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Nano-configured Opto-electric Ceramic Systems for Photo-electrochemical Hydrogen Energy

  • Pramod H. BorseEmail author
Living reference work entry

Abstract

Functional materials such as electro-optic or opto-electric ceramics are of fundamental as well as of technological interest in the context to energy application. Natural resources those include sunlight, wind, water, are available in abundance on our planet earth, ever-growing human energy requirements necessitates and demands a way to make their use for generation of renewable energy. Ceramics are excellent candidates in view of their exciting optical, mechanical, thermal, electrical, and corrosion-resistant properties. Photocatalytic material systems have fascinating ability to split water molecules under the presence of photon and electrical energy, by virtue of their suitable band energetics with respect to water redox levels. The water splitting phenomenon is an important wrt hydrogen energy technology which demands energy production via renewable energy sources. Photo−/electrocatalysts which are capable of efficiently splitting water molecule with a sustainable performance are highly desirable. The physicochemical study of materials to identify best suited photocatalyst has been a topic of prime interest. The present chapter discusses nano-configured photocatalysts reported till date and compares their performance and scope wrt their commercialization for hydrogen-producing technologies.

Keywords

Photoelectrochemical Hydrogen energy Nano photocatalyst Opto- Electric ceramics Metal oxides Sulfides Ferrites Nitrides Phosphides Photo assisted energy Functional Energy 

Introduction

Opto-electric or electro-optic ceramics are the special class of functional materials which find a rewarding place in photo-associated energy applications [1, 2]. Especially their functionality is of utmost importance in case of energy-related applications such as wet photovoltaics and dry photovoltaics [3, 4, 5]. Here the material system is expected to be optically and electronically active so that whenever the material is under the influence of photons/electrons, it produces useful energy due to photoexcited electrons across the valence and conduction band. Especially in wet photovoltaics, the photocatalysis/photoelectrochemical mechanism necessitates the utilization of photoactive ceramic system as TiO2 [6], Fe2O3 [7], WO3 [8], BiVO4 [9], CdS [10], BP [11], TiON [12], etc. As shown in Scheme 1, there are several ceramic oxide systems as titanates, ferrites, tungstates, and non-oxide as sulfides, nitrides, carbides, etc., which display exciting opto-electric properties desirable for photocatalytic hydrogen energy production.
Scheme 1

Schematic showing categorization of known opto-electric ceramic photocatalysts

Photocatalytic (PC) and photoelectrocatalytic (PEC) hydrogen production via water splitting (WS) is one of the most attractive routes to generate renewable fuel. Such PC and PEC technology is expected to dominate the energy sector in the decades to come in case other cheaper energy technologies become inaccessible or hydrogen technology become economic [13]. This technology demands discovery of an efficient and stable hydrogen-producing material. This new PC/PEC material should exhibit well-suited electronic and optical properties which thermodynamically favor the reduction and oxidation of water to yield stoichiometric hydrogen and oxygen during complete splitting of water molecule [14, 15]. Figure 1 schematically shows band energetics of known photocatalysts (ceramic materials) indicating how their valence band (VB) and conduction band (CB) are placed wrt redox level of water. In brief it shows how water reduction level (H+/H2) and water oxidation level (O2/H2O), respectively, display their energy wrt to their VB and CB edges. The straddling VB and CB edges across the redox levels of H2O molecule facilitate the reduction or oxidation of H2O.
Fig. 1

Band energetics of some of the typical opto-electric ceramic photocatalysts

Let us understand the mechanism of PEC water splitting that generates useful hydrogen/oxygen via photoreduction or photooxidation. The schematic depicted in Fig. 2 clearly shows that a typical PEC setup consists of an electrically biased photoanode (for water oxidation) and cathode and photocathode (for water reduction) immersed in an electrolyte. For simplicity, we discuss the specific case of a set of electrode consisting of photoanode and counter electrode, where photons illuminating the photoanode (made of opto-electric ceramic) facilitate production of exciton (electron-hole pair). This yields hole (h+) at valence band and electron (e) at conduction band of the light-active photoanode. Such exciton facilitates photooxidation of water molecule by virtue of h+ at anode and via e flowing to the cathode that reacts at the cathode/electrolyte interface to yield hydrogen gas molecule. It may be noted that the electronic structure of the electrode material plays a vital role as seen in Figs. 1 and 2, to enable the material to facilitate redox reaction over it. In addition, the band energetics also controls undesirable e-h recombination reaction, a suited synergy yields the stability of the electrode material in electrolyte which is highly desirable to obtain a sustainable hydrogen production. Thus electronic and optical structure is a key to identify best suited photocatalytic material. It may be added that during the redox reaction-related charge transports in electrode-electrolyte system, the type of “material electrical conductivity” of photocatalyst either p-type or n-type, decides its applicability for the water reduction reaction or water oxidation reactions, respectively, in line with Fig. 1.
Fig. 2

Schematic explaining mechanism of working of photoelectrochemical cell for water splitting

We discuss here various opto-electric ceramics that constitute a special class of photocatalyst as indicated in Scheme 1, viz., different metal oxides as titanate, ferrite, tungstates, vanadates, etc. As a general observation, in these ceramic oxides (except titanate, niobates, tantalates), the d-orbitals play an important role in rendering the suited band energetics for the photo- and photoelectro-induced water splitting reaction.

