TiO2 for full

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Materials and Interfaces

Photothermocatalytic hydrogen evolution over Ni2P/ TiO2 for full-spectrum solar energy conversion Rui Song, Bing Luo, Jiafeng Geng, Dongxing Song, and Dengwei Jing Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00369 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Industrial & Engineering Chemistry Research

Photothermocatalytic hydrogen evolution over Ni2P/TiO2 for full-spectrum solar energy conversion Rui Song, Bing Luo, Jiafeng Geng, Dongxing Song, Dengwei Jing* Corresponding author: Tel.:+86-29-82668769; E-mail: [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

Abstract For photocatalytic solar energy conversion, the critical challenge is to enhance the solar utilization efficiency. Many efforts focused on the development of broadband response nanomaterials. Here, we propose an alternative approach. Wherein, over Ni2P/TiO2 nanoparticles without noble metal, the UV-visible part of solar energy was absorbed and converted by semiconductor and its infrared part was separately collected and converted into thermal energy to heat the photocatalytic reaction to a certain temperature. The photothermocatalytic hydrogen activity was 3.6 times of the sum of the photocatalytic and thermocatalytic reactions respectively. The in situ generated oxygen vacancies in Ni2P/TiO2 during the photothermocatalytic reaction was found to be responsible for the enhanced activity. Moreover, the photocurrent transient responses results revealed the faster transfer of electrons from TiO2 to Ni2P at higher temperature which is vital for the significantly enhanced photothermocatalytic hydrogen production. Long-term test also shows the stability of the proposed reaction system. Key Words: Photothermocatalytic; Ni2P/TiO2; Hydrogen production; Oxygen vacancies. 1. Introduction Converting solar energy into clean energy owns tremendous potential to solve the increasingly serious global energy crisis and the environmental pollution caused by the burning of fossil fuels1,2. Hydrogen, as a safe, sustainable and environmental friendliness energy compatible source, can be produced by photocatalytic water splitting using solar light as energy source3-5. 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Photocatalysis for H2 evolution is an effective, environmental friendly route for addressing the current energy crisis6-8. In the field of solar energy conversion by photocatalysis, one of critical challenge is to enhance the utilization efficiency by extending the solar spectrum response from the UV to the visible, even to the infrared (IR) region9,10. For the full spectrum utilization, most recent efforts focused on the development of broadband response nanomaterials, which are capable of wide band absorption of solar light by modifying the host materials with surface or structure engineering technology, i.e., by integrating with plasmonic and/or up-conversion materials11-14. On the other hand, it is well known that solar energy can also be used for thermal energy generation15. While the thermal energy has often been intentionally removed and thus wasted during the photocatalytic hydrogen evolution process. Therefore, one is expected to find an alternative approach. Wherein, the UV-visible part can be absorbed and used directly over a certain semiconductor photocatalysts, and the infrared part of solar energy would be separately collected and converted into thermal energy to heat the photocatalytic reaction to a certain temperature. In this regard, a specially designed and a new catalytic material capable of exploiting both photo and thermal part of the solar energy is highly desired. Since the electrocatalysis was coupled into photocatalytic process, much attention has been paid for the development of new method for hydrogen production. Recently, an approach combining photo, thermal, electric and chemical processes together have been proposed which can maximize the efficiency and the conversion rate of thermal radiation to chemical potential in the water splitting process16,17. The photothermocatalysis coupling photocatalysis and thermocatalysis in one reaction to achieve enhanced energy utilization efficiency has drawn much attention. Until now, the photothermal effect of various nanometals has been widely investigated for such applications 2


