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Alternatives to water oxidation in thephotocatalytic water splitting reaction for solarhydrogen production
Yaqiang Wu,aTakuya Sakurai,a

Takumi Adachia and Qian Wang*a,b

The photocatalytic water splitting process to produce H2 is an attractive approach to meet energy
demands while achieving carbon emission reduction targets. However, none of the current photocatalytic
devices meets the criteria for practical sustainable H2 production due to their insuﬃcient eﬃciency andthe resulting high H2 cost. Economic viability may be achieved by simultaneously producing more valuable products than O2 or integrating with reforming processes of real waste streams, such as plastic andfood waste. Research over the past decade has begun to investigate the possibility of replacing water oxidation with more kinetically and thermodynamically facile oxidation reactions. We summarize how
Received 17th January 2023,
Accepted 4th March 2023
DOI: 10.1039/d3nr00260h
rsc.li/nanoscalevarious alternative photo-oxidation reactions can be combined with proton reduction in photocatalysis toachieve chemical valorization with concurrent H2 production. By examining the current advantages andchallenges of these oxidation reactions, we intend to demonstrate that these technologies would contribute to providing H2 energy, while also producing high-value chemicals for a sustainable chemical industry
and eliminating waste.

As humanity moves forward into the 21st century, the supplementation of clean and sustainable energy is one of ourgreatest scientific and technical challenges. The photocatalytic
water splitting process using nanoparticulate photocatalysts
and solar energy is a leading candidate to achieve low-carbon
production of H2 as a clean and sustainable energy carrier. In
this process, water is split into H2 and O2 via the following
redox reactions (eqn (1) and (2); pH 0 in aqueous solution
versus (normal hydrogen electrode) NHE, 25 °C, 1 atm gas
pressure, and 1 M for other solutes):2Hþ þ 2e ! H22H2 O ! O2 þ 4Hþ þ 4e

E° ¼ 0 V
E° ¼ þ1:23 V

ð1Þ
ð2Þ

Water splitting is a non-spontaneous chemical reaction
involving water oxidation to O2, which requires four electrons
to simultaneously transfer from at least two water molecules.
Photocatalysis is a light-driven chemical process that involves
oxidation and reduction reactions by the photogenerated electrons and holes. To achieve the photocatalytic water splitting
reaction, the conduction band minimum must be more negaa
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan. E-mail: wang.qian@material.nagoya-u.ac.jp;
Tel: +81-527893250bInstitute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8601, Japan

This journal is © The Royal Society of Chemistry 2023

tive than the H+ to H2 reduction potential, while the valenceband maximum must be more positive than the H2O to O2 oxidation potential. Consequently, the minimum theoreticalenergy required to initiate the water-splitting reaction is 1.23
eV. However, O2 evolution from water leads to sluggish kinetics
and high additional kinetic overpotentials,1,2 making photocatalytic H2 evolution need the use of sacrificial reagents toprovide an electron source.3–5 Specifically, the overpotential
losses of the oxygen evolution reaction (OER) can reach up to
30%, resulting in a significant amount of wasted energy.6 In
addition, the lack of eﬀective methods to separate the produced O2 and H2 gases is a key problem for the practical application of photocatalytic water splitting.7 As a result of thesechallenges, increasing attention is being focused on emerging
replacements for the OER that may support an expanding H2economy globally.
In this minireview, we highlight a variety of key reactions
that are well suited as potential replacements for the OER,
focusing on the recent developments in the study of photoreforming and photochemical oxidation reactions to produce
value-added organic and inorganic products that have received
considerable attention in the literature, such as formic acid(∼1.1 $ per kg) production from waste PET (≤0 $ per kg) andH2O2 (∼0.7 $ per kg) formation by water (∼2.9 × 10−4 $ per kg)
oxidation.8,9 These reactions are appealing because they would
enable (1) the production of more valuable chemicals than O2(∼0.1 $ per kg) alongside the production of H2; (2) the elimin-

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NanoscaleFig. 1 (a) Diagram of the photooxidation reactions rather than water-to-O2 oxidation combing with solar H2 production. (b) Band structures ofrepresentative photocatalysts and standard redox potentials for some common oxidation reactions. Data were obtained from ref. 6.

ation of the need for a H2 and O2 mixture separation unit,
thereby lowering the cost, improving the safety, and simplifying the design of photocatalytic processes; and (3) reduced
overpotential due to the simper 1 or 2 electron net oxidation
mechanisms, which would result in an increased conversion
eﬃciency and reaction rate.6 Under light irradiation, an inorganic or organic substrate is oxidized by the photoexcitedholes at the valence band, which subsequently generated valuable chemicals or CO2 or intermediate products and protons(Fig. 1a). The protons are reduced by electrons at the conduction band to produce H2. While the direct oxidation of alternative substrates combined with the production of clean H2 canbe feasibly carried out using various technologies including
electrolysis,
photoelectrochemical
andphotocatalytic
systems,6,7,10 we have targeted cases that have been demonstrated using nanoparticulate photocatalysts due to theirpotential for large-scale operations.

Qian Wang is currently an
Associate Professor at Nagoya
University, Japan. She received
her Ph.D. in 2014 from the
University of Tokyo, Japan andthen worked as a postdoctoral
researcher
for
the
Japan
TechnologicalResearch
AssociationofArtificial
Photosynthetic
ChemicalProcesses (ARPChem) project at
the University of Tokyo. In 2018,
she became a Marie SklodowskaQian Wang
Curie Research Fellow at the
University of Cambridge. She joined Nagoya University as an
Associate Professor in May 2021 and established her researchgroup, which is currently developing new materials, approaches,
and technologies for solar energy storage in the form of renewablefuels via artificial photosynthesis.

6522 | Nanoscale, 2023, 15, 6521–6535

Photoreforming
Photocatalytic reforming that harnesses solar energy toconvert sustainable or waste feedstocks for H2 generation has
become one of the most rapidly evolving applications in thefield due to its ability to valorize a range of feedstocks
especially biomass, food and plastics (≤0 $ per kg) to both
energy and value-added products (such as formic acid (∼1.1 $per kg), methanol (∼0.6 $ per kg), acetaldehyde (∼2 $ per kg)
and lactic acid (∼3 $ per kg)), as listed in Table 1.9 Research
trends highlight the shift from the early work which focusedon simple model substrates such as ethanol, glycerol, and cellulose to more recent work that utilizes real waste, making theprocess more complex but more economically and environmentally sustainable.11 The energetics of coupling the oxidation of a variety of common organic waste substrates with
H2 evolution are nearly neutral (Fig. 1b), for example, glucose
oxidation (eqn (3)), making it more favorable than water splitting, although these oxidation reactions involve multiple electron transfer steps.12 Theoretically, coupling photoreformingwith H2 production would no longer restrict the use of semiconductors with deep valence bands, allowing the applicationof those with a smaller bandgap. The photocatalytic reforming
could therefore be driven by a large portion of incident sunlight including visible and IR parts.
C6 H12 O6 þ 6H2 O ! 6CO2 þ 24Hþ þ 24eE°¼  0:001 V vs: NHE

