Nanomaterials for photoelectrochemical water

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 4 3 ( 2 0 1 8 ) 4 8 0 4 e4 8 1 7

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Review Article

Nanomaterials for photoelectrochemical water splitting e review Josny Joy, Jinu Mathew, Soney C. George* Centre for Nanoscience and Technology, Amal Jyothi College of Engineering, Kottayam, Kerala, 686518, India

article info

abstract

Article history:

Photoelectrochemical (PEC) water splitting using nanomaterials is one of the promising

Received 28 September 2017

techniques to generate hydrogen in an easier, cheaper and sustainable way. By modifying a

Received in revised form

photocatalyst with a suitable band width material can improve the overall solar-to-

1 January 2018

hydrogen (STH) energy conversion efficiency. Nanomaterials can tune their band width

Accepted 16 January 2018

by controlling its size and morphology. In many studies, the importance of nanostructured

Available online 21 February 2018

materials, their morphological and crystalline effects in water splitting is highlighted. Charge separation and transportation is the major concern in PEC water splitting. Nano-

Keywords:

materials are having high surface to volume ratio which facilitates charge separation and

Nanomaterials

suppress electron-hole pair recombination. This review focuses on the recent de-

H2

velopments in water splitting techniques using PEC based nanomaterials as well as

Water splitting

different strategies to improve hydrogen evolution.

Band gap

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Quantum efficiency

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4805 Basic mechanism involved in PEC water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4805 Efficiency calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4806 Factors affecting the PEC efficiency of nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807 Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807 Dimensionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807 Temperature and pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807 Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807 Band gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808 pH dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808 Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808 Strategies for hydrogen generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808 Role of nanomaterials in PEC water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808 Titanium dioxide (TiO2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808

* Corresponding author. E-mail address: [email protected] (S.C. George). https://doi.org/10.1016/j.ijhydene.2018.01.099 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.


4805

Zinc oxide (ZnO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4809 Quantum dots (QDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4809 Hematite (a-Fe2O3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4810 Tungsten trioxide (WO3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4811 Bismuth vanadate (BiVO4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4811 Graphene and graphene based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4811 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4813 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4813

Introduction The greatest challenge in the near future of scientific era to be addressed is the huge energy requirements. Researchers are emphasizing their efforts more on investigating clean, safe and sustainable energy resources to cure out the expected shortage of non-renewable energies and to control the pollution. Hydrogen is one such fuel with no emission of pollutants when burned in oxygen. It is a very promising renewable fuel, used in vehicles, spacecraft propulsion, aircraft and electric devices. Hydrogen is locked up in water, hydrocarbons, and other organic matter. Techniques are introduced for the separation of hydrogen from these compounds. One of the exciting ways to extract hydrogen is water splitting process. Water splitting is the process of separation of water into oxygen and hydrogen. Various methods for water splitting have been issued like photoelectrochemical (PEC) [1], photocatalytic [2], radiolysis [3], photobiological [4] and thermal decomposition [5]. Radiolysis produces nuclear waste as a by-product [3]. Photobiological water splitting is with the aid of algae bioreactor, switches from oxygen production (normal photosynthesis) yielding low-H2 production rates [4]. The major drawbacks in thermal decomposition are its low yield of hydrogen and high temperature requirement. Simplest, efficient, cheap and clean methods for the production of hydrogen are photoelectrochemical and photocatalytic water splitting.

Basic mechanism involved in PEC water splitting Fujishima and Honda introduced the photoelectrochemical water splitting with high efficiency and low cost using a semiconducting material [6]. The basic principle behind PEC water splitting is the conversion of solar energy to hydrogen by applying an external bias on to the photovoltaic materials immersed in an electrolyte which contains redox couple, of which one is made of semiconductor, exposed to light and is able to absorb light. The electricity is then used for water electrolysis. Semiconductors are having a uniqueness to function as a photocatalysts. So, it plays a vital role in activating the chemical reduction and oxidation process in the presence of light. These photocatalyst electrodes are capable of capturing the light, which provides energy for the reactions and the additional voltage required to carry out the reaction is provided by an externally applied electric/chemical bias. This externally applied bias overcomes the slow kinetics and

