Valorization of cellulose and waste paper to graphene oxide quantum dots

Biobased graphene oxide quantum dots (GOQD) were derived from cellulose via carbon nanospheres (CN) as intermediate products. Solid CN were synthesized from cellulose through microwave-assisted hyd ...


Introduction
Graphene, 1 an sp 2 -hybridized carbon sheet, has arisen as a fascinating new carbon material.5][6][7] However, its potential in biomedical applications is limited due to poor solubility in common solvents and inadequate optoelectronic properties. 8raphene oxide, as a direct derivative of graphene, consists of small graphene domains surrounded by carboxyl, epoxyl and hydroxyl groups that provide H 2 O solubility. 9 When the size of the sheets is less than 100 nm they are referred to as graphene oxide quantum dots (GOQD).Nano-sized graphene oxide has attracted attention due to its low C/O ratio and enhanced colloidal stability. 10rimarily, graphene oxide was mainly used as a precursor for graphene but the focus has now shied due to its interesting heterogeneous and electronic structure with simultaneous conducting p-states and energy bandgap. 111][22] Exfoliation is used for separating the oxygen-functionalized graphite into single sheets. 23Therefore, it can be considered that every carbon structure with graphite content is a possible source of graphene oxide including carbon nanospheres (CN) formed of graphitic akes. 246][27] Among the possible precursors, cellulose is especially interesting because it is an almost inexhaustible noneditable and renewable resource. 28The b-(1-4)-glycosidic linkages in cellulose are protected within the crystalline structure and by the intense intra-and intermolecular hydrogen bonding in the polymer backbone and between the brils. 29Conversely, this production route for synthesis of CN generally requires harsh conditions. 27,29Many of the processes used are not suitable for scaling up since they are not based on green chemistry principles.Also, they are costly and non-selective, which leads to several purication steps.
1][32] Our hypothesis here was that the CN produced through green microwave-assisted synthesis, could be further oxidized to novel value-added GOQD materials.This production route further provides the advantage of using and/or upgrading renewable and waste resources to value-added products.The oxidation-degradation reaction of the microwave synthesized cellulose based CN was performed in an O-rich environment and under mild conditions forming nano-sized sheets of GOQD.The CN precursors and obtained GOQD were characterized by multiple techniques to demonstrate a successful reaction as well as to show the uorescence properties of the synthesized GOQD.

Microwave-assisted synthesis of CN
CN were derived from cellulose or waste paper through microwave-assisted synthesis according to our previously reported procedure. 30,31The reaction was performed in a MES-1000 instrument under dynamic mode (CEM Corporation with maximum power of 950 Watts AE 50).Briey, a-cellulose was degraded in 20 ml of 0.01 g ml À1 H 2 SO 4 solution in 100 ml Teon PFA vessels.Temperature and pressure were followed by optical probe inserted in one of the vessels.The temperature was rst set to increase to 160 C under a RAMP-time of 20 minutes and then kept at isothermal conditions by input irradiation for 2 hours with a maximum pressure of 150 psi.The power effect was 100% for the duration of the reaction.Aer the reaction, the vessels were taken out and they were put into an ice bath to cool down.The resulting black solid CN were ltered from the solution and washed with 20 ml of deionized H 2 O.The CN samples were dried in a vacuum oven overnight at 25 C.

GOQD synthesis
For the preparation of GOQD, a 15 ml solution of CN in HNO 3 (1 : 100, w/w) was kept in a 100 ml one-neck round-bottom ask and sonicated at 45 C for 30 minutes in sonication bath.GOQD-30 and GOQD-60 were obtained by 30 or 60 minutes heating of the sonicated solutions in an oil bath at 90 C with magnetic stirring.The solutions were then poured into 50 ml of cold deionized H 2 O (15 C) to stop the reaction and dilute the acidic medium.An orange/red solid was gained aer removal of the acidic H 2 O by rotary evaporation.The products were kept in a vacuum oven for one week to remove any residues of H 2 O and acid.The GOQD prepared from waste paper derived CN were synthesized according to the same procedure as GOQD-30.

