Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4

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Constructing multi-functional metallic Ni interface layers in the g-C3N4 nanosheets/amorphous NiS heterojunctions for efficient photocatalytic H2 generation Jiuqing Wen, Jun Xie, Hongdan Zhang, Aiping Zhang, Yingju Liu, Xiaobo Chen, and Xin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02701 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 3, 2017

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ACS Applied Materials & Interfaces

Constructing multi-functional metallic Ni interface layers in the g-C3N4 nanosheets/amorphous NiS heterojunctions for efficient photocatalytic H2 generation Jiuqing Wena,b, Jun Xiea,b, Hongdan Zhang,a,b Aiping Zhang,a,b Yingju Liu,b Xiaobo Chen,c Xin Li a,b* a

College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants

Resource and Utilization, Ministry of Agriculture, South China Agricultural University, Guangzhou 510642, PR China b

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, PR

China c

Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, 64110, USA.

* Corresponding author at: College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, P. R. China. Tel.: +86 20 85282633; fax: +86 20 85285596. E-mail address: [email protected] (X. Li).

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Abstract:

The

construction

of

exceptionally

robust

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and

high-quality

semiconductor-cocatalyst heterojunctions remains a grand challenge towards highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-C3N4) nanosheets decorated with multi-functional metallic Ni interface layers and amorphous NiS co-catalysts were fabricated via a facile three-step process: the loading of Ni(OH)2 nanosheets, high-temperature H2 reduction and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron co-catalysts to markedly boost the visible-light H2 evolution over g-C3N4 semiconductor. The optimized g-C3N4-based photocatalyst containing 0.5 wt% Ni and 1.0 wt% NiS presented the highest hydrogen evolution of 515 µmolg-1h-1, which was about 2.8 and 4.6 times as those obtained on binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multi-functional roles in enhancing the visible-light H2 evolution, which could first collect the photo-generated electrons from g-C3N4, and then accelerate the surface H2-evolution reaction kinetics over amorphous NiS co-catalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron co-catalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-C3N4. Comparatively speaking, the multi-functional metallic Ni layers are dominantly favorable for separating and transferring photo-excited charge carriers from g-C3N4 to amorphous NiS co-catalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanoparticles are mainly advantageous for decreasing the thermodynamic overpotentials for surface H2-evolution reactions. It is hoped that the implantation of


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multi-functional metallic interface layers can provide a versatile approach to enhance the

photocatalytic

H2

generation

over

different

semiconductor-cocatalyst

heterojunctions. Keywords: Photocatalytic Hydrogen Evolution, dual-layer electron co-catalysts, metallic Ni interface layers, g-C3N4 nanosheets, amorphous NiS, H2-evolution kinetics

1. Introduction Nowdays, there has been an increasing interest in the fields of sustainable and environmentally friendly heterogeneous semiconductor photocatalytic hydrogen production driven by renewable solar energy,1,2 since the photoelectrochemical water splitting over a Pt-attached TiO2 cell was first discovered by Fujishima and Honda in 1972.3 Among a variety of heterogeneous inorganic and organic semiconductors exploited for photocatalytic H2 generation,4-10 the metal-free g-C3N4, an encouraging n-type semiconductor, has garnered considerable attention in the applications of photocatalytic H2 evolution because of their numerous interesting electric, optical, structural and physiochemical properties.11 Especially, since Wang and his coworkers pioneerly reported the photocatalytic H2 evolution over Pt-loaded g-C3N4 using triethanolamine as sacrificial agent in 2009,12 the state-of-the art progresses have been achieved in the photocatalytic H2 evolution over g-C3N4-based semiconductors.11,13,14 However, several prominent challenges of bulk g-C3N4 material, including the ultrafast electron–hole recombination rate, low surface area of g-C3N4 (~10 m2/g) and small surface active sites for sluggish up-hill H2-evolution reactions, remain to be 3 ACS Paragon Plus Environment


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addressed.11,15,16 In this context, more and more endeavors are devoted to developing heterogeneous H2 generation photocatalysts through different thermodynamic (such as doping and sensitization) and kinetics (such as constructing heterojunctions or all solid-state Z-scheme systems, fabricating micro/nano architectures and loading proper co-catalysts) modifications of g-C3N4.11,14,17-22 Among these various kinds of modification strategies, loading proper co-catalysts is extensively considered one effective approach to boost the photocatalytic H2 generation over g-C3N4-based photocatalysts, which can simultaneously achieve the promoted charge separation, accelerated surface reaction kinetics and suppressed surface back reactions.2,23,24 So far, various kinds of co-catalysts, including precious metals (e.g., Au, Pt, Pd, Ag, and PtPd),12,25-28 metal-free nanocarbons29-33 and cheap metals (e.g., Co, Fe, W, Ni, and Cu) and their compounds,23,34-37 have been decorated on g-C3N4 to obtain the improved H2 evolution. Compared to the low natural abundance and high cost of noble metals, the earth-abundant metals and their compounds seem to be more promising for practical applications in a large scale. Accordingly, the earth-abundant transition metal electrocatalysts, including Fe-, Co-, Ni- and Cu-based (hydr)oxides, sulfides, nitrides, phosphide, carbide and borides, have been found to be the most important electrocatalysts and co-catalysts for H2 evolution driven by renewable electricity or solar energy.23,29,35 In particular, various kinds of Ni-based electrocatalysts, including Ni,38,39 Ni(OH)x,40-42 Ni12P5,43 NiSx,44-49 [Ni(TEOA)2]Cl2,50 NiOx,51,52 and Ni(dmgH)253,54 as co-catalysts, can accelerate its photocatalytic activity for different applications.36 However, the Ni-based co-catalysts modified g-C3N4 photocatalysts generally exhibit much smaller H2-evolution activity

