We report for the first time the green synthesis of carbon nanodots (g-CDs) from the peels of T. cucumerina by simple hydrothermal route. An in-situ liquid phase process was adopted for the design of the monoclinic tungsten trioxide (m-WO3)- based nanostructure with g-CDs to obtain a bi-functional g-CDs/m-WO3 system.
Journal of Science: Advanced Materials and Devices (2020) 73e83 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Tungsten oxide modified with carbon nanodots: Integrating adsorptive and photocatalytic functionalities for water remediation Smrithi S Pa, Nagaraju Kottam a, *, Arpitha Vb, Archna Narula b, Anilkumar G N c, Subramanian K R Vd a Department of Chemistry, M S Ramaiah Institute of Technology (Autonomous Institute, affiliated to Visvesvaraya Technological University, Belgaum), Bangalore, 560054, India Department of Chemical Engineering, M S Ramaiah Institute of Technology (Autonomous Institute, affiliated to Visvesvaraya Technological University, Belgaum), Bangalore, 560054, India c Department of Physics, M S Ramaiah Institute of Technology (Autonomous Institute, affiliated to Visvesvaraya Technological University, Belgaum), Bangalore, 560054, India d Department of Mechanical Engineering, M S Ramaiah Institute of Technology (Autonomous Institute, affiliated to Visvesvaraya Technological University, Belgaum), Bangalore, 560054, India b a r t i c l e i n f o a b s t r a c t Article history: Received November 2019 Received in revised form 13 February 2020 Accepted 16 February 2020 Available online 25 February 2020 We report for the first time the green synthesis of carbon nanodots (g-CDs) from the peels of T cucumerina by simple hydrothermal route An in-situ liquid phase process was adopted for the design of the monoclinic tungsten trioxide (m-WO3)- based nanostructure with g-CDs to obtain a bi-functional g-CDs/m-WO3 system Characterization techniques, such as X- Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Field Emission Scanning Electron Microcsopy (FESEM), Energy Dispersion XRay Analysis (EDX), Fourier Transformed Infra-Red (FTIR), Ultra Violet Defuse Reflectance Spectroscopy (UV-DRS) and Photoluminescence Emission Spectroscopy (PLES) were employed for the morphological, dimensional, chemical and optical analysis The efficiency in the adsorption of the heavy metal, namely Cd2ỵ ions of the m-WO3 was found improved in the formated nanocomposite with g-CDs to 81% The detailed adsorption kinetics study through the adsorption isotherms revealed that the reaction mechanism can be described by the pseudo-second-order kinetic model determined by Freundlich isotherms Furthermore, the photocatalytic degradation efficiency of the crystal violet dye under the visible light was found significantly enhanced to 95% for the studied composite The photocatalytic ability of the nanocomposite was found remaining almost intact even after four repeated cycles A study in the presence of different scavengers suggested that the hydroxyl radicals play a crucial role in determining the photocatalytic activity of the nanocomposite An improvement in the structural stability and charge carrier separation kinetics was achieved by carefully growing the m-WO3 nanostructures in g-CDs These resultes have revealed the integrated adsorptive and photocatalytic capabilities of the studied nanocomposites for the waste water treatment © 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Tungsten oxide Carbon dots Green synthesis Cadmium removal Photocatalysis Introduction A highly convenient lifestyle developed by the mankind has imposed immense pollution to Mother Nature The presence of persistent non-biodegradable organics like industrial dyes as well as heavy metal ions in natural water sources as a result of the * Corresponding author E-mail addresses: knrmsr@gmail.com, nagaraju@msrit.edu (N Kottam) Peer review under responsibility of Vietnam National University, Hanoi massive industrial waste water being drained into them is one among the most urgent environmental challenges in the modern era Efforts to eliminate the contaminants from polluted water advance constantly [1,2] Also, municipal solid waste disposal has emerged as another challenging issue Hence kitchen garbage resource utilization opens up a new realm of utmost significance along with emerging green waste water technologies [3,4] Adsorption and solar photocatalysis stand out superior as economically effective water treatment technologies when compared to the conventional water purification techniques like coagulation, flocculation, membrane separation and chemical https://doi.org/10.1016/j.jsamd.2020.02.005 2468-2179/© 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 74 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 oxidation [5] Semiconductor based photocatalysis, which could mineralize wide range of organic pollutants to simple harmless molecules, is a promising solution for environmental remedy issues But meager visible-light response and rapid charge carrier recombinations are the critical drawbacks to be addressed in the semiconductor based photocatalytic applications [6] Modified approaches such as impurity doping, noble metal deposition, photosensitizer incorporation, carbon nanostructure-based modifications, electronic integration with other semiconductor nanocrystals etc were proposed to overcome the aforementioned hurdles [7] Tungsten trioxide (WO3) is an n-type indirect band gap semiconductor with an energy gap in the range of 2.5e2.8 eV with a good photo and thermal stability The versatility in the properties of tungsten trioxide makes it a potential candidate