Fluorescence “Giant” Red Edge Effect
3.2 Basic techniques for carbon quantum dot preparation
Multistep techniques are time-consuming and complicated, but they are very much effective at producing strong green luminescent carbon materials.
3.2 Basic techniques for carbon quantum dot
mbar). The cathode and anode are both graphite rods with sizes 40 mm length 3 16 mm diameter and 100 mm length36 mm diameter, respectively. In anode, a hole with size 40 mm length 3 6 mm diameter is drilled and then filled with a mixture of graphite powder and metallic catalyst. The arc discharge is produced with a 100 A current; 30 V voltage drop between the electrodes is controlled with continuous translation of the anode by keeping fixed distance (B3 mm) between it and the cathode. The catalysts used in this method are some mixtures of Ni Co, Co Y, or Ni Y in various atomic percentages; these are known to produce a chain of fascinating carbon nanostructures. The best results were obtained from the mix- ture of 4.2 at.% Ni and 1 at.% Y. Both single-shell and multi-shell CNTs are syn- thesized by arc discharge method where iron plays the role of catalyst. Here the evaporation chamber is replete with a mixture of gas consisting of methane gas and inert argon gas. A DC current of 200 A is passed through the electrodes. Single- shell tubes of 1 nm are formed in the gas phase whereas the multi-shell tubes grow on the surface of carbon cathode.
CNTs having very small diameters and a single atomic layer thickness can also be formed by cobalt and carbon vaporization in arc generators. These tubes form a web-like deposit and blend with fullerene-containing soot, which gives rubbery texture.
B, N-doped GQDs (B/N-GQDs) of 4 6 nm size can be synthesized by arc dis- charge process using graphite electrodes and chemical shearing in the atmosphere of hydrogen (H2), diborane (B2H6), helium (He), and H2, He, ammonia (NH3) mix- tures, respectively. Besides that, CQDs and CQDs TiO2 composite can be pro- duced by direct current arc discharge between graphite electrodes of high purity.
Here TiO2plays a vital role in enhancing the photolytic activity of CQDs and slow- ing down the electron hole recombination rate. Under the catalysis of visible light, the prepared CQDs TiO2composite shows stronger applicability than TiO2[1].
Laser ablation
Laser ablation is a procedure where solid materials can be removed from the sur- face of that solid material upon irradiation of a laser beam. Higher flux converts the materials into plasma, whereas in presence of lower flux the material evaporates or sublimes by absorbing heat energy. The advantages of this process are:
G It is simple.
G It is rapid.
G The end product has easily tunable surface properties.
The carbon target was prepared by hot-pressing a mixture of graphite powder and cement, followed by roasting, curing, and annealing in argon flow. For ablation, a Q-switched Nd:YAG laser (10 Hz, 1064 nm) was used, where water vapor carry- ing argon gas passes through the carbon target at 900C and 75 kPa. As a result, aggregated carbon nanoparticles (CNPs) of different sizes are produced. As the sample does not exhibit any detectable photoluminescence, surface passivation of CQDs is done by attaching simple organic species such as amine-terminated poly- ethylene glycol (PEG1500N) and poly(propionylethyleneimine-coethyleneimine)
(PPEI-EI). The surface passivation of carbon dots (CDs) with the help of organic moieties imparts bright luminescence emission[2,3]. Fluorescent CQDs can also be made with laser irradiation of a suspension of graphite power dispersed in three dif- ferent organic solvents: diamine hydrate, diethanolamine, and PEG200N. The surface properties of the CQDs could be revised by varying organic solvents in order to accomplish tunable light emission[4,5].
CQDs of visible and tunable photoluminescence can be prepared through laser ablation of nanocarbon particles of some common organic solvents such as ethanol, acetone, or water. Here the raw nanoparticles are smaller than 50 nm in average size and have a turbostratic structure. After ultrasonication, the suspension is dropped into a glass made cell which is coated with quartz for laser irradiation. An Nd:YAG pulsed laser consisting of a second harmonic wavelength (532 nm) is used to excite the suspension. After that the supernatant solution containing the CQDs is obtained by centrifuging the suspension. The nanostructure of carbon had changed to a kind of core shell structure after laser irradiation[5,6]. The experimental setup is illustrated inFig. 3.3.
Magnetic Stirrer Carbon Suspension
Glass Cell Metal Cover Reflector LASER Nd:YAG
532 nm 8 ns
Figure 3.3 Schematic illustration of laser ablation experimental setup.
Source: Adapted from X. Li, H. Wang, Y. Shimizu, A. Pyatenko, K. Kawaguchi, N. Koshizaki.
Chemical Communications, 47 (2011) 932 934.