provides sufficient voltage for the PEC cell to drive the reaction at a desired rate/current density. The photoelectrode with the absorption of photon, electrons are excited and electron-hole pairs are generated via redox reaction. Thus formed holes can oxidize the molecule and the electron can reduce Hþ to H2 [7e9]. The energy level where the probability of finding electron is half (Fermi Energy (Ef)) is important when the reference electrode is used to make measurements; it compares Ef of semiconductor with its own unchanging Femi level. In an intrinsic semiconductor Ef will be exactly at the centre band gap i.e. between Ec and Ev. Depending upon the type of dopant Ef shifts towards or away from Ec as depicted in Fig. 1. Equilibration takes place at the interface by shifting the Fermi Level of the semiconductor to match with the redox couple of the electrolyte. This will result in the formation of a thin region of space charge layer close to the surface of the semiconductor leading to band bending upwards or downwards, depending on the type of semiconductor (n-type/p-type) as shown in Fig. 2 [6]. From Figs. 1 and 2 we can summarize that, there is a strong dependency on the electronic properties of the photoelectrode to improve the water splitting efficiency. To optimize all the processes in a single component is proven impossible to achieve. Many efforts have been devoted to improve the efficiency and to absorb a wide range of light spectrum. One of the efforts is the construction of heterostructure photocatalysts (n-n/n-p/p-p junctions). This helps in the migration and separation of charge carriers. The recombination of photogenerated carriers can be reduced using heterojunction structure [10e12]. PEC water splitting can be achieved through two step process known to as Z-scheme, which a kind of mimicking the natural photosynthesis. In this system two different

Fig. 1 e Schematics on Ef shift for extrinsic semiconductor.

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Fig. 2 e Z-Scheme of PEC water splitting using n-type and p-type semiconductors.

semiconductors are used for photoexcitation and a reversible shuttle redox mediator (donor/acceptor pair). A simple schematic illustration of Z-scheme is shown in Fig. 2. The visible light can be more efficiently utilized through Z-scheme in comparison with the conversional one step process. In this system, the hydrogen generation takes place via the reduction of protons by conduction band electrons, where the electron acceptor generated by the H2 evolution photocatalyst is converted to its reduced form and the oxidation of donor electron by the valance band holes. Thus, the water splitting achieved with the occurrence of redox pair cycle. The factors affecting this mechanism structural and electronic properties of the photocatalyst and the presence of cocatalyst [13]. Through the Z-scheme migration of the photogenerated charge carriers an enhancement in the photocatalytic activity can be achieved [14]. Fig. 2 (a, b) are n- and p-type semiconductors involved in water splitting respectively. In Fig. 2 (c), two different photoelectrodes are combined, in which oxidation and reduction can be done simultaneously and can utilize light energy more efficiently. To increase the efficiency of the PEC, photogenerated carrier recombination should be reduced. In nanomaterials the charge carriers are generated at the surface due to its reduced size (increased surface to volume ratio), shape and controlled morphology and so the water splitting process occurs at the surface of the nanomaterials. In many studies it is revealed that there will be 50%e90% increase in the efficiency of PEC water splitting [8]. The overall reaction involved in water splitting mechanism [10], 2H2O þ hʋ / 2H2 þ O2 DG ¼ 4.92 eV (113 kcal mol1)

(1)

4 Hþ þ 4 e / 2H2 E◦red ¼ 0 V

(2)

2H2O / 4 Hþ þ 4 e þ O2 E◦red ¼ 1.23 V

(3)

Efficiency calculations To estimate the PEC performance of the electrode with the applied potential (Eapp), the applied bias photon-to-current efficiency (h) can be estimates as [15], Jph 1:23 Eapp Еοcp hð%Þ ¼ 100% P

(4)

where, Jph is the photocurrent density (mA cm2), Eocp is the open circuit at which Jph was measured, P is the incident light power density (mW cm2). The efficiency of a PEC device can be evaluated by incident photon-current conversion efficiency (IPCE) [16], IPCE ¼ ð1240 IPH Þ l Plight

(5) 2

IPH is the generated photocurrent density (A/m ), l is the incident light wavelength (nm), Plight is the photon flux (W/ m2), and 1240 is the unit correction factor. IPCE is the measure of photogenerated electrons collected per photon irradiated on the PEC surface. The fundamental factors that affect the overall efficiency and performance of PEC water splitting are (i) efficiencies of light absorption (hA), (ii) charge separation (hCS), (iii) charge transport (hCT), and (iii) charge collection/reaction efficiency (hCR). Therefore, solar-to-hydrogen (STH) conversion efficiency (hSTH) can be expressed as [17], hSTH ¼ hA hCS hCT hCR

(6)

The efficiency of STH greatly depends on the optoelectronic properties of the anode. Opto-electronic properties depend on the size, shape and morphology.


