Tungsten oxide modified with carbon nanodots: Integrating adsorptive and photocatalytic functionalities for water remediation

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.

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Original Article

adsorptive and photocatalytic functionalities for water remediation

Smrithi S Pa, Nagaraju Kottama,*, Arpitha Vb, Archna Narulab, Anilkumar G Nc,

a Department of Chemistry, M S Ramaiah Institute of Technology (Autonomous Institute, afﬁliated to Visvesvaraya Technological University, Belgaum),

Bangalore, 560054, India

b 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

a r t i c l e i n f o

Article history:

Received 5 November 2019

Received in revised form

13 February 2020

Accepted 16 February 2020

Available online 25 February 2020

Keywords:

Tungsten oxide

Carbon dots

Green synthesis

Cadmium removal

Photocatalysis

a b s t r a c t

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 X-Ray 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

Cd2þions of the m-WO3was found improved in the formated nanocomposite with g-CDs to 81% The detailed adsorption kinetics study through the adsorption isotherms revealed that the reaction mech-anism 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-WO3nanostructures in g-CDs These resultes have revealed the integrated adsorptive and photocatalytic capabilities of the studied nano-composites 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/)

1 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

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

* Corresponding author.

E-mail addresses: knrmsr@gmail.com , nagaraju@msrit.edu (N Kottam).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2020.02.005

Journal of Science: Advanced Materials and Devices 5 (2020) 73e83

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oxidation [5] Semiconductor based photocatalysis, which could

mineralize wide range of organic pollutants to simple harmless

molecules, is a promising solution for environmental remedy

is-sues 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

modi-ﬁcations, electronic integration with other semiconductor

nano-crystals etc were proposed to overcome the aforementioned

hurdles [7

Tungsten trioxide (WO3) is an n-type indirect band gap

semi-conductor 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 andﬁeld 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

optoelec-tronics [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

elec-tron 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-WO3and

m-WO3/g-CDs via the ultrasonication assisted acid precipitation

re-action 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

Cu2þ 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 WO3were 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

syner-gistically 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 Cd2þions 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 ad-heres to environmental safety and hence is more advantageous Only a very few reports are available on WO3based 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 Cd2þion removal as well as for the visible light driven photocatalytic degradation of crystal violet dyes

2 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 Perfor-mance Materials India Ltd, Thane) were used for the synthesis of

m-WO3 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 thor-oughly 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 wasfiltered using an 0.2mm microporous mem-brane followed by continuous washing with ethanol and distilled water Further purification was achieved by loading into a dialysis bag (Mw¼ 3000) for dialysis up to 24 h against double distilled water The powdered sample wasfinally obtained from vacuum drying at 60C overnight

2.3 Synthesis of m-WO3and g-CDs/m-WO3nanoassembly 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 mag-netic agitation The reaction mixture was magmag-netically stirred for

1 h The pH of the prepared aqueous solution was adjusted by adding HNO3drop-wise The obtained solution was stirred for 1.5 h and the resultant solution was treated by ultra-sonication for

30 min at 60C 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 500C for 3 h

Na2WO4$2H2Oþ 2HNO3 /WO3þ2NaNO3þ2H2O (1) For the synthesis of g-CDs/m-WO3, a known amount of CDs dispersed via ultrasonication for 30 min 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,

m-WO and the nanocomposite in which the sample was drop cast

S.P Smrithi et al / Journal of Science: Advanced Materials and Devices 5 (2020) 73e83 74

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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 aﬁeld emission scanning

elec-tron microscope (TESCAN-MIRA 3 LMH) The elemental

composi-tion was supplemented by energy dispersive X-ray spectrometer

(EDS QUANTAX 200 with XFlash BRUKER) The elemental

compo-sition 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 cm1at an operating range of 350e4000 cm1 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

Photo-luminescence measurements were recorded using a Spectro

ﬂuo-rimeter (HITACHI F 2700) with excitation and emission slit widths

kept constant at 5 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 932

Plus-Atomic Absorption Spectrophotometer with an adjustable

spec-tral bandwidth between 0.2 and 2 nm

2.5 Batch adsorption studies

All adsorption experiments were carried out at room

tempera-ture and neutral pH In a typical experiment, 50 mg of adsorbent

and 50 mL aqueous solution containing known amount of Cd2þions

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 min 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

stir-ring At regular intervals of time, 2 mL of sample was withdrawn

without disturbing the system, centrifuged and the concentration

of CV dye was determined by monitoring the absorbance maximum

at 592 nm

3 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 parame-ters (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 ob-tained 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-Ycalobtained from the re fine-ment gives the evidence for a high degree of agreefine-ment 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/m-WO3sample proved that the effective incorporation of CDs into the