ð3Þ


y
hv
Cx Hy Oz þð2x  zÞH2 O ! xCO2 þ 2x  z þ H22

ð4Þ

Initially, photocatalytic reforming research centered on the
production of H2 from a variety of lignocellulose biomassderived feedstocks, the most abundant and non-ediblebiomass resource, including monosaccharides, such as pentoses (ribose and arabinose) and hexoses (glucose, fructose,galactose, and mannose), alcohols (methanol, propanol,
ethanol, and butanol) and organic acids (formic acid andacetic acid).12–19 Theoretically, in the proposed biomass photoreforming process, photoexcited holes lead to the oxidation of
biomass components and derivatives (denoted as CxHyOz),

This journal is © The Royal Society of Chemistry 2023

This journal is © The Royal Society of Chemistry 2023
Simulated sunlight (AM 1.5G,
100 mW cm−2)
pH 12.5
Simulated sunlight (AM 1.5G,
100 mW cm−2)
Aqueous sodium acetate and MeCN
purged with CO2
Simulated sunlight (AM 1.5G,
100 mW cm−2)
Aqueous sodium acetate and MeCN
purged with CO2
LED irradiation (λ = 365 nm)
MeCN–water solution
Simulated sunlight (AM 1.5G,
100 mW cm−2)
LiBr and LiOH in 0.1 M H2SO4
solution
Simulated sunlight (AM 1.5G,
100 mW cm−2)
Simulated sunlight (AM 1.5G,
100 mW cm−2)
UV–vis light (250 W iron doped
halide lamp)
0.6 M H2SO4 solution 403 K; 3.0 bar
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M KOH
Simulated sunlight (100 mW cm−2)
0.5 M KOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M NaOH

Pt/TiO2

Pt/S- and N-doped graphene oxide
dots (2–7 nm)

TiO2/Co(terpyridine)2 (∼20 nm)

TiO2/Co(terpyridine)2 (∼20 nm)

Cu/TiO2 nanorods

Microcrystalline
cellulose
Cellulose

GlucoseCellobiose

GlycerolCdS/CdOx quantum dots

Cheese−1−1

20 hours

Formate, carboxylic acid, CO2, etc.
H2: ∼1070 mmol per gsubstrate per 20 h

Pyruvate, etc.

H2: ∼64.3 mmol gcatalysth

4 hours

20 hours

Formate, carbonate, etc. H2: 130 mmol h−1 m−2−1

24 hours

H2: ∼5.59 mmol gcatalyst−1 h−1−1

20 hours

H2: ∼5920 mmol gcatalyst−1 h−1

20 hours

H2: ∼9350 mmol gcatalyst−1 h−1

20 hours

20 hours

Formate, etc.
H2: ∼6070 mmol gcatalyst−1 h−1

H2: ∼9900 mmol gcatalyst−1 h−1

20 hours

Formate, etc.
H2: ∼462 mmol per gsubstrate per 20 h

QE: 15.0% at 430 nm

6 days26

24 hours

37

39

24

45

45

45

45

45

45

24

35

26

50

49

48

48

24 hours

10 hours

24 hours

12 hours

HMF, erythrose, and arabinose
H2: ∼2.57 mmol gcatalyst−1 h−1
QE: 2.73% at 430 nm

QE: 3.4% at 365 nm

24 hours

24 hours

108

25

5 hours
6 days28

Ref.

16 hours

Reactiontime

H2: ∼5 μmol per 24 h
C6H12O6 and C6H10O5 compounds
H2: ∼6 μmol per 24 h
C6H12O6 and C6H10O5 compounds
H2: ∼170 μmol per 10 h

Formic acid, lactic acid, arabinose, erythrose, etc.

H2: 22 mmol per gcatalyst per 12 hMethanol, CO, and CO2
H2: ∼190 mmol gcatalyst−1 h−1Formate: ∼1.03 mmol per gcatalyst per 24 h

CO: ∼0.92 mmol per gcatalyst per 24 h

Cellobiose conversion eﬃciency: 8.6%

QE: 0.22–0.41% at 360 nm for CO;
1.05–1.45% at 360 nm for formate

CO: ∼0.43 mmol per gcatalyst per 24 hFormate: ∼1.18 mmol per gcatalyst per 24 h
H2: ∼0.72 mmol per gcatalyst per 24 h

Glucose conversion eﬃciency: 20%

QE: 23.3% at 420 nm

Glucose conversion: 100%
Selectivity (lactic acid): 86%
QE: 42.8% at 365 nm

Eﬃciency

Acetaldehyde, methanol, formate, etc.
H2: ∼0.43 mmol per gcatalyst per 24 h

H2: 100 μmol h−1

H2: 3.39 mmol g h
Lactic acidH2: 13.3 μmol h−1

Products

Nanoscale

CdS/CdOx quantum dots (diameter∼5 nm)

CNx/Ni2P

CdS/CdOx quantum dots

Apple

Municipal solid
waste
PLA

CdS/CdOx quantum dots

StarchCdS/CdOx quantum dots (diameter∼5 nm)

CdS/CdOx quantum dots

Fructose

Wooden branch

CdS/CdOx quantum dots (diameter∼5 nm)

α-Cellulose

CdS/CdOx quantum dots

Pt/P25 TiO2 (20–30 nm)

Cellulose

Municipal waste

NiP/carbon dots (<10 nm)

GlucoseCdS/CdOx quantum dots

NiP/carbon dots (<10 nm)

Cellulose

Artificial mixed waste

Pt/

Cellulose

CNx

40 W blue LED (λ = 427 nm)
10 M KOH
16 W UV-A lamp (λ = 365 nm)

Pt/C3N4 (∼ 1 μm)

GlucoseNCN

Reaction conditions

Photocatalysts

Performance metrics for some recent representative solar fuel production systems using alternative photooxidation reactions to the OER

Substrates

Table 1

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Nanoscale, 2023, 15, 6521–6535 | 6523

6524 | Nanoscale, 2023, 15, 6521–6535
6 W LED (λ = 365 ± 5 nm)

300 W Xe lamp (λ ≥ 420 nm)

300 W Xe lamp (λ ≥ 420 nm)
300 W Xe lamp (λ > 380 nm)

14.4 W blue LED (λmax = 470 nm)
9 W blue LED (λ = 455 nm)
9 W blue LED (λ = 455 nm)
Simulated sunlight (λ > 400 nm,
100 mW cm−2)
1.85 W Xe lamp (λ > 420 nm)

