Effects of the synthesis conditions on the photocatalytic activities of sulfide-graphene oxide composites

Accepted Manuscript Effects of the synthesis conditions on the photocatalytic activities of sulfide-graphene oxide composites Fengjuan Chen, Xuekun Jin, Yali Cao, Dianzeng Jia, Anjie Liu, Rong Wu, Mengqiu Long PII:

S0143-7208(18)30205-5

DOI:

10.1016/j.dyepig.2018.09.054

Reference:

DYPI 7036

To appear in:

Dyes and Pigments

Received Date: 28 January 2018 Revised Date:

13 May 2018

Accepted Date: 20 September 2018

Please cite this article as: Chen F, Jin X, Cao Y, Jia D, Liu A, Wu R, Long M, Effects of the synthesis conditions on the photocatalytic activities of sulfide-graphene oxide composites, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.09.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effects of the synthesis conditions on the photocatalytic

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activities of sulfide-graphene oxide composites Fengjuan Chena,b, Xuekun Jinb, Yali Caob, Dianzeng Jiab*, Anjie Liub, Rong Wua,

School of Physics Science and Technology, Xinjiang University, Urumqi 830046,

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a

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Mengqiu Longc

Xinjiang, PR China b

Key Laboratory of Advanced Functional Materials of Autonomous Region, Key

Laboratory of Clean Energy Material and Technology of Ministry of Education,

China

Hunan Key laboratory of Super Micro-structure and Ultrafast Process, Central South

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c

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Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, PR

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University, Changsha 410083, China

Corresponding author. Tel.: +86 0991 8583083; Fax: +86 0991 8580032.

E-mail address: [email protected]

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ACCEPTED MANUSCRIPT Abstract: Sulfide-graphene oxide composites photocatalysts were fabricated through a simple solid-state chemical reaction method under different synthesis conditions. The resulting photocatalysts were characterized by X-ray diffraction, transmission

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electron microscopy, Nitrogen adsorption-desorption specific surface areas, XPS measurements, UV-vis absorption spectroscopy, Transient photocurrent, and Photoluminescence spectra. The result suggests that ZnS and Bi2S3 nanoparticles well

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distributed on the graphene oxide nanosheets in the sulfide-graphene oxide composites.

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Nitrogen adsorption-desorption results indicate that the specific surface areas of the composites increased after the graphene oxide is introduced. XPS measurements suggest that there are interactions between sulfide and graphene oxide. The photocatalytic activities of sulfide-graphene oxide composites were measured by the

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degradation of methyl orange under UV irradiation. Variation of the synthesis conditions allowed the sulfide-graphene oxide composites possessing different structures and photocatalytic activities as revealed by transmission electron

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microscopy and photocatalytic measurements. The photocatalytic results indicated that

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the sulfide-graphene oxide composites exhibited much higher photocatalytic activities than that of pure ZnS and Bi2S3, and nearly 97 % and 90 % of methyl orange were degraded over ZnS-graphene oxide and Bi2S3-graphene oxide after irradiation for 120 min, respectively. The superior photocatalytic activities can be attributed to the high specific surface areas, which is benefical for improving photocatalytic reaction. Futhermore, introducing graphene oxide can effectively reduce the photoinduced 2

ACCEPTED MANUSCRIPT electron-hole pair recombination probability, and then improve the photocatalytic activities, which have been proved by the transient photocurrent, photoluminescence

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Keywords: ZnS; Bi2S3; graphene oxide; photocatalysis

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spectra, and scavengers measurements.

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1. Introduction Currently, the deteriorating environmental pollution promotes people to pay

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considerable attention on semiconductor photocatalysis owing to the strong capability to destroy contaminants and extensive applicability [1-4]. Among the semiconductor photocatalysts, sulfides has been been widely used as a potential photocatalyst for

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environmental treaments due to its especial photoelectric property, high efficiency, and low cost [5-8]. However, the high recombination probability of photo-generated

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carriers severely influences the photocatalytic efficiency. Therefore, acquiring new types of sulfide-based photocatalysts with excellent performance is considerable urgent.

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Hence, many improvements have been carried out to impede the electron-hole recombination and sequentially improve the photocatalytic activity, such as compositing semiconductor, noble metal deposition, and coupling sulfides with

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carbon materials [9-12]. Among them, coupling sulfides with carbon materials have

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been found to be an effective routine to reduce the photo-generated carriers recombination probability, then enhancing the photocatalytic efficiency [13,14].

Graphene, with a unique two-dimensional chemical structure, has becoming an

potential photocatalytic candidate, because of the unique electronic property, and large specific surface area [15,16]. Functionalizing graphene with hydroxyl and carboxyl groups can obtain graphene oxide (GO), which also exhibits high 4

ACCEPTED MANUSCRIPT photocatalytic performance [17], because GO can work as an electron relay mediator for efficiently accepting and transporting electrons, which is helpful to enhancing the photocatalytic activity [18]. Many studies have proved that sulfide-GO composites

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displayed high photocatalytic activity. Thus, lots of work have been devoted to obtain sulfide-GO composites with superior photocatalytic performance [19,20]. For instance, Zou et al. fabricated CdS–rGO nanocomposites via a facile mixing process with

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ethylene glycolas solvent, and the results indicate the nanocomposites exhibit superior

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photocatalytic activity and stability than that of bare CdS, ascribing to the introduciton of rGO, which facilitates the charge separation and suppresses the recombination of electron–hole pairs [21]. Although tremendous progress has been acquired in the past years, a simpler and faciler method to synthesis sulfide-GO

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composites is highly urgent yet, and the critical role of graphene oxide and other synthesis conditions in improving photodegradation has not been investigated in detail

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yet.

