NiS CoreâShell Hybrid Nanofibers with

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Direct Z-scheme TiO2/NiS core-shell hybrid nanofibers with enhanced photocatalytic H2-production activity Feiyan Xu, Liuyang Zhang, Bei Cheng, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02710 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Direct Z-scheme TiO2/NiS core-shell hybrid nanofibers with enhanced photocatalytic H2-production activity Feiyan Xu, † Liuyang Zhang, *,† Bei Cheng, † Jiaguo Yu,*,†,§ †

State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan

University of Technology, Luoshi Road 122#, Wuhan 430070, P. R. China. E-mail: [email protected], [email protected] §

Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia.

E-mails of all authors: Feiyan Xu: [email protected] Liuyang Zhang: [email protected] Bei Cheng: [email protected] Jiaguo Yu: [email protected]

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ABSTRACT Photocatalytic water splitting to generate hydrogen (H2) is a sustainable approach for solving current energy crisis. A novel TiO2/NiS core-shell nanohybrid was fabricated where few-layer NiS nanoplates were deposited on TiO2 skeletons via electrospinning and hydrothermal methods. The NiS nanoplates with a thickness of ca. 28 nm, stood vertically and uniformly upon the TiO2 nanofibers, guaranteeing intimate contact for charge transfer. XPS analysis and DFT calculation imply that the electrons in NiS would transfer to TiO2 upon hybridization, which creates a built-in electric field at the interfaces and thus facilitates the separation of useful electron and hole upon photoexcitation. In-situ XPS analysis directly proved that the photoexcited electrons in TiO2 migrated to NiS under UV-visible light irradiation, suggesting that a direct Z-scheme heterojunction was formed in NiS/TiO2 hybrid. This direct Z-scheme mechanism greatly promotes the separation of useful electron-hole pairs and fosters efficient H2 production. The hybrid nanofibers unveiled a high H2-production rate of 655 µmol h–1 g–1, which is 14.6 fold of pristine TiO2 nanofibers. Isotope (4D2O) tracer test confirmed that H2 was produced from water, rather than from any H-containing contaminants. This work provides an alternative approach to rationally design and synthesize TiO2-based photocatalysts with direct Z-scheme pathways towards high-efficiency photo-generation of H2. Keywords: TiO2 nanofiber, NiS nanoplates, hydrogen production, In-situ XPS, direct Z-scheme


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INTRODUCTION Clean hydrogen (H2) energy has attracted much attention in the context of growing energy demand and environmental pollution. H2 utilization can reduce the consumption of fossil fuels and the emission of greenhouse gases, which exhibits great potential in solving the ever-growing energy and environmental issues.1-5 Photocatalytic H2 production from water is preferable to traditional thermochemical and electrochemical techniques; the input of thermal or electric energies is replaced by inexhaustible solar energy, evincing the advantages of sustainability, low process

cost

and

reasonable

solar-to-hydrogen

efficiency.6-9

Assorted

semiconductor

photocatalysts for H2 evolution have been explored, including but not limited to metal oxides,10-11 metal sulfides12-18 and carbon nitrides.19-22 Among them, TiO2 is chemically inert, eco-friendly, cost effective and abundant.23,24 However, similar to other unitary photocatalysts, TiO2 still suffers from poor light utilization and rapid recombination of photo-induced carriers, which deteriorate its photocatalytic efficiency and limit its practical application.25,26 Hybridizing TiO2 with a narrow band gap semiconductor as sensitizer and/or electron capturer can separate electron-hole pairs upon irradiation and promote light harvesting.27-33 NiS is an inexpensive, nontoxic semiconductor with a small band gap, and it has received increasing attention in various specialized applications such as photocatalytic H2 generation, CO2 photoreduction and solar cells.34-40 For example, Lee et al.41 reported NiS-sensitized TiO2 photocatalytic system showing enhanced CO2 photoreduction performance because of the formation of p-n junctions. Liu et al. 42 and Xin et al. 43 both demonstrated that NiS could serve



