Facile preparation of mixed-phase CdS and its enhanced photocatalytic selective oxidation of benzyl alcohol under visible light irradiation

Accepted Manuscript Full Length Article Facile preparation of mixed-phase CdS and its enhanced photocatalytic selective oxidation of benzyl alcohol under visible light irradiation Houde She, Liangshan Li, Yidong Sun, Lei Wang, Jingwei Huang, Gangqiang Zhu, Qizhao Wang PII: DOI: Reference:

S0169-4332(18)31936-6 https://doi.org/10.1016/j.apsusc.2018.07.045 APSUSC 39860

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 May 2018 26 June 2018 6 July 2018

Please cite this article as: H. She, L. Li, Y. Sun, L. Wang, J. Huang, G. Zhu, Q. Wang, Facile preparation of mixedphase CdS and its enhanced photocatalytic selective oxidation of benzyl alcohol under visible light irradiation, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.045

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Facile preparation of mixed-phase CdS and its enhanced photocatalytic selective oxidation of benzyl alcohol under visible light irradiation Houde Shea, Liangshan Lia, Yidong Suna, Lei Wanga, Jingwei Huanga, Gangqiang Zhub,*, Qizhao Wanga,* a

College of Chemistry and Chemical Engineering, Northwest Normal University, Key

Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Lanzhou 730070, China b

China School of Physics & Information Technology, Shaanxi Normal University, Xian 710062, China *Corresponding author. Tel: +86 931 7972677; Fax: +86 931 7972677.

E-mail: [email protected], [email protected](Q.Wang); [email protected](Q. Zhu)

Abstract Selective oxidation is one green technique for routine organics synthesis and environmental

remediation.

However,

the

conversion

ratio

resulted

from

environment-friendly method employing traditional photocatalytic materials like CdS is relatively low as compared with homogeneous catalytic reaction. Herein, in this paper, a facile method is reported to prepare mixed-phase CdS nanocrystals at room temperature. The as-prepared CdS samples were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectra (DRS), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic oxidation of benzyl alcohol into benzaldehyde catalyzed by as-prepared CdS was carried out with high conversion of 68% generated within 60 min using O2 as a benign oxidant under ambient condition. The enhanced catalytic activity could be attributed to the mixed-phased 1

crystal structure which facilitates the separation of photogenerated charge carriers.

Keywords: selective alcohol oxidation; CdS; mixed-phase; photocatalysis; visible light

1. Introduction Aromatic aldehydes are essential chemical intermediates used for the industrial synthesis to manufacture common merchandises such as drugs, dyes and fragrances[1]. Normally, aromatic alcohol is selectively oxidized by potent oxidants like KMnO4, CrO3 and Br2 to produce aromatic aldehydes[2-5]. Nowadays, however, along with the increasingly deteriorated environment circumstance and energy crisis, this process becomes less flavored due to the immoderate energy consumption and complex post-treatment for the byproducts and residual mass with costly expenditure. To diminish and further eradicate these draw backs, the reaction using the economical O2 derived from air as oxidant, which can be stimulated by photocatalysts under mild conditions to form superoxide radical (·O2-) so as to oxidize aromatic alcohol into the objective product, seems to be the most promising alternatives. A variety of photocatalysts have been developed for this purpose, such as TiO2[6], CeO2[7], Bi4O5Br2[8], mpg-C3N4[9], BN/In2S3[10] and NH2-MIL-125(Ti)[11]. Among these diversified photocatalysts, CdS presents greater visible-light response rather than that of commercial TiO2. Yet owing to the severe recombination rate of light generated charges, the application of CdS as potocatalyst is still very limited. Actually, the reported yield of selective oxidation reactions promoted through bare CdS is around 30-40%[12]. To solve this defect, many approaches have been brought up to improve the separate efficiency of charge carriers. For instance, the study of Chen’s group demonstrated that the photoactivity of CdS in selective organic transformations could be improved by wrapping it with conductive graphene sheet[13]. Liu synthesized CdS@SnO2 core-shell structure, with facilitated separation of electron-hole pairs achieved, resultingin 40% to 68% improvement of benzaldehyde yield than pure CdS during photocatalysis process[14]. Zhang prepared Pd@CdS using Pd colloid 2