not only in gas sensing, photocatalytic and field emission applications but also in solar energy, electrochromic and photochromic devices [8] But the rapid charge carrier recombination owing to its low band gap and trouble in oxygen reduction due to the low potential of pristine WO3 catastrophically affected its advancement as a competent photocatalyst in practical applications [9] Carbon dots (CDs) emerged as a multifaceted nanomaterial with exceptional physicochemical properties, such as aqueous solubility, photo-induced electron transfer ability and intrinsic biocompatibility This combination of properties in CDs triggered intense research in all possible areas of applications including sensor technology, biomedicine, photocatalysis and optoelectronics [10e12] Abundant availability of raw “green” precursors with facile modes of synthetic approaches makes them promising nanomaterials for the current decade [13] CD can act as an electron harvester and/or converter which owes to its capability to enhance the photocatalytic efficiency of the parent semiconductor, to which it is conjugated by suppressing the charge carrier recombination [14] In the present study, we had synthesized the m-WO3 and mWO3/g-CDs via the ultrasonication assisted acid precipitation reaction route as well as the pure CDs via the hydrothermal route using Trichosanthes cucumerina (snake gourd) peel extract as the carbon source Visa et al reported the use of the WO3-fly ash composite for the photocatalytic degradation of Bemacid Blau (acid dye), Bemacid Rot (reactive dye) and also for the removal of the Cu2ỵ ions from industrial waste water [15] Tie and co-workers reported the use of WO3 nanowires on reduced graphene oxide sheets for the photocatalytic mineralization of MB dye [16] Chemically derived carbon dots combined with WO3 were reported by Yan et al for the degradation of the methylene blue dye [17] Cadmium heavy metal ions and crystal violet (CV) dye, a cationic in nature, were chosen as the model contaminants to evaluate the adsorption and photocatalytic proficiency of the synthesized nanomaterials The degradation of the CV dye was attempted using ZnO nanonails under the UV irradiation with 95% efficiency [18] Bare ZnO synthesized using the hydrothermal method was also utilized and 62% efficiency was reported [19] 100% degradation of the same dye was achieved using TiO2 under UV light [20] Development of adsorbents to remove heavy metal ions were also well explored [15] The possibility of generating a visible active photocatalyst with higher efficiency is the hurdle to overcome with respect to the studies conducted so far on the CV dye degradation The advantages of the present study are that we aimed to generate non-toxic photocatalysts with the heterojunction that are synergistically effective in adsorption and photocatalysis It was frequently reported that the heterojunction photocatalysts show poor adsorption capacity Through our study we are able to restore the considerable adsorbent capacity for the Cd2ỵ ions removal as well as reaching up to 95% visible light driven CV dye degradation efficiency in the newly developed m-WO3/g-CDs material Apart from this, the synthesis of CDs using kitchen garbage strictly adheres to environmental safety and hence is more advantageous Only a very few reports are available on WO3 based photocatalysts with effective adsorption-photocatalysis bifunction [21] To the best of our knowledge, this is the primary study involving green synthesized carbon dot-tungsten trioxide nanocomposites being used both as an adsorbent for the Cd2ỵ ion removal as well as for the visible light driven photocatalytic degradation of crystal violet dyes Experimental 2.1 Materials Snake gourd was purchased from a local vegetable market near Mathikere, Bangalore Sodium tungstate, Na2WO4$2H2O (Sisco Research Laboratories Pvt Ltd., India), Oxalic acid (COOH)2.2H2O (Nice chemicals Pvt Ltd Cochin) and nitric acid (Avantor Performance Materials India Ltd, Thane) were used for the synthesis of mWO3 nanoparticles All other chemicals were of analytic purity grades and used without further purification Double distilled (DD) water was used throughout the synthesis, washings and experiments 2.2 Preparation of carbon dots (g-CDs) The peels of T cucumerina (snake gourd) was washed thoroughly and grinded well to obtain the extract with no addition of chemicals 50 mL of the extract was sealed into a Teflon lined autoclave and heated for 12 h at 180 C The resultant brown colored solution was filtered using an 0.2 mm microporous membrane followed by continuous washing with ethanol and distilled water Further purification was achieved by loading into a dialysis bag (Mw ¼ 3000) for dialysis up to 24 h against double distilled water The powdered sample was finally obtained from vacuum drying at 60 C overnight 2.3 Synthesis of m-WO3 and g-CDs/m-WO3 nanoassembly 1M (2.52 g in 7.3 mL of DI water) of sodium tungstate (Na2WO4$2H2O) and 0.1 M (0.1 g in 7.2 mL of DI water) of oxalic acid (COOH)2$2H2O were prepared The latter was added dropwise to the prepared 1M Na2WO4$2H2O solution which was kept under magnetic agitation The reaction mixture was magnetically stirred for h The pH of the prepared aqueous solution was adjusted by adding HNO3 drop-wise The obtained solution was stirred for 1.5 h and the resultant solution was treated by ultra-sonication for 30 at 60 C and kept at room temperature for 24 h The obtained yellow precipitate was centrifuged and washed with DD water and ethanol Finally the powder was calcined at 500 C for h Na2WO4$2H2O ỵ 2HNO3/WO3ỵ2NaNO3ỵ2H2O (1) For the