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Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts
Accepted Manuscript Title: Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts Authors: Rui Song, Wu Zhou, Bing Luo, Dengwei Jing PII: DOI: Reference:
S0169-4332(17)31256-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.221 APSUSC 35902
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-1-2017 25-4-2017 26-4-2017
Please cite this article as: Rui Song, Wu Zhou, Bing Luo, Dengwei Jing, Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.221 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts
Rui Song, Wu Zhou, Bing Luo, Dengwei Jing*
Corresponding author: Tel.:+86-29-82668769; Email:[email protected] International Research Center for Renewable Energy & State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
Highlights Metal phosphides like Ni2P, NiCoP, and FeP act as electrocatalysts have been employed here as cocatalysts for photocatalytic H2 production. NiCoP(1 wt%)/TiO2 composites showed the highest photocatalytic activity toward H2 production , which was about thirteen times that of TiO2. Electrons injected from TiO2 into metal phosphides play a role similar to that of an external bias in electrocatalysis. Mott-Schottky (MS) analysis was performed to understand the mechanism of the Ni2P, NiCoP, and FeP electrocatalysts as cocatalysts for the photocatalytic H2 generation.
Abstract In this work, electrocatalysts like the metal phosphides Ni2P, NiCoP, and FeP, can serve as cocatalysts of TiO2 to form efficient composite photocatalysts for
the H2 generation hydrogen
generation from an aqueous methanol solution. On comparing Ni2P, NiCoP, and FeP and optimizing their proportions, the NiCoP(1wt%)/TiO2 composite was found to exhibit the highest activity toward photocatalytic H2 production (1.54 μmol·h-1·mg-1), which is about thirteen times 1
that of the naked TiO2 nanoparticles. Mott-Schottky (MS) analysis indicated that the large upward shift or band bending of the Fermi energy level (EF) in metal phosphides was responsible for the enhanced activity of the composites. The steady-state photoluminescence (PL) spectra and photocurrent transient response further confirmed that the enhanced photoinduced charge transfer and band separation after TiO2 was integrated with the metal phosphides. Thus, these electrocatalysts were shown to be efficient cocatalysts that can replace noble metals as low-cost photocatalytic H2 production. Key Words: Photocatalysis, Hydrogen production, Metal phosphides, TiO2
1. Introduction The increasingly serious global energy crisis and the extreme pollution caused by the burning of fossil fuels has resulted in an urgent need for the development of environment friendly and renewable energy resources [1]. Hydrogen is an ideal candidate for the replacement of fossil fuels as it is environmentally benign and recyclable [2,3]. Among the various methods of hydrogen production, photocatalysis
has receivedsignificant attention as a viable method for the
conversion of solar energy into H2 [4,5]. Various photocatalysts have been developed for this purpose [6-8]. It is generally believed that loading of a cocatalyst on an active substrate such as the TiO2 is essential for the realization of efficient photocatalytic hydrogen production[9-12]. Furthermore, noble metals like Pt areeffective for promoting evolution of H2. However, noble metals are expensive and scarce, which limits their wider applicability. It is therefore highly desirable to find alternate non-noble metal cocatalysts that are easily available and have a lowcost [13]. 2
Molybdenum disulphide (MoS2) has
previously been used as a highly active and effective
substitute for Pt in the electrochemical hydrogen evolution reaction [14]. In addition, several heterogeneous catalysts composed of earth abundant elements have been utilized for electrochemical generation of H2. These include borides [15], carbides [16], nitrides [17], sulfides [18], selenides [19], and even metal-free catalysts [20]. Recently, it has been reported that transition metal phosphides (TMPs) such as Ni2P [21], CoP [22], and FeP [23] show high electrocatalytic activity in H2 evolution. The use of TMPs in electrocatalytic processes is advantageousowing to their ability to generate high current density, and their low overpotential and excellent stability.Even though, additional voltage is often necessary to drive the electrocatalytic reaction. When photocatalysts are irradiated with energy greater than their band gap, electron-hole pairs are created that migrate to the surface of the catalyst and initiate the reduction of H+ to H2. It is assumed that the above mentioned characteristics of metal phosphides might also be exploited for photocatalytic reactions when these metal phosphides are loaded on semiconductors and used as cocatalysts. Several efforts have been made to developtransition-metal based phosphides as cocatalysts to promote the photocatalytic hydrogen evolution. Du et al. have reported that nickel phosphide (Ni2P) as a cocatalyst with one-dimensional semiconductor nanorods forms a well-designed integrated photocatalyst that shows significantly enhanced efficiency and durability for the photoinitiated generation of hydrogen from water [24]. CdS modified with Ni2P derived from a metal organic framework (MOF) has also been found toimprove the efficiency of photocatalytic H2 production [25]. In addition, CdS quantum dots have been used as light absorbing materials 3
with metallic MoP as the cocatalyst, and the optimized system showed a high H2 evolution rate of 1100 μmol∙h−1 under visible light (λ ≥ 420 nm), which is comparable to the production rate when Pt was used as the cocatalyst [26]. The decoration of Co2P nanohybrids of uniform morphology on reduced graphene oxide (RGO) produced a material that exhibited high activity toward H2 evolution under visible light irradiation after it was sensitized by Eosin Y [27]. Despite these significant advances in photocatalytic H2 production by using electrocatalysts as cocatalysts, to the best of our knowledge, a systematic investigation of the addition of metal phosphides such as Ni2P, NiCoP, and FeP as cocatalysts onto the commercially available TiO2 has not been reported till date. Owing to the specific design of such a substrate based photocatalyst, it is difficult to distinguish between the roles played by the substrate and the cocatalyst. Therefore, in this study,commercially available TiO2 has been used, and by excluding the specific contribution of the substrate, it becomes possible to understand the underlying mechanism of action of metal phosphides as cocatalysts for photocatalytic reactions. Therefore, this strategy is valuable for the discovery of efficient noble-metal free photocatalysts in the future. This work describes for the first time, the use of electrocatalytic metal phosphides (Ni2P, NiCoP, and FeP) as cocatalysts, commercially available TiO2 as the main catalyst, and methanol as a sacrificial electron donor for efficient photocatalytic H2 production. The morphology and structure of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO2 composites have been determined by X-ray diffraction
(XRD)
and
transmission
electron
microscopy
(TEM).
