Quantum dots as enhancer in photocatalytic hydrogen evolution: A review

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5

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Review Article

Quantum dots as enhancer in photocatalytic hydrogen evolution: A review Debasmita Kandi, Satyabadi Martha**, K.M. Parida* Centre for Nano Science and Nano Technology, Institute of Technical Education and Research, Siksha ‘O’ Anusandhan University, Bhubaneswar, 751030, India

article info

abstract

Article history:

Advanced energy conversion processes like photochemical and photoelectrochemical

Received 11 November 2016

water splitting now a day plays a very important role in challenging the present energy

Received in revised form

crisis of our world. The successful utilization of this process depends on development of

13 February 2017

highly efficient, more stable, low cost and outstanding environmental benign semi-

Accepted 23 February 2017

conductor materials. From recent advancements, it is revealed that quantum dots (QDs)

Available online xxx

are very outstanding and promising material for the mentioned processes due to their favorable physical and chemical characteristics like high absorption co-efficient, quantum

Keywords:

confinement effect, thermal, chemical, mechanical and optical stability, high conductivity

Quantum dots

and recyclability. In this review article, we have clearly explained the importance of QDs in

Enhancer

water splitting along with the general mechanism involved in the process. Following that

Hydrogen

the enhancement of different materials like metal oxides, layered double hydroxides (LDH),

Visible light

carbonaceous materials (g-C3N4, benzene and benzene like materials) by QDs have discussed in the field of water splitting. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of QDs for photocatalytic water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhancement of photocatalytic activity of different semiconductors by QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhancement in hydrogen generation of metal oxides by QDs modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhancement in hydrogen generation of LDHs by QDs modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhancement in hydrogen generation of g- C3N4 by QDs modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enhancement in catalytic activity of graphene and graphene like materials by QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00

* Corresponding author. Fax: þ91 674 2350642. ** Corresponding author. Fax: þ91 674 2350642. E-mail addresses: [email protected] (S. Martha), [email protected], [email protected] (K.M. Parida). http://dx.doi.org/10.1016/j.ijhydene.2017.02.166 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

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Introduction Nanoscience has become one of the most intensely studied areas of research over the last few decades. In the last three decades quantum dots as a very effective nanomaterial have drawn much attention in the field of various research and development like electrocatalysis, LEDs, solid state lighting, displays, Infrared photodetectors, photovoltaics, transistors, quantum computing, medical imaging, biosensors and many others [1e6]. QDs have also been established as a very efficient material (enhancer) in the field of photocatalytic water splitting. QDs are zero dimensional semiconductor nano crystals or nano crystallites usually composed of group IIeVI, IIIeV or IVeVI elements and are defined as particles with physical dimension smaller than the exciton Bohr radius or de-Broglie's wavelength. But now-a-days QDs of elements like carbon [7], silicon [8], bismuth [9] and noble metal [10] are established. Small QDs such as colloidal semiconductor nano crystals can be as small as 1e10 nm which contains 10e50 atoms in diameter and 100e100,000 atoms within the quantum dot volume [11]. Quantum dot is considered to be zero dimensional and named as dot because the motion of electrons, holes and excitons is restricted in all 3 dimensions. As a result of which a quasi-zero dimensional system is formed. The word ‘quantum’ is included in its name because one property of it is analogous to quantum mechanical particle in a box in which the energy is inversely related to the size of the box. In the same manner, the band gap energy of semiconductor nanocrystal (QD) increases as the particle size decreases. In other words the electron and hole energy states within the nanocrystals are discrete and band gap is a function of QD diameter. QDs are also known as “artificial atoms” because it shows the real atom behavior i.e. the electrons are confined to quantized states with discrete energies. Because of quantization there is enhancement of band gap as compared to bulk semiconductors. The difference between atom and QD is that QDs consists of hundreds or thousands of atoms having different shape, size and energy. Various reviews have been reported on synthesis methods, characterization, surface chemistry, nature of capping agents and optoelectronic properties of QDs [12e15]. Therefore; we limit our domain to depth discussion on modification of certain semiconductor materials by QDs and cover one very crucial photocatalytic application i.e. water splitting. This review has evidently clarified the importance of QDs in photocatalytic water splitting on the basis of Gerischer theory [16]. In the next section, the role of QDs for the improvement in catalytic activity of different semiconductors and carbonaceous materials has been discussed. The semiconductor materials included here are metal oxides, layered double hydroxides, and the carbonaceous materials are g-C3N4, benzene and benzene like materials. We have focused on the recent developments in the utilization of QDs as sensitizer and the mechanism of water splitting associated with above mentioned materials. For the first time, our review represents a different angle of QDs which is brand spanking new and it is expected that this review will try to make up the deficiency of current energy status. Desirably, new perspectives towards

research and developments of QDs will be accessible in a productive manner with the help of this review article. Many semiconductor photocatalysts have been developed for photocatalytic as well as photoelectrochemial water splitting but among them the role of quantum dot is very idiosyncratic. Hence the benevolent use and importance of quantum dots in water splitting is described in the following section of our review.

Importance of QDs for photocatalytic water splitting There are many efforts devoted in the last decades for utilizing quantum dots in photocatalysis. Ultra small sized quantum dots are widely delved as sensitizer because of their high extinction coefficients, quantum confinement effect and large intrinsic dipole moments. The beneficial effects of QDs in water splitting are shown pictorially in Fig. 1(a). In the field of photocatalytic water splitting the former two properties plays very vital role. In comparison to other semiconductors, QDs have high value of extinction coefficients in visible region of solar spectrum i.e. maximum probability of minimizing the recombination of charge carriers and reinforcement of charge injection processes. It can be explained as bulk semiconductor materials show weak interaction with photons while very small amount of QDs have more capacity to interact with photons resulting maximum flux of photons. This is because QDs can emit up to 3 electrons per photon of solar radiation whereas it is only one electron in case of semiconductor bulk materials. This multiple exciton generation property is only possible due to quantum confinement effect of QDs and is noticed only when the particle size is very small enough with widening of band gap and the energy level spacing is more than kT where the symbols carry their usual meaning, k is Boltzmann's constant and T is temperature. According to Gerischer theory [16], band gap of the QDs decreases with increasing size which opens up the path to cover the entire solar spectrum. Additionally, in case of large band semiconductor materials the transfer of electrons from CB of QDs to that of semiconductor can be made thermodynamically favorable and is shown in Fig. 1(b). This is because the CB edge position can be moved towards more negative potential and VB edge position towards more positive potential with decreasing the particle size or increasing the band gap (Fig. 2). As a consequence the free energy change required for migration of electron from QD to semiconductor decreases and the thermodynamic driving force increases which increases the interfacial charge transfer. Homles et al. [17] has proved this charge transfer with CdSe QDs by plotting a graph between normalized photocatalytic proton reduction rates with respect to band gap on logarithm scale results a logarithm dependence relationship between these two parameters. This relation is also supported by Gerischer theory but at illuminated semiconductoreelectrolyte surface. As per this theory kRed

" # ðDG  lÞ2 a exp 4kTl

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

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Fig. 1 e Schematic presentation of (a) the beneficial effects of QDs sensitized photocatalysts for water splitting and (b) QD sensitized photocatalysis.

Fig. 2 e Quantum size effect in nanocrystal [17].

Where, DG is the difference between the CB edge potential and the proton reduction potential. DG ¼  e (ECB E0 Red), E0 Red ¼ constant at a certain pH In addition to this, EG a ECB Thus kRed a exp (EG) So it can be concluded that with the gradual increase of band gap there is shifting of the conduction band edge position more towards negative reduction potential and increase of thermodynamic driving force which shows faster rate of reduction reaction. That means small QDs can rapidly transfer electrons in comparison to other semiconductors. Summarily thanks to quantum confinement effect i.e. the optical properties are size dependent provided the particle size is smaller than exciton Bohr radius. These effects will likely be used more often in advanced solar water splitting devices. These advantages furnished due to high exciton coefficient and quantum confinement effect of QDs for harvesting visible light radiations and exploiting the charge carriers makes them a most promising and eco-friendly candidate for splitting of water in the visible region. Utilizing these great advantages of QDs as sensitizer many research groups have improved the catalytic activities of

semiconducting as well as conducting materials in the field of photochemical and photo electrochemical water splitting which is discussed in the following section.

Enhancement of photocatalytic activity of different semiconductors by QDs In the first step of photocatalysis, absorption of photon by the catalysts excite electrons, hence it is necessary to improve their solar photon absorption capacity. For this purpose, researchers have reported various classes of semiconductor materials, among them metal oxides, layered double hydroxides and g-C3N4 show better results for water splitting. But their wider band gap, poor carrier mobility, poor light absorption performance of the developed materials limits the photocatalyic activity in the visible solar spectrum. Thus it is very vital to develop narrow band gap catalysts that can cover visible spectrum of sunlight. Various techniques like doping of metal cation and anion, formation of solid solution, making composite, developing mesoporous materials etc. have been developed to enhance the catalytic activity of such photocatalytic materials. But in recent years QDs have drawn much attention as an efficient sensitizer in the successful splitting of

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water. In order to amplify the quantum efficiency of various semiconductors, research community is now utilizing QDs vigorously as enhancer. In our review we are trying to cover the improvement of catalytic activity of some semiconductors such as metal oxides, LDHs, g-C3N4 by compositing with QDs. In addition to these semiconductors, we also include a good conductor i.e. graphene. It is diagrammatically presented in Fig. 3 and for convenience of readers, Table 1 contains list of various QDs with their method of preparation, quantum yield and application.

