Green synthesized carbon quantum dots as TiO2
sensitizers for photocatalytic hydrogen evolution
Idris Sargin
a,*, Gizem Yanalak
a, Gulsin Arslan
a, Imren Hatay Patir
baSelcuk University, Faculty of Science, Department of Biochemistry, Konya, Turkey bSelcuk University, Faculty of Science, Department of Biotechnology, Konya, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Carbon quantum dots (CQDs) were prepared from edible mushroom Agaricus bisporus.
A facile and green route was fol-lowed in production of CQDs. CQD characterization by XRD,
FTIR, XPS, TEM, UV-vis and fluo-rescence spectra analysis. CQDs were used as a sensitizer for
TiO2in photocatalytic production
of hydrogen.
Photocatalytic H2 evolution by
TiO2/CQD and TiO2/CQD/Pt was
2129 and 14143mmol g1.
a r t i c l e i n f o
Article history: Received 18 April 2019 Received in revised form 31 May 2019
Accepted 25 June 2019 Available online 17 July 2019 Keywords:
Carbon quantum dot Hydrogen evolution Edible mushroom Green synthesis
a b s t r a c t
Carbon quantum dots (CQDs) have attracted growing interest due to their superior lumi-nescent properties, which make them excellent photosensitizers for TiO2. This study
presents the green-synthesis of CQDs from edible mushroom Agaricus bisporus through microwave irradiation. In the study as-synthesized CQDs were used as a sensitizer for TiO2
in photocatalytic hydrogen evolution in aqueous triethanolamine (sacrificial reagent) under visible-light irradiation. Photocatalytic hydrogen production activity of CQD-sensitized TiO2was found to be 472mmol g1h1(without loading any noble metal
co-catalyst) and 1458mmol g1h1(with loading Pt co-catalyst). The study revealed that the
CQDs from mushroom A. bisporus can be used as an efficient sensitizer for TiO2in
pho-tocatalytic hydrogen production.
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Biochemistry, Faculty of Science, Selcuk University, Konya, Turkey. E-mail address:idris.sargin@selcuk.edu.tr(I. Sargin).
Available online at
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https://doi.org/10.1016/j.ijhydene.2019.06.168
Introduction
Energy systems based on hydrogen have emerged as prom-ising techniques in the field of renewable energy [1e3]. Semiconductor-based photocatalytic hydrogen production is widely acknowledged as an efficient and environmentally friendly system to solve global energy crisis[4,5]. Since the discovery of this system by Honda-Fujishima in 1972 [6], thousands of semiconductor materials have been developed as photocatalysts for photocatalytic hydrogen production. TiO2is one of the photocatalysts commonly used in
photo-catalytic hydrogen generation and stands out from the other semiconductors due to its chemical stability, simple synthesis route, earth-abundance, nontoxicity, low cost and energy levels[7]. However, the lower catalytic efficiency of bare TiO2
is a major drawback, which is attributed to fast recombination of the photogenerated charge carriers and low light harvesting performance because of its wide band gap (3.2 eV). Therefore, it is a challenge to enhance photocatalytic properties of TiO2if
it is used as a photocatalyst in a wide range of the solar spectrum especially in the visible region. Doping[8], use of sensitizers[9]and modification or combination with nano-particles [10] are widely employed to improve the photo-catalytic performance of TiO2.
Literature reports demonstrated that composite systems with TiO2 and nanoparticles are efficient photocatalysts for
photocatalytic hydrogen evolution [11,12]. Studies on com-posite systems with TiO2and carbon based materials such as
carbon nanotubes, fullerenes, graphenes and CQDs are encouraging[13e15]. CQDs, in particular, exhibit distinct op-tical and electronic properties suitable for photocatalytic systems but these studies are limited in number[16,17]. Pho-toluminescent properties of CQDs are generally attributed to a large amount of‒OH and ‒COOH and ‒NH2surface functional
groups. These surface groups are responsible for water solu-bility and play roles in functionalization of CQDs[18,19].
