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Facile synthesis and enhanced photocatalytic activity of single crystalline nanohybrids for

removal of organic pollutants

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Facile synthesis and enhanced photocatalytic activity of single crystalline

nanohybrids for removal of organic pollutants

Erum Pervaiz a,b

, Honghong Liu a, Minghui Yang

a*

a Dalian Institute of Chemical Physics (DICP), Dalian National Laboratory for Clean Energy

(DNL), 457 Zhongshan road, Dalian, 116023, China.

b School of chemical & Materials Engineering (SCME), Department of Chemical Engineering,

National University of Sciences & Technology (NUST), Sector H-12 Islamabad, 44000, Pakistan.

*M. Y Email: [email protected]

Tel/Fax: +86-411-84379600

Page 1 of 33 AUTHOR SUBMITTED MANUSCRIPT - NANO-112133.R2

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Abstract

Present study focused the synthesis of α-MoO3/rGO. One dimensional nanohybrids under mild

conditions and a low temperature wet chemical route produced highly pure single crystalline

orthorhombic α-MoO3 on graphene oxide (GO) sheets. Four nanohybrids as GMO-0, GMO-1,

GMO-2, and GMO-3 has been synthesized with a different mass, charging of GO as 0 mg, 40 mg,

60 mg and 100 mg respectively. Photocatalytic performance for organic pollutant reduction has

been analyzed. Presence of different amounts of GO in the prepared metal oxide hybrids alters

performance of the material as elaborated by Brunauer–Emmett–Teller (BET) surface area, UV-

Visible diffused reflectance spectra and resulted in reduction of organic dyes depicted by

photocatalytic experiments. GO as a support material and as an active co-catalyst has decreased

the band gap of α-MoO3 (2.82 eV) to lower values (2.51 eV), rendering the prepared hybrids to

be used for visible light induced photocatalysis. Large specific surface area (72m2/g) of the

mesoporous α-MoO3/rGO nanohybrid found efficient photocatalyst for elimination of azo dyes.

Highly fast reduction (100%) of Rhodamine (RhB) was observed in few min while congored

(CR) degraded (76%) in 10 min leading to formation of stable intermediates that completely

neutralized in 12-14 h under light irradiation. Amount of GO loading in the samples was limited

to a point for achieving better results. After that, raising GO amount would decrease the

degradation extent due to presence of a higher electron acceptor. Photocatalytic experiments

revealed the synergistic effect, high selectivity of the prepared nano hybrids and degradation

towards azo dyes. Kinetics of the degradation reaction has been studied and followed pseudo

first order reaction.

Keywords: 1-D nanostructures, Graphene oxide nanohybrids, Water treatment, Photocatalysis

Page 2 of 33AUTHOR SUBMITTED MANUSCRIPT - NANO-112133.R2

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1. Introduction

Rapid industrialization, population and technology have a major influence on an environmental

balance via water contamination. Every industrial process requires chemical treatments that

include various metallic & nonmetallic precursors, metal complexes, toxic anions and organic

molecules. For example, textile industry utilizes vast amounts of water containing all of the

above mentioned chemicals and polluting water streams with toxic, carcinogenic and hazardous

for health effluents. Therefore, this waste water must be processed before disposal. Numerous

techniques currently in use for the waste water treatment, most are inefficient, cost or/and energy

intensive and result in a large sludge generation [1, 2]. Past few decades have introduced waste

water treatment by photocatalysis, using advanced oxidation process (AOP). This is an ongoing

process that utilizes light induced catalytic effect for degradation of toxic chemicals in water

resulting in CO2, H2O and non-toxic compounds. This technique has attracted lots of attention

from scientists due to a degradation of organic compounds from water using metal oxides and

their composites as heterogeneous photocatalyst, metal/metal oxides, non-metal/metal oxide

hybrids under visible light [3-5]. Photocatalysis, being low cost, moderately efficient and

environmentally friendly process has opened a new road for the removal of lethal organic

pollutants by manipulating and modifying the composition, morphology and structures of

nanomaterials [6]. Many researchers have modified these metal oxides to overcome the

shortcomings of a typical semiconductors photocatalyst but still the process lacks either

efficiency (%PDE) or results in non-sustainability [7-9]. Graphene oxide (GO) is a two

dimensional nano material with availability of massive areas and a large surface sites for the

attachment of metals and metal oxides, thus, enhancing the photocatalytic degradation process. It


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is considered that GO sheets have oxygen containing groups as hydroxyl, carboxylic and ether

bridges between the carbon atoms on the basal planes [10-12].