The
steady-state
photoluminescence (PL) spectra and photocurrent transient response were studied and used to investigate the photo-induced charge transfer and separation processes. In addition, the Mott-Schottky (MS) analysis was carried out to understand the mechanism of the electrocatalytic 4
metal phosphides (Ni2P, NiCoP, and FeP) as cocatalysts for photocatalytic hydrogen evolution. 2. Experimental section 2.1 Chemicals and Materials All reagents were of analytical grade and used as received without further purification. Nickel acetylacetonate (Ni(acac)2), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), iron oxide (Fe3O4), sodium hypophosphite (NaH2PO2·H2O), sodium sulfate (Na2SO4), urea, n-hexane, toluene, acetone, methanol, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Tri-n-octylphosphine (TOP, 97%) was purchased from Strem Chemicals, Inc., while 1-octadecene (ODE, 90%) and oleylamine (70%) were obtained from Sigma-Aldrich. Commercial TiO 2 P25 was purchased from Evonik Degussa Corporation. 2.2 Preparation of the MP/TiO2 (M = Ni2, NiCo, Fe) composites Synthesis of Ni2P nanoparticles. This reaction involves the decomposition of a phosphine that may liberate phosphorus. Therefore, it should be regarded as a highly corrosive and flammable process. Such reactions should only be performed under vacuum conditions by professionally trained personnel. Typically, nickel phosphide nanoparticles were prepared following a modified literature procedure [28]. Briefly, Ni(acac)2 (250 mg, 0.98 mmol), 1-octadecene (4.5 mL, 14.1 mmol), and oleylamine (6.4 mL, 19.5 mmol) were added to a 50 mL three-necked, round bottom flask, which was equipped with a thermometer and a condenser. The reaction mixture was heated under continuous stirring in order to remove water and other low-boiling impurities. Subsequently, tri-n-octylphosphine (2 mL, 4.4 mmol) was injected into the flask which was first put under a slight vacuum. The round bottom flask was then filled with Ar 5
and heated to 300 °C within 30 min. A change in the colour of the solution was observed to black at 220 °C. After heating the solution at 300 °C for 2 h,the heating mantle was turned off until the solution cooled to 200 °C. The flask was then removed from the heating mantle and cooled to room temperature. Subsequently, the mixture was separated by centrifugation and washed three times with 1:3 (v/v) hexane:isopropanol. Finally, the precipitate was suspended and stored in toluene for further use. Synthesis of NiCoP nanoparticles. NiCoP nanoparticles were prepared byfollowing a previously reported method with some modifications [29]. In the first step, NiCo(OH)2 precursor was synthesized using a co-precipitation method. In
a typical process, 6.4 g of NaOH was
dissolved in 20 mL of deionized water and stirred continuously at room temperature for 20 min. Subsequently, 25 mM Ni(NO3)2·6H2O and 50 mM Co(NO3)2·6H2O containing 20 mL of deionized water was added dropwise to the aqueous NaOH solution. After continuous stirring for 30 min, the mixture was treated hydrothermally at 100 °C for 24 h. The NiCo(OH)2 precursor was washed three times alternatively with deionized water and ethanol , and then dried under vacuum at 60 °C. In the next step, the washed NiCo(OH)2 precursor and NaH2PO2·H2O were blended mechanically and ground in the mortar. The molar ratio of the total metal (Ni and Co) to P was 1:5. Finally, the fine powders were kept at 300 °C for 2 h in a quartz tube with a heating rate of 5 °C∙min−1 protected under an inert argon gas flow. The obtained products were thoroughly washed with deionized water to remove any residual salts and dried under vacuum at 60 °C. Synthesis of FeP nanoparticles.
FeP nanoparticles were prepared by following a
previously reported method with slight modifications [30]. Fe3O4 and NaH2PO2·H2O were blended mechanically and ground in the mortar. The molar ratio of Fe to P was 1:5. The well 6
dispersed mixture was then transferred into a quartz tube and calcined in a tubular furnace under a flow of Ar gas for 3 h with a heating rate of 2 °C∙min−1 up to 400 °C to obtain the FeP nanoparticles. The obtained nanoparticles were subsequently washed several times with deionized water and ethanol and dried under vacuum at 60 °C. Synthesis of MP/TiO2 (M=Ni2, NiCo, Fe) nanoparticles. TiO2 nanoparticles were added to 100 mL toluene and the suspension was stirred for 0.5 h. Subsequently, the well dispersed suspension of Ni2P, NiCoP, and FeP in toluene was added and the mixture was stirred for another 1.5 h. In the next step, the mixture was collected by centrifugation and washed with acetone along with the use of mild sonication. The sediments were dried at 80 °C for 0.5 h. Finally, the obtained samples were heated for 2.5 h at 450 °C under Ar atmosphere in order to remove the remnants of impurities on the surface and to make sure that a potent solid-solid interface had formed between the metal phosphides (MP) and TiO2 nanoparticles. The MP/TiO2 composite with different amounts of metal phosphide nanoparticles is defined as MP(X wt%)/TiO2, where X ranges from 0 to 5. 2.4 Characterization The particle size and morphology of the samples were analysed from TEM images obtained on a FEI Tecnai G2 F30 transmission electron microscope at an accelerating voltage of 300 kV. The crystallographic structures and other information on chemical composition of the composite particles was obtained by powder XRD using Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 10° min−1. The absorption spectra of the samples were measured on a Hitachi U4100 UV-Vis spectrophotometer over a range of 300−800 nm and the photoluminescence spectra (PL) were measured at room temperature on a PTI QM-4 fluorescence spectrophotometer. 7
2.5 Photoelectrochemical measurements Photoelectrochemical measurements were performed on a CHI 760D scanning potentiostat (CH Instruments). In a standard three-electrode cell system, the fluorine doped tin oxide glass electrode (FTO) coated with the photocatalyst, a Pt plate, and a Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. The working electrodes were prepared by dropping 200 μL of a suspension of Ni2P/TiO 2, NiCoP/TiO2, FeP/TiO2, or TiO2 (3 mg of the photocatalyst added into 3 mL ethanol and 600 μL Nafion mixed solution) onto the surface of a FTO plate (2 cm × 2 cm). Subsequently, the electrodes were dried at room temperature for 12 h. A 500 W Xenon lamp coupled with an AM1.5 filter was used as the light source and a 0.5 M Na2SO4 solution was used as the electrolyte. The photoresponsive signals of the samples were measured under chopped light at 0.6 V vs. Ag/AgCl. Mott-Schottky analyses were performed using 5 kHz frequency in the dark. The carrier densities were calculated from the Mott–Schottky equation:
1 / C 2 (2 / e0 0 N d )(V VFB kT / e0 ) where C, e0, ε, ε0, Nd, V, VFB, kT/e0 refer to the specific capacitance,
electron charge, dielectric
constant, permittivity of vacuum, carrier density, applied potential of the electrode, flat band potential, and a temperature-dependent correction term, respectively. 2.6 Photocatalytic hydrogen production The photocatalytic hydrogen activities of MP/TiO2 (M = Ni2, NiCo, Fe) were evaluated by using a 300 W Xenon lamp as the UV-Vis light source and the photocatalytic reactions were carried out in a side-irradiation reactor. The gases sampled with a syringe were analysed with a TCD SP2100 gas chromatograph (TDX-01 column) using nitrogen as the carrier gas. For a typical 8
photocatalytic H2 generation test, 20 mg of the photocatalyst powder was dispersed by a magnetic stirrer in the reactor containing 80 mL of an aqueous solution of methanol (20 vol%) used as the sacrificial reagent. The reactor was purged with N2 for 30 min before hydrogen production to completely remove oxygen and
continuously stirred to keep the photocatalyst particles
suspended throughout the reaction. All reactions were performed at room temperature. 3. Results and discussion 3.1 Morphologies and crystalline properties Fig. 1
Fig. 1 shows the XRD patterns of Ni2P, NiCoP, FeP, TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2 photocatalysts, respectively. The characteristic peaks at 40.7º, 44.6º, 47.4º, and 54.21º in Fig. 1(a) forNi2P (JCPDS NO. 03-0953) match well with the XRD pattern previously reported by other authors [31]. Fig. 1(b) as-synthesized NiCoP. The peaks
shows the XRD pattern of the
at ca. 2θ = 30.6º, 40.9º, 44.9º, 47.5º, 54.7º, and 55.3º can be
attributed to the (110), (111), (201), (002), (110), and (300) crystal planes, respectively, of the hexagonal phase (JCPDS NO. 71-2336) [29]. Similarly, there are several sharp diffraction peaks at ca. 2θ = 30.8º, 32.8º, 37.2º, 48.4º, and 50.4º that correspond to the (002), (011), (111), (103), and (211) crystal planes of FeP (JCPDF NO. 65-2595) [23], respectively, as shown in the Fig. 1(c). For TiO2 (Fig. 1(d)), very intense and narrow diffraction peaks are observed, demonstrating the high crystallinity of the photocatalyst samples. The broad peaks around 2θ = 25.3º, 37.8º, 48.1º, 53.9º, 55.1º, 62.6º, 69.0º, 70.3º, and 75.0º can be assigned respectively to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes of the anatase phase [32]. Other sharper diffraction peaks observed in the spectrum can be attributed to the indices of rutile phase. For the 9
XRD pattern of Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2, no obvious diffraction peaks corresponding to Ni2P, NiCoP, and FeP, respectively, could be detected and the spectra showed no significant difference from that of TiO2. This may be attributed to the very strong diffraction peaks of TiO 2 and the relatively small amount of metal phosphides on TiO2. Fig. 2