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than those over noble metal loaded g-C3N4, due to their lower electrical conductivity and stability. Thus, it is still challenging to further improve the photocatalytic performance of the Ni-based cocatalysts/g-C3N4 composite photocatalysts. Essentially speaking, engineering the surfaces of Ni-based cocatalysts and g-C3N4 or the interfaces between them could significantly separate and transfer photo-excited electrons from g-C3N4 to co-catalysts and improve the stability of co-catalysts, thus maximizing the electrocatalytic H2-evolution functions of Ni-based co-catalysts. For example, Li and his coworkers demonstrated that the incorporation of CdS nanorods into the interfaces between g-C3N4 nanosheets and NiS cocatalysts are beneficial for promoting the charge transfer, thus obtaining the strengthened photocatalytic H2 production.47 Furthermore, the implantation of conductive carbon interface layers (e.g., carbon black, graphene and carbon nanotubes) between g-C3N4 nanosheets and NiS co-catalysts could also significantly enhance the photocatalytic H2-evolution activity, because the carbon interface layers could achieve both the promoted charge separation in g-C3N4 nanosheets and enhanced H2-evolution kinetics.44,46,55,56 Similarly, it is clear that the metallic metal nanoparticles also exhibit the similar electrical conductivity to those of nanocarbons. More importantly, the Ni@NiO52,57,58 and Rh/Cr2O359 core/shell nanoparticles as electrocatalysts/co-catalysts have been demonstrated to exhibit significantly enhanced H2-evolution activity over different semiconductors, compared to those systems using the single NiO and Cr2O3 nanoparticles as co-catalysts, due to the implantation of multi-functional conductive metallic metal cores. Strongly motivated by these core/shell co-catalysts, it is naturally expected that the conductive metallic Ni interface layers as co-catalysts between g-C3N4 and NiS could also play



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distinguishable roles in separating and transferring electrons from g-C3N4 semiconductor to NiS co-catalysts and further accelerating the surface reaction kinetics over NiS co-catalysts. However, limited researches have revealed the positive roles of the robust dual-layer electron co-catalysts in promoting the photocatalytic H2 evolution. In the present study, to maximize H2 evolution over robust g-C3N4-based photocatalysts, the ternary g-C3N4-Ni-NiS nanohybrids were fabricated via a facile three-step process: (as shown in scheme 1). Firstly, the conductive Ni layers were constructed

through

in-situ

reduction

of

Ni(OH)2

nanosheets

on

g-C3N4 lamina under hydrogen atmosphere. Subsequently, amorphous NiS nanosheets were also loaded on g-C3N4-Ni Schottky junctions through an anion exchange of Ni(OH)2 nanosheets. The possible underlying mechanism was also proposed. This study demonstrates that the implantation of dual-layer H2-evolution co-catalysts over g-C3N4 opens a new avenue for developing high efficiency robust g-C3N4-based photocatalysts for the visible-light H2 evolution.


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Scheme 1. Fabrication of the ternary g-C3N4/Ni/NiS photocatalysts.

2. Experimental 2.1. Preparation of photocatalysts. The pristine g-C3N4 was synthesized by thermal polymerization of urea by terms of our previous reports.44 The surface hydrogenated g-C3N4 was fabricated through annealing the pristine g-C3N4 in 500 °C for 4 hours under a 5 vol% H2 atmosphere. Then, the amorphous NiS was loaded on the aforementioned surface hydrogenated g-C3N4 via in situ conversion of Ni(OH)2 nanosheets with Na2S.44 As a result, the binary g-C3N4-1.0%NiS (mass ratio) composites were fabricated. The binary g-C3N4/Ni hybrid was synthesized via hydrogen reduction of g-C3N4/Ni(OH)2. In a typical synthesis run, 1.5 g of graphite-like carbon nitride samples without hydrogen treatment was ultrasoned in 100 mL deionized water. Then, 12.78 mL Ni(NO3)2 (0.01 M) and 50 mL NaOH (0.01M) was sequentially added in



