Semiconducting quantum dots for artificial

REviEwS Semiconducting quantum dots for artificial photosynthesis Xu-​Bing Li   1, Chen-​Ho Tung1,2* and Li-​Zhu Wu1*

Abstract | Sunlight is our most abundant, clean and inexhaustible energy source. However, its diffuse and intermittent nature makes it difficult to use directly , suggesting that we should instead store this energy. One of the most attractive avenues for this involves using solar energy to split H2O and afford H2 through artificial photosynthesis, the practical realization of which requires low-​cost, robust photocatalysts. Colloidal quantum dots (QDs) of IIB–VIA semiconductors appear to be an ideal material from which to construct highly efficient photocatalysts for H2 photogeneration. In this Review , we highlight recent developments in QD-​based artificial photosynthetic systems for H2 evolution using sacrificial reagents. These case studies allow us to introduce strategies — including size optimization, structural modification and surface design — to increase the H2 evolution activities of QD-​based artificial photosystems. Finally , we describe photocatalytic biomass reforming and unassisted photoelectrochemical H2O splitting — two new pathways that could make QD-​based solar-​to-fuel conversion practically viable and cost-​effective in the near future.

1 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China. 2 School of Chemistry and Chemical Engineering, Shandong University, Jinan, China.

*e-​mail: [email protected]; [email protected] https://doi.org/10.1038/ s41570-018-0024-8

Solar irradiation is by far our largest renewable power source. The total energy received at the Earth’s surface every year from sunlight is estimated to be 1.2 × 105 TW, about 6,000 times the present human power consumption1. Solar energy can be directly utilized, but because it is both diffuse and intermittent, we need an efficient and cost-​effective approach to storing it. Doing so in a sustainable fashion with minimal adverse impact on the planet has been a longstanding scientific challenge2–4. One useful approach to storing solar energy is to use it in the production of chemical fuels, of which molecular hydrogen (H2) is an ideal example in view of its high specific enthalpy of combustion and benign combustion product (H2O)5,6. Indeed, the solar-​driven production of H2 as an energy vector appears to be a clean, direct and cost-​effective means to storing solar energy7–11. Technologies for solar-​driven H2O splitting come in two forms: photovoltaic-​driven electrolysis (PV–electrolysis) and fully integrated artificial photosynthesis12,13. We focus on the latter approach because the integration of light-​harvesting and H2O-​splitting components enables direct solar-​to-fuel conversion. Despite this unique advantage, the approach has yet to be fully developed, and we have much room for improvement. For example, a notable recent H2O-​splitting system uses a Ru-​ modified material featuring a La,Rh-​doped SrTiO3 photocathode and a Mo-​doped BiVO4 photoanode. However, the system affords a solar-​to-hydrogen (STH) energy conversion efficiency of only 1.1% at pH 6.8 (ref.14) — a result that falls well short of the 5% target set

by the US Department of Energy in order to make solar H2 generation viable. The poor performance of even our best devices means that the exploration of artificial photosystems with higher STH conversion efficiencies and stabilities remains an important pursuit in the field of renewable fuels. The fundamentals of artificial photosynthetic H2O splitting using a single semiconducting material in solution are outlined in Box 1. With the aid of solar power, the photocatalyst splits 2H2O into 2H2 and O2 simultaneously, thereby storing solar energy in the form of a chemical fuel. The thermodynamics of this reaction mean that the electronic bandgap (Eg) of the semiconductor must be at least 1.23 eV. The slow kinetics associated with the H+ reduction and H2O oxidation reactions mean that substantial overpotentials are observed. Therefore, a bandgap larger than ~1.7 eV at room temperature is necessary in practice15. Accordingly, the past decades have seen tremendous efforts to find appropriate semiconductors, which may then be decorated with cocatalysts for the half-​reactions16–20. The H2 evolution reaction (HER) occurs upon coupling 2e− with 2H+. The O2 evolution reaction (OER) requires the removal of 4e− and 4H+ from 2H2O, a process that typically proceeds at acceptable rates only at high overpotentials because it involves the cleavage of four O–H bonds and the formation of an O=O double bond21,22. Preparing a system that can facilitate both half-​reactions simultaneously is challenging, such that one often studies the HER or the OER in isolation by making use of sacrificial reductants

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Reviews or oxidants, respectively. After understanding and optimizing each half-​reaction, one would then couple them together and split H2O without using redox reagents. In this case, the reaction is driven solely by light (photocatalytic), possibly also with an external electric potential (photoelectrochemical, PEC). Conventional artificial photosystems usually feature organic dyes (for example, eosin Y23, QuPh+-NA24 or Box 1 | Fundamentals of photocatalytic h2O splitting in solution a prototypical photocatalytic system for H2O splitting features a single semiconductor with a bandgap (Eg) greater than 1.23 ev, a value corresponding to the sum of the equilibrium redox potentials for 4 H+ + 4e− ⇌ 2H2 (0 v versus normal hydrogen electrode, NHe) and 2H2O ⇌ 4H+ + 4e− + O2 (1.23 v versus NHe). the reaction can occur at the semiconductor surface itself or can instead be mediated by dedicated hydrogen evolution reaction (Her) and oxygen evolution reaction (Oer) cocatalysts on the semiconductor. even using state-​of-the-​art cocatalysts, reasonable rates often require substantial overpotentials, such that practical devices require Eg > 1.7 ev. Light absorption triggers charge separation in the semiconductor, with the photogenerated electrons travelling to the Her cocatalyst and the holes (h+) migrating to the Oer cocatalyst. However, fast charge recombination and/or sluggish kinetics of both redox reactions (especially H2O oxidation) can make this type of complete artificial photosynthetic system very inefficient, either in terms of quantum yield (QY) or solar-​to-hydrogen (stH) efficiency, as defined below. the QY of a system is the number of moles of electrons consumed in the reactions per mole of photons absorbed by the photocatalyst at a specified wavelength, expressed as a percentage. QY =

n electronsconsumed × 100% n photonsconsumed

the stH efficiency is the chemical power derivable from the H2 produced divided by the solar power from sunlight incident on the process, expressed as a percentage and quoted using a specific light source (a popular one being air mass coefficient 1.5 G (aM1.5 G)).  237nH2   × 100% STH =   PA  AM1.5G

where nH2 is the rate of evolution of H2 in mmol s−1, 237 J mmol−1 is the specific energy content of H2, P is the power of the light source per area in mw cm−2 and A is the illuminated area in cm2. very few artificial photosystems based on a single semiconductor can simultaneously split H2O into H2 and ½O2, and those that do operate at efficiencies too low for practical applications. therefore, many fundamental studies aim to optimize the individual half-​ reactions using sacrificial redox agents, after which one would hope to prepare a material that can mediate overall H2O splitting. Potential (V vs NHE) Overall: 2H2O

hν

2H2

2H2O + O2

–1.0

e– H /H2

0

Eg > 1.7 eV +1.0

4H+ HER cocatalyst

+

O2/H2O O2 + 4H+

2H2O

h+ Semiconductor QD OER cocatalyst

h, Planck constant; v, photon frequency; QD, quantum dot.

