rGO nanocomposites

Journal Pre-proofs Full Length Article The role of the Se-rich and Se-poor conditions in the photocatalytic performance of ZnSe/rGO nanocomposites Mohammad Bigdeli Tabar, S.M. Elahi, Mahmood Ghoranneviss, Ramin Yousefi PII: DOI: Reference:

S0169-4332(20)30575-4 https://doi.org/10.1016/j.apsusc.2020.145819 APSUSC 145819

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

10 January 2020 7 February 2020 15 February 2020

Please cite this article as: M. Bigdeli Tabar, S.M. Elahi, M. Ghoranneviss, R. Yousefi, The role of the Se-rich and Se-poor conditions in the photocatalytic performance of ZnSe/rGO nanocomposites, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145819

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The role of the Se-rich and Se-poor conditions in the photocatalytic performance of ZnSe/rGO nanocomposites Mohammad Bigdeli Tabar1, S. M. Elahi1, Mahmood Ghoranneviss1, Ramin Yousefi2* 1Department

of Physics, Science and Research Branch, Islamic Azad University, Tehran, Iran

2Department

of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), MasjedSoleiman, Iran.

Corresponding E-mail address: [email protected], [email protected] Abstract: Pristine Se-rich ZnSe, Se-rich ZnSe/graphene-oxide (GO), pristine Se-poor, and Sepoor ZnSe/GO nanocomposites were synthesized as the visible-light photocatalytic materials. The electron microscope images of the samples showed a micropolyhedron morphology for the pristine Se-rich ZnSe, which this morphology was changed to the nanoparticle (NP) in the ZnSe/GO. On the other hand, the pristine Se-poor ZnSe presented a nanobelt (NB) morphology, which this morphology was converted to a mixed morphology of NBs and NPs by GO. The visible-light photocatalytic activity of the products to degrade methylene blue (MB) dye showed that the GO had a synergetic effect for the photocatalytic performance in both types of samples. However, the Se-rich ZnSe/GO nanocomposites presented an enhancement photocatalytic performance. X-ray diffraction (XRD) and Raman results indicated the GO sheets were converted into reduced GO (rGO) during the synthesis process. Although, Raman and X-ray photoelectron spectroscopy (XPS) results indicated a higher reduction level for the GO sheets in the S-rich ZnSe/rGO nanocomposites. This higher reduction level of GO sheets in the Se-rich ZnSe/rGO nanocomposites led to decreasing the electrical resistance of these nanocomposites in comparison to the Se-poor ZnSe/rGO nanocomposites. This higher conductivity of the Se-rich

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ZnSe/rGO nanocomposites was the most important factor to enhance the photocatalytic performance. Keywords: Se-rich and Se-poor ZnSe nanostructures; ZnSe/rGO nanocomposites; Photocatalytic performance. 1- Introduction

The presence of organic dyes in aquatic environments can adversely affect aquatic life and human health. These dye pollutants are usually toxic, unrecoverable, and resistant to physical and chemical degradation methods. Various methods have been proposed to degrade dye pollutants from industrial wastewater such as carbon adsorption, but these methods result in incomplete degradation of the pollutants. Besides, in such methods, only pollutants are converted from one phase to another and can even generate secondary pollutants. On the other hand, advanced oxidation process (AOS) methods can create hydroxyl radical and causes oxide dye pollutants and generate useful gases such as hydrogen and oxygen [1-5]. The photocatalytic method by semiconductor nanostructures is one of the AOS methods. In addition to common photocatalytic materials such as ZnO, Ag2CO3, CuO, and TiO2, some other groups of semiconductor nanostructures as photocatalytic materials have been also paid attention in recent years. The metal-selenide nanostructures are one of these groups, which due to their suitable band-gap value for the visible-light photocatalytic activity for researchers are attractive [6-10]. Among different metal-selenide nanostructures that have been used as the photocatalytic materials with high efficiency, ZnSe has received much more attention due to its ability to tune band-gap value and morphology [11-16]. Furthermore, the ZnSe structure includes several known defects in its structure that cause energy levels to form in the band-gap space such as donor-acceptor pairs (DAPs), which can be as visible photon absorption centers and play a significant role in the photocatalytic activity. We have demonstrated and systematic study about 2