Tables 1 and 2 show various physicochemical properties of the known photocatalysts reported in literature [8]. It mostly does imply that none of the material system possesses all the properties which are desirable for a good photocatalyst. Say, for instance, titanates are electrochemically very stable and nontoxic by nature, but they exhibit a very large band gap (3.2 eV), which makes them unsuited for solar light absorption. On the contrary, ferrite, vanadates, and sulfides, though well suited for the visible light absorption (exhibit low band gap), show pH selective stability or overall a very poor stability (as in sulfides). So the material ranging from TiO2 to CdS exhibits wide differences in their properties; thus, they cannot be treated as an ideal material for the visible light photocatalysis.
Table 1

Comparision of important physico-chemical properties of titanates, ferrites, tungstates as known opto-electic ceramic photocatalysts

 

Property

TiO2

Fe2O3

WO3

Crystal structure details

Crystal phases

Anatase, rutile, brookite

Hematite, maghemite

Tungstite, meymacite, hydrotungstite

Lattice symmetry

Tetragonal

Rhombohedral

Monoclinic

Lattice parameters (nm)

a = 0.378 b = 0.951

a = 0.5035 c = 1.374

A = 0.73, b = 0.753, c = 0.768, b = 90.54

Opto-electronic details

Optical band gap (eV)

3.2

2.2

2.8

Absorption

UV region|

Visible region

Visible region

Mobility (cm2 V−1. S−1)

4

<0.01

6.5 cm2/vs

 

Dielectric constant

60

32

50

 

Stability

Stable in aqueous media and under illumination

Stable in aqueous media for pH > 3 and under illumination

Photo and chemical stability in acidic and semi acidic over pH less than 8

 

Toxicity

Nontoxic

Nontoxic

Nontoxic

 

Density (g/cm3)

3.89

5.25

7.16

Table 2

Comparison of important physico-chemical properties of vanadates, sulphides, phosphides and nitrides as know opto-electric ceramic photocatalysts

 

Property

BiVO4

CdS

BP

TiN

Crystal structure details

Crystal phases

Pucherite, clinobisvanite, dreyerite

Greenockite, hawleyite

Phosphorous

Osbornite

Lattice symmetry

Monoclinic

Hexagonal, cubic

Orthorhombic

Cubic

Lattice parameters (nm)

A = 0.309, b = 0.308, c = 0.312, b = 90.34

a = 0.413 c = 0.674

A1 = 0.36, a2 = 0.45

A = 0.424

Optoelectronic details

Optical band gap (eV)

2.6

2.4

0.3,1.5 (monolayer)

3.4

Absorption

Visible region

Visible region

Visible and near infrared region

Visible region

Mobility (cm2 V−1. S−1)

0.2

340

1000

 

Dielectric constant

0.02

8.9

12.5

 
 

Stability

Near neutral aqueous environment

Corrosive both chemically and under illumination

Easily degrades in oxides atmosphere.

Chemically stable in almost acids except nitric acid

 

Toxicity

Nontoxic

Toxic

Nontoxic

Nontoxic

 

Density (gm/cm3)

6.1

4.8

2.6

5.2

At this point, it may be added that computational simulations of electronic and optical properties of a lattice are known to guide the experimentalist for understanding the suitability of the material for solar photocatalysis. Such properties can be simulated by using density functional theory (DFT), a quantum mechanical-based modeling [12, 13]. Thus, the DFT results and their correlation for improving understanding of photocatalyst have been appropriately mentioned at various instances during the review below.

There can be an exhaustive list of the materials that can be found in different review papers in literature. Here we aim to precisely describe the important candidate in mentioned category as well as to provide a futuristic view on the opto-electric ceramic wrt its application in visible light-induced water splitting. Accordingly, the following section briefly and explicitly reviews the past and recent work for the respective systems.

Metal Titanates

TiO2 and SrTiO3

Extensive work has been carried out on titanates, viz., TiO2, SrTiO3, and BaTiO3, and a number of ternary phase metal oxides though they were proved to be very stable; however, their optical response in UV range hindered their suitability for the solar hydrogen generation.

TiO2 is an earliest example known to be useful for photoelectrowater splitting after the work of Fujishima and Honda in 1972 [3]. It [16] is known to exist in different crystalline phases, viz., anatase, rutile [17], and brookite [18], among which the anatase is known to be a better photocatalyst [19]. It exhibits tetragonal crystal structure with band gap of 3.2 eV; it is environment-friendly and shows good stability in aqueous solution under the photo-illumination. There were several efforts to engineer its band gap by method of anionic and cationic doping to enable it to absorb visible light radiation [12]. Asahi et al. doped different dopants as C, N, S, etc. at the substitutional sites of oxygen that led to induce the defect state in the band structure of TiO2, thus making it suitable to absorb visible light photons [12, 20]. However such materials did not yield very high efficiency for visible light water splitting under in view of their low coefficient of absorption [21, 22]. Table 3 lists various TiO2-based opto-electric ceramics reported in literature [4, 5] for water splitting under UV light radiation. It may be noted that due to the suited band energetics of TiO2 (see Fig. 1), it can photooxidize or photoreduce water molecule in the presence of some scavenger or co-catalyst. The anatase form of TiO2 has been studied exhaustively and is known to be better as well as efficient crystal phase compared to the other structural phases of TiO2 [28]. It may be noted that the efficiencies reported in various reports are dependent on various parameters wrt power of light source, electrolyte used, scavengers, co-catalysts, etc. Thus it becomes difficult to compare the efficiencies, though they are presented in Table 3. Essentially, it may be noted from Table 3 that mesoporous TiO2 modified with Pt shows 6925 μmol/hr. (which is the highest) of hydrogen evolution under 300 W Hg arch lamp, whereas nanostructure configuration of TiO2 yields 0.9 mA/cm2 of photocurrent generation. Due to variation in such parameters, it may not be an easy task to compare the performance of photocatalysts.
Table 3

Hydrogen evolution performance of various types of titanates (TiO2 based) during photoelectrocatalytic water splitting

S. no

Electrode type

Details of the experimental parameters and photocurrent density

Synthesis/deposition method

Co-catal./H2 (μmol/h)

Co-catal./O2 (μmol/h)

QY %

1

TiO2 (anatase) [28]

Light source, 500 W hg; electrolyte, water vapor

MCB TiO2

Rh/1497

 

29 (340 nm)

2

TiO2 (anatase) [23]

Light source, 450 W hg; electrolyte, NaOH

MCB TiO2

NiOx/32

NiOx/14

3

TiO2 (anatase, 78%) [24]

Light source, 400 W hg; electrolyte, Na2CO3

P25 TiO2

Pt/1893

Pt/957

4

TiO2 (anatase, 78%) [25]

Light source, 250 W hg; electrolyte, pure water

P25 TiO2

Pt/353

Pt/177

1.4 (300–400 nm)

5

TiO2 [26]

Light source, 300 W Xe; electrolyte, CH3OH

Hydrolysis, calcination

Pt/`3300

6

TiO2 (rutile/anatase) [27]

Light source, 300 W Xe; electrolyte, CH3OH

Impregnation, calcination

Pt/`6700

7

Colloid TiO2 [28, 29]