WO3 framework enabled the growth of crystallites Two newly identiﬁed peaks, one at 19.5Ocorresponding to the (002) graphitic facet as well as the broad peak ranging from 21.6 to 22.1Oindicating the presence of slightly amorphous carbon dots corroborates the successful incorporation of the g-CDs into the m-WO3 nano-architecture [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 1 (a) XRD pattern observed for m-WO 3 and g-CDs/m-WO 3 (b) Enlarged version exhibiting the peaks corresponding to the carbon dots and the shift observed on combining

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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

mono-dispersion of nearly spherical uniform nanostructures Energy

dispersive X-ray (EDAX) spectral peaks clearly show the

incorpora-tion of carbon content into the m-WO3matrix 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-WO3photocatalysts TEM images

of carbon dots reveal the formation of monodispersed

nano-particles with a size distribution of around 5 nm In the case of

m-WO3 and g-CDs/m-WO3 samples, the images show clear lattice

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

well-deﬁned spherical morphology to the composite nanomaterial with a reduction in average diameter to 12± 2 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 inFig 5(a)) reveals a strong band at 3400-3100 cm1 which could be either due to theeOH or the eNH stretching A

Table 1

Structural parameters obtained from Rietveld reﬁnement for m-WO 3

Crystal System Space Group Lattice Parameters (Å) Cell Volume (Å 3 ) R factors

Fig 2 Rietveld reﬁnement for m-WO 3 synthesized using the sonication assisted precipitation method FE-SEM images reveal the formation of the ultra-ﬁne particles of nano-dimension ( Fig 3 (a) and (b)).

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weak band observed at 2200-2000 cm1may be attributed to the

presence of nitrile group The strong absorption band at

1550-1650 cm1indicates the presence ofeNH2through amide (-C]O)

bonds This observation conﬁrms the formation of N-doped carbon

dots without any addition of external nitrogen source The

exis-tence ofeC-O-C is also identiﬁed from the narrow band obtained at

~1100 cm1along with the presence of the absorption band at

around 661 cm1 which arises due to the out-of- plane eOH

bending Theseﬁndings 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, theﬁngerprint region evinced by a

strong band around 450-1200 cm1corresponds 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 WO3framework

The strong band around 1620 cm1 can be assigned to the

stretching vibrations of the WeOH bonds while the one observed at

833 cm1can be attributed to the bridging oxygen atoms of the

OeWeO bonds [23] The transmission peaks at 1000-1300 cm1

could be due to the bending vibration of the OeH bonds In the case

of the composite, the band observed around 1600 cm1 for the

amide groups overlaps with the stretching vibrations of the WeOH

bonds Apart from these, two more weak bands around 2349 cm1 and 1758 cm1corresponding either to the alkenyl C]C stretch or eCN stretch and carbonyl stretch (C]O) respectively are clearly interposed in the g-CDs/m-WO3sample 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 (seeFig 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)2vs hn(Fig 6(b)), the band gap energies for the m-WO3and g-CDs/m-WO3samples 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]

Fig 4 HRTEM images of nanoparticles with particle size distribution histogram in the inset (a) m-WO 3 (b) g-CDs/m-WO 3

4000 3500 3000 2500 2000 1500 1000 500 50

60 70 80 90 100

Wavenumber (cm-1)

g-CDs

-OH/-NH

-CN

- C=O

- C-O-C/N-H stretching

- OH bending

(a)

3000 2500 2000 1500 1000 500 0

10 20 30 40

Wavenumber (cm - 1)

g-CDs/m-WO3

m-WO3

1758

1620 833 1382

2349

660

1133

(b)

Fig 5 FTIR spectra of (a) g-CDs and (b) m-WO 3 and g-CDs/m-WO 3 nanocomposite.

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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 (seeFig 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 ofFig 7 (a) and (b), respectively)

The photoluminescence (PL) spectra obtained for the as

pre-pared 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

wave-length 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

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 character-istics of the carbon dots may be accountable to the observed PL phenomenon A group of emissive traps may be formed in between thepandp* 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

3.2 Adsorption studies The effect of m-WO3as well as g-CDs/m-WO3as 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± 2C for a contact time of 45 min was evaluated

0

2

Wavelength (nm)

Fig 7 UV-visible absorption spectrum of g-CDs In the inset, images of CD suspension

1000 2000 3000 4000 5000

Wavelength (nm)

320 nm

330 nm

340 nm

350 nm

360 nm

370 nm

380 nm

390 nm

400 nm

Fig 6 (a) DRS spectra of m-WO 3 and g-CDs/m-WO 3 (b) KubelkaeMunk plot for m-WO 3 and g-CDs/m-WO 3 samples.