nanoparticles as seeds. The metal core could prolong the life span of photogenerated electrons and thus exhibited better performance in selective oxidation of alcohol than using CdS only. The principal theory these attempts lie in is introducing a second component and resultantly enhancing the photocatalytic capacity of the composites. According to this innovative concept, delicate structure design is required and the synthesis process should be controlled with great scrupulousness. Enlightened by enhanced photocatalytic performance of mixed-phase TiO2, herein, a facile way is conducted prepare a mixed-phase CdS which can facilitate the separation of electrons and holes. The photocatalytic activities of as-prepared catalysts are evaluated by selective oxidation of benzyl alcohol into benzaldehyde with ambient conditions adopted. To the best of our knowledge, it is the first time to employ mixed-phase CdS as a catalyst in transmuting alcohol into corresponding aldehyde.

2. Experimental section 2.1. Materials All reagents were analytical grade and used without further purification. Benzotrifluoride (BTF) was purchased

from Aladdin Chemical Reagent Co., Ltd.

Benzyl alcohol (C7H8O) was purchased from Alfa Aesar (China) Chemicals Co., Ltd. Oleic acid (C18H34O2), Cadmium nitrate tetrahydrate (Cd(NO3)2•4H2O), 8% of ammonium sulfide ((NH4)2S), p-benzoquinone (BQ), isopropyl alcohol (IPA), oxalic acid (OA), sodium borohydride (NaBH4), ethyl alcohol (C2H6O), carbon tetrachloride, toluene were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation Typically, Cd(NO3)2·4H2O (1.0mmol) and oleic acid (0.0375ml) were first dissolved in deionized water (50mL) in a beaker placed on a magnetic stirrer for 15min at room temperature to form solution A. NaBH4 (4.0mmol) was also dissolved in deionized water (250mL) in a beaker placed on a magnetic stirrer for 1min at room temperature to form solution B. The two solutions were mixed and quickly put into a microwave oven (M1-L213B, at a fixed frequency of 2450 MHz, Guangdong midea 3

kitchen appliances manufacturing Co., Ltd., China) followed by 20 seconds irradiation immediately. Then the Cd nanoparticles (NPs) were collected and washed using ethyl alcohol 3 times, and dispersed in 20 mL ethyl alcohol to form solution C. The Cd NPs were sulphurised when certain amount of (NH4)2S (1.0 mmol, 2.0 mmol, 3.0 mmol, 4.0 mmol, respectively) was slowly added to the solution C and stirred for 1 h at room temperature. Then the yellowish products were collected and washed using ethyl alcohol 3 times. Finally, the products were dried in drying oven at 60 °C for 4 h, namely CdS-1 (1.0 mmol (NH4)2S), CdS-2 (2.0 mmol (NH4)2S), CdS-3 (3.0 mmol (NH4)2S), CdS-4 (4.0 mmol (NH4)2S). The picture of the products is shown as Figure S1 (Supporting Information).

2.3. Characterization X-ray

diffraction

(XRD)

