Doping of strontium in MoS2 widened d-spacing of plane （101）and piezo-enhanced photocatalytic evolution of hydrogen from ammonia borane upon near-infrared irradiation

ZHOU Yiwen， HUANG Kuangzheng， LIU Wenxiao， MENG Zeda， LUO Li， LIU Shouqing

（School of Chemistry and Life Sciences，SUST，Suzhou 215009，China；Jiangsu Key Laboratory for Environment Functional Materials，Suzhou 215009，China）

Abstract： Piezo-electric effect is ascribed to the asymmetry of structure forced by mechanics. The paper reports the efficient release of hydrogen from ammonia borane containing 19.6 wt% of hydrogen mass based on the piezoelectric photocatalysis of strontium-doped MoS2（Sr-MoS2） due to the widened d-spacing of plane （101） as the catalyst under near-infrared irradiation. XRD patterns display that the doping of strontium atoms shortened the d-spacing of the （002） planes from 6.71 ? to 6.43 ?，but facilitated the crystallization of the （002） planes. It also shows that the strontium atoms in MoS2 grew and were perpendicular to the （101） planes during doping and widened the d-spacing of the （101） planes from 2.66 ? to 2.70 ? because the atom radius of strontium （2.15 ?） is larger than that of molybdenum （2.01 ?）. The more regularly arranged （002） planes were observed，and the d-spacing of 6.43 ? was in agreement with the value calculated by XRD angle，which confirms the existence of strontium in MoS2 lattices. UV-Vis-NIR diffuse reflectance spectra indicated that the band gap of Sr-MoS2 is 0.84 eV，whereas the one of the as-synthesized MoS2 is 0.94 eV. Under optimal conditions，the hydrogen yield based on Sr-MoS2 catalyst from aqueous NH3BH3 solution was 1.22 mmol·（g·h）-1，whereas that of pristine MoS2 was only 0.35 mmol·（g·h）-1 under ultrasonic and 850 nm near-infrared irradiation at 12 h. The boosted yield of hydrogen was assigned to the enhanced piezoelectric effect due to the doping of strontium in MoS2 resulting in the widened d-spacing of plane （101），because the piezo-current of Sr-MoS2 was much larger than that of pristine MoS2.

Key words： strontium-droped MoS2；piezoelectric photocatalysis；hydrogen evolution；near-infrared irradiation；ammonia borane

1 Introduction

The exploration of clean and renewable fuels has become increasingly urgent because of critical mental is sues caused by the consumption of traditional fossil fuels. Hydrogen is a clean energy carrier that can store solar and wind energy in nature via photocatalysis，photoelectrochemistry，or electrochemistry driven by wind or solar power. Due to its easy explosion，however，the application of hydrogen gas is seriously restricted[1-2].Therefore，it is necessary to develop the easy hydrogen on demand. Great efforts have been devoted to searching and finding reliable pathways which could safely and efficiently store hydrogen under ambient conditions[3]. To meet this need，some materials that hydrogen could be chemically stored，such as metal hydrides （like NaAlH4，LiNH2，LiH，etc.）[4-8]，hydrazine hydrate[9]，hydrazine borane[10]，and ammonia borane[11-12]，have been developd.Among the materials，ammonia borane （NH3BH3），a simple B-N compound，is of interest due to its high hydrogen capacity （19.6 wt%），chemical stability，non-toxicity， low molecular weight，as well as controllable hydrogen release. Hydrogen release from NH3BH3could be realized by its alcoholysis[13-17]，thermolysis[18-19]or hydrolysis[20-21].Compared with the alcoholysis and thermolysis，the hydrolysis of NH3BH3in the presence of suitable catalysts is featured with low reaction temperature，high hydrogen yield，fast hydrogen release rate，and reliable controllability[20，22]. The hydrolysis of NH3BH3could release 3 equivalents of hydrogen （1）.

To promote the release of hydrogen from aqueous NH3BH3solution，some precious catalysts such as Pt[23-25]，Pd[26-29]，Rh[30-33]，and Ru[34-39]have been researched. Their high costs obstructed the large scale application，however. For the sake of low cost，transition metal catalysts like Ni，Co，F(xiàn)e，have been explored for the fast release of hydrogen from aqueous NH3BH3solution[40-43]. These exploratory researches are very significant for leading to the scale application.