The TEM images of Ni2P, NiCoP, and FeP are shown in Fig. 2. It is evident from Fig. 2(a) that the synthesized Ni2P nanoparticles are uniform in size with an average diameter of 13 ± 2 nm. Interestingly, most of the Ni2P nanoparticles have a hexagonal shape. The corresponding high-resolution TEM (HRTEM) image (Fig. 2(b)) shows that the size of the lattice fringes of Ni2P is approximately 0.22 nm, which corresponds well to the (111) planes of Ni2P [31]. Fig. 2(c) and 2(d) are the TEM and HRTEM images of NiCoP nanoparticles, respectively. It is evident that NiCoP nanoparticles have the particle sizes ranging between 20−50 nm and the lattice distances are 0.28 and 0.33 nm, which are in good agreement with the (101) and (001) planes of NiCoP, respectively. Fig. 2(e) shows the TEM images of FeP nanoparticles which are ca. 100−200 nm in size. The corresponding HRTEM image of the selected area is given in Fig. 2(f) and reveals that the lattice fringes with interplanar distances of 0.39 nm correspond well to the (101) planes of FeP. Fig. 3
The TEM and HRTEM images of TiO2 are shown in Fig. 3(a) and Fig. 3(b), respectively. The fringes with d spacing of 0.35 nm correspond to the (101) plane of TiO 2. Fig. 3(c) and Fig. 3(d) show TEM and HRTEM images of Ni2P(X=1)/TiO2 photocatalysts, respectively. The fringes with d spacing of 0.22 nm can be attributed to the (111) crystalline plane of Ni 2P, while the d spacing of 0.35 nm corresponds to the (101) plane of TiO2 [33]. Fig. 3(e) and Fig. 3(f) show the TEM and 10
HRTEM images of NiCoP(X=5)/TiO2 respectively. In this case, the fringes with d spacing of 0.17 nm can be assigned to the (300) crystalline plane of NiCoP while those with the d spacing of 0.35 nm correspond to the (101) plane of TiO2. These results suggest that the NiCoP nanoparticles were successfully loaded on the TiO2 nanoparticles. The TEM image of FeP(X=5)/TiO2 nanoparticles is shown in Fig. 3(g). Its corresponding HRTEM image is shown in Fig. 3(h) wherein the fringes with d spacing of 0.24 nm may be assigned to the (111) plane of FeP and the d spacing of 0.35 nm ascribed to the (101) plane of TiO2. Therefore, it may be concluded that the FeP nanoparticles were also successfully loaded on the TiO2 nanoparticles. Fig. 4
The UV-Vis diffuse reflectance spectra (DRS) of the fabricated samples are shown in Fig. 4. For TiO2 nanoparticles, the light response lay within the range of 300−400 nm and has been previously attributed to the charge-transfer transition from the valence O 2p orbital to the conduction band (Ti 3d) [34]. In comparison with the spectrum of naked TiO2, the absorption edge spectrum showed no evident shift after TiO2 was loaded with Ni2P, NiCoP, or FeP,. The only difference for these three cases is that the intensity of the absorption had varied. The very small change in the light absorption edge spectra indicates that the metal phosphides are tightly anchored on the surface of TiO 2 instead of being doped into its lattice. 3.2 Photocatalytic H2 production Fig. 5
Fig. 5 shows the amount of H2 evolved over Ni2P/TiO2, NiCoP/TiO 2, and FeP/TiO2 photocatalysts with different loading amounts of Ni2P, NiCoP, and FeP, respectively. Fig. 5(a) illustrates the amount of H2 evolved when Ni2P(X wt%)/TiO 2 is used as the photocatalyst. It can 11
be seen that a negligible amount of H2 (2.11 μmol) is generated over bare TiO2 after irradiation for 3 h. However, an integration of only 0.19 wt% of Ni2P on TiO2 leads to a significantly enhanced amount of H2 generated (42.77 μmol). With further increase in the loading amounts of Ni2P on TiO2, the highest photocatalytic activity (85.65μmol) is achieved for Ni2P(0.38 wt%)/TiO 2. However, the amount of H2 evolved reduced to 30.19 μmol when 1.0 wt% of Ni2P is utilized. The excess amount of Ni2P is assumed to shelter the incident light from reaching the TiO 2 surface, resulting in a decreased photocatalytic activity. In addition to Ni2P, other metal phosphides such as NiCoP and FeP were also studied for comparison. Fig. 5(b) and Fig. 5(c) show the amount of H2 evolved over NiCoP(X wt%)/TiO2 and FeP(X wt%)/TiO2, respectively. It can be seen that the amount of H2 generated first increases and then decreases. The highest amounts of H2 generated after irradiation for 3 h are 96.36 and 47.69 μmol for NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2, respectively. The reduced photocatalytic activity on further increase in the loading of metal phosphides can be attributed to the intense light absorption by NiCoP and FeP themselves, which consequently reduces the amount of light arriving at the TiO2 surface [35]. In order to study the effect of different phosphides electrocatalysts in the role of a cocatalyst for the promotion of the photocatalytic activity in the H2 evolution reaction, the amounts of H2 evolved and the corresponding H2 production rates upon irradiation for 3 h were compared for Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2 are shown in Fig. 5(d). The amount of H2 evolved and the corresponding production rates were 225.36, 246.91, 123.02 μmol and 1.41, 1.54, 0.77 μmol·h-1·mg-1, respectively. The remarkably improved H2 production activities of the phosphide-modified TiO2 were 12.5, 13.6, and 6.8 times higher than that of naked TiO2. These results indicate that the metal phosphide electrocatalysts are efficient cocatalysts for the 12
photocatalytic production of H2. Furthermore, among the tested metal phosphides, NiCoP is most effective. 3.3 The possible mechanism of photocatalytic H2 production Fig. 6
Table 1
As shown by the results of UV-Vis spectra given in Fig. 4, the absorption edge spectrum of TiO2 does not show any particular shift after loading with metal phosphides. Therefore it may be reasonably concluded that light absorption is not the dominant factor responsible for the enhanced photocatalytic activity. In order to understand the mechanism for the enhanced H2 production activity of metal phosphides/TiO2 composites compared to the bare TiO 2 nanoparticles, a possible reaction schemeof the charge transfer processes in context of the energy band levels is required. Therefore,
Mott-Schottky
(MS)
analyses
were
conducted
for
Ni2P(X=0.38)/TiO2,
NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2. Fig. 6(a) shows the Mott-Schottky plots of TiO2 and the various metal phosphides Ni2P, NiCoP, and FeP, before their combination. The values of the flat-band potential (Efb) can be determined from the intercepts of the linear part of the Mott-Schottky plots on the x axis and can be used to approximately estimate the conduction band potentials (ECB) for semiconductors and the Fermi energy level potential (EF) values for the metal phosphides [36]. The ECB value of TiO2 and EF values of Ni2P, NiCoP, and FeP vs. Ag/AgCl were determined to be -0.47, -0.29, -0.38, and -0.15 V, and these values vs. NHE were calculated as -0.27, -0.09, -0.18, and 0.05 V, respectively, as shown in Table 1.The calculated band diagrams were obtained by combining the MS and UV-Vis results for metal phosphides and TiO2 before they were assembled together and are given 13
in Fig. 6(b) [37,38]. On comparing the obtained flat-band potential position of TiO2 with Ni2P, NiCoP, and FeP, it is evident that the ECB for TiO 2 and EF for Ni2P and NiCoP are negative values but the EF for FeP is a positive value versus 0 V of the H+/H2 potential (NHE). Therefore, theoretically, the electrons in Ni2P and NiCoP can reduce H+ to H2 while it is impossible to achieve this transformation with FeP nanoparticles. In this context, as metal phosphides come into contact with TiO2, the photogenerated electrons from ECB of TiO2 can be injected into the EF of metal phosphides because ofthe much more negative potential of the conduction band [39]. A quick charge separation taking place in this manner clearly avoids charge recombination in TiO2 and hence improves its photocatalytic activity. In addition, the injected electrons can accumulate in the metal phosphides, which may be expected to induce a much more negative EF [40]. In order to provide further evidence in support of the abovementioned hypothesis, the Mott-Schottky analyses of metal phosphides/TiO2 composites were also carried out and these results are shown in Fig. 6(c). The Efb values of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO2 composites are -0.36, -0.40, and -0.25 V (vs. Ag/AgCl), and -0.16, -0.20 and -0.05 V (vs. NHE), respectively (Table 1). It is evident that the redistribution of electrons within the composite leads to an increase EF in the metal phosphide which is more favourable for the H2 evolution reaction (alternatively described as band bending). Especially for FeP/TiO2 composites, the negative shift in EF of FeP above the reduction potential of H+/H2 imparts it the ability to engage in hydrogen evolution. It is well known for photoelectrical reactions that more negative values of EF create a larger driving force for H2 evolution [41]. Thus, it is reasonable to assume that the difference in the photocatalytic activities of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2 composites might be attributed to the different EF of the metal phosphides. 14
Fig. 7
Based on the above discussion, one may speculate that the presence of band bending can suppress the recombination of the photoexcited electrons and holes and subsequently accelerate the transfer of electrons from the conduction band of TiO2 to the metal phosphides. Therefore, photoluminescence (PL) spectra and photocurrent values were measured in order to verify that the metal phosphides serve as acceptors of the electrons generated by TiO 2 and by which the charge recombination process can be effectively restrained. Figure 7(a) shows the PL spectra for pure TiO2, Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2. When the excitation wavelength was set at 320 nm, broad emission peaks centred at ca. 450 nm can be observed for all of the photocatalysts. It is also observed that the emission intensity of pure TiO2 is the highest, indicating that the photo-generated electrons and holes recombine to a
significant degree. The PL emission intensities are in the
decreasing order of FeP/TiO2 > Ni2P/TiO2 > NiCoP/TiO2.