patterns

were

acquired

by

a

Rigaku

D/MAX-2200/PCX-ray diffractometer in the angular range of 20 ~ 90o (2θ) with Cu Κα radiation (40 kV, 20 mA). UV-Vis diffuse reflectance spectra (DRS) were acquired by a spectrophotometer (PuXin TU-1901) with an integrating sphere attachment, using BaSO4 as a reflectance sample. The morphologies of the products were obtained by field emission scanning electron microscope (FE-SEM, Ultra Plus, Carl Zeiss) and transmission electron microscope (TEM, F20, FEI). X-ray photoelectron spectroscopy (XPS) was characterized by a PHI5702 photoelectron spectrometer. The photoelectrochemical (PEC) performances of photoanodes were acquired by a three-electrode system (CHI-660D Co., Shanghai, China) under a LED lamp ( λ >420 nm, CEL-LED100) illumination. Pt wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrodes were made on the fluride-tin oxide (FTO) conductor glasses. The solid samples (10 mg) were dispersed in anhydrous ethanol and ultrasound for 20 min before they were dripped on FTO glasses. The working electrodes were dried under the infrared lamp irradiation for 30 min. The electrolyte was 0.5 M Na2SO4 (pH=7.5) aqueous solution in a quartz ware. 0.5 V of the bias voltage was used for photoelectrochemical testing. Illumination through the back-side of FTO was used with an illumination area of about 1.0 cm-2. 4

Photoluminescence (PL) spectra of the samples were recorded at room temperature under a 390 nm excitation wavelength with a fluorescence spectrophotometer (PE, LS-55).

2.4. Evaluation of photocatalytic activity The photocatalytic selective oxidation of alcohols to corresponding aldehydes was performed as follows. Aromatic alcohol (0.5 mmol) and photocatalyst (80 mg) were dispersed in BTF (5.0 mL). BTF was chosen as the solvent due to its sluggishness and high solubility of oxygen gas[15, 16]. The mixture solution was transferred to a 250 mL of round-bottom flask. A 300 W Xe lamp (CEL-HXF300, Beijing Au light) with a maximum emission at about 470 nm was used as a visible light source. The wavelength of the visible light was controlled through a 420 nm cutoff filter. After the reaction, the solution was centrifuged to remove the catalyst particles. The obtained solution was analyzed by a Gas Chromatograph (GC9600, China). The conversion rate of alcohol, yield of aldehyde, and reaction selectivity were calculated with the following equations: Conversion % = [(C0-C1)/C0]100%

(1)

Yield % = (C2/C0 ) 100%

(2)

Selectivity % = [C2/(C0-C1)] 100%

(3)

Where C0 is the initial concentration of alcohol; C1 and C2 are the concentration of the substrate alcohol and the corresponding aldehyde at a certain time after the photocatalytic reaction, respectively.

3. Results and discussion 3.1. Characterization of photocatalysts 3.1.1. XRD and UV–vis analysis Fig. 1(a) shows X-ray diffraction (XRD) patterns of the CdS products prepared with different mole ratio of sulfur to cadmium. The diffraction peaks of CdS can be indexed to hexagonal CdS and cubic CdS respectively. The peaks located at 24.8º, 26.5º, 28.2º, 36.6º, 43.6º, 47.8º, 51.8º, 66.7º, 70.8º, 72.4º, 75.5º, 80.2º, 83.2º and 86.3º are distinctly indexed to the (100), (002), (101), (102), (110), (103), (112), (203), 5

