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Water photolysis by carbon nitride
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Water photolysis by carbon nitride Roberto C. Dante Research & Development Department, 2Dto3D S.r.l.s. Via Revalanca 5, San Firmino, Revello (CN), 12033, Italy
article info
abstract
Article history:
Polymeric carbon nitride has been used with a certain success to produce hydrogen by
Received 18 November 2018
photocatalysis exploiting its properties of wide-band gap semiconductor with band gap
Received in revised form
between 2.6 and 3.0 eV, which is suitable to split water into H2 and O2 (DE ¼ 1.23 eV).
7 January 2019
Indeed, the conduction band (CB) edge is higher than the H2/Hþ potential as well as the
Accepted 21 January 2019
H2O/O2 potential is higher than the valence band (VB) edge. In addition, carbon nitride
Available online 14 February 2019
contains many active and coordination centers. This makes carbon nitride a very versatile material. However, the direct recombination of the pair electron and hole (exciton)
Keywords:
removes energy to the redox reaction. This review is indeed focused on exciton recombi-
Photocatalysis
nation and localization, and on the efficiency of carbon nitride photosystems. Semi-
Water splitting
conductors and metals has been used to modify the properties of carbon nitride such as
Carbon nitride
engineering the band gap, separating the exciton charges in order to increase hydrogen
Semiconductors
production. The hydrogen production of these systems coupling two photosystems,
Exciton
involving the Z-scheme, and co-catalysts is much higher than those based on pure poly-
Z-scheme
meric carbon nitride or only on one co-catalyst. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A great interest on carbon nitride started in the early 90s for a theoretical work on super-hard materials. This super-hard material was first proposed in 1985 by Marvin L. Cohen and Amy Y. Liu [1,2]. Examining the nature of crystalline bonds they theorized that carbon and nitrogen atoms could form a particularly short and strong bond in a stable crystal lattice in a ratio of 1: 1.3. That this material would be harder than diamond on the Mohs scale was first proposed in 1989 [1]. Although, the super-hard phase is not yet obtained in a large scale, the easy synthesized layered phase (graphitic) exhibited interesting properties. Actually, the so called graphitic carbon nitride (g-C3N4) is a supramolecular polymeric material, held together by diffuse hydrogen bonds, with a low degree of crystallinity. It was first synthesized by Berzelius and Liebig in the first decades of the XIX century, and Liebig named it
“mellon” since it was obtained from melamine [3]. Finally, Pauling et al. arrived to clarify that these substances contained a C6N7 nucleus that they called cyameluric nucleus, and currently mostly referred as either heptazine or tri-striazine nucleus [4]. There is now a certain accordance on the structure of the material as a polymer with repeating unit C6N7(NH2)NH,. Chamorro-Posada et al. [5] showed how this structure determines some morphological properties such as corrugation, crumpling, wrapping, rolling to form globular, and tubular particles depending on the synthesis conditions. On the other hand, this propensity to corrugated shapes does not favor high degree of crystallinity. This “new-old material” exhibits characteristics of a wideband gap semiconductor with low mobility of charges. The term semiconductor in this case it is used with a certain latitude, because increasing of both mobility and charge carriers is a task that it is still far to be achieved.
E-mail address: [email protected]. https://doi.org/10.1016/j.ijhydene.2019.01.202 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 4 ( 2 0 1 9 ) 2 1 0 3 0 e2 1 0 3 6
Due to its heptazine repeating units, this material has both Lewis and Brønsted sites, and cages that can coordinate metals, and form complexes. These sites make polymeric carbon nitride a very versatile material combining characteristics of polymers, catalysts and semiconductors. For instance, the reported applications are very wide, ranging from electronics and optoelectronics, photocatalysis (including specific organic reactions such as oxidative addition to aromatic rings [6]), hydrogen energy [7], sensoring of gases, and glucose [8], and to photovoltaic energy [9]. The determining factor in the use of carbon nitride as an effective photocatalyst is the separation of charges of exciton, as well as the exciton binding energy. Indeed, considering that, in carbon nitride we are dealing with Frenkel excitons, the balance between the stability and separation of exciton charges seems to be one of the key-factors to achieve a high yield in hydrogen production. Recombination of charges represents a loss of efficiency in the whole photonic process, so that those mechanisms and methods that allow us to increase exciton life are also considered. In this report, focus is pointed towards the effect of co-talysts of different nature (sometimes more than one co-catalyst per time) on the recombination of holes and electrons of carbon nitride. This indicates a defined pathway to work on, and, which it is possible to increase hydrogen yield through. The other issue is related to the light harvesting in order to increase the effective photons by means of increasing the absorption in the visible region with other semiconductors. The absorbed photons are mostly in the UV range for the band gap of carbon nitride materials which is around 3 eV, corresponding to 413 nm, at the edge between UV and visible radiation. However, the band gap can be shifted to lower values by means of increasing of the degree of polymerization and crystallinity. Nevertheless, the strategy of lowering the band gap to increase the photon absorption can decrease the overall catalyst efficiency due to the many overpotentials involved in the process of photocatalysis. On the other hand, coupling carbon nitride with semiconductors with lower band gap (allowing more photons to be absorbed) can enable the Z-scheme, which, in our case, consists of electron feeding to the valence band of carbon nitride from a semiconductor harvesting photons from visible light [10]. Both pathways exploit the versatile nature of polymeric carbon nitride (g-C3N4) to be doped, coordinate molecules and wrap nanoparticles.