This suggests that the most effective
inhibition of photo-generated charges or recombination is achieved over NiCoP/TiO2 among the three composite photocatalysts.
Photocurrent transient responses were also studied to further investigate the electronic interactions between TiO2 and metal phosphides. Fig. 7(b) shows the photocurrent transient responses for each switch-on/off operation of TiO2, Ni2P/TiO2, NiCoP/TiO 2, and FeP/TiO2 samples. Evidently, the naked TiO2 shows a rather low photocurrent. However, enhanced photocurrents are observed for Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2, of which the largest one is observed for the NiCoP/TiO2 composite. Therefore, it may be concluded that incorporation of Ni2P, FeP, and NiCoP nanoparticles into TiO2 improves its performance and the NiCoP/TiO2 composite is the most effective material for this purpose. The separation of the charge carriers in 15
the composite photocatalysts may be attributed to the matching band potentials of these two components, which results in an enhanced driving force at the interfaces of the photocatalysts [42]. Based on the above analysis, the proposed mechanism for the use of metal phosphides as cocatalysts in the photocatalytic reaction of H2 production is shown in Fig. 8. To summarize,when TiO2 nanoparticles are illuminated by light, the valence band electrons are excited to the conduction band of TiO2 and holes are left in the valence band. The photo-generated electrons rapidly transfer to the metal phosphides for reduction of H+ into H2, as shown in Fig. 8(a). Specifically, the one-way migration of electrons leads to an increase in the EF of metal phosphides, which is more favourable for H2 evolution, as shown in Fig. 8(b). Thus the role of the injected electrons is similar to that of an external bias in electrocatalysis. Meanwhile, the sacrificial reagent consumes the holes left in the valence band of the semiconductor by supplying electrons. Thus, it can be concluded that metal phosphides are promising candidates for the role of as the cocatalysts in the photocatalytic reaction of hydrogen evolution since they contribute by accelerating the charge separation and afford the active reaction sites. Fig.8
4. Conclusion In conclusion,
electrocatalytic metal phosphides have been shown to act as excellent
cocatalysts for TiO2 that display high activity in the photocatalytic production of H2 from methanol solution. After optimizing the proportions of Ni2P, NiCoP, and FeP, the NiCoP/TiO2 composite was found to exhibit the highest activity in photocatalytic H2 production, which was ca. 13.6 times that of the naked TiO2 nanoparticles. More importantly, the Fermi energy level (EF) of 16
metal phosphides in these composites shifts higher in energy which is beneficial for hydrogen evolution. The photoluminescence and photocurrent transient response further verified that the promoted photocatalytic hydrogen production activity could be attributed to the significantly enhanced charge separation by transfer of excited electrons from TiO2 to metal phosphides. The present study provides a new route for the development of active metal phosphides/semiconductor hybrid photocatalysts for efficient H2 generation. It can be contended that these types of composite photocatalysts provide a feasible strategy of using electrocatalysts as cocatalysts to substitute the traditional noble metals and reduce the cost of the photocatalytic hydrogen production
process.
5. Acknowledge The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (No. 51422604, 21276206) and National 863 Program of China (No. 2013AA050402). This work was also supported by the China Fundamental Research Funds for the Central Universities.
17
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22
Fig. 1 X-ray diffraction patterns of (a) Ni 2P, (b) NiCoP, (c) FeP, (d) TiO2, (e) Ni2P(X=0.38)/TiO2 (f), NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2 photocatalysts.
Fig. 2 (a) Low- and (b) high-resolution TEM images of Ni2P, (c) Low- and (d) high-resolution TEM images of NiCoP, (e) Low- and (f) high-resolution TEM images of FeP.
Fig. 3 (a) Low- and (b) high-resolution TEM images of TiO2, (c) Low- and (d) high-resolution TEM images of Ni2P(X=1)/TiO2, (e) Low- and (f) high-resolution TEM images of NiCoP(X=5)/TiO2, (g) Low- and (h)
high-resolution TEM images of FeP(X=5)/TiO2.
Fig. 4 UV–vis diffuse reflectance spectra of naked TiO2, Ni2P (X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP (X=1)/TiO2 photocatalysts.
Fig. 5 The amount of H2 evolution over (a) Ni2P(X wt%)/TiO2, (b) NiCoP(X wt%)/TiO2, (c) FeP(X wt%)/TiO2, (d) the comparison of H2 evolution amount and rate (inset) of TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and
FeP(X=1)/TiO2 photocatalyst. The reaction system contains 20 mg photocatalyst and 80 mL methanol aqueous
solution (20 vol%).
Fig. 6 (a) Mott-Schottky (MS) plots of TiO2, Ni2P, NiCoP and FeP nanoparticles and (b) Calculated energy band diagram of metal phosphides and TiO2, (c) Mott-Schottky (MS) plots of Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2
and FeP(X=1)/TiO2.
Fig. 7 (a) Photoluminescence spectra (PL) and (b) photocurrent response spectrum for Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2 photocatalysts.
Fig. 8 The proposed mechanism of electrocatalyst nickel phosphide as cocatalysts for photocatalytic H 2 production with the metal phosphides/TiO2 composites.
23
Fig. 1
24
Fig. 2
25
Fig. 3
26
Fig. 4
27
Fig. 5
28
Fig. 6
29
Fig. 7
30
Fig. 8
a CB
e-
VB
Methanol
CB
-0.27 eV - - - -
H2 -
'
H+
H+
3.25 eV
Light irradiation
b
Phosphides H2
h+
VB
Oxidative products
31
+ + + + 2.96eV TiO2
Phosphides
Table 1 The EF values of Ni2P, NiCoP, FeP and ECB values of the TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2.