Characterization
Mettler-Toledo TGA/SDTA 851e was utilized for the thermogravimetric analysis (TGA) of a-cellulose, CN and GOQD.3-4 mg of each sample was placed into a 70 ml alumina cup.The samples were then heated at a rate of 10 C min À1 from 30 C to 550 C with an O 2 ow rate of 80 ml min À1 .Fourier transform infrared spectroscopy (FTIR) of a-cellulose, waste paper, CN before and aer sonication in HNO 3 , GOQD-30 and GOQD-60 were obtained by PerkinElmer Spectrum 2000 FTIR spectrometer (Norwalk, CT).The instrument was equipped with attenuated total reectance (ATR) accessory (golden gate) from Graseby Specac (Kent, United Kingdom).The X-Ray Photoelectron Spectroscopy (XPS) spectra of CN and GOQD-60 were collected by a Kratos Axis Ultra DLD electron spectrometer using monochromated Al Ka source operated at 150 W. Pass energy of 160 eV were applied for wide spectra analyzer and pass energy of 20 eV for individual photoelectron lines.The surface potential was stabilized by the spectrometer charge neutralization system.The binding energy (BE) scale was referenced to the C1s line of aliphatic carbon, set at 285.0 eV.Processing of the spectra was accomplished with the Kratos soware.Powder sample for the analysis was gently hand-pressed into a pellet directly on a sample holder using clean Ni spatula.The scanning electron microscopy (SEM) images, for picturing the morphologies and sizes of a-cellulose, CN and GOQD-60, were taken by Ultra-High Resolution FE-SEM (Hitachi S-4800).The samples were sputter-coated with 2 nm thick platinum/ palladium layer.Ultraviolet-visible (UV/Vis) absorption of GOQD-60 in deionized H 2 O solution was measured by SHIMADZU UV-2550 UV/Vis.For transmission electron microscopy (TEM) images, HITACHI HT7700 instrument including the soware version 02/05 was utilized.Aqueous dispersions of CN and GOQD-60 were drop-casted on the TEM grid (ultrathin carbon coated copper grid (TED PELLA, INC.)) and the excess H 2 O was removed with a paper tissue aer 2 min to decrease the possibility of aggregation of nanoparticles or formed lms of GOQD.The samples were le to dry in a dust free chamber for at least 1 h before the analyses in either highcontrast mode TEM (HC-TEM) or high-resolution mode TEM (HR-TEM).X-ray diffraction (XRD) spectra were recorded for a-cellulose, waste paper, GCB, CN, GOQD-30 and GOQD-60.The X-ray source was CuKR radiation (l ¼ 0.1541 nm) and the diffraction was measured by PANalytical X'Pert PRO diffractometer at 25 C with a silicium mono-crystal sample holder.The intensity was determined in a 2q angular range between 5-55 with a step size of 0.017 for all analyses.Furthermore, the crystallinity index (CI) of a-cellulose was calculated according to Segal method (eqn (1)). 33 In the XRD-spectrum of a-cellulose the height of I 002 includes both crystalline and amorphous material (2q ¼ 22.7 ).I am is the minimum height between the peaks at (002) and (110) (2q ¼ 18 ) and corresponds to amorphous material.Fluorescence spectroscopy measurements were performed by Cary Eclipse spectrophotometer from Varian.The uorescence emission of GOQD-60 in deionized H 2 O solution was measured at an excitation wavelength of 330 nm.The instrument uses xenon lamp technology.A Brunauer-Emmett-Teller (BET) measurement was made to determine the surface area of CN as an indication of particle size.The measurement was done by using Micromeritics Flow Sorb II 2300 and is based on BET theory.Dynamic light scattering (DLS) measurements of CN and GOQD-30 and GOQD-60 solutions were carried out on Zetasizer Nano ZS from Malvern Instruments (Malvern, UK) where polylactide (RI 1.46) was used as standard for all measurements.A Nanoscope IIIa multimode atomic force microscopy (AFM) (Digital Instruments, Santa Barbara, CA) in tapping mode was performed to image the materials under ambient conditions (20-25 C).Freshly cleaved mica (grade V-1, Electron Microscopy Sciences) was used as a surface for all the samples.AFM images of CN and bulk GOQD-60 were acquired from their solid powder.H 2 O dispersion of GOQD-60 was also drop-casted on the mica surface under dust free conditions to be able to image smaller domains of the GOQD sheets.

