The surface terminations of the three samples were validated by Fourier Transformed Infrared (FTIR) spectroscopy as shown in Figure . A strong C=O stretching band at 1780 cm -1 related to carboxyl groups is clearly visible on ND-COOH. For ND-OH, the major contribution comes from OH stretching (3200-3600 cm -1 ) and OH bending (1640 cm -1 ) modes from hydroxyl groups, although a contribution from adsorbed water molecules cannot be excluded. The appearance of a broad band around 1100 cm -1 is however more specifically assigned to C-O stretching from hydroxyl groups on NDs. Additionally, CHx groups of primary and secondary hydroxyl functions can be seen at 2800-2900 cm -1 . A small band at 1730 cm -1 related to residual carbonyl is detected but is smaller than the band related to OH bending modes. On ND-H, no C=O band is detected but a clear double peak (2875 and 2945 cm -1 ), characteristic of CHx groups on NDs is clearly visible. In summary, the FTIR confirms the presence of carboxylic acids on ND-COOH and shows the efficient hydrogenation of ND-H after plasma treatment and hydroxylation of ND-OH after reduction of carboxyl and carbonyl groups. In the case of DUV excitation, the probe range is limited to the range 500-750 nm, due to the different probe light generation scheme. The signal obtained for NaI is similar to previously reported studies. For both excitation scenarios, ND-H exhibits a clear positive transient signal at positive delay times. As observed for VIS excitation, the TA signal peaks around 750 nm after about 1 ps, similarly to NaI. The blue shift of the TA maximum throughout the first 1 ps observed for NaI cannot be clearly distinguished for ND-H. The 2D TA traces for the other ND samples are available in the SI (Figure ). In general, the acquired TA spectra are broad and featureless, and substantially weaker than for NaI solutions. We attempted time-resolved photoemission spectroscopy (PES) to gain further insights on the electronic states of the solvated electrons. However, stabilizing a microjet of ND dispersion in vacuum for several hours, as needed for the measurements, turned out to be very challenging. Moreover, due to the low concentration of NDs and the limited probing depth of few nanometres in aqueous solutions, the transient PES signal of solvated electrons was below the detection threshold. Therefore, we concentrate in the following on the temporal profile of the integrated TA response, which provides more information on the dynamics of the electron emission process. The integrated TA traces over the probe wavelength region of 600 -740 nm for ND-H, ND-OH and ND-COOH under DUV excitation are shown in Figure . For ND-H, a strong negative TA signal around time zero is observed, which is absent in NaI solution, pure water, and for the other NDs characterized with similar pump laser intensity (Figures and). After about 1 ps, a positive signal reaches a maximum and slowly decays over the probed timescale of 30 ps. For ND-OH and ND-COOH, a strong increase of the TA signal occurs in the first 500 fs and it progressively decays similarly to ND-H. Compared to NaI, both build-up and decay kinetics in the first 2 ps appear generally faster for the NDs, with minor differences between the surface terminations. Due to the observed similarities of both their spectral and temporal features, we assign the positive TA signals in the ND dispersions to the emission of solvated electrons upon DUV excitation. The emission of solvated electrons is therefore observed for all surface terminations after DUV excitation above the diamond bandgap. Slight temporal differences are observed between ND-OH and ND-COOH, which are likely related to different solvation of the emitted electron in the first picoseconds. Both terminations have indeed opposite zeta-potential, which suggests a different organization of the first hydration shell, hence impacting the electron solvation. In general, the more ordered water hydration structure observed around NDs 38 may lead to faster kinetics compared to solvated electrons emitted from iodide. The early dynamics was also found to be faster on detonation NDs compared to larger NDs synthesized from High Pressure High Temperature (HPHT) synthesis for carboxylated surfaces (Figure ). Even though solvated electrons are detected from ND-OH or ND-COOH under DUV light, oxidized diamond surfaces have previously shown much lower photocatalytic activity than hydrogenated diamond surfaces. This can be explained by a surface-dependent diamond-water band alignment as detailed below. Furthermore, an additional ultrafast process occurs only on ND-H during the first picosecond, which will also be interpreted in the following. For VIS excitation, an analogous treatment of the 2D TA maps is shown in Figure . Compared to the previously used DUV pulses, the VIS pump light contains around 70 times more photons per pulse, which leads to a pronounced and complex coherent artefact also observed for pure water (Figure ). Apart from that, the spectro-temporal behaviour for NaI solution under VIS excitation appears quite similar to that under DUV excitation. For NDs, two notable differences are found compared to DUV excitation. Firstly, no negative TA can be observed for any of the samples, especially not for ND-H. Secondly, a substantial positive TA signal after the initial ultrafast time range is only observable for ND-H. Similar to the DUV excitation, the kinetics in ND-H are slightly faster than in the respective NaI solution. We interpret the different behaviour of ND-H compared to ND-OH and ND-COOH (further collectively referred to as ND-O) for both DUV and VIS excitation by distinct electron emission processes summarized in Figure . First of all, surface termination is affecting the diamond electron affinity (EA). The EA of (111) diamond surfaces terminated with hydrogenated and hydroxylated surfaces were calculated in vacuum and with a thin water film using DFT calculations (see SI). While both surfaces were found to have a negative EA in vacuum (-1.0 eV for H-termination and -0.4 eV for OH-termination), only the H-terminated surface maintains a negative EA with a thin adsorbed water film at neutral pH (-0.3 eV for H-termination and +0.7 eV for OH-termination). A positive EA is also expected for carboxylated diamond surface. Despite its positive EA, the electron emission is possible for ND-O because the water