Photocatalytic hydrogen production by carbon nitride In photonic processes, photons are absorbed directly without complete conversion of energy to heat. The absorber may convert part of the photon energy to chemical energy by means of a chemical reaction as for photosynthesis or water splitting into hydrogen and oxygen. In this section, photonic aspects are considered related to hydrogen production. As a starting point the definition of the efficiencies of the process is given, the limiting efficiency of any photonic process is: hp ¼
Jg Dmex fconv ; Es
(1)
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Where hp is the limiting efficiency, Jg is the absorbed photon flux with wavelength l < lg (wavelength corresponding to the gap energy), Es is the spectral irradiance, fconv corresponds to the fraction of the excited states that contribute to the generation of a useful product. while Dmex is the chemical potential of the excited state. Only the photons with energy above the gap energy Eg can be absorbed, however, this excess energy is lost as heat. Moreover, it noteworthy to point out that Dmex < Eg. Usually this chemical potential is only the 75% of the band gap energy. For this reason, a chemical efficiency hchem can also been defined: hchem ¼
Eg Uloss DG=n ¼ : Eg Eg
(2)
Where DG is the Gibbs free energy of the product and n is the number of photons involved in the reaction, in the case of H2 from water, 2 photons are needed for each molecule of hydrogen formed. Uloss is usually around 0.3e0.4 eV from entropic considerations, but in real system is even higher, and should not be confused with the loss of the excited state [11]. The efficiency for an energy storing reaction as a whole can be expressed in the following form: hc ¼ hg hchem fconv ;
(3)
Where hg is the fraction of the solar energy with energy U > Eg. Since, in our photosystem we can assume that one photon per electron is involved at least for the reaction of hydrogen reduction (2Hþ þ2 e / H2), a photosystem S2 is considered, which involves one photosystem and two electrons. On the other hand, it is possible to have a dual photosystem D2, where one electron came from a photosystem 1 and the second one from another photosystem 2. The option of D2 allows us to obtain higher efficiencies. This situation is another argument in favor of photoactive co-catalysts. The following scheme shows the difference between the S2 and D2 systems: 2hn H2 þ 1=2O2 S2 : H2 O /
(4)
D2 : H2 O hn1þhn2 / H2 þ 1=2O2
(5)
The indexes 1 and 2 in Eq. (5) correspond to the photosystems 1 and 2, which are working coupled in the case of D2. For carbon nitride, an estimation of the efficiency (in this case is solar-to-hydrogen efficiency (STH)). taking Eg between 2.7 and 3.0 eV, hg ¼ 0.3 (depending on the location) [12], and DG0/n ¼ 1.23 eV, gives that hchem (STH) ranges between 0.14 and 0.12, assuming that fconv ¼ 1. The challenge is obtaining a much higher fraction of excited states hijacked to the desired reaction. In a dual system D2 coupling polymeric carbon nitride with a semiconductor of band gap of 2.0 eV, as a cocatalyst, such as iron oxides (Fe (III)), the efficiency can be increased between 0.31 and 0.32, i.e. practically the double compared with carbon nitride alone. Another issue to be pointed out is that the rate of the two half reactions are very different, and sacrificial agents such as holes scavenger are often used to increase the production rate. The real efficiency is much lower than the ideal ones due to several phenomena involved with different time scales such as: i. photon absorption, ii. exciton separation, iii. carrier transport, iv.
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Catalytic efficiency, v. mass transfer of reagents and products [13]. In this report, the attention is focused on point ii. exciton separation, since for carbon nitride, as an organic semiconductor, is possibly the most important issue. The exciton binding energy that reduces both energy available to the reaction as well as separation of charges is the subject matter. The desired electronic property of photoactive semiconductors, including a suitable band gap, is that the exciton binding energy be Eb < 25 meV, with dielectric constant εr > 10, because thermal energy at 25 C is 25.7 eV. If Eb is lower than the thermal energy the exciton bond can be easily broken, and the charges are free to move into different directions, as occurs in inorganic semiconductor, whose Eb is between 1 and 40 meV. Density functional theory (DFT) studies by Melissen et al. estimated that Eb for polymeric carbon nitride should be 840 meV, and the optical band gap Eg ¼ 2.6 eV with a Frenkel A, similar to an atomic radius, indicating that radius Re of 1.38 exciton is behaving as a very localized particle [14]. Probably, Eb of carbon nitride is between 0.3 and 1 eV (depending on the degree of crystallinity, etc.), i.e. it can correspond to about the 7% of hydrogen binding energy (considering the maximum value of 1 eV), which is 13.6 eV. It is comparable to the binding energy of the simplest molecule, so that the photocatalytic reaction can be expressed in the following way with the “exciton atom” indicated as hþe: 2hn þ S2 þ H2 O / 2hþ e þ H2 O /H2 þ 1=2O2
(6)
However, this reaction to be effective, must be split in the following reactions: 2hþ e þ Eb þ H2 O / 2hþ þ 2e þ H2 O/H2 þ 1=2O2
(7)
Of course, this phenomenon would be much easier if the value of Eb were similar to that of thermal energy. In the case of polymeric carbon nitride, this energy barrier is the first step of catalysis to be overcome by means of traps to separate electrons and holes. Moreover, another study by Wei & Jacob showed that the hole is localized on the secondary amine bridging the heptazine units, while electron density is surrounding it, more localized on the closest atoms bonded to this nitrogen atom [15], as shown in Fig. 1. Although, this sites are the reactive ones, their localization makes difficult to use these charges for photocatalysis without any mean that would weaken the exciton bond. The position of the redox potential of water (Hþ/H2 and H2O/O2) is considerably within the conduction band (CB) and the valence band (VB) edge levels, respectively, so that the overpotential losses should not affect the reaction, compared with other semiconductors as shown in Fig. 2 [16]. On the contrary, the relative materials, based on triazine units, instead of tri-s-triazine ones, have a VB more shifted and too close the oxidation potential, making them less attractive for water photolysis [17] since holes release their free energy when are in lower levels than the uploading state. Co-catalysts can provide trapping sites for the photogenerated charges, and promote the charge separation, thus enhancing the quantum efficiency; could improve the photostability of the catalysts by timely consuming of the
Fig. 1 e Localization of the exciton with the radius of 1.38 A in polymeric carbon nitride (circle) in the bridging secondary amine. The image shows the repeating unit C6N7(NH2)NH.