Nanomaterials are exhibiting a great impact in increasing the STH efficiency due to its large surface to volume ratio. Semiconductors with high quantum efficiency (QE) produce hydrogen under visible light illumination. QE can be calculated as [18], QE% ¼

¼

number of reacted electrons 100 number of incident photons

number of evolved Hydrogen molecules 100 number of incident photons

(7)

(8)

This paper briefly reviews how different shape, size and morphology of nanomaterials influence the PEC efficiency.

Factors affecting the PEC efficiency of nanomaterials PEC water splitting efficiency and performance is mainly depending on their orderliness and uniformity and morphology. In addition to the size reduction control over the shape and the structure is also an important fact in efficient water splitting mechanism.

Crystallinity Highly ordered crystalline materials are showing much high performance as compared to amorphous materials. For example, annealed/crystalline TiO2 nanotube shows better photocurrent properties than amorphous TiO2 nanotube. Amorphous TiO2 nanotube can be crystalline at an elevated temperature of about 300 C. A comparison on the photodegradation of amorphous and annealed TiO2 nanotubes [19] (Fig. 3). Highly ordered TiO2 nanotubes shows increased hydrogen generation by PEC water splitting. The photocurrent density of extremely high ordered (anodic TiO2) nanotubes is about 2.2 times higher than the normal TiO2 nanotubes and its good charge transferring properties as electrolytes can be in direct contact with large internal surface area of the tube [20]. As the crystallinity increases the density of defects and the site for electron-hole recombination decreases, which shows that structural property can influence the photocurrent efficiency.

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Dimensionality Nanomaterials can be classified based on the dimensionality as zero one, two and three dimensional (1D, 2D, 3D and 0D respectively) structures. 1D nanostructures includes nanorod (TiO2 nanorod [21], ZnO nanorod [22e26], WO3 nanorod [27]), nanowire (a-Fe2O3 nanowire [21], ZnO nanowire [28], MoS2tipped CdS nanowires [29], SiC nanowire [30]) and nanotube (TaON nanotube [31], TiO2 nanotubes [32e34], carbon nanotubes (CNT) [35,36]) are attractive photoelectrodes in water splitting process. 2D nanostructures (TiO2 thin film [37], hematite film [38], MoS2 nanosheets [39e41], g-C3N4 nanosheets [42]) are used for oxidation of water in PEC. Due to its small thickness and high surface area, it can efficiently harvest a large portion of UV light. It facilitates easy transport of charge carries onto the surfaces and exhibits improved hydrogen generation efficiency. Nanorods and nanowires are more photoactive and transport charge carriers more efficiently compared to thin films. Even though only a very few parts is exposed to light absorption, nanotubes are having high surface area, which facilitates enhanced redox reaction rate than nanorods and nanowires [17]. 1D structure provides rapid diffusion in a single direction, yielding a low recombination of electronehole pairs. 3 D nanostructures (Dendritic a-Fe2O3) are also noticeable for its high performance photoactivity. It allows efficient light absorption by reducing the distance for the photogenerated holes to diffuse the electrolyte/electrode interface [17]. 0 D nanostructure (quantum dots (QDs)) shows efficient visible light absorption and very good photocatalytic activity. When a semiconductor is decorated with appropriate band gap QDs, the electron-hole recombination rate is reduced and hence there is an enhancement in the photoabsorption in visible region [43]. 0 D nanomaterials include carbon dots (CD) [44,45], CdS QD [46], CdSe QD [46,47], graphene QD [48e50], Co3O4 QD [51] and graphitic carbon nitride QD [52].

