Highly Active Agro-Waste-Extracted Cellulose-Supported CuInS2 Nanocomposite for Visible-Light-Induced Photocatalysis.

Agro-waste-extracted cellulose-supported CuInS2 nanocomposites were hydrothermally synthesized with significant photocatalytic activity under the influence of cellulose as a polymeric natural support that offers delay in electron-hole life. Delayed recombination process of electrons and holes was perceived by parting of cellulose as a barrier or edge during photochemical reaction, which overall enhances the lifetime of photocatalyst. The photodegradation efficiency over five consecutive cycles along with scavenging studies have been examined for RhB dye under visible light. The boosted photodegradation rate was observed at an optimum amount of cellulose (200 mg), which is ∼10 times higher than pristine CuInS2.

Introduction

Photocatalysis is one of the most effective and rapidly growing areas to overcome the issues related to energy and environment. Attractive efficiency and low cost make photocatalysts potential candidates for various applications like air purification, hydrogen production, antibacterial activities, and degradation of organic pollutants/dyes. Photocatalytic performance of a semiconductor photocatalyst highly depends on its charge separation efficiency and light absorption ability. Therefore, more electron–hole pairs will be generated if narrow-band-gap semiconductors working under visible light are used. In addition, the morphology of semiconductor photocatalyst plays an important role in affecting the charge carrier efficiency. Thus, it is a current need to develop an environment-friendly visible light photocatalyst that can degrade the toxic organic dyes.[1] While being specific about dyes, basic RhB dye exists in cationic as well as zwitterionic forms. Cationic dyes are highly toxic and known to cause skin irritations, allergic dermatitis, cancer, and other mutations. It falls in the group of xanthene dyes, which means it possesses aromatic compounds and xanthene rings, which are a major source known to pollute the environment. Intermediates used in synthesizing dyes, pesticides, polymers, etc. mainly comprise xanthene rings, which pollute groundwater, and they need to be removed to a maximum extent. Between photocatalytic and photosensitization processes, the photocatalytic method has been widely accepted for the degradation of RhB.[2]

I-III-VI2 ternary semiconductor compounds are topic of research since years due to their wide range of applications in solar cells, photocatalytic splitting of water, phosphors, light-emitting devices, pigments, and nonlinear optical devices.[3,4] Among these chalcopyrites, direct-band-gap (1.53 eV) semiconductor CuInS2 with high absorption coefficient (105 cm–1) shows excellent stability under solar radiation and easy conversion of charge carriers. CuInS2 exists in three different crystalline phases—wurtzite, chalcopyrite, and zinc blende, among which only the chalcopyrite phase with tetragonal structure is stable at room temperature.[5] The band gap of CuInS2 can be tuned further for wide visible light range by controlling the size and shape into nano-dimension. CIS QDs have been used as sensitizers in solar cells due to their enhanced efficiency of charge transfer.[6] Enhancement in electrochemical performance is visible when CIS is applied in lithium-ion batteries.[7] CIS, ZnO/CIS core–shell nanoarrays, and graphene/graphene oxide/CIS nanofilms have been used for the electrochemical water splitting.[8−10] Further, CuInS2 thin films had been widely used as absorber layers to utilize visible light in photovoltaic cell, but very few reports are available on CuInS2 as a visible-light-responding photocatalyst for degradation of organic dyes.[11,12]

Photocatalytic activity of bare CuInS2 is poor due to fast charge carrier recombination. To improve the photocatalytic performance, various efforts have been made in the past, such as nanostructure synthesis with morphology control, elemental doping, and facet engineering,[13] which aid in light harvesting and charge movement.[14] Xei et al. synthesized Pt–CuInS2 hierarchal microarchitectures as improved visible-light photocatalyst for H2 production from water.[15] However, high cost of Pt limits its widespread applications. Moreover, out of various morphologies, hollow structures possess better photocatalytic activity due to the large fraction of empty space and high surface area. Metal sulfides are believed to be the most promising photocatalysts as they possess narrow band gaps along with valence bands at negative potential in comparison to oxides. Thus, it results in an enhanced visible light photocatalytic activity. Using only CdS which also posses narrow band gap does not result in desired catalytic performance as it induces photocorrosion.[16] Most of the methods used for the synthesis of hollow spheres till date are either template-based or include Ostwald ripening[17] and the Kirkendall effect.[18] In addition, organic–inorganic hybrid synthesis is also found to be an effective way to improve physicochemical properties. Thus, to further enhance the photocatalytic activity of CuInS2 hollow structures, a cost-effective, easy, and renewable organic material as a support is needed. Hence, supporting naturally isolated polymer as base would enhance its photocatalytic efficiency.

