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Rational design of photocatalysts for ammonia production from water and nitrogen gas
Nano Convergence volume 8, Article number: 22 (2021)
Photocatalytic N2 reduction has emerged as one of the most attractive routes to produce NH3 as a useful commodity for chemicals used in industries and as a carbon-free energy source. Recently, significant progress has been made in understanding, exploring, and designing efficient photocatalyst. In this review, we outline the important mechanistic and experimental procedures for photocatalytic NH3 production. In addition, we review effective strategies on development of photocatalysts. Finally, our analyses on the characteristics and modifications of photocatalysts have been summarized, based on which we discuss the possible future research directions, particularly on preparing more efficient catalysts. Overall, this review provides insights on improving photocatalytic NH3 production and designing solar-driven chemical conversions.
Recently, there has been an increasing focus on ammonia (NH3) as not only a key commodity for chemicals widely used in industries [1, 2], but also as a liquid energy carrier  that enables transport and supply of hydrogen (H2) gas through cracking. Furthermore, NH3 is an alternative carbon-free energy source that can be utilized in energy-conversion devices, for example, direct NH3 fuel cells [4, 5]. Traditionally, NH3 production has mainly relied on the Haber–Bosch process, which is energy intensive and generates considerable volume of CO2 into the atmosphere because of these requirements, along with the needs for H2 and the extreme operation condition .
Photochemical NH3 production using N2 and water as a hydrogen source at ambient temperature and pressure can offer a promising alternative route that is less energy intensive and reduces CO2 emission, thereby significantly reducing environmental concerns . NH3 production paired with water oxidation is a thermodynamically uphill reaction; therefore, external energy input is necessary . Photocatalysts can utilize solar energy, which is supplied to Earth with sufficient solar power and abundance and facilitate NH3 production. Since TiO2 photocatalysts applied to N2 reduction, new photocatalysts and their modifications have been intensively suggested to improve photocatalytic NH3 production .
Major advances in metal oxides such as TiO2, WO3, and SrTiO3, bismuth oxyhalides, and polymeric carbon nitride photocatalysts have resulted in interests in the development of photocatalysts and their applications for NH3 production [10,11,12,13,14]. This review is composed of mainly two sections: (1) fundamentals for photocatalytic NH3 production, (2) strategies to develop photocatalysts. The first section focuses on the principles and mechanisms involved in the overall photocatalytic reaction. The second section contains recent progresses in the development of catalysts and their major advantages with regard to catalytic performance. The insights and discussions provided in this review will serve as useful resources for further development of photocatalysts and provide future research direction for photocatalytic NH3 production.
Fundamentals for photocatalytic NH3 production
Principle and mechanism
NH3 production by N2 reduction (N2(g) + 6H+ + 6e− ⇌ 2NH3(g), E0 = 0.092 V vs. SHE) paired with O2 production by water oxidation (2H2O(l) ⇌ O2(g) + 4H+ + 4e−, E0 = 1.229 V vs. SHE) is a thermodynamically uphill reaction that requires a potential of at least 1.137 eV  per electron . Moreover, three electrons are required to produce an NH3 molecule, and four holes are required to produce an O2 molecule according to eqns. (1) and (2):
A simplified schematic diagram for photocatalyic NH3 production and challenges in designing photocatalysts are summarized in Fig. 1. For overall NH3 production, photons with energies > 1.137 eV are required to generate photoexcited electron–hole pairs that carry out the desired redox reactions at the active sites on the surfaces of the photocatalysts. Thus, a photocatalyst must have an energy band gap, (Eg = ECBM-EVBM, where CBM and VBM represent conduction band minimum and valence band maximum), larger than the energy required for the overall NH3 production. In addition, a photocatalyst must have a suitable conduction and valence band alignment to drive two half-reactions using photoexcited electrons (e−) and holes (h+) under solar light irradiation.