Temperature and pressure Temperature plays a vital role in the enhancement of PEC efficiency. Usually, experiments are conducted at high temperatures. In a study, low temperature treatments show an improved PEC efficiency. The IPCE increased up to 95% at low temperature [23]. Investigators showed that, there is an enhancement in the band gap with increase in working pressure [53].

Size

Fig. 3 e Photocurrent efficiency of TiO2 nanotubes in its different crystalline forms.

With the size effects of cocatalysts, an improvement in the PEC performance can be made. The design of good catalyst is one of the strategies to improve PEC efficiency. In smaller particles the dominating factor is the electrokinetics which leads to higher electron-hole recombination. Larger particles are having band bending properties and so, improved charge extraction at the interface of electrode and the electrolyte can be achieved. So, for the better PEC performance larger cocatalysts are suited [54].

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Band gap The main advantage of nanomaterials is the fine tuning of band gap. To enhance the PEC properties is ascribed to narrow down of the band gap [55e58]. This helps in the absorption of wide range of solar spectrum. Large band gap fails to absorb the required intensities of light energy to efficiently split the water. The appropriate band gap for efficient PEC water splitting is 1.6e2.2 eV. In this range of band gap the band edge position is accurate and higher photocharge carrier mobility. Narrowing the band gap can be achieved through incorporating donor-acceptor into the semiconductor.

pH dependency PEC cells operates at varied pH conditions. The equilibrium of the water splitting reaction very much depends on the pH of the electrolyte solution, which determines the net total charge adsorbed at the surface to be positive, zero, or negative. Even under harsh and corrosive electrolytic conditions the photocorrosion of the electrode should be reduced to maintain the efficiency. The migration of ions during the reactions can weaken the surface of the electrode. Nanomaterial incorporated photoelectrodes shows stability in varied pH conditions [59,60]. The photocatalytic property and stability improved when the electrolyte solution is buffered [61].

Light The light source should be specified and spectral distribution and AM1.5G spectrum of intensity (the AM1.5 Global spectrum is having an integrated power of 1000 W/m2) [62]. Doping of semiconductors with nanomaterials enables the absorption of UV as well as visible light region [63]. Other factors that affect the efficiency of PEC water splitting are preparation-dependent grain morphology, doping affected grain nucleation, lattice impurity, pore structuredependent water adsorption/desorption kinetics, exposure of specific facts, and semiconductor/semiconductor or semiconductor/metal interfacial characteristics [64].

Strategies for hydrogen generation The strategies for improving the hydrogen evolution can be categorized into two streams (a) increasing active sites and (b) improving electrical conductivity. To proceed the overall water splitting in a more direct and smooth manner the exposure and accessibility of active sites, vectorial electron transport capability, and release of gaseous products should be enhanced [65]. Increasing the active site can be achieved by plasma treatments and by reducing the thickness of layers. This helps in exposing more active sites by edge exposure and by creating more defecting sites [66e70]. Improvement in electrical conductivity or charge transportation kinetics can be attained through straining the basal plane [71] and doping [72]. Multisite catalysis is another trending strategy for improving the rate of hydrogen energy. Hydrogen evolution reaction (HER) is a multistage reaction process in which the first step is pretty longer and this affects

the hydrogen evolution rate. This limitation can be overcome with the help of multi-electron heterogeneous redox catalysis or multisite catalysis. In this process, the density of reduction or oxidizing species accumulated on the active site for determines the rate laws for the photoelectrochemical reaction. The rate flow for photoelectrochemical can be done through direct assessments [73].

Role of nanomaterials in PEC water splitting Nanoscale sizes are comparable to carrier scattering lengths, significantly reducing the scattering rate and increasing carrier collection efficiency. Nanomaterials have strong absorption coefficients due to an increase of oscillator strength, thereby enabling high conversion efficiency. The band gap of nanomaterials (QD) can be tuned to absorb in a particular wavelength by varying size and, in principle, cover the whole solar spectrum. The electronic band structure can be controlled by doping. Bottom-up growth approaches, which use smaller components to produce larger and more complex structures which allow scalable synthesis of single crystal nanostructures on flexible substrates under mild conditions, leading to light weight and low cost [16]. Nanoparticles are the building block for photocatalyst as their mass and charge transfer is fast and they exhibit increased light absorption and reduced light scattering. The nanoparticles can be coated on the electrode or can be suspended/dispersed in reaction medium (water) which shows improved photocatalytic activities. Water splitting efficiency enhancement was observed at the semiconductor/electrolyte interface, where the metal nanoparticles were localized. This is due to the minority charge transport to the electrolyte from the electrode [74]. In an investigation, CdS nanoparticles was prepared with uniform size ranges and experimentally proven its improved photocatalytic activities as compared to bulk CdS [75].