Recently, solid-state chemical reaction method has become a potential alternative to

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other methods for synthesizing nanomaterials ascribing to the simplicity and low cost [22,23]. Because several processes were involved in the room-temperture solid-state method including (1) physical contact among reactants, (2) instant reaction at the interface, (3) the release of heat within a short time, (4) fast nucleation, and (5) limited diffusion. In the past years, many sulfides have been prepared by this method [24,25]. As far as we know, there is few research in synthesizing sulfide-GO 5

ACCEPTED MANUSCRIPT composites via the method to date. And the study about the effects of graphene oxide contents and other synthesis conditions on the structures and photocatalytic activities of the composites is also rare.

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Therefore, in the work, we have synthesized sulfide-GO composites through a simple solid-state chemical reaction method under different synthesis conditions. The

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photocatalytic properties of the resulting products were studied via the degradation of methyl orange (MO) under UV irradiation. Different contents of graphene oxide and

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different synthesis conditions have been applied to modify the as-prepared sulfide by the above method, which may play a great effect on the surface chemical states and microstructures, and in succession influence the photocatalytic activities of sulfide-GO composites. Therefore, the effects of graphene oxide contents and

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synthesis conditions on the structures and photocatalytic activities of the composites have also been investigated, and the reasons have been investigated by transient

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photocurrent, photoluminescence spectra, and scavengers measurements.

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2. Experimental sections

2.1. Synthesis of ZnS-GO composites photocatalysts

The sulfide-GO composites were fabricated by a simple solid-state chemical

reaction method under ambient temperature. Zinc acetate, Zinc chloride, bismuth nitrate, bismuth chloride, thioacetamide, sodium thiosulfate, and graphite were used as raw materials. 6

ACCEPTED MANUSCRIPT First, graphene oxide (GO) was synthesized from graphite powder by the modified Hummer’s method [26]. The ZnS-GO composites were obtained as followings. In a typical synthesis procedure, 10 m mol zinc acetate (Zn(CH3COO)2·2H2O) was

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accurately weighed and ground for 20 min. Then 0.2 % quality percentage content of GO and 10 m mol thioacetamide (NH2CSCH3) were added, and was ground for 60 min continuously. Then, the mixture was washed with distilled water and ethanol

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several times to remove by-products, and was dried at 60 oC for 2 h. Finally the

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ZnS-GO composites were successfully obtained. When the contents of GO were 0, 0.1, 0.2, 0.5, 1, and 2 wt%, the resulting ZnS-GO composites were named GZS0 (pure ZnS), GZS0.1, GZS0.2, GZS0.5, GZS1 and GZS2, respectively.

In order to discuss the effect of the surfactants, we also prepared ZnS-GO

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composites by the following method. In a typical method, 10 m mol zinc acetate (Zn(CH3COO)2·2H2O) was accurately weighed and ground for 20 min. Then 10 m

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mol sodium dodecyl sulfate (SDS) were added, and was ground for 60 min continuously. After that, 0.2 % quality percentage content of GO and 10 m mol

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thioacetamide (NH2CSCH3) fine powder were added, and was ground for 60 min to assure an entire reaction. The ZnS-GO composites prepared with surfactants SDS and PEG 400 were named GZS0.2-SDS and GZS0.2-PEG, respectively. For comparison, the ZnS-GO composites were named GZS0.2-S1, GZS0.2-S2, GZS0.2-S3, GZS0.2-S4, GZS0.2-S5 and GZS0.2-S6, when the contents of SDS were 1, 2, 5, 10, 20 and 50 m mol, respectively. The sample GZS0.2-S3 was also named as GZS-S1. 7

ACCEPTED MANUSCRIPT In order to further study the influences of metallic salt and sulfide species, we also prepared ZnS-GO composites by the following method. In a typical method, thioacetamide (NH2CSCH3) was replaced by sodium thiosulfate (Na2S2O3), others are

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the same as the above sample GZS-S1, and the as-synthesized product was named as GZS-S2. Similarly, zinc acetate was replaced by zinc chloride (ZnCl2), others are the

GZS-S3.

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2.2. Synthesis of Bi2S3-GO composites photocatalysts

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same as the above sample GZS-S1, and the as-synthesized product was named as

The Bi2S3-GO composites were obtained as followings. In a typical synthesis procedure, 10 m mol bismuth nitrate (Bi(NO3)3·5H2O) was accurately weighed and

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ground for 20 min. Then 0.2 % quality percentage content of GO and 15 m mol thioacetamide (NH2CSCH3) were added, and was ground for 60 min continuously. Finally, the mixture was washed with distilled water and ethanol several times, and

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was dried in air at 60 oC for 2 h. The Bi2S3-GO composites prepared with different

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contents of 0，0.1，0.2，0.5，1 and 2 wt% GO were named GBS0 (pure Bi2S3), GBS0.1, GBS0.2, GBS0.5, GBS1 and GBS2, respectively.

In order to discuss the effect of the surfactant SDS, we also prepared Bi2S3-GO

composites by the following method. In a typical method, 10 m mol bismuth nitrate (Bi(NO3)3·5H2O) was accurately weighed and ground for 20 min. Then 10 m mol sodium dodecyl sulfate (SDS) were added, and was ground for 60 min continuously. 8

ACCEPTED MANUSCRIPT After that, 0.2 % quality percentage content of GO and 15 m mol thioacetamide (NH2CSCH3) fine powder were added, and was ground for 60 min to assure an entire reaction. The Bi2S3-GO composites prepared with surfactant SDS were obtained. For

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comparison, the Bi2S3-GO composites were named GBS2-S1, GBS2-S2, GBS2-S3, GBS2-S4, GBS2-S5, and GBS2-S6, when the contents of SDS were 1, 2, 5, 10, 20 and 50 m mol, respectively. The sample GBS2-S5 was also named as GBS-S1.

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In order to further study the influences of metallic salt and sulfide species,

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following the same procedure of the sample GBS-S1, except that replacing thioacetamide (NH2CSCH3) with sodium thiosulfate (Na2S2O3), others are the same as above sample GBS-S1, and the as-synthesized product named as GBS-S2. Similarly, bismuth nitrate was replaced by bismuth chloride (BiCl3·2H2O), and the

2.3. Characterization

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as-synthesized product named as GBS-S3.