photogenerated charges, particularly the holes; most importantly, the co-catalysts catalyze the reactions by lowering the activation energy [18]. The strategy to use co-catalyst to alter the electronic properties of polymeric carbon nitride is very promising for water photolysis. It should be remembered that the competing reaction is the following: hþ e % hn þ S2
(8)
i.e. the recombination of charges with the releasing of a photon (with lower energy than the absorbed one and detectable as luminescence), or the emission of heat (the predominant phenomenon). Indeed, measuring luminescence is an indirect method to know the effect of co-catalysts on the photonic efficiency expressed in Eq. (1).
Co-catalysts For water photolysis, the classical co-catalyst is Pt, there are also metal oxides such as WO3 and sulfides MoS2, as well as nonmetallic elements such as I, F, B, S and Se. Other cocatalysts have been tested mainly for photo-degradation of dyes, or other partial oxidations of organic compound, cycloadditions etc. such as Ag, Fe, Cu mainly as oxide. Recently, a new compound named C2N has been also proposed as a candidate to increase the efficiency of polymeric carbon nitride by favoring exchange of electrons [19,20]. Many other examples can be provided of the increasing yield due to co-catalysts with a synergic increased efficiency also to the respect the separated catalysts, recently also Nb salts [21], NieP compounds, VS2 [22], MnO2 [23], MoS2 composites [24], and compounds of NieP [25,26] showed enhanced hydrogen evolution rate (HER), often above 1000 mmol g1h1. It is noteworthy to point out that also carbon nitride based on triazine units (tC3N4) could act as a co-catalyst towards carbon nitride based on heptazine units (h-C3N4) as hypothesized by Zhao et al. [27].
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Fig. 2 e Band edge positions of commonly reported nitride photocatalysts. The oxidation and reduction potentials of water are also shown (green dotted lines). The red dotted line represents the band edge positions of InxGaxN with x increasing from left to right (0e1). The reduction potentials of CO2 to various value added products are also shown [16]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The basic scheme of the mechanism these oxide semiconductors can work through, coupled with polymeric carbon nitride, is shown in Fig. 3. It was proposed for the case of the system Ag3PO4/g-C3N4 in relation to photo-degradation, but basically it would be the same scheme for water photolysis, i.e. the recombination of holes of polymeric carbon nitride with electrons of the VB of the metal oxide will provide energy to free the excited electron of polymeric carbon nitride [28]. This mechanism, named direct Z-scheme (for the diagram shape), decreases the direct recombination of excitons in the polymeric carbon nitride, but require that the CB of the metal oxide had higher energy of the VB but lower than that of the CB of polymeric carbon nitride. This
Fig. 3 e Energy level positions of the composites MOx/gC3N4 with illustration of direct Z-scheme of transfer of electrons from the CB of MOx to the VB of g-C3N4.