5 W LED (λ = 450 nm)

300 W Xe lamp (λ = 420–780 nm)
300 W Xe lamp (λ > 400 nm)

CdS/CdOx quantum dots(diameter∼5 nm)

CoP/Zn2In2S5 nanosheets

CoP/Zn2In2S5 nanosheets

Zn0.2In2S3.2

Ti3C2Tx/CdS nanosheets (∼60 nm)

Pd/TiO2

Ni/CdS nanosheets

Zn3In2S6 (1–3 μm)

Pt/CNx

Pt/CNxRu–ZnIn2S4 (0.6–1.6 μm)
Ru–ZnIn2S4 (0.6–1.6 μm)
Pt/porous carbon nitride

Ni/CdS nanoparticles

PtS/ZnIn2S4 nanocomposites

MoS2 foam/CdS nanorods

RuCat/Pt–g-C3N4

Ni/CdS nanosheets (thickness∼1 nm)
Ni/CdS nanosheets (thickness∼1 nm)
NiP/NCNCNx

CdS-ZnIn2S4 nanosheets

MoS2/MAPbI3

PET water bottle

Methanol

Ethanol

Benzyl alcohol

Furfural alcohol

Cetophenone

DMF

Benzyl alcohol

DMA

THF

2,5-DMF
2-MF
HMF

Benzylamine

Benzyl mercaptane

MethanolMBA

Furfural alcohol

H2O

HI

4-MBA

HMF

Simulated sunlight (AM 1.5G)

CdS/CdOx quantum dots (diameter∼5 nm)PUR

H2: 12 mmol gcatalyst−1 h−1Ethylene glycol, etc.
H2: 56 μmol per 10 hMBAd
H2: ∼5 mL per 8 h
Furfural
H2: ∼0.5 mL per 8 h
DFF
H2: ∼21.3 μmol per 24 h
4-MBAd: ∼19.8 μmol per 24 h
H2: ∼0.5 mmol gcatalyst−1 h−1
H2O2: ∼0.6 mmol gcatalyst−1 h−1
H2: ∼29.4 mmol gcatalyst−1 h−1
I3−

H2 yield: 100%
Dibenzyl disulfide

H2: ∼12 μmol per 10 h
Minisci oxidation products: ∼15 μmol per 10 h
H2: ∼9 μmol per 10 h
Minisci oxidation products: ∼9.5 μmol per 10 h
H2: ∼6.0 mmol gcatalyst−1 h−1 DFPs: ∼1.04 g gcatalyst−1 h−1
H2: ∼1.2 mmol gcatalyst−1 h−1 DFPs: ∼0.1 g gcatalyst−1 h−1
H2: 12 μmol m−2 h−1
DFF: 1.3μmol m−2 h−1
H2: 21.4 mmol gcatalyst−1 h−1
Imine

QE: 13–22% in the range of 420–850 nm
STH: 7.35%

QE: 1.63% at 400 nm

QE: 15% at 360 nm

QE: 0.28% at 400 nm

QE: 15.2% at 452 nm
QE: 1.9% at 452 nm
DFF yield: 13.8% after 6 h
Selectivity >99%
Conversion: 99%
Selectivity for imine: 97%
QE: 11.2% at 450 nm
Conversion: ∼100% after 6 h
Selectivity of dibenzyl disulfide: ∼100%
QE: 24.96% at 420 nm

QE: 6.48% at 380 nm
5.46% at 400 nm
4.02% at 420 nm
1.03% at 450 nm

QE: ∼44% at 420 nm

Conversion: 5.15%

Formate, glycolate, ethanol, acetate, lactate, glyoxylate,methylglyoxal, and methanol
H2: ∼21 mmol gcatalyst−1 h−1Ethylene glycol: 18.9 mmol gcatalyst−1 h−1
HCHO: ∼4 mmol gcatalyst−1 h−1
H2: ∼8.5 mmol gcatalyst−1 h−1
2,3-Butanediol: 3.2 mmol gcatalyst−1 h−1
CH3CHO: ∼5 mmol gcatalyst−1 h−1
H2 yield: 172%
Benzoin yield: 61%
Deoxybenzoin yield: 30%
Benzaldehyde yield: 9%
H2: 773 μmol per gcatalyst per 4 h
Furfural: 777 μmol per gcatalyst per 4 h
H2: ∼0.25 mmol per 3 h
Phenethanol: ∼0.55 mmol per 3 h
Ethylbenzene: ∼0.05 mmol pre 3 h
Acetaldehyde: ∼0.4 mmol per 3 h
H2: ∼1 mmol per 3 h
N-Benzylidenebenzylamine: ∼2 mmol per 5 h
H2: 708.8 μmol per 4 h
Benzaldehyde: 732 μmol per 4 h

QE: 2.17% at 430 nm

QE: 3.74% at 430 nm

Formate, acetate, pyruvate, and lactate
H2: ∼4.13 mmol gcatalyst−1 h−1h−1

QE: 0.14% at 430 nm−1

Eﬃciency

Formate, glycolate, ethanol, acetate, and lactate
H2: ∼0.85 mmol gcatalyst−1 h−1

H2: ∼3.42 mmol gcatalyst

Products

52

8 hours

90 hours

12 hours

102

86

57

52

8 hours

24 hours

65

66
10 hours

12 hours

61

60

45 minutes

6 hours

68
68
53

59

59

64

69

109

54

58

67

67

37

37

37

Ref.

6 hours
12 hours
6 hours

10 hours

10 hours

4 hours

3 hours

3 hours

4 hours

12 hours

12 hours

12 hours

24 hours

4 hours

4 hours

Reactiontime

Minireview

280 W Xe lamp (λ ≥ 420 nm)

Simulated sunlight (AM 1.5G,
100 mW cm−2)
300 W Xe lamp (λ > 400 nm)

8 W blue LED (λ = 450 nm)

8 W blue LED (λ = 450 nm)

14.4 W blue LED (λmax = 470 nm)

6 W LED (λ = 365 ± 5 nm)
Ethanol

300 W Xe lamp (λ = 400–780 nm)

Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M NaOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M NaOH
Simulated sunlight (AM 1.5G,
100 mW cm−2)
10 M NaOH

CdS/CdOx quantum dots (diameter∼5 nm)

