CdS photocatalysts for enhanced hydrogen evolution in water spitting and mechanism of enhancement

Accepted Manuscript RuP2/CdS photocatalysts for enhanced hydrogen evolution in water spitting and mechanism of enhancement

Limin Song, Qian Chen, Shujuan Zhang PII: DOI: Reference:

S0032-5910(18)30482-0 doi:10.1016/j.powtec.2018.06.042 PTEC 13475

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

17 September 2017 14 April 2018 25 June 2018

Please cite this article as: Limin Song, Qian Chen, Shujuan Zhang , RuP2/CdS photocatalysts for enhanced hydrogen evolution in water spitting and mechanism of enhancement. Ptec (2018), doi:10.1016/j.powtec.2018.06.042

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ACCEPTED MANUSCRIPT RuP2/CdS photocatalysts for enhanced hydrogen evolution in water spitting and mechanism of enhancement Limin Songa, Qian Chena, Shujuan Zhangb,* a

College of Environment and Chemical Engineering & State Key Laboratory of

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Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic

College of Science, Tianjin University of Science & Technology, Tianjin, 300457, P.R.

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b

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University, Tianjin 300387, P. R. China.

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E-mail address: [email protected]

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*Corresponding author. Tel./Fax: +86-22-83955458

ACCEPTED MANUSCRIPT ABSTRACT: RuP2 nanoparticles and CdS nanorods were prepared by temperature-programmed reduction and a solvothermal route, respectively. Their intrinsic properties were characterized with X-ray powder diffractometry, high-resolution transmission electron

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microscopy, ultraviolet-visible absorption spectroscopy and photoluminescence

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spectroscopy. RuP2 as a co-catalyst significantly enhanced the photocatalytic H2

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evolution performance of CdS in water spitting under visible light radiation (λ ≥ 450). The H2 evolution amount of the 10 wt.% RuP2/CdS reached 164008 µmol/g after 10

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h of radiation, which was 6.51 times that of 0.1 wt.% Ru/CdS (25238 µmol/g) and 2.3

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times that of 0.1 wt.% Pt/CdS (58359 µmol/g). The mean H2 evolution rate of 10 wt.% RuP2/CdS in 10 h is 17362 µmol/g/h, which is 8.49 times that of 0.1 wt.% Ru/CdS

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(2044 µmol/g/h) and 5.16 times that of 0.1 wt.% Pt/CdS (3364 µmol/g/h). The 10 wt.%

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RuP2/CdS after 34 h reached an apparent quantum yield of 31%. The 102-hour longtime tests showed that the 10 wt.% RuP2 improved the H2 evolution ability,

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lifespan and stability of CdS. The activity, photoluminescence and photocurrent

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experiments indicated the RuP2 cocatalyst in RuP2/CdS under light radiation intensively attracted photo-electrons and transferred them from CdS to RuP2, thus greatly promoting the separation of photo-generated charges and producing more photo-electrons to react with the H+ absorbed on RuP2 nanopartilces, which helped to improve the photocatalytic H2 evolution performance. Keywords: RuP2/CdS, Co-catalyst, H2 evolution, Enhanced mechanism

ACCEPTED MANUSCRIPT 1. Introduction H2 energy is an environmental-friendly and economical fuel owing to the 3-fold higher burning value (1.4×105 kJ/kg) than gasoline and the clean product of vapor after H2 combustion. At present, photocatalytic water spitting is considered to be the major

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direct and easy way to produce H2 [1-3]. However, the important guarantee of efficient

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photocatalytic H2 production in water spitting is the use of excellent photocatalytic

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materials [4,5]. As reported [6-9], CdS (Eg = 2.4 eV) is a suitable photocatalyst for H2 evolution under visible light radiation because it can be excited by visible light (≤ 510

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nm) and has a low conduction band (< 0 eV). It is well-known that the H2 evolution

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ability of the main catalyst in photocatalytic water spitting can be greatly enhanced by the addition of a cocatalyst. For instance, co-catalysts MoS2 [10], ZnO [11], Pt [12], Au

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[5], and Ag [13] can significantly improve the photocatalytic water spitting ability of

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CdS. However, these co-catalysts of metal sulfides, metal oxides and noble metals are not stable in photocatalytic reactions [14]. Metal sulfides and noble metals easily erode