Results and discussion
CN were synthesized from pure a-cellulose and/or waste paper by microwave-assisted reaction and utilized as novel precursors for synthesis of GOQD as shown schematically in Fig. 1.The obtained CN and the oxidized products, GOQD, were characterized by multiple techniques to establish the effect of reaction time on size and chemistry of the produced GOQD.Furthermore, the optical properties of GOQD were evaluated.

Characterization of CN
Fig. 1 displays a simplied reaction scheme for the microwaveassisted synthesis of CN and the further degradation-oxidation reaction to GOQD.5-(Hydroxymethyl)-2-furaldehyde (HMF) intermediate is formed through depolymerization of a-cellulose to glucose followed by a dehydration reaction.HMF rapidly further reacts to form either levulinic and formic acid or carbonized ake structures through polymerization and dehydration of HMF. 30,34These carbonized structures then precipitate as spherical carbon nanospheres.CN were produced during hydrothermal degradation of cellulose under conditions where the liquefaction efficiency and hydrolysis rate were high (T > 140 C).SEM images in Fig. 2 illustrate the morphological change from untreated brous a-cellulose (average size of mm in length) and (mm in width) to the obtained CN, with a spherical shape and diameter sizes in the range between nm and mm.In accordance with the typical unmodied nanoparticle behavior, some agglomeration of the carbon particles was observed.
Furthermore, the morphology and size of CN were investigated by HC-TEM.The HC-TEM images in Fig. 3 conrmed the spherical morphology of the CN, their size dispersity and aggregation tendency.Both HC-TEM and SEM of the CN were further supported by BET-results that displayed a surface area of 3.1 m 2 g À1 , indicating that the obtained CN in their inactivated form were a collection of larger spheres with low porosity.The HC-TEM image of a single carbon nanoparticle (Fig. 3c) revealed the presence of thin carbon sheets and graphitic akes located predominately on the surface of the CN.This appearance is in accordance with the previously reported morphology of CN, where graphitic akes were located concentrically at the surface.