hν

fluorescein25), metal complexes (for example, complexes of Ru (ref.26), Ir (ref.27) or Rh (ref.28)) or bulk semiconductors (for example, TiO2 (ref.29) or g-​C3N4 (ref.30)) as the light absorbers. None of the absorbers are ideal, as the molecular species have limited photostability and the wide-​bandgap materials absorb in only a very narrow region of the solar spectrum. Approximately 40% of sunlight is visible light (Fig. 1a), and an ideal absorber would harvest as much visible light (λ > aB

0

500

600

HOMO

Valence band

700

Wavelength (nm)

Fig. 1 | intrinsic advantages of semiconducting QDs in artificial photosynthesis. a | The spectrum of solar irradiation at the Earth’s surface. b | When moving from bulk semiconductors to quantum dots (QDs) to single molecules (the organic dye eosin Y is pictured here), one moves from an electronic band structure (with bandgap Eg) to an orbital structure. The diameter (D) of a QD is smaller than the Bohr exciton diameter (aB), while the size of a bulk semiconductor is much larger than aB. The larger species have smaller bandgaps and enhanced efficiency of multiexciton generation (ref.31). c | UV–visible absorption spectra of CdS, CdSe and CdTe QDs of different sizes (prepared using different growth times)114. The energy diagram in part b is adapted with permission from ref.31, American Chemical Society. Part c is adapted with permission from ref.114, American Chemical Society.

diffuse to the surface, where the electron is processed by the cathodic HER cocatalyst and the hole is processed by the anodic cocatalyst that operates on the (sacrificial) electron donor. We can summarize these processes by writing

QD + hv → QD ∗ (1Se, 1S h) QD ∗ (1Se, 1S h) + [Cat .] → QDs+ (1S h) + [Cat .]− where [Cat.] denotes the H2 evolution cocatalyst. The overall efficiency of the STH conversion depends on the efficiencies of light absorption (ηAbs.), charge separation (ηCS) and catalysis (ηCat.). Thus, although QDs have excellent light-​harvesting ability and are amenable to exciton generation, in the case of Cd chalcogenides, the facile electron–hole recombination, sluggish charge migration (especially for photogenerated holes) and relatively slow HER kinetics greatly limit their photocatalytic H2 evolution performance39. As we described above, the slow HER kinetics at the semiconductor surface can be partly overcome by decorating it with cocatalysts such as Pt (ref.48), MoS2 (ref.41) and metal complexes42 — the presence of which

can also boost photocatalytic H2 evolution. Of the many cocatalysts that have been developed and incorporated into QD-​based photocatalytic H2 evolution systems, those derived from non-​noble metals are highly desired because these metals are of low cost and high abundance. Fe complexes, especially the H2-processing diiron hydrogenase ([FeFe]-H2ase) enzymes and their active site mimics, have been widely investigated43–45. For example, [FeFe]-H2ase46 binds to the surface of CdTe QDs functionalized with 3-mercaptopropionic acid (MPA), allowing for systematic study of the charge-​ transfer kinetics and catalytic properties of the hybrids. Once optimized, the photocatalyst processes monochromatic light (532 nm) and ascorbic acid (H2Asc) with a photon-​to-H2 conversion efficiency of 9%. A more in-​ depth study has been carried out using synthetic mimics of [FeFe]-H2ase on CdTe QDs47. For this system, steady-​state and time-​resolved spectroscopies revealed that photoelectrons in the conduction band of CdTe QDs are readily transferred to the H2O-​soluble [FeFe]H2ase, where they combine with protons to afford a TON above 500 (Table 1) — a record value at a time when most studies used organic dyes instead of QDs as the chromophores. This pioneering work showed that semiconducting QDs can be superior materials from

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Reviews b

HO2C 4

CO2H S

S

hν

Ni2+/ dihydrolipoic acid

4

S

S

e–

1Se

–0.90 V h+

H2Asc Asc + 2H+

H2

2H+ Ni2+/ dihydrolipoic acid

Eg CdSe QD

0.14 V 1Sh

Potential (V vs NHE)

a

e–

–1.2

e– –0.9 V

0 +1.1

h+

S

S S

S

4

CO2H HO2C

d

0.0

0.003

A (normalized)

ΔA (normalized)

2.0 –0.4

–0.8

–1.2

0.000 1.5

–0.003

1.0

0

2

4

10

100

1,000

0.5

8,300

8,400

–0.006

Energy (eV)

Delay time (ps) QDs experimental QDs fit QDs + Ni(OH)2 experimental QDs + Ni(OH)2 fit

8,350

Laser on–off difference (a.u.)

c

CdSe QD

4

Laser on minus laser off at 120 ps delay Fit Laser off at 120 ps delay Fit

Fig. 2 | Photocatalytic h2 evolution using semiconducting QDs. a | Solar H2 evolution using CdSe quantum dots (QDs) as light absorbers, Ni(dihydrolipoate)x as a cocatalyst and ascorbic acid (H2Asc) as a sacrificial reductant40. b | Potentials (versus normal hydrogen electrode, NHE) of the 1Se and 1Sh states of CdSe QDs and a schematic showing electron transfer from the conduction band to the cocatalyst49. c | Femtosecond recovery kinetics of CdSe/CdS QDs bleaching with and without Ni(OH)2 cocatalyst at 460 nm. In the latter case, recovery is faster because the excitons are consumed by the redox cocatalysts. d | X-​ray absorption near-​edge structure of the ground state (blue trace) at the Ni K-​edge of CdSe/CdS QDs/ Ni(OH)2 photocatalysts. The difference spectrum of data for the excited state minus data for the ground state (black trace, red data points) at a 120 ps delay. A, absorbance; Eg, bandgap; h, Planck constant; v, photon frequency. Part a is adapted with permission from ref.40, AAAS. Part b is adapted with permission from ref.49, American Chemical Society. Parts c and d are adapted with permission from ref.51, RSC.