In the past few years a lot of work has been done on the GO/metal oxides nanocomposite for

photocatalytic reduction of organic pollutants, especially azo dyes[13, 14]. It is suggested that

GO and derivatives like rGO, N-doped GO can perform well for enhanced separation of photo

induced charge species and play characteristic role in a band gap thinning [15-17]. Zhang et al.

have reported GO/CdS nanostructures for enhanced anaerobic reduction of nitro compound

(benzyl alcohol). They incorporated metal ions as interfacial mediators to increase photoactivity

[18]. They explored a positive effect of GO on retarding the electron hole pair recombination

causing a shielding effect due to GO. Yanhui Zhang et al. reported GO/ZnS visible light

photoactive catalyst for aerobic selective oxidation of alcohols and epoxidation of alkenes at

ambient conditions and observed GO as a photosensitizer for ZnS other than the electron

reservoir [19]. Quanjun Xiang et al. reported GO/g-C3N4 composites as enhanced visible light

photocatalyst [20]. Zhigang Mou et al. studied TiO2 nanoparticles functionalized N�doped

graphene for photocatalytic hydrogen generation and observed superior interfacial contact and

enhanced charge separation due to GO [21]. Md. Selim Arif Sher Shah et al. have reported

AgBr/N-Doped and amine-functionalized reduced GO as an efficient photocatalysts [22]. Wen

Qian et al. report N�doped TiO2�hybridized graphene as a highly efficient photocatalyst for

water treatment [23]. Sanjaya D. Perera et al. have studied the GO/TiO2 nanotubes and found an

enhanced photocatalytic effect attributed to GO [24]. Ge Ma. et al. have prepared the

GO/ZnFe2O4/polyaniline composite and reported increased photocatalytic and antibacterial

properties of composite [25]. Sancan Han et al. have prepared GO/Fe2O3 hexagonal nanoplates

and found an enhanced photocatalytic activity for the degradation of RhB [26]. Ganesh


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Gollavelli et al. have studied the effect of smart magnetic GO for heavy metal removal and

disinfection control [27]. Yesheng Li et al. have reported GO/WO3 hierarchal nanostructures and

found the increased activity due to presence of GO [28]. Yan-Wen Wang et al. have reported the

ZnO/rGO nanocomposites and studied the antibacterial property of the material [29]. GO with

metal oxide/metal nanocomposites have been used to enhance the physicochemical properties

not only for water treatment but for other applications as well [30-32].

Molybdenum trioxide (MoO3) is an n-type semiconductor and belongs to a class of materials

with good photochromic and electrochromic properties applicable for diverse fields including

energy storage, gas sensing devices, catalytic application and electrochemical devices [33-36].

Morphology of the nanomaterials can amend the physicochemical properties hence can be

effectively utilized for various fields. In three crystalline phases (α, β & h) of MoO3, α-MoO3

perceived as a more stable and recyclable at different operating conditions [37]. There are

different synthesis routes reported in the literature that results in different morphologies of α-

MoO3 ranging from hollow spheres to 1-D nanostructures. Most of these synthesis routes use a

hydrothermal/solvothermal technique to achieve the required morphology [38, 39]. Nano belts of

α-MoO3 are considered most promising, owing to the single-crystalline nature that is believed to

possess special characteristics over polycrystalline nature of moly oxide [40].

Different techniques have been adopted for the synthesis of MoO3-graphene 1-D nanostructures

but the process still needs a high temperature and long times. Therefore challenge is persistent

for the fast, economic synthesis of single crystalline, well-ordered 1-D nanostructure of MoO3-

graphene composites.

Herein we report a facile ultrasonic coupled reflux condensation method for synthesizing high

quality MoO3/rGO nanohybrids at 85oC, much lower than via hydrothermal method. The


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proposed method will have a serious impact on a large scale synthesis of a transition metal oxide

GO hybrid important in various industrial applications. Further, we have studied the

photocatalytic activity of the prepared nanohybrids on reduction of model azo dyes, congored

and RhB. Kinetics of the photodegradation along with reaction mechanism has been carefully

examined. Limited reports are available on the photocatalytic activity of α-MoO3/rGO

nanostructures.

2. Experimental

2.1 Synthesis of GMO hybrids

All the materials used were of analytical grade and deionized water (DI) has been used

throughout the synthesis. Graphite flakes, Sulphuric acid (H2SO4)(97%), Sodium nitrate

(NaNO3), Potassium permanganate (KMnO4), Ammonium paramolybdate

((NH4)6Mo7O24•4H2O), Polyvinylpyrrolidone (PVP, K30, MW-40,000), Oxalic acid (C2H2O4),

Isopropanol (C3H8O), Methanol (CH3OH), Congored and Rhodamine-B were purchased from

Sigma-Aldrich, China. For the typical synthesis of GO reported, modified Hummers method [41]

was used with an extensive centrifugation at 18000 rpm for 3 h. For the synthesis of the

MoO3/rGO nanocomposites, GO 100 mg was treated with ultrasound in 100 ml D.I. H2O

containing 0.5 ml of 0.1M HCl. After 60 min of ultrasonic treatment 175mg of PVP and 2 mmol

of ammonium heptamolybdate were added under continued ultra-sonication for another 60 min.