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by light radiation, while metal oxides can be easily dissolved by acids. Therefore,

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looking for new high-activity and high-stability co-catalysts is still a huge challenge. Transitional metal phosphides of Ni2P [15,16], CoP [17], FeP [18], Cu3P [19] and MoP [20] are reportedly effective cocatalysts over CdS for photocatalytic H2 production, owing to their resistance against light, acids and alkali corrosion in photocatalytic water spitting as well as high stability [21]. These metal phosphides can also significantly improve the photocatalytic H2 production ability of CdS [22]. Therefore, transitional metal phosphides are suitable cocatalysts for CdS in photocatalytic water spitting. In

ACCEPTED MANUSCRIPT this study, we report a new RuP2 cocatalyst for CdS, which significantly enhances the activity and stability of CdS in photocatalytic water spitting. The mechanism and process of enhancement were discussed in detail. 2. Experimental

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2.1. Synthesis of RuP2 and CdS

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RuP2 was prepared by a temperature-programmed reduction (TPR) method.

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Specifically, appropriate amounts of RuCl3 and (NH4)2HPO4 were dissolved in 20 mL of deionized water and evaporated to dryness in a water bath. Then the pulverized

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powder was calcined at 500 ℃ for 3 h to form a precursor, which was then heated at a

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rate of 1 ℃/min to 750 ℃ in a H2 flow and was maintained there for 2 h. The final powder was RuP2. CdS nanorods were prepared by a solvothermal way. Specifically,

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appropriate amounts of NaS2·9H2O and Cd(NO3)2·4H2O were dissolved in 70 mL of

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ethylenediamine. Then the mixture was placed in a teflon-lined autoclave, heated to 200 ℃ and maintained there for 72 h. The powder in the autoclave was washed to form

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CdS nanorods.

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2.2. Characterization of RuP2 and CdS The compositions and phases of RuP2 and CdS were determined by a Rigaku D/max 2500 X-ray powder diffractometer (XRD) (Rigaku Corporation, Japan; CuKα, λ=1.5406 Å). Their light absorption performances were studied by a UV2700 ultraviolet-visible (UV−vis) spectrometer (Shimadzu, Japan; BaSO4). Their photoluminescence (PL) properties were surveyed by an F-380 PL spectrometer (Gangdong, China; excitation at 250 nm, room temperature). The morphology and

ACCEPTED MANUSCRIPT nature of particles were observed by a JEM 2100 high-resolution transmission electron microscope (HRTEM; JEOL, Japan; 200 kV). The photocurrent ability was measured in a 660E electrochemical workstation (Shanghai Chen Hua Instrument Co. Ltd., China).

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2.3. Activity measurement

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The photocatalytic H2 production activity was tested in 50-mL sealed tubes under

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visible light radiation (λ ≥ 450 nm; 10 of 5 W light-emitting diode light, 12.6 mW/cm2). Specifically, 10 mL of L-(+)-lactic acid (a hole sacrificial agent), RuP2 and CdS (50 mg)

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were added to 40 mL of deionized water. The mixture was air-exhausted by a vacuum

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pump for 30 min. The produced H2 was detected on a GC7890 gas chromatograph (Rainbow Chemical Instrument Co. Ltd., Shandong Lunan, China) equipped with a

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thermal conductivity detector (3 m × 3 mm, 5 Å molecular sieve column).

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The apparent quantum yield (AQY) was calculated as follows: AQY= 2×number of evolved H2 molecules/ number of incident photons×100%

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3. Results and discussion

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3.1. Characterization of photocatalysts Figure 1 shows the XRD patterns of the photocatalysts. The main product of ruthenium phosphide is the orthorhombic RuP2 (JCPDS No. 34-0333) (Fig. 1A). The XRD peaks of RuP2 are assigned to the (110), (020), (011), (120), (101), (210), (111), (220), (121), (130), (211), (310), (031) and (211) crystal planes (space group: Pnnm [58], a=5.116 Å, b=5.891 Å, c= 2.870 Å). The crystallinity and mean crystal size of RuP2, estimated from the strongest peak of (110) on Jade 6.0, are 83.29% and 51 nm,

ACCEPTED MANUSCRIPT respectively. The high crystallinity of RuP2 resulting from the synthesis at 750 ◦C can reduce the numbers of crystal defects and photo-generated charge recombination centers and accelerate electron mobility, which greatly improve the photocatalytic ability. Besides RuP2, there is a small amount of RuP4 (JCPDS No. 32-0977) and its