Light source, 450 W Hg; electrolyte HCl

Hydrolysis

Pt – RuO2/4000

30 (310 nm)

8

Mesoporous TiO2 [36]

Light source, 300 W Hg; electrolyte, CH3OH

Solgel method

Pt/6925

9

TiO2 nanowires [30]

Light source, 450 W Hg; electrolyte, CH3OH

Electrospinning and solgel

54

10

TiO2 nanotubes [31]

Light source, 300 W Hg; electrolyte, CH3OH

Hydrothermal

285

11

TiO2 nanosheets [32]

Light source, 300 W Hg; electrolyte, CH3OH

Hydrothermal

117.6

12

SrTiO3 [33]

Light source, 400 W Hg; electrolyte, NaOH

Alfa-Ventron

NiOx/`70

NiOx/`32

13

SrTiO3/TiO2 [34]

Light source, 150 W Hg; electrolyte, HCOOH

Solid-state reaction

560

14

TiO2 [35]

Sensitizer Ru (bpy)32+; sacrificial reagent, water, MeOH vapor; light source, 500 W Xe

Dye sensitized

0.9

15

TiO2 (anatase) [36]

Light source, Hg-Q; reactant solution, water vapor

Rh/449

 

29

16

TiO2 [23]

Light source, Hg-P; reactant solution, 3 M NaOH

NiOx/6

NiOx/2

17

TiO2 [24]

Light source, Hg-Q; reactant solution, 2.2 M Na2CO3

Pt/568

Pt/287

18

TiO2 [25]

Light source, Hg-Q; reactant solution, pure water

Pt/106

Pt/53

19

TiO2 (nanotubes) [37]

0.90 mA/cm2 at 1.23 V vs. RHE; electrolyte, 1 M KOH

Anodization

QY quantum yield

Among other titanates, SrTiO3 (STO)-based perovskite system has been of tremendous interest, and there are several reports on utilization of STO as photocatalysts [38, 39, 40]. Unfortunately, analogous to TiO2, STO also exhibits wide band gap (3.2 eV) and thus responds poorly to the visible light photons. Nonetheless, there does exist some reports on the band gap engineering of STO via method of metal-ion doping at Ti site [38, 39, 40]. Table 4 lists various perovskite, ternary titanate (SrTiO3, BaTiO3, La2TiO5 related opto-electric ceramic reported for water splitting under UV light radiation.
Table 4

Hydrogen and oxygen evolution performance of various types of titanates (SrTiO3 based) during photo−/electrocatalytic water splitting

S. no

Electrode type

Details of the experimental parameters and photocurrent density

Synthesis/deposition method

Co-catal. /H2

Co-catal. /O2

QY %

20

TiO2 (nanowalls) [41]

2.6 mA/cm2 at 1.23 V vs. RHE; electrolyte, 1 M NaOH

Hydrothermal

21

SrTiO3 (perovskite) [33]

Light source, Hg-Xe-P; reactant solution, pure water

Reduction

Rh/27

Rh/14

22

SrTiO3 (perovskite) [33]

Light source, Hg-P; reactant solution, 5 M NaOH

Impregnation

NiOx/40

NiOx/19

 

23

LaTiO5 [42]

Light source, Hg-Q; reactant solution, pure water

Solid-state reaction

NiOx/442

24

La2Ti3O9 (layered perovskite) [42, 43, 44, 45]

Light source, Hg-Q; reactant solution, pure water

Solid-state reaction

NiOx/386

25

La2Ti2O7 (layered perovskite) [46]

Light source, Hg-Q; reactant solution, pure water

Solid-state reaction (polymerized complex)

NiOx/441

12 (<360 nm)

26

KTiNbO5 (layered structure) [46]

Light source, Hg-Q; reactant solution, pure water

Polymerizable complex technique

NiOx/30

NiOx/10

27

Na2Ti6O13 (tunnel structure) [47]

Light source, Xe-Q; reactant solution, pure water

Calcination

RuO2/7.3

RuO2/3.5

It can be noted from Table 4 that though STO show capability of photooxidation and photoreduction of water, LaTiO5 and La2Ti2O7 layered structures show the highest evolution of hydrogen.

A New Member: Visible Light Active – Black TiO2

It is worth mentioning that, as the band gap of TiO2 has posed a major limitation to its usage for solar water splitting, to overcome this limitation, several researchers have attempted band gap narrowing of TiO2 via doping, composite formation, etc. However, the reports on black TiO2 have everyone in race of using TiO2 system for solar water splitting [48, 49]. In one of such attempts, it was found that hydrogenation of TiO2 lattice leads to a disordered crystal structure that yields mid-gap states in TiO2 electronic structure and uplifts the TiO2 valence band edge [69]. Effectively, this yields an optical structure suitable to absorb visible and infrared light photons thereby making black TiO2 that is suitable for visible light photocatalysis. Figure 3 shows the large photocurrent generated by black TiO2 under solar light illumination. The color is indicative of the low band gap of TiO2 compared to white TiO2.
Fig. 3

Comparison of photocurrent of white TiO2 (P25) and black TiO2 generated under solar light radiation. Inset clearly shows the difference in the color of black TiO2 indicating its low band gap characteristics. (Permission-Ref. [49])

Metal Ferrites

Among various Fe-containing oxides (hematite, magnetite, maghemite ferrites (Fe2O3 α/β/γ)), as shown in Fig. 4, hematite phase is the best known and most extensively studied photocatalyst system since it displays a band gap (~2.2 eV) in the visible light range. Iron oxide is an earth-abundant material that is the fourth most common material found in the earth’s crust [50, 51]. Iron-containing oxides display various interesting physicochemical properties based on the oxidation state of Fe (+2, +3) and structural phase iron oxide as indicated in Table 1.
Fig. 4

Schematic of different crystal structures exhibited by bulk iron oxide. Hematite phase of Fe2O3 is a suitable system useful in PEC cell photoanode

