Qiyang Zhao, Yongtao Li, Yun Song, Xiaoli Cui, Dalin Sun, Fang Fang a)
Department of Materials Science, Fudan University, 220 Handan Road, Shanghai
200433, China a) To whom correspondence should be addressed. E-mail: f_fang@fudan.edu.cn
Supplementary Information
List of supplementary informations
1
2
3
4
The experimental detail
The comparison of the response characteristics between amorphous and crystalline MgNi
0.09
film.
The detail about the QCM method
The QCM result for the MgNi
0.09
film with a thickness of 115 nm

Supplementary Information 1:
Inductively couple plasma was performed on Hitachi P-4010 to examine the composition of the prepared films. The samples for ICP were prepared by dissolving thin films with dilute nitric acid. X-ray diffraction was carried out to reveal the phase components on a Rigaku D/max 2400 with Cu K
α
radiation at 40 kV and 100 mA. Transmission electron microscope was performed on the JEOL JEM-2010 to examine the microstructure. The optical and electrical response characteristics were investigated in-situ in a tailor-made cell. The details of the tailor-made cell were described elsewhere.
1 For in-situ measurements, the optical response was monitored by a combined use of a laser diode (λ = 535 nm) and a Si photodiode, the electrical response by a four-probe resistivity measurement setup in a Van der Pauw configuration and the amount of desorbed hydrogen by quartz crystal microbalance (QCM) method during hydrogen loading/unloading.

Supplementary Information 2:
The preparation of crystalline Mg and MgNi x
films was similar to that of amorphous ones, but the deposition rates were changed to be 2.8 Å/s for Mg (100 W RF power), 0.1–0.9 Å/s for Ni (3–20 W DC power) and 2
Å/s for Pd (180 W RF power). The optical and electrical transitions characteristics of crystalline films were investigated at room temperature and the results showed that: (i) the crystalline Mg film cannot be switched to transparent state under 0.1 MPa H
2
pressure within 2 h; and (ii) the response times for crystalline MgNi x
films are about nine times longer than those for amorphous films. The results of crystalline MgNi
0.09
film are shown in Figs. S1 and S2 as a typical example and compared with amorphous MgNi
0.09
film.
From the XRD patterns in Fig. S1, only the Si peaks arising from Si substrate and weak Pd peaks can be found for amorphous MgNi
0.09
film while in addition to Si and Pd peaks, a new Mg peak at about 34.4° is presented for crystalline MgNi
0.09
film, which agrees with previous reports on crystalline Mg-based alloy film.
2-5 Furthermore, the response characteristics of amorphous and crystalline MgNi
0.09
film are compared in Fig. S2. The optical response times for amorphous and crystalline
MgNi
0.09
films are 26 s and 220 s for hydrogen loading and 22 min and
263 min for hydrogen unloading, respectively. And the electrical response times for amorphous and crystalline MgNi
0.09
films are 44 s and 262 s for

hydrogen loading and 27 min and 300 min for hydrogen unloading, respectively. Hence, it is clear that the response times of amorphous
MgNi
0.09
film are about one tenth of those of crystalline MgNi
0.09
film, suggesting that amorphization is effective in improving the optical and electrical response of Mg-based alloy films.
Fig. S1 The XRD patterns for amorphous and crystalline MgNi
0.09
films
Fig. S2 The optical and electrical responses of amorphous and crystalline
MgNi
0.09
films upon hydrogen loading/unloading.

Supplementary Information 3:
In a classical paper in 1959, Sauerbrey described how a quartz-crystal resonator can be used as a microbalance, a so-called quartz-crystal microbalance (QCM).
6 The change in resonance frequency is proportional to the mass of an added film:
∆𝑓 =
−2𝑓
0
2
√𝜇 𝑞 𝜌 𝑞
∆𝑚
(1)
𝐴 where ∆𝑓 is the measured frequency change (Hz), 𝑓
0
is the fundamental frequency of quartz crystal (Hz), A is the electrochemically and piezoelectrically active surface area of the quartz crystal (cm 2 ), 𝜌 𝑞
is the density of quartz (2.648 g/cm 3 ), and 𝜇 𝑞
is the shear modulus of quartz (2.947*10 11 g/cm·s 2 ). For the quartz crystal used here ( f
0
= 6
MHz, A = 0.3*0.3*π cm 2 ), the relationship between the frequency shift
( ∆𝑓 , Hz) and mass change ( ∆𝑚 , μg) is simply expressed as follows:
∆𝑓 = −288.25 ∗ ∆𝑚 (2)
Fig. S3 shows the frequency (a) and the corresponding mass (b) change as a function of time for MgNi
0.09
film during hydrogen loading and unloading. During hydrogen loading, the frequency decreases from
5996613 to 5996558 Hz by 55 Hz, suggesting that MgNi
0.09
film absorbs
0.19 μg hydrogen. During hydrogen unloading, the frequency increases back to 5996610, indicating that the absorbed hydrogen desorbs from film.

Fig. S3 The frequency (a) and the corresponding mass (b) change as a function of reaction time for MgNi
0.09
film during hydrogen loading and unloading.

Supplementary Information 4:
115 nm-thickness MgNi
0.09
film is prepared, which consists of 100 nm
MgNi
0.09
layer and 15 nm Pd layer. Fig. S4 shows the frequency (a) and the corresponding mass (b) change as a function of time for 115 nm-thickness MgNi
0.09
film during hydrogen loading. The frequency decreases from 5998053 to 5998024 Hz by 29 Hz, suggesting that 115 nm-thickness MgNi
0.09
film absorbs 0.10 μg hydrogen. The amount of absorbed hydrogen is slightly more than half of 0.19 μg for 215 nm-thickness MgNi
0.09
film, revealing that the hydrogen mainly store in
MgNi
0.09
layer rather than in Pd layer.
5998060
(a)
5998055
5998050
5998045
H
2
on
5998040
5998035
5998030
5998025
5998020
-2 0 2 4 6 8 10
Time (s)
12 14 16 18 20
0.12
(b)
0.10
0.08
0.06
0.04
H
2
on
0.02
0.00
-2 0 2 4 6 8 10
Time (s)
12 14 16 18 20

Fig. S4 The frequency (a) and the corresponding mass (b) change as a function of reaction time for 115 nm-thickness MgNi
0.09
film during hydrogen loading.
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