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The phase structure was performed by X-ray diffraction (XRD) on a Bruker-D8

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diffractometer system. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were carried out using a JEM-2100F (JEOL Co., Japan) microscope to characterize the microstructures of the products. Nitrogen adsorption-desorption isotherms were acquired on a Micromeritics ASAP 2050 apparatus. XPS measurements were conducted on a Thermo Scientific ESCALAB 250Xi XPS system, using Al Ka radiation and adventitious C 1s peak (284.8 eV) calibration. Transient photocurrent experiments were conducted via an electrochemical analyzer (CHI 660 9

ACCEPTED MANUSCRIPT Instruments) in a standard three-electrode system using the as-prepared electrodes as the working electrodes, a Pt plate as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. A 0.1 M Na2SO4 aqueous solution was used as the

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electrolyte. A Xe lamp (300 W) with a cutoff filter (λ = 200 - 400 nm) was used as the light source. Photoluminescence (PL) spectra were obtained using F-4500 (Hitachi, Inc., Japan) with an excitation wavelength of 400 nm. UV-vis absorption spectra were

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obtained by using a Hitachi U-3900 spectrophotometer. The photocatalytic

2.4. Photocatalytic test

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mesurements were carried out in an XPA-7 photochemical reactor.

The photocatalytic degradation experiments were performed in an XPA-7

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photochemical reactor. A 300W mercury lamp with a maximum emission at about 365 nm was vertically positioned inside the quartz reactor, and surrounded by circulating water to cool the lamp. Photocatalytic activities were studied by the degradation of

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methyl orange (MO). The initial concentration of MO was 10 mg L-1 then 50 mg

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photocatalyst was placed in 100 mL MO solution. After exposion under UV irradiation for a certain time, the reaction suspension was collected, and then analyzed by the UV-vis spectrum through recording MO at the 462 nm. All the experiments were conducted at room temperature. The degradation efficiency of MO solution was studied by the following equation:

Degradation efficiency (%) = (Co - C) / Co × 100 10

ACCEPTED MANUSCRIPT Where Co and C are the absorbance of MO solution when irradiation time is 0 and t, respectively.

In order to study the effects of relevant reactive species, 1 mM different radical

manner similar to the photocatalytic degradation experiment.

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scavengers (KI, ethanol and 1, 4-benzoquinone) were added into MO solution in a

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The stability experiments were performed in a way that the MO solution was irradiated for 60 min, in which the photocatalyst was then recollected and washed

3. Results and discussion

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with aqueous solution in each recycle run.

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3.1. Compositional and structural characterization of ZnS-GO composites

Fig. 1 is here

Fig. 1 shows the XRD patterns of GO, pure ZnS, ZnS-GO composites GZS-S1,

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GZS-S2 and GZS-S3. The GO displays a sharp peak (Fig. 1a) at about 2θ=11.2°,

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corresponding to the (001) reflection of stacked GO sheets, suggesting the formation of GO [27]. The XRD diffraction peaks of pure ZnS. The peak of pure ZnS observed at 28.2° is the (111) plane, which can be well indexed to the sphalerite ZnS (JCPDS No. 05-0566), in good agreement with the reported [28]. It can also be found that the diffraction peaks of ZnS-GO composites GZS-S1, GZS-S2 and GZS-S3 are similar to that of pure ZnS and belongs to sphalerite ZnS (JCPDS No. 05-0566). However, no 11

ACCEPTED MANUSCRIPT diffraction peaks of carbon species can be noted, probably ascribing to the low content of GO in the composites, which was also found in previous report [29].

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Fig. 2 is here.

In order to investigate the effects of metallic salt and sulfide species on the morphology of the products, we have studied the microstructures by transmission

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electron microscopy (TEM) technology. It can be found (Fig. 2a) that GO has a pleated layered structure with several laminated layers of the graphite oxide sheets

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[30]. As for pure ZnS, the spherical ZnS nanoparticles can be obviously noted in the TEM images of Fig. 2b. As can be seen from Fig. 2c, for the composites GZS-S1, spherical ZnS nanoparticles with several of nanometers in diameter are uniform

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dispersed on the graphene oxide sheets. At the same time, the ZnS nanoparticle are about 50 nm in the composites, which is reduced and size distribution are more uniform comparing with the pure ZnS, which furnishing abundant contact between

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ZnS nanoparticles and GO and then promoting carrier transport fast [31]. However,

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because of the high surface energy, an agglomeration occurs for the composites GZS-S2 (Fig. 2d). There are ZnS nanoparticles about 100 nm randomly dispersing on the graphene oxide sheets. As for the composites GZS-S3 (shown in Fig. 2e), it can be noted that about 100 nm ZnS nanoparticles are dispersed on the surface of graphene oxide. From the above statements, it is easy to known that the graphene oxide sheets can effectively stabilize ZnS nanoparticles and then prevent the agglomeration. The above results also indicated that introducing graphene oxide to the ZnS-GO 12

ACCEPTED MANUSCRIPT composites can hinder the agglomeration of ZnS. In addition, ZnS-GO composites possess different microstructures as the variation of the synthesis conditions. Moreover, to further confirm the co-presence of ZnS and GO in the composites

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GZS-S1, the HRTEM view is carried out and shown in Fig. 2f. The interplanar spacing of 0.31 nm is clearly observed, which corresponds to (111) plane of ZnS, and the amorphous part belongs to the GO. Based on the above analyses, it could be

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concluded that ZnS nanoparticles has been successfully introduced into the

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composites and has a good connection between ZnS and GO, which promotes the effective separation of electrons and holes in the composites.

Fig. 3 is here.