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little peaks are marked as “0” (Fig. 1A). The appearance of RuP4 may be attributed to

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the high-temperature calcination and the Ru-P ratio. As showed on the XRD patterns

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of CdS nanorods (Fig. 1B), the main product is pure and hexagonal CdS (JCPDS No. 89-2944), without other impureness of sulfides. All the peaks of the product can be

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assigned to the (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202)

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crystal planes (space group: P63mc [186], a=b=4.140 Å, c= 6.715 Å). The crystallinity of CdS nanorods, estimated from the strongest peak of (101) on Jade 6.0, is 77.8%.

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Their average crystal sizes were not estimated because the sizes of CdS nanorods were

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irregular.

Figure 2a shows the TEM images of CdS. Clearly, CdS is rod-like shaped with

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smooth surfaces and good dispersion, without serious stacking (Fig. 2a). The diameter

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is about 55-60 nm and the length is about several microns. The HRTEM image (Fig. 2b) shows clear crystal lattice fringes. The interplanar distance is around 0.206 nm in agreement with the (110) plane of CdS, which indicates the CdS nanorods grow in parallel to the c-axis. As showed in Fig. 1b, the (100) plane is very stronger than other peaks, indicating (100) plane is stable and more exposed on the outer surface. As reported [23], the (100) plane has the highest surface energy of 0.02657 eV/A compared with other planes of hexagonal CdS. The exposed (100) plane with high surface energy

ACCEPTED MANUSCRIPT may significantly improve photocatalytic ability. As showed on the TEM (Fig. 2c), the RuP2 particles are sphere-like with a clear outline, but without serious reunion. The mean size is about 51 nm estimated by the particle statistics in Fig. 2d, which is consistent with the XRD result, indicating RuP2 may be single crystal nanoparticles.

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Moreover, the sizes of RuP2 particles are mainly between 40-50 nm. As showed on the

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HRTEM image of RuP2 (Fig. 2e), the crystal lattice distance is about 3.88 nm

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corresponding to its (110) plane. Moreover, the selected area electron diffraction (SAED) pattern of RuP2 is a lattice structure of single crystals, which is consistent with

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the above results. The SAED dots can be assigned to the (110) and (210) planes, which

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further prove the nature of RuP2.

As showed on the UV–vis absorbance spectra of CdS nanorods and 10 wt.%

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RuP2/CdS (Fig. 3), CdS shows a strong absorption at 400-650 nm (Fig. 3A) and an

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absorption edge around 570 nm. The band gap energy (Eg) of CdS, calculated from the plots of (λhv)1/2 vs. photon energy (hv) in Fig. 3B, is 1.98 eV, indicating CdS can absorb

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and be excited by visible light. The 10 wt.% RuP2/CdS and CdS nanorods have similar

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UV–vis absorbance spectra (Fig. 3A) because the little amount of RuP2 and CdS were mixed mechanically. The 10 wt.% RuP2/CdS has a band edge of ~ 570 nm and an Eg vague of ~ 1.98 eV (Fig. 3B). However, the 10 wt.% RuP2/CdS has an obvious absorption at 570-800 nm (Fig. 3A), which is absent from the CdS nanorods. This result can be attributed to addition of RuP2 into CdS. A wide visible light absorption helps to enhance the photocatalytic ability of CdS. The valance band (VB) top (EVB) and the conduction band (CB) bottom (ECB) of CdS

ACCEPTED MANUSCRIPT were calculated as follows [24]:

EVB  X  E e  0.5Eg

ECB  EVB  Eg where X is the geometric mean electronegativity of CdS (2.14 eV); Ee is the energy of

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free electrons at the hydrogen scale (4.5 eV); Eg is the band gap value of CdS. EVB and

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ECB were calculated to be -1.27 and -3.25 eV, respectively. CdS and 10 wt.% RuP2/CdS

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have similar values of both VB and CB, but their VB levels are very low, indicating a high electron-donating ability, which is conducive to the combination with H+ to form

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H2.

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Figure 4 shows PL spectra of CdS nanorods and the 10 wt.% RuP2/CdS. A fluorescent substance is excited by light, and the excited atoms or molecules that re-

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emit electromagnetic radiation can produce the fluorescence during the de-excitation.