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the above suspension and stirred for 2 hours. After thoroughly washing and drying, the obtained g-C3N4/Ni(OH)2 powder in a tube furnace was heated to 500 °C within 2.5 hours under an H2 (Vol(H2/Ar)=5/95) flow of 150 mL/min, then stabilized at 500 °C for 4 hours. As a result, the binary g-C3N4-0.5%Ni (mass ratio) composites were fabricated. The g-C3N4-1.0%Ni was also prepared using the same method. The ternary composites with metallic Ni and amorphous NiS were prepared by the combination of the fabrication methods for g-C3N4/Ni and g-C3N4/NiS samples. That is, metallic Ni was first in-situ growth onto g-C3N4. Then, amorphous NiS was deposited on the g-C3N4/Ni surface. The as-obtain sample with 0.5 wt% Ni and 1.0 wt% NiS was labeled as g-C3N4-0.5%Ni-1.0%NiS. Other samples with different mass ratio of Ni and NiS were similarly labeled. 2.2. Characterization. The crystal phase structures of as-prepared samples were investigated by X-ray diffraction (XRD) (MSAL-XD2 diffractometry, Cu Kα radiation). Fourier transform infrared spectra (FT-IR) were record on Shimadzu UV-2500 spectrophotometer in the form of KBr pellets. The X-ray photoelectron spectroscopy (XPS) data were got on a VG ESCALAB 250 analyzer. The morphologic and structures of samples were performed on JEM-2100HR TEM at 200 kV. The UV–Vis spectroscopy was surveyed by a Daojin UV-2550PC Diffuse Reflectance Spectroscopy. The Brunauer–Emmett– Teller (BET) surface areas of the products were acquired with a Micromeritics Gemini-2360 analyzer. The steady-state photoluminescence (PL) measurements were collected on a LS 50B (Perkin Elmer, Inc., USA) at 385 nm. 2.3. Photocatalytic H2 generation.


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The photocatalytic experiments were tested in a LabSolar H2-evolution system (Perfectlight, Beijing). A Xe lamp (300 W, λ≥420 nm) was used as the light source and the light density on the reactor was 160 mW/cm2. Prior to irradiation, 50 mg photocatalyst was suspended in a 15 vol% triethanolamine solution (100 mL) by ultrasound and then the system was kept at -0.1 MPa and 15 °C. A gas chromatograph (GC-7900, TCD with N2 as the carrier gas) was used to detect the H2 on line after every 60 minutes of illumination. 2.4. Electrochemical measurements. Transient photocurrent and Mott–Schottky (MS) plots analysis were carried out on an electrochemical analyzer (BAS100 Instruments), using a standard three-electrode system. The samples, Ag/AgCl (saturated KCl) and Pt plate were utilized as the working, reference, and counter electrodes, respectively. The electrochemical impedance spectra (EIS) was measured via an IM6e electrochemical station (Zahner Elektrik, Germany) using the above-mentioned three-electrode system. The working electrodes were fabricated as follows: about 5 mg of photocatalyst powder and 20 µL of 0.25% Nafion were dispersed in 2 mL of ethanol to obtain a suspension. After ultrasonic treatment for 2 hours, 500 µL of dispersion solution was then dip-coated onto a 2×3.5 FTO glass plate. The resulted plates were dried under infrared lamp and heated in an tube furnace at 150℃ for 1 h in a N2 gas flow. The electrocatalytic hydrogen evolution was performed on an electrochemical analyzer (BAS100 Instruments) using the aforementioned three-electrode system. The working electrodes were fabricated as follows: 6 mg of samples were ultrasound in 2 mL of deionized water for 2 hours. 3 µL of above dispersion was dip-coated on the



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top of glassy carbon electrode. After drying under infrared light, 3 µL of 0.5% Nafion solution was then dip-coated onto the catalysts.

3. Results and discussion 3.1 The structures and compositions.

Figure 1. XRD patterns of (a) metallic Ni and amorphous NiS, and (b) seven photocatalysts.

The crystal phase structure and composition properties of amorphous NiS, metallic Ni and seven photocatalysts were first investigated by powder XRD measurements. Figure 1a displays the power XRD patterns of amorphous NiS and metallic Ni obtained with the same method as those of g-C3N4/NiS and g-C3N4/Ni samples without adding the g-C3N4, respectively. Obviously, no any peak can be observed for pure NiS, implying that it was amorphous crystal structures. The pure metallic Ni presented the peaks located at 78.8°, 52.3° and 44.8°, corresponding to the (220), (200)


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and (111) planes of rhombohedral metallic Ni (JCPDS 04–0850), respectively. As depicted in Figure 1b, all photocatalyst materials display XRD characteristic peaks with 2θ values of ~27.4° and ~13.1°, matching well with the hexagonal phase of polymeric g-C3N4 (JCPDS 87–1526), indicating the samples retain the original graphitic-like packing layer structure.46,47,60 The main peak at 27.4° can be assigned to the (002) plane with a d spacing of 0.326 nm in the g-C3N4. Impressively, the (002) peaks of g-C3N4/Ni and g-C3N4/Ni/NiS samples obviously show a slight right shift towards higher angles, in comparison with that of bulk g-C3N4, indicating the narrowing interlayer distance of graphitic packing layer structure and higher long-range order of interplanar structure packing in the g-C3N4 sheets, respectively, which might be due to the further condensation induced by the further high-temperature H2 treatments of the g-C3N4 sheets. These observations match with the earlier reports in the literature for high-temperature treatments of g-C3N4 under different atmosphere.45,61 Another weak peak located at 13.1° can be contributed to the (100) plane of g-C3N4, corresponding to the distance of 0.681 nm for in-plane packing of the g-C3N4, indicating the formation of the bending of 2D layered structures composed of the tris-s-triazine units (with the theoretical value of d = 0.73 nm) in g-C3N4.12 No peaks assigned to the crystal phases of metallic Ni are detected. Clearly, the unchanged features of the XRD patterns further suggested the low weight loading and highly selective dispersion of metallic Ni and NiS co-catalysts in the samples.



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Figure 2. FTIR spectrum of four photocatalysts.