PET

Reaction conditions

Photocatalysts(Contd.)Substrates

Table 1

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Nanoscaleaccompanied by hydrogen generation through the reduction of
protons by electrons (eqn (4)). The irreversible reaction of
CxHyOz with photogenerated holes results in the concomitant
suppression of electron–hole recombination and back reactions involving the produced H2, thereby increasing thequantum eﬃciency and the rate of H2 evolution.13
Nevertheless, lignocellulose refining is costly and ineﬃcient, typically necessitating acid hydrolysis, enzymatic hydrolysis, or pyrolysis to produce more manageable substrates.20
Therefore, viable H2 production systems should directly
reform lignocellulose. The main components of lignocellulosic
biomass are cellulose (35–50%), hemicellulose (25–30%), andlignin (15–30%).21,22 Cellulose,23–28 hemicellulose24,29,30 andlignin24,26,31,32 have been reported to produce H2 using various
photocatalysts such as TiO2, CdS/CdOx quantum dots, carbon
dots, carbon nitride, and CdS (Table 1). Metallic nanoparticulate cocatalysts (e.g., Pt, Au, and Pd) and non-precious cocatalysts (e.g., NiP and NiS) are frequently loaded on photosensitizers by wet impregnation and photodeposition methods, with
Pt the most commonly studied.33 As an example, CdS/CdOx
quantum dots loaded Co species as cocatalysts were reported
to perform photoreforming of cellulose, hemicellulose andlignin to H2 in alkaline solution under simulated sunlightirradiation (H2 evolution rates for cellulose, hemicellulose, andlignin were 2.57, 2.32, and 0.53 mmol gcatalyst−1 h−1,
respectively).24
Reports on using raw biomass as the substrate for photoreforming are limited. Although direct H2 production from
unprocessed biomass has the advantage of lower production
costs, it is limited by low substrate solubility. Additionally,light is scattered and absorbed by insoluble biomass andcolored components, lowing the light absorption eﬃciency.
The recalcitrance of raw biomass may lead to a large overpotential for biomass oxidation.12 TiO2 is frequently used as aphotocatalyst for raw biomass reforming, despite its large
bandgap (around 3.2 eV) and limited sunlight utilization
ability. The H2 evolution rates of TiO2 for the photoreforming

Minireview
of raw biomass, such as poplar wood chips,27 rice husk,34
paper pulp,35 chlorella algae,36 and turf,36 are in the range of
0.01–0.1 mmol gcatalyst−1 h−1, which are typically lower thanthose obtained using carbon nitride and CdS with narrower
bandgaps (carbon nitride: ∼2.7 eV; CdS: ∼2.4 eV) (Table 1).24,29
For instance, a rate of over 5 mmol H2 gcatalyst−1 h−1 wasreported through photoreforming a wooden branch on Co/
CdS/CdOx, as shown in Fig. 2. The system was stable for morethan 6 days and was capable of reforming unprocessed lignocellulose, demonstrating a cost-eﬀective approach to produce
H2 from waste biomass. The high H2 evolution rate may also
be attributed to the alkaline conditions that not only provide
an in situ pretreatment of lignocellulose by dissolving hemicellulose, lignin, cellulose, and other saccharides but also
facilitate CdOx deposition on CdS, thereby producing a photocatalyst that is active and resistant to corrosion.24
The ideal feedstock for photoreforming comes from waste
streams that cannot be recycled or reused in any other way.11
To this end, the photooxidation of food waste and nonrecycledplastics has recently become an emerging topic in thefield.11,37–44 The chemical composition of food waste varies
greatly. Due to their hydrophobicity and typically chemicallyinert hydrocarbon chain, photoreforming fats in an aqueous
solution is more diﬃcult than photoreforming proteins composed of long chains of amino acid residues (Table 1).11 Under
simulated solar irradiation, simultaneous production of H2
and the oxidation of a variety of carbohydrates, proteins andfats as well as real-world waste (apples, bread, and cheese) togenerate formate and CO2 or carbonate was observed over CdS/
CdOx quantum dots and Ni2P/CNx.45 The carbon footprints ofthese processes were estimated to be 44 600–68 800 gCO2 per
kWh H2 with a conversion eﬃciency of 1.9–22% after 3 days,
which were still high due to the need for alkaline solutions aswell as energy for stirring, and pre-treatment (accounts formore than 90% of the values).45 The carbon footprint can be
reduced to a negative value of −3200 gCO2 per kWh H2 if thewaste in the water is 100% converted to H2 and formate.

Fig. 2 (a) Diagram of the photoreforming of lignocellulose to H2 on CdS/CdOx. (b and c) Photocatalytic production rates of H2 from α-cellulose,
hemicellulose and lignin (b), and raw and waste biomass substrates (c) using Co/CdS/CdOx quantum dots in 10 M KOH aqueous solution undersimulated solar light (AM 1.5G, 100 mW cm−2). Reproduced with permission from ref. 24. Copyright 2017 Macmillan Publishers Limited, part ofSpringer Nature.

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Nanoscale

Therefore, improvements in the conversion eﬃciency andphotooxidation process selectivity are required.
In comparison with the photodegradation of plastics thatconverts plastics into CO2 and H2O, photorefoming can not
only reduce plastic pollution but also produce fuels and products with added value, including pyruvate (∼190 $ per kg),
glycolate (∼450 $ per kg), formic acid (∼1.1 $ per kg), ethanol(∼1.2 $ per kg) and acetic acid (∼1.2 $ per kg) (Table 1),9 allowing for a substantial economic advantage and a higher atom
economy. While hydrocarbon chains such as polyethylene(PE), polystyrene (PS), and polypropylene (PP) in plastics are
currently diﬃcult to reform due to their highly stable C–C
bonds, the oxygen-containing esters including polyethylene
terephthalate (PET) (eqn (5) and (6)), polylactic acid (PLA) (eqn(7) and (8)) and polyurethane (PUR) (eqn (9) and (10)) can
facilitate photoreforming after proper hydrolysis treatment.37
NaOH

ð5Þ

hv

ð6Þ

PET : C10 H8 O4 þ2H2 O ! C8 H6 O4 þC2 H6 O2
C2 H6 O2 þ2H2 O ! 2CO2 þ5H2
Photocatalyst