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as in cancer therapy, photoexcitation thermal energy and photocatalysis, etc18,19. Fang and coworkers reported that Pt/LaVO4/TiO2 is an excellent catalyst combining photocatalysis and thermocatalysis for benzene degradation20. Its photothermocatalytic activity at 70 ºC is about three times the sum of the photocatalytic activity and thermocatalytic activity. Ha et al investigated the thermal and photothermal reduction of CO2 with H2O to CH4 using 3DOM-LaSrCoFeO6-δ as a highly active catalyst16. Its catalytic activity for the reduction of CO2 with H2O vapor to CH4 under photothermal conditions is 5 times of that under thermal-only conditions. Moreover, in our previous study, we have demonstrated that nonplasmonic Pt supported on TiO2 could effectively couple photo and thermal effects and exhibit significantly improved photothermal performance21. However, employment of the noble metals obviously restrict its practical applications. Therefore, it is of great significance to find low-cost, effective, Pt-like non-noble metal cocatalysts for efficient photothermocatalytic hydrogen production utilizing both the photo and thermal parts of solar energy and the target is the full-spectrum conversion of solar energy into hydrogen. On the other hand, in recent years many efforts have been devoted to the development of transition-metal based phosphides as potential cocatalysts further promoting the activity for photocatalytic hydrogen evolution. In particular, with density functional theory calculations, nickel phosphide combining the activity of [NiFe] hydrogenase with the thermostability of a heterogeneous catalyst has predicted to be a potential hydrogen evolution catalyst composed of inexpensive and earth-abundant elements22. In addition, the ensemble effect of the Ni hollow sites and the Ni–P bridge site play a significant role in the high hydrogen evolution reaction activity. Indra et al. reported a visible-light-driven integrated heterostructure system using Ni2P as co-catalyst and sol-gel prepared CN as the photocatalyst for efficient H2 evolution23. The noble 3



metal-free system exhibited a comparable catalytic activity with Pt-loaded CN. Kumar et al. reported the facile preparation of metal-organic framework (MOF) derived earth-abundant nickel phosphide (Ni2P) by a simple, cost-effective procedure24. However, to the best of our knowledge, the photothermocatalytic effect of Ni2P nanoparticles integrated with semiconductors for hydrogen production has not been reported. Herein, for the first time, Ni2P/TiO2 nanoparticles were used for photothermocatalytic hydrogen generation in the aim of full-spectrum solar energy conversion. The effects and the mechanism of temperature on the photothermocatalytic activity of hydrogen generation were systematically investigated. In particular, the catalytic hydrogen evolution was performed between 50-90 ºC with and without light irradiation. The EPR, XRD, TEM, Raman and XPS measurements were carried out to reveal the possible change in morphology and structure of the Ni2P/TiO2 composites after photocatalytic, thermocatalytic and photothermocatalytic reactions, respectively. UV-Vis diffuses reflectance spectra measurement was performed to find the optical properties of the composites after various reactions. Moreover, photocurrent at different temperatures was investigated for further understanding of the reaction mechanism. Additionally, the intermediates related to the products in photothermocatalytic hydrogen evolution reaction were also analyzed. Finally, the tentative mechanism for the photothermocatalytic reaction over such hybrid catalyst system was proposed. 2. Experimental section 2.1 Chemicals and Materials All reagents used in the experiment were of analytical grade and without any further purification. Nickel acetylacetonate (Ni(acac)2, 95%), isopropanol, n-hexane, toluene acetone, methanol 4


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(CH3OH), formic acid(HCOOH), formaldehyde(HCOH), sodium sulfate (Na2SO4) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd.. Tri-n-octylphosphine (TOP, 97%) was purchased from Strem Chemicals, Inc.. 1-octadecence (ODE, 90%) and Oleylamine (70%) was purchased from Sigma-Aldrich. Commercial TiO2 P25 was purchased from Evonik Degussa Corporation. 2.2 Preparation of the Ni2P/TiO2 photocatalyst composites Synthesis of Ni2P nanoparticles. The Nickel phosphide nanoparticles were prepared by slightly modifying a previously reported process18. 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 50mL three-necked, round bottom flask. Meanwhile, the round bottom flask was also equipped with a thermometer and condensing unit. The above reaction mixture was heated accompanied with stirring in order to remove water and other possible low-boiling impurities. Under the condition of vacuum, tri-n-octylphosphine (2 mL, 4.4 mmol) was injected into the flask. The round bottom flask was then filled with Ar and raised to 300 °C with 30 minutes. It could be observed that the color of the solution exhibited a change and finally turned black starting at 220 °C. After heating the solution at 300 °C for 2 h, the heating mantle was turned off until the solution reached 200 °C. The flask was then kept away from the heating mantle and cooled down to room temperature. Then, the mixture was collected by centrifugation and washed with 1:3 (v:v) hexanes:isopropanol for three times, respectively. Finally, the sediment was suspended and stored in toluene for further use. Preparation of Ni2P/TiO2 nanoparticles. The Ni2P was anchored onto TiO2 nanoparticles as the previously reported method with a slight modification25. TiO2 nanoparticles was firstly added into toluene solution in order to obtain uniform dispersion. After being stirred for 0.5 h, Ni2P was 5