the effects of these defects on the photocatalytic performance of ZnSe nanostructures in our previous works [17-18]. We observed that Se-poor and Se-rich conditions had significant roles in the enhancement of the photocatalytic performance of ZnSe nanostructures. Despite the excellent photocatalytic performance for degradation dye pollutants we observed from the synthesized samples, the photocatalyst time was long. Therefore, it was decided to resolve this disadvantage. Based on our previous experiences with semiconductor/graphene-oxide (GO) nanocomposites that exhibited a rapid photocatalytic performance due to the unique properties of the GO, we decided to synthesize ZnSe/GO nanocomposites for obtaining high-speed photocatalytic materials. Therefore, two types of ZnSe/GO nanocomposites were synthesized, one Se-rich ZnSe/GO and another Se-poor ZnSe/GO nanocomposites. First of all, the effects of GO on the morphology, structure, and optical properties of these two types of ZnSe nanostructures have been investigated. In the next step, the visible-light photocatalytic performance of these two types of nanocomposites has been compared and the effects of the GO on the photocatalytic activity of the nanocomposites have been studied. 2- Experimental

2.1.

Materials and Synthesis Zn(NO3)2.6H2O (99.99%), selenium (99.99%) powders (Sigma Aldrich), and high purity GO

sheets with 6-10 layers (US Research Nanomaterials, Inc.) were used as zinc, selenium, and GO sources, respectively. Besides, 13 mM of NaBH4 and 1 mM of glycine amino acid were applied as the reducing and surfactant agents, respectively. Pristine Se-rich ZnSe (the molar ratio of Se/Zn=1.4), Se-rich ZnSe/GO composites, pristine Se-poor ZnSe (the molar ratio of Se/Zn=0.6), and Se-poor ZnSe/GO composites were synthesized by a co-precipitation method with a similar process of our previous work [18].

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2.2.

Characterizations

Structure and crystal phases of the products were studied by an X-ray powder diffractometer (XRD, Philips, X’pert, system using CuKα radiation) and Raman spectrometer (UniRAM Double, includes a solid-state laser as an excitation source with a wavelength of 785 nm). Morphology of the samples was investigated by transmission electron and field emission scanning electron microscopies (TEM, Hitachi H-7100, and FESEM, ZEISS, SIGMA VP-500). UV-visible (Perkin-Elmer) and photoluminescence spectrometers (UniRam spectrometer with a He-Cd laser as the source with the power of 200 mW and the excitation wavelength of 325 nm) were used to study the optical properties of the samples. The textural properties such as specific surface area, pore-volume, and mean pore diameter of the samples were investigated by the Brunauer−Emmett−Teller (BET) method using N2 adsorption-desorption at liquid nitrogen temperature (77 K). 2.3.

Preparation of photocatalytic degradation samples

The photocatalytic measurement conditions to degrade methylene blue (MB) dye under a visible-light source such as the light source information and dye concentration were exactly similar to our previous work [18]. 2.4.

Photoresponse and photocurrent measurements

Photoconductivity and photoresponse of the samples were measured by photocells devices, which were fabricated by the obtained products. The light source, which was applied to examine the photocatalytic activity of the samples, was also used to examine the photoconductivity of the samples. Details of these measurements were also similar to our previous work [8]. 3- Results and discussion

3.1. Characterization results

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The XRD patterns of the samples are shown in Fig 1. According to these patterns, all samples have zincblende structures that are matched with a standard card of bulk ZnSe (JCPDS 88-2345). In addition, the XRD patterns of ZnSe/GO composites do not indicate any peak at 10.2 ⁰ that belongs to the pristine GO sheet. Therefore, the XRD results show that the GO sheets were changed into reduced GO (rGO) during the synthesis process due to using NaBH4. The XRD of the pristine Se-rich ZnSe pattern indicates an extra phase that belongs to the Se with a hexagonal structure (H-Se, JCPDS No: 01-086-2246). As can be seen, the intensities of the Se peaks are increased in the Se-rich ZnSe/GO composites. On the other hand, the XRD pattern of the Sepoor ZnSe only indicates the ZnSe phase. However, the XRD patterns of Se-poor ZnSe/GO composites also indicate a small amount of the Se phase.