The major factors to consider for photocatalyst designs are (1) photon absorption followed by photoexcited carrier generation, (2) their migration and separation, and (3) their consumption by redox reactions on the surfaces of photocatalysts. The dominant photon energy spectrum obtained from the sunlight is in the range of 390–700 nm. Furthermore, because photocatalytic redox reactions require kinetic overpotentials, photocatalysts have a bandgap energy in the range of 1.6–2.4 eV. The generated photoexcited carriers can migrate to active sites on the surface where redox reactions carried out. However, since the kinetics of carrier recombination in the interiors of the photocatalysts is fast, considerable amounts of photoexcited carriers are recombined . Moreover, photoexcited carriers can be trapped in the interior and surface of the photocatalyst, which causes photocorrosion. Thus, a photocatalyst design to improve electron–hole pair migration and separation is important for further development of photochemical NH3 production .
Most semiconductors that act as light absorbers in photocatalysts are inactive in the redox reactions with the photoexcited carriers; thus, the active sites on the surface of semiconductor are decorated with cocatalysts . For example, cocatalysts, such as RuOx, CoOx, and Co-pi [20,21,22], combined on the surface of the semiconductor for facile water oxidation by providing active sites. Another critical point to note in NH3 production via photochemical N2 reduction is the facile competitive hydrogen evolution reaction by water reduction. NH3 production is kinetically much more complex than water reduction, although the thermodynamic requirements for those reactions are similar . This indicates that photochemical H2 production may be predominant, resulting in inactive NH3 production. Thus, it is critical to modify the surface that provides active sites for the desired redox reaction using suppressing competitive reactions on the semiconductors.
It is also necessary to note the importance of rate balance for photocatalytic reactions on photocatalysts. Since electrons and holes are generated in pairs under illuminations, their consumption must be coupled as well. For instance, if oxidation reactions involving photogenerated holes are sluggish, the total NH3 production rate should be similar. In this case, scavengers, such as ethanol, methanol, and sulfites, can be employed as sacrificial reagents to facilitate oxidation reaction using photoexcited holes, thereby preventing from photoexcited carrier recombination . However, since scavengers may interfere with counter reactions, here N2 reduction, or product evaluation methods, the target scavenger should be chosen carefully. It is discussed in detail in the next section.
The simplest photocatalytic NH3 production set-up is a particulate PC system where photocatalysts are dispersed in a medium, typically pure water, with N2 bubbling under simulated solar light illumination, as shown in Fig. 1b. NH3 and O2 are generated on a single photocatalyst. The produced NH3 is technically evaluated by the spectrophotometric method via the indophenol blue method or quantified using proton nuclear magnetic resonance spectroscopy (1H-NMR) with isotope labeled nitrogen 15 N as shown in Fig. 2.
The indophenol blue method is based on the principle of the Berthelot reaction, where typically, salicylic acid, citrate, hypochlorite, and ferricyanide are used in an alkaline solution. In the presence of NH4+, the color of the yellowish indicating solution, which contains the reagents, turns to blue, corresponding to 655 nm. As shown in Fig. 2a and b, thus, the concentration of NH3 can be quantified by measuring the absorbance change at 655 nm using a UV–Vis spectrophotometer . However, to quantify the concentration of NH3 generated by photocatalytic N2 reduction, multiple possible contaminants that interfere with the evaluation should be fully controlled . For example, methanol, a widely used as a hole scavenger, and its derivatives cause incorrect color changes and significantly decrease the accuracy of NH3 quantification as shown in Fig. 2c and d. Another critical point for NH3, which is photocatalytically produced, quantification is to eliminate the parts from NH3 contaminants, unintentionally present introduced through catalyst, components in set-up  or even the air. In fact, NH3 present in human breath, sanitizers, and so can rapidly accumulate in water .
The most rigorous procedure to quantify photocatalytically generated NH3 from N2 reduction is to use isotope labeled 15N2 gas. The photocatalytic reactions are conducted in a simple particulate system with 15N2 bubbling. Since the splitting of 1H resonance in 15NH4+ and 14NH4+ resulting from the difference in the scalar interaction between N (15 N or 14 N) and H is differentiated, the 15NH4+ produced, which is dissolved in an aqueous reacting solution, can be quantified by 1H-NMR . For instance, the 1H resonance in 15NH4+ can be split into two symmetric signals with a spacing of 73 Hz, whereas the resonance in 14NH4+ can be split into three symmetric signals with a spacing of 52 Hz as shown in Fig. 2e. Thus, false positive evaluations for the produced NH3 can be prevented effectively.