Recently, various efforts have been made for replacement of petrochemical-based organic materials by those derived from renewable resources.[19] To meet the challenges of sustainable development, the discarded biomass from the agro-based industries is utilized to its full extent as a source of extraction for green route approach. This discarded waste contains pectin, hemicelluloses, and cellulose and these are widely applicable in food,[20] pharmaceutical,[21] cosmetics,[22] and polymer industries.[23,24] Cellulose is one of the ubiquitous materials in nature that includes plants (cotton, wood, soy, sugarcane) and natural materials like bacteria and tunicates as their major sources. Polysaccharides play a crucial role in designing functional food, unique biomaterials, and carriers for bioactive substances as they possess versatile physicochemical properties along with edibility and biocompatibility. It is one such biodegradable and abundant organic polymer whose composites have received much attention due to its low density, chirality, hydrophilicity, biodegradability, nontoxic nature, and low cost.[25,26] It has a long straight-chain polymer packed with inter- and intramolecular H- bonding.[27,28] For fabrication of various composites, it is used as a reinforcing material due to its high mechanical strength.[29,30] Here, in this manuscript, agro-waste is used to extract cellulose, which is used in different weight ratios (in situ and ex situ) to develop cellulose-supported CuInS2 nanocomposites (Cel/CIS) via hydrothermal strategy. The hydrothermal route used here is surfactant- and template-free. The novel photocatalysts were examined for photodegradation of RhB dye under visible light by decreasing the charge recombination trend.

Results and Discussion

X-ray diffraction (XRD) patterns of pure cellulose (Cel), bare CuInS2 (Cu0), and ex situ and in situ cellulose-supported CuInS2 nanocomposites (Cu ex and Cu1-Cu5) are shown in Figure a. All of the diffraction peaks of bare CIS were indexed as tetragonal phase of CuInS2 (JCPDS file no.750106). CIS showed peaks corresponding to planes (112), (200), (220), (312), and (224). The intensity of cellulose diffraction peak in the (200) plane gradually enhanced in Cu1, Cu2, and Cu5, which confirms the formation of Cel/CIS nanocomposites with no other impurities. Figure b shows scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) elemental mapping of Cu2 photocatalyst and a clear evidence of the existence of C, O, Cu, In, and S elements in Cel/CIS nanocomposite.

Figure 1

(a) X-ray diffraction patterns of Cel, Cu ex, and Cu0 to Cu5 nanocomposites. (b) SEM- EDX, elemental mapping of Cu2 photocatalyst.

As the fruit bran already contains hemicellulose, cellulose, pectin, and lignin, the FTIR spectra shown in Figure S1 of the Supporting Information file shows the characteristic peak of cellulose at 3417 cm–1 owing to its O–H stretch. The wide band indicates the presence of strong intermolecular H-bonding in Cel. The shoulder peak at 2853 cm–1 is assigned to various types of symmetric and asymmetric vibrations of the CH2 group. The peaks in the range of 1638–1150 cm–1 are attributed to angular deformation vibrations of C–O–H. The peaks present between 1150 and 915 cm–1 correspond to vibration and elongation of −O–H linkages. Glucosidic units are evident in the range of 950–617 cm–1.[31] The field emission scanning electron microscopy (FESEM) image of cellulose (Cel) showed a rodlike morphology with an average aspect ratio of 10 (Figure a). The FESEM image of CIS shows hierarchal hollow structures made up of nanoflakes with the thickness of ∼50–55 nm, as apparent in Figure b. In Figure c, Cu2 nanocomposite shows sphere-like hierarchal structures closely knotted with irregular or rod-shaped cellulose. The corresponding EDX images indicate the formation of isolated cellulose (Cel), pure CuInS2 (Cu0), and its composite Cel/CIS (Cu2) (Figure d–f).