In order to further investigate the surface groups of the as-prepared four typical samples, the FTIR spectra were conducted. As observed in Figure 2, the broad band emerged at around 3100-3400 cm-1 is from the stretching of residual free N–H in the bridging C–NH–C units and O-H originated from physically adsorbed water species on g-C3N4 surface, respectively,11,62 while the peaks centered between 1200 and 1700 cm-1 can be ascribed to the stretching of aromatic heptazine-derived repeating units, including the typical sp2 C=N stretching modes and out-of-plane bending of the sp3 C–N bonds.34,62 The peaks located at 1640 and 1319 cm-1 represent the stretching of

C(sp2)=N

and

C(sp2)-N

units

in

aromatic

heptazine

heterocycles,

respectively.34,63,64 The peaks at 1403 and 1240 cm-1 were corresponded to the C-NH-C unit in melem.65 Additionally, the characteristic absorption peaks centered at approximately 812 and 883 cm−1 were corresponding to the characteristic breathing of tri-s-triazine cycles and the deformation mode of N–H in amino groups,


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respectively.65 Clearly, the loaded metallic Ni and amorphous NiS cannot be revealed by the similar FTIR spectrum of four photocatalysts.

Figure 3. TEM images of (a) bulk g-C3N4, (b) binary g-C3N4-0.5%Ni and (c) ternary g-C3N4-0.5%Ni-1.0%NiS.

The further morphologic and structures of the samples were investigated by a typical TEM images in Figure 3. The g-C3N4 shows an obvious 2D porous nanosheet structure. However, the hydrogenated g-C3N4 at 500 °C (Figure 3b and 3c) has much smaller thickness than g-C3N4 without further processing (Figure 3a). As presented in Figure 3b, it is found that the metallic Ni layers are well deposited on the g-C3N4 surface. More importantly, the high-temperature H2 treatments form intimate interface between the g-C3N4 nanosheets and metallic Ni layers, which are fundamentally crucial for charge transfer, thus facilitating the improvement in H2-generation activity. In addition, as shown in Figure 3c, the amorphous NiS nanoparticles are mainly distributed on the metallic Ni layers over the surface of g-C3N4 nanosheets. Clearly, the TEM results roughly confirmed the successful loading of the metal Ni layers and



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amorphous NiS nanoparticles onto the g-C3N4 nanosheets. To further accurately identify the crystal structure of metallic Ni and amorphous NiS over the g-C3N4 nanosheets, the HRTEM measurements were performed. The HRTEM images of binary g-C3N4-0.5%Ni were displayed in Figure 4a-b. The HRTEM image (Figure 4a and 4b) showed the characteristic lattice spacing of 0.203 nm, matching well with the typical (111) plane of Ni, indicating the formation of Ni0.38 As shown in Figure 4c, the lattice fringes are not clear enough to verify the facets and the d values for NiS, further indicating that NiS was amorphous crystal structures and uniformly loaded on the surface of metallic Ni layers. The selected area electron diffraction (SAED) pattern of g-C3N4-0.5%Ni-1.0%NiS presented in Figure 4d further indicated the amorphous crystal structures. The energy dispersive X-ray (EDX) results explicitly illustrated the existence of C, Ni, N and S in the region (Figure 4e), and the corresponding EDS mapping results (Figure 4f-j) further confirms the homogeneous distribution of C, S, Ni and N elements. The similar distribution density of N and C implies the porous structures on the g-C3N4 surface. The density of Ni element in some region was obviously higher than that of S element, suggesting the coexistence of large amount of metallic Ni in the interfacial region between g-C3N4 and amorphous NiS. The uniform distribution of S implies the well dispersion of amorphous NiS on the g-C3N4 surface.


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Figure 4. (a-b) HRTEM images of g-C3N4-0.5%Ni. (c) HRTEM image of g-C3N4-0.5%Ni-1.0%NiS and (d) the corresponding SAED pattern. (e) EDX spectrum and (g-j) the corresponding EDX mapping of g-C3N4-0.5%Ni-1.0%NiS at the region shown in (f).

The surface compositions and local electronic structures of g-C3N4-0.5%Ni and g-C3N4-0.5%Ni-1.0%NiS were further characterized by XPS. The XPS survey scan in Figure 5a clearly demonstrate that the g-C3N4-0.5%Ni-1.0%NiS photocatalysts mainly consist of C, Ni, N and S elements. Figure 5b revealed that the peaks of C1s centered at 284.8 and 288.1 eV corresponded to C-C and C-N functional groups, which originated from pure graphitic sp2 and sp2 carbon in N-containing aromatic



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nucleus. In Figure 5c, four fitted peaks at the binding energy of 404.3, 401, 399.0 and 398.6 eV were observed in the spectra of N 1s core levels. The peak at 398.6 eV is regarded as C=N-C, while the peak at 399.0 eV is associated with N-(C)3. The another two peaks at 401 and 404.3 eV could be attributed to C-N-H and π-excitation in the polymeric g-C3N4 structures, respectively.66 The Ni 2p core-level spectra in Figure 5d exhibits two strong peaks corresponding to the binding energies of 855.0 and 872.0 eV, which are originated from Ni 2p3/2 and Ni 2p1/2 for Ni2+ in the amorphous NiS co-catalysts. The weak peak at 852.6 eV for the g-C3N4-0.5%Ni confirmed the existence of metallic Ni0.38,39,58 The emerged Ni 2p3/2 peak at 855.0 eV for the g-C3N4-0.5%Ni might be due to the formation of NiO on the metallic Ni surface under the testing process.38 The main S 2p peaks were observed at 163.5 and 168.7 eV in Figure 5e, which were related to the binding energies of S2- ions.44,46 The XPS results further confirmed that the metallic Ni and amorphous NiS exist in the as-obtained g-C3N4-0.5%Ni-1.0%NiS sample.