Figure 1. (a) XRD patterns of the pristine Se-rich ZnSe and Se-rich ZnSe/GO composites. (b) XRD patterns of the pristine Se-poor ZnSe and Se-poor ZnSe/GO composites. The appearance of the Se phase in the Se-poor ZnSe/rGO composites could be due to electrons exchange between ZnSe and rGO sheets. These behaviors of the Se phase in both types of ZnSe/rGO composites can prove that an electronic interaction has been formed between ZnSe and rGO sheets. We did not see such an extra phase of Se in our previous work, in which

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ZnSe/rGO composites have been synthesized with the molar ratio of Se/Zn=1 [17]. Therefore, we can say that Se-rich and Se-poor conditions cause to increase in the reaction between ZnSe and rGO, which could be due to changing of the electronic structure of Se-rich and Se-poor ZnSe in comparison to the ZnSe structure with the molar ratio of Se/Zn=1. In fact, we can say that the Se plays as a bridge role between ZnSe and rGO sheets and the numbers of these bridges are higher in the Se-rich ZnSe/rGO composites due to the initial phase of the Se in the pristine Serich ZnSe. The reduction level of rGO sheets in both types of the ZnSe/rGO composites and different behavior of Se in the structure of the samples can be compared by Raman spectroscopy. The Raman spectra of the pristine ZnSe samples and ZnSe/rGO composites are shown in Fig 2. The first difference between the spectra of the pristine ZnSe samples and ZnSe/rGO composites is appearance two peaks of the longitudinal optical (LO-phonon) mode of the ZnSe structure in the ZnSe/rGO composites; while, the spectra of the pristine samples show only one LO peak. On the other hand, the Raman peaks of the ZnSe structure show a shift in the ZnSe/rGO composites in comparison to the Raman peak in the pristine ZnSe samples. These results also confirm that the Se-poor and Se-rich condition cause to present such behavior because we did not see such different Raman results between ZnSe (Se/Zn=1) and ZnSe/rGO composites in our previous work [17]. Actually, the non-stoichiometric conditions in the Se-rich and Se-poor of the ZnSe have caused the more vibration modes of the ZnSe to appear when the ZnSe has reacted with rGO. Thus, these results also confirm that the Se element is the most important factor to form a reaction between ZnSe and rGO sheets in the ZnSe/rGO composites. The transverse optical (TO) mode is only appeared in the pristine Se-poor ZnSe and it cannot be seen such mode in the pristine Se-rich ZnSe. Therefore, this mode should belong to the Se-vacancy in the ZnSe

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structure. However, this mode has also appeared in the Se-rich ZnSe/rGO composites. The appearance of the TO peak in the Se-rich ZnSe/rGO composites could be due to the transfer of Se elements from the surface of ZnSe toward rGO. As can be seen, the Raman spectra of the ZnSe/rGO composites show two main peaks of rGO at around 1600 and 1361 cm-1, which are belonged to the G and D bands of the rGO. It is known, the magnitude of the ID/IG ratio could be a sign of the reduction level of rGO sheets. As shown, the ID/IG ratio in the spectrum of the Serich ZnSe/rGO composites is greater than this ratio in the spectrum of the Se-poor ZnSe composites. Therefore, the rGO sheets in the Se-rich sample have a higher reduction level in comparison to the rGO sheets in the Se-poor sample. Since the only difference in the samples is their selenium content, it can be concluded that the only cause of this difference is the Se-rich conditions. Thus, our hypothesis about the transfer of Se elements from the surface of Se-rich ZnSe toward rGO could be true. In fact, these Se elements have been substituted with oxygen on the GO sheets and causes to reduce GO.

Figure 2. (a) Raman spectra of the pristine Se-rich ZnSe and ZnSe/rGO composites. (b) Raman spectra of the pristine Se-poor ZnSe and ZnSe/rGO composites. Consequently, we are faced with a higher reduction level in the rGO sheets that have participated in the Se-rich ZnSe/rGO composites. Surely, such a difference in the level of reduction of GO 7

sheets will have a significant impact on the photocatalytic performance of the ZnSe/rGO nanocomposites. In addition, the Raman spectra of the ZnSe/rGO composites indicate ZnO mode at around 440 cm-1, which could be due to the interaction of rGO sheets with ZnSe. A stronger ZnO peak in the Se-rich ZnSe/rGO composites could be also due to stronger interaction between Se-rich ZnSe and rGO. FESEM images of the pristine ZnSe and ZnSe/rGO composites are shown in Fig 3. Figures 3(a) and (c) clearly show that the pristine Se-rich and Se-poor ZnSe have different morphology. The pristine Se-rich ZnSe presents micropolyhedrons morphology (Fig 3(a)), while, we can see a nanobelt (NB) morphology (apparently, they are nanorods (NRs) but according to the TEM image of this sample, they are NBs [18]) for the pristine Se-poor ZnSe (Fig 3(c)).

Figure 3. FESEM image of (a), pristine Se-rich ZnSe micropolyhedrons, (b), Se-rich ZnSe/rGO nanocomposites (c) pristine Se-poor ZnSe NBs, (d) Se-poor ZnSe/rGO nanocomposites.