synthesis of g-CDs/m-WO3, a known amount of CDs dispersed via ultrasonication for 30 in 20 mL DD water was introduced into the reaction medium along with Na2WO4$2H2O and kept for another half an hour magnetic stirring Rest of the procedure followed was similar to the synthesis of the m-WO3 nanoparticles 2.4 Characterization Transmission electron microscopy was performed on g-CDs, mWO3 and the nanocomposite in which the sample was drop cast S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 onto a carbon coated copper grid on an electron microscope (Jeol/ JEM 2100) The surface morphology and size distribution of the nanostructure was observed with a field emission scanning electron microscope (TESCAN-MIRA LMH) The elemental composition was supplemented by energy dispersive X-ray spectrometer (EDS QUANTAX 200 with XFlash BRUKER) The elemental composition and phase formation was confirmed using X-ray diffraction (X'Pert PRO, Panalytical) with CueK a radiation (0.15406 nm at 50 kV/l mA) Fourier transform infrared spectra were recorded from KBr disks (PerkinElmer Spectrum 1000) with a resolution of 1.00 cmÀ1 at an operating range of 350e4000 cmÀ1 UV- visible diffuse reflectance spectroscopy was carried out using a UV-visible spectrophotometer (Specord 210 Plus, Analytic Jena) equipped with variable spectral resolution and cooled double detection Photoluminescence measurements were recorded using a Spectrofluorimeter (HITACHI F 2700) with excitation and emission slit widths kept constant at nm for all readings with a scan rate 1500 nm/min and PMT voltage, 700 V The removal of divalent cadmium ions were studied using the GBC Scientific Equipment Ltd À932 PlusAtomic Absorption Spectrophotometer with an adjustable spectral bandwidth between 0.2 and nm 2.5 Batch adsorption studies All adsorption experiments were carried out at room temperature and neutral pH In a typical experiment, 50 mg of adsorbent and 50 mL aqueous solution containing known amount of Cd2ỵ ions were added to a 250 mL beaker The solution was agitated to allow sufficient reaction time to achieve adsorption equilibrium At particular time intervals aliquots were collected, centrifuged and analyzed using an atomic absorption spectrophotometer (AAS) 2.6 Photocatalytic experiments The photodegradation studies were conducted in an indigenous photocatalytic reactor with a mercury vapor lamp (125 W) as the light source Typically, in an experiment 50 mg of photocatalyst was added to the dye solution (10 ppm) and was magnetically stirred for 30 to achieve adsorptionedesorption equilibrium between the dye and the photocatalyst under dark condition Then the stable aqueous solution was exposed to visible light under constant stirring At regular intervals of time, mL of sample was withdrawn without disturbing the system, centrifuged and the concentration 75 of CV dye was determined by monitoring the absorbance maximum at 592 nm Results and discussion 3.1 Chemical and morphological analysis The characteristic X-ray diffraction peaks of the as-synthesized samples correspond well to the lattice reflection planes (002) (020) (200) (120) (112) (022) (320) (122) (222) (320) (123) (132) (004) (040) (400) (114) (204) and (420) which match perfectly with the monoclinic phase of WO3 (Fig 1(a)) No appearance of any other impurity peak implies the high degree of purity in the synthesized monoclinic phase with space group P21/n (JCPDS 72e0677) To obtain, the detailed structural and microstructural parameters (Table 1) of m-WO3, Rietveld refinement was carried out (Fig 2) using the Full Proof software Though the mathematical procedure followed during the refinement is out of interest in the current study, important parameters like atomic and occupancy position, lattice constant, lattice strain etc can be accurately obtained from the Rietveld refinement of the diffraction patterns The obtained experimental data was fitted with the pseudo-voigt analytical function and Marquardt least square procedure was adopted to minimize the differences between the obtained and the calculated diffraction patterns Yobs-Ycal obtained from the refinement gives the evidence for a high degree of agreement between the experimental and stimulated intensity The main characteristic peaks of g-CDs/m-WO3 are well consistent with those of the parent m-WO3 implying that the introduction of carbon dots has not affected the stability of m-WO3 In fact, the high intensity of PXRD patterns observed for g-CDs/mWO3 sample proved that the effective incorporation of CDs into the WO3 framework enabled the growth of crystallites Two newly identified peaks, one at 19.5 O corresponding to the (002) graphitic facet as well as the broad peak ranging from 21.6 to 22.1O indicating the presence of slightly amorphous carbon dots corroborates the successful incorporation of the g-CDs into the m-WO3 nanoarchitecture [22] The enlarged version of three predominant peaks for both the samples (Fig 1(b)) elucidate that effective doping of carbon dots was achieved as indicated by the shift of the related peaks The presence of the carbon dots also contributes to enhancing the crystallinity of the composite which is reflected as the sharpening of peaks in the observed PXRD patterns The monoclinic phase of the tungsten oxide nanoparticles was reported Fig (a) XRD pattern observed for m-WO3 and g-CDs/m-WO3 (b) Enlarged version exhibiting the peaks corresponding to the carbon dots and the shift observed on combining both species 76 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 Table Structural parameters obtained from Rietveld refinement for m-WO3 Crystal System