NaOH

PLA : C3 H4 O2 þH2 O ! C3 H6 O3hv

C3 H6 O3 þ3H2 O ! 3CO2 þ6H2
Photocatalyst

ð7Þ
ð8Þ

NaOH

PUR : C12 H14 N2 O4 þ2H2 O ! C7 H10 N2
þC3 H8 O2 þ 2CO2hv

C3 H8 O2 þ4H2 O ! 3CO2 þ8H2
Photocatalyst

ð9Þ
ð10Þ

The reforming of plastics conjugating with proton
reduction to H2 is also nearly energy neutral.40 For instance,
the Gibbs free energy changes for the reforming of ethylene
glycol (a monomer of PET) and lactic acid (a monomer of PLA)
are +9.2 kJ mol−1 and +27 kJ mol−1, respectively. A variety ofplastics including PLA, PET, PUR, and a PET water bottle wereoxidized by photogenerated holes in CdS/CdOx quantum dots
into a range of substrate-dependent organic products (e.g.,
formate, glycolate, ethanol, acetate, and pyruvate) in 10 M
NaOH aqueous solution, while the photogenerated electrons
reduced protons to produce H2 with a rate of 3–65 mmol
gcatalyst−1 h−1 (Table 1, and Fig. 3).37 A Ni2P/CNx photocatalyst
which is inexpensive and nontoxic has also shown promising
activities for visible-light-driven PET and PLA reforming to
produce H2 fuel and a variety of organic chemicals under alkaline aqueous conditions.46 The challenge in plastic photoreforming is the competition with plastic dehydrogenation to H2
and selective oxidation to a single product rather than CO2.47
The reported conversion of plastics was up to 30%–40%, which
requires further improvement. Furthermore, these studies imply
that photoreactions can cleave C–C, C–N, and C–O bonds in thealiphatic chains of plastics as eﬀectively as hydrolysis.
It is suggested that a theoretical maximum of 310–650 Mt
of H2 could be produced each year (the equivalent of ∼6–13%
of annual global energy consumption) if the above biomass,food and plastic components were all utilized for photoreforming.11 The preliminary techno-economic and life cycle assessment of photoreforming has shown that it has a lower carbon
footprint than or is comparable to existing methods for producing H2, converting waste to fuel, and managing waste, but the
improvement in production costs and energy balance is stillrequired before industrial applications can be envisaged.
Although the product of the oxidation reaction in photoreforming is typically CO2, it is preferred that valuable organic chemicals can be produced instead of CO2 to improve the sustainability and overall process value of the system.26,28,37,46,48–50 For
example, Cu dispersed on titanium oxide nanorods wasreported to be eﬀective for the conversion of polyols and sugars
into syngas and methanol under UV light irradiation andambient conditions.49 Decreased water content inhibited theformation of CO2, whereas the copper loading amount controlled the decomposition way of the formic acid intermediate.
Controlling the oxidation half-reaction is essential to produce

Fig. 3 (a) Diagram of the photoreforming of plastic waste using a CdS/CdOx quantum dot photocatalyst. (b) Photoreforming of polymers to H2
using CdS/CdOx quantum dots under simulated solar light (AM 1.5G, 100 mW cm−2). Reproduced with permission from ref. 37. Copyright 2018 The
Royal Society of Chemistry.

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Nanoscale
high-value organic products instead of CO2.21,41 In addition,
pre-treatment, including physical pre-treatment (such as crushing, shredding, and grinding) and chemical pre-treatment (suchas hydrolysis and solubilization under highly alkaline or acidic
conditions), is one of the most critical barriers to the practicalapplication of the current photocatalytic reforming systems,
adding significant cost and time to the overall process.
Recently, it was demonstrated that metal salt hydrate solutions
allowed for the complete solubilization of biomass and could
be used as a reaction medium for the photocatalytic reformingof lignocellulose to produce H2 and organic products undermore benign conditions than the typically required extremely
alkaline aqueous solutions, though the photocatalysts suﬀeredfrom partial deactivation due to metal salt hydrate adsorption,necessitating future development.50

Organic transformation
Since the pioneering work of Kolbe that reported the electrochemical oxidation of organic molecules with the goal of developing new methods for synthesizing organic small molecules,51 a larger body of research has grown around this topic
in the ensuing decades. Over the past decade, photocatalytic
processes that use light as the energy input have emerged as a
new alternative approach in the repertoire of the organic synthesis toolbox. In particular, interest has been aroused in

Minireview
waste biomass valorization in order to produce high-valuecommodity chemicals instead of CO2. Researchers have succeeded in converting biomass-derived small molecules such as
5-hydroxymethylfurfural (HMF) and furfural alcohol intohigher-value materials (e.g., aldehydes and acids) (Fig. 4a).52–54
For example, photocatalytic H2 evolution coupled with HMF(7110 $ per kg) oxidation to 2,5-diformylfuran (DFF) (198 000 $per kg)55 using porous carbon nitride was reported to reach a
DFF yield of 13.8% with >99% selectivity after 6 h undervisible light.53
Recent research has revealed a wide range of photocatalysts
for oxidizing alcohols in conjunction with H2 production.57,58,62–65
Alcohols such as methanol have been widely used as sacrificial
reagents for photocatalytic H2 generation from water due tothe lower thermodynamic requirements, and alcohols are oxidized to CO2.3–5 We will not discuss these examples because
the focus of this work is on the co-production of high-value
chemicals with H2. For instance, under visible lightirradiation, the coupling of H2 generation with the oxidationof benzyl alcohol (273 $ per kg) yielded deoxybenzoin (57 200 $per kg) over ZnIn sulfides,55,58 and 4-methylbenzyl alcohol(2110 $ per kg) was converted into an aldehyde (4-methylbenzaldehyde; 3440 $ per kg) over a cyanamide surface functionalized with melon-type carbon nitride.55,57 Multi-carbon alcohols and polyalcohols have been reported to be photocatalytically produced by selectively activating the inert sp3 α-C–H
bonds of alcohols and directly forming C–C bonds with theFig. 4 (a) Photocatalytic oxidation of biomass-derived intermediate compounds.52,54,56 HMF: 5-hydroxymethylfurfural; DFF: 2,5-diformylfuran. (b)Photocatalytic oxidation of alcohols.57,58 (c) Photocatalytic Minisci coupling.59 (d) Photocatalytic C–N coupling reactions.56,60 (e) Photocatalytic S–S
coupling reactions.56,61

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coupling partners (Fig. 4b).62 For instance, Zn2In2S5 and CdS
were reported as visible-light-responsive photocatalysts for theselective activation of the α-C–H bond of methanol to generate
ethylene glycol with a selectivity of as high as 90%.66,67 The
CdS-based photocatalytic system was beneficial for the activation of the C−H bond in methanol without aﬀecting the O–
H group, forming ethylene glycol via a •CH2OH radical intermediate with a yield of 16% and a quantum eﬃciency (QE) ofabove 5.0% at wavelengths ≤450 nm.66 The high ethylene
glycol selectivity (90%) was maintained throughout the
100-hour reaction, with a yield of 16% after 100 hours, demonstrating an alternative nonpetroleum strategy for ethylene
glycol synthesis. The CoP/Zn2In2S5 photocatalyst also was

Nanoscaledemonstrated to be the first example of the visible-light-driven
dehydrogenative coupling of ethanol to 2,3-butanediol with aselectivity of 53%.67 The α-C–H bond of ethanol was selectively
activated to form •CH(OH)CH3 radicals for subsequent coupling to 2,3-butanediol, presenting that the α-C–H bond can bepreferentially activated in the presence of the O–H group
under mild conditions.
Aside from alcohols, the C–H bonds in furanics can also be
activated for C–C coupling reactions through photocatalysis.
The production of diesel fuel precursors from 2,5-dimethylfuran (2,5-DMF) and 2-methylfuran (2-MF), with concurrent H2
production, over a Ru–ZnIn2S4 photocatalyst was demonstrated
under visible light irradiation (Fig. 5).68 Dehydrocoupling ofFig. 5 (a) Schematic representation of the photocatalytic transformation of 2,5-DMF/2-MF into diesel fuel. (b–d) Photocatalytic dehydrocoupling of
2,5-DMF as a substrate: standard experiment (b), catalyst lifetime evaluation (c), and selectivity to oxygenated DFPs and branched-chain DFPs (d).
Reproduced with permission from ref. 68. Copyright 2019 Springer Nature Limited.