CBM might sit below the CBM of ND-O, as illustrated in Figure . The surface termination of NDs strongly influences the orientation of the first water solvation shells via the strength of the hydrogen bonds. This can in turn modify the structure of the next few hydration layers and hence the EA of this interfacial water region around the ND. Previous DFT calculations have indeed shown that the bulk EA of water (0.2 eV) is different from surface water close to a vacuum interface (0.8 eV). In any case, the electron emission from hydrogen-terminated diamond is more efficient due to the larger energy difference between diamond and water CBM, ensured by a negative EA, as well as the possibility to emit electrons through surface C-H states. Nevertheless, the H-terminated surface alone cannot explain electron emission upon VIS excitation, as it would only allow electron emission for excitation with photon energies above 4.5 eV. Another absorption process must therefore take place on ND-H upon VIS excitation. In contrast, the negative ultrafast TA signal only observed for the ND-H under DUV illumination suggests that an additional process occurs for ND-H during the first picosecond. It could be due either to stimulated emission or a ground-state bleach of a species to be identified. Raman spectroscopy showed that the surface termination of ND-H is not only composed of CHx groups but also contains small sp 2 -hybridization islands, as previously reported on similarly prepared ND-H. Sp 2 carbon quantification based on near-edge X-ray absorption fine structures (NEXAFS) on NDs hydrogenated, either by plasma or by annealing treatments, have concluded that up to 20% of the carbon atoms may be sp 2 -hybridized, predominantly those situated at the ND surface. Such sp 2hybridized carbon atoms are mostly localized on fullerene-like reconstructions (FLRs) which were also identified on annealed NDs. These non-diamond carbon atoms induce new states below the CBM of ND-H, which can contribute to visible light absorption and were found to increase significantly the photoluminescence of ND-H, compared to oxidized surfaces. To quantify the energy levels of an sp 2 -decorated ND, we performed hybrid-DFT calculations on a series of ND structures with various degrees of FLRs coverage. The structures were optimized by tight-binding DFT and subsequent single point calculations were performed on PBE0-D3/SVP level of theory. The corresponding structures and their orbital energies are depicted in Figure . The ND-H has a negative electron affinity of -0.93 eV, which is similar to the calcula on performed on H-terminated (111) diamond, and a HOMO-LUMO gap (HLG) of 6.46 eV. This HLG is higher than the bandgap of bulk diamond (5.5 eV) due to the quantum confinement effect of small-sized ND. Upon addition of sp 2 carbon on the ND, the frontier orbital energies are shifted upwards (HOMO) and downwards (LUMO), resulting in decreased HLGs. These frontier orbitals and the next few highest occupied/lowest unoccupied orbitals of the NDs partially covered by FLRs are all located in the respective sp 2 -hybridized carbon areas. Contour plots of the relevant orbitals are shown in the ESI. With increasing FLRs coverage, additional orbitals are present within the range of the ND-H's HLG. Interestingly, the HOMO energies are highest with a low amount of FLRs present and decrease with larger reconstructed areas. This decrease of HOMO energies is, however, not monotonic. In contrast, the systems' LUMO energies decrease monotonically with increasing FLRs coverage. We further computed optical absorption spectra of smaller NDs with 0, 50 and 100 at% surface reconstruction, each surrounded by an explicit layer of water molecules (ESI). The spectra confirm that the optical gap of a H-terminated ND is also significantly reduced upon surface reconstruction. These findings support the hypothesis that sub-bandgap states are introduced in NDs upon formation of FLRs on their surface, and even a small coverage (e.g., 12 % of surface atoms) is sufficient to introduce them. The additional unoccupied surface states may act as trap states that, upon DUV excitation, induce ultrafast recombination of the excited electrons, which is detrimental to the solvated electron emission as illustrated in Figure . An alternative process may be recombination of electrons from an occupied surface state with valence band holes in the excited ND. In either case, the electrons emerging from fully H-terminated areas on the ND-H surface may not be affected by these defect states and are then emitted into the water. As a result, the ultrafast TA response of the ND-H dispersion upon DUV excitation consists of two parallel processes: (i) a strong recombination at early stages due to defect states and (ii) the emission of solvated electrons from fully hydrogenated regions on the ND-H. The presence of sp 2hybridized defect states may explain the limited CO2 reduction activity of detonation hydrogenated NDs under DUV light compared to hydrogenated NDs with a higher crystallinity. In the case of ND-O, the amount of surface sp 2 carbon is significantly reduced in the process of surface termination and therefore no such recombination is evidenced. The emission of solvated electrons from ND-H upon VIS excitation can be explained by a combination of sp 2 -hybridized islands and fully H-terminated surface areas. FLRs may enable light absorption while hydrogen-terminated functional groups offer favourable negative EA properties. Such an inhomogeneous electron emission model was first proposed by Cui et al. to explain sub-bandgap electron emission from diamond surfaces in vacuum. Our work demonstrates that a similar process occurs on ND with much smaller sp 2 -hybridized surface structures. We anticipate that by controlling more precisely the ratio between hydrogenated surface groups and FLRs content, the electron emission properties of ND-H under VIS light excitation could be further optimized. While sp 2 -hybridized states function as intermediate states absorbing in the VIS region, other (mixed) covalent surface functionalization may be considered. An essential aspect for efficient solvated electron emission would be to ensure a good electron transfer between the absorbing surface states/molecules and the electron-emitting surface groups to benefit most from the unique electronic properties of H-terminated diamond surfaces.