monoclinic Space Group P21/n (14) Cell Volume (Å3) Lattice Parameters (Å ) a b c b 7.551 7.345 7.709 90.892 to be a good candidate for photocatalysis and was also found to be more stable at room temperature than other phases It is evident from the images that the undoped monoclinic tungsten oxide consists of polydispersed nanoclusters of irregular shapes while the generation of the composite resulted in a monodispersion of nearly spherical uniform nanostructures Energy dispersive X-ray (EDAX) spectral peaks clearly show the incorporation of carbon content into the m-WO3 matrix which resulted in peaks corresponding to elemental carbon, tungsten and oxygen in the nanocomposite material against the appearance of tungsten and oxygen in the bare m-WO3 The purity of the synthesized samples is affirmed by the absence of any impurity peak in the observed spectra For further clarifications with respect to the dispersivity and morphology, HRTEM images were obtained for the g-CDs, the bare tungsten oxide and the g-CDs/m-WO3 photocatalysts TEM images of carbon dots reveal the formation of monodispersed nanoparticles with a size distribution of around nm In the case of mWO3 and g-CDs/m-WO3 samples, the images show clear lattice 423.19 R factors Rp Rwp Rexp c2 RBragg RF 9.74 12.8 7.93 2.61 8.14 5.83 fringes confirming the generation of well crystallized particles The morphology suggests the occurrence of pseudo-spherical particles with slight irregularity and agglomeration in the case of m-WO3 The particle size histogram was generated taking into account, 50 particles which suggest a broad size distribution with an average size range of 20e35 nm (Fig 4(a)) The successful inclusion of the carbon dots established a welldefined spherical morphology to the composite nanomaterial with a reduction in average diameter to 12 ± nm as inferred from the particle size histogram shown in the inset (Fig 4(b)) The spots of the selective area electron diffraction (SAED) pattern spots are clearly visible which are indicative of single crystalline nature of the individual particles From these patterns, the crystal plane spacing d values were calculated and these inferred that the planes are concurrent with monoclinic phase as observed from XRD analysis Vibrational spectral analysis of the green synthesized carbon dots (shown in Fig 5(a)) reveals a strong band at 3400-3100 cm À1 which could be either due to the eOH or the eNH stretching A Fig Rietveld refinement for m-WO3 synthesized using the sonication assisted precipitation method FE-SEM images reveal the formation of the ultra-fine particles of nanodimension (Fig 3(a) and (b)) Fig FE-SEM images of (a) m-WO3 and (b) m-WO3/g-CDs S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 77 Fig HRTEM images of nanoparticles with particle size distribution histogram in the inset (a) m-WO3 (b) g-CDs/m-WO3 40 (a) Transmittance (%) Transmittance (%) 100 g-CDs -CN 90 -C-O-C/N-H stretching 80 -C=O 70 60 -OH/-NH -OH bending 50 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) g-CDs/m-WO3 (b) m-WO3 30 1133 20 1758 10 2349 1620 3000 833 660 1382 2500 2000 1500 1000 500 Wavenumber (cm-1) Fig FTIR spectra of (a) g-CDs and (b) m-WO3 and g-CDs/m-WO3 nanocomposite weak band observed at 2200-2000 cm À1 may be attributed to the presence of nitrile group The strong absorption band at 15501650 cmÀ1 indicates the presence of eNH2 through amide (-C]O) bonds This observation confirms the formation of N-doped carbon dots without any addition of external nitrogen source The existence of eC-O-C is also identified from the narrow band obtained at ~1100 cmÀ1 along with the presence of the absorption band at around 661 cmÀ1 which arises due to the out-of- plane eOH bending These findings indicate that the synthesized carbon dots are self-functionalized with the hydroxyl, carbonyl and amino groups which impart the excellent aqueous solubility and biocompatibility to the system In the FTIR spectra shown in (Fig 5(b)) for the bare m-WO3, the fingerprint region evinced by a strong band around 450-1200 cmÀ1 corresponds to the stretching mode vibration of the WeO bonds In comparison with the spectra of the nanocomposite material, the band is intensified underlining the fact that the dopant explicitly modifies the WO3 framework The strong band around 1620 cmÀ1 can be assigned to the stretching vibrations of the WeOH bonds while the one observed at 833 cmÀ1 can be attributed to the bridging oxygen atoms of the OeWeO bonds [23] The transmission peaks at 1000-1300 cmÀ1 could be due to the bending vibration of the OeH bonds In the case of the composite, the band observed around 1600 cmÀ1 for the amide groups overlaps with the stretching vibrations of the WeOH bonds Apart from these, two more weak bands around 2349 cmÀ1 and 1758 cmÀ1 corresponding either to the alkenyl C]C stretch or eCN stretch and carbonyl stretch (C]O) respectively are clearly interposed in the g-CDs/m-WO3 sample No other impurity peaks were observed in the FTIR spectra The scrutiny of the obtained spectra further confirms the successful inclusion of the C-dots into the pure m-WO3 sample and hence substantiates the composite formation The diffuse reflectance spectra of both the samples exhibit a strong edge absorption in the visible region (see Fig 6(a)) In the case of the carbon dot-tungsten trioxide composite sample, an appreciable red shift in the absorption edge is observed which may be attributed to the synergetic effect of the interaction between the functional moieties in C-dots and m-WO3 Hence the mechanism of evolving a superior visible light-sensitive photocatalyst by the effective