added with further stirring for 0.5 h. Next, the mixture was collected by centrifugation and washed with acetone along with the use of mild sonication. The sediment was dried at 80 oC for 0.5 h. Finally, the obtained samples were heated for 2.5 h at 450 oC under Ar atmosphere in order to remove the surface remainders and to ensure a potent solid-solid interface was formatted between the Ni2P and TiO2 nanoparticles. The reason we chose Ni2P(0.38 wt%)/TiO2 is due to its excellent activity near room temperature in the photocatalytic hydrogen evolution as already reported in our previous study26. 2.3 Measurement of catalytic activity The photocatalytic hydrogen activity of Ni2P/TiO2 was evaluated by using a 300 W xenon lamp as the ultraviolet-visible light source and the photocatalytic reactions were carried out in a side-irradiation reactor. The gas phase sampled with a syringe was analysed with a TCD SP2100 gas chromatograph (TDX-01 column, nitrogen as carrier gas). For the typical photocatalytic hydrogen evolution, 14 mg Ni2P/TiO2 photocatalyst powder was dispersed by a magnetic stirrer in the reactor containing 140 mL aqueous solution of methanol sacrificial agent (20 V%). The reactor was purged with N2 for 30 min before the hydrogen production to completely remove oxygen. Continuous magnetic stirring was used to keep the Ni2P/TiO2 particles suspended during the reaction. Unless otherwise mentioned, the reaction was kept at 35 ºC. Photothermocatalytic and thermocatalytic hydrogen production were carried out in the same reactor. Photothermocatalytic activity was determined at different temperatures (50-90 ºC), which were controlled by a heating tape. Thermocatalytic activity was tested as the photothermocatalytic process without light illumination. 2.3 Characterization 6


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The particle size and morphology of the samples were observed on transmission electron microscopy (TEM) images from a FEI Tecnai G2 F30 transmission electron microscope at an accelerating voltage of 300 kV. The crystallographic structure and some information on chemical composition of the composite particles were identified by Powder X-ray diffraction (XRD) using Cu Kα radiation (λ=0.15418 nm) at a scanning rate of 10°/min. The light absorbance of the samples was measured on a Hitachi U4100 UV-Vis spectrophotometer over a range of 300-800 nm. The Raman spectrum was performed at room temperature using Jobin-Yvon LabRam HR 800 micro-Raman spectrometer, and the excitation light was at 514.5 nm from an Ar+ laser with 30 mW output power. The electron paramagnetic resonance (EPR) spectrum was recorded on a Bruker A300-9.5/12 EPR spectrometer and microwave frequency = 9.40 GHz. Surface chemical analysis was performed by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis-Ultra DLD instrument, using a monochromatized Al-Kα X-ray source (150 W). Photoelectrochemical measurements were performed on a CHI 760D scanning potentiostat (CH Instruments). In a standard three-electrode system with the photocatalyst-coated FTO as the working electrode, Pt wire as the counter electrode, and an Ag/AgCl as a reference electrode. A 500 W xenon lamp coupled with an AM1.5 filter was used as the light source. A 0.5 M Na2SO4 solution was used as the electrolyte. The working electrodes were prepared by dropping a suspension (200 µL) made of Ni2P/TiO2 and TiO2 (3 mg Ni2P/TiO2 and TiO2 added into 3 mL ethanol and 600 µL Nafion mixed solution, respectively) onto the surface of a FTO plate (2 cm * 2 cm). The working electrodes were dried at room temperature. The photoresponsive signals of the samples were measured under chopped light at 0.6 V vs. Ag/AgCl. 3. Results and discussion 7



3.1 Morphologies and crystalline properties

Figure1. X-ray diffraction patterns of (a) Ni2P and (b) TiO2, Ni2P/TiO2 photocatalysts with 0.38 wt% Ni2P loading amount. (c) Low- and d) high-magnification TEM images of Ni2P/TiO2, (e-i) TEM images and the corresponding EDX elemental mapping of Ti, O, Ni and P for Ni2P/TiO2.