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Figure 5. (a) XPS survey scan of the g-C3N4-0.5%Ni and g-C3N4-0.5%Ni-1.0%NiS, and the corresponding high-resolution XPS core-level spectra of (b) C 1s, (c) N 1s, (d) Ni 2p and (e) S 2p. 3.2 Textural and optical properties

Figure 6. N2 adsorption and desorption isotherms of bulk g-C3N4, binary g-C3N4-1.0%NiS and ternary g-C3N4-0.5%Ni-1.0% NiS (Inset: the corresponding pore size distribution curves).



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Table 1. Textural properties of bulk g-C3N4, binary g-C3N4-1.0%NiS and ternary g-C3N4-0.5%Ni-1.0% NiS Photocatalysts

BET Surface

Mean pore

area(m2g-1)

diameter(nm)

(cm3g-1)

g-C3N4

58.59

31.21

0.48

g-C3N4-1.0%NiS

62.43

26.48

0.45

g-C3N4-0.5%Ni-1.0%NiS

60.90

14.19

0.30

The

textural

properties

of

the

g-C3N4,

Pore volume

g-C3N4-1.0%NiS

and

g-C3N4-0.5%Ni-1.0%NiS samples were characterized by N2 adsorption and desorption measurements. As shown in Figure 6, it is obvious that all the samples possess type V isotherms with type H3 hysteresis loops, implying the presence of mesoporous structures connected via macropores.67 The type H3 hysteresis loops suggested that the slit-like pores were originated from the aggregates of g-C3N4 sheets. The textural parameters of three photocatalysts are presented in Table 1. The mesoporous structure could be further verified by the mean pore diameters of all these samples. The meso/macroporous structures can allow for highly efficient light reflection and scattering inside them, and facilitate the adsorption of accessible reactant molecules onto the surface exposed active sites, thus achieving the enhanced photocatalytic activity.68 Furthermore, the g-C3N4-1.0%NiS (62.43 m2g−1) and g-C3N4-0.5%Ni-1.0%NiS (60.90 m2g−1) exhibit the slightly higher specific surface area than those pure g-C3N4, further suggesting that the H2 treatment at


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500 °C could slightly reduce the thickness of g-C3N4, due to the controlled delamination-etching effect.61 However, g-C3N4-0.5%Ni-1.0%NiS photocatalysts had smaller surface area, pore volume and mean pore diameter than g-C3N4-1.0%NiS, as the loading of metallic Ni interface layers may partial fill or block meso/macroporous structures in g-C3N4.69 These data verified that the increased surface area might be not a significant factor for determining the remarkable photoactivity enhancement of the ternary composite photocatalysts.

Figure 7. (A) UV-Vis spectra of samples: (a) bulk g-C3N4, (b, c) binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, (d-g) ternary g-C3N4-0.5%Ni-0.5%NiS, g-C3N4-0.5%Ni-1.0%NiS, g-C3N4-0.5%Ni-1.5%NiS, and g-C3N4-1.0%Ni-1.0%NiS. (B) Tauc plots of four samples.

The UV–vis spectra were used to study the optical character of all as-obtained photocatalysts. Figure 7A presents the typical semiconducting UV–vis absorbance



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spectra. As shown in Figure 7A, the pure g-C3N4 displays intense absorption bands with absorption edges at around 450 nm, suggesting a band gap of about 2.68 eV (Figure 7B). g-C3N4/NiS, g-C3N4/Ni, and g-C3N4/Ni/NiS exhibit significantly higher photo-absorbance intensities than pure g-C3N4, in good agreement with their color change of the composites (inset in Figure 7A). In addition, the optical band edges of g-C3N4/NiS,

g-C3N4/Ni,

and

g-C3N4/Ni/NiS

slightly

shifted

to

a

longer

wavelength. The band gap energies (Eg) of n-type g-C3N4 photocatalysts are estimated by using the Kubelka−Munk method, based on the tangent lines of (αhν)1/2 to hν plots in Figure 7B, where α is the absorption coefficient and hν is the photon energy.2 As displayed in Figure 7B, the value of band gap for ternary g-C3N4-0.5%Ni-1.0%NiS, binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni and are determined to be 2.64, 2.66 and 2.64 eV, respectively. Apparently, metallic Ni and amorphous NiS are not incorporated into g-C3N4. The high-temperature H2 treatments play a weak role in decreasing the bandgap energy of bulk g-C3N4, due to the introduced a small amount of defects during treatment.61 In a word, the small band gap narrowing and the slightly improved visible-light harvesting ability of g-C3N4 might be beneficial for enhancing its H2-generation activity. 3.3 Photoactivity and stability. Visible-light (λ≥420 nm) photocatalytic H2-production activities of different photocatalysts were measured in a triethanolamine solution. H2 formation cannot be detected

under

various

control conditions,

including

no

photocatalysts

or

non-irradiation. Figure 8A showed the typical H2-evolution kinetics of different samples under light irradiation. It should be noted that a linearly increased