Toluene also acts as a carbon precursor to make CDs by employing the unfo- cused laser irradiation approach in which the key step is further ablation of inter- mediate graphene. An Nd:YAG nonfocusing pulsed laser of 1064 nm wavelength was used to illuminate the liquid through a quartz window. Argon gas was used to protect the reaction and maintain a safe air pressure. In this approach, graphene is generated as an intermediate. By adjusting input laser intensity, controllable prepa- ration of CQDs with discrete optical properties can be realized.Fig. 3.4reveals a schematic illustration of a device which shows real-time supervision of the fluores- cent change and provides a new approach for controllable synthesis of fluorescent nanomaterials[7,8].
But this process also has some disadvantages as follows:
G Low quantum yield.
G Surface modification is required.
G Size cannot be controlled.
G Energy consuming.
Plasma treatment
Plasma treatment is the process of refining the surface of various materials utilizing nonequilibrium gas plasmas. Nonequilibrium plasmas having depleted degrees of ioni- zation, also known as low-temperature plasmas or cold plasmas, consist of free radi- cals, electronically excited atomic and molecular species, electrons, and ions. These hyperactive plasma species interact with material surfaces nonthermally. It may also be possible that these react with and bond with different substrate surfaces or associate together to produce a very thin layer of plasma and alter consequently the surface phe- nomenon. The plasma-treated nanotubes or nanoparticles with appropriate surface
Fiber Cover Toluene
Collector Reflector
Argon gas Semiconductor
laser Nd:YAG Laser
PC
ICCD Spectrometer
Rotor
Active Carbon Ethanol
Figure 3.4 The schematic illustration of the experimental setup for controllable synthesis of fluorescent nanomaterials.
Source: Adapted from H. Yu, X. Li, X. Zeng, Y. Lu, Chemical Communications, 52 (2016) 819 822.
functionalities can make strong interaction with fluid molecules and thus disperse into the base liquid to create stable suspension. Those nonequilibrium plasmas, that is, low-temperature plasmas, which can be easily created by electrical discharges under reduced pressures (e.g., 10 mTorr 10 Torr), are composed of electronically excited atomic, molecular, ionic, and free radical species. Depending on the composition of gas or plasma chemistry, these hyperactive plasma species react with materials having a clean surface, link to different substrates, or associate to form a thin nanoscale layer of plasma coat and alter consequently the characteristics of the surface[9].
Oxygen plasma is helpful to selectively convert the topmost layer of multilayer samples. Oxygen plasmas generated at both microwave (MW) and radio frequency (RF) are used to treat CNTs. Strong spatially uniform PL can be succeeded in single-layer graphene on substrates by selective plasma oxidation (oxygen plasma treatment). Remarkably, bi- and multilayer flakes remain nonluminescent, while their elastic scattering spectra indicate the formation of sandwich-like structures containing unetched layers [5,10,11]. Briefly, samples of graphene which are formed by micro-cleavage of graphite on a silicon substrate are exposed to oxygen:
argon (1:2) RF plasma (0.04 mbar, 10 W) for increasing time (1 6 seconds). The optical and structural changes were monitored by elastic light scattering and Raman spectroscopy[12,13].
CNPs can be synthesized from benzene and the surface passivation of these nanoparticles is done by grafting primary amine functional groups in one combined step in a plasma reactor having low power supply. The functionalization process is merged into the synthesis process by taking advantage of free radicals generated by a submerged arc helium atmosphere plasma to supply ethylenediamine directly after the plasma to functionalize the CNPs. The dispersibility of those CNPs in aqueous solution is dramatically improved compared to the CNPs solely synthesized from benzene[14].
3.2.1.2 Chemical methods Electrochemical synthesis
This technique is one of the most acceptable top-down methods of producing CQDs with comparatively bigger carbonaceous materials such as graphene, graphite, car- bon fiber, etc. The advantages of the electrochemical method are as follows:
G Ease of the process.
G Abundance of precursor materials.
G Potential for large-scale production.
G Cheap process.
G Greener synthetic protocol.
However, laborious purification processing of formed particles can be considered as a main disadvantage of this method.
Electrochemical synthesis is performed through oxidation reduction reactions in electrolytic cells. Carbon nanocrystals (NCs), which give off strong blue lumines- cence, can be synthesized by electrochemical treatment of MWCNTs. The
electrochemical preparation of carbon NCs is performed in a degassed acetonitrile solution with tetrabutylammonium perchlorate (TBAP) which acts as supporting electrolyte. MWCNTs used here are developed on carbon paper by chemical vapor deposition (CVD) method. In the electrochemical cell, a Pt wire acts as a counter electrode and an Ag/AgClO4wire acts as a reference electrode[5,15].