induction of C-dots is authenticated Using the KubelkaeMunk relation, from the plot of (F(R)*hn)2 vs hn (Fig 6(b)), the band gap energies for the m-WO3 and g-CDs/m-WO3 samples were found to be 2.74 and 2.63 eV respectively This observation clearly validates that the incorporation of carbon dots into the semiconductor matrix resulted in engineering the band gap slightly towards lower end This may be due to the existence of coupled electrons in the lower energy levels of the conduction band in the form of an electron gas [23] 78 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 Fig (a) DRS spectra of m-WO3 and g-CDs/m-WO3 (b) KubelkaeMunk plot for m-WO3 and g-CDs/m-WO3 samples The optical absorption peak of the carbon dots obtained from snake gourd peel extract was observed at 283 nm with its tail extending to the visible spectrum (see Fig 7) This could be ascribed to the n-p* transition of the C]O and p-p* transitions of the conjugated C]C states The green synthesized carbon dots occur as a brown colored solution under the sunlight and shows green luminescence upon illumination using a UV lamp of 350 nm wavelength (see the inset of Fig (a) and (b), respectively) The photoluminescence (PL) spectra obtained for the as prepared C-dots evinces the emission apex around 458 nm at an excitation wavelength of 380 nm The observed PL behavior for the synthesized samples maintained the signature “excitation wavelength dependent emission” of the carbon dots reported previously [10,13] The emission maxima show a slight inclination towards the red region from 430 to 458 nm upon increasing the excitation wavelength from 320 to 380 nm, respectively This drift was observed even for the excitation wavelengths greater than 380 nm g-CDs 3.2 Adsorption studies The effect of m-WO3 as well as g-CDs/m-WO3 as the adsorbent dosage (50 mg, 100 mg, 150 mg and 200 mg) on the removal of cadmium ions under neutral pH conditions and ambient room temperature 25 ± C for a contact time of 45 was evaluated 320 nm 330 nm 340 nm 350 nm 360 nm 370 nm 380 nm 390 nm 400 nm 5000 Intensity (a.u.) Absorbance (a.u.) but the intensity of the spectra are seen dropped down thereafter (Fig 8) The PL mechanism for the observed trend can be explained on the basis of surface states The intensity of the PL spectra is directly proportional to the number of particles being excited at a particular wavelength In the current study, the surface characteristics of the carbon dots may be accountable to the observed PL phenomenon A group of emissive traps may be formed in between the p and p* levels of CeC owing to the presence of various surface functionalities in carbon dots At a particular emission wavelength, a particular surface energy trap dominates the emission Altering the excitation wavelength leads to the dominancy of corresponding surface energy traps and hence the observed variation 4000 3000 2000 1000 200 300 400 500 600 700 Wavelength (nm) Fig UV-visible absorption spectrum of g-CDs In the inset, images of CD suspension under (a) sunlight and (b) UV light irradiation 300 350 400 450 500 550 600 Wavelength (nm) Fig Photoluminescence spectra of carbon dots at different excitation wavelengths S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 for the cadmium ion solution Different initial concentrations of mgLÀ1, mgLÀ1, mgLÀ1, mgLÀ1 and 10 mgLÀ1 were also used to study the variation in the adsorption ability of the synthesized catalysts At a constant 10 mgL1 of Cd2ỵ ion concentration, studies were carried out for the optimization of catalytic loading using both the catalysts The percentage of adsorption occurring at a particular time was calculated using the formula, % Adsorption¼ (C0-Ct)/C0, adsorbent in the real time samples The adsorbents collected after the batch adsorption study was regenerated by adding M acetic acid The mixture was sonicated for h to ensure the complete release of pollutants The adsorptionedesorption cycle was repeated five times and the obtained results are depicted in the graph It is evident that four repeated cycles of adsorption study unaltered the performance of the catalyst while the fifth cycle showed a slight decrease in efficiency of the adsorptive capability (2) where C0 and Ct are the initial and final concentration of the metal ion solution in mgLÀ1 The results revealed that on increasing the catalytic dosage, the adsorption efficiency was increased up to 150 mg and thereafter showing a decrement This observation could be explained as due to the accumulation/sedimentation of the catalyst in the reactant medium Hence a maximum percentage of adsorption was obtained at an optimum catalytic loading of 150 mg to a 10 ppm cadmium heavy metal ion test solution Also, the propensity of the optimized catalyst dosage was evaluated for different initial Cd2ỵ ion concentration in both the catalysts At an optimum catalytic loading of 150 mg into 50 mL of 10 mgL1 Cd2ỵ solution under neutral conditions, tungsten oxide-carbon dots nanocomposites attained adsorption equilibrium within a contact time of 20 while bare m-WO3 required 35 Fig 9(a) and (b) depict the percentage adsorption against contact time for different cadmium ion concentrations for both m-WO3 and g-CDs/m-WO3, respectively The inability of bare m-WO3 to remove Cd2ỵ from concentrations greater than ppm for even 150 mg catalytic dosage is evident from the graph and the maximum adsorption efficiency attained was found to be 68% The usage of the nanocomposite had resulted in the enhanced adsorption efficiency of 88% The abundant availability of active binding sites owing to the proper inclusion of carbonaceous