as a cocatalyst, i.e., an efficient electron capturer, affording TiO2 boosted activities for H2 production. Clearly, the formation of conventional heterojuction-type or cocatalyst-type TiO2 photocatalysts can effectively separate the electron-hole pairs and prolong their lifetimes. However, such routes inevitably deteriorate the reduction ability of electrons, since they will spontaneously flow to a more positive energy level in the hybrids upon irradiation. Recently, Z-scheme photocatalysts, especially direct Z-scheme photocatalysts, have provided a one-stone-two-birds solution to this dilemma. In a direct Z-scheme photocatalyst, the transport of charge carriers follows a Z-shape pathway.44-49 The electrons and holes migrate to more negative and positive energy levels after charge separation, respectively, as compared to those in a heterojunction- or cocatalyst-type photocatalyst.50-54 Inspired by these merits, concerted efforts have been made to the construction of TiO2-based direct Z-scheme photocatalysts for efficient photocatalytic H2 production.55-57 However, it has not come to our knowledge that whether the construction of direct Z-schemeTiO2/NiS hybrid is feasible. Herein, we reported TiO2/NiS core-shell photocatalyst for photocatalytic H2 production by hydrothermally growing NiS nanoplates onto TiO2 electrospun nanofibers. Both the direct experimental evidence from in-situ XPS technique and theoretical evidence from DFT calculation verified the Z-scheme electron transfer route at TiO2/NiS interfaces upon irradiation. The obtained TiO2/NiS hybrid nanofibers showed an improved photocatalytic H2 production performance thanks to the increased optical absorption and the virtues of direct Z-scheme mechanism.


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RESULTS AND DISCUSSION Experimental details can be found in the Supporting Information for sample preparation, characterization and performance measurement. Figure 1 schematically illustrates the preparation of TiO2/NiS hybrid nanofibers. TiO2 nanofibers were prepared by calcining the electrospun TiO2 precursors according to our previous work.58 Then, Ni(OH)2 nanosheets hydrothermally in-situ grew on the TiO2 surface to form TiO2/Ni(OH)2 nanohybrids. The solid intermediate was immersed into NaHS.xH2O solution at room temperature to generate TiO2/NiS via ion-exchange by converting Ni(OH)2 into NiS. For convenience, the obtained TiO2/NiS is labeled as TNx, where T and N represent TiO2 and NiS, respectively; x is the molar ratio of NiS in TiO2/NiS hybrids. The real compositions (see Table S1) of the prepared TN5, TN10 and TN20 samples are 1.5, 3.3 and 4.9 mol%, respectively.

Figure 1. Schematic diagram for the preparation of TiO2/NiS core-shell hybrid nanofibers.

Figure 2a shows the XRD patterns of T, TN5, TN10 and TN20. T shows characteristic 5 ACS Paragon Plus Environment


reflections corresponding to anatase phase (JCPDS No. 21-1272) and rutile phase (JCPDS No. 21-1276). No obvious reflection assigned to NiS was discernible from the XRD pattern of TN5 because of its low content. In addition to the intensive peaks originating from TiO2, both TN10 and TN20 showed extra diffraction peaks at 34.7° and 46.0°, which corresponds to the (101) and (102) planes of NiS (JCPDS, No. 02-1280). Figure 2b shows the EDX spectrum of TN10. It revealed that a minority of Ni and S elements together with dominant Ti and O elements coexisted in the resultant sample. The results confirmed the formation of NiS by hydrothermal deposition on TiO2 nanofiber.

Figure 2. (a) XRD patterns of T, TN5, TN10 and TN20. (b) EDX spectrum of TN10.