The synthetic methods of CQDs can be classified under two groups; “top-down” and “bottom-up” routes [20]. CQDs are produced using methods involving hydrothermal/sol-vothermal treatment, ultrasonic treatment, microwave irra-diation, laser ablation, electrochemical carbonization[21]. Qu et al. (2018) prepared CQDs as a co-catalyst on the surface of KNbO3and the CQDs/KNbO3photocatalytic system showed
photocatalytic activity in degradation of organic pollutants with simultaneous hydrogen evolution[22]. Saud et al. (2015) investigated the photocatalytic and antibacterial properties of CQDs/TiO2 nanofibers [23]. Zhang et al. (2013) reported the
production of CQDs through electrochemical etching method. The authors used the CQDs as light converters in the near-IR regions to improve the photoelectrochemical properties of CdSe/TiO2[24]. Zhang and co-workers (2013) designed CQDs/
TiO2 nanotube photoanodes to investigate the
photo-electrochemical properties in the visible and NIR regions. The hydrogen evolution rate was found to be 14.1 mmol h1and the photocurrent density of the CQDs/TiO2nanotube
photo-anode was recorded four times larger than the bare TiO2
nanotube photoanode[25]. In a study by Yu et al. (2014) CQDs/ P25 composite was prepared through the hydrothermal
method and the photocatalytic hydrogen activity of the composite was reported. According to the report, the CQDs acted as a photosensitizer like dye under visible light irradi-ation, and they also played role as an electron reservoir under UV-Vis light irradiation for hydrogen evolution[17]. Bian and co-workers (2014) reported the production of C dots that were used as a sensitizer to improve the photoresponse of TiO2
nanorod arrays for photoelectrochemical systems. The photocurrent density of carbon dot-modified TiO2was about
25.2% when compared to the bare TiO2 [26]. Yang and
co-workers (2015) synthesized Ni/CQD hybrid system as an electrocatalyst for hydrogen evolution reaction and reported high stability and catalytic activity due to the effect of syner-getic contribution of CQDs and Ni nanoparticles[27]. Carbon dots with TiO2were combined using hydrothermal treatment
and their photocatalytic properties were reported by Ming et al. (2012)[28].
Numerous studies demonstrated that the microwave-based synthesis of CQDs is superior to the hydro/sol-vothermal, electrochemical and pulse-laser based methodol-ogies due to its energy-efficient, time-saving and pollution-free properties [29]. In addition, through microwave treat-ment CQDs from natural sources like microorganisms, en-zymes and plants are easily produced without using any chemicals [30,31]. Recent studies with mushrooms, though limited in number, clearly showed that mushrooms can be exploited as a carbon source for production of CQDs. Mush-rooms with high phenolic compounds, flavonoids, poly-saccharides, lipids and protein contents[32]are favourable for synthesis of CQDs with high number of heteroatoms (N, S) on the surface, which is required for high quantum yield[33].
In this work Agaricus Bisporus, an edible mushroom[34], was used as a carbon source for microwave-assisted synthesis of CQDs. The CQDs were used for the sensitization of TiO2in
the photocatalytic hydrogen production experiments. The hybrid system of TiO2/CQD was synthesized by using
hydro-thermal treatment. The structures of CQDs and TiO2/CQD
were examined using Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). UVeVis absorption and photoluminescence spectra analysis of the CQDs were also performed. The photocatalytic activity and photo-electrochemical properties of TiO2/CQD were studied under
visible light irradiation.
Experimental
Materials
Edible mushrooms (Agaricus bisporus) were purchased from a local market. The mushrooms were rinsed with distilled water and dried at ambient temperature. Dried mushrooms were ground to powder and then used in the production of CQDs. TiO2powder (Degussa P25), triethanolamine (TEOA, 97%) and
ethanol were supplied by Sigma-Aldrich. Hydrochloric acid, nitric acid and hydrogen peroxide solution (30%) was obtained from Merck (Germany).