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H2-evolution amounts of all samples could be obviously observed over the entire time range of light irradiation, confirming the relatively excellent photostabilities for all samples. Figure 8B presented the average rates of H2 production over various photocatalysts. The pure g-C3N4 exhibits almost no H2 production, which could be ascribe to the poor hydrogen evolution kinetics and the ultrafast recombination of photo-generated charge carriers in the pristine g-C3N4, further suggesting the important roles of metallic Ni and amorphous NiS in promoting the surface hydrogen evolution kinetics. Comparatively speaking, the visible-light H2-evolution rates of binary g-C3N4-0.5%Ni and g-C3N4-1.0%NiS were measured to be 112 and 185 µmolg-1h-1, respectively. This indicated that the metallic Ni and amorphous NiS layers could act as co-catalysts and active sites to fundamentally accelerate the surface hydrogen-evolution kinetics. Notably, g-C3N4-0.5%Ni-1.0%NiS showed the best hydrogen evolution rate of 515 µmolg-1h-1, which was about 2.8 and 4.6 times as those obtained on binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, respectively. It is believed that the synergistic effects on metallic Ni layers and amorphous NiS nanoparticles could separate photo-generated charge carriers and accelerate H2-evolution reaction kinetics, thus significantly enhancing the H2 production activity. However, as observed in Figure 8, the photocatalytic H2-evolution activity of ternary g-C3N4-1.0%Ni-1.0%NiS and g-C3N4-0.5%Ni-1.5%NiS is prone to significantly decrease with further gradually increasing the loading of 1 wt% Ni or 1.5 wt% amorphous NiS co-catalysts. The main reason for the decreased activity is that the excessive amounts of cocatalyst may lead to the unexpected light scattering and block effects on the g-C3N4 surface, which restrict the absorbance and utilization effciency



of incident light, thus bringing about a dramatic reduction in H2 production.7,46 Therefor, the sluggish H2-evlution rates on the g-C3N4 surface could be thoroughly enhanced through the excellent synergistic effects of metallic Ni and amorphous NiS with optimum loading amounts.

600

a b c d e f g

1500 1200 900 600

(A)

400

374

300 200

1.0

1.5

2.0

2.5

3.0

0

214

191

185 112

100

0.5

(B)

515

500

300 0 0.0

Rate of H2 evolution (µmol g-1 h-1)

1800

Amount of H2 evolution (µ mol g-1)

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1.27 a b

Time (h)

c

d

e

f

g

Samples

Figure 8. (A) Time-dependant amounts and (B) the average rate of photocatalytic H2 production over different photocatalysts: (a) bulk g-C3N4, (b, c) binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, (d-g) ternary g-C3N4-0.5%Ni-0.5%NiS, g-C3N4-0.5%Ni-1.0%NiS, g-C3N4-0.5%Ni-1.5%NiS, and g-C3N4-1.0%Ni-1.0%NiS. Conditions: 0.05 g catalyst, 100 mL of 15 vol% TEOA solution, a 300 W xenon lamp (λ≥420 nm).


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Amount of H2 evolution (µmol g-1)

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1800 1600 1400 1200 1000 800 600 400 200 0

1 run

0

2 run

3

3 run

6 Time (h)

4 run

9

12

Figure 9. Recycling H2 evolution on ternary g-C3N4-0.5%Ni-1.0%NiS photocatalyst.

Moreover, the stability and reusability of a given photocatalyst should be carefully evaluated because they are of significant importance for practical photocatalytic H2 production.

The

cycling

test

of

photocatalytic

H2

evolution

over

g-C3N4-0.5%Ni-1.0%NiS for 12 h was shown in Figure 9. As observed in Figure 9, the ternary hybrid sample exhibited steady H2 production rate during each cycle and only 9% of activity loss after four cycles, suggesting that the ternary g-C3N4-0.5%Ni-1.0%NiS photocatalysts are highly durable and cannot be easily photocorroded in the photocatalytic H2-production process.

3.4 The charge-separation performances of photocatalysts To understand the key roles of metallic Ni and amorphous NiS co-catalysts in determining the overall efficiency of ternary composite photocatalysts, PL was performed to investigate the photo-induced interfacial charge dynamics. Generally, the PL spectra have been regularly utilized to study the charge separation



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performances in the excited semiconductors.70-72 Figure 10 shows the PL spectra of all samples excited by 385nm. It could be found that all photocatalysts show the similar emission trends centered at about 450 nm, corresponding to the charge recombination in the g-C3N4.47 However, when metallic nickel layers or amorphous NiS were deposited on the g-C3N4 nanosheets, the intensity of this emission peak dropped significantly, owing to the highly efficient transfer of photo-excited electrons from g-C3N4 to base metal nickel, thus greatly inhibiting the charge recombination. Notably, the binary g-C3N4-0.5%Ni sample exhibits much lower PL intensity than g-C3N4-1.0%NiS, as the metallic Ni layers are more effective for the charge separation than the amorphous NiS nanoparticles, due to the high conductivity of the former. In addition, among the ternary hybrids, the ternary g-C3N4-0.5%Ni-1.0%NiS exhibited the weakest PL intensity, consistent with the corresponding photocatalytic H2-evolution activities. Therefore, the effective charge separation has a very pronounced facilitation effect on improving the overall H2-generation efficiency.