mechanism can be assimilated to that of the D2 system previously mentioned. The effect on photoluminescence of the Ag doping on gC3N4 is paradigmatic for all the other possible example of the same type. Ag doping caused a dramatic decay of photoluminescence, especially achieving the 1 wt% as shown in Ref. [29]. The best balance between decreasing photoluminescence and hydrogen production is found at 1 wt% of Ag loading (see Fig. 4), then the luminescence decrement only indicates that other pathways are found to charges recombinations in competition with photocatalytic hydrogen production and luminescence. However, the mechanism is probably that illustrated in Fig. 3, since silver nanoparticles can behave in this way too. The other possibility is to have a redox mediator instead of the direct transfer of the electron from the CB of MOx to the VB of pymeric carbon nitride as in the Z-direct scheme. The tests were carried out by Martin et al. on BiVO4 and WO3 using as redox mediator FeCl2 and NaI. The co-catalyst were really two per sample: Pt specific to hydrogen reduction, and BiVO4 or WO3 for oxygen oxidation. The following scheme of Fig. 5 is suggested as a modification of the Z-scheme. The results are clearly in favor of the use of the redox mediators, with the best results for WO3 and NaI/NaIO3 as redox mediator, doubtless the ability of the metal oxide to adsorb the oxidized phase of the mediator is a key factor too [30]. A comparison of a series of oxide and sulfide semiconductors to work as co-catalyst is shown in Table 1. The experimental conditions of the presented results are not the same for many reasons such as the type of hole scavenger, pH etc., lamp type, power and emission spectra so that the comparison is only semi-quantitative, however the trend is clearly in favor of the use of co-catalysts such as semiconductors to exploit the Z-scheme [22]. It is noteworthy to point out that metal sulfides seem to give the best results in terms of hydrogen evolution rate (HER). In most of these cases,there is often a metallic or at least an electric conductive phase intermediate between the two photosystems as in the case of NiS (1.5% load) [32], where the presence of carbon black allowed to achieve a HER of
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Fig. 4 e Left: Comparison between the rate of H2 rates of production by photocatalysis with g-C3N4 as-is, and doped with different amounts of Ag under visible light in methanol 25 V% aqueous solution, an arc Xe lamp with cut off at 420 nm was used (0.78 mW/cm2). Right: photoluminescence of g-C3N4 as-is and doped with different amounts of Ag [29].
992 mm h1g1, higher than that of a NiS2 with a loading of 30% [33]. Actually, the use of solid phase redox mediator, instead of soluble salts, is very promising, this will represent another way to increase the efficiency of the Z-scheme in a more efficient and practical way. The intermediate heterojunction can be obtained for example with Au, Ag or other metals. The importance of this hetero-junction was highlighted by an experiment of Sun et al.[35], which showed that the Pt -Ni(OH)2 composite deposited on g-C3N4 has a double HER (ca. 3000 vs. 1500 mmol g1 h1) than the simple dual system with spatially separated co-catalysts, due to the better mediation of the conductive Pt when the hetero-contact is very intimate; moreover, Pt is also working as an HER site, while Ni(OH)2 as an oxygen evolution (OER) one. The scheme is, in these cases, MZx-CeC3N4, with a conductive medium C (M is a metal, and Z can be either O or S) between the two photosystems (PS1-C-PS2). For instance, Ag nanoparticles (NP) can be obtained irradiating silver halogenides (AgX) such as AgBr and AgI. Moreover, results of Fe2O3eAgeC3N4 showed a 2.74 higher degradation of Rhodamine B (RhB) than pure carbon nitride [32].
Fig. 5 e Z-scheme for the transfer of electrons from the CB of MOx to the VB g-C3N4 by means of a redox mediator A/D.
Table 1 e hydrogen evolution rate (HER) with different co-catalysts and conditions. C3N4 [22e30] VS [22] Ti2C [22] MoS2 [22] WS2 [22]
HER 1 1
H2mm h g
4.4e5
1748
922
376 2
568
a
WO3 [30] BiVO4 [30] 74
[22]: monochromatic light 420 nm (14.3 mW/cm ). Triethanolamine TEOA as a hole scavenger. [30]: 300 W Xe lamp, with >395 nm cut off filter. NaI, FeCl2 as redox mediators [31]: 300 W Xe Lamp with >420 nm cut off filter, TEOA [33]: 300 W Xe Lamp with >420 nm cut off filter, TEOA [34]: 300 W Xe Lamp, TEOA a with Pt loading on WO3 b 1.5% of NiS loading and 0.5% carbon black c 25.3% of NiS2 loading
15
b
NiS [31] 992
c
NiS2 [33] Ni(OH)2 [34] 968
~3000
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Conclusions [7]
In summary, co-catalysts or dual (even composite) photosystems (e.g. D2 systems) involving the Z-scheme mechanism dramatically increased the hc (STH) efficiency of polymeric carbon nitride from ca. 0.01 to ca. 0.20, as inferred from available data. These results were expected from theoretical considerations, and favored by the use of specific catalysts for the oxidation and the reduction reactions (i.e. the cocatalysts). Exciton recombination and localization emerged as the most determinant issue to be faced for water photolysis by carbon nitride. The Z-scheme mechanism with solid phase redox mediators appears to be the more promising way to decrease the extent of hole and electron recombination for this class of materials in a more efficient manner. However, the difficulty of combining different materials to create effective hetero-junctions must be overcome. Two possibilities aroused to carry out this task: i. in situ synthesis of the solid phase mediator (and possibly that of the semiconductor co-catalyst), using carbon nitride to template the solid phase mediator, which is usually obtained by decomposition at low temperatures of unstable compounds. This route can be particularly effective (for example, halogenides of noble metals such as Ag and Au); ii. another promising route, still open to intensive investigation, is that of using graphite (or graphitic materials) to transfer electrons instead of noble metals. In this case, the roles in the synthesis route can be inverted, since graphite can act as a template for the synthesis of carbon nitride in situ because it is more thermally stable than g-C3N4, and possibly providing better hetero-junctions than metals. In addition, as a consequence of using graphite, it will be interesting to investigate the Z-scheme involving C2N (as co-cotalyst possibly provided with proper OER sites and conductive mediator), and g-C3N4 with adequate HER sites. In any case, All these tasks are favored by the presence of cages and the ability of carbon nitride sheets to wrap other nanoparticles with strong interactions.