which to construct artificial photosynthetic H2 evolution catalysts. A breakthrough in using QDs for robust and efficient solar H2 evolution came with the discovery that CdSe QDs capped with lipoic acid readily bind Ni2+/ dihydrolipoic acid to afford an effective light absorber and Ni HER cocatalyst hybrid40. This H2O-​soluble catalyst forms in situ and operates at pH 4.5 using H2Asc as the reductant (Fig. 2a). Under visible-​light irradiation (520 nm), this system evolves H2 for at least 360 h (TON 600,000; QY = 36%). The Gibbs free energy change (∆G) of interfacial electron transfer from CdSe to Ni suggests that it is thermodynamically feasible49 and can happen over an extremely short timescale of ~69 ± 2 ps (Fig. 2b). Indeed, ~90% of the transient absorption signal can be

assigned to light being used for ultrafast interfacial electron transfer. The discovery of ultrafast electron transfer in this system revealed the reason for its exceptional photocatalytic H2 activity and provides guidance for designing advanced artificial photosystems. One can make use of the negatively charged surface of MPA-​functionalized CdTe QDs to bind metal ions that serve as HER cocatalysts. A simple approach involves combining the QDs, CoCl2·6H2O and H2Asc in aqueous solution to afford a robust and active hybrid H2 photoevolution catalyst decorated with Co-​ carboxylato sites50. The hybrid can continuously evolve H2 from H2Asc over 70 h to give a TON of 219,100 or 59,600 on a QD or Co basis. This methodology has been adapted to other earth-​abundant metal centres, www.nature.com/natrevchem

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Reviews Table 1 | h2 evolution photosystems based on semiconducting QDs in aqueous solution QD light absorber

Cocatalyst

Light source

electron donor

tON

her Qy

ref

CdSe

Ni(dihydrolipoate)x

520 nm LEDs

H2Asc

600,000

36%

40

CdTe

Co2+

500 W Hg lamp

H2Asc

219,100

ND

50

CdSe

Ni2+

500 W Hg lamp

i

PrOH

15,340

11.20%

53

CdS

Ni2+

500 W Hg lamp

Glycerol

38,400

12.20%

90

CdSe

[Co(1,2-benzenedithiolato)2]−

520 nm LEDs

H2Asc

300,000

24%

80

CdSe/CdS

Pt colloids

450 nm LEDs

NEt3

164,000

65%

87

CdSe/ZnS

Cobaloxime

100 W Xe lamp

N(CH2CH2OH)3 10,000

ND

68

CdTe

{Fe2[S2(CH2)3](CO)5L}

500 W Hg lamp

H2Asc

505

ND

47

CdS/CdOx

Surface Cd atoms

AM1.5G

Cellulose

80,000

1.20%

91

CdS

CoCl2

AM1.5G

Na2SO3

29,000

7.70%

85

CdSe/CdS

Ni(dihydrolipoate)x

520 nm LEDs

H2Asc

3,390

5.80%

115

CdSe/CdS

Ni(OH)2 clusters

450 nm LEDs

i

PrOH

ND

52%

51

CdTe

[FeFe]-H2ase

532 nm laser

H2Asc

ND

9%

46

CdTe

[Co(N4-macrocycle)Cl2]+

150 W Xe lamp

H2Asc

650

10%

116

CdTe

[Co(N,N′-bis(2-sulfidoethyl)-1,4diazacycloheptane)NO]

300 W Xe lamp

H2Asc

14,400

5.30%

117

CdSe

[Fe2S2(CO)6]

500 W Hg lamp

H2Asc

8,781

ND

118

CdSe

{Fe2[S2(CH2)3](CO)5}nL′

450 nm LEDs

H2Asc

27 ,135

ND

119

CdTe

{Fe2[(SCH2)2N](CO)6}

410 nm LEDs

H2Asc

52,800

ND

120

AM1.5G, air mass coefficient 1.5G (a light source that simulates solar light that has passed through 1.5 atmospheric thicknesses of air); H2Asc, ascorbic acid; HER , hydrogen evolution reaction; LEDs, light-​emitting diodes: L , H2O-​soluble isonitrile ligand; L′, isonitrile-​functionalized poly(acrylic acid); LED, light-​emitting diode; ND, not determined; QD, quantum dot; QY, quantum yield; TON, turnover number.

including Ni(ii), Cu(ii), Fe(ii) and Mn(ii), to construct highly effective photocatalysts for H2 evolution51,52. Taking Ni(ii) as an example, a combination of X-​ray absorption near-​edge spectroscopy (XANES) and X-​ ray photoelectron spectroscopy (XPS) measurements revealed that Ni(ii), under basic conditions, absorbs on the surface of QDs in the form of Ni(OH)2 nanoclusters53. Furthermore, femtosecond transient visi­ ble absorption spectroscopy (Fig. 2c) and X-​ray transient absorption (XTA) spectroscopy (Fig. 2d) revealed in detail the kinetics of photoinduced electron transfer from QDs to the Ni-​based H2 evolution cocatalysts. The Ni K-​e dge in the absorption spectrum of the excited-​state material is redshifted relative to that of the ground state, suggesting that the excited state has a more electron-​rich Ni centre — direct evidence that photoelectrons are delivered to the Ni species. Compared with other photocatalytic systems, QD-​ based photocatalysts are easier to prepare, have better H2O solubility and/or dispersibility and greater activity and stability. Moreover, QD-​based photocatalysts comprising earth-​abundant elements can be more sustainable than traditional HER catalysts derived from precious metals. Some state-​of-the-​art QD-​based systems are listed in Table 1, along with two of their important figures of merit — TON and QY. We must improve these figures for artificial photosynthesis to be practically viable for STH conversion. Central to this is a better understanding of the physical and chemical properties of QDs. The following discussion is concerned with

how the intrinsic properties of semiconducting QDs — including size, structure and surface — might influence photocatalytic H2 evolution.

Influence of QD properties on H2 evolution Size optimization. A semiconductor particle smaller than its Bohr exciton diameter (aB) can exhibit quantum confinement effects, which distinguish QDs from their bulk counterparts and molecular dyes54–56. In this case, the charge carriers become spatially confined, such that the energies of photogenerated electrons and holes are increased31,57. Accordingly, semiconducting materials such as the metal chalcogenides CdS, CdSe, CdTe, ZnS, ZnSe and ZnTe exhibit size-​dependent band positions. For instance, bulk CdSe has a moderate bandgap (Eg = 1.74 eV) (Fig. 3a) and a Bohr exciton radius of 4.8 nm (ref.58), while 2.0 nm CdSe particles have a notably large bandgap (2.88 eV). The conduction-​band energy (ECB) is increased (electrons in the conduction band become more reducing), and the valence-​band energy (EVB) is decreased. The greater effective mass of a hole relative to an electron (mh* ≫ me*) makes ECB more sensitive to confinement than EVB (refs59,60). Smaller QDs are thus expected to more easily engage in interfacial electron transfer with H2 evolution cocatalysts than are larger QDs. Indeed, according to Marcus theory61, quantum-​ confined systems can exhibit high rates of interfacial charge transfer owing to the increased energies of confined electron–hole pairs, which is of benefit to applications in solar-​to-fuel conversions, such as artificial

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Reviews photosynthetic H2 evolution. Interfacial charge transfer proceeds under thermodynamic control, such that the ∆G associated with electron transfer from QDs to cocatalysts can greatly influence the final STH conversion efficiency. The rate constant k for non-​adiabatic electron transfer between QDs and cocatalysts can be computed using a

–2.5

Potential (V vs NHE)