Reference electrode
Ni2P
NiCoP
FeP
TiO2
Ni2P(X=0.38)/TiO2
NiCoP(X=1)/TiO2
FeP(X=1)/TiO2
H+/H2
Ag/AgCl(V)
-0.29
-0.38
-0.15
-0.47
-0.36
-0.40
-0.25
0.20
NHE(V)
-0.09
-0.18
0.05
-0.27
-0.16
-0.20
-0.05
0
32
S0169-4332(17)31256-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.221 APSUSC 35902
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-1-2017 25-4-2017 26-4-2017
Please cite this article as: Rui Song, Wu Zhou, Bing Luo, Dengwei Jing, Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.221 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly efficient photocatalytic H2 evolution using TiO2 nanoparticles integrated with electrocatalytic metal phosphides as cocatalysts
Rui Song, Wu Zhou, Bing Luo, Dengwei Jing*
Corresponding author: Tel.:+86-29-82668769; Email:[email protected] International Research Center for Renewable Energy & State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
Highlights Metal phosphides like Ni2P, NiCoP, and FeP act as electrocatalysts have been employed here as cocatalysts for photocatalytic H2 production. NiCoP(1 wt%)/TiO2 composites showed the highest photocatalytic activity toward H2 production , which was about thirteen times that of TiO2. Electrons injected from TiO2 into metal phosphides play a role similar to that of an external bias in electrocatalysis. Mott-Schottky (MS) analysis was performed to understand the mechanism of the Ni2P, NiCoP, and FeP electrocatalysts as cocatalysts for the photocatalytic H2 generation.
Abstract In this work, electrocatalysts like the metal phosphides Ni2P, NiCoP, and FeP, can serve as cocatalysts of TiO2 to form efficient composite photocatalysts for
the H2 generation hydrogen
generation from an aqueous methanol solution. On comparing Ni2P, NiCoP, and FeP and optimizing their proportions, the NiCoP(1wt%)/TiO2 composite was found to exhibit the highest activity toward photocatalytic H2 production (1.54 μmol·h-1·mg-1), which is about thirteen times 1
that of the naked TiO2 nanoparticles. Mott-Schottky (MS) analysis indicated that the large upward shift or band bending of the Fermi energy level (EF) in metal phosphides was responsible for the enhanced activity of the composites. The steady-state photoluminescence (PL) spectra and photocurrent transient response further confirmed that the enhanced photoinduced charge transfer and band separation after TiO2 was integrated with the metal phosphides. Thus, these electrocatalysts were shown to be efficient cocatalysts that can replace noble metals as low-cost photocatalytic H2 production. Key Words: Photocatalysis, Hydrogen production, Metal phosphides, TiO2
1. Introduction The increasingly serious global energy crisis and the extreme pollution caused by the burning of fossil fuels has resulted in an urgent need for the development of environment friendly and renewable energy resources [1]. Hydrogen is an ideal candidate for the replacement of fossil fuels as it is environmentally benign and recyclable [2,3]. Among the various methods of hydrogen production, photocatalysis
has receivedsignificant attention as a viable method for the
conversion of solar energy into H2 [4,5]. Various photocatalysts have been developed for this purpose [6-8]. It is generally believed that loading of a cocatalyst on an active substrate such as the TiO2 is essential for the realization of efficient photocatalytic hydrogen production[9-12]. Furthermore, noble metals like Pt areeffective for promoting evolution of H2. However, noble metals are expensive and scarce, which limits their wider applicability. It is therefore highly desirable to find alternate non-noble metal cocatalysts that are easily available and have a lowcost [13]. 2
Molybdenum disulphide (MoS2) has
previously been used as a highly active and effective
substitute for Pt in the electrochemical hydrogen evolution reaction [14]. In addition, several heterogeneous catalysts composed of earth abundant elements have been utilized for electrochemical generation of H2. These include borides [15], carbides [16], nitrides [17], sulfides [18], selenides [19], and even metal-free catalysts [20]. Recently, it has been reported that transition metal phosphides (TMPs) such as Ni2P [21], CoP [22], and FeP [23] show high electrocatalytic activity in H2 evolution. The use of TMPs in electrocatalytic processes is advantageousowing to their ability to generate high current density, and their low overpotential and excellent stability.Even though, additional voltage is often necessary to drive the electrocatalytic reaction. When photocatalysts are irradiated with energy greater than their band gap, electron-hole pairs are created that migrate to the surface of the catalyst and initiate the reduction of H+ to H2. It is assumed that the above mentioned characteristics of metal phosphides might also be exploited for photocatalytic reactions when these metal phosphides are loaded on semiconductors and used as cocatalysts. Several efforts have been made to developtransition-metal based phosphides as cocatalysts to promote the photocatalytic hydrogen evolution. Du et al. have reported that nickel phosphide (Ni2P) as a cocatalyst with one-dimensional semiconductor nanorods forms a well-designed integrated photocatalyst that shows significantly enhanced efficiency and durability for the photoinitiated generation of hydrogen from water [24]. CdS modified with Ni2P derived from a metal organic framework (MOF) has also been found toimprove the efficiency of photocatalytic H2 production [25]. In addition, CdS quantum dots have been used as light absorbing materials 3
with metallic MoP as the cocatalyst, and the optimized system showed a high H2 evolution rate of 1100 μmol∙h−1 under visible light (λ ≥ 420 nm), which is comparable to the production rate when Pt was used as the cocatalyst [26]. The decoration of Co2P nanohybrids of uniform morphology on reduced graphene oxide (RGO) produced a material that exhibited high activity toward H2 evolution under visible light irradiation after it was sensitized by Eosin Y [27]. Despite these significant advances in photocatalytic H2 production by using electrocatalysts as cocatalysts, to the best of our knowledge, a systematic investigation of the addition of metal phosphides such as Ni2P, NiCoP, and FeP as cocatalysts onto the commercially available TiO2 has not been reported till date. Owing to the specific design of such a substrate based photocatalyst, it is difficult to distinguish between the roles played by the substrate and the cocatalyst. Therefore, in this study,commercially available TiO2 has been used, and by excluding the specific contribution of the substrate, it becomes possible to understand the underlying mechanism of action of metal phosphides as cocatalysts for photocatalytic reactions. Therefore, this strategy is valuable for the discovery of efficient noble-metal free photocatalysts in the future. This work describes for the first time, the use of electrocatalytic metal phosphides (Ni2P, NiCoP, and FeP) as cocatalysts, commercially available TiO2 as the main catalyst, and methanol as a sacrificial electron donor for efficient photocatalytic H2 production. The morphology and structure of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO2 composites have been determined by X-ray diffraction
(XRD)
and
transmission
electron
microscopy
(TEM).
The
steady-state
photoluminescence (PL) spectra and photocurrent transient response were studied and used to investigate the photo-induced charge transfer and separation processes. In addition, the Mott-Schottky (MS) analysis was carried out to understand the mechanism of the electrocatalytic 4
metal phosphides (Ni2P, NiCoP, and FeP) as cocatalysts for photocatalytic hydrogen evolution. 2. Experimental section 2.1 Chemicals and Materials All reagents were of analytical grade and used as received without further purification. Nickel acetylacetonate (Ni(acac)2), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), iron oxide (Fe3O4), sodium hypophosphite (NaH2PO2·H2O), sodium sulfate (Na2SO4), urea, n-hexane, toluene, acetone, methanol, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Tri-n-octylphosphine (TOP, 97%) was purchased from Strem Chemicals, Inc., while 1-octadecene (ODE, 90%) and oleylamine (70%) were obtained from Sigma-Aldrich. Commercial TiO 2 P25 was purchased from Evonik Degussa Corporation. 2.2 Preparation of the MP/TiO2 (M = Ni2, NiCo, Fe) composites Synthesis of Ni2P nanoparticles. This reaction involves the decomposition of a phosphine that may liberate phosphorus. Therefore, it should be regarded as a highly corrosive and flammable process. Such reactions should only be performed under vacuum conditions by professionally trained personnel. Typically, nickel phosphide nanoparticles were prepared following a modified literature procedure [28]. Briefly, Ni(acac)2 (250 mg, 0.98 mmol), 1-octadecene (4.5 mL, 14.1 mmol), and oleylamine (6.4 mL, 19.5 mmol) were added to a 50 mL three-necked, round bottom flask, which was equipped with a thermometer and a condenser. The reaction mixture was heated under continuous stirring in order to remove water and other low-boiling impurities. Subsequently, tri-n-octylphosphine (2 mL, 4.4 mmol) was injected into the flask which was first put under a slight vacuum. The round bottom flask was then filled with Ar 5
and heated to 300 °C within 30 min. A change in the colour of the solution was observed to black at 220 °C. After heating the solution at 300 °C for 2 h,the heating mantle was turned off until the solution cooled to 200 °C. The flask was then removed from the heating mantle and cooled to room temperature. Subsequently, the mixture was separated by centrifugation and washed three times with 1:3 (v/v) hexane:isopropanol. Finally, the precipitate was suspended and stored in toluene for further use. Synthesis of NiCoP nanoparticles. NiCoP nanoparticles were prepared byfollowing a previously reported method with some modifications [29]. In the first step, NiCo(OH)2 precursor was synthesized using a co-precipitation method. In
a typical process, 6.4 g of NaOH was
dissolved in 20 mL of deionized water and stirred continuously at room temperature for 20 min. Subsequently, 25 mM Ni(NO3)2·6H2O and 50 mM Co(NO3)2·6H2O containing 20 mL of deionized water was added dropwise to the aqueous NaOH solution. After continuous stirring for 30 min, the mixture was treated hydrothermally at 100 °C for 24 h. The NiCo(OH)2 precursor was washed three times alternatively with deionized water and ethanol , and then dried under vacuum at 60 °C. In the next step, the washed NiCo(OH)2 precursor and NaH2PO2·H2O were blended mechanically and ground in the mortar. The molar ratio of the total metal (Ni and Co) to P was 1:5. Finally, the fine powders were kept at 300 °C for 2 h in a quartz tube with a heating rate of 5 °C∙min−1 protected under an inert argon gas flow. The obtained products were thoroughly washed with deionized water to remove any residual salts and dried under vacuum at 60 °C. Synthesis of FeP nanoparticles.