Most commonly they are known to exhibit band gap value in the range of ~2 eV. Alpha phase iron oxide (α Fe2O3) has been an important photocatalyst; consequently, there are several reports on its application for photoelectrochemical hydrogen generation [52, 53, 54, 55, 56, 57, 58, 59]. It has the most suitable optical property which is desirable for photooxidizing water; however, it inherently exhibits poor electrical property like electrical mobility or low hole diffusion length. Incidentally, in the past, it failed to display expected performance as photoanode [51, 58, 59], but with the advent of new processing technologies and nanoprocessing methodologies, there are several reports on fabrication of efficient hematite photoanodes [52, 53]. It may be noted that in spite of suitable optical properties and valence band edge, low hole diffusion length (~5 nm) of the hematite [60] was a major hurdle for its utilization as photoanode. Nonetheless, impurity [61]- or nanostructuring [52]-based electrical property tuning was thus shown to yield very high photocurrents and solar-to-hydrogen (STH) conversion rates over hematite photoanodes as given in Table 5.
Table 5

Comparison of PEC results of some doped and nanostructured hematite films indicating superior performance in contrast to known poor behavior of bulk Fe2O3 photoanode

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Photo conversion efficiency

STH/PCE/APCE/IPCEa(%)

1

Nanoclusters [62]

Relative ballistic deposition on FTO

Electrolyte, 1MKOH; applied bias, 0.5 V; light source, AM1.5; photocurrent density, 0.55 mA/cm2

IPCE, 10% at 420 nm

2

Cauliflower-type nanostructures [63]

CVD

3 mA/cm2 at 1.23 V vs. RHE

NRb

3

Nanocrystalline film [64]

Spray pyrolysis

Applied bias, 0.2 V vs. SCE; light source irradiance, 50 mW/cm2; electrolyte, 1MNaOH

1.84% PCE

4

Modified nanostructured electrodes [65]

Spray pyrolysis

Light source, 150 W Xe; electrolyte, 1MNaOH

NR

5

Self-oriented nanorod array electrodes [66]

RF sputtering

Photocurrent density, 0.72 mA/cm2; irradiance, W xenon; applied bias, 0.5 V vs. ag/AgCl

NR

6

Spin-coated nanostructured electrodes [67]

Spin coating deposition solution

Photocurrent density, 15 mA/cm2; applied bias, 1.6 V vs. RHE; light source, AM1.5G

IPCE, 37% at 300 nm

7

Nanowire arrays [68]

Plasma oxidation of iron foils

Photocurrent density, 0.38 mA/cm2; electrolyte, 1 M NaOH; applied bias, 1.5 V vs. RHE, 1–5 microns thick

NR

8

Nanonet-based heteronanostructure [69]

Atomic layer deposition

Photocurrent density, 2.7 mA/cm2; applied bias, 1.53 V vs. RHE

46% at 400 nm

9

Nanostructured Fe2O3 [70]

AACVD

Photocurrent density, 540 μA/cm2; electrolyte, 1 M NaOH; ref. electrode, ag/AgCl; light source, AM1.5, class A solar simulator

NR

10

Nanostructured Fe2O3 [71]

APCVD

Photocurrent density, 600 μA/cm2; applied bias, 1.23 V vs. RHE; electrolyte, 1 M NaOH; light source, AM1.5; irradiance, 100 mW/cm2

NR

11

Nanostructured Fe2O3 [72]

Spray pyrolysis

Photocurrent density, 14 μA/cm2; applied bias, 1.23 V vs. RHE; electrolyte, 1 M NaOH; light source, AM1.5; irradiance, 100 mW/cm2

NR

12

Nanostructured Fe2O3 [72]

USP

Photocurrent density, 1070 μA/cm2; applied bias, 1.23 V vs. RHE; electrolyte, 1MNaOH

NR

13

Nanostructured Fe2O3 [73]

Spray pyrolysis

Photocurrent density, 200 μA/cm2; applied bias, 0.2 V vs. SHE; electrolyte, 1 M NaOH; light source, 50 mW Xe lamp

NR

14

Nanostructured Fe2O3 [72]

Spray pyrolysis

Photocurrent density, 700 μA/cm2; 0.7 V vs. NHE; electrolyte, 1MNaOH + Na2SO4; AM1.5, 200 W Xe lamp

NR

References [100] and references therein

STH solar to hydrogen conversion efficiency, PCE power conversion efficiency, IPCE incident photon current conversion efficiency, APCE applied potential conversion efficiency. IPCE ~EQE and ABPE ~STH

aPhoto-conversion efficiency is reported in different forms, viz., STH, PCE, IPCE, and APCE Ref. [17]

bNR, not reported

Table 5 shows comparison of PEC performance of various types of nanostructured Fe2O3 indicating that nanostructuring of Fe2O3 has yielded an improved performance as photoanode in PEC cell [61]. In a typical case, Fig. 5 clearly exhibits that in contrast to past reports on poor performance of bulk-based ferrite electrodes, a doped and nanostructured Fe2O3 photoanode indicates large photocurrent generation upon Sn and Sn, Zr doping in Fe2O3/NiOOH system of electrode. Stability also seems to improve to a large value, under solar-simulated radiation [74]. The electrical and optical property tuning is thus exploited for fabrication of iron-based photoanodes.
Fig. 5

(a) Photocurrent density as a function of applied potential in dark and illumination; (b) photocurrent density – time lot measured at 1.23 V. All measurements were performed under 1 sun simulated light illumination (AM 1.5G, 100 mW cm−2). (Permission Ref. [74])

Apart from this known system, there are various other perovskite (BiFeO3) or ternary (Sr2FeNbO6, CaFe2O4, MgFe2O4, ZnFe2O4) ferrite systems that are known to be reported in literature [51, 53, 54, 55, 56, 57, 58, 61]. Such opto-electric ceramic systems have also been found to be useful in photoanode applications in PEC cell. Dom et al. [61] have given a detailed perspective on such ferrites wrt their PEC applications. Among the ternary ferrites, ZnFe2O4 has been the most reported in various combinations of doped system, nanostructured system, and composite system [75]. Table 6 shows the comparison of some of the water photooxidation efficiencies of some reported ZnFe2O4 spinel ferrites.
Table 6

Comparison of solar water oxidation performances of ZnFe2O4 photoanodes where ZnFe2O4 was used as a single photon absorber. (Permission Ref. [114])

Electrode

Preparation method

Onset (VRHE)

J at 1.23 VRHE (mA cm−2)