nanodots into the m-WO3 sorbent molecules resulted in the elevated percentage of Cd2ỵ ion removal in the initial stage Also, the chances of complexation with the surface functional groups like the ones of carbonyl and hydroxyl in carbon dots with Cd2ỵ ions are inevitable as reported by Kahrizi et al This will also support the removal of cadmium ions from the synthetic waste water solution [24] The gradual occupancy of the active binding sites in the later stage ended up in the subsequent reduction in percentage of adsorption and attained adsorption equilibrium The catalytic stability study was carried out for Cd2ỵ metal removal using g-CDs/m-WO3 Considerable retention of adsorption efficiency advocated for an economically viable, application of 3.3 Mathematical modelling and kinetic evaluation of the adsorption phenomena Adsorbate species distribution and mechanism of the observed adsorption phenomena can be interpreted with the aid of mathematical models Taking into consideration the various possible interactions within the adsorbate species, the mode of coverage as well as the heterogeneity/homogeneity of the adsorbent moieties, Langmuir and Freundlich models were studied to simulate the Cd2ỵ adsorption on the g-CDs/m-WO3 adsorbent In this study, nonlinear methods were employed for determining the model parameters along with the standard error values The Langmuir model basically assumes no interaction between the adsorbing molecules The non-linear form of the Langmuir model equation is given as follows, qe ¼ qm KL Ce ỵ KL Ce qe ẳ Kf Ce 1=n Adsorption (%) Adsorption (%) 50 40 ppm ppm ppm ppm 10 ppm 30 20 (4) where Kf and n are equilibrium constants signifying the adsorption capacity and intensity respectively The observed experimental 100 60 (3) where qe is the amount of Cd2ỵ (mg) being adsorbed per unit mass of the adsorbent (g) and Ce, the amount of Cd2ỵ (mg) remaining in the solution per unit volume (L) at the equilibrated solution KL (LmgÀ1) is the Langmuir constant which is related to net enthalpy of the adsorption process and qm (mggÀ1), the maximum adsorption capacity at the isotherm temperature of the system The intercept and slope of the plot with Ce/qe against Ce can be used to calculate qm and KL values The Freundlich model furnishes an expression circumscribing the exponential active sites and surface heterogeneity The nonlinear Freundlich isotherm model can be expressed as follows, (b) (a) 70 79 80 60 ppm ppm ppm ppm 10 ppm 40 20 10 0 10 20 30 Time (min) 40 50 10 15 20 25 30 Time (min) Fig Plots of percentage of adsorption efficiency against time for Cd2ỵ removal using (a) m-WO3 and (b) g- CDs/m-WO3 adsorbents 80 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 3.0 Experimental data Langmuir isotherm model Freundlich isotherm model qe (mg g-1) 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Ce (mgL-1) Fig 10 Non-linear Langmuir and Freundlich isotherm models for the adsorption of Cd2ỵ ions using g-CDs/m-WO3 catalyst the surface of the adsorbent becomes positively charged due to the protonation which would prefer the adsorption of anionic species Above pHPZC of 4.9, the surface of the nanocomposite intends to be negatively charged which can be substantiated by the FTIR spectra with bands corresponding to the WeO bonds as well as to the eOH groups This explains the potential of the adsorbent to facilitate the adsorption of cationic moieties Here in this case, the adsorption studies of Cd2ỵ cation were carried out in neutral conditions at which the surface charge remains negative and hence the probability of electrostatic attraction also plays a predominant role in driving the reaction Surface area is also an important factor in determining the adsorptive capability besides the surface charges In the case of g-CDs/m-WO3 nanocomposite, HRTEM images confirm the formation of uniformly distributed nanoparticles with sizes less than those of bare m-WO3 The reduction in particle size obviously results in the higher surface area providing the exposure of more adsorbent sites for Cd2ỵ ions Hence it can be concluded that the sorption happens in the heterogeneous surface supporting sites with different affinities as put forward by the Freundlich mechanism exploiting the advantages of a negative surface charge combined with a higher surface area 3.4 Kinetics of the adsorption process The examination of the kinetics sorption of Cd2ỵ on to the surface of g-CDs/m-WO3 nanocatalysts was carried out using the pseudo first order, pseudo second order and the intraparticle diffusion models It was clearly observed in the experiment that the rate of adsorption remained proportional to the contact time until it saturated to reach equilibrium in 20 The Lagergren pseudo-first order reaction is expressed as: lnqe qt ị ẳ ln qe K1 t (5) pH initial pH at equilibrium 12 10 pH (final) data was simulated to both these isotherm (Fig 10) and the results are tabulated in Table In the non-linear Langmuir and Freundlich plots, R2 values obtained were highly comparable for the nanocomposite catalyst (0.9977 and 0.9998, respectively) Hence, an error analysis was carried out using the normalized deviation (ND) and standard normalized deviation (NSD) and it was found that ND and NSD values were 9.61 and 12.177, respectively for the Langmuir adsorption isotherm model The maximum adsorption capacity and KL constant were found to be 88.45 mg gÀ1 and 1.23 Â 10À2 (LmgÀ1) On the other hand, for the Freundlich model, ND and NSD values were 2.56 and 2.92, respectively which suggest the best fit model for this process, Kf and 1/n values determined out of the plot were 0.98 