FeP nanoparticles were prepared by following a
previously reported method with slight modifications [30]. Fe3O4 and NaH2PO2·H2O were blended mechanically and ground in the mortar. The molar ratio of Fe to P was 1:5. The well 6
dispersed mixture was then transferred into a quartz tube and calcined in a tubular furnace under a flow of Ar gas for 3 h with a heating rate of 2 °C∙min−1 up to 400 °C to obtain the FeP nanoparticles. The obtained nanoparticles were subsequently washed several times with deionized water and ethanol and dried under vacuum at 60 °C. Synthesis of MP/TiO2 (M=Ni2, NiCo, Fe) nanoparticles. TiO2 nanoparticles were added to 100 mL toluene and the suspension was stirred for 0.5 h. Subsequently, the well dispersed suspension of Ni2P, NiCoP, and FeP in toluene was added and the mixture was stirred for another 1.5 h. In the next step, the mixture was collected by centrifugation and washed with acetone along with the use of mild sonication. The sediments were dried at 80 °C for 0.5 h. Finally, the obtained samples were heated for 2.5 h at 450 °C under Ar atmosphere in order to remove the remnants of impurities on the surface and to make sure that a potent solid-solid interface had formed between the metal phosphides (MP) and TiO2 nanoparticles. The MP/TiO2 composite with different amounts of metal phosphide nanoparticles is defined as MP(X wt%)/TiO2, where X ranges from 0 to 5. 2.4 Characterization The particle size and morphology of the samples were analysed from TEM images obtained on a FEI Tecnai G2 F30 transmission electron microscope at an accelerating voltage of 300 kV. The crystallographic structures and other information on chemical composition of the composite particles was obtained by powder XRD using Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 10° min−1. The absorption spectra of the samples were measured on a Hitachi U4100 UV-Vis spectrophotometer over a range of 300−800 nm and the photoluminescence spectra (PL) were measured at room temperature on a PTI QM-4 fluorescence spectrophotometer. 7
2.5 Photoelectrochemical measurements Photoelectrochemical measurements were performed on a CHI 760D scanning potentiostat (CH Instruments). In a standard three-electrode cell system, the fluorine doped tin oxide glass electrode (FTO) coated with the photocatalyst, a Pt plate, and a Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. The working electrodes were prepared by dropping 200 μL of a suspension of Ni2P/TiO 2, NiCoP/TiO2, FeP/TiO2, or TiO2 (3 mg of the photocatalyst added into 3 mL ethanol and 600 μL Nafion mixed solution) onto the surface of a FTO plate (2 cm × 2 cm). Subsequently, the electrodes were dried at room temperature for 12 h. A 500 W Xenon lamp coupled with an AM1.5 filter was used as the light source and a 0.5 M Na2SO4 solution was used as the electrolyte. The photoresponsive signals of the samples were measured under chopped light at 0.6 V vs. Ag/AgCl. Mott-Schottky analyses were performed using 5 kHz frequency in the dark. The carrier densities were calculated from the Mott–Schottky equation:
1 / C 2 (2 / e0 0 N d )(V VFB kT / e0 ) where C, e0, ε, ε0, Nd, V, VFB, kT/e0 refer to the specific capacitance,
electron charge, dielectric
constant, permittivity of vacuum, carrier density, applied potential of the electrode, flat band potential, and a temperature-dependent correction term, respectively. 2.6 Photocatalytic hydrogen production The photocatalytic hydrogen activities of MP/TiO2 (M = Ni2, NiCo, Fe) were evaluated by using a 300 W Xenon lamp as the UV-Vis light source and the photocatalytic reactions were carried out in a side-irradiation reactor. The gases sampled with a syringe were analysed with a TCD SP2100 gas chromatograph (TDX-01 column) using nitrogen as the carrier gas. For a typical 8
photocatalytic H2 generation test, 20 mg of the photocatalyst powder was dispersed by a magnetic stirrer in the reactor containing 80 mL of an aqueous solution of methanol (20 vol%) used as the sacrificial reagent. The reactor was purged with N2 for 30 min before hydrogen production to completely remove oxygen and
continuously stirred to keep the photocatalyst particles
suspended throughout the reaction. All reactions were performed at room temperature. 3. Results and discussion 3.1 Morphologies and crystalline properties Fig. 1
Fig. 1 shows the XRD patterns of Ni2P, NiCoP, FeP, TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2 photocatalysts, respectively. The characteristic peaks at 40.7º, 44.6º, 47.4º, and 54.21º in Fig. 1(a) forNi2P (JCPDS NO. 03-0953) match well with the XRD pattern previously reported by other authors [31]. Fig. 1(b) as-synthesized NiCoP. The peaks
shows the XRD pattern of the
at ca. 2θ = 30.6º, 40.9º, 44.9º, 47.5º, 54.7º, and 55.3º can be
attributed to the (110), (111), (201), (002), (110), and (300) crystal planes, respectively, of the hexagonal phase (JCPDS NO. 71-2336) [29]. Similarly, there are several sharp diffraction peaks at ca. 2θ = 30.8º, 32.8º, 37.2º, 48.4º, and 50.4º that correspond to the (002), (011), (111), (103), and (211) crystal planes of FeP (JCPDF NO. 65-2595) [23], respectively, as shown in the Fig. 1(c). For TiO2 (Fig. 1(d)), very intense and narrow diffraction peaks are observed, demonstrating the high crystallinity of the photocatalyst samples. The broad peaks around 2θ = 25.3º, 37.8º, 48.1º, 53.9º, 55.1º, 62.6º, 69.0º, 70.3º, and 75.0º can be assigned respectively to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes of the anatase phase [32]. Other sharper diffraction peaks observed in the spectrum can be attributed to the indices of rutile phase. For the 9
XRD pattern of Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2, no obvious diffraction peaks corresponding to Ni2P, NiCoP, and FeP, respectively, could be detected and the spectra showed no significant difference from that of TiO2. This may be attributed to the very strong diffraction peaks of TiO 2 and the relatively small amount of metal phosphides on TiO2. Fig. 2
The TEM images of Ni2P, NiCoP, and FeP are shown in Fig. 2. It is evident from Fig. 2(a) that the synthesized Ni2P nanoparticles are uniform in size with an average diameter of 13 ± 2 nm. Interestingly, most of the Ni2P nanoparticles have a hexagonal shape. The corresponding high-resolution TEM (HRTEM) image (Fig. 2(b)) shows that the size of the lattice fringes of Ni2P is approximately 0.22 nm, which corresponds well to the (111) planes of Ni2P [31]. Fig. 2(c) and 2(d) are the TEM and HRTEM images of NiCoP nanoparticles, respectively. It is evident that NiCoP nanoparticles have the particle sizes ranging between 20−50 nm and the lattice distances are 0.28 and 0.33 nm, which are in good agreement with the (101) and (001) planes of NiCoP, respectively. Fig. 2(e) shows the TEM images of FeP nanoparticles which are ca. 100−200 nm in size. The corresponding HRTEM image of the selected area is given in Fig. 2(f) and reveals that the lattice fringes with interplanar distances of 0.39 nm correspond well to the (101) planes of FeP. Fig. 3
The TEM and HRTEM images of TiO2 are shown in Fig. 3(a) and Fig. 3(b), respectively. The fringes with d spacing of 0.35 nm correspond to the (101) plane of TiO 2. Fig. 3(c) and Fig. 3(d) show TEM and HRTEM images of Ni2P(X=1)/TiO2 photocatalysts, respectively. The fringes with d spacing of 0.22 nm can be attributed to the (111) crystalline plane of Ni 2P, while the d spacing of 0.35 nm corresponds to the (101) plane of TiO2 [33]. Fig. 3(e) and Fig. 3(f) show the TEM and 10
HRTEM images of NiCoP(X=5)/TiO2 respectively. In this case, the fringes with d spacing of 0.17 nm can be assigned to the (300) crystalline plane of NiCoP while those with the d spacing of 0.35 nm correspond to the (101) plane of TiO2. These results suggest that the NiCoP nanoparticles were successfully loaded on the TiO2 nanoparticles. The TEM image of FeP(X=5)/TiO2 nanoparticles is shown in Fig. 3(g). Its corresponding HRTEM image is shown in Fig. 3(h) wherein the fringes with d spacing of 0.24 nm may be assigned to the (111) plane of FeP and the d spacing of 0.35 nm ascribed to the (101) plane of TiO2. Therefore, it may be concluded that the FeP nanoparticles were also successfully loaded on the TiO2 nanoparticles. Fig. 4