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Fig. 3 shows nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves (inset of Fig. 3) of ZnS and ZnS-GO composites. It can be seen that adsorption-desorption isotherms curves of ZnS appears to be the type IV

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with H2 hysteresis loops, indicating the presence of mesoporous [32]. It is easily

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found that the composites GZS-S1 shows a type I isotherms curves, typical feature of slit pores in microporous, which can also be proved by the inset of Fig. 3b. As for the composites GZS-S2 and GZS-S3, the isotherms can be belonged to the type IV with H3 hysteresis loop [33]. The specific surface area of the composites GZS-S1 is 188.1 m2 g-1, higher than that of ZnS (45.9 m2 g-1), GZS-S2 (122.9 m2 g-1) and GZS-S3 (33.9 m2 g-1), which could be ascribed to the presence of micporoes and small size of ZnS nanoparticles in the composites GZS-S1. Hence, the composites GZS-S1 had 13

ACCEPTED MANUSCRIPT larger specific surface area than ZnS, GZS-S2 and GZS-S3, which could provide fast transport for carrier, and then promote photocatalytic efficiency [34]. Hence, it is an effective way to deal with environmental pollution via putting the resulting

Fig. 4 is here.

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composites GZS-S1 as promising photocatalysts.

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XPS was performed to further analyze the chemical states of elements in the ZnS-GO composites GZS-S1. From Fig. 4a, it can be seen that the composites is

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mainly composed of C, O, S, and Zn elements, indicating the existence of ZnS and GO. Fig. 4b-d show the high resolution XPS spectra of C 1s, S 2p, and Zn 2p states, respectively. In Fig. 4b, the high resolution C 1s spectra presented four peaks,

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attributed to C–C (284.8 eV), C–O (285.8 eV), C=O (286.6 eV), and O–C=O (288.7 eV), consistent with the reported value of GO [35]. The peaks at 161.8 eV and 163.1 eV corresponded to S 2p3/2 and S 2p1/2 states, respectively (Fig. 4c), in agreement with

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the reported value [36].The Zn 2p3/2 and Zn 2p1/2 peaks were located at 1022.57 eV

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and 1045.52 eV, respectively (Fig. 4d), illustrating formation of ZnS. The difference between the two binding energies was 23.20 eV, comparing with the standard value of 22.97 eV [37], which has a slight shift, suggesting changes in the chemical environment and there was interaction between ZnS and GO in ZnS-GO composites. The interaction will modify the original chemical states and electronic properties in the composites.

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ACCEPTED MANUSCRIPT 3.2. Photocatalytic activity

Fig. S1 is here.

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To investigate the latent application of the ZnS-GO composites in the remedition of environmental pollution, we have studied the photocatalytic activities by degradation methyl orange (MO) under UV irradiation.

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As we all known, the contents of GO have an important influence on the

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photocatalytic activities of the ZnS-GO composites. Hence, the photocatalytic ability of the ZnS-GO composites with different contents of GO were studied by degrading MO under UV irradiation for 60 min, as shown in Fig. S1. It can be noted that the sample GZS0 (pure ZnS) displays a low photocatalytic degradation efficiency on MO

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(36 %). With GO introducing, the photocatalytic efficiencies of composites enhanced. And the composites GZS0.2 reaches the highest value of 53 % when GO content is 0.2 wt%. As the amount of GO increases further, the photocatalytic properties of the

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composites decrease. It is possible that the increased GO acts as combination centers

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of photogenerated carriers, causing the decrease of the catalytic reaction rate [38]. Thus, the appropriate amount of GO plays an important role in optimizing the performances of composites photocatalysts. From the above analysis, we can see that the composites GZS0.2 exhibits the highest photocatalytic activity when the amount of GO is 0.2 wt%. Therefore, the composites GZS0.2 was selected for further study.

Fig. S2 is here. 15

ACCEPTED MANUSCRIPT In order to study the effect of the surfactants SDS and PEG 400, we have also prepared ZnS-GO composites with surfactants and studied the photocatalytic activities. Fig. S2 exhibits the photocatalytic ability of ZnS-GO composites with different

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surfactants by degrading MO under UV irradiation for 60 min. The photocatalytic efficiencies enhanced with the involving of surfactants. And the composites GZS0.2-SDS shows a higher value of 88 % when SDS is present. Therefore, the

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Fig. S3 is here.

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composites GZS0.2-SDS assisted by SDS was selected for further study.

In order to further study the effect of the surfactant SDS, we also investigated the ZnS-GO composites photocatalytic activities. Fig. S3 shows the photocatalytic

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efficienies of the ZnS-GO composites with different contents of SDS by degrading MO under UV irradiation for 60 min. It can be found that the photocatalytic efficiencies of composites enhanced when SDS is introduced. And the composites

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GZS0.2-S3 reaches the highest value of 88 % when the content of SDS is 5 m mol.

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Thus, the appropriate amount of SDS plays an important role in optimizing the activities. When the content of SDS is 5 m mol, the composites GZS0.2-S3 (that is GZS-S1) has the highest photocatalytic activity. Therefore, the composites GZS-S1 was selected for further study.

Fig. 5 is here.

The influences of the metallic salt and sulfide species upon the photocatalytic 16

ACCEPTED MANUSCRIPT activities of the ZnS-GO composites were also studied, as shown in Fig. 5. Fig. 5a shows the degradation curves of the residual MO concentration (C/C0) with irradiation time. It is obvious that the degradation rate improved after the addition of

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photocatalysts. Meanwhile, it can be seen that nearly 92 % of MO was degraded after irradiation for 60 min over the composites GZS-S1 prepared by zinc acetate, while 42 %, 47 % and 53 % of MO were degraded over pure ZnS, the composites GZS-S2

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and GZS-S3 within the same period of time, respectively. It is clear that the

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composites GZS-S1 exhibits superior photocatalytic abilities to pure ZnS, the composites GZS-S2 and GZS-S3. It indicates that the metallic salt and sulfide species play an important role in the photocatalytic activities of the composites.