mggÀ1 and 1.07, respectively which agree well with the surface heterogeneity This observation aids in explaining the mechanism that the amount of Cd2ỵ being adsorbed in the system increases infinitely with the increasing sorbate concentration and is restricted from attaining saturation [25] Also, the presence of carbon dots increased the contact chances between tungsten oxide and Cd2ỵ ions by increasing the surface area due to the reduced and well defined size and morphology CDs may also play an important role in reducing the agglomeration of the m-WO3 nanoparticles and the surface hydroxyl and carbonyl moieties may form complexes with the Cd2ỵ ions [24] which in turn could enhance the removal efficiency Though the mechanism preferring the Freundlich model seems unsatisfactory for the high coverage conditions/situations, but it is permissible at mid-range adsorbate concentrations In general, the Freundlich sorption process is expected to proceed through multiple layers against the monolayer assumption by the Langmuir model At this juncture, the study regarding the surface properties of g-CDs/m-WO3 becomes prominent which also supplements evidences to predict the mechanism of adsorption The pH drift method experiment was conducted for g-CDs/m-WO3 for a range of pH varying from to 12 as reported in [26] and the value of pHPZC was found to be 4.9 (Fig 11) This implies that below this pH, 2 10 12 pH (initial) Fig 11 Point of zero charge measurement for g-CDs/m-WO3 by pH drift method Table Langmuir and Freundlich isotherm parameters obtained for Cd2ỵ removal using g-CDs/m-WO3 catalysts Langmuir isotherm parameters Qm 88.46 KL 0.0123 Freundlich isotherm parameters R 0.9977 ND 9.6107 NSD 12.1767 Kf 0.98 1/n 1.07 R2 0.9998 ND 2.5618 NSD 2.9212 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 where qt is the amount of adsorbate at contact time, t (mggÀ1), qe is the adsorption capacity (mggÀ1) and k1 is the pseudo-first order rate constant The pseudo-second order equation is expressed as: t t ẳ ỵ qt K2 qe qe (6) where k2 is the pseudo-second order rate constant The intraparticle diffusion model was also evaluated using the expression: qt ẳ Kp t 1=2 ỵ q (7) where Kp, the intraparticle diffusion rate constant (mggÀ1minÀ1/2) which can be obtained from the plot of qt vs t1/2.The best fit values obtained for the pseudo-first order and the pseudo-second order model examination is consolidated in Table Since the kinetic data are well fitted to the pseudo esecond order model, the adsorption phenomena can be assumed to occur via the chemisorption involving either ionic or covalent bonding interaction among the catalyst and the metal ion The intraparticle diffusion model reveals a non-linear plot (R2 ¼ 0.7158) with its axis not passing through the origin which clearly rules out the possibility of particle diffusion assisted sorption in the rate determining step 3.5 Photocatalytic degradation studies for the removal of crystal violet dye In order to evaluate the visible-light driven photocatalytic ability of the as synthesized samples, the crystal violet dye was chosen as the target water pollutant It is a triarylmethane-based Table Consolidated model parameters and correlation coefficient Model gÀCDs/mÀWO Parameters qe (mgg ) k1 (minÀ1) R2 qe (mggÀ1) k2 (minÀ1) R2 Pseudo-First order Pseudo- Second order 1.0 0.708 0.2405 0.944 1.591 2.027 0.9997 (a) basic/cationic dye used not only in textile industries but also in inkjet printers, dying of paper, colorizing fertilizers, antifreezes, leather etc Before subjecting the samples to the visible light illumination, a dark field adsorption experiment was carried out for 30 and monitored by recording the decrease of the absorption intensity at wavelength 592 nm for 105 For the 10 mgLÀ1 concentration of the crystal violet dye, studies were carried out for catalytic dosages of 50 mg, 100 mg and 150 mg to find out the optimum dosage needed for the efficient dye degradation It was observed that in both photocatalysts, the percentage of photocatalytic dye degradation was found to increase when the catalytic loading increases from 50 to 100 mg and then decreased for loadings at 150 mg This could be due to the agglomeration of the photocatalysts resulting in the hindered absorption of the visible light required for the reaction to proceed Also the appreciable degradation rate of 95% was achieved involving the g-CDs/m-WO3 photocatalyst against 90% efficiency obtained for bare m-WO3 (Fig 12 (a)) The experimental results highlighted the role of carbon dots in augmenting the photocatalytic performance of WO3 Radical trapping experiments were carried out to study the influence of active species like hỵ,OH and O.2 in the dye degradation process 0.1 mmolLÀ1 of benzoquinone and mmolLÀ1 of ammonium oxalate and isopropyl alcohol were used as radical scavengers in this study Experimental results indicated no change in the photocatalytic activity on the addition of benzoquinone and ammonium oxalate which omits the signicant or predominant participation of hỵ and O.2 , respectively Upon adding isopropyl alcohol, we had observed a higher decrease in degradation rate which affirms the prominent role of OH., which is a powerful oxidizing agent for the organic dye degradation (Fig 12(b)) This observation could lead us to explain the mechanism of reaction as follows: Under the visible light irradiation, the g-CDs/m-WO3 photocatalyst gets photo-excited to generate charge carriers which emanate through the transfer of electrons from the valence band of WO3 to its conduction band These electrons could drift to the surface of the carbon dots