GOQD synthesis
Novel value-added GOQD were synthesized from CN in a bulk reaction under O-rich acidic conditions.The performed degradation-oxidation under size control broke down the 3D CN to 2D GOQD.1.5 g GOQD-60 was collected aer 60 minutes of heating, when the original amount of CN added was 1 g, strongly indicating an introduction of new functional groups, especially in the form of oxygen elements.
As shown in Fig. 4, the GOQD synthesis process was monitored by DLS, which in the case of 2D GOQD measures the approximate lateral dimensions, to follow the changes in particle size.A signicant decrease in the size of the carbon particles occurred moving from the disassociated CN (aer sonication) to GOQD-30 and GOQD-60.The particle size of the CN was 995 AE 123 nm, whereas GOQD reached approximately the average sheet sizes of 3.3 AE 0.37 and 1.2 AE 0.75 nm (dot diameter) for GOQD-30 and GOQD-60, respectively.A prolonged reaction time at a temperature of 90 C, thus, not only resulted in cleavage of the 3D carbon structures into small 2D dots but also caused further cleavage of GOQD-30 to GOQD-60.This observation revealed that, by controlling the reaction conditions, tuned graphene oxide structures with desired sizes could be obtained.
The synthesis process of the GOQD was further monitored by FTIR (Fig. 5).While the CN disassociated aer sonication in HNO 3 , there was no additional introduction of functional groups.However, the FTIR spectrum proved that different oxygen groups were added to both GOQD-30 and GOQD-60.This further conrmed the cause of the observed mass increase.Both GOQD-30 and GOQD-60 displayed characteristic modes of alcohols (stretching of C-OH at 3560 cm À1 ), carboxylic acids (stretching of C]O at 1740 cm À1 , broad stretching of C-OH at 2500-3350 cm À1 and stretching of C-O at 1100 cm À1 ), aromatic sp 2 carbons (stretching of C]C at $1570 cm À1 ), and epoxides (stretching of C-O-C at 1230 cm À1 ).Some differences were also observed between GOQD-30 and GOQD-60, including more sp 3 -  hybridized C-H stretches (2920 cm À1 ) and a broader peak of COOH stretches (3050-3350 cm À1 ) in GOQD-60.Therefore, as an outcome of increased reaction time, more defect sites were introduced in the carbon structure.Importantly, the functional groups at the edges and in the carbon structure will result in dots that are not completely at, as is the case for graphene.The oxygen groups provide both bulkiness and absorption of H 2 O that will lead to a higher monolayer thickness for the graphene oxide 11,35 in comparison to the thickness in dry state.
Fig. 6 shows the XPS analysis performed on CN and GOQD-60 (from now referred to as GOQD).It is important to notice that due to the 3D spherical structure and size of the CN, XPS spectrum shows the characteristics of the surface layers and not the core of the spheres.worthwhile to emphasize that this ratio can be even higher for the core of the CN.The concentration of the oxygen containing groups is also expected to be higher at the edges of the GOQD compared to the basal plane.The higher oxygen content of the herein synthesized GOQD compared to the previous studies on graphene oxides, 36 which typically had C/O ratios of $2.4, is most likely explained by the larger edge/basal plane ratios due to smaller size of the GOQD.
The amorphous character and solid-state structure of starting and synthesized materials were studied by XRD.CN and GOQD were originlly derived from a highly crystalline precursor, a-cellulose, with a crystal index of 64.2% (Fig. 7).While XRD revealed the dominantly amorphous character of the bulk solid-state of CN and GOQD, CN exhibited a small intensity peak at $25 corresponding to the (002) graphitic plane. 37The peak observed for CN was both much broader and appeared at a slightly lower diffraction angle in comparison to GCB, a fully graphitized carbon particle, with the (002) graphite peak at 26 . 38The larger interspacing between the crystal planes give rise to a lower diffraction angle 39 and as a consequence of the functionalized akes forming the CN, there was an increased distance between the graphitic layers.As expected, the CN, thus, had lower graphite content than GCB.For GOQD, while a tiny incurvation in the (002) graphitic plane located at $23 was detected, 40 in consistency with another report, 41 a broad peak was indicated as a sign of the loss of long-range order.
The solid-state morphology of bulk GOQD was rst monitored by SEM.As seen in Fig. 8, wrinkles, caused by the association of the 2D dots due to the electrostatic forces present between the various functional groups, were formed. 42The single dots are visualized in the defect part of the lm as shown in Fig. 8a.AFM images of the bulk GOQD surfaces (Fig. S2, ESI † le) were in agreement with the SEM image in Fig. 8b and emphasized the lm-like structure of GOQD caused by the aggregation-stacking of the dots into layers of sheets.
The differences in the chemical structure of the produced cellulose derived CN and GOQD were monitored by evaluating their thermal resistance (Fig. 9).The CN had the same main T on at around 330 C as a-cellulose but with enhanced thermal resistance in the range between 330-450 C. The improved thermal resistance probably originated from the graphitized carbon layers at the surface of the CN and their ability to resist combustion.The combustion of a-cellulose was shown to be complete with no ash content at a temperature of approximately 330 C, due to high oxygen content, whereas the CN contained an ash residue of 4.4 wt% at a temperature of 450 C. GOQD were contributing to the obtained TGA curve with several T on steps (see the DTG curves in Fig. S3 in ESI † le), which is a further indication of oxygen functionalities in the GOQD. 43,44hile the rst combustion stage started already at around 50 C, due to the loss of moisture, the second combustion stage begun at approximately 110 C and the nal observed  combustion stage took place at a temperature of 400 C. Interestingly, about 10.5% of GOQD was still stable at 550 C, even though the oxygen content is higher in comparison to CN.
However, the mass loss of GOQD was shown to be lower above $450 C as an outcome of stable oxygen functionalities. 45he size and thickness characteristics of a single GOQD layer were investigated by AFM using dilute aqueous solution of GOQD for the disassociation of sheets into dots (Fig. 10).In this regime and based on the DLS measurement of the aqueous solutions, single dots were obtained with R h $ 1 nm.While the single sheet of graphene (atomically at carbon structure) can reach van der Waals thickness of $0.34 nm, the thickness of graphene oxide would be thicker due to the covalently bonded oxygen and displacement of sp 3 carbons above and below the plane of graphene.The average thickness of the synthesized GOQD was 0.5 nm (Fig. 10b) and is considered to be an exfoliated monolayer sample of GOQD as the thickness value agrees well with previously reported monolayers of dry graphene oxide. 11,46As can be seen in Fig. 10c and d, AFM visualized sheets with an average size of $15 nm.This somewhat larger size which was also seen in HR-TEM of exfoliated single sheet of GOQD (Fig. S4, ESI † le) is probably due to association of the dots and lateral interaction between functional groups at the edges during the drying process, as well an effect of tip convolution.