(211), (114), (105), (300), (213) and (302) crystal planes of pure hexagonal phase CdS (JCPDS No. 41-1049), respectively. The highest peak locates at 26.5º. Since there is no rod-like nanocrystals observed in TEM images (shown in Fig. 2(c) below), the extraordinary height of diffraction peak at 26.5º cannot be explained by the preferential growth along c axis of hexagonal phase CdS nanocrystals. Noting that the peak at 26.5, 30.6 and 52.1º could also be assigned to (111), (200) and (311) crystal planes of cubic phase CdS (JCPDS No. 75-1546), respectively, it is clear that the sample comprise both hexagonal and cubic phase CdS. The peaks assigned to hexagonal CdS intensified when Cd:S ratio decreases from 1:1 to 1:4, indicating more CdS in hexagonal phase are formed. In the reaction, Cd(NH3)4(OH)2 complex precursors are formed by elemental cadmium, ammonium sulfide molecules, oxygen and water. Then, a tetrahedral coordination structure is formed by Cd 2+ and (NH4)2S, and subsequently to form CdS4 units in the reaction. Comparing the crystal structures of cubic and hexagonal CdS, it can be seen that the coordination of cubic CdS and hexagonal CdS are a tetrahedral cross-conformation and a tetrahedron with overlapping conformations, respectively. The body belongs to an overlapping conformation. When the molar ratio of S and Cd is relatively low, it is beneficial to form the cross-conformation of S3Cd-SCd3 group with lower steric hindrance. When the molar ratio of S and Cd is relatively high, it is favorable to form the S3Cd-SCd3 group with higher steric hindrance. With respect to the hexagonal phase structure of the overlapping conformation, the cubic CdS phase is thermodynamically a metastable phase, while the hexagonal CdS phase is the stable phase. From Fig. 1(a), we also can see that with the increase in the ratio of S and Cd, the half-width of the diffraction peak of the CdS sample gradually narrowed, with a gradual increment in the intensity of the diffraction peak. As shown in Fig. 2 (a, b) and Fig. S2, the crystal grain size of CdS gradually grows as the ratio between S and Cd increases, which increases from 10 nm to 30 nm. In order to achieve phase equilibrium, the grain growth would also promote the phase transition from cubic phase to hexagonal phase. Fig. 1(c) reveals the enlarged C(200) diffraction peak positions in the range of 30 35° of the samples. It is clearly seen that there is a weak diffraction peak around 30.6°, 6

which proves the existence of cubic cadmium sulfide[17]. UV–vis diffuse reflectance spectra (DRS) are usually used to reveal the optical property of photocatalyst. The DRS of different catalysts are shown in Fig. 1(b). The sample exhibits intense absorption in the visible region in the range of 400 to 600 nm. Furthermore, the CdS-2 shows red shift to higher wavelength than the other samples (CdS-1, CdS-3, CdS-4). For an indirect-gap semiconductor, it is well known that the equation between the absorption coefficient and bandgap energy can be described as follows[18-20]: Eg =1240/λ

(4)

Where λ，Eg are wavelength and optical band gap energy, respectively. The optical band gap energy of CdS-2 is about 2.05 eV. The result may be due to the mixing of hexagonal phase and cubic phase CdS. Therefore, CdS-2 can absorb large amounts of visible light, and it would be a suitable visible-irradiation photocatalyst. The band gaps of the samples can be estimated by using the following relationship[21-23]: Αhν = A(hν-Eg)n/2

(5)

Where α, h, A and ν correspond to absorption coefficient, Planck’s constant, proportionality and light frequency, respectively, and n is equal to 1 for the direct band gap material[24]. The value of band gap for CdS-1, CdS-2, CdS-3 and CdS-4 are determined as 2.09, 2.05, 2.36 and 2.38eV, respectively. It can be seen that the samples contain cubic and hexagonal proportions can engender different band gap. 3.1. 2. Morphology of the Samples Scanning electron microscope (SEM) images of the as-prepared CdS-2 product can be seen in Fig. 2(a, b). The sample displays a ball-like aggregate structure of varying sizes with diameter around 10-20 nm. SEM images of the other samples (CdS-1, CdS-3, CdS-4) were shown in Figure S2 (Supporting Information). In order to investigate the interface presence of CdS-2 sample, which showed the best photocatalytic activity, TEM, HRTEM and selected-area electron diffraction (SAED) pattern were performed, respectively. As shown in Fig. 2(c), the mean size of the CdS-2 particles is approximately 10-20 nm. From the HRTEM image of CdS-2 sample( Fig. 2(d)), we can see that the lattice plane with spacing of 0.358, 0.316, 7