The UV-Vis diffuse reflectance spectra (DRS) of the fabricated samples are shown in Fig. 4. For TiO2 nanoparticles, the light response lay within the range of 300−400 nm and has been previously attributed to the charge-transfer transition from the valence O 2p orbital to the conduction band (Ti 3d) [34]. In comparison with the spectrum of naked TiO2, the absorption edge spectrum showed no evident shift after TiO2 was loaded with Ni2P, NiCoP, or FeP,. The only difference for these three cases is that the intensity of the absorption had varied. The very small change in the light absorption edge spectra indicates that the metal phosphides are tightly anchored on the surface of TiO 2 instead of being doped into its lattice. 3.2 Photocatalytic H2 production Fig. 5
Fig. 5 shows the amount of H2 evolved over Ni2P/TiO2, NiCoP/TiO 2, and FeP/TiO2 photocatalysts with different loading amounts of Ni2P, NiCoP, and FeP, respectively. Fig. 5(a) illustrates the amount of H2 evolved when Ni2P(X wt%)/TiO 2 is used as the photocatalyst. It can 11
be seen that a negligible amount of H2 (2.11 μmol) is generated over bare TiO2 after irradiation for 3 h. However, an integration of only 0.19 wt% of Ni2P on TiO2 leads to a significantly enhanced amount of H2 generated (42.77 μmol). With further increase in the loading amounts of Ni2P on TiO2, the highest photocatalytic activity (85.65μmol) is achieved for Ni2P(0.38 wt%)/TiO 2. However, the amount of H2 evolved reduced to 30.19 μmol when 1.0 wt% of Ni2P is utilized. The excess amount of Ni2P is assumed to shelter the incident light from reaching the TiO 2 surface, resulting in a decreased photocatalytic activity. In addition to Ni2P, other metal phosphides such as NiCoP and FeP were also studied for comparison. Fig. 5(b) and Fig. 5(c) show the amount of H2 evolved over NiCoP(X wt%)/TiO2 and FeP(X wt%)/TiO2, respectively. It can be seen that the amount of H2 generated first increases and then decreases. The highest amounts of H2 generated after irradiation for 3 h are 96.36 and 47.69 μmol for NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2, respectively. The reduced photocatalytic activity on further increase in the loading of metal phosphides can be attributed to the intense light absorption by NiCoP and FeP themselves, which consequently reduces the amount of light arriving at the TiO2 surface [35]. In order to study the effect of different phosphides electrocatalysts in the role of a cocatalyst for the promotion of the photocatalytic activity in the H2 evolution reaction, the amounts of H2 evolved and the corresponding H2 production rates upon irradiation for 3 h were compared for Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2 are shown in Fig. 5(d). The amount of H2 evolved and the corresponding production rates were 225.36, 246.91, 123.02 μmol and 1.41, 1.54, 0.77 μmol·h-1·mg-1, respectively. The remarkably improved H2 production activities of the phosphide-modified TiO2 were 12.5, 13.6, and 6.8 times higher than that of naked TiO2. These results indicate that the metal phosphide electrocatalysts are efficient cocatalysts for the 12
photocatalytic production of H2. Furthermore, among the tested metal phosphides, NiCoP is most effective. 3.3 The possible mechanism of photocatalytic H2 production Fig. 6
Table 1
As shown by the results of UV-Vis spectra given in Fig. 4, the absorption edge spectrum of TiO2 does not show any particular shift after loading with metal phosphides. Therefore it may be reasonably concluded that light absorption is not the dominant factor responsible for the enhanced photocatalytic activity. In order to understand the mechanism for the enhanced H2 production activity of metal phosphides/TiO2 composites compared to the bare TiO 2 nanoparticles, a possible reaction schemeof the charge transfer processes in context of the energy band levels is required. Therefore,
Mott-Schottky
(MS)
analyses
were
conducted
for
Ni2P(X=0.38)/TiO2,
NiCoP(X=1)/TiO2, and FeP(X=1)/TiO2. Fig. 6(a) shows the Mott-Schottky plots of TiO2 and the various metal phosphides Ni2P, NiCoP, and FeP, before their combination. The values of the flat-band potential (Efb) can be determined from the intercepts of the linear part of the Mott-Schottky plots on the x axis and can be used to approximately estimate the conduction band potentials (ECB) for semiconductors and the Fermi energy level potential (EF) values for the metal phosphides [36]. The ECB value of TiO2 and EF values of Ni2P, NiCoP, and FeP vs. Ag/AgCl were determined to be -0.47, -0.29, -0.38, and -0.15 V, and these values vs. NHE were calculated as -0.27, -0.09, -0.18, and 0.05 V, respectively, as shown in Table 1.The calculated band diagrams were obtained by combining the MS and UV-Vis results for metal phosphides and TiO2 before they were assembled together and are given 13
in Fig. 6(b) [37,38]. On comparing the obtained flat-band potential position of TiO2 with Ni2P, NiCoP, and FeP, it is evident that the ECB for TiO 2 and EF for Ni2P and NiCoP are negative values but the EF for FeP is a positive value versus 0 V of the H+/H2 potential (NHE). Therefore, theoretically, the electrons in Ni2P and NiCoP can reduce H+ to H2 while it is impossible to achieve this transformation with FeP nanoparticles. In this context, as metal phosphides come into contact with TiO2, the photogenerated electrons from ECB of TiO2 can be injected into the EF of metal phosphides because ofthe much more negative potential of the conduction band [39]. A quick charge separation taking place in this manner clearly avoids charge recombination in TiO2 and hence improves its photocatalytic activity. In addition, the injected electrons can accumulate in the metal phosphides, which may be expected to induce a much more negative EF [40]. In order to provide further evidence in support of the abovementioned hypothesis, the Mott-Schottky analyses of metal phosphides/TiO2 composites were also carried out and these results are shown in Fig. 6(c). The Efb values of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO2 composites are -0.36, -0.40, and -0.25 V (vs. Ag/AgCl), and -0.16, -0.20 and -0.05 V (vs. NHE), respectively (Table 1). It is evident that the redistribution of electrons within the composite leads to an increase EF in the metal phosphide which is more favourable for the H2 evolution reaction (alternatively described as band bending). Especially for FeP/TiO2 composites, the negative shift in EF of FeP above the reduction potential of H+/H2 imparts it the ability to engage in hydrogen evolution. It is well known for photoelectrical reactions that more negative values of EF create a larger driving force for H2 evolution [41]. Thus, it is reasonable to assume that the difference in the photocatalytic activities of the Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2 composites might be attributed to the different EF of the metal phosphides. 14
Fig. 7
Based on the above discussion, one may speculate that the presence of band bending can suppress the recombination of the photoexcited electrons and holes and subsequently accelerate the transfer of electrons from the conduction band of TiO2 to the metal phosphides. Therefore, photoluminescence (PL) spectra and photocurrent values were measured in order to verify that the metal phosphides serve as acceptors of the electrons generated by TiO 2 and by which the charge recombination process can be effectively restrained. Figure 7(a) shows the PL spectra for pure TiO2, Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2. When the excitation wavelength was set at 320 nm, broad emission peaks centred at ca. 450 nm can be observed for all of the photocatalysts. It is also observed that the emission intensity of pure TiO2 is the highest, indicating that the photo-generated electrons and holes recombine to a
significant degree. The PL emission intensities are in the
decreasing order of FeP/TiO2 > Ni2P/TiO2 > NiCoP/TiO2.