Titanium dioxide (TiO2) TiO2 is the most widely used material for water spitting and hydrogen generation studies [8,76e81]. Bulk TiO2 is having bandgap of 3.03e3.18 eV leads to poor efficiency in the solar light absorption although its stability and low cost makes them potentially applicable in water splitting. TiO2 nanotube is the superior alternative among TiO2 nanomaterials. In bulk TiO2 faster electron-hole recombination occurs and this leads to low quantum efficiency. In order to reduce this phenomenon, materials with small band gaps were decorated along with TiO2. An alternative for decorating TiO2 with a material, black TiO2 nanoparticles were developed, which shows more efficient photocatalytic activities [76]. The size and shape of nanoscale TiO2 can have a significant impact on the conversion efficiency. Recently, nanostructured TiO2 are having important role in improving the efficiency of light absorption and hydrogen generation rate. On comparing TiO2 nanorod and nanowire, nanowire are efficient than nanorods [7,82]. There is an increased cell performance with increase in annealing temperature [19]. This is due to formation of crystalline structure which increases the charge transport. Fig. 5


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shows the effect of different annealing treatments on the morphology and structure of a TiO2 nanotube. Fig. 4a shows the morphology of these tube layers before annealing and possess a smooth inner and outer tube wall, and a welldefined tube tops. After thermal annealing the tube diameter and outer tube wall remain the same and roughening of the inner tube wall occurs (Fig. 4b). Fig. 4c shows “water annealed” tubes, with a strong change in morphology. The IeV curve in Fig. 5 shows that, the water annealed tubes have a 3 times higher efficiency than amorphous with an improvement of h < 0.01% to h z 4.68% [19].

Zinc oxide (ZnO) ZnO nanomaterials have been extensively explored in water splitting [83e87]. It possess an energy band structure (bandgap ~3.3 eV) and optoelectronic properties similar to those of TiO2. The photocatalytic efficiency of ZnO is low because of its weak visible light absorption and low photocatalytic quantum efficiency [83]. ZnO is having the band gap energy of 3.37 eV and having high exciton binding energy of 60 meV at room temperature. They are having potentially high electron mobility, chemically stable and highly transparent. Hydrogen generation is depending on the morphology, electrolyte interactions and defect density. To optimize the generation, nanostructure should tailor their fundamental properties through the deposition techniques and annealing. The photon-tohydrogen efficiency dependence on other deposition techniques is depicted on Figs. 6 and 7. The defects in thin films are having high impact on the photoresponds and PCE water splitting. As the density of the defects increases, there is a decrease in the photoresponds [83].

Fig. 5 e IeV curves for TiO2 nanotube before and after different annealing treatments [19].

Quantum dots (QDs) Semiconductor QDs exhibits several unique properties which makes their potential utility in PEC cell. Mainly, the effective band structure of QDs can be fine-tuned by altering their size or dimensions to maximize the efficiency of solar light absorption. Thus, the electronic energy levels and physical processes (such as electron-hole generation) can be controlled over the heterojunction. To improve the photoactivity of catalysts, QDs, such as CdS [18,93], CdSe, CdTe [102], Ag2S, PbS and CuInS2. QDs are having

Fig. 4 e Cross sectional SEM images of TiO2 nanotube layers: (a) as formed sample, (b) thermally annealed, (c) water annealed sample, (d) hydrothermally treated sample [19].

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Table 1 e Hydrogen evolution for nanomaterials. Nanomaterial

Fig. 6 e Photon to hydrogen efficiency of ZnO thin film deposition via pulsed laser deposition (PLD), oblique-angle deposition (OAD) and glancing-angle deposition (GLAD) under the applied potential of þ1.0 V.