Strategies to develop photocatalysts
This section is divided into mainly two parts: (1) semiconductors comprising photocatalysts, which have been widely applied as a light absorber in Fig. 3a, and (2) strategies to modify the catalysts, which have been effectively used to further increase the photocatalytic NH3 production.
Titanium(Ti)-based oxides Titanium dioxide, TiO2, possesses several advantages that make it attractive for use in photocatalytic NH3 production. TiO2 has a bandgap of ~ 3.2 eV, which absorbs UV light, and a CBM and VBM that offer sufficient overpotential for the photoexcited electrons to reduce N2 to NH3 and holes to oxidize water to O2, respectively. In addition, the inexpensive, environmentally benign, and stable nature of TiO2 is an additional advantage. Historically, the primary limitation of using TiO2 has been rapid photoexcited electron–hole recombination, which is similar to many other oxide-based photocatalysts. Strontium titanate, SrTiO3 , is another candidate showing two major benefits over TiO2 for use in the NH3 production. The CBM of SrTiO3 is slightly more negative than that of TiO2, thereby providing a stronger driving force to reduce N2 using the photoexcited electrons. In addition, intrinsic charge mobility in SrTiO3 is higher than that in TiO2, thereby suppressing photoexcited charge recombination . However, despite this thermodynamic advantage, the photocatalytic NH3 production using TiO2 or SrTiO3 is limited because of ineffective light absorption resulting from their wide bandgaps and inactive redox reactions due to the limited active sites on their surface. Recently, many attempts have been made to address these limitations, which include doping, coupling with electrocatalysts (cocatalysts), surface defect engineering, and so on [33,34,35].
Bismuth oxyhalide (BiOX) BiOX (X = Cl, Br, and I) has a layered structure where bismuth and oxygen layers (BiOx) are connected with halide (X) layers alternatingly via Van der Waals forces . Since the VBM is determined by O 2p and X \(\updelta\) p (\(\updelta\) = 3, 4, and 5, corresponding to X = Cl, Br, and I, respectively) and the CBM is mainly evaluated by Bi 6p, their band alignments and bandgap are varied depending on the element comprising the semiconductors . For example, BiOCl, BiOBr, and BiOI have bandgap energies of 2.92, 2.65, and 1.75 eV, respectively , and are thermodynamically suitable for NH3 production, they enable the production of NH3 and O2 by the overall photocatalytic N2 reduction. However, despite these advantages, BiOX semiconductors have severe drawbacks when using them as photocatalysts, mainly due to the insufficient photogenerated charge separation in the bulk and surface. Recently, their fundamental limitations have been investigated, and strategies to improve their photocatalytic performances have been explored.
Graphitic carbon nitride (g-C3N4) g-C3N4 is a polymeric semiconductor with a bandgap energy of 2.7 eV and appropriate positions for the CBM and VBM for the NH3 and O2 production, respectively . However, since rapid photoexcited carrier recombination in the bulk results in limited charge separation in the redox reactions and ineffective N2 adsorption on the surface, as an initial step for N2 reduction, multiple advancements to develop g-C3N4 have been suggested .
Strategies to enhance photocatalytic NH3 production
As described in the introduction section, photocatalysts comprising semiconductors can produce NH3 using N2 and water under illumination. However, the typical NH3 production rate and efficiency of intrinsic semiconductors using solar energy have been far lower than those expected from their bandgap energies. The major limiting factors include (1) limited light absorption because of the intrinsically large bandgaps, (2) significantly fast photoexcited electron–hole pairs recombination, and (3) slow redox reactions including N2 reduction and water oxidation. Recently, many strategies have been made to address one or more of these challenges, which include engineering defects by extrinsic doping or vacancy introducing, composites forming using multi-light absorbers, and coupling with electrocatalysts. This section will give an overview of each of these attempts and discuss how these strategies affected the photocatalytic NH3 production.