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Figure 3 (b) shows that the pristine ZnSe micropolyhedrons have been changed to nanoparticle (NPs) morphology in the Se-rich ZnSe/rGO nanocomposites. On the other hand, Fig 3 (d) shows the NBs population in the Se-poor ZnSe sample has been decreased and NPs have appeared in this sample. In fact, the rGO can play as a substrate with too many active sites to create clusters with nano-sized and these clusters cause to synthesize ZnSe with nano-sized morphology on the rGO sheets. These active sites are more in the Se-rich condition in comparison to the Se-poor condition due to the more reaction between Se-rich ZnSe and rGO. Therefore, we can see NPs morphology for the Se-rich ZnSe/rGO nanocomposites, while, the morphology of the Se-poor ZnSe/rGO nanocomposite is a mixed morphology of the NPs and NBs. More details about the effects of the rGO on the morphology of the samples can be achieved by TEM and HRTEM images. TEM images of the Se-rich ZnSe and Se-poor ZnSe/rGO nanocomposites are shown in Fig 4.

Figure 4. (a) TEM image of ZnSe1/rGO nanocomposites that shows NPs morphology for the Serich ZnSe. The inset shows an HRTEM image and a SAED pattern of the ZnSe NPs with a zincblende structure that are placed on the rGO surface. (b) TEM image of the ZnSe2/rGO nanocomposites that shows mixture morphology of the NPs and NBs for the Se-poor ZnSe. The inset shows an HRTEM image and a SAED pattern of the ZnSe nanostructures with a zincblende structure that are placed on the rGO surface. 9

Figure 4(a) shows that the morphology of the Se-rich ZnSe is NPs with an average size of around 10 nm. The inset of Fig 4 (a) shows that how rGO sheets have been decorated by ZnSe NPs with a zincblende structure. Figure 4 (b) shows a TEM image of the Se-poor ZnSe/rGO nanocomposites and this image also confirms that this sample includes a mixed morphology of the NPs and NBs. The inset of Fig 4 (b) also shows the ZnSe nanostructures that are placed on the rGO sheets have a zincblende structure. SAED patterns of both samples indicate a multiphase structure that could be due to the formation of a heterostructure by ZnSe and rGO. Figures 5 (a-e) and (a′-e′) show the elemental mapping images of the Se-rich and Se-poor ZnSe/rGO nanocomposites, respectively. These mappings show that the distribution of the elements is homogeneous in both samples. In addition, the Se/Zn in the Se-rich ZnSe/rGO nanocomposites is bigger than this ratio in the Se-poor ZnSe/rGO nanocomposites. Therefore, the Se-rich and Se-poor conditions have remained in the ZnSe/rGO nanocomposites. Furthermore, the O/C ratio in the Se-rich ZnSe/rGO nanocomposites is lower than the O/C ratio in the Se-poor ZnSe/rGO nanocomposites. Consequently, the elemental mapping also indicates that the reduction level of the rGO sheets in the Se-rich ZnSe/rGO nanocomposites is higher than the reduction level in the Se-poor ZnSe/rGO nanocomposites.

Figure 5 (a-e) Elemental mapping images of the Se-rich ZnSe/rGO nanocomposites. (a′-e′) Elemental mapping images of the Se-poor ZnSe/rGO nanocomposites.

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The interaction mechanism between the ZnSe nanostructures and rGO sheets can be discussed by the XPS results and the XPS spectra of the Se-rich and Se-poor ZnSe/rGO nanocomposites are shown in Fig 6. As shown, C-1s peak in both samples is an asymmetric peak (Figs 6 (a) and (e)). As can be seen, oxygen group content has been reduced in the Se-rich ZnSe/rGO nanocomposites. In addition, a peak at around 283 eV has appeared in the C-1s of the Se-rich ZnSe/rGO nanocomposites that according to the previous results in the literature, it belongs to the C-Se interaction [19]. A comparison between O-1s peaks of the samples indicates an asymmetric for the O-1s peak of the Se-rich ZnSe/rGO nanocomposites (Fig 6 (b)), while, we can see a symmetric peak for the O-1s of the Se-poor ZnSe/rGO nanocomposites (Fig 6 (f)). This different feature in the O-1s peaks of the samples could be due to stronger interaction between Se-rich ZnSe NPs and rGO sheets in comparison to the interaction between Se-poor ZnSe nanostructures and rGO sheets. Se-3d peaks of the samples also present different features (Figs 6 (c) and (g)).