Fig. 7 e ZnO thin film deposition: A) GLAD, B) PLD, and C) OAD, all show significant photoresponse, but the GLAD nanoparticle samples have superior characteristics with a photocurrent of 142 mAcm¡2 at 1.0 V. The PLD sample (D), and the OAD sample (E), show increased dark current under bias of þ0.8 to þ1.1 V, while the GLAD sample (F) shows no discernible dark current [83].

narrow band gap and are used as dye sensitizers, which is utilized to sensitize the semiconductors with wide band gap in the visible spectrum. From the recent reports (Table 1), CdS QD-sensitized TiO2/ Pt photocatalyst with a hydrogen generation rate of 0.8 mmolh1 g1. CdS/vermiculite is having enhanced water splitting efficiency (QE: 17.7%) under solar light, which is much higher than that for CdS/attapulgite (QE: 8.2%). The wide band gap of vermiculite (3.5 eV) can be altered by CdS, which possess narrow band gap energy (2.4 eV). Its hydrogen generation rate is as high as 92 mmolh1, which is 10 times higher than pure CdS. Photocurrent density of CdS/vermiculite is 0.4 mAcm2 at 1.90 V, which is five times more than that of the bare vermiculite film [93]. CdS QD sensitized TiO2 nanowire electrode with photocurrent density of 500 mV bias is 0.2 mA/cm2 whereas, no visible

MoS2eCdS CdS TiO2 nanotube TiO2 thin film mesoporous SrTiO3 nanocrystal SrTiO3 nanocrystal b-SiC nanoparticle CdS QD-sensitized TiO2/Pt pure MoS2 spheres Au multimer@MoS2 spheres CD-CdS TiO2 Nanoparticle CD- TiO2 nanoparticle nanostructured CdS CdS/GQDs nanohybrids 1D CdS nanowire/2D MoS2 nanosheet CNTs/MnO2eC3N4 CNTs/C3N4 TiO2eNi(OH)2/CNT/CdS TiO2eNi(OH)2/CdS CNTs/ZnFe2O4 graphitic carbon nitride QD/TiO2 nanotube pure TiO2 GQDs/TiO2

Hydrogen production rate

Ref.

5.24 mmol/h/g 0.31 mmol/h/g 2.32 mmol/h/g 59.8 mmol after 8 h 188 mmol/h/gcat

[88] [89] [90] [37] [91]

276 mmol/h/gcat 0.85 mmol/g/h 0.8 mmol/h/g 881.6 mmol/g 2997.2 mmol/g 2.55 mmol/h 61.85 mL/h/cm2 110.45 mL/h/cm2 2945 mmol/h 95.4 mmol/h 9.73 mmol/h/g

[91] [92] [93] [94] [94] [44] [45] [45] [95] [96] [97]

122 mmol/h 44.64 mmol/h 12 mmol/h/g 5.36 mmol/h/g 18 mmol/g/h 22.0 mmol/h/cm2

[98] [98] [99] [99] [100] [52]

18.74 mmol 41.26 mmol

[101] [101]

light response for the unmodified TiO2 nanowire, i.e., the photocurrent density doubles for CdS QD sensitized TiO2 nanowires as compared to the unmodified TiO2 nanowires [18]. NiO/CdTe QDs electrode is having good PEC water splitting activity and stability for H2 generation at low potential. The observed photocurrent density is 26 mAcm2. It shows high efficiency in hydrogen evolution with 100% Faradaic efficiency [102]. Main characteristics of QDs are (i) tunable absorption properties, (ii) high stability, (iii) suitable for solar spectrum and (iv) multiple band gap sensitizations.