X-ray powder diffraction (XRD) was utilized to explore the crystal phase of the obtained products. XRD pattern of Ni2P (Figure 1(a)) shows that the crystal structure of the as-prepared nanocrystals can be assigned to the hexagonal phase (JCPDS NO. 03-0953)27. No other impurities are found. Additionally, the XRD patterns of the TiO2 and Ni2P/TiO2 are shown in Figure 1(b). Evidently, no obvious diffraction peaks belonging to Ni2P can be observed and all the reflections show no significant difference from the TiO2, probably due to the strong diffraction peaks of TiO2 and the relatively small size and low amount of Ni2P. However, as shown in Figure1(c) and (d), the TEM and high-resolution transmission electron microscopy (HRTEM) images of Ni2P/TiO2 clearly exhibit the lattice spacing of 0.22 nm and 0.35 nm, which well correspond to the (111) plane of Ni2P and (101) plane of TiO2, respectively27,28. In addition, Figure 1(e) gives the scanning TEM (STEM) image. And the corresponding EDS elemental mapping images of Ti, O, Ni and P 8


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are exhibited in Figure 1(f-i), respectively. It reveals that Ti and O elements are dispersed uniformly in the scanned area. And the Ni and P mapping image shows the same outline as the spot in Figure 1(e). Above results imply that the Ni2P nanopartilces have been successfully loaded onto the TiO2 surface. 3.2 Photothermocatalytic hydrogen production

Figure 2. (a) Hydrogen production over Ni2P/TiO2 under photo and photothermocatalytic(50-90 °C) conditions, respectively; (b) the comparison of 3 h H2 generation over Ni2P/TiO2 under photocatalytic (35 °C), thermocatalytic and photothermocatalytic (50-90 °C) reaction conditions, respevtively. The system contains 14 mg Ni2P/TiO2 photocatalyst in a 140 mL aqueous solution containing 20 vol% methanol.

The activities of hydrogen evolution reaction over Ni2P/TiO2 under photocatalytic condition at photo (35 ºC), and photothermocatalytic and thermocatalytic (without light irradiation) conditions within the range of 50-90 ºC were then determined. Figure 2(a) shows the amount of photothermocatalytic hydrogen evolution versus time over Ni2P/TiO2. It reveals that the increase of the temperature can greatly promote the photothermocatalytic activity. The detailed values of the catalytic performance under various temperature after 3 h were found to be 64.5, 108.3, 135.8, 172.5, 223.1, 277.4 µmol, respectively. To confirm the synergetic effect of photothermocatalytic reactions, the photocatalytic hydrogen evolution amount at 35 ºC was simply added with the hydrogen evolution amount under thermocatalytic reaction condition, at each temperature, as shown in Figure 2(b). It can be clearly seen that the amounts of hydrogen released at 9



photothermocatalytic are much higher than the sum of hydrogen produced by photocatalytic and thermocatalytic reactions at all temperatures. For instance, the photocatalytic hydrogen evolution amount at 35 ºC is 75.5 µmol, and the thermocatalytic hydrogen evolution amount at 90 ºC is 1.63 µmol. The total amount is far less than the photothermocatalytic hydrogen evolution amount at 90 ºC (277.4 µmol). The photothermocatalytic hydrogen activity (90 ºC) is about 3.6 times of the sum of the photocatalytic and thermocatalytic hydrogen production. Thus, at present stage, it can be concluded that the photothermocatalytic hydrogen evolution reaction is not simply the sum of the photocatalytic and thermocatalytic hydrogen evolution reactions but their synergetic effects. That is to say, the Ni2P/TiO2 may also exhibit excellent photothermocatalytic synergetic effect similar to that of Pt/TiO2 reported by our previous work19.

Figure 3. The photothermocatalytic hydrogen evolution stability of the Ni2P/TiO2 catalyst. The system contains 14 mg Ni2P/TiO2 photocatalyst in a 140 mL aqueous solution containing 20 vol% methanol at 90 ºC. After every 3 hours, the produced H2 was evacuated.

To evaluate the photothermocatalytic hydrogen evolution stability of the Ni2P/TiO2 catalyst, the recycling experiment at 90 ºC was also performed. In each of the four runs, the reaction system was refreshed by bubbling N2 into the solution for 30 min. As shown in Figure 3, there is no obvious decrease in the H2 production after the four cycles. It suggests that the Ni2P/TiO2 catalyst 10