0.179 and 0.336 nm match (100) crystallographic plane, (101) crystallographic plane, (200) crystallographic plane and (002) crystallographic plane of hexagonal CdS, respectively. It can be observed that the lattice plane with spacing of 0.291 nm is in consistence with (200) crystallographic plane of cubic CdS. In Fig. 2(d), the inset is SAED pattern of CdS-2. SAED pattern is composed of diffraction spots and diffraction rings and thus, debunking the polycrystalline characteristic of the CdS-2[25] that conforms to the polycrystalline structure reflected by XRD results. 3.1.3. XPS and photoelectrochemical (PEC) performances analysis The surface nature of the as-prepared CdS-2 sample was characterized by XPS and PEC measurements in Fig. 3. The specific peaks related to Cd and S elements in the CdS-2 can be clearly observed in the survey XPS spectrum in Fig. 3(c). As shown in Fig. 3(a), the peaks at the binding energy of 404.9 and 411.7 eV correspond to Cd 3d5/2 and Cd 3d3/2, respectively[26-28]. The Cd spin orbit separation is found to be 6.8 eV. The above results indicate that the valence state of element Cd is divalent in CdS-2. Fig. 3(b) indicates the S 2p peak bifurcates as two peaks of S 2p3/2 and S 2p1/2, corresponding the binding energy of 161.4 eV and 162.7 eV, respectively[29]. The above results indicate that the element S is performed as S2− in CdS-2 sample[30, 31]. Fig. 3(d) shows electrochemical impedance spectroscopy (EIS) Nyquist plots of CdS electrodes. Electrochemical impedance spectroscopy (EIS) mainly focuses on the separation efficiency of the electrons and holes. The smaller arc radius of the electrochemical impedance spectroscopy (EIS) Nyquist plot corresponds to the highly separation efficiency of the electron-hole[32-35]. CdS-2 shows the smallest impedance arc radius, therefore CdS-2 has better electrical conductivity and higher the photocatalytic efficiency rather than the other measured samples[36-38]. 3.1.4 The transient photocurrent response (I–t curves) and PL analysis As shown in Fig. S4(a), the CdS-2 sample shows the highest current density, indicating that the light-induced separation rate is greater for the electron-hole pair in the CdS-2 sample. The photocurrent response results correspond to photocatalytic performance test, which is related to the amount of electrons generated by light irradiation[39, 40]. Fig. S4(b) shows the PL spectra of CdS-1, CdS-2, CdS-3 and 8

CdS-4 catalyst samples for the excitation wavelength of 390 nm at room temperature. From Fig. S4 (b), it can be seen that the catalyst sample has a strong signal peak at about 540 nm. In general, the higher the intensity of the photoluminescent signal, the higher the recombination probability of photo-generated electrons (e-) and holes (h+), and the lower the activity of the catalyst sample[41]. In this regard, it is seen from Fig. S4(b) that the activity of the catalyst CdS-2 is significantly higher than that of the pure sample, which is in agreement with the experimentally measured activity of the catalyst. 3.1.5 Mott–Schottky analysis To study the band structures of the samples, the Mott-Schottky analysis has been carried out in 0.5 M Na2SO4 aqueous solution (pH=7.5). The flat band potential of the samples can be obtained from the Mott-Schottky test as given in Fig. S5. The flat band potentials of CdS-1, CdS-2, CdS-3 and CdS-4 are related to −0.66, −0.52, −0.62 and −0.54 eV, respectively. Assuming the difference between flat band potentials and conduction band minimum is negligible for n-type semiconductors, so the determined flat band potentials can be approximated as conduction band edge[42]. In Fig. 1(d), the band-gap of the samples has been estimated according to the following band gap calculation formula [40]: Eg = EVB − ECB

(6)