Production of CQDs
Optimization tests were done to determine the optimum operational conditions. In the tests to determine the solvent type, 5.0 g of mushroom in 50 mL of water was microwave-irradiated for 5 min at three different microwave energy levels (400, 800 and 1600 W). The same procedure was fol-lowed for two other solvents; hydrogen peroxide solution (15% by volume) and aqua regia solution (50% by volume). The CQDs produced in hydrogen peroxide solution at 400 W irra-diation energy exhibited higher fluorescence intensity (exci-tationlexat 365 nm and emissionlemat 530 nm). Then, the
other operational conditions were studied; the ratio of hydrogen peroxide to water: from 0% to 30%; the amount of dry mushroom: 1.0e9.0 g and the time of microwave irradia-tion: 1e15 min tests (Fig. S1).
The synthesis of CQDs was performed at the specified conditions according to the results of the optimization ex-periments: 5.0 g of mushroom powder in 50 mL of hydrogen peroxide solution (15%) was irradiated by microwave for 7 min (at 400 W). Subsequently, the crude solution was centrifuged at 4500 rpm (3260 g) for 30 min, the supernatant was recov-ered, filtered using a Millex-GP Syringe Filter Unit, 0.22mm, (Millipore Co., Cork, Ireland), dialysed against distilled water in dialysis tubing (Viskase Sales Corp., Seamless Cellulose Tubing, Size: 16/32, Lot: 208001) (the dialysis medium was constantly refreshed with distilled water to get rid of any H2O2
or free radicals) and finally freeze-dried. Synthesis of the TiO2/CQDs heterostructure
The TiO2/CQD composites were prepared using CQDs and TiO2
as a raw material and water/ethanol mix solution as a solvent. In a typical synthesis, 4 mL of CQDs (2 mg mL1) solution and 0.4 g of TiO2were dissolved in 20 mL distilled water and 6 mL
ethanol solution according to a literature report[17]. To obtain a homogeneous mixture distribution, the solution was stirred vigorously for 4 h at room temperature. The mixture was then transferred into a Teflon-lined autoclave reactor and heated at 140C for 4 h. After the cooling to the room temperature, the obtained TiO2/CQD composite was collected with
centrifuga-tion (at 4500 rpm for 10 min) and thoroughly washed with distilled water three times and left to dry overnight.
Photocatalytic and photoelectrochemical hydrogen evolution tests
The typical photocatalytic hydrogen evolution experiment was carried out in a Pyrex flask with a total volume of 135 mL and sealed with rubber septum for sampling. Firstly, TEOA solution (0.33 M) was prepared with water and pH of the so-lution was adjusted at different values (from 7 to 10). After deoxygenation process with nitrogen for TEOA, 10 mg of TiO2/
CQD (or TiO2/CQD/Pt) was added into 20 mL of TEOA solution
in the glovebox. Herein, we also used Pt co-catalyst to enhance the hydrogen evolution rate of TiO2/CQD photocatalyst. In this
case, 0.25 mM of chloroplatinic acid (H2PtCl6) was added to
reaction cell and Pt metal catalyst photodeposited on the TiO2/
CQD to form TiO2/CQD/Pt[35]. Prior to the irradiation, the
mixed solution was sonicated to ensure a complete dispersion
of the content. The prepared photocatalytic system was irra-diated by using 300 W Xe lamp equipped with al 420 nm cuteoff filter under constant stirring. Gas chromatography was used to detect the photogenerated hydrogen amount in the sample.
The photoelectrochemical hydrogen evolution reaction was measured by using linear sweep voltammetry (LSV) and chronoamperometry methods run under on/off visible light illumination (l 420 nm). Constant potential (0 V) was applied in the chronoamperometry method. The pure TiO2and
CQD-modified TiO2were studied using CH Instruments 760D
elec-trochemical working station by glassy-carbon as a working electrode, Ag/AgCl and platinum wire as a reference and counter electrode, respectively, in combination with TEOA and Na2SO4as the electrolyte solution.