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has quite good stability toward photothermocatalytic hydrogen evolution and sustains its photothermocatalytic activity throughout the H2 evolution reaction. 3.3 Photoinduced charge carrier transportation at different temperatures In order to understand the origin of photothermocatalytic synergetic effect, the mechanism of thermocatalytic and photocatalytic reaction is discussed first. Under the thermocatalytic reaction, there is barely hydrogen produced, while after coupling photo with thermal conditions, significantly enhanced hydrogen production can be obtained(Figure 2(b)). For photocatalytic reaction, the mechanism have been explained by photoinduced electrons(e-) and holes(h+) and subsequent transfer of e- to reactants for hydrogen evolution. Tentatively, considering the mechanisms of thermocatalysis with photocatalysis, it can be deduced that there exist some factors that could bridge the gap between thermocatalytic and photocatalytic process. Our previous study revealed that the incorporation of Ni2P to TiO2 could efficiently enhance the separation of photoinduced electrons and holes and promote the photocatalytic hydrogen evolution activity26. Thus, it is speculated that the further enhanced photoinduced electrons and holes separation efficiency with the increasing temperature may be one of the reasons attributed to the photothermocatalytic synergy effect. Therefore, the electrons and holes separation property versus temperature among the photothermocatalytic hydrogen evolution reaction has been studied firstly. (a) 10

90 °C 80 °C 70 °C 60 °C 50 °C

8

(b) -19.0

2

R =0.998 -1

Eact=23.16 kJ⋅mol

-19.2

-19.4

lnk

Photocurrent density (µA⋅cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

-19.6

-19.8

4

-20.0

2 30

61

92

123

154

185

216

2.70

2.75

2.80

2.85

Time (s)

2.90

2.95 -3

3.00

3.05

3.10

3.15

-1

1/T(×10 K )

Figure 4. (a) Photocurrent of Ni2P/TiO2 at 50-90 ºC, (b) Hydrogen production plot of lnk vs. 1/T and a straight regression lines for Ni2P/TiO2 under photothermocatalytic reaction conditions in the presence of 20 vol% CH3OH. 11



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k is defended as rate constants.

Figure 4(a) displays photocurrent transient responses with each switch-on/off operation of Ni2P/TiO2 nanohybrids at different temperatures. Undoubtedly, larger photocurrents are observed at higher temperature. Therefore, the enhanced electron–hole separation ability occurred at higher temperature could be an important contribution to the excellent photothermocatalytic hydrogen production activity. Hussein et al. proposed that apparent activation energy is insensitive to reactants resulted from trap-hindered transport of photoexcited carriers29,30. Namely, mobility of photoexcited carriers would be a factor responsible for the positive apparent activation energy for water splitting reaction because detrapping of photoexcited carriers could be promoted by increase the reaction temperature. According to the result in Figure 2(b), the apparent activation energy (Eact) for Ni2P/TiO2 presuming Arrhenius behavior at 50-90 °C, is estimated to be 23.16 kJ·mol-1 (Figure 4(b)), which is somewhat lower than the Eact of the water photolysis of the gold nanorods deposited on a single TiO2 crystal (Eact = 28 kJ·mol−1)31. Apparently, the lower apparent activation energy in our case further indicates that the fast transfer and separation of electrons and holes at higher temperatures, which can efficiently suppress the recombination of electron–hole and, consequently, enhance the hydrogen production activity32. 3.4 The effect of intermediates on the photothermocatalytic hydrogen production The intermediates related to the products in photothermocatalytic hydrogen evolution reaction were also analyzed. Generally, when the CH3OH is used as sacrificial regent, the reaction steps relating H2 formation are as follows33: Ni2P/TiO2+hv→e-+h+

(1)

H2Oads+h+→OH+H+

(2)

12


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OH+CH3OH→CH2OH+H2O

(3)

2H++2e-→H2

(4)

It is believed that the CH3OH decomposition process as raw material for H2 production also occurs when using CH3OH as sacrificial regent. According to the literature34-37, the hydrogen produced process are as follows: CH3OH+2h+→CH2O+2H+

(5)

CH2O+H2O+2h+→HCOOH+2H+

(6)

HCOOH+2h+→CO2+2H+

(7)

HCOOH→CO+H2O

(8)

2H++2e-→H2

(4)

Thus, in order to estimate which intermediate products are mainly influenced by the reaction temperature in the photothermocatalytic hydrogen evolution process, 20% CH3OH, 20% HCOH and 20% HCOOH, respectively, were employed and compared for photothermocatalytic hydrogen evolution experiments under different temperatures. (a)

(b)

HCOH HCOOH CH3OH

HCOH HCOOH

8

Photothermocatalytic

Thermocatalytic

CH3OH

H2 Production (µmol)

250

H2 Production (µmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

150

100

6

4

2 50

0

50

60

70

80

0

90

50

Temperature (°C)

60

70

80

90

Temperature (°C)

Figure 5. The hydrogen evolution amounts at different temperature in the presence of CH3OH, HCOH and HCOOH under (a) photothermocatalytic and (b) thermocatalytic reaction conditions over Ni2P/TiO2.