Figure 2

(a–c) FESEM images of Cel, Cu0, and Cu2 photocatalysts. (d–f) EDX analysis of corresponding photocatalysts.

To further investigate the surface morphology and structure, the transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns for Cel (a–c), Cu0 (d–f), and Cu2 (g–i) are shown in Figure . The TEM images of Cel show a sheetor rodlike morphology, whereas the SAED patterns clearly show the crystalline nature of cellulose. The interplanar distance (d) of Cel is found to be 0.35 nm with the corresponding plane (200). The hierarchal hollow spheres of Cu0 are made up from nanoflakes with size range of 35–40 nm that is clearly seen in the FESEM image, too. The d-spacing value is noted as 0.33 nm in Cu-0, which indicates the corresponding plane (112). Similarly to the FESEM image, Cu2 shows spherical particles on the cellulose surface, but in the TEM image, the size of particles being in micron range are relatively larger to identify at a high-magnification. The SAED pattern of the composite shows diffused rings that indicates the polycrystalline nature of the sample. The d-spacing values for Cu2 are found to be 0.37 nm and 0.34 nm corresponding to planes (200) of cellulose and (112) of CIS. All of the calculations for size and d-spacing were done by setting scale through Image J software.

Figure 3

HRTEM images and SAED patterns of (a–c) Cel, (d–f) Cu0, and (g–i) Cu2 photocatalysts.

The surface chemistry and oxidation state of different atomic species present in sample Cu2 were investigated by recording their X-ray photoelectron spectroscopy (XPS) images in the binding energy range of 0–1100 eV. Figure a presents the survey scan of sample Cu2, and it reflects the peaks related to Cu, In, S, C, and O only, which confirms the phase purity of the synthesized sample. The high-resolution spectra of each element, i.e., Cu 2p, In 3d, S 2p, C 1s, and O 1s, respectively, are shown in Figure b–f. Figure b displays the XPS image of S 2p core level and it inferred one broad peak with a shoulder, which deconvoluted into two peaks, one at 161.3 eV (2p3/2) and other at 162.3 eV (2p1/2) by fitting with an energy difference of 1 eV and another peak at 169 eV. The peaks at 161.3 and 162.3 are assigned to sulfur coordinated to Cu and In.[32] The peak observed at 169 eV indicates coupling between the CIS nanostructures and cellulose through −C-SOx–Cu/In bonding. The XPS image of C 1s (Figure c) represents two peaks located at 284.7 and 286.1 eV assiged to C–H and C–O interaction in the cellulose.[33] However, compared to XPS data of pure cellulose reported earlier, higher intensity of C–H peak in the present work ensures the cellulose–CIS interaction and simultaneous carbonization of cellulose with CIS. In Figure d, two peaks are visible, one at binding energy 445.1 eV and other at 452.7 eV with peak splitting of 7.6 eV were assigned to In 3d5/2 and In 3d3/2 respectively, indicating oxidation state In3+ which match well with the previous reports of CuInS2. One single peak in the O 1s spectrum (Figure e) at 532 eV is associated with cellulose only and is consistent with the reported data.[34]Figure f represents the XPS image of Cu 2p core level and reveals two peaks at 931.9 eV (2p3/2) and 952 eV (2p1/2) with a spin–orbit separation of 20.1 eV, which confirms that the copper is present in the Cu+ state.[35] The absence of satellite peak at around 945 eV and the presence of LMNN peak in survey scan also supports this finding.[35] Furthermore, a careful investigation indicates the asymmetric nature of the Cu 2p3/2 peak, which was deconvoluted into two peaks by fitting. This asymmetric behavior may be due to the interaction of cellulose with CIS nanostructures or due to the presence of Cu2+ to a very small extent.

Figure 4

XPS images of Cu2 photocatalyst: (a) survey, (b) S 2p, (c) C 1s, (d) In 3d, (e) O 1s, and (f) Cu 2p.