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Water photolysis by carbon nitride Roberto C. Dante Research & Development Department, 2Dto3D S.r.l.s. Via Revalanca 5, San Firmino, Revello (CN), 12033, Italy
article info
abstract
Article history:
Polymeric carbon nitride has been used with a certain success to produce hydrogen by
Received 18 November 2018
photocatalysis exploiting its properties of wide-band gap semiconductor with band gap
Received in revised form
between 2.6 and 3.0 eV, which is suitable to split water into H2 and O2 (DE ¼ 1.23 eV).
7 January 2019
Indeed, the conduction band (CB) edge is higher than the H2/Hþ potential as well as the
Accepted 21 January 2019
H2O/O2 potential is higher than the valence band (VB) edge. In addition, carbon nitride
Available online 14 February 2019
contains many active and coordination centers. This makes carbon nitride a very versatile material. However, the direct recombination of the pair electron and hole (exciton)
Keywords:
removes energy to the redox reaction. This review is indeed focused on exciton recombi-
Photocatalysis
nation and localization, and on the efficiency of carbon nitride photosystems. Semi-
Water splitting
conductors and metals has been used to modify the properties of carbon nitride such as
Carbon nitride
engineering the band gap, separating the exciton charges in order to increase hydrogen
Semiconductors
production. The hydrogen production of these systems coupling two photosystems,
Exciton
involving the Z-scheme, and co-catalysts is much higher than those based on pure poly-
Z-scheme
meric carbon nitride or only on one co-catalyst. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A great interest on carbon nitride started in the early 90s for a theoretical work on super-hard materials. This super-hard material was first proposed in 1985 by Marvin L. Cohen and Amy Y. Liu [1,2]. Examining the nature of crystalline bonds they theorized that carbon and nitrogen atoms could form a particularly short and strong bond in a stable crystal lattice in a ratio of 1: 1.3. That this material would be harder than diamond on the Mohs scale was first proposed in 1989 [1]. Although, the super-hard phase is not yet obtained in a large scale, the easy synthesized layered phase (graphitic) exhibited interesting properties. Actually, the so called graphitic carbon nitride (g-C3N4) is a supramolecular polymeric material, held together by diffuse hydrogen bonds, with a low degree of crystallinity. It was first synthesized by Berzelius and Liebig in the first decades of the XIX century, and Liebig named it
“mellon” since it was obtained from melamine [3]. Finally, Pauling et al. arrived to clarify that these substances contained a C6N7 nucleus that they called cyameluric nucleus, and currently mostly referred as either heptazine or tri-striazine nucleus [4]. There is now a certain accordance on the structure of the material as a polymer with repeating unit C6N7(NH2)NH,. Chamorro-Posada et al. [5] showed how this structure determines some morphological properties such as corrugation, crumpling, wrapping, rolling to form globular, and tubular particles depending on the synthesis conditions. On the other hand, this propensity to corrugated shapes does not favor high degree of crystallinity. This “new-old material” exhibits characteristics of a wideband gap semiconductor with low mobility of charges. The term semiconductor in this case it is used with a certain latitude, because increasing of both mobility and charge carriers is a task that it is still far to be achieved.
E-mail address: [email protected]. https://doi.org/10.1016/j.ijhydene.2019.01.202 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Due to its heptazine repeating units, this material has both Lewis and Brønsted sites, and cages that can coordinate metals, and form complexes. These sites make polymeric carbon nitride a very versatile material combining characteristics of polymers, catalysts and semiconductors. For instance, the reported applications are very wide, ranging from electronics and optoelectronics, photocatalysis (including specific organic reactions such as oxidative addition to aromatic rings [6]), hydrogen energy [7], sensoring of gases, and glucose [8], and to photovoltaic energy [9]. The determining factor in the use of carbon nitride as an effective photocatalyst is the separation of charges of exciton, as well as the exciton binding energy. Indeed, considering that, in carbon nitride we are dealing with Frenkel excitons, the balance between the stability and separation of exciton charges seems to be one of the key-factors to achieve a high yield in hydrogen production. Recombination of charges represents a loss of efficiency in the whole photonic process, so that those mechanisms and methods that allow us to increase exciton life are also considered. In this report, focus is pointed towards the effect of co-talysts of different nature (sometimes more than one co-catalyst per time) on the recombination of holes and electrons of carbon nitride. This indicates a defined pathway to work on, and, which it is possible to increase hydrogen yield through. The other issue is related to the light harvesting in order to increase the effective photons by means of increasing the absorption in the visible region with other semiconductors. The absorbed photons are mostly in the UV range for the band gap of carbon nitride materials which is around 3 eV, corresponding to 413 nm, at the edge between UV and visible radiation. However, the band gap can be shifted to lower values by means of increasing of the degree of polymerization and crystallinity. Nevertheless, the strategy of lowering the band gap to increase the photon absorption can decrease the overall catalyst efficiency due to the many overpotentials involved in the process of photocatalysis. On the other hand, coupling carbon nitride with semiconductors with lower band gap (allowing more photons to be absorbed) can enable the Z-scheme, which, in our case, consists of electron feeding to the valence band of carbon nitride from a semiconductor harvesting photons from visible light [10]. Both pathways exploit the versatile nature of polymeric carbon nitride (g-C3N4) to be doped, coordinate molecules and wrap nanoparticles.