Hematite (a-Fe2O3) Hematite (a-Fe2O3) has been found to be a most promising material in water splitting. It is having a narrow band gap ~2.1 eV, which facilitates it to absorb about 40% of solar light. While comparing with other photoanodes hematite possess excellent stability under most of the aqueous environment in addition to compatibility, abundance and low-cost [103]. The synthesis route and morphology of a-Fe2O3 plays a major role in controlling the PEC efficiency. In PEC test, a-Fe2O3 plumelike hierarchical nanoplatelet showed a photocurrent of 1.39 mA/cm2 at 1.55 VRHE [104]. Higher PEC performance can be seen in a-Fe2O3 nanorods as it is having small diameter, large number of holes, vertical growth of the array on the substrate along [109] axis and higher density of donor electrons [105]. Temperature affects the alignment of nanorod. Highly ordered nanorods shows enhanced photoelectrochemical response, mainly attributed to the reduced resistance of the charge transfer inside the electrode and across interface of


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Fig. 8 e Z-scheme system consisting of a Pt-loaded metal sulfide photocatalyst with a p-type semiconductor character and RGO-TiO2 composite photocatalyst for water splitting [131]. electrode and electrolyte [106]. The stability and photocurrent efficiency can be improved by doping with a CoePt over layer [107]. The current density of a-Fe2O3 increased by 180% through pulse reverse electrodeposition when compared to the normal electrodeposition method [108].

Tungsten trioxide (WO3) WO3 nanomaterials brings new opportunities for the development of PEC water splitting with superior performance [109e117]. It is having a band gap around ~2.5e2.7 eV. The merits of WO3 nanomaterials are high crystallinity, porosity, the ability of capturing 12% of the solar illumination, moderate hole diffusion length, good chemical stability and easy and cheap preparation methods. The photocurrent density depends on the substrate and the treatment temperature due to the charge transport resistance imposed on the substrate. The effect of substrate can be ascribed as the function of temperature. The photocurrent density increases with increasing temperature. At a higher temperature there is a transition from crystalline to amorphous state and also the bulk recombination of charge carriers occurs which results in lowering of photocurrent density and affects the stability of the electrode [112]. Photocurrents of 2.02 mAcm2 and 0.62 mAcm2 for the WO3 photoanodes on FTO and metal substrates observed at 65 C [113]. WO3 nanoflakes with hierarchical architecture improved photoelectrochemical activity because of the multiple light scattering which facilitates better light absorption and the exposure of active sites resulted in enhanced interfacial charge transfer [114]. The efficiency of PEC with WO3 can be improved by using dopants [109,110] and composites [115]. Nanocomposites can enhance the PEC performance. The electron-hole recombination rate can be reduced and also improves the permeation properties and aids the transport of charge carriers. With the incorporation of rGO into WO3 the H2 conversion efficiency

increased by two folds (0.6% for WO3 to 2.7% for rGO-WO3) [115e117].

Bismuth vanadate (BiVO4) BiVO4 nanoparticle has gained considerable attention in the field of water splitting due to its good absorption capacity with band gap energy 2.4e2.6 eV [118e127]. The well-polished band edge, moderate band gap energy and stability even in acidic aqueous solution makes BiVO4 nanoparticle well suited to be considered as a photoanode for the use of water splitting. Through electrostatic spray pyrolysis low cost and flexible BiVO4 thin films can be prepared with photocurrent density that reach to 0.8 mA/cm2 at an applied potential of 1.9 V versus RHE [118]. For the further improvement of photocurrent density doping can be adopted. Doped BiVO4 thin films can performs double times better than undoped BiVO4 thin films [122,124,125].

Graphene and graphene based materials Graphene is an active area in water splitting due to its uniqueness in adsorption and reaction properties [128e131]. Graphene, a single layer of sp2-bonded carbon atoms, shows excellent electronic mobility as all the atoms are surface atoms; it has a large exposed area and a p-conjugation structure. Due to these properties graphene sheets can efficiently accept and transport the electrons from the excited semiconductor, suppress charge recombination, improve the interfacial charge transfer processes and provides much more active adsorption sites and photocatalytic reaction centres, which consequently enhances the photocatalytic H2 production activity. Hydrogenation of graphene can be easily done even at low temperatures. Presence of metal catalysts in graphene system can improve the capability of hydrogen storage. Process is fully reversible (complete desorption when