Enhancement in hydrogen generation of metal oxides by QDs modification Generally metal oxides (MO) nanomaterials are used in photocatalytic and PEC hydrogen generation which show splendid mechanical flexibility, high specific area and chemical stability. One of the major shortcomings of most MOs absorb in UV region and very few in visible region. Hence in order to make them as good photocatalysts for splitting of water under visible radiation, band gap modulation is very necessary. Generally the lowest unoccupied molecular orbital of metal cations make the conduction bands of oxide semiconductors and O 2p orbitals make the valence band. Thus in order to make narrow band gap in metal oxide based system a new valence band or an electron donor level (DL) of elements excluding O 2p orbitals must be created as conduction band level never be lowered [18]. To make it visible light active several attempts have been taken; doping of metal [19e23], doping of non-metal [24e39], co-doping of metal and nonmetal [40e42], combining the large gap semiconductor with low band gap material [43], single phase material [44,45], and quantum dots sensitization [46,51e59]. It has been seen that there is significant enhancement of photocatalyic activity of metal oxides after sensitized with

Fig. 3 e Schematic illustration of QDs and QDs modified materials.

QD. Lian et al. [46] have developed a unique structure i.e. plasmonic silver quantum dots with size of 1.3e21.0 nm coupled with hierarchical TiO2 nanotube arrays (H-TiO2NTAs) photoelectrodes. The photo conversion efficiency has been increased significantly and the H2 evolution rate is found to be approximately 124.5 mmol cm2 h1 and shows photocurrent density of 0.104 mA/cm2 at a potential of 0.7 V versus saturated calomel electrode (SCE). Under visible light irradiation, the excited hot electrons from Ag QDs (sensitizer) are transferred to the CB of H-TiO2-NTA surmounting the Schottky barrier. In the meantime, it is expected that excited electrons move along the wall of TiO2 nano tube arrays, which furnish the fast transfer paths. In addition, the fast transfer of electrons has been achieved by the ultra-small sized Ag QDs. The external bias voltage motivates the electrons to go from photoanode to cathode and produce H2 on the surface of platinum foil (cathode) meanwhile the holes are utilized in various oxidation reactions like the conversion of ethylene glycol to glyoxal, oxalic acid or other additional products. With some modifications Lee group [47] have designed and synthesized a photocatalysts by coating a thin layer of amorphous TiO2 (a- TiO2) on CdSe NC QDs (2.5 nm) for hydrogen evolution from water and 436 mmol/g$h amount of hydrogen have been liberated. This coreeshell hetero structure facilitates the migration of electron from the conduction band of CdSe core to the CB of a-TiO2 whereas the holes are localized to the valence band of CdSe NCs. The migrated electrons are utilized in the reduction of water to produce hydrogen. It is examined that high potential level of CB of smaller QDs facilitates rate of electron transfer and consequently speed up the photoreduction of water. Hensel et al. [48] have observed that CdSe QD sensitized N-doped TiO2 nanostructures shows greater photo response in comparision to N-doped TiO2 (TiO2: N) or CdSe QD sensitized TiO2. QD sensitized TiO2 shows photocurrent density of 2.75 mA/cm2 which is higher than pristine TiO2 or TiO2: N. The reason behind the enhancement is due to intense absorption of visible light by small sized CdSe QDs (average diameter of 2.6 nm). The mechanism can be understood by the following model shown in Fig. 4. It is proposed on the basis of electronic states of various constituents of the nanocomposite. Since the conduction band of bulk CdSe is more negative i.e. 0.8 eV versus normal hydrogen electrode NHE [49] than that of the TiO2 i.e. at 0.5 eV versus NHE [50] so electrons are transferred efficiently from CB of CdSe QD to that of TiO2. The CB of CdSe QD position can be made more negative by decreasing the size of particle which is according to quantum confinement effect [51,52]. As shown in Fig. 4 electrons are jumped from VB of CdSe QDs to some intermediate steps i.e. V0 of TiO2. As a result the charge separation and photocurrent efficiency is increased tremendously due to longer lifespan of electrons and holes [53,54]. Wang et al. [55] have fabricated CdS and CdSe QD cosensitized ZnO nanowire arrayed photoanode. This nanostructure shows high IPCE of ~45% at 0 V vs. Ag/AgCl as it exhibits strong absorption sensitized photoanodes (CdS-ZnOZnO-CdS and CdSe-ZnO-ZnO-CdSe). This is due to the suitable band alignment of the QDs and the ZnO NWs which facilitates efficiently the migration of photoinduced electrons from the QDs to the NWs. The Fermi levels of CdS, CdSe QDs and ZnO NW are aligned and conduction band positions of the QDs are

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

Sl.no a

1

QD

Method

Ag QD/H- TiO2-NTAs composites

electrochemical anodic oxidation and electrodeposition method

2

CdSe QD/a- TiO2

a

3

CdSe QD sensitized N-doped TiO2

Ligand exchange and reflux method CBD and hydrothermal method

a

4

CdS and CdSe QD co-sensitized ZnO nanowire

Hydrothermal and CBD method

a

5

CdSe QD-sensitized Au/TiO2 nanocomposite

6 a 7

Pt-(CdS QD/TiO2) film CdSe/H- TiO2

emulsion-based bottom-up selfassembly (EBS) method and CBD method Doctor blade and CBD method e

8

CdS QD-g-C3N4 composite

Chemical impregnation method

9

CdS/g-C3N4 composites

Solvothermal method

a

Chemical etching

11

Si quantum dots/TiO2 Nanotube Arrays Composite Electrodes ZnS/CdSe/CdS QDs

12

ZnO/CdS heterostructures magnetically cilia film

Hydrothermal method

13

QDs-Cu2S/BiOBr

Precipitation method

14

InN/InGaN QDs

15

CdS QD/Cd2SnO4

Plasma-assisted molecular beam epitaxy at 460  C Solution combustion method

a

Graphene QD @ ZnO nanowires (GQDs@ZnO NWs) (i) CuInS2 QD (ii) TiO2/CuInS2 QDs/CdS

Chemical oxidation and hydrothermal method Solvothermal synthesis

18

g-C3N4 quantum dots

Thermal-chemical etching process

a

CdS/Cu2S co-sensitized TiO2 branched nanorod arrays

SILAR method

10

16

17

19

SILAR and CBD method

Light source/reaction solution 300 W Xe lamp, l > 420 nm/2 M ethylene glycol and 0.5 M Na2SO4 solution 300 W Xe lamp, l > 420 nm/0.25 M of Na2S and 0.35 M of Na2SO3 1000 W Xe arc lamp/0.25 M Na2S and 0.35 M Na2SO3 solution 1000 W Xe arc lamp/0.25 M Na2S and 0.35 M Na2SO3 solution 1000 W Xe arc lamp/0.2 mol/L Na2S and 0.3 mol/L Na2SO3 150 W Xe lamp Sunlight illumination 100 mW cm2/0.24 M Na2S and 0.35 M Na2SO3 300 W Xe arc lamp (l >400 nm)/25% methanol solution 300-W xenon lamp (l > 420 nm)/ 0.1 M L-ascorbic acid 500 W Xenon lamp/0.5 M Na2SO4 Solar radiation of 75 mW cm2/ 0.25 mol L1 Na2S and 0.125 mol L1 Na2SO3 300 W xenon lamp/0.1 M Na2S and 0.1 M Na2SO3 300 W Xe lamp (l <400 nm)/0.1 M Na2S and 0.5 M Na2SO3 1000 W Xenon arc lamp/3 M H2SO4 with 0.5 M Na2SO4 salt 1 sun illumination/0.24 M Na2S and 0.35 M Na2SO3. 150 W Xe lamp (100 mW cm2)/ 0.5 M Na2SO4 Visible light l > 400 nm/Na2S (0.35 M)/Na2SO3 (0.25 M) visible light irradiation (l > 420 nm)/triethanolamine 300 W Xe lamp (100 mW cm2) 0.35 M Na2S and 0.25 M Na2SO3

H2 evolution

References

124.5 m mol cm2 h1

[46]

436 mmol/g$h

[47]

photocurrent density: 2.75 mA/cm2

[48]

Photocurrent density of ~12 mA/cm2 at 0.4 V vs Ag/AgCl. IPCE: 45% 0.8 mA/cm2 IPCE: 50%

[55]

5.407 mmol cm2 h1 Photocurrent density: 16.2 mA/cm2

[57] [59]

17.27 mmol h1

[88]

4.494 mmol h1 g1

[89]

Photoconversion efficiency 1.6 times than pristine TiO2 3.2 mmol min1

[114]

2.7 times than individual components

[116]

717 mmol/g

[117]

133 mmol h1 cm2

[118]

20 mmol h1 cm2,

[119]

0.34 mA/cm2 IPCE:0.42%

[120]

3610 mmol h1, (i)Quantum efficiency (QE): 4.74%. (ii)QE: 41% 137.84 mmol h1

[121]

[122]

13.65 mA

[123]

[56]

[115]

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(continued on next page)

5

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Table 1 e List of various QDs with their method of preparation, quantum yield and application.