After this, certain amount of oxalic acid had been added and the solution was transferred to a 250

ml round bottom flask equipped with a reflux condenser. The solution was then subjected to a

thermal treatment at 85oC for 3 h under reflux, subsequently cooled to room temperature, filtered

in vacuum and washed several times with ethanol and water. The obtained gray powder after

drying at 70 oC was used for further characterization. Different composites with varying wt (mg) %


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GO (0, 40, 60,100) were prepared and mentioned as GMO-0, GMO-1, GMO-2, GMO-3. GMO-0

represent the pure MoO3 prepared by the same technique in the absence of GO. In order to see

that GO was reduced during the above mentioned synthesis route, GO without a molybdenum

precursor was treated using the above mentioned procedure.

2.2 Characterization techniques

The crystalline structures of the samples were characterized by X-ray diffraction (XRD) using a

Miniflex 600 X-ray diffractometer with monochromatic Cu Kα radiation (λ=0.1542 nm,

accelerating voltage 40 kV, applied current 15 mA) at a scanning rate of 1°/min. The

morphology and compositions were performed using a field emission scanning electron

microscopy (FESEM) instrument (JSM-7800F, Japan). Transmission electron microscopy (TEM)

characterization was performed on a JEM-2100 electron microscope operating at 200 kV. UV-

Vis Diffuse Reflectance Spectra (UV-Vis DRS) were recorded with a Hitachi U-3900

spectrometer in the range of 200–850 nm using a BaSO4 standard as the reference. XPS

measurements were carried out on an X-ray photoelectron spectrometer (ESCALAB250Xi)

using Al Kα (1486.6 eV) X-rays as the excitation source. C 1s (284.6 eV) was chosen as the

reference. Raman spectra (532 nm excitation) were recorded on Bruker Optics Senterra. Surface

area measurements were performed by a nitrogen adsorption using Brunauer-Emmet-Teller

(BET) area method on Accelerated Surface Area and Porosimetry System (ASAP 2420), to

obtain the value of specific surface area, pore volume and mean pore size.

2.3 Photocatalytic analysis

The photocatalytic measurements were carried out in an open double jacket glass thermostatic

photo reactor. Before light irradiation, a suspension containing 50 mg L-1

congored solution (100

mL) and the solid catalyst (0.01g) was kept in the dark for 30 min to ensure adsorption-


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desorption equilibrium. The suspension was irradiated under continuous stirring using a visible

light of 300 W Microsolar 300 UV - Xe lamp with an UV-cutoff filter (420 nm) and was

positioned 10 cm away from the reactor (removing the effect of light on photocatalysis). All the

experiments were performed at 25 °C; constant stirring rate was maintained throughout. At the

regular time intervals of irradiation, 2 mL aliquots were removed, centrifuged and subjected to

the UV analysis for a measurement of a remnant concentration of the dye at a characteristic

absorption peak near 498 nm (λmax).

3. Results and discussion

3.1 Structural and surface area analysis by XRD & BET

In order to comprehend the crystalline structure and crystallite sizes of the synthesized samples

PXRD patterns were recorded as shown in Fig.1 (a). All of the patterns are well fitted for pure α-

MoO3 (orthorhombic phase) crystal structure without presence of any undesired phase. All of the

peaks can be indexed according to a standard Pdf card (JCPDS 05-0508) space group Pbnm(62)

with characteristic peaks (020), (040) and (060) at 2-theta positions of 12.5o, 26

o and 39.3

o

respectively. Fig. S1 (a-b) shows the XRD patterns for GO and rGO. The diffraction peak (002)

at 2-Theta 10.7o attributed to GO, shows that the graphite is well oxidized with lattice spacing of

0.87 nm, but due to a high intensity of α-MoO3 diffraction peaks in GMO composites, the

diffraction peak for GO is not visible at 10.7o. Disappearance of (002) peak at 10.7

o can also be

ascribed for the reduction of GO to rGO (2θ=26o) during ultrasonic synthesis in the presence of

oxalic acid (Fig. S1 (b)). It can also be observed from the patterns that all of the samples show

much higher peak intensities, indicating well crystalline structure, whereas GMO-3 has relatively

weak peak intensities as compared to other nano hybrids owing to the less crystalline phase.

With the low peak intensity, shift to a higher, 2-theta angles have also been observed (Fig. 1 (b)),


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thus reports a larger crystallite size with a wide distribution as can be seen in Table 1. This

increase in crystalline size with a relatively low peak intensities can be attributed to the increased

amount of GO in the samples which possibly inhibits the defined crystal growth in the preferred

growth direction. Using the d(hkl) spacing, full width at half maximum (FWHM) β values from

indexed XRD patterns, crystallite sizes D(hkl), unit cell volume (V) and lattice parameters (a,b &

c) were calculated and presented in Table 1.