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Moreover, the PL energy intensity (Y axis) reflects the quenched degree of photogenerated charges under light radiation. A high-intensity fluorescent peak indicates the

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sample is excited to produce a large amount of high-energy photons. Therefore, the

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CdS PL spectrum shows a strong peak at 501 nm, indicating the excited CdS can reemit the photons at 501 nm (Fig. 4). However, the excitation of the 10 wt.% RuP2/CdS shows a relatively weak peak at 501 nm compared with CdS nanorods. The RuP2 added into CdS can absorb photo-electrons and transfer them from CdS to RuP2, so RuP2 shows a lower-energy PL peak. It is indicated that the addition of RuP2 into CdS can greatly accelerate the separation of photo-generated charges under light radiation, producing more photo-electrons, which helps to improve the photocatalytic

ACCEPTED MANUSCRIPT performance. 3.2 Photocatalytic activity The H2 evolution experiment in water splitting was conducted under visible-light radiation (λ ≥ 450 nm) in home-made 50 mL tubes. Figure 5A shows the H2 evolution

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data of 0.05 wt.%, 0.1 wt.%, 0.2 wt.% Pt/CdS; 5 wt.%, 10 wt.%, 15 wt.%, 40 wt.%

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RuP2/CdS; 0.05 wt.%, 0.1 wt.%, 0.2 wt.% Ru/CdS after 10 h of visible light radiation.

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The H2 evolution amounts for the ten compounds are 54589, 58359, 43148, 107535, 164008, 64666, 25238 24762, 25238 and 20606 µmol/g, respectively, indicating the H2

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evolution amounts of all the RuP2/CdS compounds except 40 wt.% RuP2/CdS are

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higher compared with the 0.1 wt.% Pt/CdS and 0.1 wt.% Ru/CdS. The activity of 0.1 wt.% Pt/CdS is the best among the 0.05 wt.%, 0.1 wt.% and 0.2 wt.% Pt/CdS, the

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activity of 0.1 wt.% Ru/CdS is the best among the 0.05 wt.%, 0.1 wt.% and 0.2 wt.%

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Ru/CdS, and the H2 evolution amount of 0.1 wt.% Pt/CdS is higher than that of 0.1 wt.% Ru/CdS because Pt has a larger work function than Ru (5.65 vs. 4.71 eV). The Pt with

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large work function can intensively absorb photo-electrons, accelerate the separation of

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photogenerated charges and promote the electron utilization efficiency. The H2 evolution amount of 10 wt.% RuP2/CdS is 6.5 times that of 0.1 wt.% Ru/CdS and 2.3 times that of 0.1 wt.% Pt/CdS, indicating the addition of RuP2 significantly improves the H2 evolution ability of CdS. It is clear that the H2 evolution amount of RuP2/CdS rises with increasing ratio of RuP2 within 5 to 10 wt.%. When the ratio of RuP2 rises to 15 and 40 wt.%, the H2 evolution amount greatly decreases (Fig. 5A). Therefore, the H2 evolution amount was maximized at the ratio of 10 wt.% RuP2. As showed in Fig.

ACCEPTED MANUSCRIPT 5B, the mean H2 evolution rates of 0.1 wt.% Pt/CdS; 5 wt.%, 10 wt.%, 15 wt.%, 40 wt.% RuP2/CdS; 0.1 wt.% Ru/CdS after 10 h of visible light radiation are 3364, 6841, 17362, 3968, 2045 and 2044 µmol/g/h, respectively. The mean rate of 10 wt.% RuP2/CdS is 5.16 times that of 0.1 wt.% Pt/CdS and 8.49 times that of 0.1 wt.% Ru/CdS. This result

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further proves that the RuP2 added to CdS can significantly improve the H2 evolution

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ability of CdS. Compared with all the samples, the ratio of 10 wt.% RuP2 in RuP2/CdS

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is the optimal dosage. The AQYs for H2 evolution of the ten compounds after 10 h of visible light radiation are 4.75%, 5.08%, 3.84%, 9.37%, 14.28%, 5.63%, 4.05%, 2.08%,

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2.20% and 1.83%, respectively (Fig. 5C). In all samples, the 10 wt.% RuP2/CdS still

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shows the highest AQY, as its AQY is 2.81 times that of 0.1 wt.% Pt/CdS and 6.4 times that of 0.1 wt.% Ru/CdS. The blank catalytic experiment over RuP2 is shown in the Fig.