6

Sl.no

QD

Method

20

ZnO QD

Solegel method

21

SILAR method

22 23

CdS QDs decorated screw-likeSnO2 nanostructures Ultra-thin TiO2 nanosheets decorated Pd QD CdSe QDs/zirconium titanium phosphate (ZTP)

24

CdS QDs/ZnO nanorod

SILAR method

25

CdS QDs/Au nanoparticles

Self-assembly method

26

Graphene QDs/TiO2 nanotube array

Impregnation method

27

Au@CdS coreeshell nanostructure

Self-assembly method

a

28 29

CdSe QDs/a-Fe2O3 CoSx QDs/TiO2

Spray pyrolysis. Precipitationedeposition method

30

CdSeeZnS QDs/Au-Pt alloy

Surface linkage method

a

ZnO QDs/a-Fe2O3

Electrodeposition method

32

CdS/H-3D-TiO2/Pt-wire

Hydrothermal and CBD method

33

CdS and CdTeS QDs

Hydrothermal technique

a

WO3/Ag/CdS NRs

Hydrothermal method

31

34

e Solvothermal method

H2 evolution

Light source/reaction solution Xe lamp/0.35 M Na2S and 0.25 M Na2SO3 500 W Xe lamp/0.25 M Na2S and 0.35 M Na2SO3 formaldehyde solution visible light irradiation (l  420 nm), 20 mL of 0.02 M Na2S, Xe lamp (100 mW cm2), 1 M Na2S 500 W Xe lamp (l  420 nm), Na2S 0.1 M, Na2SO3 0.1 M 300 W Xe lamp, 1 M Na2S and 0.1 M Na2SO3 0.1 M Na2SO3 and 0.1 M Na2S, visible light (l  420 nm) 150 W Xenon lamp, 1 M NaOH Ethanol, 300 W Xe lamp, 0.5 M Na2SO4 300-W Xe lamp, Na2S (0.5 M) and Na2SO3 (0.5 M) 150 W Xe arc lamp, 1 M NaOH Visible light:100 mWcm2 0.5 M Na2Se0.5 M Na2SO3 Visible light, ascorbic acid and CoCl2$6H2O 1 M H2SO4

References

260 mmol h1 g1,

[124]

159.6 mmol (h cm2)1

[125]

250 mL g1 min1 905.4 mmol in 3 h

[126] [127]

490 mmol h1, photocurrent density: 6.51 mA/cm2 550 mmol in 10 h

[128]

18.5 mmol h1

[130]

201.9 mmol h1g1

[131]

Photocurrent density of 550 mA cm2 838.9 mmol h1 g1

[132] [133]

190 mmol in 2 h

[134]

6 mL h1 cm2 at 0.75 V/SCE.2.84 mA/cm2 at 0.75 V/SCE 18.42 mmol cm2 h1, 8.77 mA/cm2,

[135] [136]

9.3, 8 mmol g1 h1

[137]

1.38 mA/cm2 at 0.6 V vs Ag/AgCl

[138]

[129]

As compared to bulk semiconductors, the efficiency of QDs is much better in the field of photocatalytic water splitting. The reason of high efficiency of QDs as compared to bulk semiconductors have already been explained briefly in Section Importance of QDs for photocatalytic water splitting with special emphasis on quantum confinement effect. For comparison the efficiency of bulk semiconductor and the respective QDs are given in Table 2. a Tested for photoelectrochemical hydrogen production.

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Table 1 e (continued )

300 W Xe lamp 300 W Xe lamp

(a) CdSe QDs (b)CdSe/CdS QDs CuO QDs/TiO2 ZnO QDs/a-Fe2O3 8 9

(a) CdSe QRs (b) CdSe/CdS DIRs CuO/TiO2 ZnO/a-Fe2O3 7

CdS and CdSe QDs/ZnO NRs MoS2 QDs/CdS Core/Shell ZnO/CdS NWs MoS2/CdS Nanosheet 5 6

PbS QDs/ZnO NW PbS/ZnO NW 4

CdS QDs/TiO2 NTAs CdS QDs-Au-TiO2 2 3

7

[151,152] [153,135]

(a) 691 m mol (b) 67.7 m mol 99,823 mmol h1 g1 2.84 mA/cm2 (a) 149 m mol (b) 15.9 m mol 71.6 mmol h1 g1 1.5 mA/cm2

[150]

15 mA/cm2 312.75 mmol h1 g1 3.58 mA/cm2 49.80 mmol g1 h1

AM 1.5 illumination, 0.5 M Na2S visible light irradiation, 0.02 M Na2S and 0.025 M NaSO3 70 mW, 520 nm light-emitting diode (10 h) Solar light, glycerol-water solution 150 W Xe lamp, 1 M NaOH

[146,147] [148,149]

18 mA/mm2 0.6 mA/cm2 500 W Hg lamp

[144,145]

3.98 mA/cm2 7.06 mA/cm2 1.462 mA/cm2 5.6 mA/cm2

[140,141] [142,143]

4.494 mmol h1 g1 4152 mmol h1 g1 300 W Xe lamp

Visible light l  420 nm solar illumination (100 mW cm2) Visible light (l > 400 nm), 0.1 M of Na2S and Na2SO3. AM 1.5 illumination (100 mW cm2), 0.1 M Na2SO4 500 W Xe lamp, 0.5 M Na2S visible-light irradiation (l  420 nm), 10 vol. % lactic acid 70 mW, 520 nm light-emitting diode (10 h) 400 W Hg lamp, 10 vol % methanol 150 W Xe lamp, 1 M NaOH CdS QDs/g-C3N4

CdS/g-C3N4 Core shell CdS/TiO2 NTAs TiO2-Au-CdS 1

Photocatalytic activity

QDs QDs

Light source/reaction solution

Bulk semiconductor QDs Bulk semiconductor

Catalyst Sl no.

Table 2 e Comparison study of photocatalytic activity of bulk semiconductor and QDs towards water splitting.

Bulk semiconductor

Ref.

[139,89]

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Fig. 4 e Proposed models for electron transfer at CdSe/TiO2 interface in a CdSe-TiO2: N sample [Reproduced from Ref. [48]].

closely present, as a result the photoexcited electrons formed in CdSe is transported to the ZnO NWs through the CdS intermediate layer. At ZnO NWs the electrons are passed to the cathode where hydrogen is produced in PEC device and the holes left behind are consumed at anode-electrolyte interface for oxidation process. The IPCE value is greatly increased simply due to the electron transfer from CdSe QDs to ZnO NW which is favored by the band alignment between CdS and CdSe. Liu and coworkers have reported an average enhancement of photo electrochemical performance of 50% by synthesizing CdSe QD-sensitized Au/TiO2 nanocomposite for PEC applications. Due to light scattering by Au NPs (6 nm), the light absorption of CdSe QDs is increased which contributes substantial improvement of PEC performance [56]. Similar type of work has done by Hong et al. [57]. They have designed Pt-(CdS/ TiO2) film by doctor blade method followed by chemical bath deposition (CBD) method. CdS contents and size of it (5e7 nm) in the nanocomposite efficiently control the rate of evolution of hydrogen. Higher the content of CdS higher is the electron loss and henceforth the reduction of hydrogen production. It is because the electrons inside the photocatalyst may get dissipated and the CdS QDs are coagulated with each other in spite of depositing on TiO2. The hydrogen evolution has found to attain a maximum value of 5.407 mmol cm2 h1 in Pt-(CdS/ TiO2) film-typed photocatalysts having saturated value of Cd/ Ti ratio of 0.197. The IPCE value has been further increased by Fan et al. [58] and chemical bath deposition technique was used to fabricate a highly efficient TiO2 photoelectrode by growing seeds (4.2 nm CdSe QDs) on TiO2 thin film. After that the film was covered by ZnS layer followed by post sintering process at 400  C. The power conversion efficiency is found to be 3.21% and IPCE peak values is 73%. Photocurrent of 14.86 mA/cm2 has been reported which is surprisingly very higher than photoelectrodes prepared by only seed growing method, CBD method and non-post sintering methods. In comparison to CdSe QD modified thin film TiO2, hybrid

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mesoporous TiO2 shows better photocatalytic activity. This work has been established by Kim et al. [59] and they have synthesized a QD sensitized mesoporous which is a sandwich like heterostructure. The top layer is made up of mesoporous TiO2 and the bottom is a surface textured TiO2 inverse opal layer. It is reported that maximum photocurrent density (~1.6 mA/cm2) is exhibited by CdSe/H-TiO2 and which is 35% more efficient than CdSe/P25. Under filtered exposure conditions CdSe/H-TiO2 shows a greatest current density of 14.2 mA/cm2 in visible region and under UVevisible irradiation the efficiency is nearly 88%. The mechanism involves the transfer of electrons from quantum dots to the conduction band of TiO2, from where the electrons are withdrawn to the counter electrode and the hydrogen generation reaction occurs. Meanwhile, the holes diffused to the surface of TiO2 or CdSe oxidize S2. From the above discussion, it can be concluded that CdSe QDs sensitized hybrid TiO2 (H-TiO2) shows better activity than that of CdSe QDs sensitized N-doped TiO2 which in turn shows better result than CdSe QDs sensitized Au-doped TiO2. As Au and CdSe QDs act as light trapping as well as scattering agent, CdSe QDs/Au/TiO2 showed fewer enhancements in photocurrent but in case of N-doped TiO2 the enhanced photocurrent is due to strong visible light absorption of CdSe QDs and the formation of oxygen vacancy in TiO2 lattice by the doped N-atoms. It can be explained as the excited electrons migrated from N0 levels to V0 level which is situated below than the H2 reduction potential; the electrons are efficiently utilized to enhance the photocurrent instead of H2 generation. In comparison to chalcogenide QDs modified TiO2, noble metal QD like Ag QDs modified TiO2 have showed nominal current. This is because of larger size of Ag QDs (1.3e21.0 nm) compared to chalcogenide QDs (2.5e7 nm) which could defense the multireflection of absorbed light inside the TiO2 NTAs.