–2.0 –1.5 –1.0

Conduction band

–0.5 0.0 0.5

Valence band

1.0 1.5

2.0

2.3

2.4

2.8

3.0

4.2

bulk

Size (nm)

b

H2 hν

Ec

e–

Cd0

2H+

Eg CdSe SO4 + 2H 2–

+

h+

SO32– + H2O SO32– + H2O

Normalized absorptioncorrected H2 evolution

c

Ev

hν SO42– + H2 CdSe

1.0 0.8 0.6 0.4 0.2 0.0 2.0

2.5

3.0

3.5

4.0

4.5

Size (nm)

Fig. 3 | Quantum confinement effect controlled photocatalytic h2 evolution. a | The potentials of the conduction and valence bands of CdSe change as the size of the particles vary from nanoscale to bulk. The upper circles illustrate the corresponding size variation58. b | Photocatalytic H2 evolution using CdSe quantum dots (QDs) as light absorbers, with surface Cd atoms serving as the active sites for H+ reduction. SO32− is oxidized at the surface of the QDs. c | The relative photocatalytic H2 evolution rates normalized with regard to photocatalyst amount and absorbed photons. Ec, conduction-​band energy ; Eg, electronic bandgap; Ev, valence-​band energy ; h, Planck constant; v, frequency of the photon; NHE, normal hydrogen electrode. Data in parts a and c from ref.58. Part b is adapted with permission from ref.58, American Chemical Society.

k=

2π h

 (λ + ΔG )2   exp −  4πλkBT  4πλkBT   H

2

(1)

where ΔG is the driving force derived from the difference between the oxidation potential of the exciton in a QD and the reduction potential of the cocatalyst, H is the strength of the electronic coupling between the QD and the cocatalyst, h is the Planck constant, kB is the Boltzmann constant, T is the absolute temperature and λ is the total reorganization energy of electron transfer. Therefore, because the size of a QD affects the band energies and thus ∆G, it also has a direct influence on k. In this way, altering the particle size is a means to tune the performance of QDs for photocatalytic H2 evolution, when the HER cocatalyst active site is either a surface metal centre (for example, Cd2+) or an external material. The way in which quantum confinement affects photocatalytic H2 evolution is best explained by considering the simple case of CdSe QDs in the absence of external cocatalysts39. CdSe QDs with diameters of 1.8–6.0 nm can be passivated with 2-mercaptoethanol and subjected to photocatalytic H2 evolution using 0.1 M aqueous Na2SO3 as the reductant. Under visible-​light irradiation, electrons in excited QDs are transferred to surface Cd2+ ions to afford reduced, Brønsted basic sites that bind H+ and evolve H2 (Fig. 3b). The photogenerated holes move to the surface and are quenched by SO32−. While bulk CdSe and 6.0 nm QDs are not competent photocatalysts, the smaller particles are, with the normalized rate of H2 evolution increasing when smaller QDs are used (Fig. 3c). We reiterate: a larger driving force for interfacial electron transfer to cocatalysts results in faster H2 photogeneration. When using a synthetic [FeFe]-H2ase mimic as the HER co-​catalyst, CdSe QDs are superior chromophores to [Ru(2,2′-bipyridine)3]2+ in evolving H2 from H2Asc62. Spectroscopic and electrochemical analyses revealed that the driving force for electron transfer from the QDs to the [FeFe]-H2ase mimic is greater than that from [Ru(2,2′-bipyridine)3]2+. From the above examples, it is clear that optimizing the QD size distribution is an effective and facile pathway to increasing the rate of solar H2 evolution from QD-​based photosystems. The electronic coupling (H) between QDs and cocatalysts can also influence the efficiency of interfacial photoinduced electron transfer and, in turn, the photocatalytic activity. When fabricating a system for H2 photoevolution, one must carefully consider the interactions between semiconducting QDs and cocatalysts, although these aspects will not be discussed in detail here. Homostructure or heterostructure? Although quantum confinement greatly increases the energy of electrons and holes, it is also important to consider the location at which an exciton is generated and its propensity to undergo annihilation (possibly at a trap state). Also important is the efficiency with which high-​energy charges tunnel to their respective catalytic active sites to participate in the redox reactions. Hence, we can conclude that exciton dynamics have a key role in determining the STH conversion efficiency of H2 evolution, and control over these dynamics appears to be an effective pathway to further www.nature.com/natrevchem

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Reviews Core CB

c

Ligands

VB

r2ψ2

Rx

Shell CdSe/ZnS type I

Electron Hole CB

CdTe/CdSe type II

r2ψ2

Energy (eV)

VB

Core–shell quantum dots

0

10

20

30

40

Rate of H2 photogeneration (ml h–1)

b

Energy (eV)

a

5

4

3

2

1

0

0.00

0.55

Distance from centre (Å)

1.10

1.65

2.20

2.75

3.30

Shell thickness (monolayers)

Fig. 4 | heterostructured semiconducting QDs for photocatalytic h2 evolution. a | A schematic illustration of a core/shell quantum dot (QD) and its effective exciton radius Rx. b | Radial distribution function of lowest energy (1S) conduction-​band (CB) electrons (red traces) and valence-​band (VB) holes (blue lines) in CdSe/ZnS type I (upper panel) and CdTe/CdSe type II (lower panel) QDs. Both structures have a 1.4 nm-​thick core and a five-​monolayer-thick shell. Vertical dashed lines indicate the core/shell and shell–ligand interfaces, respectively. c | Rates of H2 photogeneration from CdS/CdSe core/shell QDs with varying shell thicknesses. Visible light (λ > 420 nm) was used, along with ascorbic acid (H2Asc) as the sacrificial reductant and Ni(3-mercaptopropionate)2 as the cocatalyst. r, distance from centre; Ψ, electric potential. Parts a, b and c are adapted with permission from refs66,69,72, respectively , American Chemical Society.

improve photocatalytic performance. To achieve this goal, we need to inhibit competing channels of exciton annihilation, such as charge trapping and photocorrosion. To establish an ideal artificial photosynthetic system, it is highly desirable to efficiently generate and preserve high-​energy charges (e− and h+) with suitable mobility to access the solution to initiate redox reactions — in this case, the reduction of H+ and oxidation of H2O or a sacrificial reductant. One way to enhance exciton delocalization, decrease electron–hole wavefunction overlap, passivate trap states and/or suppress photo-​oxidation is to replace the homostructured QDs we have described here with the corresponding heterostructures, such as core–shell QDs63–66 (Fig. 4a). One can use type I heterostructures, in which the core has a narrower bandgap than the shell67 (Fig. 4b, upper panel). For example, when moving from CdSe QDs to CdSe/ZnS core–shell QDs, the introduction of the ZnS shell leads to the slight dilatation of both the e− and h+ wavefunctions, which leak into the surrounding shell, albeit only weakly. Another advantage of this type I structure is that the surface is passivated because undercoordinated Cd sites are not accessible and ZnS is resistant to photo-​oxidation. On the basis of these effects, type I core/shell structures should be useful for solar H2 evolution in terms of both activity and stability. Indeed, CdSe/ZnS core/shell QDs outperform CdSe QDs for solar H2 evolution in the presence of a cobaloxime HER cocatalyst68. However, if the ZnS shell is too thick, it introduces a large tunnelling barrier that slows both e− and h+ transfer to the corresponding reduction and oxidation active sites67. Thus, enhancing the performance of QDs in solar H2 evolution requires the shell thickness to be optimized. One possible way to enable effective charge separation and interfacial charge migration simultaneously is