Figure 6 (a-d) XPS spectra of the Se-rich ZnSe/rGO nanocomposites. (a′-d′) XPS spectra of the Se-por ZnSe/rGO nanocomposites.

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It can be seen, a strong peak of SeOx in the Se-3d of the Se-rich ZnSe/rGO nanocomposites, which could be due to the formation of a selenium dioxide structure in the interface of ZnSe and rGO sheets. In fact, this structure could be formed after Se substitution with oxygen in the rGO structure resulting in oxygen extracted from the rGO structure and interacts with the selenium on the surface of the ZnSe as well as Zn. We observed a strong ZnO mode in the Raman spectrum of the Se-rich ZnSe/rGO nanocomposites that can confirm this claim. The Zn-2p peaks of the samples are shown in Figs 6 (d) and (h). As can be seen, the spin-orbit splitting of the Zn-2p3/2 and Zn-2p1/2 is 23 eV, which indicates the Zn atoms are totally bonded with Se in both samples. Absorption spectra of the samples have been obtained by UV-vis spectroscopy and these spectra are shown in Fig 7 (a). As can be seen, all samples have an absorption in the visible region of the electromagnetic spectrum. Therefore, all of these samples can be used as the visible-light photocatalytic materials. Since ZnSe is known as a direct band gap semiconductor, the band-gap values of the samples can be estimated by Tauc’s plots according to the direct band-gap rule and the Tauc’s plots results are shown in Fig 7 (b). As shown, different Se conditions cause to change the band-gap value and the pristine Se-rich ZnSe micropolyhedrons show larger band-gap than the band-gap value of the pristine Se-poor ZnSe NRs. The H-Se is also a direct band-gap semiconductor with a band-gap value of 1.6 eV. Therefore, this shift in the band-gap of the Se-rich ZnSe micropolyhedrons could be due to the band offsets and Fermi energy levels of two semiconductors. In addition, the ZnSe/rGO nanocomposites show lower band-gap energy in comparison to the pristine ZnSe. These shifts could be also due to band offsets between ZnSe nanostructures and rGO sheets.

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Figure 7. (a) Absorption spectra and (b) Tauc plots of the pristine Se-rich ZnSe, Se-poor ZnSe, Se-rich ZnSe/rGO, and Se-poor ZnSe/rGO nanocomposites. 3.2. Photocatalytic measurements The visible-light photocatalytic activity of the samples to degrade MB dye has been examined by comparison Ct/C0 ratio of the samples, which C0 and Ct are the initial absorbances and the absorbance at a time t of the dye, respectively. The Ct/C0 ratios of the samples after exposure of the visible-light at 30-min intervals have been drawn and the results are shown in Fig 8 (a). As can be seen, the samples have some absorption in the dark condition that could be due to surface absorption. The higher dark absorption of the Se-rich ZnSe/rGO nanocomposites could be due to their NP morphology. As shown, in both types of samples, the rGO causes to enhance the photocatalytic performance. Although, the Se-rich ZnSe/rGO nanocomposites in comparison to the Se-poor ZnSe/rGO nanocomposites show a faster photocatalytic performance. The photocatalytic rate constants for the degradation of the dye (k), which is one of the useful factors to compare the photocatalytic activity of the samples, that can be estimated from the first-order plot using the following equation: 𝐶0

𝑙𝑛 𝐶𝑡 = 𝑘𝑡 (1)

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The k plot results of the samples are shown in Fig 8 (b). As shown, the ZnSe/rGO nanocomposites have a higher k than the pristine ZnSe samples. Although, this plot shows that the highest k belongs to the Se-rich ZnSe/rGO nanocomposites. Therefore, it can be claimed the Se-rich conditions have been a noteworthy role to enhance the photocatalytic performance of the ZnSe/rGO nanocomposites due to several reasons, which some of them will be discussed in the next part.

Figure 8. (a) Ct/C0 results for the different time interval during the photocatalytic degradation of MB dye for different samples. (b) Degradation rate constants of MB dye for different samples as well as dark condition. The reusability test of the ZnSe/rGO nanocomposites has been examined by a cyclic photocatalytic activity for four times to degrade of MB dye under the visible-light irradiation and results are shown in Fig 9 (a). In this process, the nanocomposites were reused by centrifuging the MB and washing the nanocomposites after each photocatalytic activity. The results show that the rate constant, k, for the Se-rich ZnSe/rGO nanocomposites has been only reduced around 4%, after four times photocatalytic activity, while, this reducing for the Se-poor ZnSe nanocomposites is around 21%. Therefore, the Se-rich ZnSe/rGO nanocomposites not only 14