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2,5-DMF and 2-MF yielded mainly H2, dimers and trimers as
products with a small number of tetramers produced in 12 h.
Photogenerated holes oxidized the furfuryl C–H bond of 2,5DMF/2-MF, delivering protons and furfuryl radicals that,through C–C coupling, form the desired DFPs. Over Ru–
ZnIn2S4, the QE for the dehydrocoupling of 2,5-DMF was
15.2% at 452 nm, which was approximately double that of
ZnIn2S4 because Ru dopants substituted for indium ions in
the ZnIn2S4 matrix facilitated charge separation eﬃciency andthus accelerated C–H activation. Diesel fuel precursors were
produced with selectivities of more than 96% in conjunction
with H2 generation and were subsequently transformed into
diesel fuels composed of straight- and branched-chain alkanes
that were comparable to petroleum diesel in terms of alkane
constituents. Future work should focus on improving theyields of C–C coupling products and the QE by the use of more
eﬃcient catalysts.
Lately, it has been demonstrated that H2 production could
be coupled with more complex organic synthesis. The Minisci
reaction is a known strategy for the direct functionalization ofC–H bonds in heteroarenes, involving the radical coupling ofelectron-deficient heteroarenes with oxidatively-generated
nucleophilic radicals.59 A visible light-mediated Minisci
coupling of N-heteroarenes with ethers, alcohols, andamides in the presence of aerobic oxygen as an oxidant wasreported to conjugate with proton reduction to H2 using a
cyanamide functionalized carbon nitride photocatalyst(Fig. 4c).59
Photocatalytic C–N coupling of amines for the eﬃcient synthesis of imines may have important applications in pharmaceutical and agricultural chemistry research (Fig. 4d).56 Ni/CdS
was reported to photocatalyze the non-oxygen coupling ofamines to yield their corresponding imines under visible lightirradiation, concurrent with H2 generation.60,69 In particular,
the photocatalyst showed a high conversion of benzylamine(99%) with 97% selectivity for imines, associated with a QE of
11.2% at 450 nm for simultaneous H2 evolution.60 By constructing in situ photoimmobilized Ni clusters on two-dimensional ultrathin CdS nanosheets, the generation of imines was
dramatically improved with a QE of ∼44% at 420 nm for theconversion of 4-methoxybenzylamine.69 It was proposed thatphotogenerated holes initiated the oxidation of amines via the
deprotonation process with the creation of C-centered α-amineradicals to form aldimine intermediates and release protons.
The protons were reduced by the photogenerated electronsand produced H2 with the assistance of a Ni cocatalyst.
Additionally, the photocatalytic system converted a wide rangeof primary and secondary amines (i.e., heterocyclic, aliphatic,
and N-heterocycles) to form their corresponding imines withhigh yields (≥84%) and selectivity (≥95%), thereby oﬀering apromising and versatile pathway for organic chemical
synthesis.
The photocatalytic synthesis of disulfides through the coupling of thiols is also of interest because disulfides function asprotecting groups in synthetic applications and as vulcanizing
agents for rubber (Fig. 4e). One of the examples is the visible-

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Minireview
light irradiation of CdSe quantum dots to result in virtuallyquantitative coupling of a variety of thiols, exemplified by
3-mercaptopropionic acid (MPA), to produce disulfides and H2
in aqueous solution.70 The conversion of MPA reached 99% in
1.5 hours and the yield of 3,3′-dithiodipropanoic acid was near
100%. Likewise, PtS/ZnIn2S4 completely transformed benzyl
mercaptan to produce dibenzyl disulfide in 6 h with a selectivity of ∼100% under optimal conditions.61 These examples
demonstrated an attractive alternative to existing proceduresfor synthesizing disulfides from thiols.
Controlling the selectivity of products is the most important
issue of organic synthesis, as well as the existence of various
potential reactive oxygen species in photocatalysis, including
superoxide radicals (•O2−), hydrogen peroxide (H2O2), hydroxyl
radicals (•OH) and singlet oxygen (1O2).71 For selective oxidation, the reactive oxygen species and adsorption–desorption
behavior must be carefully controlled.

Inorganic oxidation
Photocatalytic H2 production along with more valuable inorganic products rather than O2 via a two-electron oxidation
pathway represents an alternatively appealing way. For
instance, hydrogen peroxide (H2O2) production from water via
the two-electron oxidation process is kinetically more favorable
than the water-splitting reaction, which requires four electrons
to produce O2, despite the O2 generation (1.23 eV vs. NHE)
showing a thermodynamic advantage over the H2O2 production (1.78 eV vs. NHE). The photocatalytic reaction to simultaneously produce H2 and H2O2 from water (eqn (11)) has
attracted increasing attention in recent years because it
obtains clean H2 energy by utilizing abundant water
resources and inexhaustible sunlight,72,73 as well as being
environmentally friendly for producing the value-added H2O2(∼0.7 $ per kg),9 which was widely utilized in the chemicalindustry, and disinfection, energy, and environmental
fields.74–77 In addition, this reaction not only realizes theautomatic separation of the oxidation product with gaseousH2 but also eﬀectively avoids the reverse reaction of generated
H2 and O2 existing in the thoroughly overall water splitting
reaction.78,79
2H2 O ! H2 þ H2 O2

ð11Þ

Recent eﬀorts, including optimizing reaction conditions,
doping, heterojunctions, cocatalyst loading, and using biological hybrids, have been made to enhance the carrier utilizationeﬃciency of the photocatalyst and increase the selectivity for
H2O2 production, thereby advancing this technology. Graphitic
carbon nitride (g-C3N4), for example, as a notable photocatalyst with advantageous features of low cost and tunableoptical and electronic properties was widely employed in thisfield. A sulfur doping treatment was performed on thin g-C3N4
nanosheets to compensate for the light absorption loss causedby the exfoliation process while improving the charge transfer
eﬃciency.80 Moreover, CoxNiyP nanoclusters were loaded onto