Electrolyte

IPCE at 400 nm and 1.23 VRHE

Stability

ZnFe2O4 [56]

Aerogel-assisted chemical vapor deposition

0.88

0.35

pH 14 NaOH

10%

ZnFe2O4 [76]

Drop-casting a Zn solution on FeOOH followed by annealing and microwave treatment

0.64

0.24

pH 14 NaOH

7%

3 h stable

ZnFe2O4 [77]

Drop-casting a Zn solution on FeOOH followed by annealing and H2 treatment

0.75

0.32

pH 14 NaOH

3 h stable

ZnFe2O4/NiFeOx [78]

Drop-casting a Zn solution on FeOOH followed by annealing and H2 treatment

0.53

0.35

pH 14 NaOH

8% at 1.1 VRHE

24 h stable

TiO2/ZnFe2O4/NiFeOx [79]

Drop-casting a Zn solution on FeOOH followed by microwave treatment and H2 treatment

0.62

0.92

pH 14 NaOH

8%

11 h stable

ZnFe2O4/NiFeOx [80]

Drop-casting a Zn solution on FeOOH followed by annealing and H2 heat treatment

0.85

1.00

pH 14 NaOH

6%

ZnFe2O4 [81]

Atomic layer deposition on an inverse opal-structured substrate

0.90

0.26

pH 13 NaOH

2%

3 h stable

ZnFe2O4 [82]

Spray pyrolysis, Ti doping

0.95

0.35

pH 14 NaOH

3%

Metal Tungstates

The next opto-electrically active ceramic candidate that has tremendous potential wrt solar hydrogen production is W-based oxide ranging from tungsten trioxide to its ternary oxide. It is another suitable photocatalyst (n-type) preferred after Fe2O3, due to its low cost, eco-friendliness, and narrow band gap (2.2–2.8 eV) [83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93]. It is known to be highly stable in acidic condition. It is known to have long hole diffusion length of 150 nm in contrast to 4–5 nm of Fe2O3; however, similar to Fe2O3, WO3 also has high (0.4 V) potential for water oxidation. In spite of its several advantages, it is not a very efficient photoanode in individual form due to its thermodynamic stability wrt anodic photocorrosion. In view of the importance of WO3 wrt its well-suited physicochemical properties, it was studied to achieve a photoanode with best PEC performance [85, 86, 87, 88]. Its PEC properties were tailored based on nanostructuring, doping, or composite formation [86, 88, 89, 90, 91, 92]. Accordingly, there are several opto-electric ceramic systems, viz., NiWO4 [93], Bi2WO6, BiFeWO6, and MWO4 (M = Ni, Cu, Co, Ni).

It is worth mentioning that in spite of tremendous work being carried in using tungstate systems, they have not been shown (Table 7) to demonstrate very high performances.

Table 7

Comparison of solar water splitting performances of metal tungstate photoanodes/photocatalyst

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Co-catal. /H2 (μmol/h)

Co-catal. /O2 (μmol/h)

1

WO3 [93, 94]

As received

Light source, Xe-L42

65

2

PbWO4 (scheelite) [95]

Solid-state reaction

Light source, hg-Xe-Q; reactant solution, pure water

RuO2/24

RuO2/12

3

Bi2WO6 [96]

Calcination

Light source, Xe-L42

3

Ref. [4] and references are therein

Metal Vanadates

Vanadates such as BiVO4 (BVO), BiCu2VO6, etc. are the recent material systems [96, 97, 98, 99, 100] that have been explored as photocatalytic active candidates especially due to their narrow band gap characteristics. Especially, BiVO4 [97] has been shown to perform efficiently in 2015, which led to an exponential rise in its publications. It is known to exist in three polymorphs as pucherite, dreyerite, and clinobisvanate, the last clinobisvanate with monoclinic crystal structure being the most photoactive phase. It is known to exhibit band gap of 2.4–2.5 eV, whereas monoclinic scheelite phase shows low band gap due to V 3d–O 2pπ levels. However it too poses a serious limitation of very slow transfer rate of photogenerated holes from BVO to the electrolyte. Figure 6 shows that V 3d and O 2pπ levels straddle the redox level of water. It was expected that theoretically BVO can yield good stability and STH of 9.2%. Experimentally, however BVO still exhibits problem of poor charge transport and low absorption coefficient. To enhance the charge transfer rate, it is thus necessary to dope V+5 in BVO by metal ions as W+6 or Mo+6 as well as nanostructure the electrode to attain high photocurrents [103, 104].
Fig. 6

Energy-level diagram summarizing the findings of this work for the electronic structure of monoclinic scheelite BiVO4. The dominant orbital character in each region is represented with respect to the vacuum level. Experimental spectra of X-ray absorption to the empty states of the conduction band and X-ray and photoelectron emission from filled states of the valence band are presented on the right. The energy positions of the relevant water splitting redox reactions H+/H2 and O2/H2O are indicated on the energy axis at 4.44 and 5.67 eV below the vacuum level. The Fermi energy position at the surface of the material, determined by photoelectron spectroscopy, is specific to the analyzed thin film because it is a function of doping and surface band bending (Permission Ref. [101])

Table 8 shows that BVO shows maximum oxygen evolution. There has been exhaustive number of reports, but no expected high performance has been achieved; thus, there is a lot of scope for further research. We have not included exhaustive list as the main objective here is to project the possible opto-electric ceramic family that finds applications as visible light photocatalyst.
Table 8

Comparison of solar water oxidation performances of BiVO4 photoanodes and other vanadate photoanodes

S. no

Electrode type

Details of the experimental parameters and photocurrent density

Co-catal. /O2

1

BiVO4 [101, 102]

Light source, Xe-L42

421

2

BiVO4 (n-type) [155, 156]

Mobility (μ) ` 0.02 cm2 V−1 s−1; band gap, 2.4–2.5, 6.7 mA/cm2 at 1.23 volts vs. RHE

Not reported

3

BiCu2VO6 [105]

Light source, Xe-L42

2.3

4

BiZn2VO6 [106]