This suggests that the most effective
inhibition of photo-generated charges or recombination is achieved over NiCoP/TiO2 among the three composite photocatalysts.
Photocurrent transient responses were also studied to further investigate the electronic interactions between TiO2 and metal phosphides. Fig. 7(b) shows the photocurrent transient responses for each switch-on/off operation of TiO2, Ni2P/TiO2, NiCoP/TiO 2, and FeP/TiO2 samples. Evidently, the naked TiO2 shows a rather low photocurrent. However, enhanced photocurrents are observed for Ni2P/TiO2, NiCoP/TiO2, and FeP/TiO 2, of which the largest one is observed for the NiCoP/TiO2 composite. Therefore, it may be concluded that incorporation of Ni2P, FeP, and NiCoP nanoparticles into TiO2 improves its performance and the NiCoP/TiO2 composite is the most effective material for this purpose. The separation of the charge carriers in 15
the composite photocatalysts may be attributed to the matching band potentials of these two components, which results in an enhanced driving force at the interfaces of the photocatalysts [42]. Based on the above analysis, the proposed mechanism for the use of metal phosphides as cocatalysts in the photocatalytic reaction of H2 production is shown in Fig. 8. To summarize,when TiO2 nanoparticles are illuminated by light, the valence band electrons are excited to the conduction band of TiO2 and holes are left in the valence band. The photo-generated electrons rapidly transfer to the metal phosphides for reduction of H+ into H2, as shown in Fig. 8(a). Specifically, the one-way migration of electrons leads to an increase in the EF of metal phosphides, which is more favourable for H2 evolution, as shown in Fig. 8(b). Thus the role of the injected electrons is similar to that of an external bias in electrocatalysis. Meanwhile, the sacrificial reagent consumes the holes left in the valence band of the semiconductor by supplying electrons. Thus, it can be concluded that metal phosphides are promising candidates for the role of as the cocatalysts in the photocatalytic reaction of hydrogen evolution since they contribute by accelerating the charge separation and afford the active reaction sites. Fig.8
4. Conclusion In conclusion,
electrocatalytic metal phosphides have been shown to act as excellent
cocatalysts for TiO2 that display high activity in the photocatalytic production of H2 from methanol solution. After optimizing the proportions of Ni2P, NiCoP, and FeP, the NiCoP/TiO2 composite was found to exhibit the highest activity in photocatalytic H2 production, which was ca. 13.6 times that of the naked TiO2 nanoparticles. More importantly, the Fermi energy level (EF) of 16
metal phosphides in these composites shifts higher in energy which is beneficial for hydrogen evolution. The photoluminescence and photocurrent transient response further verified that the promoted photocatalytic hydrogen production activity could be attributed to the significantly enhanced charge separation by transfer of excited electrons from TiO2 to metal phosphides. The present study provides a new route for the development of active metal phosphides/semiconductor hybrid photocatalysts for efficient H2 generation. It can be contended that these types of composite photocatalysts provide a feasible strategy of using electrocatalysts as cocatalysts to substitute the traditional noble metals and reduce the cost of the photocatalytic hydrogen production
process.
5. Acknowledge The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (No. 51422604, 21276206) and National 863 Program of China (No. 2013AA050402). This work was also supported by the China Fundamental Research Funds for the Central Universities.
17
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Fig. 1 X-ray diffraction patterns of (a) Ni 2P, (b) NiCoP, (c) FeP, (d) TiO2, (e) Ni2P(X=0.38)/TiO2 (f), NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2 photocatalysts.
Fig. 2 (a) Low- and (b) high-resolution TEM images of Ni2P, (c) Low- and (d) high-resolution TEM images of NiCoP, (e) Low- and (f) high-resolution TEM images of FeP.
Fig. 3 (a) Low- and (b) high-resolution TEM images of TiO2, (c) Low- and (d) high-resolution TEM images of Ni2P(X=1)/TiO2, (e) Low- and (f) high-resolution TEM images of NiCoP(X=5)/TiO2, (g) Low- and (h)
high-resolution TEM images of FeP(X=5)/TiO2.
Fig. 4 UV–vis diffuse reflectance spectra of naked TiO2, Ni2P (X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP (X=1)/TiO2 photocatalysts.
Fig. 5 The amount of H2 evolution over (a) Ni2P(X wt%)/TiO2, (b) NiCoP(X wt%)/TiO2, (c) FeP(X wt%)/TiO2, (d) the comparison of H2 evolution amount and rate (inset) of TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2, and
FeP(X=1)/TiO2 photocatalyst. The reaction system contains 20 mg photocatalyst and 80 mL methanol aqueous
solution (20 vol%).
Fig. 6 (a) Mott-Schottky (MS) plots of TiO2, Ni2P, NiCoP and FeP nanoparticles and (b) Calculated energy band diagram of metal phosphides and TiO2, (c) Mott-Schottky (MS) plots of Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2
and FeP(X=1)/TiO2.
Fig. 7 (a) Photoluminescence spectra (PL) and (b) photocurrent response spectrum for Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2 photocatalysts.
Fig. 8 The proposed mechanism of electrocatalyst nickel phosphide as cocatalysts for photocatalytic H 2 production with the metal phosphides/TiO2 composites.
23
Fig. 1
24
Fig. 2
25
Fig. 3
26
Fig. 4
27
Fig. 5
28
Fig. 6
29
Fig. 7
30
Fig. 8
a CB
e-
VB
Methanol
CB
-0.27 eV - - - -
H2 -
'
H+
H+
3.25 eV
Light irradiation
b
Phosphides H2
h+
VB
Oxidative products
31
+ + + + 2.96eV TiO2
Phosphides
Table 1 The EF values of Ni2P, NiCoP, FeP and ECB values of the TiO2, Ni2P(X=0.38)/TiO2, NiCoP(X=1)/TiO2 and FeP(X=1)/TiO2.
Reference electrode
Ni2P
NiCoP
FeP
TiO2
Ni2P(X=0.38)/TiO2
NiCoP(X=1)/TiO2
FeP(X=1)/TiO2
H+/H2
Ag/AgCl(V)
-0.29
-0.38
-0.15
-0.47
-0.36
-0.40
-0.25
0.20
NHE(V)
-0.09
-0.18
0.05
-0.27
-0.16
-0.20
-0.05
0
32