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a P-doped g-C3N4 photocatalyst, and the doping and cocatalyst
eﬀects enhanced the photocatalytic water reduction and oxidation with a H2 evolution rate of 239.3 μmol gcatalyst−1 h−1 andstoichiometric H2O2 production.81 It is also reported that theliving Chlorella vulgaris could accelerate the photocatalytic
activity of g-C3N4 to produce H2O2 and H2 from water, giving a
QE of 0.86% at 420 nm.82 The defect control is also a proposed
way applicable to increase H2O2 production. Although defects
are typically regarded as charge recombination sites, it has
been demonstrated that if properly controlled, they can also
contribute to photocatalytic reactions.83 Defects can introduce mid-gap states in the forbidden band of semiconductor
photocatalysts to harness more visible light, serve as active
sites for reactants, and oﬀer more trapping sites to hamper
the recombination of carriers.84 For example, the introduced
carbon vacancies in C3N4 can not only extend the lightabsorption range and improve carrier separation eﬃciency
but also alter the H2O2 production pathway from a two-stepindirect reaction to a one-step direct reaction, thereby enhancing the H2O2 production.85 Construction of heterojunctions
appears to be an eﬀective strategy for inorganic semiconductor photocatalysts to generate H2 and H2O2 simultaneously. For instance, a system consisting of CdS andZnIn2S4 hollow cubes was employed for the stoichiometric
generation of H2 and H2O2 from pure water, achieving a QE
of 1.63% at 400 nm.86 Noble metal deposition remains acommon but eﬀective way to promote photocatalytic reactions. Production of H2 and H2O2 with rates of 9.8 μmol
mgcatalyst−1 h−1 and 8.2 μmol mgcatalyst−1 h−1, respectively,from pure water ( pH ∼ 7) was realized using Pt/porous brookite TiO2 nanoflutes as a photocatalyst, and the QE reached43.4% at 365 nm.87 The simulation and experimental results
revealed that a two-electron reaction is kinetically favorable
on the surface of brookite TiO2 due to the unique surficial
chemistry micro-circumstance, promoting the adsorption ofactivated hydroxyl groups and the subsequent step of hydrogen peroxide generation.
Though some promising progress has been made, photocatalytic H2O2 production is still far from the market requirement for practical applications, and the industrial productionof H2O2 still mainly relies on the anthraquinone oxidation
process (accounting for over 95% of production).88 Facing thesame problems with other semiconductor photocatalysis,
photocatalytic H2O2 production also suﬀers from the drawbacks of low light utilization ability and the high recombination of photogenerated carriers. Another tricky problem is the
rapid decomposition of the generated H2O2 occurs at thesurface of the photocatalyst/cocatalyst, which can be caused by
UV irradiation. More eﬀorts should be devoted to the surface
modification and fine control of the photocatalyst/cocatalyst
and new cocatalyst development to promote H2O2 evolution
and inhibit its decomposition process. For example, the combination of common inorganic semiconductor photocatalysts
with bacteria, biomaterials, and organic semiconductors may
have some positive eﬀects. Additionally, a deeper understanding of the photocatalytic mechanism and H2O2 evolution

6530 | Nanoscale, 2023, 15, 6521–6535

Nanoscaleprocess is significant and will contribute to the optimizationand design of highly active photocatalysts.
2HX ! H2 þ X2

ð12Þ

2Cl ! Cl2 þ 2e

E° ¼ þ1:36 V

ð13Þ

2Br ! Br2 þ 2e

E° ¼ þ1:09 V

ð14Þ

2I ! I2 þ 2e

E° ¼ þ0:54 V

ð15Þ

Photocatalytic hydrohalic acid (HX, X = Cl−, Br−, I−) splitting (eqn (12)) is another attractive approach for producing H2
with a valuable byproduct X2, which has numerous applications in the energy and hygiene industries.89–91 Halide ion(Br−, I−) oxidation (eqn (13)–(15)) is thermodynamically mucheasier than H2O oxidation, and more importantly, these photocatalytic reactions are conducted under the conditions of a
relatively high H+/X− concentration, allowing for a faster reaction rate in dynamics. In addition, the oxidation of halide ionsdoes not involve a net proton transfer, distinct from the H–O
breakage and proton loss during O2 formation in H2O splitting, and thus are kinetically favorable. As an example, chloride oxidation catalyzed by silver ions in concentrated Cl− solution has been reported, and the chloride complex-assisted catalysis mechanism has been proposed.92 The research on chloride oxidation could promote the development of photocatalytic
seawater splitting, which is still in its infancy due to its low
product yield and a plausible catalytic mechanism involving
the eﬀect of inorganic ions, yet having been studied for years.93,94
Photocatalytic HBr and HI splitting to produce H2 based on
halide perovskites has aroused a new research upsurge inrecent years since the first report about the hydrogen generation from hydriodic acid using MAPbI3 as a photocatalyst in2016 (Fig. 6a–c).95 Subsequently, material compositing andheterojunction construction were extensively employed toboost hydrohalic acid splitting. Reduced graphene oxide (rGO)
was first used to composite with MAPbI3 perovskites for thephotocatalytic hydrogen evolution reaction, resulting in ahydrogen evolution rate (93.9 μmol h−1 under visible-lightirradiation) 67 times higher than that of pure MAPbI3.96
Afterwards, various materials such as TiO2, Ni3C, carbon dots,
black phosphorus, MoS2, and CoP were used as cocatalysts orheterojunction
counterparts
topromotecarrier
separation.97–103 In particular, a MoS2/MAPbI3 composite
exhibited a remarkable photocatalytic hydrogen evolution
activity of 29.4 μmol gcatalyst−1 h−1 under visible light and an
H2 production of 103 mL day−1 under outdoor natural sunlightirradiation.103 In addition to the heterostructures, composition
regulation is also a feasible way. For instance, a graded andgradient halide ion distribution was realized in both organometal and all-inorganic mixed halide perovskites,104,105 and a
resulting bandgap funneling was constructed for eﬃcientcarrier transfer, causing an enhanced photocatalytic activity
and a solar-to-H2 conversion eﬃciency (STH) of 1.05% for
MAPbBr3−xIx/Pt.104 In addition, single-atom decoration is also
an eﬀective strategy to improve the perovskite photocatalytic
HI splitting eﬃciency,106,107 an excellent photocatalytic H2

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Fig. 6 (a) Schematic band diagram of MAPbI3 powder for the HI splitting photocatalytic reaction. (b) Quantitative comparison between the evolved
H2 and I3−. (c) Stable photocatalytic H2 evolution produced by MAPbI3 powder in the saturated solution for 160 h (H3PO2 was added to the HI solution). Reproduced with permission from ref. 95. Copyright 2016 Springer Nature Limited. (d) Mechanism illustration of the synthesis process of Pt/
FAPbBr3−xIx. (e) Photocatalytic H2 evolution activities of ω-FPBI (FAPbBr3−xIx loaded ω% Pt in mass percentage). Reproduced with permission fromref. 107. Copyright 2022 The Royal Society of Chemistry.

evolution rate of 682.6 mmol h−1 under simulated sunlightirradiation (area of π cm) and a corresponding STH of 4.5%
were recorded using Pt/FAPbBr3−xIx (Fig. 6d and e).107
Halide perovskites possess several excellent material properties, such as a wide light absorption range, a high absorption coeﬃcient, and a long carrier diﬀusion length, allowing
them to be ideal photocatalysts. However, satisfactory performance of this kind of material for photocatalytic H2 production
has not been achieved. More eﬀorts should be devoted to theinvestigation of the mechanism and fundamental principles ofthe physical/chemical process during photocatalysis for thisspecial system, where exists a dynamic equilibrium of dissolution-crystallization between the catalyst and the corresponding saturated solution.