Brunauer–Emmett–Teller (BET) surface areas of Cel, Cu0, and Cu2 were found to be 48, 263, and 139 m2/g, respectively (Table S1). Here, the BET surface area of pure cellulose (Cel) is minimum as expected due to the crystalline nature of the material. Cu0 shows hollow hierarchical-type morphology with small cavities due to arrangements of petals in the form of a sphere. Due to these cavities and porelike structure, the material shows maximum surface area (263 m2/g). In composite (Cu2), the surface area of catalyst improved compared to pure cellulose due to the interaction of cellulose support with CIS hollow structures. Figure shows Barrett, Joyner, and Halenda (BJH) pore size distribution of the Cel, Cu0, and Cu2 photocatalysts.

Figure 5

BJH pore size distribution of Cel, Cu0, and Cu2 photocatalysts.

Figure a,b shows kinetic linear simulation plots for Cel, Cu ex, and Cu0 to Cu5 nanocomposites. The photocatalytic activity of all of the photocatalysts has been examined under the influence of (Tungsten) W light source, as shown in Figure a. The photocatalytic activity of pure cellulose and commercial cellulose was checked which showed only 10 and 20% degradation in 35 min, while composites expressed photodegradation faster compared to bare CIS (Cu0) and Cel. Cu1 photocatalyst degraded up to 70% in 25 min, Cu2 up to 80%, and Cu5 degraded up to the level of 75% in the same time. However, CIS shows photodegradation 70% in 25 min, which was equal to Cu1, as very less amount of cellulose (100 mg) is present. These experiments conclude that overall composites (Cel/CIS) showed good photocatalytic activity compared to bare CIS and Cel. Here, the optimal amount of cellulose was also checked where the maximum degradation was achieved. The photodegradation efficiency was observed for all photocatalysts using the following equation. Photodegradation efficiency = (Cn – C0)/Cn × 100. The photodegradation rates of all of the photocatalysts have also been calculated. The photodegradation rate was calculated using the ln C0/Cn versus t plot and fitting in linear first-order rate kinetics. The degradation rates and half-lives of all of the photocatalysts are shown in Table . Cu0 expressed a rate of 0.046 min–1, while the Cu2 photocatalyst, which showed the highest photodegradation, showed a rate of 0.056 min–1. Thus, it is concluded that the composite showed its maximum activity with the chalcopyrite semiconductor in existence of cellulose support at a weight ratio of 200 mg and achieved rate higher in comparison to Cu0.

Figure 6

(a, b) Kinetic linear simulation plots of Cel, Cuex, and Cu0 to Cu5 nanocomposites for photodegradation of RhB dye under visible light. (c, d) Scavenger study and recyclability tests for Cu2 photocatalyst under visible light for 25 min.

Table 1

Degradation Rates and Half-Lives of Cel, Cu ex, and Cu0 to Cu5 Photocatalysts

Sample code	rate/min	Half-life	% efficiency
Cel	0.0014	693	5
Cu0	0.047	14.81	70
Cu1	0.040	17.32	70
Cu2	0.056	12.42	80
Cu5	0.052	13.58	75
Cu ex	0.039	17.76	67

Figure c,d shows scavenging and recyclability tests of Cu2 photocatalyst. In the presence of EDTA, it showed photodegradation of only 49%, 47% for p-benzoquinone, and 30% for isopropanol, which means that the photocatalyst shows less photodegradation even after inhibiting the process. Recyclability was done for the Cu2 photocatalyst up to five cycles. Negligible change was observed in photodegradation efficiencies, and even after five cycles, it showed 71% degradation efficiency. Figure a shows time-resolved photoluminescence spectra of Cel, Cu0, and Cu2 photocatalysts. Cel showed an average lifetime of 1.5 ns. The average lifetime of bare Cu0 was found to be 265 ns, and Cu2 shows 200 ns, which is very close to that of bare Cu0. Time-resolved data fitting is shown in Table S2. Figure b depicts a schematic representation of the proposed photocatalyst mechanism of Cel/CIS nanocomposites for degradation of RhB dye in visible light source (W) of 200 W. The possible mechanism of photodegradation of dye can be explained as shown in the schematic diagram, which explains that when visible light source is switched on, the electron–hole pairs of chalcopyrite semiconductor CIS get separated. Excited electrons reach conduction band (CB) from valence band (VB), making hole in VB. Photogenerated electrons reduce O2 molecules, which are adsorbed on surface of catalyst, leading to the formation of O2–• radicals. Charge transfer may take place between Cel and CIS. The active holes react with H2O to generate •OH. This •OH radical degrades organic dyes in the presence of visible light irradiation. Cellulose influenced the photodegradation process due to delayed charge recombination period by being a support. Cu0 also has active adsorption sites due to its hollow and porous nature. More photogenerated electrons and holes are developed while using Cu2 photocatalyst because of creation of edge or interface between two materials (cellulose and CuInS2).