As shown in Figure 5(a), for CH3OH, HCOH and HCOOH, the hydrogen evolution amount all increase with the increasing temperature under photothermocatalytic reaction condition. However, under thermocatalytic reaction, the highest hydrogen evolution amount is 8.0 µmol after 3 h as

13



shown in Figure 5(b). Considering that in the photothermocatalytic hydrogen production condition, the amount of HCOH and HCOOH, as intermediate products, should be very small. So the effects of these species can be neglected. The above results indicate that intermediate products are not the mainly influencing factor for the excellent photothermocatalytic hydrogen evolution over our hybrid catalyst. 3.5 The in situ generation of oxygen vacancies during the photothermocatalytic hydrogen production Therefore, characterizations of Ni2P/TiO2 before and after various reactions were carried out for the further understanding of the photothermocatalytic reaction mechanism. For the convenience of comparison, Ni2P/TiO2 after photothermocatalytic reaction at 90 °C in methanol is labeled as Ni2P/TiO2-PT throughout the following characterization and discussion. Similarly, Ni2P/TiO2 after photocatalytic reaction at 35 °C and after thermocatalytic reaction at 90 °C in methanol are labeled as Ni2P/TiO2-P and Ni2P/TiO2-T, respectively. (b)

Ni2P/TiO2 Ni2P/TiO2-P

Ti 2p

458.6

(c)

Ni2P/TiO2 Ni2P/TiO2 -P Ni2P/TiO2 -T

Ni2P/TiO2-PT

Ni2P/TiO2 -PT

Intensity (a.u.)

Ni2P/TiO2-T

Ni2P/TiO2

Intensity (a.u.)

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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464.3 458.5

Ni2P/TiO2-PT 464.2

1.96

1.98

2.00

2.02

2.04

466

464

g Value

462

BE (eV)

460

458

456

10

20

30

40

50

60

70

80

2 Theta (degree)

Figure 6. (a) EPR spectra of Ni2P/TiO2, Ni2P /TiO2-P, Ni2P /TiO2-T and Ni2P /TiO2-PT; (b) the Ti 2p XPS spectra of Ni2P/TiO2 and Ni2P /TiO2-PT, (c) XRD patterns of Ni2P/TiO2, Ni2P /TiO2-P, Ni2P /TiO2-T and Ni2P /TiO2-PT.

Figure 7. TEM images of Ni2P /TiO2, Ni2P /TiO2-PT and TiO2-PT.

EPR

spectra

Ni2P/TiO2

before

and

after

photocatalytic,

14


thermocatalytic

and

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photothermocatalytic tests are shown in Figure 6(a). As reported in the literatures, the isotropic signal appeared at about g = 2.005 is corresponding to the electrons trapped in the oxygen vacancy (defined as VO)38,39. For Ni2P/TiO2 after various reactions, the isotropic signal showed stronger intensity, indicating the higher concentration of oxygen vacancy (VO). Evidently, Ni2P/TiO2-PT is the most prominent one. It is worth noting that no direct signal of Ti3+ is observed because of its weak signal intensity40 and also just a small portion of Ti3+ existing in the thin surface shell. The changing of oxygen vacancy concentration may accompany the change of bonding environment of Ti atoms. Thus, XPS measurements were carried out, shown in the Figure 6(b). Obviously, after photothermocatalytic hydrogen reaction, the Ti 2p peaks shift to lower binding energies. This effectively proves the presence of Ti3+ caused by oxygen vacancies41. While in the XRD patterns (Figure 6(c)), the weaker diffraction peak at 2θ = 27.5° for Ni2P/TiO2-PT also suggests the more disordered surface structure and the generation of oxygen vacancies42. The disordered surface structure of the Ni2P/TiO2-PT and TiO2-PT sample can be further confirmed by TEM images(Figure

7).