Instrumentation
TEM images of the CQDs were obtained using JEOL JEM-2100 (UHRe Ultra High Resolution) instrument. XRD patterns of the CQDs were recorded on a Bruker D8 Advance X-ray diffractometer; 2q angle was scanned between 0and 90 at
scan rate of 3min1. FT-IR spectra of the CQDs were recorded on a Bruker Vertex 70 FT-IR spectrometer in the range of 4000 and 500 cm1. XPS analysis was performed on a PHI 5000 VersaProbe Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer. A microwave oven (MARS CEM) was used for microwave irradiation. UVeVis absorption spectra of the CQDs in water were obtained on a spectrophotometer (Shi-madzu UV1800 UVeVis absorption spectrophotometer). Pho-toluminescence of the CQDs was studied on a Perkin Elmer LS 55 instrument (Perkin Elmer, Cambridge, UK); aqueous solu-tions of the CQDs were excited at 365 nm (lex) and the
corre-sponding fluorescence emissions were recorded. Photocatalytic hydrogen evolution activity of the CQDs under the visible light irradiation was investigated on a Solar Light XPS-300™ (with cut-off filter l 420 nm). An online gas chromatography system was used to determine the amount of hydrogen evolved in the system (Shimadzu GC-2010 Plus, molecular sieve 5A, thermal conductivity detector (TCD) and argon as the carrier gas).
Results and discussion
Optimization of the synthesis of CQDs from mushroomA. bisporus
The results of the optimization tests are given inFig. S1. Upon determining the solvent and the irradiation energy, i.e., hydrogen peroxide solution and microwave irradiation energy of 400 W (Fig. S1a), the water/hydrogen peroxide ratio was studied and the ratio of 1:1 by volume led to the highest fluorescence intensity (Fig. S1b). Then, the varying amount of mushroom (1.0, 3.0, 5.0, 7.0 and 9.0 g) in 50 mL of hydrogen peroxide solution (15%) was irradiated for 5 min and the highest fluorescence was recorded for 5.0 g of mushroom (Fig. S1c). Finally, 7 min of irradiation of 5.0 g of mushroom in 50 mL of hydrogen peroxide solution (15%) at 400 W gave the best result (Fig. S1d).
Characterization of the CQDs
TEM images of the CQDs and TiO2/CQD composite
TEM micrographs of the CQDs (Fig. 1a) and the TiO2/CQD
composite (Fig. 1b) are presented (Please also refer toFig. S2). The CQDs (with size lower than 20 nm) were nearly spherical in shape. They had graphite-like crystalline structure; the arrow inFig. 2points the lattice fringes in the structure of the CQDs. The observation of lattice fringes indicated that the CQDs had also crystalline cores[36,37].
XRD pattern of the CQDs and the TiO2/CQD composite
In the X-ray diffraction pattern of CQDs the peak at approxi-mately 2W ¼ 16was attributed to the presence of amorphous
carbon (Fig. 3a). The peaks observed at about 22 and 26 showed that the CQDs had graphitic crystallinity[36,38].
In the X-ray diffraction pattern of the TiO2/CQD composite
the peaks at 2W ¼ 25.7, 38.2, 48.4, 54.3 and 63.0indicated the
anatase phases of TiO2(Fig. 3b). XRD survey spectrum of the
TiO2is presented inFig. S3. Comparison ofFig. 3b andFig. S3
also showed the anatase phases of TiO2. Rutile peaks were
also observed at 2W ¼ 26.5, 54.2 and 63.0[39,40]. The presence
of CQDs did not affect the crystalline structure of TiO2. This
could be due to the low content of CQDs. FT-IR spectrum analysis of the CQDs
In the FT-IR spectrum (Fig. 4), the broad absorption peak at 3274 cm1and the shoulder at about 3370 cm1are attributed to the stretching vibrations of OeH and NeH. The absorption band at 1622 cm1is assigned to C¼C stretching vibration and the band at 1072 cm1to CeO stretching vibration. The ab-sorption bands at 2937 and 2853 cm1 are assigned to the asymmetric and symmetric stretching vibrations ofeCH2. The
band at 1385 cm1is assigned to the bending vibrations of CeH and at 1196 cm1to CeN[36,41]. The band at 629 cm1
corresponds to SeC[37].