Conclusions and outlook
Although the research in this field is at too early a stage to be
towards industry-level operation and deployment, exciting
work in combining H2 fuel generation with value-added chemical production or waste valorization is already showing to have awide impact on large-scale challenges in global energy(Table 1). When looking at both inorganic and organic chemistries, the water-to-O2 conversion can be replaced by a variety ofphotoreactions that can occur in an aqueous solution.
In any proton-to-H2 generation, the availability of substrates
at the scale required for the process is a key consideration.
Biomass, food, and plastic wastes are abundant, inexpensive,and carbon- and hydrogen-rich feedstocks. Photocatalytic
methods oﬀer upcycling routes that yield higher-value products, as opposed to conventional mechanical strategies thattypically downcycle to lower-quality products.11,40 In recent

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years, much eﬀort has been devoted to the study of the simultaneous production of H2 and chemicals with added value
through reduction and oxidation, respectively. A preliminary
techno-economic assessment demonstrates that photoreforming of waste can become economically competitive with existing technologies if the eﬃciency and lifetime can be substantially improved.11 For large-scale applications to be economically and environmentally viable, H2 evolution rates must
increase by a factor of at least 50 (rates of >0.004 mol H2gsubstrate−1 h−1, conversions of >50% per day, and QEs of>50%). It will be imperative to develop more eﬀective anddurable photocatalytic systems. In addition, the current pretreatments commonly reported for photoreforming to improve
the solubilization of the feedstock are unlikely to be economically viable on a large scale.
The photocatalytic oxidation of organic compounds, including alcohols, hydrocarbons, aromatics, and amines, diﬀers
from conventional methods conducted at high temperaturesand pressures in that it is environmentally friendly, operates
under mild conditions, and is sustainable. However, selective
photocatalytic oxidations present huge challenges. Ideally, theoxidation process would selectively produce a single highvalue product. Controlling reactive oxidation species andadsorption–desorption behavior is the primary challenge inselective oxidation. Therefore, it may be necessary to observe
the reaction mechanism in situ for a better understanding of
the reaction kinetics.71 To yield a single higher-value productin both organic transformation and waste valorization processes, selective oxidation cocatalysts may be required.
For inorganic reactions, the H2O-to-H2O2 conversion andhalide oxidation present themselves as an attractive alternative
to O2 evolution due to the higher value of the products and thesimpler mechanism resulting from the lower electron demand.

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Table 2

Summary of the developed alternative photooxidation reactions to the OER

Oxidation
reactionOpen Access Article. Published on 07 March 2023. Downloaded on 9/23/2024 4:24:10 PM.
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Nanoscale

Reforming

Organics

Inorganics

Substrates

Main products

Challenges

Lignocellulose biomass-derived feedstocks (pentoses,
hexoses, alcohols, and organic acids)

CO2

Cellulose, hemicellulose, and lignin

CO2

Raw biomass

CO2

Waste streams (biomass, food, and plastic)

CO2, formate• Costly and ineﬃcient lignocellulose refining
processes required• Low-value products• Low substrate solubility• Low-value products• Low substrate solubility• Costly and environmentally unfriendly pretreatment processes required• Low-value products• Low substrate solubility• Costly and environmentally unfriendly pretreatment processes required• Competition with water reduction and plastic
dehydrogenation to H2• Low selectivity• Low selectivity

Alcohols
FuransAminesThiolsWater
HX (X = Cl−, Br−, I−)

Nonetheless, the reaction eﬃciency of these reactions must be
enhanced.
The H2 production cost for a waste photoreforming pilot
plant (400 m2, 200 kgwaste per day, 14.4 kgH2 per day) was estimated to be £11.80 per kg, which is more than H2 sale targetsof £3–5 per kg and predictions for H2 produced by photocatalytic water splitting (£1–3 per kg).1,11 It can, however, be
reduced by increasing the plant scale and photoreforming
eﬃciency. If the plant scale is enlarged 30 times, the cost of
producing H2 can be decreased to £3 per kg. Furthermore,
economic gains can be realized by producing high-value compounds such as pharmaceutical components at the same time.
As an example, oxidizing biomass to 3-hydroxybutyrolactone(3-HBL), a precursor for chiral drugs, could cut H2 production
costs to £3 per kg even if the photoreforming conversion
eﬃciency is as low as 0.2% per day. However, although 3-HBL
and its hydrolyzed form 3,4-dihydroxybutyric acid (3,4-DHBA)
have been reported to be produced from biomass (glucose, glycolic acid, and xylose) via biological synthesis using bacteria(Escherichia coli), enzymes and yeast, its production via a
photocatalytic process has not yet been realized.110–112 Future
eﬀorts would combine photocatalysts with microbes to create
abiotic–biotic hybrids capable of producing photocatalytic H2
while oxidizing biomass to 3-HBL. Likewise, the H2 production
costs can be reduced by photocatalytic transformations to
produce value-added organic and inorganic chemicals. For
instance, a recent techno-economic analysis suggested that byoxidizing water to produce H2O2 instead of O2 using the photoelectrochemical technique, a levelized cost of hydrogen of$6.19 per kg can be obtained with an STH of 10.1% and an
H2O2 price of $0.85 per kg, which is a significant improvement
over the values found in an analogy of the photoelectrochemical water splitting system.113 Aside from producing costeﬀective H2, the procedures would also reduce the cost of6532 | Nanoscale, 2023, 15, 6521–6535

Dimers andtrimers
Imine
Disulfides
H2O2
X2

organic and inorganic products. A minimal selling price of glucaric acid was predicted to be $6.94 per kg under a base casescenario employing a potential photo-biorefinery approach forsunlight-driven electro-oxidation of glucose, which was 70%
lower than the current market price.114
In spite of the obstacles listed in Table 2, significant progress has been made in the study of coupling H2 production
with diverse oxidation chemistry and will continue. With continued material and technological advancements, thesetechnologies have the potential to produce fuels and chemicals
with added value, while also reducing waste.

Conﬂicts of interest
There are no conflicts to declare.

Acknowledgements
This work was supported by the JSPS Leading Initiative for
Excellent Young Researchers Program (to Q. W.), the JST Fusion
Oriented REsearch for Disruptive Science and Technology
Program (no. 21470281, to Q. W.), and the JSPS Grant-in-Aid forYoung Scientists (Start-up) (no. 21K20485; to Q. W.).

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Nanoscale
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