have presented a higher photocatalytic activity but also have shown higher stability photocatalytic performance in comparison to the Se-poor ZnSe/rGO nanocomposites. Moreover, the phase stability of the nanocomposites after four times photocatalytic activity has been examined by XRD characterization and results are shown in Fig 9 (b). As shown, both samples present a stable phase after 4 times reusability, approximately. Therefore, more reduction in the rate constant, k, of the Se-poor ZnSe/rGO nanocomposites could be due to their morphology, which was a mixed morphology of the NBs and NPs. In fact, the washing process of these nanocomposites with such mixed morphology cannot be completed and resulting it can cause to reduce the photocatalytic activity after several times reusability.

Figure 9. (a) Degradation rate constants, k, of MB dye by the ZnSe/rGO nanocomposites before and after 4 times photocatalytic activity. (b-c) XRD patterns of the Se-rich and Se-poor ZnSe/rGO nanocomposites after 4 times photocatalytic activity. 3.3. Photocatalytic mechanism The Se concentration effects on the textural properties of the Se-rich and Se-poor ZnSe/rGO nanocomposites were investigated by N2 adsorption-desorption measurements using the BET method. Figure 10 demonstrates the N2 adsorption-desorption isotherms of the Se-rich and Sepoor ZnSe/rGO nanocomposites. Both samples show a hysteresis loop that indicating the samples include a mesoporous structure. The hysteresis loop shape of the samples is similar, thus they contain similar pore shapes.

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Figure 10 (a) N2 adsorption–desorption isotherm of the Se-rich and Se-poor ZnSe/rGO nanocomposites. (b) The pore size distribution of the Se-rich and Se-poor ZnSe/rGO nanocomposites. Figure 10 (b) shows the pore size distribution of the samples. The Se-rich ZnSe/rGO nanocomposites show narrower pore size distribution that could be due to their NP morphology. As can be seen, the textural properties of the ZnSe/rGO nanocomposites have been improved by Se-poor conditions. Therefore, the textural properties do not play a significant role in the enhancement photocatalytic performance of the Se-rich ZnSe/rGO nanocomposites. Table 1. Textural properties of the Se-rich and Se-poor ZnSe/rGO nanocomposites Sample Surface area Mean pore diameter (nm) Pore volume (m2/g) (cm3/g) Se-rich ZnSe/rGO 30.02 2.21 8 × 10-3 nanocomposites Se-poor ZnSe/rGO 135.80 4.08 130 × 10-3 nanocomposites The electrical conductivity of the nanocomposites was compared by I-V measurements and photoresponse test. The I-V plots have been drawn under dark (D) and source light (L) illumination conditions and the plots are presented in Fig 11 (a). As can be observed, the resistance of the Se-rich ZnSe/rGO nanocomposites is smaller than those of the Se-poor ZnSe/rGO nanocomposites under illuminated and dark conditions. The photoresponse test of the samples also shows higher photocurrent as well as the smaller rise and fall times for the Se-rich ZnSe/rGO nanocomposites than those for the Se-poor ZnSe/rGO nanocomposites (Fig 11 (b)).

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The faster response and higher photocurrent of the Se-rich ZnSe/rGO nanocomposites indicate a stronger electron-hole separation in the Se-rich ZnSe/rGO nanocomposites. One of the most important characterization techniques to show electron-hole pair lifetime is PL spectroscopy. The PL spectra of the ZnSe/rGO nanocomposites are shown in Fig 11(c). Usually, the PL spectrum of the ZnSe nanostructures shows two emission peaks from the visible region. One blue emission that is belonged to the near-band-emission (NBE) and another a red emission that is belonged to the defects [17]. The PL spectra of the nanocomposites show that the rGO causes to decrease the defect emission. On the other hand, the blue emission split into two peaks that could be due to the Se phase on these nanocomposites. But the most important characteristic of the PL results is the magnitude of the PL intensity of the Se-poor ZnSe/rGO nanocomposites compared to the PL intensity of the Se-rich ZnSe/rGO nanocomposites. It is known, the PL intensity is one of the factors that can be used to measure electron-hole pair lifetime [20]. The lower PL intensity of the Se-rich ZnSe/rGO nanocomposites shows a higher electron-hole pair lifetime in comparison to the Se-poor ZnSe/rGO nanocomposites. Therefore, these results also like the Raman and XPS results show that the reduction level of rGO has been increased by the Se-rich conditions and such condition has been caused to increase the conductivity of the Se-rich ZnS/rGO nanocomposites. Consequently, more electron-hole pairs have been created by photons’ energies and more carriers can transfer toward the interface of the nanocomposites and dye and resulting in higher photocatalytic efficiency has been happened by Se-rich ZnSe/rGO nanocomposites.