A cavity microelectrode (CME) is applicable for the synthesis of hybrid compo- sites made of conducting polymer poly(N-methylpyrrole) and CNTs electrochemi- cally. This electrode imparts proper nanometric coating of the polymer on the surface of CNT, without using any additive-like surfactant molecule. The CME helps to characterize the existence of the polymer covering on the CNT surface by the cyclic voltammetry (CV) method[16].
The single carbon source that can be used for the production of fluorescent CDs by one-pot electrochemical carbonization under basic conditions is alcohol having low molecular weight. Here two Pt sheets are used as the auxiliary working electro- des, and a calomel electrode kept on a Luggin capillary is used as the reference electrode. The electrolyte is NaOH/EtOH [17]. The electrochemical oxidation of alcohols in alkaline solution results in the formation of a large number of carbo- nium ions, alkoxy radicals, and hydroxyl free radicals [18,19]. CDs are produced by electrochemical oxidation and dehydration of ethanol at a suitable potential. The applied potential controls the size of the CDs. The higher the applied potential, the greater the number of alcohol molecules oxidized and free radicals and carbonium ions that might be produced to undergo crosslinking and dehydration to form CNPs, leading to larger CDs[17].
Chemical ablation/oxidation
Strong oxidizing acids such as hydrogen peroxide, nitric acid, and sulfuric acid car- bonize small organic molecules to carbonaceous materials, which can be further cut into small sheets by controlled oxidation. Various types of carbon precursors can be used in this method which makes the method more facile and more accessible. But there are some drawbacks of this process such as:
G Harsh reaction conditions.
G Involving more than one step.
G Nanoparticle size is not retained.
The concentration-dependent CDs can be obtained by the reaction of coal pitch powder with the mixed solution of hydrogen peroxide (H2O2) and formic acid (HCOOH) without any heating. But this preparation process is very hazardous if heated due to the liberation of toxic gases. So the small crystalline carbons can be exfoliated from coal pitch with the mixture of H2O2and HCOOH and then can be grown up by their assembly. In a typical experiment, the abovementioned com- pounds are mixed in an open flask by vigorous stirring, and then coal pitch powder of mild temperature is added to the mixed solution. Subsequently, the resultant muddy is stirred for 20 hours without any external heating. Then the unreacted powder is removed by centrifugation, and the suspension is obtained which contains CDs[7,20].
Excitation-independent fluorescent CDs are also synthesized by the oxidation of carbon fibers (CFs) in nitric acid (HNO3) under reflux condition followed by ultra- filtration. The photoluminescence properties of CDs are primarily determined by their size and amount of surface oxidation which can be modified by fluctuating temperature, reaction time, and nitric acid concentration[7,21].
Fluorescent CDs can be synthesized by chemical oxidation of sugarcane carbon followed by exfoliation. Sugarcane dry pulp is one of the vital solid wastes obtained from agriculture. It is useful as a natural source for mass production of carbon- based nanomaterials because it is plentiful, low priced, environment friendly. The sugarcane bagasse pulp is cut into tiny pieces and dried in sunlight for 6 days so that combustion of the pulp occurs at 60C in the open atmosphere to form carbon.
The yielded carbon is added to toluene and continuously stirred for one day at room temperature to achieve complete dispersion. Then, the ultrasonication of the dispersed carbon solution is done for 1 hour followed by centrifugation for 30 min- utes at room temperature. After that the black precipitate is removed and the super- natant liquid is collected. Ethanol is added to dilute the supernatant liquid and the CDs are collected (Fig. 3.5)[22].
3.2.2 Bottom-up approach
Bottom-up approach is suitable for assembly and building short-ranged orders at nanoscale dimensions. Bottom-up or self-assembly nanofabrication methods use physical or chemical forces or forces which operate on the nanoscale to bring together the basic units to form bigger structures. Since the size of components Figure 3.5 Schematic diagram of the synthesis of CQDs from sugarcane bagasse pulp.
Source: Adapted from S. Thambiraj, D.R. Shankaran. Applied Surface Science, 390 (2016) 435 443.
during nanofabrication decreases, the bottom-up approach provides a major alterna- tive to top-down technology.
3.2.2.1 Microwave-assisted method
CQDs can be prepared from organic compounds upon microwave irradiation which is a fast and cost-effective method. Fluorescent self-passivated CQDs can be formed by using PEG as both the carbon precursor and solvent in different atmospheres including O2, CO2, N2, and air by microwave irradiation method (Fig. 3.6). Oxygen can be used in expediting the synthesis of CQDs from PEG and can significantly influence the properties of the CQDs using PEG as a single component precursor without passivation reagent. In the microwave-assisted formation of CQDs, PEG undergoes successive reactions, including dehydrogenation, polymerization, cycli- zation, and aromatization[23]. This process has a limitation in that the size of the synthesized nanoparticles cannot be controlled properly. But still, this method is one of the most convenient methods because of its rapidity, cost-effectiveness, sus- tainability, and scalability.
