2D Ni2P quantum dot loaded TiO2(B) nanosheet photothermal catalysts for enhanced hydrogen evolution

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Hybrid 0D/2D Ni2P quantum dot loaded TiO2(B) nanosheet photothermal catalysts for enhanced hydrogen evolution ⁎

Xiao Luoa,b, Rong Lia,c, Kevin Peter Homewooda, Xuxing Chena, , Yun Gaoa,

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a Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science & Engineering, Hubei University, Wuhan 430062, China b School of Sciences, Hubei University of Automotive Technology, Shiyan 442002, China c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

A R T I C LE I N FO

A B S T R A C T

Keywords: TiO2(B) nanosheets Ni2P quantum dots 0D/2D architecture Photocatalytic H2 evolution Synergistic photothermal catalysis

The development of low cost, stable, robust photocatalysts to convert solar energy into hydrogen energy is an important challenge. Here, we describe a simple solvothermal method to successfully fabricate a catalyst with a hybrid 0D/2D Ni2P quantum dot/TiO2(B) nanosheet architecture. HRTEM shows that Ni2P quantum dots about 5 nm in size were dispersed on ultrathin TiO2(B) nanosheets. The optimum photocatalytic H2 evolution rate with 10 wt% Ni2P/TiO2(B) (3.966 mmol g−1 h−1) was superior to Pt loaded TiO2(B) (3.893 mmol h−1 g−1), which was 15 times higher than pure TiO2(B) nanosheets. Significantly, the new catalyst shows high stability and reusability in multiply cycled H2 production runs for a 30 h period. The H2 production rate can be considerably increased furthered by using synergistic photothermal H2 evolution (20.129 mmol g−1 h−1 at 90 °C).

1. Introduction One of the most urgent current issues for the global environment and ever increasing energy demand is developing clean, renewable replacements for conventional fossil fuels [1,2]. Photocatalytic H2 evolution over semiconductors harnessing solar irradiation has received much attention as a promising solution [3–10]. Ever since the discovery of photocatalytic H2 generation in 1972 [11], various semiconductors have been intensively investigated for this application. These include TiO2 [12–15], SrTiO3 [16,17], g-C3N4 [18–22], CdS [23,24] and MoS2 [25]. Of these, TiO2 has been the most extensively studied, due to its high chemical stability, non-toxicity, low cost, and high earth abundance [26–28]. However, conventional bulk TiO2 materials show low photocatalytic efficiency due to rapid recombination of e−–h+ pairs, and slow surface reaction kinetics. Consequently, numerous efforts have been made to overcome its drawbacks (such as low surface area, large band gap, fast recombination of e−–h+ pairs). For example, Asahi et al. extended TiO2 absorption from ultra-violet to visible light by nitrogen doped method [29]. Hydrogenation induced disordered TiO2 nanocrystal is an effective way to enhance visible light absorption of TiO2 [30,31]. Surface heterojunction [32] and graphene hybridization [33] can also optimize photocatalytic efficiency. Moreover, photothermal catalytic H2 evolution is a promising route to achieve high solar spectrum conversion efficiency due to synergetic effect of

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photocatalysis and thermalcatalysis. The visible and even the infrared light can be used for generating thermal energy and heat the reaction system to a higher temperature [34]. Therefore, photothermal catalytic H2 evolution can enhance utilization efficiency of solar light. What is more, the morphology and dimension of materials play an important role on catalytic activity. In particular ultra-thin 2D materials have been shown to boost photocatalysis due to their unique structure, abundant active surfaces, high ratio of surface atoms, large number of active sites, and unique optical, electronic and chemical properties are attractive candidates [35,36]. The ultra-thin thickness considerably decreases the distance needed for the separation and migration of e−–h+ pairs reducing their recombination. Recently, our group reported on the synthesis of 2D ultra-thin TiO2(B) nanosheets with very high surface area and very high surface densities of active sites [37]. However, the efficiency of pure TiO2(B) nanosheets is still far from ideal due to fast carrier recombination and sluggish reaction kinetics. Using cocatalysts is an efficient strategy for boosting photocatalysis. Unfortunately, the most effective cocatalysts are dominated by the noble metals, especially Pt [38], but these have the major shortcomings of low earth abundance and very high cost making their large scale application impractical. Efficient, robust, low cost cocatalysts which can substitute for Pt for practical industrial scale H2 production are an urgent requirement [39,40]. Many non-noble metal based materials have been demonstrated to be excellent cocatalysts for high efficiency

Corresponding authors. E-mail addresses: [email protected] (X. Chen), [email protected] (Y. Gao).

https://doi.org/10.1016/j.apsusc.2019.144099 Received 19 June 2019; Received in revised form 10 September 2019; Accepted 18 September 2019 Available online 17 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Xiao Luo, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144099

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Fig. 1. Schematic illustration of the synthesis of the 0D/2D Ni2P quantum dot loaded TiO2(B) nanosheet architecture photocatalyst. The Ni2P quantum dots were anchored on ultra-thin TiO2(B) nanosheets by a simple solvothermal method.