to construct a core/shell heterostructure with a staggered type II band alignment69 (Fig. 4b, lower panel), in which the lowest energy conduction-​band electron and valence-​band hole wavefunctions can be preferentially localized in the shell and core materials, respectively. The decreased electron–hole overlap extends the lifetime of the excitons while decreasing the exciton spin relaxation rate and Auger recombination70, desirable effects for light-​harvesting materials in photocatalytic applications. For example, the photocatalytic H2 evolution activities of CdSe/CdS core/shell structures are greater than those of bare CdSe QDs71. The difference is attributed to the former structures promoting electron– hole separation and passivation of surface-​deep trap states with energies below the 2H+/H2 couple. Similarly, CdS/CdSe core/shell structures are superior to CdS QDs owing to enhanced light absorption and electron–hole separation72 (Fig. 4c). These examples indicate that the construction of core/shell QDs featuring either type I or type II band alignment is emerging as an effective strategy for the photocatalytic HER. Surface design. Up to now, we have focused our discussion on optimizing the size and structure of the QD inorganic framework such that it can better absorb solar light to generate and deliver separated charges to cocatalysts. We note that the surface of the framework typically features a layer of ‘ligands’, such as organic molecules, polymers, metal-​free inorganic ions or molecular metal chalcogenide complexes. This layer can have important effects on the synthesis, properties, processing and applications of QDs73–75. For instance, surface ligands impart solubility on QDs, prevent aggregation, passivate surface trap sites and control the exciton coherence length and energy76–79. The redox catalysis cannot proceed unless

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Reviews the charges generated can move through the inorganic framework, tunnel through the ligand layer and make it to the active sites. In this section, we discuss the progress in our understanding of how the surface ligand layer can affect the outcome of photocatalytic H2 evolution. In the early days of employing colloidal semiconducting QDs as light absorbers in artificial photosynthesis, the semiconductor core and surface ligand layer were regarded as a whole. These materials typically feature ligand-​capped QDs (usually thiolates) that absorb incident photons and deliver photoelectrons to molecular HER cocatalysts, such as [FeFe]-H2ase mimics47 and Ni complexes48. The surface ligands were thought of solely in terms of stabilizing the aqueous colloidal QDs towards aggregation, which would be to the detriment of the solar H2 evolution performance. Thus, multidentate ligands have been used to further stabilize colloidal QDs. For example, the trithiol 2,2,2-tris(mercaptomethyl)acetic acid can bind CdSe QDs tightly, which serve as stable light absorbers that combine with [Co(1,2benzenedithiolato)2]− for solar H2 evolution in the presence of H2Asc80. The trithiol binds the CdSe surface more tightly than monothiol or dithiol capping agents, ensuring good dispersity and stability in H2O, along with superior photocatalytic properties. With the above advances in the present field came the recognition that large organic ligands at the surface of colloidal QDs play vital roles in influencing H2 photogeneration. Such ligands affect surface trap states and related charge separation and serve as an insulating layer to determine the efficiency of charge movement in and out of the core material. Recently, it has been found that the surface-​trap-related charge separation efficiency is affected by surface ligands, such that photocatalytic H2 evolution is also distinctly ligand-​dependent81. For example, in the case of almost monodispersed CdSe/ CdS QDs, it has been found that using ligands with more thiol groups decreases the photoluminescence intensity and increases the efficiency of H2 photogeneration. The shielding effect of the organic ligand layer can be reduced by removing some ligands or exchanging them for inorganic ligands82–84. For example, ligand removal from stabilized CdSe QDs enables them to function as better catalysts for the photocatalytic conversion of thiols into their corresponding disulfides and H2 (ref.56). It was recently found that the rate of H2 photogeneration at ‘bare’ CdS QDs is 175 times greater than that at MPA-​ capped CdS QDs in the presence of the HER cocatalyst CoCl2 and the sacrificial electron donor Na2SO3 (ref.85). In this case, the presence of organic ligands is detrimental because they represent a physical barrier restricting the access of cocatalysts to CdS and more generally limit mass and charge diffusion to and from the QD surface. The utility of the inorganic ligand strategy is demonstrated by the inorganic ligand-​stabilized CdSe/CdS QDs, resulting in an eightfold increase in H2 evolution activity relative to MPA-​stabilized CdSe QDs, with both experiments being conducted under the same conditions using Ni(OH)2 clusters as HER cocatalysts53. The shell of the hybrid features excess S2−, resulting in the electrostatic stabilization of colloidal QDs in H2O and strong electronic coupling between the colloidal QDs

and the cocatalyst Ni(OH)2 clusters. The latter is one of the main reasons for the superior photocatalytic activity of the core–shell structure. The discovery of the importance of electronic coupling was followed by efforts to specifically design surface ligands to enhance the coupling between QDs and external cocatalysts and/or sacrificial reagents. For instance, CdSe QDs stabilized with polyethyleneimine (PEI)86 allow for rapid solar H2 evolution from poly(acrylic acid)-immobilized [FeFe]-H 2ase mimics and H 2Asc (Fig. 5a) . The strong coordinate bonds between the amino groups in PEI and the surface Cd 2+ centres not only ensure the excellent dispersion of CdSe QDs under acidic conditions but also enhance their interaction with H 2Asc (Fig. 5b). The concentration of HAsc− around the QDs can be computed using

 eΨ   C S = C B exp  kBT   

(2)

where CS is the concentration of HAsc− around the surface, CB is the bulk concentration of HAsc−, e is the elementary charge, kB is the Boltzmann constant, T is the absolute temperature and λ is the electric potential. The presence of the PEI layer leads the concentration of HAsc− absorbed at the QD surface to increase from 0.07 mol L−1 to 0.27 mol L−1, with an ~30-fold increase in the rate of hole transfer from the QDs to HAsc−. This enhanced interfacial charge transfer led to a TON of 83,600, the highest value for photocatalytic H2 evolution using [FeFe]-H2ase mimics as cocatalysts. A similar study that aimed to increase coupling made use of acrylate-​ stabilized CdSe/CdS core/shell QDs, which could be crosslinked to form a network that also included Pt nano­ particles bridged to the light absorbers87. The resulting short distance between the QDs and the HER cocatalyst particles enables ultrafast electron transfer (~65 ps), as confirmed using femtosecond transient absorption spectroscopy and X-​ray transient absorption measurements. The enhanced electronic communication contributes to the overall system, giving an initial H2 evolution QY of ~65% and a TON > 1.64 × 107 per Pt nanoparticle under visible light. Based on our discussion above, we can conclude that photocatalytic H2 evolution from QD-​based artificial photosystems depends not only on the QD size and structure but also on the surface characteristics of the QD light absorber. Additional influences on the STH conversion efficiency of a given system include the nature of the cocatalyst, the concentration ratio of light absorber to cocatalyst and the interactions between these two components. Thus, the design of a highly efficient photocatalytic H2 evolution system requires many factors to be taken into consideration in order to arrive at the optimal conditions.