Recently，piezophotocatalytic technique，an interesting edge research，can harvest both solar and mechanic powers to expedite the evolution of hydrogen[44-45]，because the photogenerated free electron/hole pairs can be effectively separated by the piezo-potential arising from exerting a mechanical stress. Furthermore，Zhang and Wang utilized glutathione to decorate the surface of MoS2，achieving a higher H2evolution[46]；Cobalt，a transition metal，doped into MoS2for promoting the hydrogen evolution，has been reported by our group[47]. The main group elements in the periodic table have not been doped in MoS2，and their impact on the piezo-release of hydrogen still is unknown. Herein we explore the effect of strontium-doped MoS2（Sr-MoS2） on piezo-photocatalytic evolution of hydrogen.

2 Experimental section

2.1 Chemicals

Sodium molybdate dihydrate（Na2MoO4·2H2O），thiourea（CS（NH2）2） and strontium chloride hexahydrate（SrCl2·6H2O） were purchased from Sinopharm Chemical Reagent Co.，Ltd for synthesizing pristine MoS2and Srdoped MoS2. Ammonium borate（NH3BH3） was obtained from Alddin Ltd for preparing hydrogen of hydrolysis. Indium tin oxide（ITO） conductive glass with 10 Ω was acquired from Zhuhai Kaivo Optoelectronic Technology Co.Ltd for measuring the piezo-current and piezo-voltage. The ITO glass was ultrasonically treated in 30 mL of deionized（DI） water for 1 h，and then it was rinsed with 10 mL of DI water，finally，it was dried under air conditions for measuring the piezo-current and piezo-voltage of pristine MoS2and Sr-MoS2. All reagents used were of analytical grade and applied without further purification. All solutions were prepared with 18.2 MΩ·cm DI water.

2.2 Synthesis of pristine MoS2 and Sr-MoS2

The doping of strontium in MoS2was conducted as follows. First，Na2MoO4·2H2O（2.42 g，0.01 mol） and（NH2）2CS（3.05 g，0.040 mol） were separately dissolved in 30.0 mL of DI water and ultrasonically dispersed for 30 min. SrCl2·6H2O（0.400 g，0.001 5 mol） was dissolved in 10.0 mL of DI water. Then，30.0 mL of （NH2）2CS solution and 10.0 mL of SrCl2·6H2O solution were dripped in Na2MoO4·2H2O solution. Finally，DI water of 10.0 mL was also added in the Na2MoO4·2H2O solution，producing 80.0 mL of mixed solution. The as-mixed solution was ultrasonically dispersed for 1 h again and then it was transferred into a 100 mL Teflon-lined stainless-steel autoclave，which was subsequently sealed and placed in an incubator for maintaining at 200 °C for 24 h.The as-synthesized black products were collected and washed carefully with DI water and anhydrous ethanol in sequence，and finally dried at 60 °C in a vacuum chamber for 6 h to obtain Sr-doped MoS2catalyst. The Sr-MoS2sample was utilized for the characterization and piezo-photocatalytic hydrogen production. Similarly，pristine MoS2and Sr-doped MoS2with different doping concentrations were synthesized for comparison.

2.3 Structural characterization

X-ray diffraction（XRD） was performed with an X’Pert-Pro MPD X-ray diffractometer （Panalytical，Netherlands）. The X-ray source emitted Cu-Kα radiation with a wavelength of 0.154 nm at a tube voltage of 40 kV and a tube current of 40 mA. Morphological observations were conducted via transmission electron microscopy（TEM；Tecnai G220；FEI，USA）. The Sr-doped MoS2and pristine MoS2powders were dispersed in anhydrous alcohol by an ultrasonic device，then it was dripped on carbon-coated copper grids by a dropper for drying under ambient conditions，finally，it was placed into the aforementioned TEM system for observing morphology and measuring the lattice structure and sizes. UV-Vis-NIR diffuse reflectance spectra （UV-Vis-NIR DRS） were recorded on a Shimadzu UV-Vis NIR spectrometer（UV-3600 plus，Japan）within the wavelength scope of 190～3 400 nm.

X-ray photoelectron spectrometry（XPS） with an XSAM 800 apparatus was applied to measure the composition of the Sr-doped MoS2catalysts and the valence states of Sr，Mo，and S in the Sr-doped MoS2to reveal the piezo-catalytic and photocatalytic reaction mechanism. The carbon 1 s peak at 284.60 eV was utilized to calibrate the binding energy（Eb） scale. The aluminum Ka 1.2 line（hv 1 486.60 eV） worked as the X-ray excitation source. To achieve maximum instrumental resolution，the spectra were recorded in fixed analyzer transmission mode. The instrument was run under a vacuum of 1×10-9Torr in the analysis chamber. Wide and high-resolution spectra were recorded at a constant pass energy of 50 eV and channel widths of 1.0 and 0.1 eV.