Lattice parameters were calculated using following relation;

��

��

��

��

Fig. 1: PXRD patterns of (a) GMO nanohybrids (b) peak shift in 2θ (25o-28

o)

Table 1: Crystallite sizes, Lattice parameters, Unit cell volume, BET surface area, Pore size and

Pore volume for GMO nanohybrids


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Sample Crystallite size D(040) nm

Lattice paramet

er (Ǻ)

Unit cell volume

(Ǻ3)

BET surface

area (m2/g)

BJH pore size

(nm)

Total pore

volume (cm3/g)

GMO-0 33.29 (2) a=3.966 b=13.89 c=3.72

204.92 29.89 8.78 0.291

GMO-1 30.78 (2) a=3.974 b=13.921 c=3.699

204.63 43.46 9.23 0.256

GMO-2 29.89 (2) a=3.962 b=13.858 c=3.697

202.98 72 9.54 0.261

GMO-3 41.76 (2) a=3.9896 b=13.874

1 c=3.723

206.07 10.43 7.9 0.245

As the specific surface area and pore sizes of the materials have a direct effect on the

performance efficacy, we analyzed BET surface area to see the influence on photocatalytic

performance of the prepared GMO hybrids. Typical BET isotherms have been presented for

GMO-2 and GMO-3 nanohybrids in Fig. 2 (a-b). The isotherms depict a low hysteresis between

adsorption and desorption path for both samples with an observation of mesoporous nature of the

prepared nano hybrids. After careful analysis, pore sizes for both samples were 9.54 nm and 7.9

nm respectively for GMO-2 and GMO-3, whereas pore size distribution was observed to lie in a

wide range of pore diameters ranging from 3.5 nm to 67 nm. Multipoint BET analysis was used

to place the data in order to evaluate the specific surface areas, which come out to be 72 m2/g and

10.2 m2/g for both samples respectively. The calculated values of the surface area can be clearly

related to the photocatalytic efficiency and will be discussed in further sections. Low aspect ratio

of the prepared nanohybrids containing large amount of GO (GMO-3) as observed in FESEM

and TEM images, has lesser surface area as compared to others. This resulted in a low crystalline


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nano hybrid, with broken edges along with an agglomeration of α-MoO3 nanobelts on the GO

surface, resulting in a thick mass of nano hybrid with less surface sites available for the reactive

species adsorption and subsequently decreasing the photocatalytic performance.

Fig. 2: BET Isotherms (a) GMO-2(b) GMO-3 (Inset) Multipoint BET analysis

3.2 Morphological analysis be SEM and TEM

The microstructure and morphological analysis of obtained GO and α-MoO3/rGO nanohybrids

were performed by FE-SEM and TEM respectively as shown in Figs. 3 and 4. It can be observed

from the SEM image of pure GO (Fig. S2 (a)) that Graphite has been well exfoliated to GO with

few (3) layers thickness as confirmed by TEM and HRTEM images of GO.

Fig. 3 (a-c) shows FE-SEM images of the GMO-0 and GMO-2 (Fig. 3 (d)) composites. In Fig. 3

(a-c) it can be perceived clearly that a uniform 1-D structure with a smooth surface was obtained

by the proposed synthesis route at low temperatures. This shows that proposed route has

manufactured well ordered 1-D nanostructure of α-MoO3 as compared to hydrothermally

prepared materials [36, 37]. It is obvious that increasing the amount of GO in the composites has


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influenced the width and uniformity of the one dimensional structure as can be seen by the

comparison of Fig. 3 (c & d). Well defined grain structure of as synthesized nanohybrids is

visible in all SEM images. Although grain size distribution is wider for the samples but due to

the presence of a large grain boundaries, the prepared nanohybrids can be suitable for the

efficient photocatalysis, as a larger grain surface is available for light irradiation hence

generation of more charge carriers. PVP plays the vital role in the presence of oxalic acid for

structure directed growth of uniform α-MoO3 nano structures as compared to hydrothermally

synthesized samples. Amounts of PVP and oxalic acid were optimized after several experiments

and the best conditions reported here. PVP observed to play a role as a surfactant and oxalic acid

as a formation of structured columns of α-MoO3 on well dispersed GO sheets. In the absence of

PVP no 1-D structure was observed, while in the absence of oxalic acid molybdenum precursor

was observed to be oxidized into mixed phases with unconverted molybdenum. Average sizes of

the nanobelts as estimated by SEM were in a close agreement with TEM calculations and shown

below. Also due to growth of a thick mass on a large surface area α-MoO3 nanobelts randomly

distributed on GO sheets, presence of GO sheets was not obvious in all SEM images but a

presence of nanobelts grown on GO sheets was confirmed by TEM and shown in Fig. 4 (c).