Figure 7

(a) Time-resolved photoluminescence spectra for Cel, Cu0, and Cu2 photocatalysts. (b) Schematic representation of the mechanism of dye degradation process.

Table shows the literature from recent years, where CIS is used with various other semiconductors for different applications. It has been used with g-C3N4 and other inorganic materials. From this, we realize that polymer derived from waste has not been used yet, which is cost-effective and eco-friendly. Table depicts sources used and time taken to degrade RhB dye from the literature.

Table 2

CIS-Based Nanocomposites for Photocatalysis and Other Applications

CIS composites	Applications	Reference
CIS/ZnS/AgInS2	H2 evolution	(39)
CIS/CdS	reduction of Cr (VI)	(40)
CIS/TiO2/SnO2	air decontamination	(41)
ZnO/CIS	photocatalytic studies on crystal violet under visible light	(42)
CIS/g-C3N4	photocatalytic study on degradation of tetracycline	(43)
ZnS/CIS	photocatalytic degradation of RhB	(44)
CIS/ TiO2	H2 evolution in solar light	(45)
bio-based cellulosic CIS	optoelectronic applications	(39)
ZnO nanorods/r-GO/CIS quantum dots	photocatalytic degradation of RhB under 300 W xenon lamp	(46)
ZnO/CIS	photocatalytic degradation of methyl orange and 4-chlorophenol	(47)
MoS2 nanosheet-modified CIS	H2 production under visible light	(11)

Table 3

Degradation of RhB Dye by Various Photocatalysts within Different Time Intervals and Irradiation Sources

catalyst used	source	time (min)	reference
Bi2WO6/TiO2	sunlight	120	(48)
H2O2	UV light	25	(49)
Mn3O4/CeO2	300 W Xe	200	(50)
graphene/Au	visible light	250	(51)
graphene/SnO2	350 W Xe	200	(52)
Ag3PO4/TiO2	solar light	120	(53)
ZnO	UV illumination	70	(54)
fluorinated Bi2WO6	500 W Xe	360	(55)
Bi5O7Br	300 W Xe	120	(56)
Ag/AgBr/BiOBr	300 W halogen lamp	20	(57)
Bi2Mo6	500 W Hg lamp	400	(58)
Cel/CIS	200 W tungsten bulb	25	our work

Here are a few reports where RhB is degraded using different semiconductor catalysts. Mostly, the source used is UV and visible light, but even if the visible light is being used, the time achieved to degrade RhB is quite longer than what we acheived in Cel/CuInS2. Moreover, the starting material is developed from waste peels, which are discarded mostly. Hence, our developed material is cost and time effective.

Conclusions

Here, we reported novel Cel/CIS photocatalyst synthesized hydrothermally with controlled morphology. The unique point in our work is isolation of cellulose from the waste and its use for environmental remediation by developing visible-light-induced photocatalyst. These Cel/CIS have superior visible-light-driven photocatalytic activity as electrostatic interaction is witnessed between cellulose and CIS which aids in delaying the charge carrier. Thus new, low-cost, and green route would influence the production of catalyst on a large scale and can lay the foundation of hybrid photocatalytic mechanism for other potential applications.

Materials

Indium chloride and sodium chlorite were purchased from Sigma-Aldrich, India. Copper sulfate and sulfuric acid were bought from HiMedia. Thiourea, N,N-dimethyl formamide, and potassium hydroxide were purchased from SRL, India.