Enhancement in hydrogen generation of LDHs by QDs modification Layered double hydroxides (LDHs) are two dimensional hydrotalcite like anionic clays containing positively charged metals and replaceable interlayer anions and water molecules with the formula [M 2þ1-x M 3þx (OH)2](A n) x/n$m H2O where M 2þ and M 3þ are divalent and trivalent metals, respectively; A n is the interlayer anion [60]. Most important characteristics property of LDH is that it possesses high surface area which is helpful in the adsorption and amiable exchange of anions. Moreover, it has both hydrophobic and hydrophilic nature. Their specific structure and flexibility in chemical composition proves it as a very efficient semiconductor material in the field of photocatalytic water splitting and degradation of pollutants [61e64]. In addition to this, highly active mixed metal oxides are formed when LDH decomposes in the temperature range of 300e600  C which have been used in various photocatalytic processes [65e67]. But on the other side poor charge carrier mobility and fast electron hole recombination process suppresses their catalytic efficiency. So to improve the activity several attempts like incursion of carbon materials [68e72], conductive organic polymers [73,74] and quantum dots [75,76] have been executed.

Till today there are few works have been done on QDs sensitized LDH materials and we hope this review article will be the source for further wide research. Quantum dot shows high photocatalytic activity for visible-light-induced H2 generation. QDs sensitized LDH have been investigated as an efficient photocatalyst for the effective generation of hydrogen under visible light irradiation. Tang et al. [75] have designed CQD (carbon quantum dot)/NiFe- LDH complex which show better photocatalytic activity (Fig. 5) than the pure NiFe-LDH. Overpotential of ~235 mV in 1 M KOH has reported by this group at a current density of 10 mA/cm2. The electrocatalytic property of NiFe-LDH is further enhanced by CQDs on account of fast electron transfer and electron storage characteristic. Due to the presence of functional groups on surface of CQDs, it shows some electrostatic interaction with the LDH and significantly boosts the electrocatalytic activity and durability of LDH. These special characteristics are due to the small size of CQDs (5 nm) which provide specific surface area for electro catalytic reactions which in turn increases the electro catalytic activities and longevity of NiFe-LDH. In addition to these, size dependent up-converted PL behavior, electron storage property and supernal conductivity of CQDs play a key role in the photocatalytic activities. Hence it is concluded that the CQD/NiFe-LDH composite is a better oxygen evolution reaction (OER) catalyst than either the pristine NiFe-LDH or the other NiFe composites.

Enhancement in hydrogen generation of g- C3N4 by QDs modification Graphitic carbon nitride, g-C3N4 is the most thermally and chemically stable visible light active photocatalyst (band gap 2.7 eV) and metal free allotrope of carbon nitride. g-C3N4 possesses flake like structure analogous to graphite containing alternative double bonds and the interlayer distance is 0.326 nm. This p-conjugated system facilitates the easy delocalization of electrons throughout the framework. The tris-triazine units interconnected with planar amino groups constitutes the graphitic planes of g- C3N4 [77]. g-C3N4 shows better photocatalytic activity for splitting of water into hydrogen and oxygen under visible light irradiation [78e80] because of smaller electronegativity of N and

Fig. 5 e Schematic model for the roles of CQDs in the high electrocatalytic activity of CQD/NiFe-LDH nanocomposites [Reproduced from Ref. [75]].

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Fig. 6 e Schematic of photogenerated charge transfer in the CdS QDs/g-C3N4 system under visible light irradiation [Reproduced from Ref. [88]].

correspondingly smaller band gaps of nitrides. The g-C3N4 photocatalyst is stable under light irradiation in water solution as well as in acid (HCl, pH ¼ 0) or base (NaOH, pH ¼ 14) solutions. Moreover, the non-toxic nature of the precursors and the simpler synthetic methods of g-C3N4 render it a desirable photocatalyst for water splitting. It is capable of producing both hydrogen and oxygen but on the contrary it has many disadvantages. The main disadvantage is the high recombination rate of its photogenerated electronehole pairs [81]. To overcome this problem and to enhance its photocatalytic performance, a number of methods have been exploited, such as porous structures [82], doping and coupling of g-C3N4 with metals [83], graphene [84,85], activated by protonation [86], and organic dyes [87]. But recent development shows that when g-C3N4 is coupled with semiconductor QDs, its photocatalytic activity increases tremendously. It is believed that the enhanced photocatalytic activity is due to the interfacial transfer of photogenerated electrons and

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holes between g-C3N4 and QD which leads to charge separation on both parts. The mechanism of charge transfer in QDs/ g-C3N4 composite can be explained by taking the example of CdS QDs/g-C3N4 composite which is shown in Fig. 6 [88]. Due to difference in conduction band edge potentials the photoexcited electrons in CB of g-C3N4 are migrated to the VB of CdS QDs. And due to quantum size effect, VB of CdS QDs is lower than that of g-C3N4; hence the photogenerated holes on the VB of CdS QDs are transferred to the VB of g-C3N4. Hydrogen gas is generated on the Pt nanoparticles by accumulating electrons from VB of CdS QDs. Therefore, the photogenerated electrons from CdS QDs and the efficient electronehole separation in the CdS QDs/g-C3N4 heterostructure conduct a noticeable increment of H2 production. Here, it is very important to mention that after incorporation of CdS QD, the g-C3N4 has shown efficient separation of the photogenerated charge carriers and there is enhancement of the visible light photocatalytic H2 evolution activity. Ge et al. [88] reported that 30 wt. % CdS QD-g-C3N4 composite have showed the H2 evolution rate was 17.27 mmol h1under visible light irradiation which is ~9 times that of pure g-C3N4. The presence of Pt co-catalyst plays major role in enhanced H2 evolution. After this Cao et al. [89] prepared in situ growth of CdS QDs on g-C3N4 nanosheets. The 12 wt % CdS QD and g-C3N4 composite have shown a significant evolution rate of 4.494 mmol h1 g1 and this is surprisingly 115 times greater than that of neat g-C3N4. From this experiment it is clearly revealed that CdS content in the composite plays a very crucial role in the charge separation process. Optimum amount of CdS can uniformly disperse on the surface of g-C3N4. As the CdS content increases, the interface area of CdS- g-C3N4 also increases which facilitates more transfer of electrons and holes between CdS QDs and gC3N4. But excess deposition of CdS QDs leads to aggregation which leads to the reduction of interfacial surface area and the also the charge separation efficiency. From this discussion, it is concluded that TGA capped CdS QDs involving chemical impregnation method is more efficient than CdS QDs prepared by using DMSO by solvothermal method. The loaded amount and the size of CdS QDs also determine the rate of H2 evolution. It should be mentioned here that TGA

Fig. 7 e Schematic illustration of the PEC mechanism over the GNs-CdS QDs multilayered films [Reproduced from Ref. [107]]. Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

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capped CdS QDs are in the range of 2e5 nm whereas CdS QDs prepared by DMSO are of 5 nm. Hence, it can be concluded that smaller the size of QDs more is the rate of H2 evolution.