Defect engineering has been widely suggested to control bulk and surface properties of catalysts, for example, by heteroatoms doping, vacancies formation, and so on. This defect engineering enhances photocatalytic NH3 production by changing the light absorption properties, improving charge separation efficiencies, or facilitating surface redox reactions, and it generally influences the multi-mechanistic steps of light harvesting simultaneously .
Heteroatom doping Heteroatom doping involves intentional introduction of impurity atoms into the lattices of materials to change their optical and electrical properties [42,43,44]. Depending on the species and degree (concentration) of dopants, changes, in the bandgap energy, band alignment, charge separation efficiency, and N2 adsorption ability are expected. However, since dopants can provide electron–hole recombination sites or prohibit charge transports, identifying types and concentrations of dopants appropriate for the photocatalytic process is important.
Zaicheng Sun and coworkers prepared a nickel-doped TiO2 (Ni-x-TiO2, where x = concentration of the Ni precursor controlled in the preparation step) photocatalyst using the sol–gel method  show Their optimized sample (Ni-0.8-TiO2) exhibited an NH3 production rate of 46.80 µmol·g−1·h−1 under simulated solar light irradiation, which is seven times higher than the rate using pure reference TiO2. The Ni-0.8-TiO2 sample extends the light absorption range to the visible light region with a bandgap energy of 2.92 eV, whereas pure TiO2 absorbs only UV light with a bandgap energy of 3.2 eV as shown in Fig. 3b. As Ni atoms having 2 + valence that replace the Ti site having 4 + valence, oxygen vacancies (Ov) are created naturally for charge neutrality within the host semiconductors, and they vary the band position of the CBM and VBM in TiO2, which results in a decrease in the bandgap energy. Besides changes in the optical properties, N2 adsorption, the initiation step for NH3 production from N2, is enhanced by Ni doping on TiO2, as investigated using a N2 temperature programmed desorption spectroscopy (N2-TPD) and a computational analysis.
Tierui Zhang and coworkers prepared a copper-doped TiO2 nanosheet (x%-TiO2, where x = molar ration of Cu/Ti controlled in the preparation step) photocatalyst using a hydrothermal method . The optimized sample, 6%-TiO2, exhibited an NH3 production rate of 78.9 µmol·g−1·h−1, whereas pristine TiO2 nanosheets exhibited a rate of 0.34 µmol·g−1·h−1. As Cu heteroatoms replaced the sites of Ti in the TiO2 nanosheets, small amounts of Ti3+ valence states in Ti4+-dominant TiO2 and Ov sites are created, as observed using X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy. Furthermore, owing to the difference in size between the Cu2+ dopants and Ti4+ hosts, Cu-doped TiO2 has a compressive strain, which is evaluated by DFT calculations, and it causes changes in the distribution in the electron densities around O and Ti in the materials. The combination of non-stoichiometry and lattice strain in Cu-doped TiO2 narrows its bandgap energy, corresponding to the extended light absorption range of 600–800 nm, and electrons are accumulated around the sites of O atoms, thereby affecting the changes in the N2 adsorption affinity.
Jing Zhang and coworkers prepared a Fe-doped SrTiO3 (FexSr1-xTiO3, where x = the stoichiometric ratio of Fe precursor and Sr precursor controlled in preparation step) photocatalyst using the hydrothermal method followed by calcination . The Fe0.1Sr0.9TiO3 sample exhibited the best NH3 production rate of 30.1 μmol·g−1·h−1, which is 3.2 times higher than that of pristine SrTiO3. By replacement of the Fe3+ ion, which has a smaller size, on the Sr2+ site in SrTiO3, the size of particles is reduced along with the corresponding increase in the surface area. Furthermore, the Fe0.1Sr0.9TiO3 photocatalyst exhibits significantly enhanced N2 chemisorption and activation ability, which Fe dopants are the major contributor to.