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Fig. 9 e Scheme of the surface phase junction in anatase¡graphene¡rutile heterojunction photocatalysts: (i) electrons excited from VB to CB; (ii) electrons moved to Pt particles on TiO2 surface; (iii) electrons moved from rutile CB to Pt particles on graphene surface; (iv) electrons moved from rutile CB to the surface-defect state of anatase; (v) proton reduction on Pt particles, (b) Time-dependent H2 production results of samples with different Anatase/Rutile ratios [133]. heated) [128]. Graphene is hydrophobic in nature and so it cannot be used directly in water splitting. Graphene is a zero band gap semiconductor as the bonding orbitals p and antibonding orbitals p* touch at the Brillouin zone corners. Due to the overlapping of electronic bands, the electrons and holes in graphene sheet behave as massless charges. The electronic properties of graphene can be altered by introducing heteroatoms or functional groups. For instance, the electronic properties of graphene can be tuned by adding oxygen atoms and forms graphene oxide (GO). GO is hydrophilic and exhibits photocatalytic activity. On oxygen adsorption, p and p* orbitals separation occurs and creates a band gap in the sheet. GO can made either p-doped material or n-doped material by proper substitution of edge site oxygen atoms. Oxygen is more electronegative than carbon atoms and on doping GO with more quantity of oxygen can makeup p-type semiconductors and on replacing edge site oxygen

functionalities with nitrogen group, it can be transformed to p-type semiconductors. Thus, GO is having the flexibility of altering its properties, which makes it an excellent choice in water splitting field. Photo-generated electrons may participate in one of the two following ways: reacting with oxygencontaining functional groups on the GO sheets to form reduced-GO and transferring through reduced-GO sheet to react with Hþ ions forming H2. The reduced-GO sheet can serve as an electron collector and transporter to efficiently separate the photo-generated electron-hole pairs whereas, the oxygen-containing functional groups on the GO sheets can compete with the Hþ ions to consume the photo-generated electrons. When GO content is excessive, GO reduction reaction costs a lot of photo-generated electrons and thereby reduces QE and it restrains the hydrogen production. For example, Ptcomposite electrode loaded with 1 wt% of GO is having


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Fig. 10 e Scheme of the a-Fe2O3 modified with rGO and its dependency on photocurrent density [134].

22.5% QE while, the same electrode loaded with 40 wt% is having 0.6% QE [129]. Lengthening the lifetime of the charge carriers effectively only if the GO content is proper [130]. The incorporation of GO with semiconductors creates the p-n junction, which also considerably improves the separation of photogenerated charges (Fig. 8) [131]. The electrons which are excited to the conduction band of the semiconductor are injected into the reduced graphene in the graphene/semiconductor as the graphene/graphene-redox potential is lower than the wide band gap semiconductors [132]. The unpaired p-electrons in GO can bind to wide band gaped semiconductors to provide a heterojunction, which in turn facilitates increased absorption of visible light. The higher H2 production rate can be obtained by anatasegraphenerutile heterojunction system (Fig. 9) [133]. a-Fe2O3 nanostructure with rGO sheet shows enhanced photocurrent density (1.06 mAcm2), which is 1.47 times higher than that of the pristine Fe2O3 (Fig. 10) and also it improves charge separation efficiency by 1.82 times and charge injection efficiency by 1.67 times [134]. GO can be used as a nanoparticle segregation and dispersion in water. This can be done by channelling the electrons in narrow band gaped nanoparticles to GO, which suppresses the electron-hole recombination and prevents from aggregation. GO can be tuned for multiple functions. Reduced or partially reduced GO can be blended with metal electrodes as they serve as electron sink to provide electron-hole exciton separation and allows transport of charge carriers due to its pconjugation and large surface area.

Conclusion Various nanomaterials such as nanotubes, nanowire, and nanorods, as well as nanosheets of semiconductors and graphene based materials were reviewed for PEC electrolysis.

The electronic properties of nanomaterials can be altered by tuning the energy band gap levels. The incorporation of nanomaterials in semiconductors reduces the band gap and allows more solar light absorption. Nanostructures can efficiently transport charges and can inject electrons and holes at the water-nanomaterial interface for oxidation and reduction process. However, the efficiency of the overall PEC cell for water splitting is still quite low, it is necessary to improve the efficiency and practicality of the photo-water splitting system particularly the development of highefficiency and cost-effective materials. The unique properties of nanomaterials provide great opportunities for developing systems with high photocatalytic efficiency for water splitting using sunlight.

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