between STO and SNO can be confirmed by the time-resolved photoluminescence. For semiconductor materials, the dynamic electron migration process can be estimated by time-resolved photoluminescence experiments, and the normalized decay profiles were shown in Figure 8b; the TRPL

Figure 7. Stability test of 20%-STO/SNO heterojunction under visible-light irradiation.

PL spectra.58,59 The PL spectra of the STO, SNO, 10%-STO/ SNO, 20%-STO/SNO, 30%-STO/SNO, and 40%-STO/SNO were presented in Figure 8a, excited with a wavelength of 397

Figure 8. (a) Photoluminescence (PL) spectra of different photocatalysts at room temperature with an excitation wavelength of 397 nm. (b) The time-resolved fluorescence spectra for as-prepared photocatalysts monitored at 469 nm using a 337 nm laser at room temperature. 9754

DOI: 10.1021/acssuschemeng.7b01548 ACS Sustainable Chem. Eng. 2017, 5, 9749−9757

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ACS Sustainable Chemistry & Engineering decay signals were fitted by a single-exponential decay kinetics function:61

I(t ) = I0 exp( −t/τ )

production and further applications in the field of environment and energy.

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(2)

The calculated lifetime values of bare STO, SNO, and 20%STO/SNO are 0.58, 0.60, and 0.72 ns, respectively. Obviously, the 20%-STO/SNO heterojunction possessed a longer lifetime. The prolonged lifetime can prove that the 20%-STO/SNO heterojunction retarded the recombination of photogenerated e−h, thereby increasing the activity of the STO/SNO heterojunction for photocatalytic H2 evolution. Accordingly, a reasonable reaction mechanism for photocatalytic H2 evolution under visible-light irradiation (λ> 420 nm) over the STO/SNO is illustrated in Figure 9. ECB and EVB

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Deli Jiang: 0000-0002-8409-7772 Min Chen: 0000-0001-8167-7707 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the financial support of the National Nature Science Foundation of China (No. 21406091, 21576121, and 21606111), Natural Science Foundation of Jiangsu Province (BK20140530 and BK20150482), and China Postdoctoral Science Foundation (2015M570409).

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REFERENCES

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Figure 9. Proposed mechanism of charge transfer in the STO/SNO 0D/2D heterojunctions under visible-light irradiation.

of STO were determined to be −0.77 and 2.43 eV, and those of SNO were −0.99 and 1.6 eV. When SNO is excited by visible light, the excited e− in the valence band (VB) of SNO would easily inject into the conduction band (CB) of SNO, and then it immigrated to the more positive conduction band (CB) of STO through the close contact interface. At last, the excited electrons tend to be trapped by the Pt nanoparticles due to its high Fermi level,62 which could reduce H+ and produce H2 molecules effectively.63−65 Simultaneously, the corresponding holes that accumulated at the VB of SNO can oxidize the electron donor (CH3OH). Hence, the STO/SNO 0D/2D heterojunction with matched energy level and interfacial interaction could increase the charge separation efficiency, leading to an enhancement of photocatalytic H2 production activity.

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CONCLUSIONS In summary, novel STO/SNO 0D/2D heterojunctions with an interfacial interaction for photocatalytic H2 evolution were successfully prepared by means of a facile hydrothermal method. It is demonstrated that the optimal 20%-STO/SNO 0D/2D heterojunctions exhibited the highest hydrogen evolution of 17.16 μmol under visible-light irradiation, which is about 298 and 2 times higher than those on the pure STO and SNO. This enhanced photocatalytic H2 production activity can be mainly ascribed to the improved interfacial chargetransfer efficiency, which resulted from the constructed 0D/2D heterojunctions. This work is expected to afford more valuable guidance for the rational design and preparation of novel 0D/ 2D heterojunction photocatalysts for highly efficient H2 9755

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