Cost-​effective solar H2 evolution using QDs Colloidal semiconducting QDs with optimized size distribution, shell material and thickness and surface ligands come with advantages in light absorption, exciton www.nature.com/natrevchem

© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Reviews a NH2

N

H2N

NH2

HN

N

N H

H N

N

NH2 n

N

H2N

NH2

2H+ e

e

–

hν

–

H2 CdSe QD HAsc–

h+

h+ Asc + H+

CO2H

O C

CONHR m

n

S

R=

NC

S

Fe

CO CO

Fe C CO O

b

HAsc–

HAsc–

Polyethyleneimine modification

HAsc– HAsc– HAsc– HAsc

HAsc– HAsc–

HAsc– HAsc–

–

HAsc–

HAsc–

HAsc– HAsc–

HAsc–

Fig. 5 | influence of surface ligands on photocatalytic h2 evolution. a | Schematic illustration of a CdSe quantum dot (QD) with a poly(acrylic acid) coating bearing diiron H2 evolution cocatalysts. The material can be further decorated with polyethyleneimine, a branched polymer that is protonated in aqueous solution to afford a QD with a cationic exterior. b | The charge on QDs featuring polyethyleneimine causes electrostatic attraction to hydrogen ascorbate (HAsc−). h, Planck constant; v, photon frequency. Figure adapted from ref.86, CC-​BY-4.0.

generation and charge separation that lead to unprecedented activities and stabilities in photocatalytic H2 evolution. However, the promising performances of QDs in solar fuel generation have typically come when using sacrificial reductants, which are neither sustainable nor cost-​effective. Photocatalytic generation of H2 in a more environmentally benign and cost-​effective manner is highly desired, and we now describe some recent promising approaches towards this. Photocatalytic valorization of biomass to give H2. Biomass — from raw materials to intermediates — is the most abundant renewable chemical resource on Earth88,89 and an affordable and renewable precursor to H2 gas. The photocatalytic reforming of biomass is attractive because oxidation of the biomass and the accompanying H2 evolution can proceed at ambient temperature and pressure using solar light as the sole energy input. Biomass reforming is energetically more favourable than H2O splitting, and further purification

of H2 is not needed because it is the only gaseous product in most such reforming processes. Thus, directly reforming unprocessed biomass and its intermediates using colloidal QDs has the potential to provide affordable and clean energy from locally sourced materials and/or waste. The first example of H2 production from the reforming of a bio-​based chemical mediated by QDs and non-​n oble metal cocatalysts came in 2014, with an in situ-​generated photocatalyst from MPA-​coated CdS QDs and NiCl2 decomposing glycerol and evolving H2 gas (~74.6 mmol h−1 mgQDs−1)90. It was proposed that photoelectrons are transferred to Ni, which attaches to two surface S2− ligands and serves as the HER active site. Concomitantly, the photogenerated holes can be trapped by H2O or OH− on the surface of CdS QDs to give OH∙ radicals, which, in turn, convert glycerol into oxidized species such as hydroxy ethyl acetate, 2,5-dimethyl-1,4-dioxane and 2,6-dimethyl-1,4-dioxane. In addition, photogenerated holes can also react directly with glycerol at the catalyst surface. Lignocellulose (Fig. 6a) is the most abundant form of biomass in the world, and its valorization to give H2 gas is an attractive avenue for renewable fuel production. However, state-​of-the-​art approaches to solar-​driven reforming of lignocellulose to H2 gas at ambient temperature are currently limited to noble-​metal-based photosystems that exhibit very low activity under UV light. In an encouraging recent study, a photocatalytic system based on CdS/CdOx QDs generated in situ under highly alkaline conditions was shown to photoreform cellulose, hemicellulose and lignin into molecular H2 at room temperature91 (Fig. 6b). The layer of CdOx on the QD surface not only ensures light absorption by the CdS core but also prevents photo-​ oxidation of the QDs. Surprisingly, this system is stable after operation for at least 6 days and is even able to reform unprocessed lignocellulose, such as grass, wood and paper (Fig. 6c). The system operates under solar irradiation at room temperature, such that it is an inexpensive route to using waste biomass. Mechanistic investigations indicated that the HER occurs at surface Cd0 sites generated on the QD surface, while lignocellulose oxidation occurs through the formation of OH∙ radicals. Photoelectrochemical H2O splitting. A great challenge associated with the overall H2O splitting reaction is how to design a single photocatalytic system that can absorb visible light and access excited states whose e− and h+ energies match the redox potentials of both H+ reduction and H2O oxidation. The sluggish kinetics of charge separation and the propensity of materials to undergo oxidative decomposition greatly limit overall H2O splitting in aqueous solution. However, these disadvantages can be overcome by using integrated PEC H2O-​splitting approaches (Box 2). A conventional PEC system typically includes at least one photoactive electrode — either the cathode or the anode — as the working electrode (Box 2; see the figure, panels a and b). The dark counterelectrode, usually a Pt disk, is connected to the photoactive working electrode through an external

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Reviews The utilization of QDs in constructing photoresponsive electrodes has been of intense interest. Compared with QD-​sensitized photocathodes, QD-​sensitized photoanodes are much easier to construct, as described in useful recent reviews92,93. We instead focus on QD-​based photocathodes for H2 evolution, the first of which was reported in 2011 (ref.94). The photocathode featured a Au substrate linked to InP QDs through 1,4-benzenedithiolate ligands. The light absorbers are complemented by the [FeFe]-H2ase active site mimic [Fe2S2(CO)6], which serves as the H2 evolution cocatalyst. Under 395 nm light irradiation and a potential of 400 mV versus Ag/AgCl, this photocathode could evolve H2 gas to give nA-​scale

circuit. The combination of the above two electrodes immersed in the electrolyte solution comprises the basis of a PEC cell. In most cases, the introduction of a photoelectrode can contribute only part of the driving force for H2O splitting, such that an external bias is still needed in single-​photoelectrode PEC systems. However, if both the cathode and the anode are photoresponsive, then one can realize overall H2O splitting solely using solar light in an unassisted PEC cell (Box 2; see the figure, panel c). This device can be regarded as an ideal choice for solar-​to-fuel conversion, in which H2O and solar light are the only inputs required for H2 and O2 production. b

a

2H2O

OH OH

O HO

OH

Cd

O OH

H2 + 2OH–

hν

O

HO O

O

n

Cellulose

S HO2C

O

O HO O

O AcO

OH

OAc

O

O

O O HO

Hemicellulose

OH

O

e– O

O

OH

O

h+

HO

Lignocellulose O

OH

HO

HCO2– + CO32– + oxidized organics

OH OH

O

Lignocellulose

O

OMe

HO OMe

O

O OMe

Lignin

c

Grass

Paper

Wood

Substrate Suspended (wood) CdS/CdOx

d 14

Amount of gas evolved (μmol)