XPS spectrum analysis of the CQDs and the TiO2/CQD
composite
In the XPS survey spectrum of the CQDs three major peaks corresponding to C1s, O 1s and N 1s and minor peaks for P 2s, P 2p, Si 2s, Si 2p and O 2s were observed (Fig. 5). Their atomic percentages were recorded as; C 1s: 71.7; O 1s: 21.8; N 1s: 4.8; Si 2p: 1.1 and P 2p: 0.6%. The results showed that the CQDs from A. bisporus consisted mainly of carbon, oxygen and nitrogen atoms. The CQDs were also rich in oxygen and were N- and
P-doped. Nitrogen and phosphorus content can be ascribed to the protein and the nucleic acid contents of the mushroom. The peak at 99 eV was from the silicon that was used as a substrate in the preparation of sample. Deconvolutions of the high resolution of C 1s, O 1s and N 1s signals are presented in
Fig. 5. The peaks correspond to the groups that were observed in the analysis of FT-IR spectrum of the CQDs.
In the XPS survey spectrum of TiO2/CQD composite three
major peaks were observed; O 1s (56.2%), C 1s, (25.1%) and Ti 2p (18.7%) (Fig. 6). High resolution C 1s, O 1s and N 1s signals were fitted into deconvoluted peaks. The assignment of the peaks are given inFig. 6. Compared to the bare CQDs, lower binding energy values were observed for TiO2/CQD composite.
For example, the C 1s signal was fitted into three deconvoluted peaks at 283.1, 285.3 and 286.6 eV. In case of the composite, the deconvolution of the C 1s signal was fitted into two peaks at 283.0 and 285.0 eV. A similar observation was recorded for O 1s signals; O 1s signal of the CQDs: 530.0, 531.1 and 532.1 eV, O 1s signal of the TiO2/CQD composite: 528.0 and 530.0 eV.
Inter-action of the CQDs with TiO2led to a shifting in the binding
energy values of C 1s and O 1s. This shifting can be due to the transfer of the electron pairs from the CQDs to the vacant d-orbitals of Ti. Two binding energy values at 456.7 and 462.4 eV of Ti were assigned to the 2p1 and 2p3 states of Ti4þ[42]
(Please refer toFig. S3for XPS survey spectrum of TiO2).
Fig. 1e TEM images of the CQDs from mushroom A. bisporus (a) and TiO2/CQD composite (b).
Fig. 2e HR TEM image of the CQD from mushroom A. bisporus; the arrow points the lattice fringes in the structure of the CQDs, which is an indication of the presence of crystalline cores.
UV-Vis absorption and fluorescence spectra of the CQDs In the UV-Vis absorption of the CQDs the maximum peak at 217 nm was attributed to II-II* transition of conjugated C¼C units (Fig. 7)[37]. UVevis absorbance spectrum of the CQDs
showed a broad absorbance peak. This broad absorbance to 600 nm can ascribed to the presence of multiple energy absorbing functionalities on the CQDs' surface as reported by Pacquiao et al. (2018)[37]. Excitation of aqueous solution of the CQDs at 345 nm led to the highest emission peak at 427 nm. The CQDs also showed the typical behaviour of CQDs by producing excitation-dependent fluorescence emission [21]. Excitation wavelength was increased from 305 to 425 nm and the corre-sponding fluorescence emission was recorded at higher wave-length by shifting from blue to yellow. When the CQDs were excited at 425 nm, the maximum of corresponding emission shifted to 506 nm. Addition of TiO2to CQD, the intensity of
fluorescence emission of CQD decreased as depicted inFig. S4. Photocatalytic and photoelectrochemical hydrogen evolution The photocatalytic hydrogen evolution experiments were carried out by using CQDs sensitized TiO2 (TiO2/CQD) as a
photocatalyst and TEOA as a sacrificial agent. First, to deter-mine the optimum pH, photocatalytic measurements were carried out from pH 7 to 10. pH 9 was found to be the optimum Fig. 3e X-ray diffraction patterns of the CQDs from A. bisporus mushroom (a) and the TiO2/CQD composite (b).
Fig. 4e FT-IR spectrum of the CQDs from A. bisporus mushroom.