Hence, at pH=7.5, the VB potentials of CdS-1, CdS-2, CdS-3 and CdS-4 are located at 1.42, 1.53, 1.74 and 1.84 eV, respectively. 3.2. Evaluation of photocatalytic activity Fig. 4(a) shows the photocatalytic performance of selective oxidation of a range of CdS to corresponding aldehydes under visible light irradiation for 1 h. It is clear that the CdS-2 sample has better performance of oxidation of benzene methanol with the conversion of 68%, the yield of 63%, the selectivity of 94%, respectively. A series of controlled trials have been done (see Table1, Supporting Information). In order to investigate the solvent effect on selective oxidation of benzyl alcohol, the solvents of BTF, carbon tetrachloride, toluene and water were employed, respectively. The results are shown in Fig. 4(b). The conversion of benzyl alcohol to benzaldehyde in BTF 9

solvent is greater than in the other solvents. When illuminated for 1 h, the selectivity is 94% in the BTF solution, 88% in the acetonitrile, in 50% in the toluene, 90% in the carbon tetrachloride and 100% in the water solution. The results show that the selectivity of benzaldehyde is different in the solvents (BTF> acetonitrile >carbon tetrachloride > toluene > water). Because the solubleness of oxygen gas is different in the solvents and BTF can dissolve more oxygen[36]. This mixture (0.5 mmol benzyl alcohol, 5.0 mL BTF and 80 mg CdS-2 sample) was transferred into a 20 mL weighing glass bottle filled with molecular oxygen in 100 ml reactor (Figure S3 Supporting Information) at a pressure of 0.1 MPa and stirred for 20 min to make blend evenly. The conversion is 81.2%. Molecular oxygen plays an important role in reaction system. Fig. 5(a) shows the selective oxidation of benzyl alcohol, p-methoxybenzyl alcohol and p-chlorobenzyl alcohol to corresponding aldehydes under irradiation for 1h. The results show that the catalyst exhibits high selectivity for oxidation of three aromatic alcohols under the same condition. But, the conversion and yield of oxidation of benzyl alcohol are higher than the other aromatic acohols, the conversion is 68% and yield is 63%. Photocatalytic stability is one of the most important factors. So the cyclic experiments were carried out, using CdS-2 nanoparticles as a photocatalyst. The results are shown in Fig. 5(b). After 4-cycle experiments, it can be seen that the yield, the conversion and the selectively all changed obviously. It is proposed that the bad stability of CdS-2 photocatalyst may be attributed to photocorrosion of CdS-2 photocatalyst caused by reaction solvents. 3.3. XRD patterns of fresh and used CdS-2 and possible reaction mechanism As shown in Fig. 6(a), the XRD patterns of CdS-2 sample used for 4 times and the fresh sample are almost identical. To understand the possible reaction mechanism for the photocatalytic oxidation of benzyl alcohol using CdS-2 catalyst, a series of controlled experiments using different radical scavengers were carried out and the results are shown in Fig. 6(b). Different radical scavengers were added to a photocatalytic oxidation of benzyl6 alcohol reaction system for the purpose of scavenging the corresponding active species (•OH, •O2-, h+, e-), such as 10

p-benzoquinone (BQ) for scavenging superoxide radicals (•O2−), isopropyl alcohol (IPA) for removing •OH, and oxalic acid (OA) for capturing holes (h+)[43-46]. As shown in Fig. 8(b), when the scavengers of BQ were added to the photocatalytic oxidation of benzyl alcohol reaction system, the conversion of benzyl alcohol decreased rapidly. When IPA was added to the photocatalytic oxidation of benzyl alcohol reaction system, the conversion of benzyl alcohol decreased slightly. When OA was added to the photocatalytic oxidation of benzyl alcohol reaction system, the conversion of benzyl alcohol also decreased rapidly Based on the above results, it is clear that the selective oxidation of benzyl alcohol to benzaldehyde is mainly triggered by •O2- and h+. It has been reported that the photo excited electrons and holes migrate in two sub-layers of CdS nanosheets under the visible light irradiation[3, 47-50]. In Fig. 7, •O2− was formed by molecular oxygen capture photo excited e− in CB of CdS. As shown in Fig. 11, it is proposed that the reaction for the selective oxidation of benzyl alcohol to benzaldehyde may contain two processes. The major process is that •O 2− attacks a substrate in the system, followed by the formation of benzyl alcohol anion. h+ can be captured by benzyl alcohol with its moderate oxidation potential, oxidizing the benyl alcohol into benzaldehyde with the release of H+[29, 51]. Therefore, •O2− and h+ play important roles in the selective oxidation of aromatic alcohols to corresponding aromatic aldehydes.