The morphologies of samples were observed by using SEM and TEM and the results are shown in Figure 3. The shape of pristine TiO2 (T) is nanofiber with smooth surface, which has a diameter around 300 nm and a length of tens of micrometers (Figure 3a). After hydrothermal synthesis, numerous Ni(OH)2 nanoflakes grew out of TiO2 nanofibers as shown in Figure S2. NiS was then transformed from them upon sulfidation while maintaining the nanoflake morphology 6 ACS Paragon Plus Environment

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as shown in Figure 3b. The crystalline phase of T and TN10 was also examined by using HRTEM. Figure 3c shows clear lattice fringes with d spacings of 0.352 and 0.325 nm, which correspond to anatase TiO2 (101) facets and rutile TiO2 (110) facets, respectively. Apart from the lattice fringes of TiO2, the HRTEM image of TN10 shows additional lattice fringes with d spacing of 0.255 nm belonging to NiS (101) facets (Figure 3d). The result further proved the generation of NiS on TiO2 nanofibers, in congruency with the XRD results. The EDX mapping images of TN10 (Figure 3e) indicated that the sample contained Ti, O, N and S elements, suggesting the co-existence of TiO2 and NiS. The thickness of the NiS nanoplates was determined by AFM analysis. Figure 3f shows the AFM height image of NiS nanoplates, which were peeled off from the TiO2 nanofibers by bath ultrasonication. The corresponding height profiles of each NiS nanoplate (A, B and C), as shown in Figure 3g, demonstrated an average thickness of 28.0 nm.



Figure 3. SEM images of (a) T and (b) TN10; insets on the upper right are the corresponding enlarged SEM images. HRTEM images of (c) T and (d) TN10. (e) EDX elemental mappings of Ti, O, Ni and S in TN10. (f, g) AFM image of TN10 and the corresponding height profiles of the NiS 8 ACS Paragon Plus Environment

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nanoplates.

The optical absorption properties of samples were studied to determine an appropriate light source for photocatalytic experiments (Figure 4). Pristine TiO2 only adsorbs UV light with wavelengths below 400 nm, whereas pristine NiS can strongly absorb visible light due to its narrow band gap and dark color. The TiO2/NiS hybrid exhibits slightly enhanced light harvesting in the visible light range with increasing loading of NiS because of the strong light absorption of NiS nanoplates, indicating the successful deposition of NiS nanoplates on TiO2 nanofibers. These results indicate that the TiO2/NiS heterostructures only can be excited by UV light, but can utilize some visible light to produce photo-induced carriers and thus promote the photocatalytic activity, as corroborated in the following context.

Figure 4. UV-vis diffuse reflectance spectra of T, TN5, TN10, TN20 and N.

N2 sorption technique was employed to measure the specific surface areas (SBET), pore volumes (PV) and average pore sizes (APS) of the samples (Figure 5). Each sample shows 9 ACS Paragon Plus Environment


IV-type isotherm according to BDDT (Brunauer, Deming, Deming and Teller) classification and an apparent H2-type hysteresis loop in the P/P0 range of 0.45~0.9, indicating the existence of ink-bottle pores due to the interparticle aggregation. In another P/P0 range of 0.9~1.0, the hysteresis loops are H3-type, implying the existence of narrow slit-like pores that could derive from the randomly-distributed NiS nanosheets.59 The pore size distribution was also estimated from the desorption branch of sorption isotherms (inset of Figure 5). The SBET, PV and APS of the resultant samples were summarized in Table S1 (see supporting information). TN5 and TN10 show a similar SBET but an elevated PV and APS as compared with T. TN20 shows a significant decrease in SBET and APS due to the overloading of NiS.

Figure 5. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution (inset) of T, TN5, TN10 and TN20.