Figure 10. PL spectra over various photocatalysts excited at 385 nm. (a) bulk g-C3N4, (b,c)

binary

g-C3N4-1.0%NiS

and

g-C3N4-0.5%Ni,

(d-f)

ternary

g-C3N4-0.5%Ni-0.5%NiS, g-C3N4-0.5%Ni-1.0%NiS and g-C3N4-0.5%Ni -1.5%NiS. 24 ACS Paragon Plus Environment

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Figure 11. Visible-light photocurrent responses obtained on four electrodes in 15 vol % TEOA solutions.

The enhanced efficiency of charge separation and transfer was also verified by the photocurrent-time responses of different photocatalysts. It is clear that the photocurrent responses are commonly employed to reveal the visible-light photoelectrochemical performance occurring on the photocatalyst surface.63,73-76 The transient visible-light photocurrent responses (I–t curves) recorded for several cycles under chopped visible-light irradiation (λ>420 nm) are shown in Figure 11. The photocurrent intensity quickly decreases to zero and maintains stable values when the light was turned off and on, respectively. The g-C3N4-0.5%Ni-1.0%NiS produced a higher photocurrent than binary g-C3N4/Ni and g-C3N4/NiS hybrid materials and pure g-C3N4 in the same condition, suggesting the ternary g-C3N4-0.5%Ni-1.0%NiS photocatalyst had lower recombination rare and a more efficient charge separation, thus enhancing the photocatalytic H2-evolution activity. The EIS Nyquist plots were performed, as an effective approach to study the charge



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transfer and recombination process.63,64,77,78 As shown in Figure 12, the arc radiuses on the EIS Nyquist plots of ternary g-C3N4-0.5%Ni-1.0%NiS and binary g-C3N4-0.5%Ni were much smaller than that of g-C3N4 and g-C3N4-1.0%NiS without irradiation (Figure 12), suggesting that the metallic Ni loading could greatly improve the charge transfer and separation of g-C3N4. The binary g-C3N4-0.5%Ni displays much smaller EIS arc radius than g-C3N4-1.0%NiS, indicating the significant importance of high conductivity for achieving the effective charge separation in the g-C3N4. Thus, Ni and NiS co-catalysts in the ternary hybrids could result in the accelerated charge separation and transport, thus dominantly improving the H2 generation.

Figure 12. Nyquist plots for four samples in 0.1M Na2S and 0.02 M Na2SO3 aqueous solutions in the dark.

3.5 Proposed photocatalytic mechanism The photocatalytic H2-evolution reactions on the surface active sites are similar to those in electrolysis.2 Thus, co-catalysts have been widely loaded on the H2-evolution


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photocatalysts to dramatically reduce the over-potential for H2 evolution over these semiconductors, which could be further identified by the electrocatalytic H2 evolution activity. Figure 13 shows the polarization curves of metallic Ni, amorphous NiS, pure and modified g-C3N4 electrodes in the dark in 0.5 M H2SO4 solution. As observed in Figure 13, the onset potentials of amorphous NiS nanoparticles and metallic Ni layers for electrocatalytic H2 evolution were about -0.4 and -0.6 V (vs. Ag/AgCl, in 0.5 M H2SO4), respectively, corresponding to the overpotentials of -0.8 and -1.0 V (vs. Ag/AgCl, at pH=7). Clearly, the amorphous NiS nanoparticles exhibited much lower overpotential for electrocatalytic H2 evolution than that of metallic Ni layers, indicating that the amorphous NiS nanoparticles are better co-catalyst than metallic Ni for photocatalytic H2 evolution over g-C3N4. The obtained cathode current ranging from -0.8 to -1.5 V (vs. Ag/AgCl, in 0.5 M H2SO4) can be ascribed to the electrocatalytic H2 evolution over four different composite electrodes. Obviously, the ternary Ni/NiS-loaded g-C3N4 shows a much higher electrocatalytic H2 production than g-C3N4-1.0%NiS, g-C3N4-0.5%Ni and pristine g-C3N4, indicating that the synergistic effects of metallic Ni and amorphous NiS as cocatalysts could mainly decrease the overpotential for efficient electrolytic H2 evolution on the co-catalyst-modified g-C3N4 electrode. However, it should be noted that the g-C3N4-0.5%Ni sample exhibits a slight lower overpotential for electrocatalytic H2 evolution than that of g-C3N4-1.0%NiS in the dark, which might be due to the intimate interface between the g-C3N4 nanosheets and metallic Ni layers and the good conductivity of metallic Ni. In a word, the polarization curves of different samples further confirmed the major roles of amorphous NiS as cocatalysts in decreasing the



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H2-evolution overpentials.

Figure 13. Polarization curves of metallic Ni, amorphous NiS and four different photocatalysts were measured in 0.5 M H2SO4.