3.2.2.2 Hydrothermal method
CQDs can be synthesized by low-temperature hydrothermal carbonization which is easily achieved in the one-step green method using the cabbage as the natural car- bon source. The surfaces of procured CQD are rich with a hydroxyl group (OH2) as well as nitrogen and so no further modification is required [24]. The synthesis route is shown in Fig. 3.7. This process is very cheap, rapid, nontoxic, and eco- friendly. But the sizes of the QDs cannot be maintained thoroughly.
Highly green-fluorescent CDs are synthesized by one-step hydrothermal treatment of orange juice at comparatively low temperature (120C) and in less time (150 min- utes). The mechanism for the formation of CDs includes hydrothermal carbonization of the greater constituents of orange juice, mainly glucose, sucrose, fructose, ascorbic acid, and citric acid. The synthesis procedure has been illustrated in Fig. 3.8. The interesting part of this synthesis method is that neither strong acid nor postsynthetic surface passivation is required. The starting material can be obtained from a single
Microwave O2
HO
O H n
HO
O
O O O
OH OH
Figure 3.6 Microwave-assisted synthesis of CQDs from polyethylene glycol.CQDs, Carbon quatum dots.
natural source and the synthesis method is cost-effective and green. As the synthe- sized CDs are functionalized with carboxylic acid, hydroxyl, carbonyl, and epoxy groups, they are highly water soluble without being chemically modified[24,25].
Besides orange juice, waste orange peels are also employed as a carbon source to synthesize fluorescent CDs. The orange peels are processed by the hydrothermal carbonization at a mild temperature (180C). The carbonization process and func- tionalization process take place through the dehydration reaction of the orange peels, which results in the formation of fluorescent carbon particles with very small size (in nm range). The prepared hydrothermal CDs contain a large quantity of oxy- gen functional groups at their surface[26].
Cabbage Cleaning with DI
water
Hydrothermal treatment at 140ºC
for 5hours
Brown solution obtained and filtered
Centrifuge at 12000 rpm for 15 minutes
Dialyzed 48 hour by 1kDa membrane
Figure 3.7 Schematic presentation of the synthesis procedure of CQD using cabbage with the hydrothermal treatment.CQDs, Carbon quatum dots.
Figure 3.8 Illustration of formation of CDs from hydrothermal treatment of orange juice.
Source: S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Chemical Communications, 48 (2012) 8835 8837.
Similarly strawberry juice is treated hydrothermally in one step to synthesize fluorescent nitrogen-doped carbon nanoparticles (FNCNPs) with N2 content of 6.88%. These FNCNPs are used for sensitive and selective detection of Hg21[27].
The hydrothermal approach of grass at 180C also leads to the creation of photolu- minescent, water-soluble, carbon-rich, nitrogen-doped, polymer nanodots which can detect Cu21from water samples[28].
3.2.2.3 Ultrasound-assisted method
The ultrasound-assisted approach for CQDs procurement exploits the ultra- sonic wave of high energy to break carbon compounds into nanoparticles in the presence of alkali, acid, or oxidant, which is regarded as a novel technique of CQDs synthesis. The use of energetic ultrasonic waves bypasses the need for complex posttreatment processes, therefore realizing the effortless synthe- sis of CQDs with smaller sizes. Monodispersed water-soluble fluorescent CNPs are prepared directly from C6H12O6 by a one-step ultrasonic treatment under acidic or alkaline conditions (Fig. 3.9). The hydroxyl groups available on the CNP surface make them water soluble[7,29]. Potato starch can play the role of carbon precursor to produce water-soluble CQDs via an acid-assisted ultrasonic route[30].
High-quality PEG-functionalized functional carbonaceous nanomaterials (FCNs) are synthesized via a dehydration reaction of cigarette ash and thiol group- containing PEG through a facile one-pot ultrasonic irradiation treatment[31]. The synthesis procedure is shown inFig. 3.10.
Fluorescent nitrogen-doped CQDs (N-CQDs) can be produced from dopamine which undergoes ultrasonic treatment in presence of dimethylformamide for 8 hours. The CQDs exhibit water stability and dispersibility, bright fluorescence in different ionic strength and pH, high photostability, and low cytotoxicity[32].
20 CQDs
Filteri Dialysis, 72 HCl+H20 Ultrasonic,
25ºC, 6h
90ºC, Heatin Glucos
Figure 3.9 The synthesis illustration of the CQD.CQDs, Carbon quatum dots.