Experimental Section

Isolation of Cellulose

Waste fruit rinds of specific fruits in equal quantity (banana, orange, sweet lime, pomegranate) were washed, ground, sieved by standard 200 mesh ASTM standard to prepare bran. The ground peels were then boiled for 10 min to remove sugars, phenolic compounds, and water-soluble polysaccharides. This bran (50 g) was subjected to alkali hydrolysis with 5% KOH for 16 h. This was followed by bleaching treatment with 2% sodium chlorite solution maintaining pH 5 for 1 h at 70 °C to decolorize the solution. The residue was further treated with 2% sulfuric acid for 1 h at 80 °C. These treatments were repeated twice or thrice as desired. In each step, the residue was neutralized and centrifuged for the next step. The steps followed were slightly modified from those given in refs (36, 37).

Synthesis of CIS Hollow Spheres

CIS hollow spheres were synthesized via facile hydrothermal approach using 5 mmol InCl3 and CuSO4 were dissolved in 40 mL of N,N-dimethyl formamide (DMF) to form a solution by dynamic stirring. Thiourea (0.1 g) was added to it and the resulting homogeneous solution was transferred into a Teflon-lined stainless steel autoclave maintained at 160 °C for 25 h.[38] The black precipitates were centrifuged and dried at 60 °C and used for further characterization.

Synthesis of Cel/CIS Nanocomposite

Synthesis of cellulose-supported copper indium sulfide (Cel/CIS) was carried out by adding different weight ratios (100, 200, 500 mg) of cellulose in CIS solution via in situ wet chemical approach. A similar procedure was repeated, and the solution was kept in a hydrothermal autoclave at 160 °C for 25 h. For ex situ composite, 25 mg of bare CIS and isolated cellulose (1:1 weight ratio) were taken in beaker, stirred, and sonicated for 30 min. The products were termed as Cu-0 for pure CuInS2, cellulose-supported copper indium sulfide with different weight ratios (100, 200, 500 mg) as Cu-1, Cu-2, Cu-5, and Cu-ex for ex situ synthesized nanocomposites.

Material Characterization

The phase purity of all of the photocatalysts was determined by powder X-ray diffraction (PXRD) using Bruker D8 equipped with Cu Kα monochromatized incident radiation of wavelength 0.1540 nm. Fourier transform infrared (FTIR) spectra were recorded by PerkinElmer Sp65. The band gaps of all photocatalysts were determined using Jasco 670 DRS. Field emission scanning electron microscopy (FESEM) images were taken using JEOL JSM 6390LV along with their energy-dispersive X-ray (EDX) spectra. Surface area measurements were carried out by a Quanta Chrome Nova Win station for Brunauer–Emmett–Teller (BET) and BJH analyses. High-resolution transmission electron microscopy (HRTEM) images were collected using Jeol/JEM 2100. Lifetime measurements of photocatalyst were done by Jobin Vyon Fluorocube with excitation wavelength of 390 nm. Omicron ESCA, Germany, was used for obtaining XPS images. Aluminum anode was used for samples with an approximate energy of 1486.7 eV. The angle between source and analyzer was 85°. Monochromatic X-ray was used, which has its resolution confirmed by FWHM of 0.6 eV.

Photocatalytic Activity

Photocatalytic experiments for the developed novel Cel/CIS composites were performed using 10 mg of photocatalyst in 25 mL of RhB dye solution (1 μM) under visible light irradiation (Tungsten, 200 W). The dye solution along with the photocatalyst was absorbed in the dark for 15 min and later kept under a light source until it showed maximum degradation. The kinetic linear simulation plots were carried out from the absorption data. Out of all of these photocatalysts, the best sample was chosen, which gave the highest degradation efficiency, and further characterization was done for that composite only. The recyclability of the photocatalysts was analyzed by repeating the similar experiments up to five cycles. Different scavengers such as EDTA, benzoquinone, and isopropanol in 1 mM concentrations were used to understand the mechanism of photogenerated electrons and holes during the entire process.