Photocatalytic hydrogen production by carbon nitride In photonic processes, photons are absorbed directly without complete conversion of energy to heat. The absorber may convert part of the photon energy to chemical energy by means of a chemical reaction as for photosynthesis or water splitting into hydrogen and oxygen. In this section, photonic aspects are considered related to hydrogen production. As a starting point the definition of the efficiencies of the process is given, the limiting efficiency of any photonic process is: hp ¼
Jg Dmex fconv ; Es
(1)
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Where hp is the limiting efficiency, Jg is the absorbed photon flux with wavelength l < lg (wavelength corresponding to the gap energy), Es is the spectral irradiance, fconv corresponds to the fraction of the excited states that contribute to the generation of a useful product. while Dmex is the chemical potential of the excited state. Only the photons with energy above the gap energy Eg can be absorbed, however, this excess energy is lost as heat. Moreover, it noteworthy to point out that Dmex < Eg. Usually this chemical potential is only the 75% of the band gap energy. For this reason, a chemical efficiency hchem can also been defined: hchem ¼
Eg Uloss DG=n ¼ : Eg Eg
(2)
Where DG is the Gibbs free energy of the product and n is the number of photons involved in the reaction, in the case of H2 from water, 2 photons are needed for each molecule of hydrogen formed. Uloss is usually around 0.3e0.4 eV from entropic considerations, but in real system is even higher, and should not be confused with the loss of the excited state [11]. The efficiency for an energy storing reaction as a whole can be expressed in the following form: hc ¼ hg hchem fconv ;
(3)
Where hg is the fraction of the solar energy with energy U > Eg. Since, in our photosystem we can assume that one photon per electron is involved at least for the reaction of hydrogen reduction (2Hþ þ2 e / H2), a photosystem S2 is considered, which involves one photosystem and two electrons. On the other hand, it is possible to have a dual photosystem D2, where one electron came from a photosystem 1 and the second one from another photosystem 2. The option of D2 allows us to obtain higher efficiencies. This situation is another argument in favor of photoactive co-catalysts. The following scheme shows the difference between the S2 and D2 systems: 2hn H2 þ 1=2O2 S2 : H2 O /
(4)
D2 : H2 O hn1þhn2 / H2 þ 1=2O2
(5)
The indexes 1 and 2 in Eq. (5) correspond to the photosystems 1 and 2, which are working coupled in the case of D2. For carbon nitride, an estimation of the efficiency (in this case is solar-to-hydrogen efficiency (STH)). taking Eg between 2.7 and 3.0 eV, hg ¼ 0.3 (depending on the location) [12], and DG0/n ¼ 1.23 eV, gives that hchem (STH) ranges between 0.14 and 0.12, assuming that fconv ¼ 1. The challenge is obtaining a much higher fraction of excited states hijacked to the desired reaction. In a dual system D2 coupling polymeric carbon nitride with a semiconductor of band gap of 2.0 eV, as a cocatalyst, such as iron oxides (Fe (III)), the efficiency can be increased between 0.31 and 0.32, i.e. practically the double compared with carbon nitride alone. Another issue to be pointed out is that the rate of the two half reactions are very different, and sacrificial agents such as holes scavenger are often used to increase the production rate. The real efficiency is much lower than the ideal ones due to several phenomena involved with different time scales such as: i. photon absorption, ii. exciton separation, iii. carrier transport, iv.
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Catalytic efficiency, v. mass transfer of reagents and products [13]. In this report, the attention is focused on point ii. exciton separation, since for carbon nitride, as an organic semiconductor, is possibly the most important issue. The exciton binding energy that reduces both energy available to the reaction as well as separation of charges is the subject matter. The desired electronic property of photoactive semiconductors, including a suitable band gap, is that the exciton binding energy be Eb < 25 meV, with dielectric constant εr > 10, because thermal energy at 25 C is 25.7 eV. If Eb is lower than the thermal energy the exciton bond can be easily broken, and the charges are free to move into different directions, as occurs in inorganic semiconductor, whose Eb is between 1 and 40 meV. Density functional theory (DFT) studies by Melissen et al. estimated that Eb for polymeric carbon nitride should be 840 meV, and the optical band gap Eg ¼ 2.6 eV with a Frenkel A, similar to an atomic radius, indicating that radius Re of 1.38 exciton is behaving as a very localized particle [14]. Probably, Eb of carbon nitride is between 0.3 and 1 eV (depending on the degree of crystallinity, etc.), i.e. it can correspond to about the 7% of hydrogen binding energy (considering the maximum value of 1 eV), which is 13.6 eV. It is comparable to the binding energy of the simplest molecule, so that the photocatalytic reaction can be expressed in the following way with the “exciton atom” indicated as hþe: 2hn þ S2 þ H2 O / 2hþ e þ H2 O /H2 þ 1=2O2
(6)
However, this reaction to be effective, must be split in the following reactions: 2hþ e þ Eb þ H2 O / 2hþ þ 2e þ H2 O/H2 þ 1=2O2
(7)
Of course, this phenomenon would be much easier if the value of Eb were similar to that of thermal energy. In the case of polymeric carbon nitride, this energy barrier is the first step of catalysis to be overcome by means of traps to separate electrons and holes. Moreover, another study by Wei & Jacob showed that the hole is localized on the secondary amine bridging the heptazine units, while electron density is surrounding it, more localized on the closest atoms bonded to this nitrogen atom [15], as shown in Fig. 1. Although, this sites are the reactive ones, their localization makes difficult to use these charges for photocatalysis without any mean that would weaken the exciton bond. The position of the redox potential of water (Hþ/H2 and H2O/O2) is considerably within the conduction band (CB) and the valence band (VB) edge levels, respectively, so that the overpotential losses should not affect the reaction, compared with other semiconductors as shown in Fig. 2 [16]. On the contrary, the relative materials, based on triazine units, instead of tri-s-triazine ones, have a VB more shifted and too close the oxidation potential, making them less attractive for water photolysis [17] since holes release their free energy when are in lower levels than the uploading state. Co-catalysts can provide trapping sites for the photogenerated charges, and promote the charge separation, thus enhancing the quantum efficiency; could improve the photostability of the catalysts by timely consuming of the
Fig. 1 e Localization of the exciton with the radius of 1.38 A in polymeric carbon nitride (circle) in the bridging secondary amine. The image shows the repeating unit C6N7(NH2)NH.