Fig. 5e XPS survey spectra of the CQDs from A. bisporus mushroom and the high resolution XPS spectra of C 1s, O 1s and N 1s signals (Red lines are the cumulative fit peaks). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
value for hydrogen evolution as shown inFig. S5. Also, TiO2/
CQD photocatalyst was tested for hydrogen evolution reaction in the presence or absence of Pt co-catalyst under visible light irradiation (l 420 nm). As seen inFig. 8a, when we compared hydrogen evolution performance of the TiO2/CQD
photo-catalyst, the hydrogen amount was recorded as 1458mmol g1
(with Pt co-catalyst) and 472mmol g1(without Pt co-catalyst). The rate of hydrogen evolution was increased by using Pt co-catalyst. The photodeposition of Pt metal catalyst, which was obtained by reducing H2PtCl6, was conducted to achieve
deposition of metallic platinum over TiO2/CQD surfaces. After
8 h, the photocatalytic hydrogen evolution for TiO2/CQD and
TiO2/CQD/Pt composites reached to 2129 mmol g1 and
14143mmol g1, respectively. No hydrogen gas was detected
using only either CQD or TiO2in this photocatalytic HER
sys-tem. The reusability of TiO2/CQD/Pt composite was also
investigated for the photocatalytic hydrogen evolution (Please refer toFig. S6). As depicted inFig. S6, the catalyst CQDs/TiO2/
Pt can be reused without diminishing activity.
As shown inFig. 8b, the photoelectrochemical activity to-wards HER was evaluated by chronoamperometry (CA) and linear sweep voltammetry (LSV) (Fig. S7) in 0.1 M Na2SO4and
0.3 M TEOA aqueous solution as an electrolyte and electron donor, respectively. For comparison, bare TiO2and TiO2/CQD
electrodes were tested similarly by the chronoamperometry method. Herein the bare TiO2 had very low photocurrent
density as shown as blue line inFig. 8b. However, TiO2/CQD
electrode had much higher photocurrent density than that of Fig. 6e XPS survey spectrum of the TiO2/CQD composite and the high resolution of XPS spectra of C 1s, O 1s and Ti 2p1/3
signals (Red lines are the cumulative fit peaks). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7e UV-Vis absorption (a) and fluorescence emission spectra of the CQDs from A. bisporus mushroom at varying excitation wavelength (b).
bare TiO2under the same conditions (red line inFig. 8b). The
enhanced photocurrent density of TiO2/CQD was proportional
to the rate of photocatalytic reactions, which was also attrib-uted to the synergic effect between the CQDs and TiO2,
facil-itating photoinduced charge carriers separation effectively
[21,22]. Moreover, when the light source was turned on, the photocurrent density of TiO2/CQD increased rapidly.
Never-theless, the formation of CQDs aggregates could block the active sites of TiO2photocatalyst, which led to a decrease in
the photocurrent value[21].
The photocatalytic system of TiO2/CQD for hydrogen
evo-lution can be explained by the electron transfer mechanism. Electrochemical band levels of TiO2/CQD and Pt are shown in Fig. 9. Under the visible light irradiation (>420 nm), CQDs play a key role as a photosensitizer. Firstly, the electrons of the II conjugated CQDs are excited from the valance band to the conduction band, which is started by absorption of photon. Then, the photogenerated electrons are donated to the con-duction band levels of TiO2, leading to the photocatalytic H2
production. The electron donor TEOA is also used as a hole scavenger to supply the regeneration of the system[17,43].
Conclusions
This study revealed that photocatalysis in the hydrogen evo-lution systems comprising CQDs as sensitizer are more effi-cient. The CQDs were easily prepared from the edible mushroom A. bisporus using a microwave-assisted method. As-synthesized CQDs were used for the sensitization of TiO2.
TiO2/CQD system showed a better photocatalytic hydrogen
evolution performance over the bare TiO2 system. The
hydrogen evolution rate is reported as 472mmol g1h1for the
TiO2/CQDs. Also, the addition of Pt catalyst resulted an
in-crease in the catalytic activity with 1458mmol g1h1, when
compared to a non-catalysed reaction. Under the illumination of sunlight, the photocurrent density of the TiO2/CQD was
approximately 140 times larger than that of TiO2. Preparation
and application of CQDs from renewable sources will likely continue to attract research interest in the renewable energy field. Further studies are needed especially in the of field hydrogen evolution by the photocatalytic systems with CQDs.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.ijhydene.2019.06.168.
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