Fig. 2. (a) XRD patterns and (b) Raman spectra of pure TiO2(B), Ni2P and the Ni2P/TiO2(B) composites with varying weight ratios.

chemical properties are investigated by XRD, TEM, XPS and show the successful construction of the Ni2P/TiO2(B) 0D/2D architecture. Transient photocurrent measurements and Electrochemical Impedance Spectroscopy (EIS) and linear sweep voltammetry, were used to investigate the mechanism producing the enhanced H2 generation.

H2 evolution, such as MoS2 [41–43], CoSx [44] and NiS [45] amongst others. Although these cocatalysts effectively promote H2 evolution rate they usually suffer from major long-term instability. Recently, Ni2P has attracted extensive attention as a low cost, stable and long-lasting cocatalyst for H2 evolution [46,47]. Ni2P consists of cheap earth abundant elements, exhibits unique physicochemical properties such as extraordinary electrocatalytic activity, excellent electrical conductivity and unique metalloid characteristics. Moreover, density function theory (DFT) computation demonstrates an ensemble effect between Ni-P bridge and Ni hollow sites which expedites hydrogen evolution [48]. Recently, much research interest has focused on Ni2P nanocrystals as cocatalyst hybrids with other semiconductors, such as CdS [49], TiO2 [50] and g-C3N4 [51]. However, the particle size of Ni2P synthesized by conventional calcination is too large [52]. A method to synthesize very small Ni2P particles is needed. 2D ultra-thin TiO2(B) nanosheets are anticipated to be good candidates to obtain small Ni2P particles. Their huge surface area provides a high density of sites for Ni2P crystal nucleation whilst the numerous active sites inhibit their growth. Here a novel 0D/2D architecture, fabricated by anchoring Ni2P quantum dots as cocatalysts on ultra-thin TiO2(B) nanosheets, was successfully synthesized via the solvothermal method. The as-prepared Ni2P/TiO2(B) photocatalysts exhibit significantly enhanced photocatalytic H2 evolution performance. As the reaction temperature increases, H2 evolution activity radically increases due to a synergetic photothermal interaction. The structural and

2. Experimental 2.1. Sample preparation All chemicals and materials were of analytical grade acquired from Sinopharm Chemical Reagent Co., Ltd and used without further purification. The 2D ultra-thin TiO2(B) nanosheets were synthesized by our previously reported method [37]. The Ni2P/TiO2(B) photocatalysts were prepared through a solvothermal reaction using red phosphorus (red P) and NiCl2·6H2O as precursors. First, Red P was ground using a pestle and mortar to obtain the fine P powder. 0.2 g TiO2(B), red P powder and NiCl2·6H2O were mixed into 80 mL of ethylene glycol (EG). The weight ratio of red P to NiCl2·6H2O is 1:2. After stirring for 30 min, the mixture was poured in to a 100 mL autoclave and held at 150 °C for 10 h. The final product was collected by centrifuging and rinsed several times in ethanol, followed by vacuum drying at 60 °C for 10 h. As prepared Ni2P/TiO2(B) samples were denoted as x-NT where “x” indicates the weight ratio of Ni2P to TiO2(B) calculated from the initial NiCl2·6H2O precursor. Pure Ni2P was also prepared from the same 2

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Fig. 3. (a) SEM image of pure Ni2P; (b) TEM image and (c, d) HRTEM images of 10-NT. EDX elemental mapping of Ti, O, Ni, and P for 10-NT (bottom of the figure).