LSV was carried out with a CHI660C electrochemistry workstation （Chen Hua Instruments，Shanghai，China）in a conventional three-electrode system，where a ITO electrode （1 cm×1 cm） was utilized as the working electrode，a saturated calomel electrode （SCE） as the reference electrode，and a platinum plate as the counter electrode，respectively. All potentials were converted to the reversible hydrogen electrode （RHE，E=EVvs/SCE+0.242+0.059pH）.

The procedures for measuring piezo-currents of Sr-MoS2were conducted as follows. First，a sheet of 10 mm×18 mm ITO glass was utilized as the bottom electrode，about 20 mg of Sr-MoS2was uniformly distributed on a square of 10 mm×10 mm ITO sheet，and then another ITO sheet was utilized as the top electrode to fabricate a sandwich piezo-measurement cell[47]. A swallow clincher（15 mm×6.5 mm） was utilized to clamp the sandwich piezo-measurement cell to maintain its stability，and another swallow clincher was utilized to bring pressure on or relax the piezo-measurement cell by closing or opening it，respectively. Each of the swallow clincher exerted about （1.2±0.1） kg force on the measurement cell （that is，（1.2±0.1） kg force was exerted on a 10 mm×10 mm sample when the swallow clincher was closed）.

2.4 Piezo-photocatalytic hydrogen release

NH3BH3of 0.05 mol·L-1in 100 mL aqueous solution was utilized to conduct the piezoelectric catalysis and piezophotocatalysis for hydrogen release in a sealed reaction system （Labsolar 6A photocatalytic system，Perfect light Co. Ltd.，Beijing，China）. A 50 W near-infrared（NIR） LED lamp with λ=850 nm worked as NIR irradiation source. An ultrasonic generator with 200 W power supplied mechanical energy onto Sr-MoS2catalyst for piezocatalysis at 40 kHz，which was coupled with a DC-0506 liquid thermostatic bath to maintain a constant temperature of （23±2） ℃. The reaction system with volume of 465 mL was evacuated to the initial internal pressure at 0.01 kPa before run. It was matched with a GC-7806 gas chromatograph（Shiwei Puxin Instruments Co. Ltd.，Beijing，China） to measure the concentration of H2formed during the piezocatalytic or/and piezophotocatalytic process at 1 h intervals. A chromatographic column of 5 m （length）×2 mm i.d. filled with 5 ? molecular sieve was utilized as a separating column. High-purity argon was utilized as the carrier gas. The flow rate of the carrier gas was set at 23 mL·min-1，and the thermal conductivity tank was used to detect H2. The column temperature was set at 80 °C，the inlet temperature at 120 °C，and the detector temperature at 150 °C.

3 Results and discussion

3.1 Characterization of Sr-MoS2 nanoflowers

Fig.1A indicated the XRD patterns of pristine MoS2and Sr-MoS2，which is indexed to the typical hexagonal structure （JCPDS 37-1492，space group：P36/mmc，a=3.12 ?，c=12.555 ?，α=β=90° and γ=120°） of crystalline MoS2[48]. Three diffraction peaks at 2θ=13.19°，33.66°，57.40° were assigned to the Bragg diffractions from the（002），（101） and （110） planes of MoS2，respectively. In particular，the peak at 2θ=13.19° that originated from the （002） diffraction planes of Sr-MoS2with the d-spacing value of 6.43 ?，became higher than that of pristine MoS2sample，indicating the doping of strontium facilitated crystallization of MoS2. By comparison，the diffraction peaks after the doping of strontium in MoS2showed fraction shifts. The peak positions and plane distances were listed in table 1.

Figure 1 （A）XRD patterns of the as-synthesized （a）Sr-MoS2 and （b）MoS2 nanoflowers；（B）TEM image of the as-synthesized Sr-MoS2 nanoflowers；（C）High-resolution TEM image of Sr-MoS2 with 0.643 nm spacing-d value

Table 1 Diffraction positions and plane distances

As seen in table 1，the d-spacing value of the （002） plane shifted from 6.71 ? of pristine MoS2to 6.43 ? of Sr-MoS2，revealing the doping of strontium in MoS2led to a shortened distance between the （002） planes.Similarly，the doping of strontium also shortened the distance of the （110） plane a little. However，the doping of strontium lengthened the distance between the （101） planes from 2.66 ? to 2.70 ?. It revealed that strontium atoms grew and arranged perpendicular to the（101） planes in MoS2，because the atom radius of strontium（2.15 ?） is larger than that of molybdenum （2.01 ?）. The diffraction peak of the （002） planes derived from Sr-MoS2is higher than that from pristine MoS2，indicating larger atoms that grew perpendicular to the （101）planes in MoS2facilitated the arrangement and crystallization of atoms along with the （002） planes in MoS2.