Enhancement in catalytic activity of graphene and graphene like materials by QDs Graphene is a flat monomer of carbon atoms firmly held together to form a 2 dimensional honey comb lattice like structure, and is an elementary building block for other dimensional graphitic materials. It can be enfolded into 0 D fullerenes, rolled into 1D nanotube or stacked into 3D graphite [90]. Andre Geim and Konstantin Novoselov collected Nobel Prize in 2010 in physics for revolutionary experiments about graphene. Till today three types of graphene have been found these are single layer graphene (SG), bilayer graphene (BG) and few layer graphene (FG, number of layer 10). Under ambient conditions the charge carrier mobility m can overstep 15,000 cm2 V1 S1 [91e94]. This can be significantly improved up to 100,000 cm2 V1 S1 [90]. Electron donor and acceptor molecules alter the electrical conductivity. Conductivity increases and decreases when graphene is associated with electron acceptor and electron donor molecules respectively. Due to its high surface area it is accepted as an efficient photocatalyst. Among the three types of graphenes, single layer graphene is estimated to possess huge surface area of about 2600 m2 g1 [95]. Also few layer graphene show large surface area i.e. 270e1550 m2 g1 [96]. This value have been measured by the BrunauereEmmetteTeller (BET) method. Hence it is concluded that the surface area of some of few layer graphene approaches the value of single layer graphenes. Instead of having these benefits graphene has some bottlenecks. Single layer graphene are not stable in solution. They get aggregated back to form graphite. Moreover the production of single layer graphene is quite low by various mass production methods [97e107] as the main product is usually multiple layer graphene. These serious problems can be overcome effectively by making composite with quantum dots. Some few works in the field of photocatalysis have been done till today and are explained below. Xiao et al. [108] suggested that under visible light irradiation, promising photo catalytic and photo electrochemical performances are exhibited by graphene nano sheets (GNS)CdS QDs multilayered films because of intimate interfacial contact between GNs and CdS QDs. This intimate interfacial contact leads to a stacked structure in which CdS QDs and GNs are present alternately. This stacking structure is beneficial for shuttling of photo generated electrons from CdS QDs to GNs. In Fig. 7 it is shown that the work function of GNs is ca.4.42 eV [109,110] and work function of fluorine doped tin oxide (FTO) substrate is about 4.4 eV [111,95]. In the meantime CdS QDs with diameter of nearly 5.6 nm have band gap of about 2.25 eV. The position of conduction band of the CdS QDs is at ca.3.98 eV and that of valence band is 6.23 eV [112,96]. From Fig. 7 it is clear that the location of CB of CdS above the work function of GNs facilitates the transfer of electron from the CB of CdS QDs to GNs upon visible light irradiation. The transferred electron is captured by GNs which is further transferred to fluorine-doped tin oxide (FTO) substrate. As a

Table 3 e List of abbreviations and their full forms used in the review. Abbreviations BET BG CB CBD CQD DIRs DL DMSO EBS EPR FG FTO GNS GO GQD IPCE LDH LED NGO NHE NP NS NTA NW PEC PL QD QRs SG SILAR TEM VB

Full form BrunauereEmmetteTeller Bilayer Graphene Conduction Band Chemical Bath Deposition Carbon Quantum Dot Dot in Rods Donor level Dimethyl sulfoxide Emulsion Based bottom up Selfassembly Electron Paramagnetic resonance Few layer Graphene Fluorine-doped Tin Oxide Graphene Nanosheet Graphene Oxide Graphene Quantum Dot Incident Photon to Charge carrier Efficiency Layered Double Hydroxide Light Emitting Diode Nitrogen doped Graphene Oxide Normal Hydrogen Electrode Nano Particles Nano Sheet Nano Tube Array Nano Wires Photo Electro Chemical Photo Luminescence Quantum Dot Quantum Rods Single layer Graphene Successive Ionic Layer Absorption and Reaction Transmission Electron Microscopy Valence Band

result the electronehole charge separation efficiency and the photocurrent density are increased. Cao et al. [113] synthesized Graphene eCdS (G-CdS) directly from graphene oxide (GO) in dimethyl sulfoxide (DMSO). The decoration of CdS QDs reduces the aggregation of itself and also the single layer graphene. The above mentioned two problems can be surpassed by the direct one pot synthesis of G-CdS from GO. The one pot reaction is composed of the simultaneous reduction of GO and loading of CdS on graphene sheet. Most interestingly no molecular linkage is needed to bind CdS QDs and graphene sheets. They have shown the ultrafast transfer of electrons in the range of picoseconds from CdS QDs to the graphene sheets. This can be utilized in various photochemical applications by further experimentations. The list of the abbreviations used in the review with their full form is provided in Table 3.

Summary and outlook In summary, the area of quantum dot research has drawn much attention by the scientific community. It has gained extensive applicability, recognition and much interest in recent years for

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the development of solar energy conversion process. This review gives a comprehensive description of basics of quantum dot, type, uniqueness, modifications and applications towards photocatalytic and photoelectrochemical water splitting for hydrogen energy production. Quantum dots are well known for its efficient visible light absorption in whole over the solar spectrum. So the metal oxides are widely modified by the quantum dots for the increment of the photocatalytic activity. Very recently carbonaceous materials like g-C3N4, graphene and benzene like materials are modified with quantum dots which showed very good photocatalytic activity. Researchers have developed quantum dot modified materials with aimed at reproducibility and enhanced productivity which is the prime reason for industrialization of hydrogen production technology. The practical fabrication of quantum dot materials in terms of size, shape, morphology, crystallinity and chemical composition remains an active area of study in photocatalytic research. The synthetic procedure to obtain highly visible light active quantum dot materials is still a challenging task in terms of long term stability under prolonged solar irradiation. Till to date, InN/InGaN quantum showed highest PEC activity with IPCE 56% but still stability is a question mark in terms of practical applications. It is expected that the inexpensive, environmentally friendly quantum dot based materials will play an important role in the hydrogen production under solar irradiation and contribute much to the coming hydrogen based economy. Due to the special nanostructure, size and shape selectivity of the quantum dot based materials, it is not only helpful for hydrogen economy but also extends its application towards fuel cell application, lithium ion batteries, sensors and solar cell. This review gives a pathway to develop new kind of quantum dot based materials with special optical and electronic properties. As the review covers from basic to applications of quantum dot materials, it is more helpful for the researcher those who are preliminary working on quantum dot based materials.

references

[1] Govindhan M, Mao B, Chen A. Novel cobalt quantum dot/ graphene nanocomposites as highly efficient electrocatalysts forwater splitting. Nanoscale 2016;8:1485e92. [2] Ramasamy P, Kim B, Lee MS, Lee JS. Beneficial effect of water in the colloidal synthesis of InP/ZnS core-shell quantum dots for optoelectronic applications. Nanoscale 2016;8:17159e68. [3] Wierer JJJ, Tansu N, Fischer AJ, Tsao JY. III-nitride quantum dots for ultra-efficient solid-state lighting. Laser Phot Rev 2016;10:612e22. [4] Zhou R, Niu H, Ji F, Wan L, Mao X, Guo H, et al. Bandstructure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics. J Power Sources 2016;333:107e17. [5] Song X, Zhang Y, Zhang H, Yu Y, Cao M, Che Y, et al. Highperformance ambipolar self-assembled Au/Ag nanowire based vertical quantum dot field effect transistor. Nanotechnology 2016;27:405201e7. [6] Weng X, Neethirajan S. A microfluidic biosensor using graphene oxide and aptamer-functionalized quantum dots

[7]

[8]

[9]

[10]

[11] [12] [13]

[14] [15]

[16] [17]

[18]

[19]

[20]

[21]

[22]

[23]

[24] [25]

[26]

11

for peanut allergen detection. Biosens Bioelectron 2016;85:649e56. Zhaoa C, Lia W, Lianga Y, Tian Y, Zhang Q. Synthesis of BiOBr/carbon quantum dots microspheres with enhanced photoactivity and photostability under visible light irradiation. Appl Catal A General 2016;527:127e36. Kang BZ, Liu Y, Tsang CHA, Ma DDD, Fan X, Wong NB, et al. Water-soluble silicon quantum dots with wavelengthtunable photoluminescence. Adv Mater 2009;21:661e4. Apte SK, Garaje SN, Naik SD, Waichal RP, Kale BB. Environmentally benign enhanced H2 production from abundant copious waste H2S using size tuneable cubic bismuth (Bi0) quantum dotseGeO2 glass photocatalyst under solar light. Green Chem 2013;15:3459e67. Rastogi SK, Denn BD, Branen AL. Synthesis of highly fluorescent and thio-linkers stabilize gold quantum dots & nano clusters in DMF for Bio-labeling. J Nanopart Res 2012;14:673e84. Murphy CJ. Optical sensing with quantum dots. Anal Chem 2002;74:520e6. Liu S, Su X. The synthesis and application of IeIIIeVI type quantum dots. RSC Adv 2014;4:43415e28. Hines DA, Kamat PV. Recent advances in quantum dot surface chemistry. ACS Appl Mater Interfaces 2014;6:3041e57. Green MJ. The nature of quantum dot capping ligands. Mater Chem 2010;20:5797e809. Kershaw SV, Susha AS, Rogach AL. Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties. Chem Soc Rev 2013;42:3033e87. Gerischer H. The impact of semiconductors on the concepts of electrochemistry. Electrochim Acta 1990;35:1677e99. Holmes MA, Townsend TK, Osterloh FE. Quantum confinement controlled photocatalytic water splitting by suspended CdSe nanocrystals. Chem Commun 2012;48:371e3. Martha S, Sahoo PC, Parida KM. An overview on visible light responsive metal oxide based photocatalysts for hydrogen energy production. RSC Adv 2015;5:61535e53. Anpo M. Preparation, characterization, and reactivities of highly functional titanium oxide-based photocatalysts able to operate under UVevisible light irradiation: approaches in realizing high efficiency in the use of visible light. Bull Chem Soc Jpn 2004;77:1427e42. Yamashita H, Harada M, Misaka J, Takeuchi M, Ikeue K, Anpo M. Degradation of propanol diluted in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts. J Photochem Photobiol A 2002;148:257e61. Kitano M, Matsuoka M, Ueshima M, Anpo M. Recent developments in titanium oxide-based photocatalysts. Appl Catal A 2007;325:1e14. Borgarello E, Kiwi J, Gratzel M, Pelizzetti E, Visca M. Visible light induced water cleavage in colloidal solutions of chromium-doped titanium dioxide particles. J Am Chem Soc 1982;104:2996e3002. Cao Y, Yang W, Zhang W, Liu G, Yue P. Improved photocatalytic activity of Sn4þ doped TiO2 nanoparticulate films prepared by plasma-enhanced chemical vapor deposition. New J Chem 2004;28:218e22. Domen K. Metal ion and N co-doped TiO2 as visible-light photocatalyst. J Mater Res 2004;19:2100e8. Choi Y, Umebayashi T, Yoshikawa M. Fabrication and characterization of C-doped anatase TiO2 photocatalyst. J Mater Sci 2004;39:1837e9. Shen M, Wu Z, Huang H, Du Y, Zou Z, Yang P. Carbon-doped anatase TiO2 obtained from TiC for photocatalysis under visible light irradiation. Mater Lett 2006;60:693e7.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5