Light source, Xe-L42

6

Metal Sulfides

Most of the sulfides such as CdS exhibit high conduction band position wrt the reduction level of water. They also show very suitable optical properties that are useful in solar light absorption. They display an electronic structure where metal cation exhibits d10 electronic configuration. Accordingly, conduction band of metal sulfides is composed of d and sp. orbitals, whereas valence band is made of s 3p levels. These valence band edges are thus more negative in contrast to O 2p orbitals. In essence in sulfides the conduction band is negative enough to reduce water and shows a narrow band gap desirable for solar light absorption. As shown in Scheme 2, the genealogy of various sulfide photocatalysts is categorized as IIB–VIA, IIB–IIIA–VIA, IA–IIIA–VIA, and IB–IIB–IVA–VIA sulfides based on elemental stoichiometry [107].
Scheme 2

Type of opto-electric sulfides based on their position in the periodic table. They exhibit suitable properties for PEC water splitting. Ref. [107]

There is an exhaustive work on sulfides that demonstrated their roles as photocatalyst or photoanode for water splitting [107, 108, 109, 110, 111, 112, 113, 114]. Cadmium sulfide is the most efficient and economic single system photocatalyst known among all the existing materials [108]. Bulk CdS has a band gap of 2.4 eV; it exhibits hexagonal and cubic crystal phases. It has shown best efficiencies in most of the reports; however, the sulfides suffer because of photocorrosive instability [109, 110, 111]. Specifically, its low water oxidation kinetics gives rise to accumulation of photogenerated holes, thereby leading to photocorrosion. There are several attempts to arrest this photocorrosion. Still, there is big need to address this challenge of instability of sulfide, which would thus yield a sustainable performance from sulfides. It has been found that CdS in conjunction with ZnS leads to improved performance. Nonetheless, ZnS also yields very high hydrogen generation under UV radiation as shown in Table 10; however, it is necessary to produce hydrogen under solar radiation. Nanostructuring of CdS [120] yielded very large hydrogen production of 27,333 μmole/h-g as shown in Table 9. There are several reports on various types of sulfides, viz., CdS-ZnS [114], ZnInS4 [115], CdIn2S4 [116], AgGa2In3S8 [132], and AgInZn7S9 [117], but their performance, in individual form, was not found to be better than CdS, so they have not been discussed. However, the later section does show finer discussion of modified sulfides wrt hydrogen evolution performance.
Table 9

Comparison of solar water reduction performances of cadmium sulfide-related systems

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Co-catal. /H2 (μmol/h)

QY %

1

Pt/CdS [117]

Mixing and grinding

Light source, 500 W Hg; reactant solution, Na2SO3

40

35 (at 436 nm)

2

CdS [111]

Spray pyrolysis deposition

Solar simulator AM 1.5G; IPCE

5

3

ZnS [118]

Single jet

Light source, 200 W Hg; reactant solution, Na2SO3 + H3PO2 + NaOH

13,000

90 (at 313 nm)

4

CdS-ZnS [119]

Stirring and precipitation

Light source, 300 W Hg; reactant solution, Na2SO3 + Na2S

250

0.60

5

Nanosheet CdS [120]

Hydrothermal

Visible light via filter λ ˜ l420 nm

27,333

60 (420 nm)

6

CdS [121]

Precipitation

Light source, 300 W Xe (>420 nm); reactant solution, lactic acid

MoS2/5400a

93 (420 nm)

7

CdS [122]

Precipitation and hydrothermal method

Light source, 300 W Xe (>420 nm); reactant solution, Na2S + Na2SO3

Pt-PdS/29233a

 

8

CdS [123]

Precipitation

Light source, 500 W Hg (>420 nm); reactant solution, Na2S + Na2SO3

WC/`1350a

 

9

CdS-ZnS [124]

H2S thermal sulfurization

Light source, 350 W Xe (>430 nm); reactant solution, Na2S + Na2SO3

900a

10.2 (420 nm)

a Unit of H2 evolution micro mol/h/g

Table 10

Comparison of solar water reduction performances of different sulfide systems

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Co-catal. /H2

QY %

10

AgInZn7S9 [125]

Precipitation and calcination

Light source, 300 W Xe (>420 nm); reactant solution, Na2S + Na2SO3

Pt/3164.7a

20 (420 nm)

11

ZnIn2S4 [126]

Hydrothermal method

Light source, 300 W Xe (>420 nm); reactant solution, Na2S + Na2SO3

Pt/231a

12

ZnIn2S4 [127]

Surfactant-assisted hydrothermal method

Light source, 300 W Xe (>430 nm); reactant solution, Na2S + Na2SO3

Pt/562a

18.4 (420 nm)

13

AgGa0.9 In0.1S2 [128]

Solid-state reaction

Light source, 300 W Xe (>420 nm); reactant solution, Na2S + Na2SO3

Pt/3500 a

14

CdIn2S4 [129]

Hydrothermal method

Light source, 450 W Xe (>420 nm); reactant solution, H2S + KOH

6960 a

17.1 (500 nm)

15

CdIn2S4 [130]

Surfactant-assisted hydrothermal method

Reactant solution, KOH (photodecomposition of H2S); incident light >420 nm

6476

16

Cd0.1Zn0.9S [131]

Hydrothermal method

Reactant solution, Na2SO3 and Na2S; incident radiation, visible light

21,850

17

CdS. ZnS [132]

Coprecipitation method

Reactant solution, Na2SO3 and Na2S; incident radiation, sunlight

2283.9

18

ZnIn2S4 [133]

Hydrothermal method

Reactant solution, Na2SO3 and Na2S; incident light >420 nm

Pt/8420

34.3 (at 420 nm)

19

AgGa2In3S8 [134]

Heat-treated solid-state reaction method

Reactant solution, K2SO3 and Na2S; incident light >420 nm

Rh/3433.3

15 (at 460 nm)

20

CuGa2In3S8 [135]

Heat-treated solid-state reaction method

Reactant solution, K2SO3 and Na2S; incident light >420 nm

Rh/10666.7

15 (at 560 nm)

21

AgInZn7S9 [135]

Coprecipitation and heat-treated method

Reactant solution, Na2SO3 and Na2S; incident light >420 nm

Pt/3133

35 (at 420 nm)

22

(CuIn)xCd2(1-x)S2 [134]

Low-temperature hydrothermal method

Reactant solution, Na2SO3 and Na2S; incident light >420 nm

649.9, Pt/2456

Pt/ 26.5 (at 420 nm)

23

ZnS-CuInS2-AgInS2 [135]

Coprecipitation and heat-treated method

Reactant solution, K2SO3 and Na2S; incident light >420 nm

Ru/7733.3

a Unit of H2 evolution micro mol/h/g

Other New Systems (Nitrides, Phosphides)

Nitrides

Nitrides are the structurally diverse class of important opto-electric ceramic material system that can be categorized in the following way. Accordingly, group IVA, C3N4, Si3N4, Ge3N4, and Ta3N5, are useful for photocatalytic and catalytic applications; group IV B, TiN, Zr3N4, and Hf3N4, are useful for hard coating and biomedical applications; and group IIIA, AlN, InN, and GaN are useful for solid-state lightning and high-power electronics applications [136].