Optical properties of GOQD in H 2 O solution
The special chemical structure of GOQD enables them to absorb UV and visible light of lower wavelengths.In accordance with previous works 20,47 our GOQD particles displayed two absorption peaks in the UV-range including a small peak at 320 nm corresponding to n / p* transition in C]O and a peak at $200 nm having a transition in C]C by p / p*, Fig. 11.Interestingly, GOQD synthesized by utilizing the longer heating time (GOQD-60) exhibited a bathochromic shi ($20 nm red shi) at absorption peak corresponding to n / p* transition in C]O, which is most likely due to the increase in the concentration of carboxyl groups (see Fig. S5, ESI † le). 48OQD possess uorescence properties due to their optoelectronic state.This was clearly visible under UV light in a H 2 O solution with GOQD concentration of 0.1 mg ml À1 .The uorescence emission was also evaluated with uorescence spectroscopy showing emission at 450 nm, at the maximum intensity, by an excitation wavelength of 330 nm (close to the rst absorption transition in the UV).A small emission peak appeared at the emission of 650 nm, which is not attributed to the uorescence of GOQD.It is a result of overlap of second order emission associated to the excitation wavelength. 49,50g. 10 AFM images of GOQD, (a) in height mode at a magnification of 100 nm, (b) displaying the thickness in 3D picture, (c) in height mode at a magnification of 50 nm and (d) height profile taken along the line in (c).The samples were prepared from a concentration of 0.05 mg ml À1 GOQD-60 in deionized H 2 O and drop-casted on a mica plate.

Preparation of the GOQD from waste papers
Finally, we demonstrated that similar GOQD could be synthesized by utilizing low quality waste paper.The schematic representation of the produced CN and GOQD-30 and their diameters in the single particle regime (dilute regime) are presented in Fig. 12.During the rst step, as described above for pure cellulose, the microwave-assisted recycling of the waste paper produced amorphous CN as a result of the polymerizationprecipitation reaction of HMF synthesized by dehydration reaction of glucose.The synthesized CN was utilized as a precursor for the preparation of the GOQD-30 via an oxidation reaction in the presence of HNO 3 as oxidizing agent.The corresponding FTIR and XRD spectra can be seen in Fig. S6, ESI † le.

Conclusions
Novel value-added graphene oxide quantum dots (GOQD) were synthesized from abundant non-editable biomass, as well as from a large volume paper waste product through a facile process.The CN (D ¼ 995 nm) intermediates produced by microwave-assisted hydrothermal degradation of cellulose were oxidized to 2D GOQD under O-rich acidic condition (HNO 3 ) via a simple 2 steps reaction: (1) sonication for disintegration of CN aggregations and (2) heating at 90 C for the main oxidationdegradation process.It was revealed that the size of nal GOQD could be tuned by heating time as 30 and 60 minutes heating produced GOQD with $3 and $1 nm diameters, respectively.Furthermore, UV/Vis and uorescence spectroscopy showed optical properties of synthesized GOQD.A novel methodology for easy synthesis of green GOQD with interesting properties from bio-or waste resources was, thus, demonstrated.