XPS was performed to characterize the chemical states of the samples (Figure 6). The Ti 2p XPS spectra (Figure 6a) show two symmetrical peaks of Ti 2p3/2 around 458.7 eV and Ti 2p1/2 around 464.6 eV, corresponding to Ti4+ in TiO2. The O 1s XPS spectra (Figure 6b) shows two 10 ACS Paragon Plus Environment

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kinds of O species including lattice O (~ 529.8 eV) and -OH (~531.5 eV). Notably, the binding energies (BEs) of Ti 2p and O 1s for TN10 shift to lower values by 0.2 eV in comparison with that for T, suggesting that some electrons in NiS have transferred to TiO2 upon hybridization.55 The result is reasonable because TiO2 has a more negative Fermi level than NiS (see DFT calculation), which forces electrons to migrate from NiS to TiO2 until their Fermi levels are equalized.60 Such electron transfer between TiO2 and NiS will create an internal electric field at the interface directing from NiS to TiO2, which could promote the charge separation and enhance the photocatalytic activity for H2 production.



Figure 6. In-situ and ex-situ XPS spectra of (a) Ti 2p and (b) O 1s of T and TN10; in-situ and ex-situ XPS spectra of (c) Ni 2p and (d) S 2p of N and TN10. In-situ XPS spectra were recorded under light irradiation.

The Ni 2p XPS spectrum of TN10 (Figure 6c) contains Ni 2p3/2 and Ni 2p1/2 and the respective satellite peaks in NiS, respectively. The S 2p XPS spectra (Figure 6d) reveals the presence of S2together with a minority of SO42- species, originating from the formation of NiS nanoplates and the disproportionation reaction of NaHS, respectively. The BEs of Ni 2p and S 2p for TN10 show a positive shift in comparison with those of pristine NiS, reaffirming the electrons transfer from NiS to TiO2, which agrees with the opposite BE shift of Ti and O. Such BE shifts also prove an intimate contact between TiO2 and NiS at their interfaces, favoring the formation of direct Z-scheme TiO2/NiS photocatalyst without the aid of a redox mediator. The photocatalytic H2-production activity of samples was examined by splitting water from methanol aqueous solution under UV-visible light irradiation. No appreciable amount of H2 can be found without a photocatalyst or light irradiation. Figure 7a shows the H2-production rates of T, TN5, TN10 and TN20 under irradiation. T has the lowest H2-production rate because of the poor light utilization and rapid electron-hole recombination in pristine TiO2. After coupling with NiS, the obtained TNx shows an improved photocatalytic activity. The H2-production rate increases as the loading of NiS (x) is increased and comes to the maximum of 655 µmol h–1 g–1 over sample TN10. Nevertheless, further increment in NiS amount was harmful, which would reduce the photocatalytic efficiency (TN20). The increased electron-hole recombination and shielding effect 12 ACS Paragon Plus Environment

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towards light absorption of an overdose of NiS may be responsible for the performance deterioration.

Figure 7. (a) Comparison of photocatalytic H2-production activities of all the samples. (b) GC-MS spectra obtained after injecting 0.5 mL samples of the gas phase species produced by D2O splitting over TN10 photocatalyst in a sealed Pyrex flask under UV-visible light irradiation for 2 h. (c) Cyclic H2-production curves for the TN10. The reaction system was bubbled with N2 for 30 min in every 3 h to remove the H2 inside.

In order to trace the origin of H2, the isotopic tracer experiment was conducted and the results are shown in Figure 7b. The detected product contains both D2 and H2. Beyond doubt, D2 came from H2O splitting, and trace amounts of H2 co-existed due to the proton exchange between D2O and H2O in air at the same time. The m/z signals at 28, 32 and 40 are accredited to N2, O2 and Ar gas, respectively.61,62 Photocatalytic hydrogen production rates for four successive runs are displayed in Figure 7c. It can be seen that activity decay is hardly perceptible after four cycles, revealing good stability of TN10. The cycled TN10 sample well maintained its original morphology as shown in Figure S3, 13 ACS Paragon Plus Environment


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revealing good structural robustness. We also compared the H2-production rates of the current photocatalysts with previous TiO2-based ones. As can be seen in Table 1, TiO2/NiS in this work outperforms most of the binary photocatalysts reported previously and even shows comparable activity to the ternary TiO2-based photocatalyst of CuS/NiS/TiO2.63 Therefore, NiS, as an inexpensive and nontoxic semiconductor, can show great potential for achieving efficient photocatalytic H2 production.