Among the nitrides, as shown in Table 11, Ta3N4, GaN, and graphitic C3N4 have been reported as potential photocatalyst or photoanode systems for water splitting [137]. In general, the valence band of nitrides is composed of N 2p levels, and the conduction band is composed of metal-based orbitals, like Ta 5d in Ta3N4. Among the reported ones, Liu et al. have cited highest photocurrent of 12.1 mA/cm2 at 1.23 V vs. RHE over a Ta3N4 electrode modified by Co cubane/Ir complex/Ni (OH)x/ferrihydrite/TiOx under 1 sun AM 1.5 G radiation. Similarly, GaN and C3N4 also showed very efficient performance for water splitting in GaN and hydrogen generation in C3N4 nitride. It may be noted at this point that nitrides and related systems show poor stability.
Table 11

Comparison of solar water splitting performances of nitride systems

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Co-catal. /H2 (μmol/h)

Co-catal. /O2 (μmol/h)

QY %

1

Ta3N5 [138]

Nitridation

Band gap, 2.1 eV

Pt/10

420

0.1 (H2), 10 (O2), (at 420–600 nm)

2

Ta3N5 [137]

Anodization/nitridation

Co(OH)x decoration, 5.3 mA cm− 2 at 1.2 VRHE in 1 M KOH under simulated AM 1.5G

3

GaN [139]

MBE

Methanol/AgNO3 300 W Xe lamp

Rh/Cr2O3/5

/50

0. 5 AQE

4

C3N4 [140]

Heat treatment

Solar simulated light; triethanolamine

Pt/5261

29.2% at 400 nm

Black Phosphorous

Black phosphorous [141] is another new class of two-dimensional (2D) materials that has joined the 2D family of graphene and transition metal chalcogenides (TMD) and emerged as a promising nanomaterial. It has shown exotic electronic and optical properties as shown in Table 2. Accordingly, it has shown tremendous potential for photovoltaic, opto-electronic, and biological applications. This V group element of the periodic table is known to exist in three main allotropes: white, red, and black types. Among these allotropes, black phosphorous is thermodynamically known to be the most stable, practically non-inflammable, chemically less reactive, and insoluble in most solvents. It exhibits three crystal structures, viz., simple cubic, orthorhombic, and rhombohedral. Recent reports have shown that black phosphorous (BP) can be a promising candidate for photo-induced water splitting [142], owing to its exotic optical and electronic properties of its layered structure. Importantly it exhibits a layer-dependent band gap of 0.3–1.5 eV as one looks at the bulk BP to a few layered nanosheets of BP. Table 12 compares the performance of BP-based photocatalyst and PEC systems [142, 143, 144, 145, 146]. It can be seen that unlike graphene layers or MoS2 nanosheets, BP nanosheets can be directly used as photocatalyst/electrode for water splitting application; in one such work, Tian et al. [146] showed how modification of BP sheets with CoP nanoparticles yielded very high apparent quantum efficiency of 42.55% under 430 nm photons.
Table 12

Comparison of photo-induced water splitting performances of black phosphorous system

S. no

Electrode type

Synthesis/deposition method

Details of the experimental parameters and photocurrent density

Co-catal. /H2 (μmol/h)

QY %

1

BP (nanosheets) [143]

Solvothermal

300 W Xe lamp coupled with a UV cutoff filter (λ > 420 nm); electrolyte, pure water

Pt/447 g−1

4 (at 420 nm)

2

BP/CN (heterostructured) [144]

Ball milling

A blue LED lamp (440–445 nm); electrolyte, water + IPA

786 g−1

4

BP (nanoflakes) [142]

Solvent exfoliation method

Light source, solar simulator 100 mWcm−2; ethylenediaminetetraacetic acid (EDTA)

RGO – Pt/3.4 g−1 (>420 nm), 0.84 g−1 (>780 nm)

8.7 (at 420 nm), 1.58 5 (at 780 nm)

5

BP (quantum dots) [145]

Facile solution-based method

Under visible light irradiation (>420 nm)

6

BP (nanosheets) [146]

Solvothermal

Xe lamp irradiation (λ ≥ 420 nm); electrolyte, pure water

CoP/131.6 g−1

AQE 42.55 (at 430 nm)

Efficient Nano-Configured Ceramics for Hydrogen Energy Application

Earlier sections have clearly revealed that till date there is no individual material system that can, in its single form, facilitate water splitting. This can be correlated to the inefficacy of the materials to yield ideal physicochemical properties and thus enable an ideal photo-splitting of water molecule. To state further, as shown in Scheme 3, it is thus necessary to tune the band gap, control electron-hole recombination kinetics, and charge transport dynamics in any PEC system. The tunability of the properties can be achieved by making use of (1) composite approach, (2) anionic/cationic doping, or (3) nanostructuring. Scheme 3 clearly indicates that interplay of material property tuning and nanostructuring can be used to attain an ideal type of photocatalyst/photoelectrocatalyst. This section clearly shows how material property tuning yields improved performance toward water splitting under radiation. Table 13 shows various aspects of property tuning that yields near to ideal performance desirable for economic, sustainable, and efficient hydrogen generation under sunlight.
Scheme 3

Art of designing and fabrication of efficient and stable new generation photocatalyst

Table 13

Comparison of solar water splitting performances of nano-configured photocatalyst/photoelectrodes fabricated using various opto-electric ceramic systems