HO

Charge H2 O2

12 10 8 6 4 2 0

H2 bubble

0

30

60

90

Time (min)

Fig. 6 | Cost-​effective solar h2 evolution using QDs. a | Lignocellulose exists as microfibrils in plant cell walls and is comprised cellulose surrounded by the less crystalline polymers hemicellulose and lignin. b | Under visible-​light irradiation, these components can be photoreformed into H2 using CdS quantum dots (QDs) coated with CdOx (the oxide surface is believed to contain some OH− ligands; H atoms omitted for clarity). Light absorption by CdS generates e−–hole (h+) pairs, which travel to the CdOx surface and participate in H+ reduction and lignocellulose oxidation, respectively. c | The core/shell QD is a highly robust photocatalyst able to generate H2 from crude sources of lignocellulose, such as grass, paper or wood. Sunlight exposure of QDs in alkaline solution triggers photoreforming, as evidenced by H2 bubbles. d | Instead of using sunlight to split biomass, one can also split H2O by using a photoelectrochemical cell constructed of an array of CdS QD-​modified TiO2 nanorods serving as the photoanode and CdSe QD-​modified NiO nanosheets serving as the photocathode. H2 and O2 are evolved in a 2:1 molar ratio as charge passes from the anode to the cathode through an external circuit. h, Planck constant; v, photon frequency. Parts a–c are adapted from ref.91, Springer Nature Limited. Part d is adapted with permission from ref.105, American Chemical Society. www.nature.com/natrevchem © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Reviews photocurrent at a Faradic efficiency of ~60%. Despite the extremely low current densities observed, this simple PEC system laid the foundation for the construction of QD-​sensitized photocathodes. A wave of progress in both current density and Faradic efficiency came by using NiO as a hole-​transfer layer to construct QD-​s ensitized photocathodes 95.

Box 2 | Fundamentals of PeC cells for overall h2O splitting Photoelectrochemical (PeC) cells consisting of a photoelectrode (anode or cathode) and a dark electrode usually require both an external bias and light irradiation for overall H2O splitting (panels a and b of the figure). the introduction of a photoelectrode lowers the voltage required by replacing some of the electrical power with solar power. a typical integrated PeC H2O-​splitting system comprises a photocathode and a photoanode sensitized by two different light absorbers (panel c of the figure). For an ideal PeC system, pH-​neutral aqueous electrolyte contacts the two electrodes, and the energy of the visible light enables H2O to be split into H2 and O2 in a 2:1 molar ratio at the cathode and anode, respectively. under visible-​light irradiation, the photocathode will absorb photons with energies equal to or greater than the bandgap, thus generating e−–h+ pairs. the photogenerated electrons migrate to the active sites to reduce protons into H2, while the holes transfer to the counterelectrode through the external circuit to oxidize H2O into O2. thus, a well-​designed PeC cell featuring both a photocathode and photoanode can mediate overall H2O splitting in the absence of a sacrificial reagent. Moreover, the H2 and O2 products form in different locations such that gas separation is not required, a practical advantage of this approach.

a

e–

V

e–

½O2

H2

hν

2H+

2H+

H2O

2H+

Photoanode

Membrane

b

e–

V

Cathode e–

½O2

H2 2H

2H

+

hν

+

H2O

2H+

Anode

Membrane

c

e–

Photocathode e–

½O2 hν

H2 2H+

Photoanode h, Planck constant; v, photon frequency.

hν

2H+

H2O

2H+

Membrane

Photocathode

A NiO film can serve as the substrate onto which one can anchor CdSe QDs to construct a sensitized photocathode. With the aid of H2 evolution cocatalysts — either [Co(bdt)2]− (bdt2− = 1,2-benzenedithiolate) or [Ni(DHLA)x](2−x)+ (DHLA− = dihydrolipoate) — a current density of approximately −2 mA cm −2 was obtained, and H2 gas evolved with a Faradic efficiency of 100 ± 2%. However, the electrolyte contains large concentrations of Cl−, which can undergo oxidation in place of H2O at the counterelectrode. Related later work described a PEC system also based on a CdSe QD-​sensitized NiO photocathode but using Na2SO4 electrolyte without any molecular cocatalysts. The system can afford a photocurrent density of approximately −60 µA cm −2 at a bias of −0.1 V versus the normal hydrogen electrode (NHE) and near quantitative Faradaic efficiency with no distinct decline over 45 h of operation96. More importantly, electron paramagnetic resonance spectra suggested the presence of OH∙ radicals on the Pt anode surface, directly confirming the oxidation of H2O. Further investigations indicated that the introduction of a hole-​transfer (redox-​active) ligand, such as phenothiazine97, on the surface of the QDs can increase the current density to −180 μA cm−2, such that H2 is evolved at a rate of ~3,000 μmol  h−1 gQDs−1 cm−2. Recent studies aimed at developing more active PEC H2 evolution systems have focused on promoting interfacial charge transfer. These systems feature regular NiO arrays98, 3D graphdiyne on Cu (ref.99) and rainbow photocathodes constructed by spin-​coating CdSe QDs of different sizes in a sequential manner100 to construct QD-​based photocathodes. QDs made from other metal semiconductors, such as CdTe (ref.101) and InP/ZnS (ref.102), have also been used and can split H2O when combined with H2 evolution cocatalysts such as MoS2 (ref.103) or cobaloximes104. However, the PEC systems we have mentioned here each have only a single working photoelectrode (usually a QD-​sensitized photocathode) and thus still require an external bias to split H2O. How can we split H2O into H2 and O2 from neutral H2O in the absence of an external bias? In principle, a PEC system featuring a single photoelectrode could have the potentials necessary for overall H2O splitting. In practice, it is very difficult to simultaneously meet the bandgap and redox potentials required to split H2O. Solving this dilemma could involve using integrated PEC systems consisting of a photocathode and photoanode with matched band-​position alignment. For example, an unassisted QD-​based PEC cell comprising a CdS QD-​modified TiO2 nanorod photoanode coupled to a CdSe QD-​modified NiO nanosheet photocathode105 mediates spontaneous overall H2O splitting in neutral H2O under visible-​light irradiation (λ > 400 nm). The optimized system with appropriate surface protection affords average gas evolution rates of 2.24 (H2) and 1.07 (O2) mmol h−1 cm−2, corresponding to a Faradic efficiency of 95% for H2 evolution (Fig. 6d) . Although the STH conversion efficiency is low (0.17%), it is comparable to the efficiency of natu­ ral photosynthesis12 and validates the concept of STH conversion devices for spontaneous overall H2O splitting.