coupled plasma atomic emission spectrometry (ICP-AES). The photoluminescence (PL) was measured by a fluorescence spectrophotometer (Perkin Elmer LS55) with 315 nm excitation light. Photoelectrochemical and electrochemical measurements were made on an electrochemical workstation (CHI 660E, Shanghai Chen Hua Instrument Company, China). A platinum wire, a saturated Ag/ AgCl electrode and a fluorine-doped tin oxide (FTO) glass deposited with a sample film served as counter, reference, and working electrodes, respectively, in the standard three-electrode system. For the working electrode preparation, the FTO glass was first rinsed by acetone, ethanol and deionized water for 30 min respectively. 5 mg of the sample was then suspended in a solution containing 100 μL nafion solution and 2 mL of ethanol. Lastly, 50 μL of the suspension was injected onto a FTO glass (1 cm2) substrate and the electrodes dried in atmosphere at ambient temperature for 24 h. A 300 W Xenon lamp (Perfect Light PLS-SXE 300) was used as the light source for the transient photocurrent response. The EIS spectra were recorded over a frequency range of 1–106 Hz. A 0.1 M Na2SO4 aqueous solution was used as the electrolyte.

method by a solvothermal reaction using red P and NiCl2·6H2O in the absence of TiO2(B) nanosheets. For comparison, TiO2(B) loaded with 1 wt% Pt were prepared by photodeposition. The synthesis of the 0D/ 2D Ni2P quantum dot/TiO2(B) nanosheets is illustrated schematically in Fig. 1.

2.2. Characterization Phase and structure of the samples were investigated by powder XRD using a D8-Advance diffractometer with Cu Kα radiation source (λ = 0.15418 nm) and Raman spectroscopy (Renishaw inVia, U.K.) at a laser excitation of 532 nm. An FESEM (ZEISS SIGMA 500), TEM (FEI TECNAI G2 20 S-TWIN), and HRTEM (FEI Titan G2 60-300) were used to examine the sample morphologies. The specific surface area and pore size distributions were measured using a QDS-MP-30 surface area analyzer. X-ray photoelectron spectra (XPS) were collected by an ESCALAB 250Xi spectrometer: all the binding energies were calibrated to the 284.6 eV C 1s peak of the adventitious carbon present on the surface. UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained on a spectrophotometer (Japan SHIMADZU UV-3600). The Ni/Ti mass ratios of the Ni2P/TiO2(B) composite were analyzed by inductively 3

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hydrogen using a TCD detector. Before testing, dissolved oxygen and air was removed by vacuumizing the photoreaction system. A 300 W Xe lamp equipped with a 313 nm ( ± 10 nm) line pass filter was used to measure the apparent quantum efficiency (AQE). The AQE was calculated with the following equation:

AQE(\%) = =

number of reacted electrons ×100 number of incident photons

2×number of evolved hydrogen molecules ×100 number of incident photons

(1)

Synergistic photothermal catalytic and thermal catalytic hydrogen production were determined at different temperatures (50–90 °C) using a water bath. Thermal catalytic activity was measured as for the photothermal process but without illumination. 2.4. Density functional theory (DFT) calculation Density functional theory calculations: First-principle calculations were carried out using the plane-wave pseudopotential formulation [53–55] as implemented in the Vienna ab-initio Simulation Package (VASP). Perdew-Burke-Ernzerhof (PBE) [56], with the generalized gradient approximation (GGA), was used to describe the exchange and correlation energies. A cutoff energy of 400 eV for the plane-wave basis and single k-point (Γ-point) for the calculations was applied to the calculations to ensure an energy convergence of 1 meV and residual force acting on each atom of less than 0.03 eV/Å. The support was modeled by constructing the TiO2(1 0 0) surface with a 4 × 2 supercell for the loading of Ni2P nanoparticles cut from the unit cell of Ni2P possessing two layers of the (1 1 1) surface. To eliminate interactions between the TiO2 layer and its periodic images, we used a vacuum distance larger than 14 Å for the supercell geometry.

Fig. 4. UV–vis absorbance spectra of TiO2(B), Ni2P/TiO2(B) composites and Ni2P. The inserted image are optical photographs of TiO2(B), 1-NT, 5-NT, 10NT, 15-NT, 20-NT, 50-NT, and Ni2P (from left to right), and a Kubelka-Munk function plot of TiO2(B) nanosheets.