TEM and HRTEM images for α-MoO3/rGO nanobelts are presented in Fig 4 (a-e). Fig. 4 (a-b)

shows low resolution TEM images GMO-0 sample that reveals uniform nanobelts with a smooth

surface and round edges. TEM of GMO-1 nanohybrid (Fig. 4 (c)) shows α-MoO3 nanobelts were

well grown on to GO sheets with uniform surface and homogenous thickness, while selected area

electron diffraction (SAED) pattern of a single nanobelt revealed the single crystalline nature

(spot pattern) of prepared nanohybrids. SAED pattern recorded perpendicular to the growth axis


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of the single nanobelt could be attributed to the [0ī0] zone axis diffraction of orthorhombic

MoO3, and suggests that rods grew along the [001] direction with a high degree of crystallinity.

Fig. 3: FESEM images of nanohybrids (a-c) GMO-0 (d) GMO-2

These figures revealed very uniform nanobelts with an average width of 150 nm and length of

several hundred nanometers showing a good aspect ratio with a thickness of around 50 nm.

Calculated aspect ratio for one rod on average was 15, which can be attributed to a length along

the growth axis [001] to the width perpendicular to the growth axis [100] [35]. TEM images of

GMO-3 sample presented in Fig. S3 (a-b), clearly shows the broken edges and low aspect ratio

(less diameter with small length) nanobelts as compared to GMO-1 nanohybrid. This clearly


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depicts the effect of the increased amount of GO that inhibits the growth of nanobelts with a

uniform surface throughout the length of belt. HRTEM images of GO and GMO-1 are presented

in Fig. S3(c-d) respectively. GO sheets with 3 layers thickness can be observed in HRTEM of

GO. From HRTEM of α-MoO3, calculated d-spacing of 0.346 nm (040) and 0.39 nm (110) were

well in accordance with the XRD data. The results indicate that the samples are single crystalline

nanobelts with a high crystallinity.

Fig. 4: TEM & HRTEM images of (a-b) GMO-0 (c) GMO-1 (d) SAED pattern of single belt (e)

Single belt

3.3 Raman, UV-Vis DRS and XPS analysis

In order to observe the molecular dynamics and presence of structural defects, Raman

spectroscopy is a respected technique. Fig. 5 (a) represents the Raman spectra of GO (inset) and


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GMO-1 nano composites. In the Raman spectrum of GO two characteristic peaks corresponding

to D and G bands along with a wide 2D band were observed at 1354.6 cm-1

and 1600 cm-1

and

2900 cm-1

respectively. Intensity of G band is higher as compared to D band (ID/IG=0.94)

showing that GO has been exfoliated with more ordered sp2 layered structure and has lesser

defects after it was synthesized by an extended centrifugation at 18000 rpm.

In the Raman spectrum of GMO-1 nanohybrid, very small peaks related to D and G bands,

clearly revealed that a large mass of high surface area nanobelts covered the GO sheets

completely. However there has been a slight shift in the positions of D and G bands towards

large wavenumbers (1407.36cm-1

and 1629.25cm

-1) that reveals the increase in surface area with

decrease in the grain size of the composite and the growth of multilayers during the synthesis of

α-MoO3 on GO sheets. In the Raman spectrum for GMO-1, three intense peaks due to

symmetrical and asymmetrical stretching vibration of α-MoO3 at 665.8 cm-1

(O-Mo-O), 815.10

cm-1

and 993.75cm-1

(Mo═O) can be observed [42].

Fig. 5 (b) shows the diffused reflectance spectra of the GMO composites taken in a powdered

form. It can be observed that all GMO composites have strong absorption edge in the visible

light region and shows that the prepared GMO composites are suitable for the photocatalytic

application using visible light. In Fig. 5 (b) (Inset) Tauc plots of the composites was drawn using

Kubelka-Munk relation to compare the effect of GO mass charging on the band gap of GMO

composites. As obvious from the Tauc plots the band gap of pure α-MoO3 nanobelts shifted from

2.82 eV to lower values 2.51 eV as the amount of GO in the samples was increased. This effect

is useful for the application of the material using visible light induced photocatalysis [43]. The

decrease in the band gap can be attributed to the presence of a graphitic carbon that condenses

the visible light reflection hence shifting the absorption edge to a lower wavelength. It is


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indicated that the samples show a visible-light response as gradually increasing the amount of

GO.