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for the cadmium ion solution Different initial concentrations of

2 mgL1, 4 mgL1, 6 mgL1, 8 mgL1and 10 mgL1were also used to

study the variation in the adsorption ability of the synthesized

catalysts At a constant 10 mgL1of Cd2þion 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,

where C0and Ctare the initial andﬁnal concentration of the metal

ion solution in mgL1 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 Cd2þion concentration in both the catalysts At an

optimum catalytic loading of 150 mg into 50 mL of 10 mgL1Cd2þ

solution under neutral conditions, tungsten oxide-carbon dots

nanocomposites attained adsorption equilibrium within a contact

time of 20 min while bare m-WO3required 35 min.Fig 9(a) and (b)

depict the percentage adsorption against contact time for different

cadmium ion concentrations for both m-WO3and g-CDs/m-WO3,

respectively The inability of bare m-WO3 to remove Cd2þ from

concentrations greater than 6 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

abun-dant availability of active binding sites owing to the proper

inclu-sion of carbonaceous nanodots into the m-WO3sorbent molecules

resulted in the elevated percentage of Cd2þ 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 Cd2þions 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

reduc-tion in percentage of adsorpreduc-tion and attained adsorpreduc-tion

equilib-rium The catalytic stability study was carried out for Cd2þmetal

removal using g-CDs/m-WO3 Considerable retention of adsorption

efﬁciency advocated for an economically viable, application of

adsorbent in the real time samples The adsorbents collected after the batch adsorption study was regenerated by adding 1 M acetic acid The mixture was sonicated for 1 h to ensure the complete release of pollutants The adsorptionedesorption cycle was repeatedfive 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

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 mathe-matical models Taking into consideration the various possible in-teractions 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 Cd2þ adsorption on the g-CDs/m-WO3 adsorbent In this study, non-linear methods were employed for determining the model pa-rameters 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,

qe¼ qmKLCe

where qeis the amount of Cd2þ(mg) being adsorbed per unit mass

of the adsorbent (g) and Ce, the amount of Cd2þ(mg) remaining in the solution per unit volume (L) at the equilibrated solution KL (Lmg1) is the Langmuir constant which is related to net enthalpy

of the adsorption process and qm(mgg1), the maximum adsorp-tion capacity at the isotherm temperature of the system The intercept and slope of the plot with Ce/qeagainst Cecan be used to calculate qmand KLvalues

The Freundlich model furnishes an expression circumscribing the exponential active sites and surface heterogeneity The non-linear Freundlich isotherm model can be expressed as follows,

where Kfand n are equilibrium constants signifying the adsorption capacity and intensity respectively The observed experimental

0 10 20 30 40 50 60 70

Time (min)

2 ppm

4 ppm

6 ppm

8 ppm

10 ppm

(a)

0 20 40 60 80 100

2 ppm

4 ppm

6 ppm

8 ppm

10 ppm

Time (min)

(b)

Fig 9 Plots of percentage of adsorption efﬁciency against time for Cd 2þ

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data was simulated to both these isotherm (Fig 10) and the results

are tabulated inTable 2 In the non-linear Langmuir and Freundlich

plots, R2values obtained were highly comparable for the

nano-composite 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 g1 and 1.23 102

(Lmg1) On the other hand, for the Freundlich model, ND and NSD

values were 2.56 and 2.92, respectively which suggest the bestﬁt

model for this process, Kfand 1/n values determined out of the plot

were 0.98 mgg1and 1.07, respectively which agree well with the

surface heterogeneity This observation aids in explaining the

mechanism that the amount of Cd2þbeing 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 Cd2þions 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-WO3nanoparticles and

the surface hydroxyl and carbonyl moieties may form complexes

with the Cd2þions [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

sup-plements evidences to predict the mechanism of adsorption The

pH drift method experiment was conducted for g-CDs/m-WO3for a

range of pH varying from 2 to 12 as reported in [26] and the value of

pHPZCwas found to be 4.9 (Fig 11) This implies that below this pH,

the surface of the adsorbent becomes positively charged due to the protonation which would prefer the adsorption of anionic species Above pHPZCof 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 Cd2þ cation were carried out in neutral conditions at which the surface charge remains negative and hence the proba-bility 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 Cd2þions 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 Cd2þon to the sur-face 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 min

The Lagergren pseudo-ﬁrst order reaction is expressed as:

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Experimental data Langmuir isotherm model Freundlich isotherm model

q e

-1 )

C e (mgL -1 )

Fig 10 Non-linear Langmuir and Freundlich isotherm models for the adsorption of

Cd2þions using g-CDs/m-WO 3 catalyst.