3. Results and discussion 3.1. Characterization of as-prepared photocatalysts The XRD measurements are displayed in Fig. 2a. For pure TiO2(B), the main diffraction peaks occur at 14.2°, 24.9°, 28.6°, and 48.5° correspond to (0 0 1), (1 1 0), (0 0 2), and (0 2 0) crystal faces of the monoclinic TiO2(B) crystalline phase (JCPDS card No. 46-1237), respectively. For pure Ni2P, the distinct diffraction peaks at 40.7°, 44.6°, 47.4° and 54.2° correspond to the (1 1 1), (2 0 1), (2 1 0) and (3 0 0) planes of Ni2P (JCPDS card No. 89-4864, hexagonal), respectively, confirming pure Ni2P crystals were successfully synthesized by our solvothermal method. For the samples of Ni2P/TiO2 composites, both diffraction peaks for TiO2(B) and Ni2P are seen with no additional peaks. Notably, all peaks were significantly broadened compared to bulk TiO2(B), which indicates that the nanosheets are ultra-thin. In contrast, for pure Ni2P, all peaks were sharp. The corresponding crystallite size of the Ni2P is 35 nm derived by the Scherrer formula. For the Ni2P/TiO2 composites, the width of the TiO2(B) diffraction peaks does not change as the dosage of Ni2P increases, indicating the morphology of TiO2(B) was not changing. When growing the composites, the intensity of Ni2P peaks gradually decreased and broadened with decreasing Ni2P dose, indicating a decrease in the size of the Ni2P dots. The crystal structure of the samples was also confirmed by Raman spectroscopy (Fig. 2b). The pure TiO2(B) nanosheets show clear characteristic Raman peaks at 210, 256, 382, 422, 477, 553, 632, and 828 cm−1 corresponding to the vibrational features of pure TiO2(B) [57], as previously reported [58]. The as-prepared Ni2P/TiO2(B) composites show the same Raman peaks as pure TiO2(B), showing the TiO2(B) did not change phase after solvothermal reaction. The morphology and microstructure of the samples were studied by field emission scanning electron microscopy (FESEM) and field emission transmission electron microscopy (FETEM). As shown in Fig. 3b, after one step of solvothermal reaction, the Ni2P/TiO2(B) composite

Fig. 5. Nitrogen adsorption-desorption isotherms of TiO2(B), Ni2P/TiO2(B) composites and Ni2P. Table 1 The Ni/Ti mass ratios (from ICP-AES), specific surface area (SBET), and pore volume of the as-prepared samples. Samples

Ni/Ti (ICP)

SBET (m2 g−1)

Pore volume (cm3 g−1)

TiO2(B) 1-NT 5-NT 10-NT 15-NT 20-NT 50-NT Ni2P

0 1.39% 7.37% 12.86% 18.24% 24.37% 61.90% –

260.88 251.68 256.36 265.74 248.13 209.64 148.07 0.92

0.348 0.331 0.289 0.321 0.279 0.214 0.162 0.018

2.3. Photocatalytic and synergistic photothermal catalytic hydrogen production Photocatalytic hydrogen generation tests were carried out in a quartz reactor. First, a 10 mg catalyst sample was suspended in 50 mL of 20% methanol solution (10 mL methanol and 40 mL H2O) in the reactor. The pH value was adjusted to 4 with lactic acid. The Xe lamp (Perfect Light PLS-SXE 300) was used to simulate solar light. The reactor was connected to an on-line evaluation system with a closed gas circulation (CEL-SPH2N, CEAuLight) to monitor the generated 4

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Fig. 6. XPS spectra of TiO2(B), 10-NT and Ni2P aggregates. (a) Ti 2p; (b) O 1 s; (c) Ni 2p; (d) P 2p.

shows 2D nanosheet morphology. The edges of the nanosheets roll up due to surface tension. The HRTEM image in Fig. 3c shows Ni2P quantum dots with size of 3–5 nm well-dispersed on TiO2(B) nanosheets and in intimate contact. The HRTEM image of 10-NT (Fig. 3d) simultaneously reveals (1 1 1) lattice plane of Ni2P (0.22 nm) as well as the (0 2 0) and (0 0 3) plane of TiO2(B) (0.19 nm and 0.21 nm). The EDX elemental mapping of Ti, O, Ni, and P was shown in the bottom of Fig. 3, further confirm the homogeneous distribution of the elements Ni and P on TiO2(B) nanosheet. These results prove that the Ni2P quantum dots were successfully loaded onto the surface of TiO2(B) nanosheets. In the synthesis, a control experiment was performed using the same method, but without TiO2(B) to obtain a pure Ni2P sample. As seen in Fig. 3a, the as-prepared pure Ni2P forms aggregates with morphology and size obviously different from the quantum dots. This result indicates that the huge surface area of TiO2(B) can provide a high density of sites for Ni2P crystal nucleation, and the numerous active sites inhibit Ni2P growth at the same time, leading to the formation of Ni2P quantum dots on TiO2(B). The light harvesting properties of the samples were investigated by UV–vis diffused reflectance spectra (Fig. 4). Pure TiO2(B) nanosheets show an absorption edge at 363 nm, the Kubelka-Munk function plot of pure TiO2(B) nanosheets (inset of Fig. 4) shows a direct band gap of 3.60 eV, which is 0.4 eV higher than reported TiO2(B) [59], as a consequence of quantum confinement. After loading with Ni2P, the absorption edge of the TiO2(B) does not shift showing that the Ni2P deposited onto the TiO2(B) nanosheet surface is not doping into the TiO2(B) lattice. Furthermore, compared to pure TiO2(B) nanosheets, all the Ni2P/TiO2(B) composites display enhanced absorption in the visible region, potentially beneficial for improved photocatalysis. The absorption of the Ni2P/TiO2(B) composites increases with the Ni2P