Fig. 5 (a) Raman spectra of GMO-1 nanohybrid (b) UV-DRS spectra of GMO hybrids

XPS measurements were taken to analyze the surface oxidation states of the elements present in

the nanohybrids. Fig. S4 (a) represents the typical binding energy curve of GMO-1 nanohybrid

and inveterate the presence of Mo, C and O as the only elements present in the hybrids. Presence

of C1s electrons binding energies with a lower intensity as compared to Mo3d confirmed the

thick mass of α-MoO3 on GO sheets. Fig. S4 (b-d), high resolution survey of binding energies for

Mo3d, O1s and C1s electronic states are presented after de-convolution of XPS data. Two

distinct peaks for Mo6+

at 232.81 eV and 235.94 eV is typical of Mo3d5/2 and Mo3d3/2

respectively along with a satellite peak (234 eV) due to surface oxidation of MoO3 in the air. No

other oxidation states of Mo (Mo4+

, Mo5+

) were observed in XPS measurements. O1s resolved

binding energies (Fig. S4 (c)) had peaks centered at 530.585 eV and 531.851 eV related to O2-

present in α-MoO3 lattice and bound to carbon of GO as C-O respectively. The C1s resolved

spectra of α-MoO3/rGO (Fig. S4 (d)) corresponds to the binding energy of carbon sp2 (C-C,


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284.36 eV) and epoxy/hydroxyls groups (C–O, 285.26 eV), whereas C1s electrons with binding

energies around 289 eV (carboxylates O-C═O) and 286.9 eV (C═O)[36] are not observed

showing that GO is reduced well to rGO during the ultrasonic assisted reflux condensation

method in the presence of oxalic acid. All results are well in agreement with the previous reports

available for binding energies of α-MoO3 metal oxide with rGO.

3.4 Photocatalytic degradation analysis

In order to analyze the photocatalytic efficiency of the synthesized GMO composites,

degradation of congored (CR) has been considered as model industrial pollutant. All experiments

were performed under controlled temperature in ambient conditions. Initial concentration (CA0)

of the dye was 50 ppm and the amount of the catalyst was 0.01g, although CA0 and catalyst

loadings both were varied to see the effect on kinetics of the reaction. Degradation of CR in the

presence of photocatalyst under light was recorded with definite time intervals as shown in Fig. 6.

While maintaining an adsorption-desorption steady state, there is no influence on the absorbance

peak of CR after 1 hour stay in dark. Fig. 6 (a) clearly shows a successive decrease in the

maximum absorbance of the dye in the presence of GMO-0 photocatalyst under light irradiation.

In the absence of photocatalyst, no change in the initial concentration of the pollutant dye has

been observed for long time (Inset). Photocatalytic performance of GMO-2 (0.01g) for CR can

be observed in Fig. 6 (b), where 76 % of the initial concentration has been reduced within 10 min

under light. The indicative color of the CR gradually changes from purple to blue with peak

λmax=498nm disappearance and shift to a higher wavelength by GMO-2 nanohybrid. Gradually

this blue color disappears to white but after long time of 12-14 h under light irradiation. HPLC-

MS analysis were performed to estimate the degraded products at various time intervals as

shown in Fig. S9. Clearly there was no large carbon or toxic compound but a mixture of some


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stable intermediates with definite life span was observed as degradation product, mentioned on

HPLC plot. Fig. S5 shows the degradation of the dye (CR) in the presence of GMO-2 (0.05g). It

was observed that with 0.05 g of photocatalyst more than 50% degradation was achieved in just

one minute of light irradiation. This enhanced activity of prepared nanohybrid towards reduction

of CR has been repeated three times to realize the validity of the results. Degradation efficiency

of GMO nanohybrids in current work was a match with other recent reports and greatly

enhanced activity of the synthesized material was observed. Dhanavel et al. reported α-MoO3

composite with a poly-aniline for an adsorptive dye removal and reported more than 51%

reduction of CR and RhB in 60 min[44]. Yiming et al. found α-MoO3/g-C3N4 composite to be an

efficient light driven photocatalyst with increased rate constant however, the reaction took 120

min for the complete removal of methyl orange [45]. In another work, Anukorn et al. presented

Eu doped MoO3 for degradation of methylene blue and observed 98% removal of dye in 60 min

with a high rate constant [46]. Further, we checked the photocatalytic performance of GMO-2 for

another toxic pollutant known as Rhodamine-B, shown in Fig. 6 (c). Within one minute of the

visible light irradiation, more than 80% reduction of Rh-B was observed whereas no change was

obvious in the concentration of Rh-B in the presence of GMO-0 photocatalyst as shown in the

Fig. S6. These experiments confirmed the activity and efficiency of the hybrids along with the

influence of GO on the photocatalytic process. Presence of non-metal with metal oxides can alter

the photocatalytic performance in two ways. One is introducing the defect levels secondly to

behave as traps for light induced charge species [47]. For photocatalytic degradation reaction, the

presence of GO can greatly inhibit the recombination of photogenerated charge carriers due to its

excellent electron-accepting and transporting properties, thus greatly enhancing the efficiency of


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photocatalytic reaction. Table 2 present a mild comparison of photocatalytic performance of

hybrid material in current work with some recent reports.