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Figure 11. (a) I-V measurements and (b) photoresponse of the Se-rich and Se-poor ZnSe/rGO nanocomposites. (c) PL spectra of the Se-rich and Se-poor ZnSe/rGO nanocomposites. Since the morphology of the ZnSe/rGO does not show a layer of ZnSe on the rGO sheets, we can assume that there is a heterostructure that has been formed by ZnSe/rGO and ZnSe. The band alignment of a heterostructure can be drawn by the estimating of the valence band maximum (VBM) positions of each part of the heterostructure by the valence band spectroscopy (VB). Figure 12 shows the VB spectra of the pristine ZnSe and ZnSe/rGO nanocomposites with different Se conditions. As shown, the pristine Se-rich and Se-poor ZnSe samples have different VBM positions, which show the electronic structure of the ZnSe can be changed by different Se concentration in the ZnSe. In addition, the VBM positions of the ZnSe/rGO nanocomposites have been shifted in comparison to the VBM positions of the pristine ZnSe samples. However, the amount of this shifting is different (1.1 eV for the Se-rich ZnSe/rGO and 2.3 eV for the Sepoor ZnSe/rGO nanocomposites). By using VBM position energy and band-gap values of the samples, the conduction band minimum (CBM) position of the samples can be also estimated from the following equation: ECB=EVB – Eg (2)

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Figure 12. The VB spectra of (a) pristine Se-rich ZnSe and Se-rich ZnSe/rGO nanocomposites and (b) pristine Se-poor ZnSe and Se-poor ZnSe/rGO nanocomposites. According to the obtained ECB from the above equation, Eg from UV-vis results, and EVB from the VB spectra of the samples, the band diagram of the samples can be drawn schematically which is shown in Fig 13. All VBM positions consider relative to EF (Fermi energy), which is placed at 0 eV. Therefore, the VBM positions have been considered below of the EF and CBM positions above the EF. These diagrams indicate both nanocomposites are type-II heterostructure. In addition, the diagrams clearly show how photogenerated electrons and holes transfer between two parts of the heterostructure. The redox process by the ZnSe/rGO nanocomposites can be written by the following reactions: ZnSe + hv → ZnSe (h+ + e-)

(3)

ZnSe (h+ + e-) + rGO → ZnSe (h+) + rGO (e-)

(4)

rGO (e-) + O2 → rGO + •O-2

(5)

2eˉ + 2H+ + •O-2 → H2O2

(6)

eˉ + H2O2 → OHˉ + •OH

(7)

h+ + H2O → •OH + H+

(8) 19

2eˉ + 2H+ + O2(ads) → H2O2

(9)

eˉ + O2(ads) → •O-2

(10)

•O -

(11)

2

+ H+ → •OOH

•OOH

+ H+ + eˉ→ H2O2

(12)

eˉ+ H2O2 → HO- + •OH

(13)

H2O2 + •O-2 → •OH + OH- + O2

(14)

H2O2 + hv → 2•OH

(15)

MB + •OH → intermediate products → CO2 + H2O

(16)

These reactions show that three factors are the most important factors in the redox process, which are oxygen radicals (•O2), holes (h+), and hydroxyl radicals (•OH).

Figure 13. Schematic of the photocatalytic activity a heterostructure that were generated by (a) pristine Se-rich ZnSe and Se-rich ZnSe/rGO nanocomposites and (b) pristine Se-poor ZnSe and Se-poor ZnSe/rGO nanocomposites. To understand which of these parameters has the most significant role in the redox process by different nanocomposites, the scavengers have been used. 2-propanol (2-ProH), ammonium 20