photogenerated charges, particularly the holes; most importantly, the co-catalysts catalyze the reactions by lowering the activation energy [18]. The strategy to use co-catalyst to alter the electronic properties of polymeric carbon nitride is very promising for water photolysis. It should be remembered that the competing reaction is the following: hþ e % hn þ S2
(8)
i.e. the recombination of charges with the releasing of a photon (with lower energy than the absorbed one and detectable as luminescence), or the emission of heat (the predominant phenomenon). Indeed, measuring luminescence is an indirect method to know the effect of co-catalysts on the photonic efficiency expressed in Eq. (1).
Co-catalysts For water photolysis, the classical co-catalyst is Pt, there are also metal oxides such as WO3 and sulfides MoS2, as well as nonmetallic elements such as I, F, B, S and Se. Other cocatalysts have been tested mainly for photo-degradation of dyes, or other partial oxidations of organic compound, cycloadditions etc. such as Ag, Fe, Cu mainly as oxide. Recently, a new compound named C2N has been also proposed as a candidate to increase the efficiency of polymeric carbon nitride by favoring exchange of electrons [19,20]. Many other examples can be provided of the increasing yield due to co-catalysts with a synergic increased efficiency also to the respect the separated catalysts, recently also Nb salts [21], NieP compounds, VS2 [22], MnO2 [23], MoS2 composites [24], and compounds of NieP [25,26] showed enhanced hydrogen evolution rate (HER), often above 1000 mmol g1h1. It is noteworthy to point out that also carbon nitride based on triazine units (tC3N4) could act as a co-catalyst towards carbon nitride based on heptazine units (h-C3N4) as hypothesized by Zhao et al. [27].
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Fig. 2 e Band edge positions of commonly reported nitride photocatalysts. The oxidation and reduction potentials of water are also shown (green dotted lines). The red dotted line represents the band edge positions of InxGaxN with x increasing from left to right (0e1). The reduction potentials of CO2 to various value added products are also shown [16]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The basic scheme of the mechanism these oxide semiconductors can work through, coupled with polymeric carbon nitride, is shown in Fig. 3. It was proposed for the case of the system Ag3PO4/g-C3N4 in relation to photo-degradation, but basically it would be the same scheme for water photolysis, i.e. the recombination of holes of polymeric carbon nitride with electrons of the VB of the metal oxide will provide energy to free the excited electron of polymeric carbon nitride [28]. This mechanism, named direct Z-scheme (for the diagram shape), decreases the direct recombination of excitons in the polymeric carbon nitride, but require that the CB of the metal oxide had higher energy of the VB but lower than that of the CB of polymeric carbon nitride. This
Fig. 3 e Energy level positions of the composites MOx/gC3N4 with illustration of direct Z-scheme of transfer of electrons from the CB of MOx to the VB of g-C3N4.