3.5 Kinetic data of congored reduction analysis

Fig. 7 (a) presented the comparison of a degradation efficiency of all of the prepared composites

for various time intervals, where GMO-2 has highest degradation efficiency of 100 percent only

in 30 min. GMO-0, GMO-1 and GMO-3 had lesser percentage of degradation in the said time

interval.

Fig. 6: Photocatalytic degradation of CR by (a) GMO-0 (b) GMO-2 (0.01g) (c) Degradation of

RhB by GMO-2 (0.01g) (d) successive degradation of CR in first 10 min by GMO-2 nanohybrid


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GMO-3 with more percentage of Graphene oxide had a lesser efficiency; this can be due to the

presence of the excess charge species in recombination centers that can decrease the light

induced charge species hence, reducing the percentage degradation efficiency of the

photocatalyst. Percentage degradation of CR have been monitored by the following relation,

� � �� − �� × ��

In order to investigate the photocatalytic activity of the as-prepared GMO-2 nano hybrids, a

kinetic study was performed for the photodegradation process using differential method of

analysis. To gather the kinetic data careful experiments (Fig. 6 (d)) were performed for GMO-2

in first 10 min as 76 % conversion was observed for this time interval.

Known initial concentration and concentration values at the various time intervals were used to

find out the rate (-dCA/dt) of degradation reaction as shown in Fig. 7 (b). Further the rate time

data for GMO-2 was used to calculate the actual order (n) and the rate constant (k) of the reaction

at a room temperature as shown in the inset of Fig. 7 (b). The order of photocatalytic reaction

was 1.19≈1, whereas k was 0.245min-1

.The principal equations used for the rate data are as

follows,

−�� −��

�� −��


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Fig. 7: (a) comparative PDE (%) of CR by GMO nanohybrids (b) Concentration vs time plot for

kinetic analysis (Inset) Linear fit by Differential method of analysis

In order to find the reusability of the catalyst, spent photocatalyst was recovered by filtration

followed by washing with ethanol and water respectively. The separated GMO-2 nanohybrid was

reused for photocatalytic reduction of CR up to three cycles as shown in Fig. S7 (ESI). The

current nanohybrid photocatalyst retained its percent activity for CR with minor change in the

calculated values of conversion in first 10 min of reaction under visible light irradiation.

3.5 Photocatalytic mechanism and possible reaction Pathways

As photons of a larger energy (> Eg) interact with semiconducting materials, transition from

valence band to conduction band results in positive negative carrier species with a tendency to

recombine once again without utilization in redox reaction. Speed of charge species

recombination process (completes in few nanoseconds) must be less as compared to a charge

transfer mechanism for remarkable photochemical change. To avoid this rejoin of charged

species trapping, surface recombination and interfacial transfer are the possible ways. There can

be different arguments that can support the ongoing photocatalytic performance of the prepared


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GMO hybrids to justify the remarkable activity. Firstly, GO with 2-D structure having strong π-π

interactions with the metal oxide species, can serve as a support for the photocatalyst to enlarge

the specific surface area as can be allied to the BET results. 1-D nano structure on 2-D co-

catalyst can provide plenty of surface reactive sites and 2-D GO serves as electron dumping sites

thereby increasing the life span of the charged species [12]. Secondly, α-MoO3 has a large

number of surface functional groups (hydroxyl) that anchors the active dye part (amine) on the

surface and the dye gets adsorbed for instance on the surface sites by the weak electrostatic

forces[48]. Dye immobilization on the surface of the photocatalyst increased the speed of a

photogeneration of charges and resulted in the efficient reduction of adsorbed dye. Molybdenum

oxide is considered as a layered structure attached by the weak covalent bonds to each other

(interlayer). These layers can be detached via application of either a mechanical energy or a

strong light absorption. That can generate a 2D structure on applying specific motive. These

layers can generate multi electron reactions effect. GO 2-D structure with 0 eV bang gap can

absorb the whole light spectrum without generating any active charge centers that perform redox

reaction so can accomplish different exploitation of light12

. Moreover, a band alignment occurs

as a result of a right band structure as Fermi level of graphene is less than conduction band of

most of the semiconductor photocatalysts [44]. But the increased amount of the electron trapping

species hinders the photocatalytic performance by absorbing more electrons and less efficient

redox reaction. So GMO-3 has less photocatalytic performance in respect of GMO-2.