oxalate (AO), and benzoquinone (BQ) have been used as the scavengers of •OH, h+, and •O2, respectively [21]. The effects of scavengers on the degradation rate constant of MB over the pristine ZnSe and ZnSe/rGO nanocomposites under the light irradiation are shown in Fig 14. The degradation rate constant of MB over the pristine ZnSe and ZnSe nanocomposite in both types of the samples was decreased after the addition of the scavengers in the reaction system compared with the photocatalytic degradation of MB with no scavengers’ conditions. Therefore, these results indicate that all of the three factors have been generated during the photocatalytic process. Although, these factors have been more generated in the ZnSe/rGO nanocomposites in comparison to the pristine ZnSe and as a result, the ZnSe/rGO nanocomposites presented a higher photocatalytic activity in comparison to the pristine ZnSe. However, the contribution of these factors to the degradation of dye pollution by the two types of nanocomposites is not the same. The inhibition level pursues the order of 2-PrOh>AO>BQ for the Se-rich ZnSe nanocomposites and AO>BQ>2-PrOH for the pristine Se-poor ZnSe/rGO nanocomposites. Therefore, it can be concluded that the •OH and h+ are essential in the photocatalytic activity of the Se-rich ZnSe/rGO nanocomposites, while, the most important factor for the photocatalytic activity of the Se-poor ZnSe/rGO nanocomposites has only been the photogenerated h+. Consequently, two factors play in the redox process of the Se-rich ZnSe/rGO nanocomposites, while, only one factor has more role in the photocatalytic activity of the Se-poor ZnSe/rGO nanocomposites. Such different factors that have participated in the redox process cause to enhance the photocatalytic performance of Se-rich ZnSe/rGO nanocomposites in comparison to the Se-poor ZnSe/rGO nanocomposites.

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Figure 14. Scavenger effect results on the photocatalytic rate constants of (a) Se-rich ZnSe/rGO nanocomposites and (b) Se-poor ZnSe nanocomposites. 4. Conclusion The Se-poor and Se-rich conditions on the photocatalytic performance of the ZnSe/rGO nanocomposites have been investigated. The morphology studies of the samples showed that the Se conditions had a significant role in obtaining different morphology. In addition, rGO could affect the morphology of the ZnSe and caused to change Se-rich ZnSe micropolyhedrons to ZnSe NPs and Se-poor ZnS NRs changed to a mixture ZnSe NBs and NPs. The characterization results indicated extra selenium was placed on the surface of the Se-rich samples and played the role of a bridge between the ZnSe NPs and the rGO sheets. The photocatalytic performance of the nanocomposites showed higher photocatalytic activity for the Se-rich samples in comparison to 22

the Se-poor samples. The BET results indicated the textural properties no role in the photocatalytic performance of the ZnSe/rGO nanocomposites. On the other hand, Raman, XPS, and photoresponse results showed that the rGO in the Se-rich ZnSe/rGO nanocomposites had a higher reduction level and resulting in higher conductivity. This higher conductivity was the most important factor to enhance the photocatalytic performance of the Se-rich ZnSe/rGO nanocomposites. In fact, the Se elements placed in the rGO structure and caused to increase reduction level of rGO and the resulting increase in the conductivity of the rGO in the ZnSe/rGO nanocomposites. Finally, the redox process to degrade dye’s molecules by the Se-rich ZnSe/rGO and Se-poor ZnSe nanocomposites were analyzed and it was observed •OH and h+ had the most important role in the photocatalytic activity of the Se-rich ZnSe/rGO, while, only holes had a significant role in the redox process of the photocatalytic activity of the Se-poor ZnSe/rGO nanocomposites. Acknowledgment R. Yousefi gratefully acknowledges Islamic Azad University (I.A.U), Masjed-Soleiman for partial supporting of this research. References [1] M. Samadi, M. Zirak, A. Naseri, E. Khorashadizade, A.Z. Moshfegh, Recent progress on doped ZnO nanostructures for visible-light photocatalysis, Thin Solid Films 605 (2016) 2–19. [2] J. Wang, C. Liu, S. Yang, X. Lin, W. Shi, Fabrication of a ternary heterostructure BiVO4quantum dots/C60/g-C3N4photocatalyst with enhanced photocatalytic activity, Journal of Physics and Chemistry of Solids 136 (2020) 1091642. [3] W. Shi, C. Liu, M. Li, X. Lin, F. Guo, J. Shi, Fabrication of ternary Ag3PO4/Co3(PO4)2/gC3N4 heterostructure with following Type II and Z-Scheme dual pathways for enhanced visible-

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Graphical abstract

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Highlights  Obtaining different morphology of ZnSe nanostructure by changing Se concentration.  GO causes to change micro-sized ZnSe to nano-sized ZnSe.  Se-rich ZnSe/rGO presents higher photocatalytic activity than Se-poor ZnSe/rGO.

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Dear Prof. Ernst van Faassen The experiment of this research work has been carried out by Mr. Bigdeli who is my Ph.D student and all process of the experimental and discussion of the research have been guided by my supervision. Prof. Elahi and Ghoranneviss are the advisories of my student and they helped us to discuss the results. Ramin Yousefi (Ph.D)

Conflict of interest: There are no conflicts of interest to declare.

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