Vacancies Vacancy, one of defect formation strategies, leads to changes in the band structure and chemical adsorption nature on the surface . This section gives an overview of intentionally induced anion vacancies in semiconductor photocatalysts and discusses how they affected the photocatalytic NH3 production.
Zhong Jin and coworkers compared the NH3 production activities using BiOBr semiconductors in the presence (Vo–BiOBr) or absence (BiOBr) of oxygen vacancies . The Vo-BiOBr samples were prepared using the hydrothermal method with the addition of polyvinylpyrrolidone surfactants leading to in-situ generation of abundant vacancies, whereas pristine BiOBrs were synthesized without adding the surfactants. The Vo-BiOBr photocatalysts produced NH3 at a rate of 54.70 μmol·g−1·h−1 under UV–Vis irradiation, which is 10 times higher than the production rate using BiOBr. The major contribution of this significant improvement is enhanced N2 adsorption that initiates the intermediate formation for NH3 production. For example, the amount of adsorbed N2, estimated based on N2 adsorption isotherms, on Vo-BiOBr is considerably higher than that using BiOBr as shown in Fig. 4a. Furthermore, by introduction of the oxygen vacancies, the bandgap energy is reduced with the shifts in the CBM and VBM positions, which leads to increased light absorption as shown in Fig. 4b.
Chuanyi Wang and coworkers prepared metal-free g-C3N4 semiconductors by thermal decomposition of melamine and induced nitrogen vacancies (V-g-C3N4) within the semiconductors by additional calcination under a N2 atmosphere . The V-g-C3N4 samples produced NH3 at a rate of 1240 μmol·g−1 h−1 using pure N2 and water under visible light irradiation, whereas pristine g-C3N4 produced negligible amounts of NH3. Further, morphologies and bandgap energies were not significantly varied depending on the intentional creation of N vacancies; however, two factors changed considerably. Due to the presence of N vacancies on the semiconductor, the signal collected using photoluminescence spectra decreased, indicating the reduction in the carrier recombination and enhancement of their separation. Furthermore, the N vacancies enable selective adsorption and activate N2, and it was experimentally confirmed that negligible amount of NH3 produced on g-C3N4 selectively blocked the vacancies using Pd particles.
Coupling with multi-light absorbers
Coupling with multi-light absorbers to form heterojunctions has been widely investigated to improve light absorption capacity under single illumination and separation of photoexcited electron–hole carriers through their designed band alignments. This strategy for catalyst design is effective in improving the photocatalytic NH3 production.
Zhong Jin and coworkers prepared Bi2MoO6/Ov-BiOBr composites, where Ov denotes the oxygen vacancies using two-step methods, and synthesized Bi2MoO6 by solution-phase reflux process followed by the hydrothermal process to couple Ov-BiOBr on as-prepared Bi2MoO6 . Despite minimal difference in light absorption before and after coupling, the Bi2MoO6/Ov-BiOBr composites showed a significantly enhanced NH3 production rate (90.7 μmol·g−1 h−1), which is almost 30 and 3 times higher than the rates evaluated using Bi2MoO6 and Ov-BiOBr, respectively as shown in Fig. 5a. The major role of coupling is the formation of cascaded band alignments at the interface between two semiconductors. The photoexcited holes are transported and oxidize the electron donor on Bi2MoO6 where the VBM is located relatively higher, whereas the photoexcited electrons are transported and reduce N2 on Ov-BiOBr where the CBM located relatively lower. Furthermore, oxygen vacancies on BiOBr show synergistic effects on the NH3 production by providing N2 adsorption and activation sites as shown in Fig. 5b.
Jimmy Yu and coworkers prepared Ov-TiO2 using the hydrothermal method followed by loading Au nanoparticles. The Au nanoparticles generate hot electrons under visible light illumination , which is known as plasmonic phenomena. The hot electrons are injected into the CBM of TiO2 and then trapped in vacant sites where N2 is reduced into NH3. The generated holes remain on Au and oxidize the electron donors, methanol, in solution as shown in Fig. 5c and d. The optimized Au/TiO2-Ov samples produced NH3 with a rate of 78.6 μmol·g−1·h−1 under visible light irradiation, which is at least 98 and 35 times higher those using Au/TiO2 and TiO2-Ov photocatalysts, respectively.