The UV-Visible NIR diffuse reflectance spectra in Fig.2A indicated that Sr-MoS2boosted the absorption peak at 900 nm（curve b），implying the doping of strontium atoms in MoS2facilitated the absorption for NIR irradiation. Aside with the peak，a new peak at 736 nm appeared，further confirming the strontium atoms were doped in MoS2，compared with curve a，and revealing Sr-MoS2can harvest more light irradiation. Based on the UV-Visible NIR diffuse reflectance spectra and the Tauc formula，there is the linear relationship of （αhν）1/2against hν，showing the indirect transition of light-excited carriers in the catalyst. In the case of Sr-MoS2，an intercept of 0.84 eV，which is smaller than that of pristine MoS2，was obtained. In general，the indirect band gap of MoS2is 0.94 eV. The narrowed band gap is due to the defects formed in the process of strontium doped in MoS2[49-51]. XPS analysis also detected the strontium element doped.

Figure 2 （A）UV-Vis-NIR DRS of the as-synthesized MoS2（a） and Sr-MoS2（b） samples；（B）Tauc plots for the indirect band gaps of MoS2（a） and Sr-MoS2（b） samples

As seen in the XPS survey spectrum （Fig.3A），strontium，molybdenum，sulfur，and oxygen in Sr-MoS2were detected，whereas no strontium was found in pristine MoS2. The binding energies located at 228.9 and 232.6 eV were attributed to ones of 3d5/2and 3d3/2of Mo（IV），respectively，and the shoulder peak at 226.1 eV corresponded to the binding energy of S 2p（Fig.3B）. The peaks at 232.0 and 236.0 eV revealed a fraction of Mo（VI） in Sr-MoS2，showing that Mo（VI） was not completely reduced to Mo（IV） during the hydrothermal reaction[52]. A fraction of MoO3was possibly present in the as-synthesized species because oxygen was also detected in the XPS survey spectra. On the basis of the high-resolution S 2p spectrum in Fig.3C，the doublet peaks located at 161.7 and 162.9 eV were assigned to the 2p3/2and 2p1/2of S2-in Sr-MoS2，whereas the peak located at 169.2 eV was ascribed to S6+. The Sr 3d spectra were fitted with doublets（Fig.3D）. Peaks at 133.4 and 135.0 eV were assigned to the binding energies of Sr 3d5/2and Sr 3d3/2[53-54]，respectively. Together with the data of XRD，it further revealed that strontium atoms were doped in MoS2lattices.

3.2 Hydrogen evolution by piezo-photocatalysis

3.2.1 Hydrogen yield boosted by doping

Hydrogen yield was raised remarkably by doping strontium in MoS2as shown in Fig.4. The yield of hydrogen approached to 1.46 mmol （1.22 mmol·（g·h）-1，curve a） where Sr-MoS2worked as catalyst in 12 h，while the yield was only 0.42 mmol （0.35 mmol·（g·h）-1，curve b） where pristine MoS2was utilized as catalyst under similar conditions. It revealed that the doping of strontium in MoS2played a key role in boosting hydrogen yield.

3.2.2 Effect of strontium content on hydrogen yield

A series of Sr-MoS2samples with different atomic ratios of strontium to molybdenum were synthesized for examining the effect of strontium content on hydrogen yield. The as-synthesized Sr-MoS2catalysts of 0.10 g were soused in 100 mL solutions of 0.05 mol·L-1NH3BH3，respectively. Then they were under ultrasonic and 850 nm irradiation for 12 h，the results were presented in Fig.5. As seen in Fig.5，at the initial stage，the yield rose with the content of strontium doped in MoS2. Then it reached to 1.46 mmol，a top value，when the ratio of strontium to molybdenum is 15.0%. Finally，it started to drop. The rising yield at the initial stage may be derived from the enhanced piezoelectric effect and conductivity （also，see mechanism section），which also confirmed that the doping of strontium can significantly improve the hydrogen yield.