Fig. 7. Simulated charge distributions for Ni2P/TiO2(B) composite model, shows the electronic properties of the interactions between Ni2P and TiO2(B) surfaces.

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volume. X-ray photoelectron spectroscopy (XPS) was used to probe the surface chemistry of the samples. High-resolution Ti 2p spectra of TiO2(B) and 10-NT are shown in Fig. 6a. For TiO2(B), a pair of peaks located at 457.9 and 463.6 eV could be ascribed to the binding energy of Ti 2p3/2 and Ti 2p1/2. After coupling with Ni2P, the shape of the Ti 2p spectrum of 10-NT is similar to the TiO2(B) Ti 2p spectrum. Note that the characteristic peaks of Ti 2p3/2 and Ti 2p1/2 shifted slightly up to 458.3 and 464.1 eV. Fig. 6b presents the O 1s spectra of pure TiO2(B) and 10-NT. For TiO2(B) O 1s spectrum, the deconvoluted peaks correspond to three oxygen-containing chemical bonds: oxide (Ti-O-Ti) 529.6 eV, hydroxide (Ti-O-H) at 530.8 eV and water molecule (H-O-H) at 532.5 eV [31,60,61]. After coupling with Ni2P, the Ti-O-Ti and Ti-O-H peak of 10-NT shifted slightly to 529.8 and 530.9 eV, respectively. The weak peak appearing at 529.5 eV in 10-NT could be ascribed to the Ni-O bond [62]. In the Ni 2p spectra (Fig. 6c), three distinct peaks at 853.1, 856.2 and 861.5 eV respectively belong to Niδ+ (0 < δ < 2) in Ni2P, the oxidized Ni species (Ni2+) and the satellite of Ni 2p3/2 peak, respectively [51,63]. Furthermore, three additional distinguishable signals at 870.2, 874.0 and 879.8 eV correspond to Niδ+ in Ni2P, the oxidized Ni species and the satellite of the Ni 2p1/2 peak, respectively [51,63]. The ratio of the peak areas of Niδ+ to Ni2+ in 10-NT is 2.25, which is larger than that of Ni2P (1.18), indicating that Ni2P in 10-NT obtains electrons from TiO2(B) and then part of the Ni2+ converts into Niδ+. The P 2p spectra (Fig. 6d) displays two characteristic peaks at 129.5 and 133.0 eV, which correspond to the Pδ− in Ni2P and surface nickel phosphate species due to air contact [63]. Compared to pure Ni2P sample, the Ni and P signals of 10-NT are weak due to the low Ni2P content embedded on TiO2(B). The observed Ni 2p and P 2p peaks are in accordance with other reports [64–67], confirming the formation of Ni2P in the product. For 10-NT, the binding energies of Ti 2p3/2 and Ti 2p1/2, and Ti-O-Ti and Ti-O-H peaks in 10-NT exhibit positive shift compared to pure TiO2(B), which can be ascribed to the electrons transfer from the TiO2(B) to Ni2P at the interface. As for Ni 2p and P 2p spectra, all peaks of 10-NT shift negatively compare to pure Ni2P, further proves the migration of electrons from TiO2(B) to Ni2P [68]. The electrons transfer will further be proved by UPS spectra, DFT calculation, electrochemical measurements in following sections. To illustrate the interface charge transfer between Ni2P and TiO2(B), the electronic properties of the interactions between Ni2P and TiO2(B) surfaces were further studied by density functional theory (DFT) computational calculation. As shown in Fig. 7, the acceptable lattice mismatch (< 6%) and good dangling bond saturation allow the desired intimate interface between Ni2P and TiO2(B), for high interfacial charge transfer efficiency. To further understand the photogenerated e−–h+ separation, migration and charge transfer between Ni2P and TiO2(B) in the photocatalysis reaction, the band structures of Ni2P and TiO2(B) were investigated by the XPS valence band (VB-XPS) spectra and UPS spectra. Fig. 8a shows the VB-XPS spectra; TiO2(B) and 10-NT exhibited edge values of 2.42 and 2.20 eV, respectively, consistent with previous reports [69]. The VB-XPS value of Ni2P is −0.10 eV, indicating Ni2P has metallic characteristics [70,71]. The UPS spectra show the valence band maximum (EVBM) (Fig. 8b) and secondary electron cutoff (ESECO) (Fig. 8c) of TiO2(B) and Ni2P. The work function of TiO2(B) and Ni2P are 4.55 eV and 4.77 eV, respectively, suggesting a Schottky junction exists between the metalloid Ni2P and TiO2(B), which resulted in effective separation of e−−h+ pairs and reduction of charge recombination. The build-in electric field drives electrons flow to Ni2P, and boosts the H2 evolution rate.