Table 2

Langmuir and Freundlich isotherm parameters obtained for Cd2þremoval using g-CDs/m-WO 3 catalysts.

2 4 6 8 10

12 pH initial

pH at equilibrium

pH (initial)

Fig 11 Point of zero charge measurement for g-CDs/m-WO 3 by pH drift method S.P Smrithi et al / Journal of Science: Advanced Materials and Devices 5 (2020) 73e83

80

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where qtis the amount of adsorbate at contact time, t (mgg1), qeis

the adsorption capacity (mgg1) and k1is the pseudo-ﬁrst order

rate constant

The pseudo-second order equation is expressed as:

t

K2qe2þ t

where k2 is the pseudo-second order rate constant The

intra-particle diffusion model was also evaluated using the expression:

where Kp, the intraparticle diffusion rate constant (mgg1min1/2)

which can be obtained from the plot of qtvs t1/2.The bestﬁt values

obtained for the pseudo-ﬁrst order and the pseudo-second order

model examination is consolidated inTable 3 Since the kinetic data

are wellﬁtted 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 (R2¼ 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

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 illu-mination, a darkﬁeld adsorption experiment was carried out for

30 min and monitored by recording the decrease of the absorption intensity at wavelength 592 nm for 105 min For the 10 mgL1 concentration of the crystal violet dye, studies were carried out for catalytic dosages of 50 mg, 100 mg and 150 mg tofind out the optimum dosage needed for the efficient dye degradation It was observed that in both photocatalysts, the percentage of photo-catalytic dye degradation was found to increase when the photo-catalytic loading increases from 50 to 100 mg and then decreased for load-ings 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 in-fluence of active species like hþ,OH.and O2.-in the dye degradation process 0.1 mmolL1of benzoquinone and 1 mmolL1of ammo-nium 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 signiﬁcant or predominant participation of hþ and O2.-, 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 photo-catalyst 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 ac-ceptors [24] This will prevent the traditional charge separation model and will induce Z-scheme mechanism of the efﬁcient

Table 3

Consolidated model parameters and correlation coefﬁcient.

0 20 40 60 80 100 120 0.0

0.2 0.4 0.6 0.8

g-CDs/m-WO 3

C 0

Time (min)

(a)

0 20 40 60 80 100 120 0.0

0.2 0.4 0.6 0.8

AO IPA BQ

C 0

Time (min)

(b)

Fig 12 (a) Variation of C/C 0 with respect to time in the degradation of crystal violet (b) Effect of different scavengers on the photocatalytic dye degradation efﬁciency of

g-CDs/m-S.P Smrithi et al / Journal of Science: Advanced Materials and Devices 5 (2020) 73e83 81

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charge-carrier separation This ends up with an increased redox

potential of the photogenerated charge carriers which in turn

re-sults in the elevated redox ability of the composite photocatalysts

Concurrently, hþleaves 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 O2to produce O2.-which in

this case might have instantly produced hydroxyl radicals as

evi-denced by the active species trapping experiment Precisely we

could comprehend that the generation of hydroxyl radicals may be

assisted in the presence of WO3either through the reductive path

by electrons on the surface of carbon dots or by the direct oxidation

of OHby the holes in the valence band produced during the charge

separation, which are indicated by the following reactions

2WO3þsunlight/WO3(hþ)þ WO3(e) (8)

WO3(e)þ O2.-þ Hþ/WO3þ HO2. (10)

AND/OR

WO3(hþ)þ OH¡þ Hþ/WO3þ OH þ H þ (13)

To examine the stability and the regeneration potential of the

g-CDs/m-WO3nanocomposite, we had carried out multiple cycles of

the dye degradation studies under the same experimental

condi-tions 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

suc-cessful reusability of g-CDs/m-WO3is also afﬁrmed by these set of

studies which is vital as far as a promising, industrially-relevant

nanophotocatalyst is concerned

4 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

ab-sorption spectra, PL and FTIR studies XRD and Rietveld reﬁnement

conﬁrmed the purity of the crystalline monoclinic phase of

tung-sten 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

measure-ments 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

nano-composite adsorbent compared to classical m-WO3 Cadmium

up-take efﬁciency of 81% was achieved from 10 mgL1of the Cd2þion

solution using 150 mg adsorbent in 20 min which remained stable

up to four repeated cycles The observed dataﬁtted well with the

Freundlich multilayer adsorption isotherm with its kinetic

param-eters 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% in 2 h of

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 g-CDs/m-WO3owes 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 func-tionalities in the same system to address environmental remediation

Declaration of Competing Interest The authors declare that there is no conﬂict of interest Acknowledgements

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

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