As photocatalytic reaction is considered as a chain reaction involving formation of a certain

intermediate, without identification of all possible intermediates, it is difficult to propose a

reaction mechanism. Therefore, we conducted a scavenger experiment to clearly identify the

presence of most active radical produced during photocatalytic reaction. Oxygen molecules


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always play a strident role in the assembly of active radicals, namely singlet oxygen (1O2),

hydroxyl radicals (OH•) and superoxide radicals (O2•−). Photoinduced electrons and holes

react with the associated O2 and H2O to breed reactive radicals (•OH) that advance in chain

reaction leading to a degradation of organic pollutants[49]. Azo dyes with -N═N- are difficult to

break, require stronger radical leading to the formation of a very unstable intermediate with low

activation energy. In order to confirm the presence of hydroxyl radical, we used selective

quenchers namely, isopropanol (0.1M) and methanol (0.1M) using the same photocatlytic

experiment in the absence of water. The photodegradation of congored decreases to 20 % and 49%

in the presence of isopropanol and methanol respectively as compared to an aqueous system in

first 10 min (Fig. S7). This indicates that isopropanol inhibited the photochemical reaction by

scavenging the hydroxyl radicals produced as intermediates from the redox reaction of

photoinduced charge species. Based upon this scavenging experiment, proposed reaction

mechanism for redox reaction can be as follows: Negative charges of the conduction bands

somehow trapped by GO and the rest can react with a dissolved O2 to transform it into more than

one of active radicals, as shown below, subsequently removing conjugative bonds in the

congored. Positive carriers in the valence band can further react with adsorbed hydroxide ions

from the water to generate more hydroxyl radicals (•OH)[50]. A possible reaction pathway with

all of possible intermediates for degradation of CR can be expressed as follows;

� !" � �# → �% � &'

�% �(�! → !(∙ �(%

�"(��*+*��!+,�' � �(% � �&' → �"(��*+*��!+,�' ∙ &' � !� →• !�'

• !�' �(% → (!�' �(% → �!(∙


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• !�' �(�! →• !!( � !('

2• !!( → !� �(�!�

(�!� �• !�' →• !( � !(' � !�

OH• + Organic molecules → degradation products

Table 2: Performance comparison of current work with recent reports

Material/catalyst Organic Pollutant/dye

Synthesis method Degradation & Time

Ref.

α-MoO3/PANI RhB, CR, Oxidative polymerization 78% ,80% in 60 min [44]

E-MoO3 MB Hydrothermal 80% in 60 min [46]

α-MoO3/g-C3N4 MO Thermal treatment 100% in 120 min [45]

α-MoO3 MB Hydrothermal ≥80% in 160 min [39]

ZnO CR Chemical precipitation ≥85% 120 min [51]

ZnO CR Sol-gel ≥94% in 60 min [52]

AgX/C3N4 MO Water bath method ≥80% in 20-180 min [53]

CdS/rGO RhB Condensation method ≥92% in 30 min [54]

TiO2 /rGO MB Modified Hydrothermal ≥90% in 180 min [55]

Cu2O/rGO MO Hydrothermal 100% in 100 min [56]

Α-MoO3/rGO

nanohybrid

CR, RhB Reflux condensation CR-100% in 30 min

RhB-80% in 1 min

This

work

4. Conclusions

One dimensional GMO-nanohybrids with a different percentage of GO were synthesized using a

novel low temperature route yielding a high purity crystalline uniform nanobelts. High aspect

ratio nanobelts with a uniform size distribution were synthesized by oxalic acid as an oxidizing

agent in the presence of PVP that plays a structure directing role. Successive increase in GO

consequences a large surface area (72m2/g) and a remarkable catalytic performance under light

irradiation but limited to a point after which a decrease was observed of the both properties.

Addition of GO in the nanohybrids reduced the bag gap (Eg) of the synthesized samples from


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2.82 eV to 2.51 eV thereby increased the visible light absorption tendency of the material. GO

served as an efficient co-catalyst for the electron capping and dye immobilization on α-MoO3

surface. Synergic adsorption effect of GO with α-MoO3 resulted in 100 % removal of RhB in

few minutes, whereas for CR degradation undergoes chain reaction mechanism with formation

of less toxic intermediates with definite life span leading to 100% removal, following a pseudo

first order kinetics. Hydroxyl radical have been observed as an active intermediate for the

photochemical reaction of congored removal under light irradiation.

Acknowledgements

This work is supported by National Natural Science Foundation of China through grant

21471147 and Liaoning Provincial Natural Science Foundation through grant 2014020087. M.

Yang would like to thank the National "Thousand Youth Talents" program of China. Erum

Pervaiz would like to thank the Talented Young Scientist Programme (TYSP) by Ministry of

Science & Technology (MoST) China.

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driven AgX/graphite-like C3N4 (X=Br, I) hybrid materials with synergistic

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[56] Zhang W, Li X, Yang Z, Tang X, Ma Y, Li M, Hu N, Wei H and Zhang Y 2016 In situ

preparation of cubic Cu2O-RGO nanocomposites for enhanced visible-light degradation

of methyl orange Nanotechnology 27 265703


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