Fig. 8. (a) XPS valence band spectra with the top of the valence bands marked. (b and c) UPS spectra of pure TiO2(B) nanosheets and pure Ni2P in (b) valence region and (c) secondary electron cutoff region.

dosage. As a result, the colour of the Ni2P/TiO2(B) powder changed gradually from white to dark (as shown in the inserted images). Fig. 5 shows the N2 adsorption-desorption isotherms of pure TiO2(B), Ni2P/TiO2(B) composites with varying doses of Ni2P and pure Ni2P. The TiO2(B) and Ni2P/TiO2(B) composites show type IV nitrogen adsorption-desorption isotherms with type H3 hysteresis loops. Table 1 summarizes the Ni/Ti mass ratios, specific surface areas and pore volume of the as-prepared samples. When the loading of Ni2P ranges from 1% to 15%, their specific surface areas remained large at about 248–265 m2 g−1, almost the same as that of pure TiO2(B) (260.88 m2 g−1). In contrast, when the loading of Ni2P exceeds 15%, the specific surface areas reduced dramatically. For a small amount of Ni2P precursor, the Ni2P occupies nucleation sites on the nanosheet surface forming quantum dots. Once nucleation sites are saturated, further increase of Ni2P precursor leads to the aggregation of Ni2P to form large particles decreasing the specific surface area and pore

3.2. H2 production of Ni2P/TiO2(B) composites H2 evolution potential is highly dependent on the pH of the solution, the H2 evolution potential value shifts with pH by 0.05916 V per pH unit so it would be beneficial to carry out the photocatalysis in a solution with lower pH values. However, according to previous reports, 6

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Fig. 9. (a) Photocatalytic H2 evolution rates of 10-NT measured at room temperature with different pH values; (b, c) Photocatalytic H2 evolution performance of all the as-prepared samples; (d) Cycling runs for photocatalytic H2 evolution of 10-NT. Table 2 Comparation of photocatalytic H2 evolution performance with other recent TiO2 based photocatalysts. Photocatalyst* NiCoP/TiO2(A/R) NiO/TiO2(A) TiO2(A/R)/NiS Co(OH)2/TiO2(A/R) rGO/Pt-TiO2(A/R) CuS/TiO2(A) Ni2P/TiO2(B)

H2 production rate −1

−1

1.54 mmol g h 0.337 mmol g−1 h−1 0.655 mmol g−1 h−1 1.964 mmol g−1 h−1 1.075 mmol g−1 h−1 1.262 mmol g−1 h−1 3.966 mmol g−1 h−1

Light resource

Sacrificial reagent

Ref.

300 W 300 W 350 W 300 W 500 W 300 W 300 W

20 vol% methanol aqueous solution 25 vol% methanol aqueous solution 20 vol% methanol aqueous solution 25 vol% methanol aqueous solution 10 vol% triethanolamine aqueous solution 0.35 M Na2S and 0.25 M Na2SO3 aqueous solution 20 vol% methanol aqueous solution

[50] [75] [14] [76] [77] [78] This work

Xe-lamp Xe-lamp Xe-lamp Xe-lamp Xe-lamp AM 1.5G filter Xe-lamp Xe-lamp

* A: anatase, R: rutile.