[27] Ohno T, Mitsui T, Matsumura M. Photocatalytic activity of S-doped TiO2 photocatalyst under visible light. Chem Lett 2003;32:364e5. [28] Tesfamichael T, Will G, Bell J. Nitrogen ion implanted nanostructured titania films used in dye-sensitised solar cells and photocatalyst. Appl Surf Sci 2005;245:172e8. [29] Gandhe AR, Naik SP, Fernandes JB. Selective synthesis of Ndoped mesoporous TiO2 phases having enhanced photocatalytic activity. Microporous Mesoporous Mater 2005;87:103e9. [30] Zhao Y, Qiu X, Burda C. The effects of sintering on the photocatalytic activity of N-doped TiO2 nanoparticles. Chem Mater 2008;20:2629e36. [31] Zhang G, Ding X, Hu Y, Huang B, Zhang X, Qin X, et al. Photocatalytic degradation of 4BS dye by N,S-codoped TiO2 pillared montmorillonite photocatalysts under visible-light irradiation. J Phys Chem C 2008;112:17994e7. [32] Fang J, Wang F, Qian K, Bao H, Jiang Z, Huang W. Bifunctional N-Doped mesoporous TiO2 photocatalysts. J Phys Chem C 2008;112:18150e6. [33] Liu Y, Chen X, Li J, Burda C. Photocatalytic degradation of azo dyes by nitrogen-doped TiO2 nanocatalysts. Chemosphere 2005;61:11e8. [34] Babu VJ, Kumar MK, Nair AS, Kheng TL, Allakhverdiev SI, Ramakrishna S. Visible light photocatalytic water splitting for hydrogen production from N-TiO2 rice grain shaped electrospun nanostructure. Int J Hydrogen Energy 2012;37:8897e904. [35] Liu G, Sun C, Yan X, Cheng L, Chen Z, Wang X, et al. Iodine doped anatase TiO2 photocatalyst with ultra-long visible light response: correlation between geometric/electronic structures and mechanisms. J Mater Chem 2009;19:2822e9. [36] Lee JS. Photocatalytic water splitting under visible light with particulate semiconductor catalysts. Catal Surv Asia 2005;9:217e27. [37] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269e71. [38] Lin WC, Yang WD, Huang IL, Wu TS, Chung ZJ. Hydrogen production from methanol/water photocatalytic decomposition using Pt/TiO2xNx catalyst. Energy Fuels 2009;23:2192e6. [39] Nishijima K, Kamai T, Murakami N, Tsubota T, Ohno T. Photocatalytic hydrogen or oxygen evolution from water over S- or N-Doped under visible light. Int J Photoenergy 2008;2008:173943e50. [40] Miyauchi M, Takashio M, Tobimatsu H. Photocatalytic activity of SrTiO3 codoped with Nitrogen and lanthanum under visible light illumination. Langmuir 2004;20:232e6. [41] Wang J, Yin S, Komatsu M, Sato T. Synthesis of La/N Codoped SrTiO3 using polymerized complex method for visible lightp. J Eur Ceram Soc 2005;25:3207e12. [42] Wei W, Dai Y, Guo M, Yu L, Huang B. Density functional characterization of the electronic structure and optical properties of N-doped, La-doped, and N/La-codoped SrTiO3. Phys Chem C 2009;113:15046e50. [43] Reddy KH, Martha S, Parida KM. Facile fabrication of Bi2O3/ BieNaTaO3 photocatalysts for hydrogen generation under visible light irradiation. RSC Adv 2012;2:9423e36. [44] Parida KM, Reddy KH, Martha S, Das DP, Biswal N. Fabrication of nanocrystalline LaFeO3: an efficient solegel auto-combustion assisted visible light responsive photocatalyst for water decomposition. Int J Hydrogen energy 2010;35:12161e8. [45] Yin J, Zou Z, Ye J. Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M¼Sr, Ba). Chem Phys Lett 2003;378:24e8.

[46] Lian Z, Wang W, Xiao S, Li X, Cui Y, Zhang D, et al. Plasmonic silver quantum dots coupled with hierarchical TiO2 nanotube arrays photoelectrodes for efficient visiblelight photoelectrocatalytic hydrogen evolution. Sci Rep 2015;5:1e10. [47] Lee S, Lee K, Kim WD, Lee S, Shin DJ, Lee DC. Thin amorphous TiO2 shell on CdSe nanocrystal quantum dots enhances photocatalysis of hydrogen evolution from water. J Phys Chem C 2014;118:23627e34. [48] Hensel J, Wang GM, Li Y, Zhang JZ. Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation. Nano Lett 2010;10:478e83. [49] Wang CJ, Shim M. Guyot-sionnest, P. Electrochromic nanocrystal quantum dots. Science 2001;291:2390e2. [50] Graetzel M. Review article Photoelectrochemical cells. Nature 2001;414:338e44. [51] Kongkanand A, Tvrdy K, Takechi K, Kuno M, Kamat PV. Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSeTiO2 architecture. J Am Chem Soc 2008;130:4007e15. [52] Norris DJ, Bawendi MG. Measurement and assignment of the size-dependent optical spectrum in CdSe quantum dots. Phys Rev B 1996;53:16338e46. [53] Prabakar K, Minkyu S, Inyoung S, Heeje K. CdSe quantum dots co-sensitized TiO2 photoelectrodes: particle size dependent properties. J Phys D Appl Phys 2010;43:012002e5. [54] Liu L, Wang G, Li Y, Li Y, Zhang JZ. CdSe quantum dotsensitized Au/TiO2 hybrid mesoporous films and their enhanced photoelectrochemical performance. Nano Res 2011;4:249e58. [55] Wang G, Yang X, Qian F, Zhang JZ, Li Y. Double-Sided CdS and CdSe Quantum dot co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogen generation. Nano Lett 2010;10:1088e92. [56] Ruben L, Houtepen AJ, Talgorn E, Schoonman J, Goossens A. Study of electronic defects in CdSe quantum dots and their involvement in Quantum dot solar cells. Nano Lett 2009;9:856e9. [57] Hong E, Kim JH, Yu S, Kim JH. Effect of CdS contents on H2 production from Pt-(CdS/TiO2) film-typed photocatalysts. Korean J Chem Eng 2011;28:1684e7. [58] Fan SQ, Kim D, Kim JJ, Jung DW, Kang SO, Ko J. Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications. Electrochem Commun 2009;11:1337e9. [59] Kim K, Kim MJ, Kim SI, Jang JH. Towards visible light hydrogen generation: quantum dot-sensitization via efficient light harvesting of hybrid-TiO2. Sci Rep 2013;3:1e8. [60] Mohapatra L, Parida K. A review on the recent progress, challenges and perspective of layered double hydroxides as promising photocatalysts. J Mater Chem A 2016;4:10744e66. [61] Gong M, Li YG, Wang HL, Liang YY, Wu JZ, Zhou JG, et al. An advanced NieFe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc 2013;135:8452e5. [62] Zhao MQ, Zhang Q, Huang JQ, Wei F. Hierarchical nanocomposites derived from nanocarbons and layered double hydroxides - properties, synthesis, and applications. Adv Funct Mater 2012;22:675e94. [63] Zhao YF, Chen PY, Zhang BS, Su DS, Zhang ST, Tian L, et al. Highly dispersed TiO6 units in a layered double hydroxide for water splitting. Chem Eur J 2012;18:11949e58. [64] Zhao YF, Zhang ST, Li B, Yan H, He S, Tian L, et al. A family of visible-light responsive photocatalysts obtained by dispersing CrO6 octahedra into a hydrotalcite matrix. Chem Eur J 2011;17:13175e82.