mechanism can be assimilated to that of the D2 system previously mentioned. The effect on photoluminescence of the Ag doping on gC3N4 is paradigmatic for all the other possible example of the same type. Ag doping caused a dramatic decay of photoluminescence, especially achieving the 1 wt% as shown in Ref. [29]. The best balance between decreasing photoluminescence and hydrogen production is found at 1 wt% of Ag loading (see Fig. 4), then the luminescence decrement only indicates that other pathways are found to charges recombinations in competition with photocatalytic hydrogen production and luminescence. However, the mechanism is probably that illustrated in Fig. 3, since silver nanoparticles can behave in this way too. The other possibility is to have a redox mediator instead of the direct transfer of the electron from the CB of MOx to the VB of pymeric carbon nitride as in the Z-direct scheme. The tests were carried out by Martin et al. on BiVO4 and WO3 using as redox mediator FeCl2 and NaI. The co-catalyst were really two per sample: Pt specific to hydrogen reduction, and BiVO4 or WO3 for oxygen oxidation. The following scheme of Fig. 5 is suggested as a modification of the Z-scheme. The results are clearly in favor of the use of the redox mediators, with the best results for WO3 and NaI/NaIO3 as redox mediator, doubtless the ability of the metal oxide to adsorb the oxidized phase of the mediator is a key factor too [30]. A comparison of a series of oxide and sulfide semiconductors to work as co-catalyst is shown in Table 1. The experimental conditions of the presented results are not the same for many reasons such as the type of hole scavenger, pH etc., lamp type, power and emission spectra so that the comparison is only semi-quantitative, however the trend is clearly in favor of the use of co-catalysts such as semiconductors to exploit the Z-scheme [22]. It is noteworthy to point out that metal sulfides seem to give the best results in terms of hydrogen evolution rate (HER). In most of these cases,there is often a metallic or at least an electric conductive phase intermediate between the two photosystems as in the case of NiS (1.5% load) [32], where the presence of carbon black allowed to achieve a HER of
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Fig. 4 e Left: Comparison between the rate of H2 rates of production by photocatalysis with g-C3N4 as-is, and doped with different amounts of Ag under visible light in methanol 25 V% aqueous solution, an arc Xe lamp with cut off at 420 nm was used (0.78 mW/cm2). Right: photoluminescence of g-C3N4 as-is and doped with different amounts of Ag [29].
992 mm h1g1, higher than that of a NiS2 with a loading of 30% [33]. Actually, the use of solid phase redox mediator, instead of soluble salts, is very promising, this will represent another way to increase the efficiency of the Z-scheme in a more efficient and practical way. The intermediate heterojunction can be obtained for example with Au, Ag or other metals. The importance of this hetero-junction was highlighted by an experiment of Sun et al.[35], which showed that the Pt -Ni(OH)2 composite deposited on g-C3N4 has a double HER (ca. 3000 vs. 1500 mmol g1 h1) than the simple dual system with spatially separated co-catalysts, due to the better mediation of the conductive Pt when the hetero-contact is very intimate; moreover, Pt is also working as an HER site, while Ni(OH)2 as an oxygen evolution (OER) one. The scheme is, in these cases, MZx-CeC3N4, with a conductive medium C (M is a metal, and Z can be either O or S) between the two photosystems (PS1-C-PS2). For instance, Ag nanoparticles (NP) can be obtained irradiating silver halogenides (AgX) such as AgBr and AgI. Moreover, results of Fe2O3eAgeC3N4 showed a 2.74 higher degradation of Rhodamine B (RhB) than pure carbon nitride [32].
Fig. 5 e Z-scheme for the transfer of electrons from the CB of MOx to the VB g-C3N4 by means of a redox mediator A/D.
Table 1 e hydrogen evolution rate (HER) with different co-catalysts and conditions. C3N4 [22e30] VS [22] Ti2C [22] MoS2 [22] WS2 [22]
HER 1 1
H2mm h g
4.4e5
1748
922
376 2
568
a
WO3 [30] BiVO4 [30] 74
[22]: monochromatic light 420 nm (14.3 mW/cm ). Triethanolamine TEOA as a hole scavenger. [30]: 300 W Xe lamp, with >395 nm cut off filter. NaI, FeCl2 as redox mediators [31]: 300 W Xe Lamp with >420 nm cut off filter, TEOA [33]: 300 W Xe Lamp with >420 nm cut off filter, TEOA [34]: 300 W Xe Lamp, TEOA a with Pt loading on WO3 b 1.5% of NiS loading and 0.5% carbon black c 25.3% of NiS2 loading
15
b
NiS [31] 992
c
NiS2 [33] Ni(OH)2 [34] 968
~3000
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Conclusions [7]
In summary, co-catalysts or dual (even composite) photosystems (e.g. D2 systems) involving the Z-scheme mechanism dramatically increased the hc (STH) efficiency of polymeric carbon nitride from ca. 0.01 to ca. 0.20, as inferred from available data. These results were expected from theoretical considerations, and favored by the use of specific catalysts for the oxidation and the reduction reactions (i.e. the cocatalysts). Exciton recombination and localization emerged as the most determinant issue to be faced for water photolysis by carbon nitride. The Z-scheme mechanism with solid phase redox mediators appears to be the more promising way to decrease the extent of hole and electron recombination for this class of materials in a more efficient manner. However, the difficulty of combining different materials to create effective hetero-junctions must be overcome. Two possibilities aroused to carry out this task: i. in situ synthesis of the solid phase mediator (and possibly that of the semiconductor co-catalyst), using carbon nitride to template the solid phase mediator, which is usually obtained by decomposition at low temperatures of unstable compounds. This route can be particularly effective (for example, halogenides of noble metals such as Ag and Au); ii. another promising route, still open to intensive investigation, is that of using graphite (or graphitic materials) to transfer electrons instead of noble metals. In this case, the roles in the synthesis route can be inverted, since graphite can act as a template for the synthesis of carbon nitride in situ because it is more thermally stable than g-C3N4, and possibly providing better hetero-junctions than metals. In addition, as a consequence of using graphite, it will be interesting to investigate the Z-scheme involving C2N (as co-cotalyst possibly provided with proper OER sites and conductive mediator), and g-C3N4 with adequate HER sites. In any case, All these tasks are favored by the presence of cages and the ability of carbon nitride sheets to wrap other nanoparticles with strong interactions.
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