Ni2P exhibits very high activity in alkaline solutions, ~7 mV overpotential in alkaline solutions, and ~30 mV overpotential in acidic solutions [72]. Consequently, the pH of the solution is likely to have a significant effect on the photocatalytic performance of the Ni2P/ TiO2(B) composites. To establish the optimum conditions for H2 evolution, we studied the H2 evolution of 10-NT samples in different pH values as shown in Fig. 9a. The pH values were adjusted with lactic acid. As expected, when the pH decreased from 7 to 4, the photocatalytic H2 rate of 10-NT was promoted reaching a maximum at pH 4. Less amounts of H2 were obtained when the pH was further decreased to pH 2. The photocatalysis efficiency of the as-prepared samples was evaluated at pH 4 for the hydrogen production experiments. The performance of the photocatalysts is given in Fig. 9c. For pure TiO2(B) nanosheets, only a small amount of H2 (0.262 mmol h−1 g−1) was produced. The photocatalytic H2 evolution rate of pure Ni2P is extremely small (see Fig. 9b), indicating that it is inactive as a photocatalyst. When the dose of Ni2P increased from 1% to 10%, the performance of the composite improved remarkably. The highest activity is

Fig. 10. Hydrogen production rates of 10-NT for the thermal catalytic, photocatalytic and synergistic photothermal catalytic processes. The thermal and photothermal processes were measured at 50 °C, 70 °C and 90 °C.

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Fig. 11. (a) EIS Nyquist plots of TiO2(B),10-NT and Pt-TiO2(B); (b) Transient photocurrent responses (I–t curves) of TiO2(B), 10-NT and Pt-TiO2(B) in 0.1 M Na2SO4 aqueous solution under dark and light irradiation; (c) Linear sweep voltammetry of TiO2(B), Pt-TiO2(B) and 10-NT; (d) Photocurrents of 10-NT at 25 °C, 50 °C, 70 °C and 90 °C.

photocatalytic H2 evolution activity during six runs with a total continuous reaction time of 30 h. Negligible degradation was found. This result further confirms the stability and reusability of Ni2P/TiO2(B). The excellent photocatalytic stability of Ni2P/TiO2(B) is due to high chemical stability of Ni2P for H2 evolution in aqueous media [73,74]. The photothermal and thermal catalysis (without light irradiation) of 10-NT were further investigated as a function of temperature. As illustrated in Fig. 10, the increased temperature remarkably enhances the H2 evolution giving evolution rates of 6.752, 14.271, 20.129 mmol g−1 h−1, at 50 °C, 70 °C and 90 °C, respectively. Note the photothermal H2 rates far exceed the sum of the photocatalytic and the thermal catalytic H2 evolution rates. The higher temperature not only facilitates charge carrier migration but also accelerates the desorption of the molecular hydrogen formed at the nanosheet surfaces, this synergy greatly enhances the H2 evolution rate of the photothermal catalysis. Fig. 12. Photoluminescence spectra of TiO2(B) and Ni2P/TiO2(B) composites.

3.3. Photocatalytic H2 evolution enhancement mechanism achieved with 10-NT, giving remarkable H2 evolution of 3.966 mmol h−1 g−1, superior to that of the optimum amount of 1 wt% Pt loaded TiO2(B) (3.893 mmol h−1 g−1), 15 times higher than that of TiO2(B) nanosheets and 81 times higher than that of commercial Degussa P25. What’s more, compared with other TiO2 based photocatalysts (listed in Table 2), the photocatalytic H2 evolution activity of Ni2P/TiO2(B) composite is very competitive. The AQE of 10-NT measured at 313 nm is as high as 31%. On further increasing the Ni2P loading, the photocatalytic efficiency fell, due to the Ni2P on the surface shielding some of the light from the underlying catalyst. As well as high H2 evolution rates, stability is a crucial requirement for a photocatalyst in practical applications. The stability of 10-NT was checked by cycling runs as shown in Fig. 9d. Impressively, it shows highly stable

Light-absorption, migration of charge, and surface reaction rate are vital for a photocatalytic system. Since the photons absorbed by the Ni2P nanoparticles give negligible contribution to the photocatalytic reaction, its role as a cocatalyst is its remarkable impact on facilitating charge migration and accelerating surface reaction rates of the TiO2(B) nanosheets. The charge-transfer resistances can be directly evaluated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 11a, the arc radius in the Nyquist plots of 10-NT (50 O) was smaller than pure TiO2(B) (100 O) and even smaller than that of Pt-TiO2(B) (55 O). The small resistance value for electron transfer of the Ni2P cocatalyst shows it has increased electron transfer at the photocatalyst/solution interface. Further evidence is provided by the transient photocurrent 8

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Fig. 13. Proposed photocatalytic mechanisms of Ni2P/TiO2(B) composite.