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5

[65] Zou L, Li F, Xiang X, Evans DG, Duan X. Self-generated template pathway to high-surface-area zinc aluminate spinel with mesopore network from a single-source inorganic precursor. Chem Mater 2006;18:5852e9. [66] Meng WQ, Li F, Evans DG, Duan X. Photocatalytic activity of highly porous zinc ferrite prepared from a zinc-iron(III)sulfate layered double hydroxide precursor. J Porous Mater 2004;11:97e105. [67] Shu X, He J, Chen D. Tailoring of phase composition and photoresponsive properties of Ti-containing nanocomposites from layered precursor. J Phys Chem C 2008;112:4151e8. [68] Zhou GM, Wang DW, Yin LC, Li N, Li F, Cheng HM. Oxygen bridges between NiO nanosheets and graphene for improvement of lithium storage. ACS Nano 2012;6:3214e23. [69] Zhu PN, Nair S, Peng SJ, Yang SY, Ramarkrishna S. Facile fabrication of TiO2egraphene composite with enhanced photovoltaic and photocatalytic properties by electrospinning. ACS Appl Mater Interfaces 2012;4:581e5. [70] Mu JB, Shao CL, Guo ZC, Zhang ZY, Zhang MY, Zhang P, et al. High photocatalytic activity of ZnOcarbon nanofiber heteroarchitectures. ACS Appl Mater Interfaces 2011;3:590e6. [71] Tettey K, Yee MQ, Lee D. Photocatalytic and conductive MWCNT/TiO2 nanocomposite thin films. ACS Appl Mater Interfaces 2010;2:2646e52. [72] Zang YY, Xie D, Wu X, Chen Y, Lin YX. Enhanced photovoltaic properties in graphene/polycrystalline BiFeO3/ Pt heterojunction structure. Appl Phys Lett 2011;99:132904e6. [73] Han JB, Dou YB, Zhao JW, Wei M, Evans DG, Duan X. Flexible CoAl LDH@PEDOT core/shell nanoplatelet array for highperformance energy storage. Small 2013;9:98e106. [74] Mahanta D, Manna U, Madras G, Patil S. Multilayer selfassembly of TiO2 nanoparticles and polyaniline-graftedchitosan copolymer (CPANI) for photocatalysis. ACS Appl Mater Interfaces 2011;3:84e92. [75] Tang D, Liu J, Wu X, Liu R, Han X, Han Y, et al. Carbon quantum Dot/NiFe layered double-hydroxide composite as a highly efficient electrocatalyst for water oxidation. ACS Appl Mater Interfaces 2014;6:7918e25. [76] Dong S, Liu F, Lu C. Organo-modified hydrotalcite-quantum dot nanocomposites as a novel chemiluminescence resonance energy transfer probe. Anal Chem 2013;85:3363e8. [77] Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, et al. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat Mater 2009;8:76e80. [78] Dong F, Zhao Z, Xiong T, Ni Z, Zhang W, Sun Y, et al. In situ construction of g-C3N4/g-C3N4 metal-free heterojunction for enhanced visible-light photocatalysis. ACS Appl Mater Interfaces 2013;5:11392e401. [79] Liu G, Niu P, Sun C, Smith SC, Chen Z, Lu GQ, et al. Unique electronic structure Induced high photoreactivity of sulfurdoped graphitic C3N4. J Am Chem Soc 2010;132:11642e8. [80] Zhang J, Chen X, Takanabe K, Maeda K, Domen K, Epping JD, et al. Synthesis of a carbon nitride structure for visible-light catalysis by copolymerization. Angew Chem Int Ed 2010;49:441e4. [81] Zhang YJ, Mori T, Ye JH, Antonietti M. Phosphorus-doped carbon nitride solid: enhanced electrical conductivity and photocurrent generation. J Am Chem Soc 2010;132:6294e5. [82] Yuan X, Zhou C, Jin Y, Jing Q, Yang Y, Shen X, et al. Facile synthesis of 3D porous thermally exfoliated g-C3N4 nanosheet with enhanced photocatalytic degradation of organic dye. J Colloid Interface Sci 2016;468:211e9.

13

[83] Chen XF, Zhang JS, Fu XZ, Antonietti M, Wang XC. Fe-gC3N4-catalyzed oxidation of Benzene to phenol using hydrogen peroxide and visible light. J Am Chem Soc 2009;131:11658e9. [84] Xiang QJ, Yu JG, Jaroniec M. Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites. J Phys Chem C 2011;115:7355e63. [85] Zhang YJ, Mori T, Niu L, Ye JH. Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion. Energy Environ Sci 2011;4:4517e21. [86] Zhang YJ, Thomas A, Antonietti M, Wang XC. Activation of carbon nitride solids by protonation: morphology changes, enhanced ionic conductivity, and photoconduction experiments. J Am Chem Soc 2009;131:50e1. [87] Yan HJ, Huang Y. Polymer composites of carbon nitride and poly (3-hexylthiophene) to achieve enhanced hydrogen production from water under visible light. Chem Commun 2011;47:4168e70. [88] Ge L, Zuo F, Liu J, Ma Q, Wang C, Sun D, et al. Synthesis and efficient visible light photocatalytic hydrogen evolution of polymeric g-C3N4 coupled with CdS quantum dots. J Phys Chem C 2012;116:13708e14. [89] Cao SW, Yuan YP, Fang J, Shahjamali MM, Boey FYC, Barber J, et al. In-situ growth of CdS quantum dots on gC3N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation. Int J Hydrogen Energy 2013;38:1258e66. [90] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [91] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666e9. [92] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Two-dimensional atomic crystals. Proc Natl Acad Sci U. S. A 2005;102:10451e3. [93] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005;438:197e200. [94] Zhang Y, Tan JW, Stormer HL, Kim P. Experimental observation of the quantum Hall effect and Berry's phase in graphene. Nature 2005;438:201e4. [95] Peigney A, Laurent C, Flahaut E, Bacsa RR, Rousset A. Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon 2001;39:507e14. [96] Rao CNR, Sood AK, Subrahmanyam KS, Govindaraj A. Graphene: the new two-dimensional nanomaterial. Angew Chem Int Ed 2009;48:7752e77. [97] Kashuba AB. Conductivity of defectless graphene. Phys Rev B 2008;78:085415. [98] Hernandez Y, Nicolosi V, Lotya M, Blighe FM, Sun Z, De S, et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nat Nanotechnol 2008;3:563e8. [99] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat Nanotechnol 2008;3:101e5. [100] Nethravathi C, Rajamathi M. Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide. Carbon 2008;46:1994e8. [101] Fan X, Peng W, Li Y, Li X, Wang S, Zhang G, et al. Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation. Adv Mater 2008;20:4490e3. [102] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

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[103]

[104]

[105]

[106]

[107] [108]

[109]

[110]

[111]

[112]

[113]

[114]

[115]

[116]

[117]

[118]

[119]

[120]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5

nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558e65. Berger C, Song ZM, Li XB, Wu XS, Brown N, Naud C, et al. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006;312:1191e6. Dedkov YS, Fonin M, Rudiger U, Laubschat C. Rashba effect in the Graphene/Ni(111) system. Phys Rev Lett 2008;100:107602e5. Li X, Wang X, Zhang L, Lee S, Dai H. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 2008;319:1229e32. Yang S, Guo D, Su L, Yu P, Li D, Ye J, et al. A facile method for preparation of graphene film electrodes with tailor-made dimensions with Vaseline as the insulating binder. Electrochem Commun 2009;11:1912e5. Sutter PW, Flege JI, Sutter EA. Epitaxial graphene on ruthenium. Nat Mater 2008;7:406e11. Xiao FX, Miao J, Liu B. Layer-by-layer self-assembly of CdS quantum dots/graphene nanosheets hybrid films for photoelectrochemical and photocatalytic Applications. J Am Chem Soc 2014;136:1559e69. Liu ZF, Liu Q, Huang Y, Ma YF, Yin SG, Zhang XY, et al. Organic photovoltaic devices based on a novel acceptor material: graphene. Adv Mater 2008;20:3924e30. Czerw R, Foley B, Tekleab D, Rubio A, Ajayana PM, Carroll DL. Substrate-interface interactions between carbon nanotubes and the supporting substrate. Phys Rev B 2002;66:033408e11. Helander MG, Greiner MT, Wang ZB, Tang WM, Lu ZH. Work function of fluorine doped tin oxide. J Vac Sci Technol A 2011;29:011019e22. Li J, Cushing SK, Zheng P, Senty T, Meng F, Bristow AD, et al. Solar hydrogen generation by a CdS-Au-TiO2 sandwich nanorod array enhanced with Au nanoparticle as electron relay and plasmonic photosensitizer. J Am Chem Soc 2014;136:8438e49. Cao BA, Liu Z, Chu S, Wu M, Ye Z, Cai Z, et al. A facile onestep method to produce grapheneeCdS quantum dot nanocomposites as promising optoelectronic materials. Adv Mater 2010;22:103e6. Li Z, Cui X, Hao H, Lu M, Lin Y. Enhanced photoelectrochemical water splitting from Si quantum dots/TiO2 nanotube arrays composite electrodes. Mater Res Bull 2015;66:9e15. Antoniadou M, Sfaelou S, Lianos P. Quantum dot sensitized titania for photo-fuel-cell and for water splitting operation in the presence of sacrificial agents. Chem Eng J 2014;254:245e51. Peng F, Zhou Q, Zhang D, Lu C, Ni Y, Kou J, et al. Bio-inspired design: inner-motile multifunctional ZnO/CdS heterostructures magnetically actuated artificial cilia film for photocatalytic hydrogen evolution. Appl Catal B-Environ 2015;165:419e27. Wang B, An W, Liu L, Chen W, Liang Y, Cui W. Novel Cu2S quantum dots coupled flower-like BiOBr for efficient photocatalytic hydrogen production under visible light. RSC Adv 2015;5:3224e31. Alvi NH, Rodriguez PEDS, Aseev P, Hernandez VJG, Alvi AH, Hassan W, et al. InN/InGaN quantum dot photoelectrode: efficient hydrogen generation by water splitting at zero voltage. Nano Energy 2015;13:291e7. Kelkar S, Ballal C, Deshpande A, Warule S, Ogale S. Quantum dot CdS coupled Cd2SnO4 photoanode with high photoelectrochemical water splitting efficiency. J Mater Chem A 2013;1:12426e31. Guo CX, Dong Y, Yang HB, Li CM. Graphene quantum dots as a green sensitizer to functionalize ZnO nanowire arrays