Obviously,

the

Ni2P/TiO2

sample

is

well-crystallized,

displaying

clearly-resolved and well-defined lattice fringes, even at the edge of the nanocrystals, while the Ni2P/TiO2-PT sample exhibits a unique crystalline core/disordered shell structure, characterized by a ~1.5 nm-thick disordered surface layer. However, no disordered surface layer appears around the TiO2 particles after photothermocatalytic reaction, which illustrate that Ni2P plays an essential role for realizing the disordered surface of TiO2.

15



(b)

Ni2P/TiO2 Ni2P/TiO2 -P

200

150

160

170

-1

22

υ )22 (α hυ

Ni2P/TiO2-PT

2.0

180

2.5

3.0

400

600 -1

800

300

350

400

3.5

hυ

Raman Shift (cm ) Eg(3) A1g+B1g

Raman Shift (cm )

Ni2P/TiO2-PT

Ni2P/TiO2-T

Intensity (a.u.) B1g

Ni2P/TiO2

Ni2P/TiO2-P

Ni2P/TiO2 -PT

140

Eg(2)

(c)

Ni2P/TiO2

Ni2P/TiO2 -T

450

500

4.0

550

Wavelength (nm)

Intensity (a.u.)

Eg(1)

Intensity (a.u.)

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.0

3.5

3.0

2.5

2.0

1.5

1.0

Binding energy (eV)

Figure 8. (a) Raman spectra (Inset: the enlarged region of the most intense Eg peak) and (b) UV-vis diffuse reflectance spectra (Inset: Plots of the Kubelka−Munk function against photon energy) of Ni2P/TiO2, Ni2P /TiO2-P, Ni2P /TiO2-T and Ni2P /TiO2-PT; (c) Valence band XPS spectra of Ni2P/TiO2 and Ni2P /TiO2-PT.

Also, the Raman spectra Figure 8(a) reveal that the broadening of the Eg(1) modes and the position is slightly shifted toward higher wavenumber detected in Ni2P/TiO2 after photocatalytic, thermocatalytic and photothermocatalytic reactions, which are attributed to the oxygen vacancies generated in the oxides43. Meantime, the larger blue-shift and broadening Raman bands in Ni2P/TiO2-PT can be related to its more severely disordered surface structure44. The disordered surface structure generated in semiconductors can yield mid-gap centers, forming a continuum extending to and overlapping with the conduction band edge and sequentially enhancing the light harvesting11. From the UV-Vis diffuse reflectance spectra (DRS) (Figure 8(b)) the band gap of the Ni2P/TiO2-PT, calculated based on the K-M function(the inset of Figure 8(b)), is slightly lower than that of Ni2P/TiO2. Combined with the valence band XPS(Figure 8(c)) that suggest unchanged valence band, the conduction band might have a mild transfer. Namely, there may be conduction band tail states resulted from oxygen vacancy that extend below the conduction band minimum. It is presumably responsible for the enhanced optical absorption. On the other hand, the chemical states of Ni2P before and after photothermocatalytic reaction have been investigated by the XPS measurements, and the results are shown in the Figure 9. No

16


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observable Ni 2p and P 2p signals, owing to the composition of Ni2P, can be detected due possibly to its low mass loading. Regarding to the weak signal centered at 133.8 eV, it is assigned to P-O species from the surface oxidation23,45,46. Therefore, compared to the P-O species signal, the relative low intensity of P ஔି signal around 129.5 eV resulted in the unobservable signals in our experiments23,45,46. However, P-O species can still be detected after photothermocatalytic reaction, indicating the good stability of Ni2P in the reaction process, which is consistent with the stability test in the Figure 3. (a)

(b)

Ni 2p

P 2p

Ni2P/TiO2-PT

Ni2P/TiO2

880

875

870

865

860

855

Ni2P/TiO2-PT

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


850

Binding energy (eV)

128

Ni2P/TiO2

130

132

134

136

138

140

Binding energy (eV)

Figure 9. The (a) Ni 2p and (b) P 2p XPS spectra for Ni2P/TiO2 and Ni2P /TiO2-PT, respectively.