S. no

Electrode type

Details of the experimental parameters

Performance

1

TiO2 nanotubes [31]

Light source, 300 W Hg; electrolyte, CH3OH

H2 generation, 285 μmolh−1 g−1

2

TiO2 (nanotubes) [147]

Light source, 95 mWcm−2 UV lamp; electrolyte, 1 M KOH

Photocurrent density (J), 26 mA/cm2; efficiency (ŋ) = 16.5%

3

TiO2 (nanowalls) [41]

Electrolyte, 1 M NaOH; applied potential, 1.23 V vs. RHE

Photocurrent density (J), 2.6 mA/cm2

4

CdS (flower-like microsphere) [148]

Co-catalyst, 0.5% wt. Pt; sacrificial agent, 10 vol% lactic acid

H2 generation, 9374 μmolh−1 g−1; QE, 24.7% (at 420 nm)

5

ZnInS4 (flower-like microsphere) [115]

Co-catalyst, 1.0% wt. Pt; sacrificial agent, 0.25 M Na2SO3 + 0.35 M Na2S

H2 generation, 8420 μmolh−1 g−1; QE, 34.3% (at 420 nm)

6

TiO2/ZnO (hedgehogs and fan blades) [149]

Photodegradation

Target pollutant, MO; activity (Kapp,10−3 min−1), 30 min/97% (0.117)

7

Bi2S3 [150]

Photodegradation

Target pollutant, MO; activity (Kapp,10−3 min−1), 97%/97% (77.6)

8

γ-Fe2O3/ZnO [151]

Photodegradation

Target pollutant, MB; activity (Kapp,10−3 min−1), 50 min/95.2%

9

CdTe-ZnO-N [152]

Light source, 100 mW UV lamp; electrolyte, 0.5 M Na2SO4

Photocurrent density (J), 0.46 mA/cm2; photo conversion (ŋ) = 1% at 0.5 V vs Ag/AgCl

10

RGO-Fe2O3 [153]

Light source, 500 mWcm−2 UV lamp; electrolyte, 1 M NaOH

Photocurrent density (J), 6.7 mA/cm2; efficiency (ŋ) = 0.76%

11

BiVO4 + Fe2O3 (dual photoanode) + 2 – Si(PV)-Pt(dark cathode) [154]

Dual photoanode and Si cathode

Water splitting application; solar to energy conversion efficiency, 7.70%

12

SnO2/graphene [155]

Electrolyte, 0.1 M NaHCO3; applied potential, −1.8 V vs. RHE

FE, 93.6%

13

MoS2/Mo – Terminated edges [156]

Electrolyte, EMIM-BF4∗∗; applied potential, −0.77 V vs. RHE

FE, 98%; CO production

14

InP NWs/MOSx [157]

Open circuit potential, +0.55 V vs. RHE; J0 vs. RHE = 22.0 mA/cm−2; Jsc = 22.0 mA/cm−2; STH, 17.0%; robustness, 7% current loss after 1 h at 0 VRHE

15

Pt/ITO/(n+-ia-Si)/n-Si/(i-pa-Si)?ITO [158]

HER∗∗∗ application; solar to energy conversion efficiency, 13.26%

16

Pn+-Si/au mesh (nano porous) [159]

Electrolyte, 0.2 M KHCO3; applied potential, −0.03 V vs. RHE

FE, 91%; CO production

17

pn+-Si/F/SnO2/TiO2/Ir [160]

HER∗∗∗ application; solar to energy conversion efficiency, 10.90%

18

GaAS/InGaP/TiO2/Ni (photoanode)-Pd-C(dark cathode) [161]

CO2 reduction; solar to energy conversion efficiency, 10.00%

19

P – Si/SrTiO3/Ti/Pt [162]

Open circuit potential, +0.45 V vs. RHE; J0 vs. RHE = 25 mA/cm−2; Jsc = 35 mA/cm−2; STH, 4.9%; robustness, over 35 h at −0.2 VRHE

20

GaAS/AuGe/Ni/Au/Pt [163]

Open circuit potential, +1.022 V vs. RHE; J0 vs. RHE = 22.5 mA/cm−2; Jsc = 22.5 mA/cm−2; STH, 6.4%; robustness, 11% current loss after 8 day at 0 VRHE

21

Cu (In, Ga)Se2/CdS/Ti/Pt [164]

Open circuit potential, +0.65 V vs. RHE; J0 vs. RHE = 25.0 mA/cm−2; STH, 5.4% at +0.3 VRHE; robustness, 3 h at +0.3 VRHE; lost 20% activity

22

Si/n++ − GaN/InGaN/p+ − GaN [165]

Open circuit potential, +0.5 V vs. RHE; J0 vs. RHE = 40.6 mA/cm−2; Jsc = 40.6 mA/cm−2; STH, 8.7% at +0.33 VRHE; robustness, stable for 3 h at 0.06 VRHE

23

P – Cu2O/AZO/TiO2/MoS2 + x [166]

Open circuit potential, +0.48 V vs. RHE; J0 vs. RHE = 6.3 mA/cm−2; STH, 7.7% at +0VRHE; robustness, degraded gradually over 10 h

FE Faradic efficiency, EMIM–BF4 1-ethyl-3-methylimidazolium tetrafluoroborate, HER hydrogen evolution reaction, J0 dark saturation current density, JSC photocurrent density at short circuit, W-H tungsten-halogen lamp, STH solar to hydrogen conversion efficiency

Conclusion

Opto-electric ceramics showing suitable low band gap (~2.2 eV), desirable band energetics, electrochemical properties, and catalytic sites can be used as visible light photocatalysts/photoelectrocatalysts for splitting water under solar light radiation. Till date there is no effective water splitting system that has been found to exist, but nano-configured photocatalysts/photoelectrocatalysts can be suitably fabricated for making the best ideal material for sustainable generation of hydrogen energy under solar light.

Notes

Acknowledgments

The authors thank the support of the Director, ARCI, DST Lab, India.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Center for NanomaterialsInternational advanced research center for Powder Metallurgy and New Materials, (ARCI)HyderabadIndia

Section editors and affiliations

  • Pramod H. Borse
    • 1
  1. 1.NanomaterialsInternational Advanced Center for Powder Metallurgy & New Materials (ARCI)HyderabadIndia

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