Table 1: Comparisons of photocatalytic H2-production performance of various TiO2-based photocatalysts. Photocatalyst

Sacrificial agent

Light

Yield (µmol h-1 g-1 )

Ref.

NiS/TiO2 nanofibers

methanol

350 W Xe lamp

655

This work

NiS/TiO2 nanosheets

methanol

300 UV Xe lamp

313.6

43

CuS/NiS/TiO2

methanol

500 W Xe lamp

800

63

C-TiO2@g-C3N4

methanol

300 W Xe lamp

35.6

28

500

64

150 W Au/TiO2

methanol

CERAMIC-Metal-Halide Lamp

N-TiO2/g-C3N4

methanol

350 W Xe lamp

296

65

CdS/Pt/TiO2

lactic acid

300 W Xe lamp

636.2

66

Pt/black TiO2

methanol

300 W Xe lamp

103.3

67

NiTiO3/TiO2

methanol

300 W Xe lamp

680

68

Ag-TiO2-graphene

methanol

300 W Xe lamp

129.5

69

g-C3N4/TiO2/Cu(OH)2

methanol

300 W Xe lamp

378

70

Co(OH)2/TiO2

methanol

300 UV Xe arc lamp

46.93

71

To explore the evolution of the charge carriers in TiO2/NiS nanofibers, photoluminescence (PL)


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emission spectra of T and TN10 with wavelength ranging from 350 nm to 600 nm were taken (Figure 8a). Both samples show similar PL spectra, which embraces three emission peaks located at 396, 451 and 468 nm. The strong emission around 396 nm corresponds to the band gap transition of TiO2. In addition, the PL peaks at 451 and 468 nm are attributed to band edge free excitons, and others at 483 and 493 nm can be ascribed to bound excitons.25 Note that the intensity of TN10 is marginally lower than that of T, indicating that the introduction of NiS can significantly retard the recombination of charge carriers in TiO2 via the direct Z-scheme mechanism. Electrochemical impedance spectroscopy (EIS) was used to explore the charge transfer capability of the samples (Figure 8b). TNx (x = 5, 10 and 20) shows a semicircle with smaller radius in the middle-frequency region in comparison to T, which indicates that the hybridization of NiS can promote the electron transfer and thus leads to enhanced activity for photocatalytic H2 production. The polarization curves of T and TN10 under UV-visible light irradiation are shown in Figure 8c. The overpotential (η5) for TN10 to achieve current density of 5 mA cm−2 was 840 mV, which was much lower than that of T (η5 = 1041 mV), indicating that TN10 presented better hydrogen evolution reaction (HER) performance. That is to say, the reduction capability of TN10 is stronger than that of T.



Figure 8. (a) Photoluminescence (PL) spectra of T and TN10 samples. (b) Nyquist plots of T, TN5, TN10 and TN20 in 0.5 M Na2SO4 aqueous solution; inset demonstrating the fitted equivalent circuit model and (c) polarization curves of T and TN10 at a scan rate of 5 mV s−1 in 0.5 M Na2SO4 under UV-visible light irradiation.

From above analysis, the superior activity of TN10 cannot only be attributed to the enhanced light harvesting because TN10 has a limited UV-visible light absorption. Contrarily, its superior activity is due to the formation of direct Z-scheme heterojunction between TiO2 and NiS. As we know, the electron transfer can be determined by the Fermi energy (EF) difference between two semiconductors. Electrons will migrate from the semiconductor with higher EF to the one with lower EF at their interfaces upon contacting. In our case, DFT calculation shows that the work functions of TiO2 and NiS are 6.57 and 5.12 eV, respectively (Figure 9). It means that the Fermi level of NiS is higher than that of TiO2, leading to the electron transfer from NiS to TiO2 upon contact until their Fermi levels are aligned. The transfer of electrons creates an internal electric field at the TiO2/NiS interface pointing from NiS to TiO2. The DFT results are consistent with the XPS analysis, where the BE shifts of Ti 2p, O 1s, Ni 2p and S 2p are evidenced. 16 ACS Paragon Plus Environment

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Figure 9. Calculated electrostatic potentials for (101) face of (a) TiO2 and (b) NiS. The blue and red dashed lines denote the Fermi level and the vacuum energy level, respectively.