The possible in situ generation of oxygen vacancy mechanism can be explained by the simultaneous deeper hydrogenation occurred on the Ni2P/TiO2 in the photothermocatalytic water splitting reaction as follow40,47-49: Ni2P/TiO2+hv→e-+h+

(1)

H2Oads+h+→OH+H+

(2)

CH3OH+h+→CH2OH+H+

(3)

H++e-+~Ni2P/Ti-O-Ti-O~→Ni2P/~Ti-OH+Ni2P/VO-Ti-O~

(9)

Specifically, in the photocatalytic H2 evolution process over Ni2P/TiO2, when TiO2 nanoparticles are illuminated by light, the valence band electrons are excited to the conduction band with holes left in the valence band, and then the photo-generated electrons rapidly transfer to Ni2P for reduction H2O into highly active atomic hydrogen [H] species and then generation H2 (route 1 and 2 in Figure 10). Accordingly, during this process, a small portion of the highly-reactive [H] species will further migrate along the Ni2P surface to the TiO2 nanoparticles in 17



the vicinity and react with them to tinily reduce the lattice Ti4+ and generate point defects such as oxygen vacancies in the TiO2 surface (route 3 in Figure 10)40,47. Evidently, when introducing thermal energy into the photocatalytic system, the reaction between the discrete [H] and TiO2 can be significantly promoted inducing a strengthened hydrogenation and creating a more disordered surface on TiO2. The oxygen vacancies in turn can promote the light adsorption due to the decrease of band gap and facilitate the interfacial transfer of photogenerated electrons (route 4 and 5 in Figure 10)40,47. Moreover, the presence of oxygen vacancies on the shell of TiO2 would also increase the adsorption of the methanol molecules50,51, which are spontaneously dissociated on the surfaces with oxygen vacancies by forming alkoxide and hydroxide groups. These alkoxide species, more photocatalytically reactive than the physisorbed species52,53, can quickly scavenge the photogenerated holes54, thus, partially resulting in enhanced H2 evolution as well.

Figure 10. The illustration of the possible mechanism of the photothermocatalysis for hydrogen evolution over Ni2P/TiO2 nanoparticles.

According to the literatures and our experimental characterization, we speculate that there are three main roles of thermal energy for promoting hydrogen evolution. Firstly, coupling thermal energy to photocatalytic H2 evolution reaction can reduce the recombination of photoinduced electron-hole as the temperature increases, which has been proved by the increased transient photocurrent intensity with the gradually promoted temperature. Secondly, as a typical synergistic effect, when introducing thermal energy into the photocatalytic reaction, the reaction between the discrete [H] and TiO2 surface can significantly promote the hydrogenation of TiO2 to create a disordered surface (oxygen vacancies). The generated oxygen vacancies in the shell in turn

18


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promote the light adsorption due to the local states below the conduction band edge and facilitate the transfer and separation of photo-generated electrons. Moreover and thirdly, the presence of oxygen vacancies on the shell of TiO2 would also increase the adsorption of the methanol molecules50,51, which are spontaneously dissociated on the surfaces with oxygen vacancies by forming alkoxide and hydroxide groups. These alkoxide species, more photocatalytically reactive than the physisorbed species52,53, can quickly scavenge the photogenerated holes54, thus, partially resulting in enhanced H2 evolution as well. Finally, the better H2 evolution performance is achieved in the photothemocatalytic reaction in the methanol aqueous solution after coupling photocatalytic reaction with thermal energy. 4. Conclusion In summary, in the aim of the full-spectrum exploitation of solar energy, the photothermocatalytic hydrogen evolution reaction over a Pt-free integrated catalytic system has been attempted. The photothermaocatalytic amounts of H2 were found to be much higher than that of the sum of photocatalytic and thermocatalytic reactions at each temperature. It can be concluded that Ni2P/TiO2 nanocomposites can effectively couple photo and thermal energy and thus exhibit significantly improved photothermal performance much higher the simple sum of the both. Moreover, the possible reaction mechanism of the photothermocatalytic was proposed in detail. Based on the results, the excellent photothermocatalytic hydrogen evolution was attributed to the enhanced charge transportation properties and in situ generation of oxygen vacancies by simultaneous hydrogenation of Ni2P/TiO2. In addition, the long-term stability of the Ni2P/TiO2 catalyst was also proved by several runs of repeated cycles of photothermocatalytic hydrogen evolution tests, indicating its excellent stability and great potential for toward practical use. In 19



summary, we have reported a promising approach of coupling thermocatalytic with photocatalytic effects for the full-spectrum use of solar energy over a noble metal free catalyst system. 5. Acknowledge The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (No. 51422604, 51776165). This work was also supported by the China Fundamental Research Funds for the Central Universities. Reference ( 1)

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