unrestrained through the photo-induced charge transfer process and hence help in the restriction of the electronehole recombination It is very well established that the carbon dots can serve both as electron donors and electron acceptors [24] This will prevent the traditional charge separation model and will induce Z-scheme mechanism of the efficient m-WO 1.0 g-CDs/m-WO 0.8 0.8 0.6 0.6 C/C C/C À1 0.4 0.2 0.0 0.0 20 40 60 80 Time (min) 100 120 (b) No Scavengers AO IPA BQ 0.4 0.2 81 20 40 60 80 100 120 Time (min) Fig 12 (a) Variation of C/C0 with respect to time in the degradation of crystal violet (b) Effect of different scavengers on the photocatalytic dye degradation efficiency of g-CDs/mWO3 82 S.P Smrithi et al / Journal of Science: Advanced Materials and Devices (2020) 73e83 charge-carrier separation This ends up with an increased redox potential of the photogenerated charge carriers which in turn results in the elevated redox ability of the composite photocatalysts Concurrently, hỵ leaves the valence band to migrate towards the surface of WO3 where it reacts with the superficially adsorbed water molecules to form OH In the meantime, electrons on the surface of the carbon dots can react with O2 to produce O.2 which in this case might have instantly produced hydroxyl radicals as evidenced by the active species trapping experiment Precisely we could comprehend that the generation of hydroxyl radicals may be assisted in the presence of WO3 either through the reductive path by electrons on the surface of carbon dots or by the direct oxidation of OHÀ by the holes in the valence band produced during the charge separation, which are indicated by the following reactions þ À 2WO3þsunlight/WO3 (h ) þ WO3 (e ) À WO3 (e ) ỵ O2 /WO3ỵ O.2 (8) The authors declare that there is no conflict of interest Acknowledgements (10) HO.2ỵ Hỵ/H2O2 (11) H2O2ỵ e/OH (12) ỵ OH AND/OR WO3 (hỵ)ỵ OHĂỵ Hỵ/WO3ỵ OH Declaration of Competing Interest (9) þ WO3 (eÀ)þ O.2 þ H /WO3þ HO2 visible light irradiation which remained unaltered for multiple cycles The active species trapping experiment indicated the role of hydroxyl radicals as the main oxidative moiety in the degradation study The exalted adsorption and photocatalytic degradation in gCDs/m-WO3 owes to the negative surface charge, reduced particle size and effective charge separation offered by the photo-induced charge transfer properties of the supportive carbon dots This study shows the possibility of developing rationally-designed environmentally benign carbon dots based nanoheterostructures that could result in the successful integration of multiple functionalities in the same system to address environmental remediation Authors wish to acknowledge the management of M.S Ramaiah Institute of Technology, Bangalore for the constant support and encouragement through RIT seed funding No: MSRIT/Admin/2019/ 111 Project code: 2019/RIT/R&D/IF/001 References ỵ Hỵ (13) To examine the stability and the regeneration potential of the gCDs/m-WO3 nanocomposite, we had carried out multiple cycles of the dye degradation studies under the same experimental conditions After every study, the catalyst was recovered by centrifuging and washing repeatedly with distilled water followed by drying Up to four consequent cycles, we observed negligible variation in the degradation rate Anyhow, feeble decrease was observed in the degradation efficiency during the fifth cycle of study This study implies the low degree of byproduct accumulation on the surface of the photocatalysts and confirms their excellent stability The successful reusability of g-CDs/m-WO3 is also affirmed by these set of studies which is vital as far as a promising, industrially-relevant nanophotocatalyst is concerned Conclusion Green synthesis of carbon dots utilizing kitchen garbage such as snake gourd (T cucumerina) peel via the hydrothermal treatment was effectually carried out and confirmed by TEM, UV-visible absorption spectra, PL and FTIR studies XRD and Rietveld refinement confirmed the purity of the crystalline monoclinic phase of tungsten oxide with the P21/n space group in the case of pure m-WO3 The inclusion of carbon dots into the composite sample was confirmed using various analytical techniques The DRS measurements revealed a slight decrease in band energy for g-CDs/m-WO3 which could result in the extended photoresponse with respect to bare m-WO3 The adsorption studies revealed a fast and efficient adsorption of the heavy metal cadmium ion using the nanocomposite adsorbent compared to classical m-WO3 Cadmium uptake efficiency of 81% was achieved from 10 mgLÀ1 of the Cd2ỵ ion solution using 150 mg adsorbent in 20 which remained stable up to four repeated cycles The observed data fitted well with the Freundlich multilayer adsorption isotherm with its kinetic parameters determined by the pseudo-second order kinetic model The photocatalytic degradation experiments of the CV dye using the same nanocomposite yielded a degradation rate of 95% 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For further clarifications with respect to the dispersivity and morphology, HRTEM images were obtained for the g-CDs, the bare tungsten oxide and the g-CDs/m-WO3 photocatalysts TEM images of carbon. .. water [15] Tie and co-workers reported the use of WO3 nanowires on reduced graphene oxide sheets for the photocatalytic mineralization of MB dye [16] Chemically derived carbon dots combined with. .. 30 at 60 C and kept at room temperature for 24 h The obtained yellow precipitate was centrifuged and washed with DD water and ethanol Finally the powder was calcined at 500 C for h Na2WO4$2H2O