[121]

[122]

[123]

[124]

[125]

[126]

[127]

[128]

[129]

[130]

[131]

[132]

[133]

[134]

[135]

[136]

[137]

on F-doped SnO2 glass for enhanced photoelectrochemical water splitting. Adv Energy Mater 2013;3:997e1003. Li TL, Cai CD, Yeh TF, Teng H. Capped CuInS2 quantum dots for H2 evolution from water under visible light illumination. J Alloys Compd 2013;550:326e30. Wang W, Yu JC, Shen Z, Chan DKL, Gu T. Photocatalytic hydrogen evolution on graphene quantum dots anchored TiO2 nanotubes-array. Chem Commun 2014;50:10148e50. Tang L, Deng Y, Zeng G, Hu W, Wang J, Zhou Y, et al. CdS/ Cu2S co-sensitized TiO2 branched nanorod arrays of enhanced photoelectrochemical properties by forming nanoscale heterostructure. J Alloys Compd 2016;662:516e27. Xu X, Zhou G, Feng B, Bao Z. Hu ZnO quantum dots arranged by hole scavenger groups for enhanced and stable photocatalyic hydrogen generation. J Mater Lett 2016;165:196e9. Zhang Z, Gao C, Wu Z, Han W, Wang Y, Fu W, et al. Toward efficient photoelectrochemical water splitting by using screw like SnO2 nanostructures as photoanode after being decorated with CdS quantum dots. Nano Energy 2016;19:318e27. Li S, Hu H, Bi Y. Ultra-thin TiO2 nanosheets decorated with Pd quantum dots for high-efficiency hydrogen production from aldehyde solution. J Mater Chem A 2016;4:796e800. Biswal N, Parida KM. Enhanced hydrogen production over CdSe QD/ZTP composite under visible light irradiation without using co-catalyst. Int J hydrogen energy 2013;38:1267e77. Liu Z, Wang Y, Wang B, Li Y, Liu Z, Han J, et al. PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties. Int J hydrogen energy 2013;38:10226e34. Xing X, Liu R, Yu X, Zhanga G, Cao H, Yao J, et al. Selfassemble of CdS quantum dots with polyoxometalates encapsulated gold nanoparticles: enhanced photocatalytic activities. J Mater Chem A J Mater Chem A 2013;1:1488e94. Yu Y, Ren J, Meng M. Photocatalytic hydrogen evolution on graphene quantum dots anchored TiO2 nanotubes-array. Int J hydrogen energy 2013;38:12266e72. Ma X, Zhao K, Tang H, Chen Y, Lu C, Liu W, et al. New insight into the role of gold nanoparticles in Au@CdS coreeshell nanostructures for hydrogen evolution. Small 2014;10:4664e70. Sahai S, Ikram A, Rai S, Dass S, Shrivastav R, Satsangi VR. CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen. Int J hydrogen energy 2014;39:11860e6. Yu Z, Meng J, Xiao J, Li Y, Li Y. Cobalt sulfide quantum dots modified TiO2 nanoparticles for efficient photocatalytic hydrogen evolution. Int J hydrogen energy 2014;39:15387e93. Yu W, Noureldine D, Isimjan T, Lin B, Gobbo SD, Abulikemu M, et al. Nano-design of quantum dot-based photocatalysts for hydrogen generation using advanced surface molecular chemistry. Phys Chem Chem Phys 2015;17:1001e9. Ikram A, Sahai S, Rai S, Dass S, Shrivastav R, Satsangi VR. Enhanced photoelectrochemical conversion performance of ZnO quantum dots sensitized a-Fe2O3 thin films. Int J hydrogen energy 2015;40:5583e92. Yan J, Li X, Yang S, Wang X, Zhou W, Fang Y, et al. Design and preparation of CdS/H-3D-TiO2/Pt-wire photocatalysis system with enhanced visible-light driven H2 evolution. http://dx.doi.org/10.1016/j.ijhydene.2016.08.188. Baslak C, Aslan E, Patir IH, Kus M, Ersoz M. Photocatalytic hydrogen evolution based on mercaptopropionic acid

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 5

[138]

[139]

[140]

[141]

[142]

[143]

[144]

[145]

stabilized CdS and CdTe quantum dots. Int J hydrogen energy 2016;41:20523e8. Liu Z, Wu J, Zhang J. Quantum dots and plasmonic Ag decorated WO3 nanorod photoanodes with enhanced photoelectrochemical performances. Int J hydrogen energy 2016;41:20529e35. Zhang J, Wang Y, Jin J, Zhang J, Lin Z, Huang F, et al. Efficient visible light photocatalytic hydrogen evolution and enhanced photostability of core/shell CdS/g-C3N4 nanowires. ACS Appl Mater Interfaces 2013;5:10317e24. Chen YC, Liou JY, Lan WJ, Chen JY, Wang HW. Hydrogen generation from CdS Modified-titania nanotube arrays. Adv Mater Res 2012;463e464:743e7. Su F, Lu J, Tian Y, Ma X, Gong J. Branched TiO2 nanoarrays sensitized with CdS quantum dots for highly efficient photoelectrochemical water splitting. Phys Chem Chem Phys 2013;15:12026e32. Kima Minju, Kim Young Kwang, Lim Sang Kyoo, Kim Soonhyun, In Su-Il. Efficient visible light-induced H2production by Au@CdS/TiO2 nanofibers: synergistic effect of coreeshell structured Au@CdS and densely packed TiO2nanoparticle. Appl Catal B Environ 2015;166e167:423e31. Nguyen VM, Cai Q, Grimes CA. Towards efficient visiblelight active photocatalysts: CdS/Au sensitized TiO2 nanotube arrays. J Colloid Interface Sci 2016;483:287e94. Li X, Li J, Cui C, Liu Z, Niu YS. PbS Nanoparticles sensitized ZnO nanowire arrays to enhance photocurrent for water splitting. J Phys Chem C 2016;120:4183e8. Shia XF, Xiaa XY, Cuia GW, Denga N, Zhaoa YQ, Zhuob LH, et al. Multiple exciton generation application of PbS quantum dots in ZnO@PbS/graphene oxide for enhanced

[146]

[147]

[148]

[149]

[150]

[151]

[152]

[153]

15

photocatalytic activity. Appl Catal B Environ 2015;163:123e8. Bai Z, Yan X, Li Y, Kang Z, Cao S, Zhang Y. 3D-Branched ZnO/CdS nanowire arrays for solar water splitting and the service safety research. Adv Energy Mater 2016;6:1501459e66. Lin CJ, Kao LC, Huang Y, Banares MA, Liou SYH. Uniform deposition of coupled CdS and CdSe quantum dots on ZnO nanorod arrays as electrodes for photoelectrochemical solar water splitting. Int J hydrogen energy 2015;40:1388e93. Yin X, Leilei L, Jiang WJ, Zhang Y, Zhang X, Wan LJ, et al. MoS2/CdS Nanosheets-on-nanorod heterostructure for highly efficient photocatalytic H2 generation under visible light irradiation. Appl Mater Interfaces 2016;24:15258e66. Liang S, Zhou Z, Wu X, Zhu S, Bi J, Zhou L, et al. Constructing a MoS2 QDs/CdS core/shell flower like nanosphere hierarchical heterostructure for the enhanced stability and photocatalytic activity. Molecules 2016;21:213e25. Qiu F, Han Z, Peterson JJ, Odoi MY, Sowers KL, Krauss TD. Photocatalytic hydrogen generation by CdSe/CdS nanoparticles. Nano Lett 2016;16:5347e52. Xu S, Du AJ, Liu J, Ng J, Sun DD. Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water. Int J hydrogen energy 2011;36:6560e8. Kumara DP, Shankar MV, Kumari MM, Sadanandam G, Srinivas B, Durgakumari V. Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation. Chem Commun 2013;49:9443e5. Hsu YK, Chen YC, Lin YG. Novel ZnO/Fe2O3 CoreShell nanowires for photoelectrochemical water splitting. Appl Mater Interfaces 2015;7:14157e62.

Please cite this article in press as: Kandi D, et al., Quantum dots as enhancer in photocatalytic hydrogen evolution: A review, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.166