To investigate the electronic band structures, Mott–Schottky (MS) plots were conducted to approximately estimate the potential change of the CB edge of the g-C3N4 nanosheets at 1 kHz in a Na2SO4 solution. As displayed in Figure 14a, the positive tangent slopes revealed that four photocatalysts exhibited the characteristics of n-type semiconductors, in good agreement with the previous literatures. The calculated flat-band potentials for bulk g-C3N4, g-C3N4-1.0%NiS, g-C3N4-0.5%Ni and g-C3N4-0.5%Ni-1.0%NiS in an aqueous solution of Na2SO4 are -1.21, -1.14, -0.94, and -0.90 V (vs. Ag/AgCl), respectively. Apparently, the flat-band potentials of binary and ternary composites exhibited a positive shift compared to that of pure g-C3N4. The slope of the linear region for g-C3N4-0.5%Ni-1.0%NiS electrode is lower than that of g-C3N4-1.0%NiS, suggesting a higher donor density due to the good electrical conductivity of metallic nickel. It is known that the CB position is generally more


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negative than the flat-band potential by -0.2 V,2 and the VB position can be calculated by the CB and band gap energy. Moreover, the VB positions for bulk g-C3N4, g-C3N4-1.0%NiS, g-C3N4-0.5%Ni and g-C3N4-0.5%Ni-1.0%NiS are estimated to be 1.27, 1.32, 1.50 and 1.54 V (vs. Ag/AgCl), respectively. Therefore, the electronic band structures of these four different semiconductors can be illustrated in Figure 14b. In general, the lower valence band edge can accelerate the electron-hole separation efficiency and further enhance the photoactivity, owing to the strengthened oxidation ability.

Figure 14. (a) Mott–Schottky (MS) plots and (b) band gap structures of four different photocatalysts. The MS plots were measured at 1 kHz in Na2SO4 (0.1 M).

Thus, a possible underlying photocatalytic mechanism is proposed in Scheme 2 for the ternary g-C3N4-Ni-NiS composites under visible-light irradiation. The electrons could be excited from VB of g-C3N4 to its CB by visible light, leaving the holes in its VB. On the one hand, the loading of metallic Ni and amorphous NiS on g-C3N4 can enhance its VB levels, which could further strengthen the oxidation of TEOA



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adsorbed on the g-C3N4 surface, thus achieving the improved charge separation and activity enhancement. On the other hand, the CB potential of g-C3N4 in the ternary composites is about -1.1 V (vs Ag/AgCl, at pH=7), which is still more negative than those overpotentials of the metallic Ni interface layers (-1.0 V vs Ag/AgCl, at pH=7) and amorphous NiS nanoparticles (-0.8 vs Ag/AgCl, at pH=7).Thus, both metallic Ni and amorphous NiS can act as co-catalysts accept the photo-generated electrons from g-C3N4, to significantly enhance the photocatalytic H2 evolution over g-C3N4 semiconductor (Schemes 2a and 2b). For the ternary nanohybrids (Scheme 2c), after implantation of the highly conductive Ni interface layers between g-C3N4 photocatalyst and amorphous NiS co-catalysts, these photo-excited electrons in the CB of g-C3N4 can rapidly transfer to metallic Ni layers due to the formation of the intimate Schottky junctions between metallic Ni and g-C3N4, thus achieving the rapid electrons-hole separation. More interestingly, it was also demonstrated that the metallic Ni interface layers exhibited much higher over-potentials for electrocatalytic H2 evolution than that of the amorphous NiS nanoparticles. Therefore, the collected electrons in metallic Ni interface layers could still possess enough overpotentials to drive the electrocatalytic H2 evolution over the amorphous NiS. Therefore, the synergetic effect between metallic Ni and amorphous NiS on the g-C3N4 surface can simultaneously achieve the effective separation of photo generated electron-hole pairs and enhance the H2-evolution and TEOA oxidation kinetics, thereby leading to the significant improvement of the photocatalytic activity over the ternary hybrid photocatalysts.


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Scheme 2 Proposed photocatalytic H2 production mechanisms over the (a) g-C3N4-1.0%NiS, (b) g-C3N4-0.5%Ni and (c) ternary g-C3N4-Ni-NiS composites under visible-light irradiation. 4. Conclusions. The g-C3N4/Ni/NiS nanohybrids are successfully fabricated with a facile process: the loading of Ni(OH)2 nanosheets, high-temperature H2 reduction and further deposition of amorphous NiS nanosheets. Both robust metallic Ni interface layers and

amorphous

NiS

are

utilized

as

dual-layer

electron

co-catalysts

to

enormously enhance the H2-evolution activity over g-C3N4 semiconductor due to effective charge separation and decreased overpotentials for H2 evolution. The optimized g-C3N4-0.5%Ni-1.0%NiS composite showed the highest hydrogen evolution of 515 µmolg-1h-1, 2.8 and 4.6 times higher than g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, respectively. The excellent synergetic effects between the multi-functional metallic Ni interface layers and amorphous NiS can effectively improve the charge separation/transportation, enhance both H2-production and TEOA-oxidation kinetics, thus resulting in the significantly boosted photocatalytic H2 evolution over the ternary composite photocatalysts. The herein reported strategy of constructing dual-layer electron co-catalysts allows for improving the photocatalytic



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H2 production and photoreduction of CO2 and O2 over various 2D semiconductor nanosheets. More importantly, this interesting design approach could be readily extended to many other fields, including electrocatalysis, solar cells and photoelectrochemical devices.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X. Li). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by National Natural Science Foundation of China (51672089 and 21475047), the Science and Technology Planning Project of Guangdong Province (2015B020215011) and the State Key Laboratory of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology) (2015-KF-7).


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Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4

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