Fig
Fig. S1 (in ESI †) shows the FTIR spectrum of the raw material, a-cellulose, and the produced CN. a-Cellulose has its typical vibration modes of hydroxyl (C-OH) (3300-3500 cm À1 ), sp 3 -hybridised carbon (C-H) ($2980 cm À1 ), C-O group (1050 cm À1 ) and ether linkage (C-O-C) (1160 cm À1 and 950 cm À1 ).Formation of carbonous functional groups in the CN was conrmed by the appearance of carboxylic acid (COO-H stretching as broad band in the region 3300-2500 cm À1 , C]O stretching at 1705 cm À1 and peaks in the region at 1000-1260 cm À1 due to C-O stretching) and aromatic groups (sp 2 -hybridized carbon C]C stretching at $1507 and 1618 cm À1 ).SEM images in Fig.2illustrate the morphological change from untreated brous a-cellulose (average size of mm in length) and (mm in width) to the obtained CN, with a spherical shape and diameter sizes in the range between nm and mm.In accordance with the typical unmodied nanoparticle behavior, some agglomeration of the carbon particles was observed.Furthermore, the morphology and size of CN were investigated by HC-TEM.The HC-TEM images in Fig.3conrmed the spherical morphology of the CN, their size dispersity and aggregation tendency.Both HC-TEM and SEM of the CN were further supported by BET-results that displayed a surface area of 3.1 m 2 g À1 , indicating that the obtained CN in their inactivated form were a collection of larger spheres with low porosity.The HC-TEM image of a single carbon nanoparticle (Fig.3c) revealed the presence of thin carbon sheets and graphitic akes located predominately on the surface of the CN.This appearance is in accordance with the previously reported morphology of CN, where graphitic akes were located concentrically at the surface.24

24
Fig. S1 (in ESI †) shows the FTIR spectrum of the raw material, a-cellulose, and the produced CN. a-Cellulose has its typical vibration modes of hydroxyl (C-OH) (3300-3500 cm À1 ), sp 3 -hybridised carbon (C-H) ($2980 cm À1 ), C-O group (1050 cm À1 ) and ether linkage (C-O-C) (1160 cm À1 and 950 cm À1 ).Formation of carbonous functional groups in the CN was conrmed by the appearance of carboxylic acid (COO-H stretching as broad band in the region 3300-2500 cm À1 , C]O stretching at 1705 cm À1 and peaks in the region at 1000-1260 cm À1 due to C-O stretching) and aromatic groups (sp 2 -hybridized carbon C]C stretching at $1507 and 1618 cm À1 ).SEM images in Fig.2illustrate the morphological change from untreated brous a-cellulose (average size of mm in length) and (mm in width) to the obtained CN, with a spherical shape and diameter sizes in the range between nm and mm.In accordance with the typical unmodied nanoparticle behavior, some agglomeration of the carbon particles was observed.Furthermore, the morphology and size of CN were investigated by HC-TEM.The HC-TEM images in Fig.3conrmed the spherical morphology of the CN, their size dispersity and aggregation tendency.Both HC-TEM and SEM of the CN were further supported by BET-results that displayed a surface area of 3.1 m 2 g À1 , indicating that the obtained CN in their inactivated form were a collection of larger spheres with low porosity.The HC-TEM image of a single carbon nanoparticle (Fig.3c) revealed the presence of thin carbon sheets and graphitic akes located predominately on the surface of the CN.This appearance is in accordance with the previously reported morphology of CN, where graphitic akes were located concentrically at the surface.24

Fig. 4
Fig. 4 Average sizes and size distributions of CN after sonication (black), GOQD-30 (red in the middle) and GOQD-60 (red) as determined by DLS at 25 C.

Fig. 11
Fig. 11 UV/Vis and fluorescence spectra of GOQD-60 at a concentration of 0.1 mg ml À1 in deionized H 2 O.The chosen excitation wavelength for the fluorescence measurement was 330 nm.Included are images of GOQD in deionized H 2 O solution (yellow) and visualized green fluorescence.