Fig. 5b shows the absorption spectra of MO with different times over the

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composites GZS-S1 under UV irradiation. After irradiation for 80 min, the absorption peak almost disappears, indicating the complete degradation of MO. The above result

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further indicates that the composites GZS-S1 displays excellent photocatalytic activity,

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which is in good agreement with the statement in Fig. 5a.

The superior photocatalytic abilities of the composites GZS-S1 can be attributed to

the high specific surface areas and the decrease of carrier recombination probability. As we all known, photocatalytic process mainly occurs on the surface of catalysts. Hence, specific surface areas have an improtant effec upon the phocatalytic activity. The specific surface area of the composites GZS-S1 was 188.1 m2 g-1, which was much higher than that of ZnS (45.9 m2 g-1), GZS-S2 (122.9 m2 g-1) and GZS-S3 (33.9 17

ACCEPTED MANUSCRIPT m2 g-1). The studies suggest that large surface area can also offer more opportunities and active sites in degradation reaction [39]. Hence, the high specific surface areas of the composites GZS-S1 could promote the photocatalytic performance. On the other

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hand, introduction of GO is helpful for improving the photocatalytic activity, which can efficiently decrease the recombination probability of photogenerated carriers. Similar situations have also been verified in other studies on rGO/TiO2 nanotube and

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rGO/g-C3N4 [40,41].

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3.3. Mechanism underlying the enhanced photocatalytic activity of ZnS-GO composites

Fig. S4 is here.

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The UV-vis absorption spectra of the ZnS-GO composites (GZS-S1, GZS-S2 and GZS-S3) and pure ZnS were shown in Fig. S4. Comparing with pure ZnS, the

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ZnS-GO composites all display an obvious blue-shift ascribing to the quantum size-effect. At the same time, it can be found that the composites GZS-S1 and GZS-S3

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both exhibit an enhanced absorption in the UV region due to the presence of graphene oxide. Therefore, introducing graphene oxide into the composites can enhance the light absorption ability, and then improve the photocatalytic activity.

Fig. 6 is here.

For the convenience of observation, UV-vis diffuse reflectance spectrum of the composites GZS-S1 was shown in Fig. 6. It can be found that the composites GZS-S1 18

ACCEPTED MANUSCRIPT displayed an absorption edge at about 470 nm. Moreover, the band gap energy (Eg) of the samples can be estimated using the following equation:

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αhν = A(hν − Eg)n/2 Where α, h, ν, A and Eg are the absorption coefficient, Planck constant, light frequency, proportional constant and band gap energy, respectively. The band gaps of

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the composites GZS-S1 is calculated from the plot of (αhν)1/2 versus hν (n = 1 for direct transition) [42]. From the inset of Fig. 6, the band gap energies of the

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composites GZS-S1 was estimated to be 2.62 eV, lower than that was reported earlier [43], suggesting that the band gap could be controlled by changing synthesis conditions.

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Fig. 7 is here.

In order to further examine the ZnS-GO composites which possessed excellent

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electron–hole pair separation efficiency, the photocurrent response spectra of the as-prepared samples were measured under UV irradiation. As can be seen in Fig. 7a,

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the pure ZnS exhibited the lowest photocurrent responses, which was attributed to the relatively higher recombination rate of photo-generated charge carriers. As for the ZnS-GO composites, they all exhibit higher photocurrent than that of pure ZnS. Among them, the composites GZS-S1 exhibited the highest photocurrent due to the introduction of GO and the appropriate band structure which is helpful for GO effectively trapped photo-excited electrons from the CB of the ZnS, thereby greatly 19

ACCEPTED MANUSCRIPT enhancing the photo-induced charge separation ability.

Photoluminescence (PL) spectra were further carried out to study the recombination of photo-generated electrons and holes in the semiconductors. Fig. 7b shows the room

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temperature PL spectra of the samples, which exhibited a broad emission peak around 445-700 nm. Obviously, all the ZnS-GO composites displayed significantly weakened

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PL intensity compared with pure ZnS, revealing that introducing of GO led to an inhibition effect on the electron-hole pair recombination. In addition, the composites

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GZS-S1 showed the lowest PL intensity, which illustrated the highest electron-hole separation efficiency. Combined with photocurrent analysis, we further proved that the composites GZS-S1 had a unique ability for photo-induced charge separation.

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In order to confirm the active radical species involved in the photocatalytic process, control experiments were carried out for the composites GZS-S1. The 1,4-benzoquinone, ethanol, and KI were used to detect the superoxide (˙O2−), the

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hydroxyl radical (˙OH), and the hole (h+), respectively [44]. As shown in Fig. 7c, the

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photocatalytic activity of the composites GZS-S1 to the degradation of MO increases slightly by the addition of the the hole but reduces obviously with the addition of superoxide radical scavengers. The results indicated that ˙O2− is the main reactive species for the degradation of MO.

It is important to further understand the intrinsic reasons for the generation of the ˙O2

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radicals in the photocatalytic processes with the composites GZS-S1. From the 20

ACCEPTED MANUSCRIPT XPS valence band spectra (Fig. 7d), it can be found that composites GZS-S1 had a valence band (VB) at 1.47 eV. According to the calculation of UV-vis absorption spectra, the band gap of the composites GZS-S1 was 2.62 eV. The minimum of the

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conduction band (CB) was determined using the formula ECB = EVB − Eg and was −1.15 eV for the composites GZS-S1. As is already known, the redox potentials of − ˙OH/OH

and O2/˙O2− were +2.38 and −0.046 eV (vs. NHE) [45], respectively, and the

−

. Nevertheless, the VB value of the composites GZS-S1 (1.47 eV) were not

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˙O2

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CB value of the composites GZS-S1 (−1.15 eV) was negative enough to reduce O2 to

sufficient to oxidize ˙OH to OH−. These results were consistent with those of the free radical trapping experiments.

composites GZS-S1

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3.4. Effects of photocatalytic experiment parameters on MO degradation over ZnS-GO

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Fig. 8 is here.