4. Conclusions To sum up, CdS-2 sample with a band gap of 2.05 eV was prepared using Cd(NO3)2·4H2O and (NH4)2S as precursors. The as prepared CdS-2 sample was used for the selective oxidation of benzyl alcohol to benzaldehyde. The conversion reaches 68%, the yield is 63% and the selectivity is 94% after being illuminated for 1 h under visible light irradiation. Through above experiments, we can find that the catalyst, oxygen and the visible light irradiation are present, the selective oxidation of benzyl alcohol to benzaldehyde is able to proceed. ·O2- and h+ are the important oxidizing agents in the reaction system. 11

Acknowlegements This work was financially supported by the National Natural Science Foundation of China (21261021, 21663027, 51262028), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Program for the Young Innovative Talents of Long yuan and the Program for Innovative Research Team of Northwest Normal University (NWNU-LKQN-15-2).

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Figure Caption Fig. 1 Fig. 1 (a) XRD patterns of the as-prepared samples and (b) UV–vis diffuse reflectance spectra of the as-prepared samples, (c) C(200) diffraction peak positions in the range of 30 - 35° and (d) estimation of band gap energies of the as-prepared samples. Fig. 2 SEM images of CdS-2 (a, b), (c) TEM image of CdS-2 and (b) HRTEM image of CdS-2, insert is SAED pattern of CdS-2. Fig. 3 (a) high resolution spectra of Cd 3d , (b) high resolution spectra of S 2p, (c) XPS survey spectra and (d) EIS Nyquist plots of CdS electrodes measured under the open-circle potential and visible light irradiation (λ> 420 nm) in 0.5 M Na2SO4 (pH=7.5) solution Fig. 4 (a) Photocatalytic performance for selective oxidation of benzyl alcohol to benzaldehyde using the samples and (b) photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over CdS-2 in different solvents under irradiation for 1 h. ( C, Y and S mean Conversion, Yield and Selectivity, respectively.) Fig. 5 (a) Photocatalytic performance of selective oxidation of a range of aromatic alcohols (benzylalcohol; p-methoxybenzyl alcohol; p-chlorobenzyl alcohol) to corresponding aldehydes using CdS-2 catalyst under irradiation for 1h and (b) cyclic experiments of CdS-2 microsphere photocatalyst.

( C, Y, S mean

conversion, yield and selectivity, respectively.) Fig. 6 (a) XRD patterns of fresh and used CdS-2 after photocatalytic reaction and (b) benzyl alcohol of conversion using CdS-2 photocatalyst in the presence of different radical scavengers under irradiation for1h. Fig. 7 Schematic diagram of the proposed mechanism for selective oxidation of benzyl alcohol to benzaldehyde using CdS-2 under irradiation for 1h

16

H(002) C(111)

a

H(110)

H (100)

C(220)

H(101)

b

H(112)

H(103) C(200) H(102)

CdS-2

Absorbance (a.u.)

Intensity (a.u.)

C(311)

CdS-4 CdS-3

CdS-1 CdS-3

CdS-2

CdS-4

CdS-1

20

c

30

40

50 60 2θ (degree)

70

80

90

300

400

500 600 Wavelength (nm)

700

800

d

C(200)

2 (α hv) (a.u.)

Intensity (a.u.)