cocatalyst on the surface of ultra-thin TiO2(B) nanosheets designed to give high-efficiency of hydrogen production. The Ni2P nanoparticles were firmly anchored onto the TiO2(B) nanosheets to form the hybrid 0D/2D architecture. The photocatalytic H2 evolution results demonstrated Ni2P can act as an earth-abundant, low-cost, noble-metal-free cocatalyst and remarkably enhances the photocatalytic properties of TiO2(B). Noticeably, the 10-NT sample exhibited an extremely high H2 evolution rate (3.966 mmol h−1 g−1), 15 times higher than TiO2(B) nanosheets and even superior to optimum amount of 1 wt% Pt loaded TiO2(B). Importantly, the Ni2P/TiO2(B) catalyst displayed excellent stability during cycling tests. For the synergistic photothermal catalysis H2 evolution is substantially enhanced further. The H2 evolution rate reached 20.129 mmol g−1 h−1 at 90 °C. EIS, transient photocurrent responses, SLV and PL spectra revealed that during photocatalysis the Ni2P quantum dots promote charge transfer, hinder charge recombination and accelerate the reaction rate. It is anticipated that this work will serve as a demonstrator for developing low cost, Pt replacement photocatalysts for the generation of green sustainable H2 energy.

responses shown in Fig. 11b. 10-NT exhibited higher photocurrent density than pure TiO2(B) and even higher than Pt-TiO2(B) in agreement with both the EIS measurements and the photocatalysis. Linear sweep voltammetry (LSV) was also carried out as shown in Fig. 11c, the fastest increased cathodic current density for sample 10-NT indicates more active sites and faster electron migration for the hydrogen evolution reaction. Fig. 11d displays the photocurrents of 10-NT at different temperature. The higher photocurrents are observed with the increase of temperature showing the separation of charge carriers is promoted at higher temperatures. The following key conclusions can be drawn from these results about the remarkably improved H2 production performance. Ni2P acts as an excellent cocatalyst by promoting charge migration both on the surface of the photocatalyst and at the photocatalyst/solution interface and so speeds up surface reactions for hydrogen generation. We employed photoluminescence (PL) spectrum to detecting the separation and recombination tendency of the photogenerated e−–h+ pairs of the photocatalysts. Fig. 12 shows the PL spectra of TiO2(B) and Ni2P/TiO2(B) composites. The broad emission peaks from 350 to 550 nm corresponds to the absorption onset of TiO2(B). After hybrid with Ni2P, the PL intensity obviously declined, indicates Ni2P can effective promote the charge transfer thereby inhibit the charge recombination in TiO2(B). The proposed pathway for the photocatalysis is illustrated schematically in Fig. 13. Photogenerated electrons are excited from the valence band to the conduction band of the TiO2(B) nanosheets under light illumination. The intimate interface between Ni2P and TiO2(B) favors for high interfacial charge transfer efficiency. The Schottky junction between metallic Ni2P and TiO2(B) can suppress the backflow of injected electrons from semiconductor to metallic Ni2P. The Ni2P quantum dots act as the electron acceptor and captured electrons from TiO2(B). The less mobile photogenerated holes in the valence band of the TiO2(B) are quickly quenched by the methanol scavenger (Fig. 13a). As previously reported electrocatalyst Ni2P can serve as superior cocatalyst to facilitate charge separation electrons. Electrons injected into phosphides raise the quasi-Fermi level, resulting in a greater driving force for hydrogen evolution, facilitating efficient charge migration at the photocatalyst/solution interface [79]. Finally, the Ni2P quantum dots on TiO2(B) nanosheets act as an electron reservoir accelerating the reaction rate. H+ is then reduced to produce molecular hydrogen on the Ni2P quantum dots (Fig. 13b).

Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 11574076, 21801071, 11874144, 11674087, 51602096), 111 project (D18025), the Natural Science Foundation of Hubei Provincial (Grant No. 2018CFB171), Wuhan Science and Technology Bureau (2018010401011268) and the open foundation (No. 20180030) of the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. References [1] N. Armaroli, V. Balzani, The future of energy supply: challenges and opportunities, Angew. Chem. Int. Ed. 46 (2006) 52–66. [2] A. Meng, L. Zhang, B. Cheng, J. Yu, Dual cocatalysts in TiO2 photocatalysis, Adv. Mater. 31 (2019) 1807660. [3] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 414 (2001) 625–627. [4] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Photocatalyst releasing hydrogen from water, Nature 440 (2006) 295. [5] Z. Wang, C. Li, K. Domen, Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting, Chem. Soc. Rev. 48 (2019) 2109–2125. [6] B. Dong, J. Cui, Y. Gao, Y. Qi, F. Zhang, C. Li, Heterostructure of 1D Ta3N5 nanorod/

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