The effects of photocatalytic experiment parameters (including initial pH, and

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dosage of photocatalyst) on the efficiencies of composites GZS-S1 in degradation of MO were also studied, and the results are shown in Fig. 8. Fig. 8a shows MO degradation at pH = 1, 3, 5, 7, 9 and 11. It can be found that the degradation rate firstly increases and then decreases as the solution pH increasing. When the solution pH was 5, the degradation rate approached to 0.08 with irradiation for 60 min. With the solution pH continuously increasing, the degradation rates reduce quickly. The 21

ACCEPTED MANUSCRIPT above statements revealed that the acidic conditions were more favorable for MO degradation over the composites GZS-S1. It indicate that the solution pH plays a key role in the photocatalytic degradation of MO.

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Fig. 8b displays MO degradation using various dosages of composites GZS-S1 from 100 to 1500 mg L-1. It can be known that the degradation rates quickly increased as dosages of the composites GZS-S1 increasing. Especially, when the composites

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dosages increased from 100 to 500 mg L-1, the degradation rates changed from 0.7 to

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0.08 with irradiation for 60 min. As the dosages increased continuously, the degradation rate grows but the change is not obvious. These results indicate that larger dosage of photocatalyst is helpful for increasing the photocatalytic activity of the composites. It is probably because the larger dosage of photocatalyst can provide

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more surface active sites for photocatalytic reactions [46].

Fig. S5 is here.

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In order to evaluate stability of the composites GZS-S1, cyclic experiment for

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photocatalytic degradation of MO was carried out with irradiation for 60 min as shown in Fig. S5. At the first run, 92 % MO was removed. Then the degradation efficiencies were slightly reduced to 90 % (second run), 89 % (third run), 87% (fourth run), 86 % (fifth run), and 83 % (sixth run), respectively, suggesting the relatively high stability for the composites.

3.5. Characterization and photocatalytic activity of Bi2S3-GO composites 22

ACCEPTED MANUSCRIPT Fig. S6 is here.

The phase structures of the GO, pure Bi2S3, and Bi2S3-GO composites were characterized by XRD, as shown in Fig. S6. It can be seen from Fig. S6, the

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diffraction peak of near 11.2° is ascribed to GO (001), indicating the presence of GO [27]. Pure Bi2S3 is belonging to the orthorhombic Bi2S3 (JCPDS No. 65-2431). The

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main diffraction peaks of the composites GBS-S1 and GBS-S2 prepared under different conditions are similar to that of the pure Bi2S3. However, the diffraction

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peaks of the composites GBS-S3 prepared by sodium thiosulfate are not consistent with that of pure Bi2S3, indicating that sulfide species play a key role on the phase structure of composites.

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Fig. 9 is here.

Fig. 9 presents the TEM images of Bi2S3 and the Bi2S3-GO composites. From Fig. 9a, it can be found that Bi2S3 nanoparticles is about 80 nm, and occurred a serious

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aggregation. As for the Bi2S3-GO omposites GBS-S1, it can be found that Bi2S3

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nanoparticles are randomly distributed on the surface of GO (Fig. 9b). As shown in Fig. 9c, about 10 nm of Bi2S3 nanoparticles are uniformly dispersed on the GO sheet in the composites GBS-S2, which can provide enough contact surface between GO and Bi2S3 nanoparticles and more active sites for carrier transport [34]. Comparing with pure Bi2S3, Bi2S3 nanoparticles in the composites GBS-S2 decreases obviously and distributes more uniformly. As can be seen from Fig. 9d, a large number of Bi2S3 23

ACCEPTED MANUSCRIPT nanoparticles are distributed on the surface of GO, and the distribution is uniform. The above results show that Bi2S3-GO composites can efficiently prevent the agglomeration of Bi2S3 nanoparticles. At the same time, it can be seen that composites

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with different morphologies are obtained under different synthesis conditions, probably due to the different reaction rates of the system with the change of the synthesis conditions. The interplanar spacing of 0.30 nm is corresponding to (112)

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plane of Bi2S3, and the amorphous GO can also be found (Fig. 9e). From the above

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statements, it can be known that there is a well interaction between Bi2S3 and GO in the composites, which will lead to an enhancement of the carrier separation.

Fig. 10 is here.

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Fig. 10 shows nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves (insets) of Bi2S3 and the Bi2S3-GO composites. It can be found that the N2 isotherms of all the samples are to be type IV, feature of mesoporous

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materials with H3 hysteresis loop [33]. The corresponding pore size distribution

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curves (insets of Fig. 10) also verified that Bi2S3 and the Bi2S3-GO composites are mainly constituted by mesopores and macropores. Such porous structures can provide more opportunities and reactive sites for the photocatalytic reaction [39]. The specific surface areas of the composites GBS-S2 is calculated to be 22.7 m2·g-1, larger than that of Bi2S3 (8.6 m2·g-1), GBS-S1 (21.8 m2·g-1), and GBS-S3 (3.2 m2·g-1), which is helpful for improving the photocatalytic activity.

24

ACCEPTED MANUSCRIPT Fig. S7 is here.

In order to study the effect of GO content on photocatalytic activity of Bi2S3-GO composites, the degradation ability of MO for the composites synthesized at different

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contents of GO at 60 min under UV irradiation were studied and shown in Fig. S7. It can be noted that the photocatalytic efficiencies of the Bi2S3-GO composites improved

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with GO introducing. And the composites GBS2 gets the highest value of 46 % when the content of GO is 2 wt%. Therefore, the composites GBS2 was selected and studied

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further.

Fig. S8 is here.

Fig. S8 shows the degradation ability of Bi2S3-GO composites synthesized at

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different contents of SDS at 60 min under UV irradiation. It can be seen that the composites GBS2-S5 has the highest value of 46 % when SDS content is 20 m mol.

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Therefore, the composites GBS2-S5 (named GBS-S1) was selected for further study.

Fig. 11 is here.