Surface modification and reaction engineering
As discussed above, photocatalytic activities have been developed significantly by modifying photocatalysts, for example, defects engineering and composite formation. However, although photogenerated charge generation and separation are advanced by multiple possible strategies, if the surfaces of the semiconductors exhibit slow charge injection and are poorly catalytic for N2 reduction, the NH3 production rate may not be enhanced directly. The surface modification on the semiconductors using a robust electrocatalyst (cocatalyst) for N2 to NH3 conversion is the simplest way to improve the limiting factor at the interface between the semiconductors and liquid where reactants are present. Therefore, the modification of the semiconductor surface with various NH3 production catalysts, such as Ru, Cu, and Au, has been investigated [20, 34, 53].
In contrast, since the rates of reduction and oxidation reactions always correspond to one another, the sluggish photo-oxidation using holes may become a limiting factor for the photocatalytic NH3 production. To overcome the possible limitation, it has been widely suggested to load appropriate electrocatalysts (cocatalysts) for water oxidation on the semiconductor or provide a more facile hole acceptor (electron donors) than water, such as methanol or ethanol (Table 1).
Summary and outlook
In summary, we reviewed the mechanistic and experimental steps for photocatalytic NH3 production and examined various strategies to modify photocatalysts employed to enhance the production activities. This review clearly shows that although significant development of photocatalyts has been reported thus far, further advances for practical NH3 production are required. First, the efficiency for charge generation via solar light absorption should be enhanced. Considering that the majority of incident photons are in the range of visible light, development of absorbers, can harvest the incident solar light, is required by determining appropriate materials and engineering semiconductors.
The charge separation effect is also required for photocatalytic NH3 production through suppression of their recombination. For example, strategies for doping and composite formation will exhibit advances in the future and may increase the photon absorption efficiency simultaneously. In addition, morphology engineering [72,73,74,75,76] for photocatalysts will be promising and can be achieved by controlling the distances for carrier transportation and lifetime duration between the photo-induced generation and recombination. Furthermore, this may change facets and surface area, thereby providing active sites for the redox reactions using transported carriers. Moreover, if the facets are optimized with increasing surface area, morphology engineering can be significantly beneficial for NH3 production.
With improvements in photon absorption and charge separation, surface engineering will be able to provide active sites where N2 reduction using electrons and oxidation using holes is important. Thus, defect engineering, for example, formation of vacancies, cocatalyst coupling, and applying sacrificial reagents have been suggested. It is noteworthy that N2 diffusivity control close to photocatalysts make an important contribution to practical photocatalytic NH3 production. For instance, importance of three-phase interfaces, where a catalyst and two phases reactants such as water and active gas faced, has been demonstrated . This indicates that even if highly robust catalysts are prepared, if the supply of N2 gas and water is insufficient at the region close to the catalyst, the NH3 production is limited. Thus, environmental engineering, for example, via hydrophobicity control on the catalysts, will be significantly effective. The mechanism, experimental methods, discussion, insights, and suggestions contained in this review provide a good fundamental and foundation for further improvement of photocatalytic NH3 production.
Availability of data and materials
The datasets used and analysed during the current study are available from the corresponding references listed.
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This work was supported by the research fund of Hanyang University (HY202000000002708), the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) (No. 20202020800330), the Graduate school of Post Plastic specialization of Korea Environmental Industry & Technology Institute grant funded by the Ministry of Environment of Republic of Korea and National Research Foundation (NRF) Grant funded by the Korea government (No. 2020R1G1A1100104, No. 2019M1A2A2065612, No. 2018R1A2A1A05077909).
The authors declare that they have no competing interests.
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Choe, S., Kim, S.M., Lee, Y. et al. Rational design of photocatalysts for ammonia production from water and nitrogen gas. Nano Convergence 8, 22 (2021). https://doi.org/10.1186/s40580-021-00273-8