Figure 3 （A）XPS survey spectra of Sr-MoS2 and pristine MoS2；（B）High-resolution Mo 3d XPS spectra；（C）High-resolution S 2p XPS spectra；（D）High-resolution Sr 3d XPS spectra

Figure 4 Hydrogen yield in the presence of catalysts upon 850 nm NIR irradiation under ultrasonic vibration. The volume of solution containing 0.05 mol·L-1 NH3BH3 is 100 mL，the temperature is （23±2） ℃（（a）0.10 g Sr-MoS2；（b）0.10 g pristine MoS2；（c）no catalyst）

Figure 5 Dependence of hydrogen yield on strontium content in MoS2

Control tests have been conducted under similar conditions in order to illustrate the role of piezoelectricity.The results were shown in Fig.6. Compared with 0.6 mmol of hydrogen released only in the presence of 0.10 g Sr-MoS2catalyst at 12 h（curve a），the ultrasonic vibration boosted the hydrogen yield to 0.9 mmol（curve b） under similar conditions. Based on the hydrogen evolution （curve c） upon 850 nm NIR irradiation，the addition of ultrasonic vibration also resulted in an efficient amplification（curve d） under similar conditions again. Therefore，it is concluded that Sr-MoS2is capable of harvesting mechanic energy efficiently to promote the hydrogen release. Curve c indicated that Sr-MoS2can harvest NIR irradiation for hydrogen evolution due to its narrow band gap[55].

3.3 Mechanism of hydrogen release

The piezoelectric currents of pristine MoS2and Sr-MoS2were measured by i-t technique with CHI660C electrochemical station. A dovetail clip was utilized to apply pressure between the ITO electrodes to observe the piezoelectric current. The results were shown in Fig.7.

The results displayed that both Sr-MoS2and pristine MoS2produced piezo-current. Moreover，the piezocurrent of Sr-MoS2is larger than one of pristine MoS2，which confirmed that the doping of strontium in MoS2boosted the piezoelectric effect. It accounts for the enhanced hydrogen release derived from Sr-MoS2，compared with that of pristine MoS2.

Furthermore，a series of Sr-doped MoS2samples were adhered on ITO for electrochemical linear scan voltammetry in 1.0 mol·L-1KOH solution. The resulting voltammetric curves were shown in Fig.8.

Figure 6 Piezoelectric effect of Sr-MoS2 on hydrogen evolution in the presence of 0.10 g Sr-MoS2 catalyst in 100.0 mL solution containing 0.05 mol·L-1 NH3BH3（（a）neither under ultrasonic vibration nor upon 850 nm NIR irradiation；（b）only under ultrasonic vibration；（c）only upon 850 nm NIR irradiation；（d）both under ultrasonic vibration and upon 850 nm NIR irradiation）

Figure 7 Piezoelectric i-t curves measured by applying (1.1±0.1) kg·cm-2 pressure between ITO electrodes. About 20.0 mg of Sr-MoS2 or MoS2 sample was sandwiched in between ITO electrodes. The potential of the top electrode was set as 0.00 V versus reference or working electrode(（a）Sr-MoS2；（b）pristine MoS2)

As seen in Fig.8，the cathodic current for reducing hydrogen ions raised with the Sr-doped content at the initial stage when the overpotential was 0.4 V，until the ratio of Sr-to-Mo was equal to 15.0%，then the cathodic current started to drop. The relationship between the cathodic current and the content of strontium doped in MoS2is in agreement with the dependence of hydrogen yield on the content of strontium doped in MoS2（see Fig.5）.Combining the results of linear scan voltammetry and piezoelectric current with the yield of hydrogen release during the piezo-photo-catalysis，it is concluded that the doping of strontium in MoS2boosted the built-in electric field in the cell lattices of MoS2and conductivity，the boosted piezoelectric effect and conductivity in Sr-MoS2subsequently raised the yield of hydrogen release significantly.

There are two kinds of hydrogen atoms in NH3BH3molecule. One is charged positively（H+），which is linked to nitrogen atom；the other is charged negatively（H-），which is linked to boron atom. Under consideration of piezocatalytic and piezo-enhanced photocatalytic hydrogen release，the reaction mechanism was suggested as follows.

On the one hand，the hydrogen atoms charged negatively donated electrons to both photo-generated holes （pathway a） and piezo-holes（pathway a’） to form hydrogen gas. On the other hand，the hydrogen atoms charged positively accepted electrons to both photo-generated holes（pathway b） and piezo-holes （pathway b’） to form hydrogen gas.The a-b pathway is photocatalytic production of hydrogen and the a’-b’ pathway is piezo-electrical production of hydrogen. Therefore，piezoelectricity can boosts hydrogen yield via strontium-doped MoS2catalyst upon nearinfrared irradiation.

Figure 9 Mechanism of hydrogen release from NH3BH3 solution by piezoelectric catalysis and piezo-enhanced NIR photocatalysis

4 Conclusions

The doping of strontium widened the d-spacing of the （101） planes，shortened the d-spacing of the （002）planes，and facilitated the crystallization of the （002） planes. The enhanced asymmetry of structure boosted the piezoelectricity and conductivity of MoS2and facilitated the separation of photogenerated electrons and holes，thereby raises hydrogen yield.