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The influences of the metallic salt and sulfide species upon the photocatalytic

activities of the Bi2S3-GO composites were also studied, as shown in Fig. 11. It can be seen that nearly 90 % of MO was degraded after irradiation for 120 min over the composites GBS-S2 prepared by sodium thiosulfate, while 58 %, 81 % and 51 % of MO were degraded over pure Bi2S3, the composites GBS-S1 and GBS-S3 within the same period of time, respectively (Fig. 11a). It is clear that the composites GBS-S2 25

ACCEPTED MANUSCRIPT exhibits superior photocatalytic abilities to pure Bi2S3, the composites GBS-S1 and GBS-S3. It indicates that the metallic salt and sulfide species play an important role upon the photocatalytic activities of the Bi2S3-GO composites.

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Fig. 11b shows the concentration changes of MO with irradiation time over composites GBS-S2. The absorption peak disappears almost after irradiation for 120

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min, suggesting the nearly complete degradation of MO under UV irradiation. It further indicates that the composites GBS-S2 has excellent photocatalytic activity on

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MO, which can also be found in Fig. 11a.

The superior photocatalytic activity of the Bi2S3-GO composites can be attributed to the following two aspects. Firstly, from the above study, we know that photocatalytic

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activity of the composites has a relationship with specific surface areas. As the specific surface areas larger, the photocatalytic activities are better. As shown in Fig. 10, the determined specific surface areas of Bi2S3-GO composites GBS-S2 is 22.7 m2·g-1,

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larger than that of Bi2S3 (8.6 m2·g-1), GBS-S1 (21.8 m2·g-1), and GBS-S3 (3.2 m2·g-1),

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thus displaying excellent photocatalytic activities (as shown in Fig. 11a). In addition, introducing GO is beneficial for enhancing the photocatalytic activity, which can effectively suppresses the recombination of photogenerated carriers [47].

Fig. 12 is here.

XPS was also used for the Bi2S3-GO composites GBS-S2. There are C, O, S, and Bi elements in the composites GBS-S2 (Fig. 12a), indicating the presence of Bi2S3 26

ACCEPTED MANUSCRIPT and GO. Fig. 12b-d shows the high resolution XPS spectra of C 1s, S 2p, and Bi 4f states, respectively. In Fig. 12b, four peaks were presented in the high resolution C 1s spectra, corresponding to C–C (284.8 eV), C–O (285.8 eV), C=O (286.6 eV), and

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O–C=O (288.7 eV), suggesting the presence of GO. In Fig. 12c, the high resolution S 2p spectra presented two peaks at binding energies of 159.3 eV (S 2p3/2) and 164.7 eV (S 2p1/2), assigning to S2+ in Bi2S3. The Bi 4f7/2 and Bi 4f5/2 peaks were located at

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159.4 eV and 164.7 eV (Fig. 12d), illustrating formation of Bi2S3 [48]. Comparing

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with the reported value [49], the peaks of Bi 4f7/2 and Bi 4f5/2 shifted to lower binding energies, indicating that there was interaction between Bi2S3 and GO in Bi2S3-GO composites.

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Fig. 13 is here.

In order to further examine the electron–hole pair separation efficiency of the Bi2S3-GO composites, photocurrent response spectra, PL spectroscopy, and

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scavengers were also studied as shown in Fig. 13. It can be found that the composites

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GBS-S2 displayed the highest photocurrent and possessed the lowest PL intensity, both revealing that electron–hole pair separation efficiency was excellent. In Fig. 13c, the photocatalytic efficiency for MO degradation distinctly diminished with 1,4-benzoquinone introduced, indicating that ˙O2− was the leading reactive species for the degradation of MO.

Fig. 14 is here. 27

ACCEPTED MANUSCRIPT The effects of dosage and initial solution pH on the MO degradation efficiencies of Bi2S3-GO composites GBS-S2 were also studied, as shown in Fig. 14. It can be seen that dosage and initial pH both play key roles in the photocatalytic degradation of MO

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by composites GBS-S2. There are mainly two aspects, on the one hand, larger dosage of photocatalyst is helpful for the photocatalytic rdegradation of MO. On the other

composites GBS-S2.

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Fig. S9 is here.

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hand, the acidic conditions were more favorable for MO degradation over the

The stability of the Bi2S3-GO composites GBS-S2 were also studied for degradation MO after irradiation for 60 min. After six consecutive cycles, no apparent

4. Conclusions

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variety was observed, implying the high stability of the composites GBS-S2 (Fig. S9).

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In summary, sulfide-graphene oxide composites with different structures and photocatalytic activities have been successfully obtained by the variation of the conditions

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synthesis

via

a

simple

solid-state

chemical

reaction

method.

Sulfide-graphene oxide composites possessing different structures and photocatalytic activities can be verified by transmission electron microscopy and photocatalytic measurements. N2 adsorption-desorption results indicated that the specific surface areas of the composites increased after introducing GO. The resulting composites showed much higher photocatalytic activities in degradation MO than pure ZnS and 28

ACCEPTED MANUSCRIPT Bi2S3 within the same situations, because of the large specific surface area and the decrease of photo-generated carrier recombination probability, which have been proved by the transient photocurrent, photoluminescence spectra, and scavengers

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measurements. Hence, the composites can be considered as potential candidates for the treatment of organic pollutants. Moreover, the work offers a simple route to study the effects of graphene oxide contents and other synthesis conditions on the structures

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and photocatalytic activities of GO-based composites by adjusting synthesis

Acknowledgments

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conditions.

This work was financially supported by the Natural Science Foundation of Xinjiang

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University (No. BS150212), the National Natural Science Foundation of China (No. 21666036), the Scientific and Technological Personnel Training Project of Xinjiang Autonomous Region (No. QN2016BS0038), the Natural Science Foundation of

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Xinjiang Province (No. 2015211C280), and Key Laboratory of Industrial and

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Biocatalysis Ministry of Education of Tsinghua University (No. 2015201).