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5D. The H2 evolution amount of the 10 wt.% RuP2/CdS reached 164008 µmol/g after

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10 h of radiation, which was 90.6 times that of RuP2 (1811 µmol/g). Transitional metal phosphides of Ni2P, CoP and FeP are also effective cocatalysts over CdS for

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photocatalytic H2 production. As shown in Fig. 5E, the H2 evolution amount of the 10

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wt.% RuP2/CdS reached 164008 µmol/g after 10 h of radiation, which was 1.25 times that of 10 wt.% Ni2P/CdS (131022 µmol/g) and 2.3 times that of 10 wt.% CoP/CdS (70282 µmol/g) and 3.0 times that of 10 wt.% FeP/CdS (55129 µmol/g). The long-time lifespan tests over 10 wt.% RuP2/CdS are illustrated in Fig. 6A. Clearly, the H2 evolution amount of 10 wt.% RuP2/CdS sharply increases to 355573.8 µmol/g before 34 h, but after 34 h, its activity stably rises to 412360.5 µmol/g with time. During the test, the activity rises all the time, without any reduction within 102 h. This result

ACCEPTED MANUSCRIPT indicates the RuP2/CdS has an excellent lifetime under the tested conditions. Figure 6B shows the AQYs of 10 wt.% RuP2/CdS during 102 h. The AQYs of 10 wt.% RuP2/CdS is 31% at 34 h, and increase all the time, which further proves the high efficiency and long life of RuP2/CdS. Figure 7 exhibits the recycled stability of 10 wt.% RuP2/CdS,

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after the test was repeated every 4 h under the same conditions. The H2 evolution is

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stable without obvious change in three cycles, which proves that RuP2/CdS has a long

3.3 Photocatalytic process and mechanism

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lifetime and strong reusability.

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The PL analysis shows that the RuP2 on the surfaces of CdS can intensively attract

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photo-electrons under light radiation, which accelerates the migration of photogenerated electrons and promotes the separation of photogenerated electron-hole pairs,

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thereby improving the utilization efficiency of photo-generated electrons. A substantial

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increase in the number of photoelectrons must produce a stronger photocurrent, so we tested the contrast photocurrent-time (I-t) curves of CdS and 10 wt.% RuP2/CdS. The

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addition of RuP2 in 10 wt.% RuP2/CdS significantly intensified the photocurrent

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compared with pure CdS nanorods (Fig. 8). Since photocurrent is formed by the directional movement of photo-electrons, the high- intensity photocurrent further proves the strong extracting electrons from CdS and improves the electron utilizing efficiency of RuP2. The proposed enhancement mechanism is illustrated in Fig. 9. Specifically, the photogenerated holes remain in the VB under visible light radiation, while the photo-generated electrons migrate to the CB and are attracted onto the surfaces of RuP2 particles. Meanwhile, the H+ adsorbed on RuP2 particles combines

ACCEPTED MANUSCRIPT with the photo-generated electrons to form •H, and then two •H radicals bind to form H2. The photo-generated holes combined with lactic acid (electron donor) are exhausted, which further promotes the photo-generated electrons and H+ reaction to form H2. 4. Conclusions

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We successfully prepared RuP2 particles by a traditional route and synthesized a

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series of RuP2/CdS photocatalysts. We used X-ray powder diffractometry, high-

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resolution transmission electron microscopy, ultraviolet-visible absorption spectroscopy and so on to explore their intrinsic properties and tested their H2 production activities

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through experiments. The RuP2/CdS shows excellent activity, lifetime and stability

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during H2 production. In particular, 10 wt.% RuP2/CdS displays the best activity that the H2 evolution amount reached 164008 µmol/g after 10 h of radiation. As for the

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mechanism, RuP2 enhances the photocatalytic ability of H2 production by intensively

Acknowledgements

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attracting electrons and improving electron utilizing efficiency.

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This work was supported by Natural Science Foundation of Tianjin of China (Grant

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14JCYBJC20500).

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ACCEPTED MANUSCRIPT Highlights

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RuP2 is utilized as a novel cocatalyst RuP2 with strong electron attraction ability Improved migration, utilization, separation efficiency of photo-electrons

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Graphics Abstract

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