Under UV-vis light irradiation, the valence band (VB) electrons of TiO2 and NiS are photoexcited to their conduction bands (CB). On account of the internal electric field at the TiO2/NiS interfaces and the coulomb repulsion between photoelectrons in TiO2 CB and NiS CB, the electrons in TiO2 CB will recombine with the holes in NiS VB. This implies the presence of direct Z-scheme charge transfer between TiO2 and NiS. Such Z-scheme transfer can effectively separate the useful photoinduced electron-hole pairs, leading to a high concentration of electrons and holes accumulated in NiS CB and TiO2 VB, respectively.72-75 As a result, the photogenerated electrons in NiS CB would effectively reduce H2O molecules (or H+) to produce H2. Meanwhile, TiO2 can function as oxidation active sites, where the accumulated holes could be consumed by the sacrificial agent of HCHO. The schematic Z-scheme heterostructures before and after their contact along with the charge transfer and separation over TiO2/NiS are illustrated in Figure 10. 17 ACS Paragon Plus Environment


Figure 10. Schematic illustration of TiO2 and NiS direct Z-scheme heterojunction (a) before and (b) after contact along with the charge transfer and separation in TN10 under UV-visible light irradiation.

Such direct Z-scheme pathway of charge transfer was further authenticated by using an in-situ XPS under light irradiation. Noticeably, the BEs of Ti 2p and O 1s of TN10 under irradiation (Figure 6a-b) exhibit a positive shift of ca. 0.4 eV comparing to those measured in darkness. Conversely, the BEs of Ni 2p and S 2p of TN10 (Figure 6c-d) shift by 0.5 eV towards a lower value under irradiation. These BE shifts unquestionably prove that the photoinduced electrons in the TiO2 CB can transfer to NiS under UV-vis light irradiation determined by the direct Z-scheme TNx heterojunction, which further proves the proposed photocatalytic mechanism as elucidated above.

CONCLUSIONS 18 ACS Paragon Plus Environment

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A novel TiO2/NiS core-shell hybrid direct Z-scheme nanostructure was fabricated by growing NiS nanoplates vertically on the electrospun TiO2 nanofiber. Benefiting from the close contact deriving from the direct growth and the direct Z-scheme mechanism, the as-obtained TiO2/NiS hybrid nanofibers exhibited superior photocatalytic activity towards H2 production under UV-visible light irradiation as compared with pristine TiO2. Isotope tracer test confirmed that the products exclusively originated from the water. DFT calculation shows that TiO2 has a larger work function than NiS, resulting in electron transfer from NiS to TiO2 upon their contact and thus creating an internal electric field in the interfaces, which can promote the useful electron-hole separation upon photoexcitation. In-situ XPS analysis further indicated the photoexcited electrons in TiO2 could flow to NiS under UV-vis light irradiation, confirming a direct Z-path of charge transfer. This work can provide new insight into the design of other TiO2-related direct Z-scheme photocatalysts for efficient photocatalytic H2 production.

Supporting Information Available: Experimental section, SEM images of TiO2/Ni(OH)2 precursor and the cycled TN10, and tabular physical properties of TNx. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] 19 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by NSFC (51320105001, 21573170, U1705251 and 21433007), NSFHP (2015CFA001) and Innovative Research Funds of SKLWUT (2017-ZD-4).

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TiO2/NiS core-shell hybrid nanofibers show improved photocatalytic H2-production activity due to the formation of direct Z-scheme heterojunction between TiO2 and NiS.


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