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Reviews Table 2 | Current reported PeC h2 evolution systems based on semiconducting QD photocathodes Photocathode

electrolyte (ph)

Bias

Current density

Faradic efficiency

Au/poly(3-hexylthiophene)/CdSe QDs/Pt

0.1 M phosphate buffer (pH 7)

0 V versus RHE

−1.2 mA cm−2

80–100%

121

NiO/CdSe QDs/NiS

0.5 M Na2SO4 (pH 6.8)

0 V versus Ag/AgCl

−130 µA cm

95%

105

NiO/CdSe QDs/ [Co(1,2-benzenedithiolate)2]−

0.1 M KCl (pH 7)

−0.28 V versus RHE

−2 mA cm−2

100 ± 2%

95

NiO/CdSe QDs

0.1 M Na2SO4 (pH 6.8)

−0.1 V versus NHE

−60 µA cm−2

~100%

96

NiO/CdSe QDs/cobaloxime

0.1 M Na2SO4 (pH 6.8)

0 V versus NHE

−110 µA cm−2

~81%

104

NiO/CdTe QDs/NiS

0.1 M phosphate buffer (pH 6)

−0.222 V versus Ag/AgCl

−40 µA cm−2

~100%

101

NiO/phenothiazine/CdSe QDs

0.1 M Na2SO4

−0.1 V versus NHE

−180 µA cm−2

~100%

97

Graphdiyne/CdSe QDs

0.1 M Na2SO4

0 V versus NHE

−70 µA cm

90 ± 5%

99

NiO/CdSe QDs/ [Co(1,2-benzenedithiolate)2]−

0.2 M hexamethylenetetramine/ HCl buffer with 0.1 M KCl (pH 6)

0 V versus Ag/AgCl

−115 μA cm−2

99.5%

100

NiO/CdSe QDs/[Fe2S2(CO)6]

0.1 M Na2SO4 (pH 6.8)

−0.1 V versus NHE

−56 μA cm−2

52%

122

−2

−2

ref

NHE, normal hydrogen electrode; PEC, photoelectrochemical; QD, quantum dot; RHE, reversible hydrogen electrode.

In considering the state-​of-the-​art QD-​based PEC systems for H2O splitting (Table 2), we note that most of the devices can give a photocurrent density only in the realm of μA. Future research will be motivated by improving these current densities and STH efficiencies while maintaining device stability. Many strategies exist to achieve these objectives, including the design and synthesis of QDs with exceptional solar-​light response, perhaps with structures and/or surfaces engineered for favourable interfacial charge separation and migration. Novel materials for the anodic reaction would promote hole capture and transfer, while efficient H2 evolution cocatalysts would rapidly mediate the cathodic HER. With these materials in hand, one would then turn to developing new approaches for PEC device construction.

Conclusions and outlook In this Review, we have summarized the state-​of-the-​art artificial photosynthetic H2 evolution systems based on IIB–VIA semiconducting QDs. We described how size optimization, structure modification and surface design enable us to improve the photocatalytic H2 evolution activity of QD-​based artificial photosystems. Once optimized in terms of size distribution, shell material and thickness and surface ligands, QDs have excellent light absorption, exciton generation and charge separation properties. In this way, QDs have been shown to be ideal components in photocatalysts with unprecedented activity and stability for photocatalytic H2 evolution in the presence of sacrificial reagents. Generating molecular H2 in an environmentally benign manner could involve either photocatalytic H2 generation from biomass valorization or PEC H2O splitting using semiconducting QDs. The low cost and wide abundance of biomass make it a 1.

2.

Armaroli, N. & Balzani, V. The future of energy supply: challenges and opportunities. Angew. Chem. Int. Ed. 46, 52–66 (2007). Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar energy utilization. Proc. Natl Acad. Sci. USA 103, 15729–15735 (2006).

3. 4. 5.

useful feedstock for STH conversion. We can also construct unassisted PEC devices to effect overall H2O splitting from pH-​neutral solution at almost 100% Faradic efficiency. Translating either of these approaches into practical technologies will require fundamental research into modifying QDs, exploring new cocatalysts and rationally designing and constructing electrodes. Only then will we attain useful QD-​based PEC devices with the STH efficiencies and stabilities required for energy conversions. In addition to the desirable electronic properties of semiconducting QD light absorbers, they have certain intrinsic properties that will be advantageous in building artificial photosystems for large-​scale and facile solar-​ to-fuel conversions. We have every reason to believe that producing sustainable, usable and renewable forms of energy from solar light will become viable on a large scale in the near future. First, the QDs in typical usage consist of IIB and VIA elements —materials that are free of expensive noble metals. Second, after decades of development, we have mature approaches for the low-​cost, large-​scale, aqueous-​phase synthesis of high-​ quality QDs106. Third, the fabrication of highly efficient solar energy conversion photosystems from environmentally friendly QDs is just around the corner. Indeed, very recently, some Cd-​free and visible-​light-responsive QDs or nanocrystals of ZnSe (ref.107), ZnTe (refs108,109), ZnS (refs102,110) and their heterostructures111–113 have been incorporated into solar-​to-fuel conversion systems. Thus, we foresee a future in which semiconducting QDs will be an ideal choice to address many questions arising in energy research and beyond. Published online xx xx xxxx

Gray, H. B. Powering the planet with solar fuel. Nat. Chem. 1, 7 (2009). Nocera, D. G. Solar fuels and solar chemicals industry. Acc. Chem. Res. 50, 616–619 (2017). Lubitz, W. & Tumas, W. Hydrogen: an overview. Chem. Rev. 107, 3900–3903 (2007).

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Armaroli, N. & Balzani, V. The hydrogen issue. ChemSusChem 4, 21–36 (2011). Kim, D., Sakamoto, K. K., Hong, D. & Yang, P. Artificial photosynthesis for sustainable fuel and chemical production. Angew. Chem. Int. Ed. 54, 3259–3266 (2015).

www.nature.com/natrevchem © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

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18.

19. 20.

21.

22. 23.

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26.

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28.

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31.

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Acknowledgements The authors are grateful for financial support from the Ministry of Science and Technology of China (2014CB239402 and 2017YFA0206903), the National Science Foundation of China (21390404, 21861132004 and 21603248), the Strategic Priority Research Program of the Chinese Academy of Science (XDB17000000), Key Research Program of Frontier Science of the Chinese Academy of Sciences (QYZDY-​ SSW-JSCO29) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2018031).

Author contributions All authors contributed to researching the article, discussing the content and writing and editing of the article.

Competing interests The authors declare no competing interests.

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