Supporting Information Available The photocatalytic ability of ZnS-GO composites under different contents of GO is

shown in Fig. S1. The photocatalytic ability of ZnS-GO composites GZS0.2-SDS and GZS0.2-PEG with surfactants SDS and PEG 400, respectively (Fig. S2). The photocatalytic ability of ZnS-GO composites under different contents of SDS is shown 29

ACCEPTED MANUSCRIPT in Fig. S3. The UV-Vis absorption spectra of ZnS-GO composites with different metallic salt and sulfide species are shown in Fig. S4. Photocatalytic efficiencies of composites GZS-S1 for MO degradation of six cycles after irradiation for 60 min is

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shown in Fig. S5. XRD patterns of GO, pure Bi2S3, Bi2S3-GO composites (GBS-S1, GBS-S2 and GBS-S3) are shown in Fig. S6. The photocatalytic ability of Bi2S3-GO composites under different contents of GO is shown in Fig. S7. The photocatalytic

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ability of Bi2S3-GO composites under different contents of SDS is shown in Fig.S8.

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Photocatalytic efficiencies of composites GBS-S2 for MO degradation of six cycles after irradiation for 60 min is shown in Fig. S9.

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Figure Captions: Fig. 1. XRD patterns of samples GO, pure ZnS, ZnS-GO composites (GZS-S1,

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GZS-S2 and GZS-S3).

Fig. 2. TEM images of (a) GO, (b) ZnS, (c) ZnS-GO composites GZS-S1, (d) GZS-S2, (e) GZS-S3, and (f) HRTEM image of composites GZS-S1.

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Fig. 3. Nitrogen adsorption-desorption isotherms and the corresponding pore size

GZS-S3.

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distributions (insets) of (a) ZnS, (b) ZnS-GO composites GZS-S1, (c) GZS-S2, and (d)

Fig. 4. XPS spectra of the ZnS-GO composites GZS-S1: (a) survey spectrum; (b) C 1s;

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(c) S 2p; and (d) Zn 2p.

Fig. 5. (a) Degradation curves of MO over pure ZnS and ZnS-GO composites under UV irradiation for certain times; (b) The concentration changes of MO with

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irradiation time over the composites GZS-S1.

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Fig. 6. UV-Vis absorption spectra of composites GZS-S1, inset is the plot of (αhν)1/2 versus photon energy (hν) for the composites.

Fig. 7. (a) Transient photocurrent response of the ZnS-GO composites and pure ZnS under UV irradiation, (b) PL spectra of the samples, (c) Effects of different reactive species scavengers on the degradation of MO by the composites GZS-S1, and (d) XPS valence band spectra of the composites GZS-S1. 39

ACCEPTED MANUSCRIPT Fig. 8. Effects of (a) initial pH of MO aqueous solution, and (b) dosage of photocatalyst on the efficiencies of the composites GZS-S1.

(d) GBS-S3, and (f) HRTEM image of composites GBS-S2.

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Fig. 9. TEM images of (a) Bi2S3, (b) the Bi2S3-GO composites GBS-S1, (c) GBS-S2,

Fig. 10. Nitrogen adsorption-desorption isotherms and the corresponding pore size

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distributions (insets) of (a) Bi2S3, (b) the Bi2S3-GO composites GBS-S1, (c) GBS-S2,

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Fig. 11. (a) Degradation curves of MO over pure Bi2S3 and Bi2S3-GO composites under UV irradiation for certain times; (b) The concentration changes of MO with irradiation time over the composites GBS-S2.

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Fig. 12. XPS spectra of the Bi2S3-GO composites GBS-S2: (a) survey spectrum; (b) C 1s; (c) S 2p; and (d) Bi 4f.

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Fig. 13. (a) Transient photocurrent response of the Bi2S3-GO composites under UV irradiation, (b) PL spectra of the composites, and (c) Effects of different reactive

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species scavengers on the degradation of MO by the composites GBS-S2.

Fig. 14. Effects of (a) initial pH of MO aqueous solution, and (b) dosage of photocatalyst on the efficiencies of the composites GBS-S2.

Fig. S1. The photocatalytic ability of ZnS-GO composites with different contents of GO. 40

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Fig. S3. The photocatalytic ability of ZnS-GO composites under different contents of

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Fig. S4. The UV-vis absorption spectra of ZnS-GO composites with different metallic

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Fig. S5. Photocatalytic efficiency of ZnS-GO composites GZS-S1 for MO degradation of six cycles after irradiation for 60 min.

Fig. S6. XRD patterns of samples GO, pure Bi2S3, Bi2S3-GO composites (GBS-S1, GBS-S2 and GBS-S3).

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Fig. S7. The photocatalytic ability of Bi2S3-GO composites under different contents of

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SDS.

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Fig. S8. The photocatalytic ability of Bi2S3-GO composites under different contents of

Fig. S9. Photocatalytic efficiency of Bi2S3-GO composites GBS-S2 for MO degradation of six cycles after irradiation for 60 min.

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Effects of the synthesis conditions on the photocatalytic

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activities of sulfide-graphene oxide composites Fengjuan Chena,b, Xuekun Jinb, Yali Caob, Dianzeng Jiab*, Anjie Liub, Rong Wua,

School of Physics Science and Technology, Xinjiang University, Urumqi 830046,

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Mengqiu Longc

Xinjiang, PR China b

Key Laboratory of Advanced Functional Materials of Autonomous Region, Key

Laboratory of Clean Energy Material and Technology of Ministry of Education,

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Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, PR China c

Hunan Key laboratory of Super Micro-structure and Ultrafast Process, Central South

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University, Changsha 410083, China

There are mainly four highlights: 1.

Sulfide-GO composites were synthesized by a simple solid-state method.

2. Effects of GO contents and others on activity of composites were investigated. 3.

The composites exhibited superior activities to pure sulfides.

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The superior activity is proved by transient photocurrent, PL and scavengers

measurements.