CdS-4

CdS-3 CdS-2

CdS-1 CdS-2 CdS-3 CdS-4

2.36eV CdS-1

30

31

32 33 2θ (degree)

2.05 eV

34

35

1.8

2.0

2.09 eV

2.2

2.4

2.38 eV

2.6

2.8

3.0

Energy (eV)

Fig. 1 (a) XRD patterns of the as-prepared samples and (b) UV–vis diffuse reflectance spectra of the as-prepared samples, (c) C(200) diffraction peak positions in the range of 30 - 35° and (d) estimation of band gap energies of the as-prepared samples.

Fig. 2 SEM images of CdS-2 (a, b), (c) TEM image of CdS-2 and (b) HRTEM image of CdS-2, insert is SAED pattern of CdS-2.

17

a

b

Cd 3d

S 2p

Cd 3d 3/2

404

406 408 410 412 Binding energy (eV)

S 2p 1/2

159

414

6x104

S 2p

S 3p

0

160

d

5x104

161 162 163 Binding energy (eV)

CdS-1

CdS-2

164

CdS-3

165

CdS-4

-Z''(ohm)

Cd 3p3 Cd 3p1

4x104

S 1s

C 1s

Intensity (a.u.)

Cd 3d5

c

O 1s

402

S 2p 3/2

Intensity (a.u.)

Intensity (a.u.)

Cd 3d 5/2

3x104 2x104 1x104

200

400 600 Binding energy (eV)

800

0

1000

0

2000

4000

6000 Z'(ohm)

8000

10000

Fig. 3 (a) high resolution spectra of Cd 3d , (b) high resolution spectra of S 2p, (c) XPS survey spectra and (d) EIS Nyquist plots of CdS electrodes measured under the open-circle potential and visible light irradiation (λ> 420 nm) in 0.5 M Na2SO4 (pH=7.5) solution

Percentage (%)

80

100

a

b

Y C S

Y C S

80 Percentage (%)

100

60 40 20

60 40 20

0 CdS-1

CdS-2 CdS-3 Samples

CdS-4

0 BTF

acetonitrile carbon tetrachloridetoluene

water

Solvents

Fig. 4 (a) Photocatalytic performance for selective oxidation of benzyl alcohol to benzaldehyde using the samples and (b) photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over CdS-2 in different solvents under irradiation for 1 h. ( C, Y and S mean Conversion, Yield and Selectivity, respectively.)

18

100

100

a

Y C S

80 Percentage (%)

80 Percentage (%)

b

Y C S

60 40

60 40 20

20 0

0

1st

benzyl alcohol p-methoxybenzyl alcohol p-chlorobenzyl alcohol

2nd

3rd

4th

Recycled Times

Fig. 5 (a) Photocatalytic performance of selective oxidation of a range of aromatic alcohols (benzylalcohol; p-methoxybenzyl alcohol; p-chlorobenzyl alcohol) to corresponding aldehydes using CdS-2 catalyst under irradiation for 1h and (b) cyclic experiments of CdS-2 microsphere photocatalyst.

( C, Y, S mean conversion, yield

and selectivity, respectively.) 80

a

b Conversion (%)

Intensity (a.u.)

60 Fresh

40

20

Used

10

20

30

40 50 60 2θ (degree)

70

80

90

0 Blank

BQ

IPA

OA

Fig. 6 (a) XRD patterns of fresh and used CdS-2 after photocatalytic reaction and (b) benzyl alcohol of conversion using CdS-2 photocatalyst in the presence of different radical scavengers under irradiation for1h.

19

Fig. 7 Schematic diagram of the proposed mechanism for selective oxidation of benzyl alcohol to benzaldehyde using CdS-2 under irradiation for 1h.

20

Graphical abstract

21

Highlights ·The mixed-phase CdS was prepared by a simple microwave method. ·The selective catalytic oxidation of benzyl alcohol to benzaldehyde has highly

Conversion and Selectivity under visible light. ·Superoxide anions, hole and electron were proved that they play important roles in

photocatalysis in the paper.

22